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The surface effects of plasma on xerographic photoreceptors Yiu, Julian CH 2014

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The Surface Effects of Plasmaon Xerographic PhotoreceptorsbyJulian CH YiuB.Sc., The University of British Columbia, 2011A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF SCIENCEinThe Faculty of Graduate and Postdoctoral Studies(Chemistry)THE UNIVERSITY OF BRITISH COLUMBIA(Vancouver)July 2014c© Julian CH Yiu 2014AbstractXerographic photoreceptors play a large role in the printing process and advancements inphotoreceptor robustness and efficiency will have a positive impact on both the economyand environment. During the printing process, the photoreceptor is exposed to plasmawhich causes degradation leading to a loss of print quality over time and eventual replace-ment of the part. To this end, we utilize a combination of static SIMS, ATR-FTIR, andAFM to characterise both chemical and physical changes on the photoreceptor surfaceafter the photoreceptor had been brought to an end of life state. We also attempt tocorrelate changes in the photoreceptor to visible print defects in printed test pages. Ourresults point to evidence of binding polymer degradation at two specific locations in themolecule with water from the environment playing a large role in this process. Photorecep-tor stressing leads to deep regular undulations in the photoreceptor surface which can beseen via AFM. However, we did not find any significant observable differences in printedtest pages. Although we obtained evidence and clues on the general mechanism of photore-ceptor degradation, more work is required to properly identify degradation pathways andproducts. Further work is also required to properly determine the way in which chemicaldegradation manifests itself onto printed pages.iiPrefaceThis work was a collaboration between the Grant Lab at the University of British Columbiaand the Xerox Research Centre of Canada. Commercial samples were supplied by Xeroxas well as some pre-treated samples. The majority of sample preparation and stressingwas done by myself. Secondary ion mass spectrometry was performed by the InterfacialAnalysis and Reactivity Lab at the University of British Columbia. Other experimentswere carried out by myself. Analysis of mass spec data, FTIR data, and test pages wasalso done by myself although the code and program for test page analysis already existedprior to my involvement. All of the following thesis was written by myself.iiiTable of ContentsAbstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiPreface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iiiTable of Contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ivList of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viList of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viiAcknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 An Introduction to Xerography and the Xerographic Process . . . . . . . 11.2 Organic Photoreceptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31.2.1 Dual Layer vs. Single Layer Photoreceptors . . . . . . . . . . . . . 31.2.2 The Charge Transport Layer . . . . . . . . . . . . . . . . . . . . . 41.2.3 The Charge Generating Layer . . . . . . . . . . . . . . . . . . . . . 61.3 Current Knowledge on Photoreceptor Degradation and Our Research . . . 92 Instrumental Theory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.1 Secondary Ion Mass Spectrometry (SIMS) . . . . . . . . . . . . . . . . . . 112.2 Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3 Atomic Force Microscopy (AFM) . . . . . . . . . . . . . . . . . . . . . . . 153 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173.1 Preparation and Analysis of Drum Photoreceptor Samples . . . . . . . . . 173.2 Preparation and Analysis of Film Photoreceptor Samples . . . . . . . . . 183.3 Test Page Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193.4 SIMS Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20ivTable of Contents3.5 ATR-FTIR Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213.6 IPA Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223.7 AFM Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 224 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.1 Authentic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.1.1 SIMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234.1.2 ATR-FTIR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244.2 SIMS of Control and Worn Photoreceptors . . . . . . . . . . . . . . . . . 274.2.1 Drum Photoreceptors . . . . . . . . . . . . . . . . . . . . . . . . . 274.2.2 Film Photoreceptors . . . . . . . . . . . . . . . . . . . . . . . . . . 314.3 ATR-FTIR of Control and Worn Photoreceptors . . . . . . . . . . . . . . 314.3.1 Drum Photoreceptors . . . . . . . . . . . . . . . . . . . . . . . . . 314.3.2 Film Photoreceptors . . . . . . . . . . . . . . . . . . . . . . . . . . 344.4 AFM Images of Control and Worn Photoreceptor Surfaces . . . . . . . . . 374.5 Test Page Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 Photoreceptor Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . 415.1 Chemical Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 415.2 Physical Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 Closing Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51AppendixA Appendix A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54vList of Tables4.1 Table of Authentic Compound IR Bands . . . . . . . . . . . . . . . . . . . 264.2 Table of Drum Sample IR Bands . . . . . . . . . . . . . . . . . . . . . . . 334.3 Table of Film Sample IR Bands . . . . . . . . . . . . . . . . . . . . . . . . 35viList of Figures1.1 Simple Diagram of the Xerographic Process . . . . . . . . . . . . . . . . . 21.2 Basic Structure of Single and Dual Layer Photoreceptors . . . . . . . . . . 41.3 Common Binder Polymers Used in CTLs . . . . . . . . . . . . . . . . . . . 51.4 Common Charge Transport Moieties . . . . . . . . . . . . . . . . . . . . . 61.5 Examples of Charge Generating Pigments . . . . . . . . . . . . . . . . . . 71.6 Model for Internal Conversion . . . . . . . . . . . . . . . . . . . . . . . . . 82.1 Layout of a Secondary Ion Mass Spectrometer . . . . . . . . . . . . . . . . 122.2 Static SIMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132.3 Schematic of a FTIR Spectrometer . . . . . . . . . . . . . . . . . . . . . . 142.4 Atomic Force Microscope Schematic . . . . . . . . . . . . . . . . . . . . . . 153.1 Custom Built Photoreceptor Charging Apparatus . . . . . . . . . . . . . . 183.2 Close Up of Charging Apparatus . . . . . . . . . . . . . . . . . . . . . . . 193.3 Molecular Structures of Photoreceptor Components . . . . . . . . . . . . . 214.1 Mass Spectra of Individual Photoreceptor Components . . . . . . . . . . . 234.2 IR Spectrum of Pure PTFE . . . . . . . . . . . . . . . . . . . . . . . . . . 244.3 IR Spectrum of Pure TPD . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.4 IR Spectrum of Pure PCZ . . . . . . . . . . . . . . . . . . . . . . . . . . . 254.5 SIMS of an Unused Piece of Drum Photoreceptor . . . . . . . . . . . . . . 274.6 SIMS Spectra of the PTFE Region for Different Sample Types . . . . . . . 284.7 SIMS Spectra of the PCZ Region for Different Sample Types . . . . . . . . 294.8 SIMS Spectra of the TPD Region for Different Sample Types . . . . . . . . 304.9 SIMS Spectra of the Zinc Region for Different Sample Types . . . . . . . . 304.10 SIMS Spectrum of Film Photoreceptor . . . . . . . . . . . . . . . . . . . . 314.11 Drum Sample Before IPA Treatment - 2700 cm−1 - 3300 cm−1 . . . . . . . 324.12 Drum Sample After IPA Treatment - 2700 cm−1 - 3300 cm−1 . . . . . . . . 324.13 Drum Sample Before IPA Treatment - 1000 cm−1 - 1900 cm−1 . . . . . . . 34viiList of Figures4.14 Drum Sample After IPA Treatment - 1000 cm−1 - 1900 cm−1 . . . . . . . . 344.15 Film Sample Before IPA Treatment - 2700 cm−1 - 3300 cm−1 . . . . . . . . 354.16 Film Sample After IPA Treatment - 2700 cm−1 - 3300 cm−1 . . . . . . . . 354.17 Film Sample Before IPA Treatment - 1000 cm−1 - 1900 cm−1 . . . . . . . . 364.18 Film Sample After IPA Treatment - 1000 cm−1 - 1900 cm−1 . . . . . . . . 364.19 Top Down AFM Image of Control Photoreceptor . . . . . . . . . . . . . . 374.20 3D AFM Image of Control Photoreceptor . . . . . . . . . . . . . . . . . . . 374.21 Top Down AFM Image of Toner A Sample Before IPA Treatment . . . . . 384.22 3D AFM Image of Toner A Sample Before IPA Treatment . . . . . . . . . 384.23 Top Down AFM Image of Toner A Sample After IPA Treatment . . . . . . 384.24 3D AFM Image of Toner A Sample After IPA Treatment . . . . . . . . . . 384.25 Top Down AFM Image of Toner B Sample Before IPA Treatment . . . . . 394.26 3D AFM Image of Toner B Sample Before IPA Treatment . . . . . . . . . 394.27 Top Down AFM Image of Toner B Sample After IPA Treatment . . . . . . 394.28 3D AFM Image of Toner B Sample After IPA Treatment . . . . . . . . . . 395.1 Weakest Bonds in Bisphenol A Polycarbonate . . . . . . . . . . . . . . . . 425.2 Possible Location of Degradation in Bisphenol Z Polycarbonate . . . . . . 435.3 Degradation Mechanism 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . 455.4 Degradation Mechanism 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . 465.5 Degradation Mechanism 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . 46A.1 Halftone Page Density Distribution . . . . . . . . . . . . . . . . . . . . . . 54A.2 Halftone Page Dot Radii . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54viiiAcknowledgementsI would like to thank Dr. Edward Grant from the University of British Columbia and Dr.Richard Klenkler from the Xerox Research Centre of Canada for their insight and advice inall aspects of this project. I send my gratitudes to Dr. John Kim at the Interfacial Analysisand Reactivity Lab at the University of British Columbia for his time in assisting me withmass spec. I would also like to extend my thanks to fellow members Najmeh Tavassoli,Zhiwen Chen, Jachin Hung, Hossein Sadeghi, Ashton Cristy, and Markus Schulz-Weiling ofthe Grant group for assistance in various areas from coding to technical advice. Withoutyour encouragement and help, this would not have been possible. Thanks go out to myfriend Leo Li for his expert insight on AFM.ixChapter 1Introduction1.1 An Introduction to Xerography and theXerographic ProcessIn 1938, Chester Carlson invented xerography, a dry photocopy technique used in today’sprinters and photocopiers [1]. Its introduction quickly revolutionized both industry andhouseholds worldwide, as quick and convenient automated printing/photocopying provedindispensable in everyday life. In 2006, worldwide consumption and disposal of printercartridges reached 375 million units and global expenditures for digital imaging suppliespassed the $100 billion mark [2]. Furthermore, the production of a single printer cartridgeconsumes one gallon of petroleum and releases 4.8 kg of CO2 [3]. The widespread depen-dence on xerographic printing demands constant improvements in printing efficiency andxerographic cartridge lifetime for both environmental and economic reasons. Our researchaims to identify and classify changes occurring in photoreceptors during regular use thateventually leads to degradation. By understanding the nature of photoreceptor damage,print cartridges with longer service lifetimes can be designed. This has not only economicbenefits, but environmental benefits as well. To better understand this research, we requiresome background information on xerography. In the following sections, I will give a briefoverview of xerography and the common organic photoreceptor. Reviews by Duke et al.[1], Weiss and Abkowitz [4], and Pai and Springett [5] are very thorough and provide thebasis of the following introduction sections. For a deeper understanding of the xerographicprocess, readers are encouraged to refer to these articles.The xerographic process consists of six steps, beginning with uniform charging of thephotoreceptor. This can be done via a corona discharge or a charge contact roller. In coronadischarge, a wire heated to extreme temperatures ionizes surrounding gases to create aplasma that charges the photoreceptor. In the case of contact roller charging, a urethaneroller charged to high voltages accomplishes the same goal. Here, plasma formation occursnear the point of contact between the charging roller and the photoreceptor surface. Adigital signal or a scanner then relays the necessary information of the print job to the11.1. An Introduction to Xerography and the Xerographic Processprinter. In recent times, most machines convert scanned images into a digital signal ratherthan directly using the reflected light from the scanned image to perform the print job.This allows for manipulation of the image prior to printing. Following this, a light sourcesuch as a laser or a set of LEDs discharges specific areas on the photoreceptor surface basedon the received signal to create a latent image of the print job. As the photoreceptor rollspast the developer housing, it attracts and holds charged toner particles electrostaticallyon specific areas corresponding to the latent image. In subsequent steps, the photoreceptorcomes into contact with the final substrate (such as paper) and transfers over the toner.A heater fuses the toner securely onto the substrate before finally releasing the finishedproduct. A thorough cleaning procedure then prepares the photoreceptor for its nextprinting cycle. Light floods onto the photoreceptor to completely remove any remainingcharges on its surface while also rendering remaining toner particles easily removable. Acleaning blade or brush then physically removes excess toner. This cleaning mechanismensures a consistent level of print quality. Figure 1.1 below outlines this process.Figure 1.1: Simple Diagram of the Xerographic Process. The photoreceptor plays anintegral part in the printing process. Uniform charging followed by application of a writinglaser generates a latent image of the print job. Following this, toner adheres to areas onthe photoreceptor corresponding to the latent image. The photoreceptor then transfers thetoner to a blank page. To complete the cycle, light is used to remove any remaining surfacecharges on the photoreceptor while a cleaning blade physically removes leftover toner.21.2. Organic Photoreceptors1.2 Organic PhotoreceptorsAs discussed earlier, the photoreceptor plays an essential role in xerography and it is nosurprise that it has evolved many times in the past few decades. In 1938, the photo-conductor used by Chester Carlson was merely a layer of sulphur on a zinc plate. Theintroduction of inorganic selenium and selenium/arsenic alloy drums as photoreceptors inlater years proved very efficient in large production machines using large gas lasers and nodoubt helped propel the field of xerography forward. In 1970, IBM released the Copier Ifeaturing the first organic photoreceptor to be used in xerography. Due to environmentalissues, it did not have a long commercial lifetime, but it opened the gateway to futuredevelopment of organic photoreceptors.Today, organic photoreceptors outclass inorganic photoreceptors by far in both effi-ciency and versatility. Organic synthesis enables the creation and modification of a vastnumber of photoconducting materials and polymers. This allows the usage of cheaper andsmaller lasers near the infrared wavelengths or LEDs rather than large blue lasers. Inaddition to this, the design of the photoreceptor can be biased towards economy or effi-ciency. For example, smaller home printers may be outfitted with economic single layerorganic photoreceptors whereas large printers demanding high quality and volume may beoutfitted with efficient but more costly dual layer photoreceptors.1.2.1 Dual Layer vs. Single Layer PhotoreceptorsAs mentioned earlier in section 1.2, organic photoreceptors in printers can either be duallayer or single layer. Figure 1.2 shows the structures of both single layer and dual layerphotoreceptors and their basic mode of operation. In a single layer photoreceptor, thesurface can become positively or negatively charged. A light source writes on certainareas of the photoreceptor surface, sending photons into the photoreceptor bulk. As thephotons move into the photoreceptor, charge generation occurs randomly. The single layerphotoreceptor benefits from simplicity and a low fabrication cost but sacrifices efficiency.For instance, hole and electron recombination readily occurs due to the fact that bothelectrons and holes move through the same bulk. It is interesting to note that although thesurface of a single layer photoreceptor can be positively or negatively charged, a negativecharge is preferred. This is because hole transport occurs much quicker than electrontransport. If the surface was positively charged, electrons could remain trapped in thebulk of the photoreceptor due to their slow movement speed.To remedy the issue of electron-hole recombination, dual layer photoreceptors contain31.2. Organic Photoreceptorstwo layers: a thick charge transport layer (CTL) on top of a thin charge generating layer(CGL). Unlike single layer photoreceptors where charge generation occurs anywhere in thebulk, only the CGL possesses charge generating moieties. Incoming light penetrates theCTL to reach the CGL where electrons and holes are generated. Additionally, the surfaceof dual layer photoreceptors are almost always negatively charged. Due to this, the fastermoving holes travel upwards into and through the CTL to the surface whereas slower mov-ing electrons move downwards through the thinner CGL. This leads to higher efficiencywith reduced chances of electron-hole recombination and charge trapping. Although fab-rication costs are higher, more and more machines use dual layer photoreceptors becauseof their increased lifetimes and efficiencies.Figure 1.2: Basic Structure of Single and Dual Layer Photoreceptors. Latent image genera-tion in a single layer photoreceptor (left) versus a dual layer photoreceptor (right). Chargegeneration can occur anywhere in the bulk of the single layer photoreceptor whereas it islimited to the charge generating layer in the dual layer photoreceptor. This reduces thechances of electron-hole recombination and leads to higher charge transport efficiency. Theabbreviations used are as follows; CTL = charge transport layer; CGL = charge generatinglayer; BL = blocking layer. Figure adapted from Wong et al. [6]1.2.2 The Charge Transport LayerIn a dual layer photoreceptor, the charge transport layer (CTL for short) moves generatedcharges (usually holes) to the surface within the time constraints of the xerographic processwhile ideally having a low rate of charge trapping. The CTL consists of a binding polymerdoped with approximately 40-50% weight charge transport molecules. The binder polymermay sometimes include an active functional group acting as the charge transport moiety,but having a separate charge transfer species dispersed within the binding polymer is seenmore frequently. Common binding polymers shown in Figure 1.3 include the family of41.2. Organic Photoreceptorsbisphenol polycarbonates such as polycarbonate A and polycarbonate Z, polysulfones, andpoly (methyl methacrylate). Hole transport molecules are typically aromatic amines whichare classified into families based on their functional groups. Examples of these are shownbelow (Figure 1.4).Figure 1.3: Common Binder Polymers Used in CTLs. Several example polymers and theiracronyms that are commonly used as binders in charge transport layers.The term ’molecularly dispersed polymer’ (MDP) is also used to describe the morphol-ogy of the photoreceptor [7]. The general consensus is that charge transport takes placevia a ‘hopping’ mechanism. In the case of hole transport, this refers to the transfer of anelectron from a neutral molecule to a cation radical through the highest occupied molecularorbital (HOMO) through chain redox reactions over the length of the photoreceptor [7]. Ithas been found that drift mobility for MDPs generally decreases with both electric fieldand temperature [7].51.2. Organic PhotoreceptorsFigure 1.4: Common Charge Transport Moieties. Molecule structures of commonly usedcharge transport moieties and their respective acronyms and families. Picture above are tri-p-tolylamine (TPA), N,N’-diphenyl-N,N’- bis(3-methylphenyl)-[1,1’-biphenyl]-4,4’-diamine(TPD), and 4,4’-(diphenylmethylene)bis(N,N-diethylaniline).1.2.3 The Charge Generating LayerThe charge generating layer (CGL) is a thin layer (approximately 2 microns) found belowthe charge transport layer. This layer contains a variety of pigments which absorb incominglight to generate charge. As discussed earlier, different printing machines use different lightsources. Therefore, many different charge generating pigments have also been used. Twoexamples of charge generating materials are seen in Figure 1.5. The bisazo dye abosrbsvisible light whereas the phathalocyanine dye abosrbs near-infrared light.Although the mechanism of charge generation differs slightly from one charge generatingspecies to another, the general mechanism through internal conversion remains the same.This is illustrated in Figure 1.6. Firstly, incoming light excites the charge generatingcompound to an upper state, S*, which then de-excites to the first excited state termedS1. At S1, three different processes compete: radiative decay (kr), nonradiative decay (kn)and geminate pair formation (kgp). In the event of pair formation, the geminate pair will61.2. Organic PhotoreceptorsFigure 1.5: Examples of Charge Generating Pigments. Two examples of charge generatingpigments. The bisazo dye is sensitive to visible light whereas the phthalocyanine complexis sensitive to light in the near-infrared region.either recombine or separate and migrate with the assistance of the applied field. The actof geminate pair separation gives rise to the charges required in the xerographic process.[8]71.2. Organic PhotoreceptorsFigure 1.6: Model for Internal Conversion. Seen above is an illustration of the internalconversion process. The system is first excited from the ground state (S0) to an excitedstate (S*) by incoming light. Following de-excitation to the first excited singlet state (S1),geminate pair formation (kgp) competes with nonradiative decay (kn) and radiative decay(kr). The geminate pair can then recombine or dissociate into free carriers. Figure adaptedfrom Weiss et al. [4].81.3. Current Knowledge on Photoreceptor Degradation and Our Research1.3 Current Knowledge on PhotoreceptorDegradation and Our ResearchAs discussed above, the generation of charge and maintenance of that charge on the surfaceof an organic photoreceptor is of utmost importance to the xerographic process. Ideally,the photoreceptor acts as a perfect insulating parallel plate capacitor in the dark in whichthe charge density on the surface (Q) equals the product of capacitance (C) and surfacepotential (V): Q = CV. However, an unavoidable phenomenon known as dark decay (V/s)occurs in which the potential of the surface drops between charging and toner development.As the photoreceptor ages, the rate of dark decay increases over time until it noticeablyaffects print quality. At high levels of dark decay, areas of the photoreceptor may becomeunintentionally discharged, leading to unwanted toner development on the final product.Dark decay occurs when surface charge is neutralized unintentionally. A major contributorto dark decay is the transport of holes to the surface. These holes are not intentionallygenerated and can arise through a variety of processes. For example, hole injection from thesurface can occur when radicals are released due to oxidation of nearby molecules. Althoughmost photoreceptors include a dielectric layer to prevent this, the metallic electrode can alsoinject charge due to the high electric fields present during charging. Finally, charge can beinjected from the bulk of the photoreceptor. These charges result from charge trapping inprevious cycles and migrate throughout the photoreceptor as electric field increases duringcharging. Several studies have been conducted to investigate the nature of trapped chargescontributing to dark decay. For instance, it was discovered that increased exposure timesto light increased the amount of trapped charge [3]. In addition to this, it was also foundthat a thicker charge generating layer or photoreceptor tended to trap more charges [3].Montrimas et al. found a build up of charge at the CGL and CTL interface which wasreleased during charging and resulted in lower surface charge acceptance than normal [9].As mentioned above, the generation of radical species on the surface causes charge in-jection. The degradation of surface photoreceptor material can also lead to a general loss offunctionality. For example, degradation and cross-linking of the photoreceptor polymer canimpede charge transport. Additionally, thinning of photoreceptor material due to degra-dation can affect electrical properties such as the electric field [6]. This in turn can affectmigration times, cause dark decay, and lead to print defects. Our research has focused onthe chemical changes that occur on the photoreceptor surface during the charging process.Although modern photoreceptors have a protective layer, the extreme environment duringcharging coupled with physical wear over time can still cause degradation on the surface.91.3. Current Knowledge on Photoreceptor Degradation and Our ResearchThe generation of new species and the degradation of photoreceptor material can impedethe overall xerographic process and cause print defects.It can be said without exaggeration that the photoreceptor plays a central role inthe xerographic process. Companies all over the world work continuously to engineerphotoreceptors with greater efficiencies and extended lifetimes. Through empirical testingof different materials, better photoreceptors and xerographic systems have been inventedand used. Prior research at Xerox Canada points to degradation of polycarbonate aswell as a connection between water contact angle and degradation. However, the detailedmechanism of photoreceptor degradation remains largely unknown. Our research seeks tolearn new details about the pathway and end products of photoreceptor degradation. Weintend to accomplish this by bringing the photoreceptor to an end of life state in a systemsimilar or identical to a common high volume laser printer. We shall use ATR-FTIF, TOF-SIMS, and AFM, which are highly sensitive surface techniques to characterize chemical andphysical changes occurring on the photoreceptor surface. By comparing the surface of thephotoreceptor before and after charging, we hope to elucidate the nature of degradationcaused by plasma.10Chapter 2Instrumental Theory2.1 Secondary Ion Mass Spectrometry (SIMS)Mass spectrometry serves widely as an analytical technique to elucidate the chemical com-position of complex materials. Analysis involves steps of sample ionization followed by oneof several mass filtering methods, such as time of flight (TOF), quadrupole, or ion trap.Many methods of ionization are available; these include hard ionization methods such aselectron impact which induces large amounts of fragmentation and soft ionization methodssuch as MALDI (matrix assisted laser desorption/ionization) which aim to keep moleculesintact.Due to the nature of our photoreceptor samples, many existing ionization techniquescurrently available do not work well. For example, hard ionization techniques causinglarge amounts of fragmentation would result in large amounts of data loss due to sampledestruction. Furthermore, plasma degradation only occurs on the surface, which in turndemands a surface sensitive technique. This rules out some softer ionization methods suchas MALDI and electrospray (ESI) which require sample preparation that would disturband mix degraded surface molecules with untouched bulk material.Secondary ion mass spectrometry (SIMS) can analyse sample surfaces with little to nomolecular damage with high sensitivity. This combined with the ability to perform depthprofiling experiments makes SIMS the ideal technique for our experiments. An image ofa basic SIMS machine is shown in Figure 2.1. A typical SIMS machine consists of an ionsource such as the commonly used Cs+ ion gun or a liquid metal gun such as Au+ or Ga+which fires primary ions towards the sample. These primary ions are focused, pulsed andfiltered in an ion column on its way towards the sample to hit sub-micrometer areas fordetailed area-specific analysis. Primary ions from the ion gun strike the surface of thesample with an energy in the range of 5keV to 25keV. Energy transfer to sample moleculesleaves a ’crater’ of damage in the surrounding area while simultaneously ejecting neutraland ionized molecules or fragments as seen in Figure 2.2. It is important to note that whileimpaction by the primary ion causes structural change in surrounding molecules, many of112.1. Secondary Ion Mass Spectrometry (SIMS)Figure 2.1: Layout of a Secondary Ion Mass Spectrometer. Primary ions strike the sampleto sputter off secondary ions which are collected and filtered through the mass analyserbefore arriving at a detector. Figure adapted from Van Vaeck et al. [10]the ejected molecules retain their original form. An electron flood gun compensates forany charge imbalance and the ionized species (known as secondary ions) are sent towardsthe detector. Typical mass filters used in SIMS include sector, quadrupole and TOF. [10][11]Dynamic SIMS and static SIMS are two different methods of analysis that can beperformed on a singular SIMS machine by changing experimental parameters. DynamicSIMS employs a large dosage of primary ions to repeatedly bombard a specific area. This‘digs’ into the sample layer by layer and enables depth profiling experiments. Static SIMSon the other hand, utilizes an extremely low ion dosage such that only one primary ionimpacts a particular area. This ensures that damaged molecules resulting from primaryion impaction are not ejected, thus preserving structural information. Furthermore, staticSIMS works very well in surface analysis as only the topmost layer of molecules is sampled.It is for these two reasons that static SIMS is extremely well suited for this experiment;we are able to analyse the surface of our polymer film while preserving structural integrity.[10] [11]122.2. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR)Figure 2.2: Static SIMS is a specials mode of SIMS. Energy transfer from an incomingprimary ion causes sputtering of both neutral molecules and ions as well as moleculardamage radiating outwards from the site of impact. The use of a low ion dosage ensuressubsequent impactions at damaged areas do not occur. Figure adapted from Van Vaeck etal. [10]2.2 Attenuated Total Reflectance Fourier TransformInfrared Spectroscopy (ATR-FTIR)Fourier transform infrared spectroscopy (FTIR) obtains chemical information on a com-pound based on interactions due to vibrational properties of the sample. Due to its speedand convenience in measuring both liquid and solid samples, FTIR has become an ex-tremely widespread technique. A basic FTIR machine works in the way depicted in Figure2.3. A broadband light source sends collimated light towards a beam splitter. The splitbeams are sent towards two mirrors: one stationary and the other moving. The stationarymirror merely reflects the beam but the moving mirror causes a systematic change in theoptical wavelength of its reflected beam as it moves. Due to interference upon recombi-nation, a different wavelength of light results for each position of the moving mirror. Aparameter known as retardation refers to the optical path length difference between thetwo mirrors with the zero position having complete constructive interference. Raw datain the form of an interferogram measures the signal for each value of retardation. A pro-cess known as Fourier transform then converts the interferogram into the absorption ortransmission spectrum we are familiar with.[12] [13]As the molecules of interest reside on the surface of our sample, we need a surfacesensitive technique. Shining light through the entire sample would yield much less utility132.2. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR)Figure 2.3: Schematic of a FTIR Spectrometer equipped with an ATR accessory. Theaccessory redirects incoming light towards the ATR crystal. Inset depicts a zoomed inview of the ATR crystal along with the sample.compared to sampling only the top layers. Attenuated total reflectance (ATR) IR allowsmeasurements of sample surfaces with typical penetration depths of 0.5 µm to 2 µm.Many typical FTIR spectrometers can be outfitted with an accessory equipped with anATR crystal. Different crystals are used for different types of samples and measurements.Common materials include germanium, zinc selenide, KRS-5 (Thallium Bromide-Iodide),silicon and diamond. Measurements are performed by placing the solid or liquid sampledirectly onto the surface of the ATR crystal. The accessory re-directs incoming light intothe crystal at an angle such that total internal reflection occurs. Evanescent waves format the boundary between sample and the ATR crystal which penetrates into the topmostlayers of the sample. Following this, the outgoing beam containing information pertainingto the sample is sent back towards the detector. This whole process can be seen above inFigure 2.3. [14] [15] [16]142.3. Atomic Force Microscopy (AFM)2.3 Atomic Force Microscopy (AFM)To get a clear idea of the photoreceptor’s surface morphology, we turn to atomic forcemicroscopy, a scanning probe microscopy technique used in imaging and molecule manipu-lation on the nanometer scale. AFM probes are quite tiny, consisting of a several millimetersized holder plate to which a micrometer sized cantilever is attached. A tip with a radius ofseveral nanometers rests on the end of the cantilever and interacts with the sample surfaceduring measurements. I will only give a very brief description of AFM and some of itsoperating modes here, for more information, the reader can consult the text written byHaugstad [17] or the extensive review by Giessibl [18].Figure 2.4: Atomic Force Microscope Schematic. Variations on the sample surface causesdeflections in the cantilever which changes the amount of light reaching the photodiodes.This is converted into an electrical signal which describes the location of the cantilever.This signal also works in a feedback loop which moves the tip to keep it at a constantdistance from the sample surface.Figure 2.4 depicts a typical set-up used in atomic force microscopes. The mountedcantilever is brought into close proximity with the sample surface. As the tip interactswith the sample surface, repulsive and attractive forces between the sample and the tipcause the tip to deflect. Typical interacting forces include mechanical contact, electrostatic,magnetic, capillary, and van der Waal’s forces. Some tips may also be engineered to152.3. Atomic Force Microscopy (AFM)include molecules that interact with the sample; one can measure the strength of interactingproteins for example. A laser positioned to reflect off the cantilever and into an arrayof photodiodes serves to monitor the degree of cantilever deflection. As the position ofthe cantilever changes, the varying amount of light reaching the photodiodes causes it togenerate different voltages. This serves as a feedback loop, allowing piezoelectrics to adjusttip height as the scan runs while also being the raw data from which the image is compiled.Other methods of data collection are also possible, such as utilizing the feedback signalrequired to keep the tip at a constant position relative to the sample surface.AFM can be set up in a variety of ways to measure different surfaces, with the maincategories being non-contact, contact, and tapping mode. Contact mode, also known asstatic mode, involves physically dragging the tip across the sample surface. This can al-low detailed images but may result in sample and tip degradation, and is not suitable forsoft samples. Non-contact, or dynamic mode keeps the cantilever oscillating at a resonantfrequency above the sample surface. This method of imaging works well with softer biolog-ical samples and does not suffer from sample and tip degradation. Finally, tapping modeworks as an in between method. Occasionally, keeping the tip at a small distance abovethe sample proves quite difficult due to attractive forces between the sample and the tip.By oscillating the tip at larger amplitudes, this ‘tapping’ method can offer a well resolvedimage similar to that of non-contact mode while minimizing sample damage. Since oursample is quite robust, we chose to use contact mode.16Chapter 3Methodology3.1 Preparation and Analysis of DrumPhotoreceptor SamplesConsumer product photoreceptors in photocopiers and printers are typically coated onto analuminium drum and are housed in a drum cartridge containing two other important com-ponents: the charging roller and the cleaning blade. We selected a set of drum cartridgesand installed them into consumer Xerox printers (a Docucolor 250 and a 700 Digital ColorPress). Following this, we stressed the photoreceptors to an end of life state by printingpages with a variety of text and images. Photoreceptors were stressed over a period of3 weeks to 100,000 prints at regular office conditions (22 degrees Celcius and 45% roomhumidity) inside an environmental chamber. We ran two tests concurrently with bothtests utilizing the same photoreceptor but different toners. For the purpose of this thesis,these two toners will be referred to as ‘toner A’ and ‘toner B.’ Toner A was run on theDocucolor machine whereas toner B was run on the Digital Color Press. The differencein the toner lies in particles of zinc stearate which act as a lubricant of sorts. Toner Auses 22 µm zinc stearate beads whereas the toner B uses much smaller 5 µm zinc stearatebeads. Empirical tests showed that photoreceptors in printers using toner B had a longerlifespan when compared to the same photoreceptor coupled with the toner A. It wouldprove informative to compare the differences in damage when using different toners as thismay lead to further understanding of photoreceptor degradation.After photoreceptors were stressed and removed from the printers, one end of the drumwas physically crushed in a vice clamp. This causes the photoreceptor film to cleanlyremove itself from the drum in the vicinity of the physical damage. We removed thesepieces of film with cleaned forceps and cut them into 1 cm by 1 cm squares for furtheranalysis. For the purpose of this thesis, these samples will be referred to as ‘drum samples.’173.2. Preparation and Analysis of Film Photoreceptor SamplesFigure 3.1: Custom Built Photoreceptor Charging Apparatus. The charging apparatus andstepper motor can be seen on the right along with a high voltage power supply (middle)and waveform generator (left).3.2 Preparation and Analysis of Film PhotoreceptorSamplesAlthough stressing of photoreceptor drums in a consumer model printer provides a veryrelevant model, the cleaning blade shaves off a tiny fraction of film from the photoreceptorwith every cycle. This poses a potential problem since most of the chemistry in xerographyoccurs on the surface of the photoreceptor. Layers of disturbed photoreceptor and by-products of degradation could potentially be removed by the cleaning blade. To remedythis situation, we built a charging apparatus to separately charge a piece of photoreceptorfilm with a charging roller. The experimental setup is shown in Figures 3.1 and 3.2. Ascan be seen in Figure 3.2, the machine includes a movable charging roller without the tonerand the cleaning blade. Although we cannot track print resolution with test pages usingthis machine, it serves as a good model of the charging and discharging process. Personnelat the Xerox Research Centre of Canada used the formulation of the drum photoreceptorto re-create the photoreceptor in a film form for use in this part of the experiment. Fromthis point onwards, these samples will be referred to as ‘film samples.’To prepare film samples for charging, we cut pieces of film into 10.5 cm by 7 cmrectangles. Dichloromethane is used to remove a 1 cm strip of photoreceptor material alongthe short side to expose the metal substrate layer. The photoreceptor is directly attachedto a 10.5 cm by 7 cm metallic stage with double sided tape. Grounding is accomplished byattaching a piece of copper tape from the exposed metal strip of the photoreceptor to themetallic stage of the apparatus. A 5 cm wide urethane contact charge roller is mountedabove the photoreceptor and controlled by a Velmex VMX stepper motor. The roller was183.3. Test Page AnalysisFigure 3.2: Close Up of Charging Apparatus. The charging apparatus consists of a mount-ing stage and a movable urethane charging roller. The piece of photoreceptor is directlyattached to the metal stage using two sided tape.set to cover a length of 8 cm out of the total 10.5 cm and the motor was set to move theroller at a rate of 5 cm per second. A high voltage power supply coupled to an amplifier sent1500V of AC at 1kHz along with a -500V DC bias to the charge contact roller. Each ‘pass’over an area on the photoreceptor was counted as one cycle and the photoreceptor wasstressed for 100k cycles to mimic an end of life photoreceptor. A broadband light sourcewas used to discharge the photoreceptor during this charging process. Following stressing,1 cm by 1 cm squares are cut from the stressed area for SIMS and FTIR analysis. Specificsof charging were based upon work by Kawamoto and Satoh at Fuji Xerox Co Ltd [19].3.3 Test Page AnalysisDuring stressing, photoreceptor drums were removed from the printer every 25,000 printsto print test pages for test page analysis. All photoreceptors were installed in a newprinter using the corresponding toner to eliminate extraneous factors that could influenceprint quality. The test pages consisted of Xerox standard files containing halftone boxes(halftones are dots of various size and density used to reproduce a solid-looking area),horizontal and vertical lines of varying thickness, and negative lines as well. To analysethe Xerox print quality pages, a high resolution scanner was used to feed image data into aspecifically designed image analysis software. For halftone pages, we used a high resolution193.4. SIMS AnalysisCanon camera to take a picture of the page which we then analysed with MATLAB, a highlevel computing environment.The image quality software used analysed each page for several parameters. One im-portant parameter is background graininess which increases when toner develops on whitebackground areas. Another important parameter in print quality control known as mi-crouniformity measures uniformity in halftone areas. Good microuniformity results in auniform looking area of toner whereas bad microuniformity results in grainy prints due tosmall scale variations in toner density. Mottle is a parameter similar to microuniformityexcept that it measures long range variation in toner density rather than short range. Wealso measured negative and positive lines widths in both horizontal and vertical direc-tions. These measurements allow us to determine the printer’s ability to print lines andleave negative space at a high resolution. Analysis of some halftone pages was done withMATLAB; we looked for differences in dot radius, density of dots (the ratio of toner towhite background in a given area) and displacement of a dot from its supposed location.In particular, we were curious to see whether these variations (if any) showed some sortof pattern that could correspond to physical domains of degradation on the photoreceptoritself.3.4 SIMS AnalysisTOF-SIMS analysis on photoreceptor samples was done at the Interfacial Analysis andReactivity Laboratory (IARL) in the Advanced Materials and Process Engineering Labo-ratory (AMPEL) at the University of British Columbia on a Trift V nanoTOF-SIMS fromPhysical Electronics. The Xerox Research Centre of Canada provided pure samples of themain components of the photoreceptor to serve as standards to use in these tests. Theseincluded bisphenol polycarbonate Z (PCZ), N,N’-diphenyl-N,N’- bis(3-methylphenyl)-[1,1’-biphenyl]-4,4’-diamine (TPD), and polytetrafluoroethylene (PTFE); their structures canbe seen in Figure 3.3. Authentic compounds were run in their solid powdered form whereasdrum and film samples were cut into 1 cm by 1 cm squares with sterilized tools. The purepolycarbonate powder presented challenges during analysis as the machine was unable toget any signal from the pure powder. To remedy this, we dissolved the powder in a smallamount of dichloromethane. This solution was then dropped onto a clean glass slide andallowed to air dry into a thin film which was then used for SIMS analysis. Spectral acqui-sition was performed using Au+ primary ions in the static regime with a dosage of 1012ions/cm2 over a mass range of 0 - 1800 amu in both positive and negative polarity. Actual203.5. ATR-FTIR Analysissample areas used for analysis measured 400 µm by 400 µm. Following this, the spectrawere matched against online libraries to identify reference peaks.Figure 3.3: Molecular Structures of Photoreceptor Components are shown: PCZ, TPD andPTFE. These three compounds make up the photoreceptor, with PCZ and TPD being inabundance with small amounts of PTFE.3.5 ATR-FTIR AnalysisATR-FTIR analysis was done using a Perkin Elmer FTIR available from the shared in-strument facility at the University of British Columbia. Spectra of the three authenticcompounds were taken by compressing a small amount of solid powder on top of a dia-mond ATR crystal while drum and film samples were placed face down onto the crystal. Aminimal amount of applied force held the sample in place. Spectra for powdered standardsand film samples were averaged over 10 scans and generally taken over a range of 600wavenumbers to 4000 wavenumbers with a resolution of 2 wavenumbers. Perhaps due tothe frailty and natural curvature of drum photoreceptor samples, the signal to noise ratiowas quite low. In an attempt to improve this, each spectrum was averaged over 15 scansand taken with a resolution of 1 wavenumber as opposed to 2.213.6. IPA Treatment3.6 IPA TreatmentDrum and film photoreceptor samples were cleaned by briefly dipping the sample into asmall volume of isopropylalcohol and wiping with a cotton swab. This removes any looseparticles on the surface. These IPA treated pieces of photoreceptors were subjected toFTIR and AFM analysis afterwards. From the MSDS of the photoreceptor components,we know that the photoreceptor will not dissolve in IPA.3.7 AFM ImagingAtomic force microscopy was performed in contact mode using Aspire conical contact tipsfrom nanoScience (part CCSR-10) with a spring constant of 0.1N/m on a nanoSciencenanoSurf machine. The cantilever was 255 µm in length, had a width of 42 µm, and was0.9 µm thick. Drum photoreceptor samples were cut and taped onto a small magneticmounting stage. Imaging was performed on a 20 µm square with 128 points per line at 1second per line. Contact was measured by DeflVolts and had a trigger point of 1.000 V.AFM was only performed on drum photoreceptors and for each piece of photoreceptor, wetook several scans at different locations.22Chapter 4Results4.1 Authentic Compounds4.1.1 SIMSFigure 4.1: Mass Spectra of Individual Photoreceptor Components PTFE, PCZ, and TPDare shown. Circled are reference peaks used to identify the presence of these compounds.The y-axis corresponds to total counts while the x-axis measures atomic mass units.Figure 4.1 shows the mass spectra of the three individual photoreceptor components. Peaksfrom these spectra were matched against an online database of known spectra. Matcheswere found for PTFE and TPD and representative peaks are circled in Figure 4.1. In thecase of PTFE, we see representative peaks at 131 (C3F5+), 69 (.C2F2+), and 31 (CF+).For TPD, the defining peak at 516 corresponded with its molecular mass. We were unableto match PCZ to any results in the online database, presumably due to it being studiedrarely. Thus, we took all four peaks as seen above to represent PCZ.234.1. Authentic Compounds4.1.2 ATR-FTIRFigures 4.2, 4.3, and 4.4 show the IR spectra of our authentic compounds with assignmentsin Table 4.1. Notations on the IR spectra label the associated vibrational modes; assign-ments of bands are based upon the National Standard Reference Data System [20]. Thesebands were used as reference points when looking at the spectra of the photoreceptor films.Although not apparent now, take note of the PCZ spectrum, as interesting changes occurin the aliphatic C-H stretching region.Figure 4.2: IR spectrum of pure PTFE.244.1. Authentic CompoundsFigure 4.3: IR spectrum of pure TPD.Figure 4.4: IR spectrum of pure PCZ.254.1. Authentic CompoundsSample Name Peak (cm−1) AssignmentPTFE 638.68 C-F deformation1147.33 C-F stretch1202.77 C-F stretchPCZ 817.84 aromatic C-H out of plane bend1011.86 aromatic C-H in plane bend1158.96 C-O stretch1189.85 C-O stretch1220.43 C-O stretch1452.68 aromatic C=C stretch1503.68 aromatic C=C stretch1770.73 C=O stretch2859.20 aliphatic C-H (CH2) sym. stretch2934.49 aliphatic C-H (CH2) asym. stretch3042.10 aromatic C-H stretchTPD 694.94 aromatic C-H out of plane bend756.25 aromatic C-H out of plane bend1152.60 C-N stretch1271.84 C-N stretch1483.97 aromatic C=C stretch1581.91 aromatic C=C stretch3030.20 aliphatic C-H (CH3) stretch3059.30 aromatic C-H stretchTable 4.1: Table of Authentic Compound IR Bands. The above details relevant peaks andtheir corresponding assignment for our three authentic substances.264.2. SIMS of Control and Worn Photoreceptors4.2 SIMS of Control and Worn Photoreceptors4.2.1 Drum PhotoreceptorsWe performed our initial tests on photoreceptor samples obtained from drums stressed inXerox printers. Figure 4.5 shows the SIMS spectrum of a piece of control photoreceptor inthe mass ranges of PTFE, PCZ, and TPD. By contrasting this spectrum with those of theauthentic compounds in the previous section, we were able to ascertain the presence of allthree compounds in our photoreceptors.Figure 4.5: SIMS of an Unused Piece of Drum Photoreceptor. Circled peaks correspondto reference peaks found in the spectra of the pure photoreceptor components. The y-axiscorresponds to total counts while the x-axis measures atomic mass units.The lower mass region indicates many additional fragments but we were able to pickout ions with mass 31 and 69, indicative of PTFE. In the PCZ region, we see the four peaksfound in the spectrum of pure PCZ, thus confirming the presence of PCZ. TPD was alsofound and confirmed by the presence of a fragment at mass 516. The presence of massesat 73, 147, 207, 221, and 281 amu indicates PDMS (polydimethylsilane) contaminationcommonly found in machine oils. As can be in Figure 4.5, some of these PDMS signals,such as that at mass 73, are quite large, but they do not obscure our peaks of interest.274.2. SIMS of Control and Worn PhotoreceptorsContamination most likely originated from the printer itself and seems largely unavoidable.Figure 4.6 compares the positive SIMS spectra of our control and two types of stressedphotoreceptor with that of PTFE. We are able to pick out the three signals indicative ofPTFE in all our photoreceptor samples. Furthermore, the relative ratios of these peaksseem unchanged as well. Although not shown here, we also see a large peak at 19 in thenegative mass spectrum for all samples, representative of F−.Figure 4.6: SIMS Spectra of the PTFE Region for Different Sample Types. Positive SIMSspectra is shown for our pure standard as well as control and two sample photoreceptors.There are many other signals in the region, but PTFE is still identifiable. Circles markfragments used to identify the presence of PTFE.Figure 4.7 shows a comparison of negative mass spectra in the 200 amu - 300 amu massrange. Concerning the control and the photoreceptor stressed with toner B, we observepeaks similar to those found in the spectra of pure PCZ. However, our spectrum of thephotoreceptor running with toner A shows a striking difference at mass 223. Compared topure PCZ, in which mass 223 is smaller than the neighbouring signal at 209, this stressedsample shows a massive signal at 223. Although this signal is also somewhat larger in boththe control and the other stressed photoreceptor, it is not to the scale seen in the toner Asample.284.2. SIMS of Control and Worn PhotoreceptorsFigure 4.7: SIMS Spectra of the PCZ Region for Different Sample Types. Negative SIMSspectra in the region where PCZ is found. PCZ signals in the control photoreceptor andthe photoreceptor sample using toner B appear similar to that of the pure PCZ. Thephotoreceptor using toner A showed an increased peak at mass 223. Circles mark thisdifference.Figure 4.8 shows the positive SIMS spectrum of TPD in all our samples. We see severalunidentified peaks in our stressed samples that appear neither in our control nor the purestandard, such as the large set of peaks at around 545 amu in the toner A sample. However,we do find the TPD peaks unchanged throughout all our samples.As the two toners differed only in the size of zinc stearate beads, we also looked fortraces of this difference on the photoreceptor surface. We were able to detect zinc in itscorrect isotopic abundances in all our drum samples as seen in Figure 4.9. However, asophisticated method of internal standardization will be required to deduce the amount ofzinc on each surface.294.2. SIMS of Control and Worn PhotoreceptorsFigure 4.8: SIMS Spectra of the TPD Region for Different Sample Types. Positive massspectra showing TPD present in all samples with peaks in stressed samples similar to thosein the control and standard. TPD identifying peaks are circled.Figure 4.9: SIMS Spectra of the Zinc Region for Different Sample Types. Mass spectrashowing zinc present in all three samples. Small horizontal bars represent known isotopicabundances of zinc.304.3. ATR-FTIR of Control and Worn PhotoreceptorsFigure 4.10: SIMS Spectrum of Film Photoreceptor. Mass spectrum of a piece of filmphotoreceptor in the PCZ region before and after stressing. Although care was taken tochoose an area that physically appeared less damaged, we still see many random peaks.4.2.2 Film PhotoreceptorsWe also performed tests on stressed film samples but unfortunately, our results were verynoisy. An example of this can be seen in Figure 4.10 in the negative mass spectrum from 200amu to 300 amu where we normally find PCZ. The control spectrum looks normal and ourPCZ reference peaks can be identified, but the spectrum after stressing is very noisy. Wesee fragments of high intensity at almost every mass; some which we find in our control andsome which are completely new. Unfortunately, these fragments may have arisen due to theformation of some black ‘gunk’ during stressing. Without a cleaning blade, degraded piecesof charging roller and extraneous contamination build-up can interfere with the chargingprocess. We tried various methods to remedy this problem such as changing the inputvoltage and motor speed but the problem persisted.4.3 ATR-FTIR of Control and Worn Photoreceptors4.3.1 Drum PhotoreceptorsFigures 4.11 and 4.12 show IR spectra of our control and stressed drum photoreceptorsbefore and after IPA cleaning in the 2700 cm−1 - 3300 cm−1 region. Table 4.2 details thecorresponding assignments of these peaks. A comparison with the IR spectrum of ourauthentic compounds shows that the majority of the features in these spectra belong to314.3. ATR-FTIR of Control and Worn PhotoreceptorsPCZ. However, we do observe the formation of a new band at approximately 2920 cm−1in all samples before and after IPA treatment. Prior to IPA treatment, this band is themost intense in toner A samples. Upon cleaning with IPA, both stressed samples revertto a state similar to that of the control photoreceptor, with the new peak being reduced.Regarding the aromatic C-H stretch region, there seems to be no significant change betweenany samples regardless of IPA treatment.Figure 4.11: Drum Sample Before IPA Treat-ment - 2700 cm−1 - 3300 cm−1.Figure 4.12: Drum Sample After IPA Treat-ment - 2700 cm−1 - 3300 cm−1.324.3. ATR-FTIR of Control and Worn PhotoreceptorsSample Name Peak (cm−1) AssignmentDrum Control 2857.20 PCZ aliphatic C-H (CH2) sym. stretch (both)2919.50 new peak (both)2933.20 PCZ aliphatic C-H (CH2) asym. stretch (both)3038.10 aromatic C-H stretch (both)Drum Toner A 2856.80 PCZ aliphatic C-H (CH2) sym. stretch (both)2920.30 new peak (both)2933.90 PCZ aliphatic C-H (CH2) asym. stretch (both)3038.10 aromatic C-H stretch (both)Drum Toner B 2854.00 PCZ aliphatic C-H (CH2) sym. stretch (both)2920.00 new peak (both)2933.20 PCZ aliphatic C-H (CH2) asym. stretch (both)3038.10 aromatic C-H stretch (both)Table 4.2: Table of Drum Sample IR Bands. A table detailing relevant bands and theircorresponding assignment for drum samples before and after IPA treatment. Bands ap-pearing only prior to IPA treatment are labelled with ‘before,’ while bands appearing onlyafter IPA treatment are labelled with ‘after.’ Bands that are present before and after arelabelled with ‘both.’Figures 4.13 and 4.14 shows lower wavenumber ATR-FTIR spectra of the stressed andcontrol drum samples before and after IPA treatment. Surprisingly, we see little changein the carbonyl stretching band (far right) across all our samples. The control samples dopossess a band at 1500 cm−1 which is absent in the stressed samples. Cleaning with IPAdoes not affect this in any way.334.3. ATR-FTIR of Control and Worn PhotoreceptorsFigure 4.13: Drum Sample Before IPA Treat-ment - 1000 cm−1 - 1900 cm−1.Figure 4.14: Drum Sample After IPA Treat-ment - 1000 cm−1 - 1900 cm−1.4.3.2 Film PhotoreceptorsFigures 4.15 and 4.16 shows the aliphatic stretching region for control and stressed filmphotoreceptors. Table 4.3 gives the corresponding band assignments. Unsurprisingly, thespectra are extremely similar to those for drum photoreceptors, with the exception ofhaving a better signal to noise ratio. As in the drum photoreceptors, we once again seethe formation of a new feature at 2920 cm−1 in both control and stressed samples priorto IPA washing. Similar to the drum photoreceptors, this band shows a much higherintensity in stressed samples compared to unstressed samples. In the case of the stressedfilm photoreceptor, we also see the formation of a second new feature at 2850 cm−1. Uponcleaning with IPA, both of these bands disappear. Again, we see no change in the aromaticC-H stretching bands across all spectra.Inspection of IR spectra show little difference between stressed samples and controlsamples at lower wavenumbers. More significant differences occur in the C-H stretchingregion ( 3000 cm( − 1)).344.3. ATR-FTIR of Control and Worn PhotoreceptorsFigure 4.15: Film Sample Before IPA Treat-ment - 2700 cm−1 - 3300 cm−1.Figure 4.16: Film Sample After IPA Treat-ment - 2700 cm−1 - 3300 cm−1.Sample Name Peak (cm−1) AssignmentFilm Control 2858.68 PCZ aliphatic C-H (CH2) sym. stretch (both)2920.00 new peak (before)2932.93 PCZ aliphatic C-H (CH2) asym. stretch (both)3038.10 aromatic C-H stretch (both)3060.20 aromatic C-H stretch (both)Film Stressed 2850.40 new peak (before)2858.68 PCZ aliphatic C-H (CH2) sym. stretch (both)2919.80 new peak (before)2932.93 PCZ aliphatic C-H (CH2) asym. stretch (both)3038.10 aromatic C-H stretch (both)3060.20 aromatic C-H stretch (both)Table 4.3: Table of Film Sample IR Bands. A table detailing relevant bands and theircorresponding assignment for stressed and control film samples before and after IPA treat-ment. Peaks that only show up prior to IPA treatment are labelled with ‘before,’ whilepeaks that only appear after IPA treatment are labelled with ‘after.’ Peaks that are presentbefore and after are labelled with ‘both.’Figures 4.17 and 4.18 shows lower wavenumber ATR-FTIR spectra for film photore-ceptors. Again, like the drum photoreceptors, we see little change in this region regardless354.3. ATR-FTIR of Control and Worn Photoreceptorsof stressing or IPA cleaning.Figure 4.17: Film Sample Before IPA Treat-ment - 1000 cm−1 - 1900 cm−1.Figure 4.18: Film Sample After IPA Treat-ment - 1000 cm−1 - 1900 cm−1.364.4. AFM Images of Control and Worn Photoreceptor Surfaces4.4 AFM Images of Control and WornPhotoreceptor SurfacesThis section presents AFM images of control and worn drum photoreceptors produced incontact mode. On the left we have a 2D image while the right is a 3D representation ofthe same image. The images selected here are representative of many replicate analyses ofthese materials.Figure 4.19: Top Down AFM Image ofControl Photoreceptor.Figure 4.20: 3D AFM Image of ControlPhotoreceptor.Upon taking an AFM image of the control photoreceptor, we observe a smooth surfaceas seen in Figures 4.19 and 4.20.Figures 4.21 and 4.22 show AFM images of toner A samples prior to IPA treatment.We see large regular grooves spaced several microns apart that span approximately 400nanometers in height. Figures 4.23 and 4.24 shows images of the toner A samples cleanedwith IPA. Upon cleaning with IPA, the large grooves still remain but appear smoother.Figures 4.25 and 4.26 shows the surface of photoreceptors stressed using toner B priorto IPA treatment. The surface shows large repeating grooves similar to that of the samplesstressed with toner A. However, unlike the previous samples, the grooves span a maximumof 100 nanometres in height. Additionally, they seem to be more finely spaced, occurringmore frequently in a fixed space when compared to samples stressed with toner A. Uponcleaning with IPA, these grooves shrink even more to yield an almost smooth surface asseen in Figures 4.27 and 4.28.374.4. AFM Images of Control and Worn Photoreceptor SurfacesFigure 4.21: Top Down AFM Image ofToner A Sample Before IPA Treatment.Figure 4.22: 3D AFM Image of Toner ASample Before IPA Treatment.Figure 4.23: Top Down AFM Image ofToner A Sample After IPA Treatment.Figure 4.24: 3D AFM Image of Toner ASample After IPA Treatment.384.4. AFM Images of Control and Worn Photoreceptor SurfacesFigure 4.25: Top Down AFM Image ofToner B Sample Before IPA Treatment.Figure 4.26: 3D AFM Image of Toner BSample Before IPA Treatment.Figure 4.27: Top Down AFM Image ofToner B Sample After IPA Treatment.Figure 4.28: 3D AFM Image of Toner BSample After IPA Treatment.394.5. Test Page Analysis4.5 Test Page AnalysisSurprisingly, analysis of test pages revealed very little change in print quality after 100,000prints. There were trivial changes in background graininess in prints made before andafter stressing. There was also little difference in background graininess regardless ofwhich toner was used. We observed the same situation for positive and negative lines.The printer printed lines of similar thickness across all variables we tested. Mottle alsoappeared quite similar across all samples. However, we did observe one consistent differencein microuniformity. The microuniformity noise started at approximately 1.50 in the controland stayed somewhat constant after stressing when toner B was used; when toner A wasused, this value increased to approximately 1.70. Our analysis of halftone pages alsorevealed some differences in halftone pages printed with different photoreceptors. Forexample, we saw the mean radii of toner dots increase by 1 pixel (from 46 pixels to 47pixels) after stressing. Variations in toner density did not appear completely random,suggesting domains with increased rates of degradation.However, any differences we saw were quite small and could arise very easily fromvariations in the printing process not related to the photoreceptor. For example, the veryfirst page a printer prints may differ greatly from a page printed after the machine haswarmed up. Toner falling onto the page randomly from a faulty developer housing canalso introduce variations not due to the photoreceptor. We also observed wide dark stripeson the halftone pages for which we had no explanation. These considerations cast doubtson the significance of our test page analysis. As these photoreceptors are engineered tobe extremely robust, it is possible that chemical changes had yet to advance to a degreethat manifested as noticeable print defects. Since suppliers tend to urge their customersto replace parts far before actual break-down occurs, it is possible our stressing times werenot long enough. It is apparent that more sophisticated methods of test page printingneed to be formulated to isolate the effects of photoreceptor degradation. One end goal ofthis research was to correlate domains of degradation on the test page to similar domainson the photoreceptor itself. This remains to be achieved but selected data and exampleanalyses can be found in the appendix.40Chapter 5Photoreceptor Degradation5.1 Chemical DegradationAs a photoreceptor charges during the xerographic process, a plasma forms in the gap be-tween the charge contact roller and the photoreceptor surface [19]. Printers usually operateunder regular ambient conditions and as a result, plasma discharge occurs in atmosphericair. Water in the air interacting with excited atoms, molecules, and free electrons in theplasma can chemically change the surface of the photoreceptor and lead to degradationin performance over time. Furthermore, energy released as photons can photo degradesensitive photoreceptor components. Results from the SIMS and ATR-FTIR point to pho-toreceptor degradation involving polycarbonate Z (PCZ) whereas TPD and PTFE remainmostly unchanged. Analytic diagnostics discussed earlier show few changes in signaturefeatures corresponding to PTFE and TPD when comparing the controls to the stressedsamples. In contrast, we see differences in PCZ features in both SIMS and FTIR. Forthis reason, the majority of this chapter will be dedicated to the issue of polycarbonatedegradation.Although there has been little work done on bisphenol polycarbonate Z, much is knownabout bisphenol polycarbonate A (a common polymer in plastics) which should share manyproperties with PCZ. Davis and Golden utilized mass spectrometry, viscosity measure-ments, and gas chromatography to study PCA degradation. They observed competitionbetween condensation and hydrolysis reactions leading to chain scission and gel formation.They concluded that PCA degrades mainly via random chain scission at carbonate centers[21]. Jang and Wilkie studied thermal degradation of PCA in air. They proposed thatthe majority of degradation occured at the isopropylidene centre followed by hydrolysisand decarboxylation at the carbonate centres [22]. McNeill and Rincon also studied thedegradation of PCA at high temperatures and suggested a homolytic cleavage mechanismfollowed by re-oligomerization to produce a variety of cyclic molecules [23]. The authorsmentioned above proposed various mechanisms for PCA degradation, all with one elementin common. Initial degradation seems to always occur at the side chain C-C bond or the415.1. Chemical Degradationbackbone carbonate linkage as indicated in Figure 5.1. According to Jang and Wilkie,these two bonds are the weakest in the polymer, with dissociation energies of 251 kJ/moland 330 kJ/mol [22].Figure 5.1: Weakest Bonds in Bisphenol A Polycarbonate. The two weakest bonds inthe bisphenol A polycarbonate monomer unit. Indicated are dissociation energies. Figureadapted from Jang and Wilkie [22].The two aromatic rings on either side of the indicated C-C bond help to stabilize anyresulting products produced via bond scission. The loss of one methyl group via homolyticcleavage leaves a stabilized radical which then reacts further, such as by Fries photo-rearrangement [24] [25]. The carbonyl carbon is typically reactive and it is no surprisethat the C-O bond is also weak. A hydrolysis at the carbonyl group results in monomericbisphenol A coming off of the parent polymer chain with the release of carbon dioxide [23][22]. Although we are not studying PCA, PCZ has much of the same substituents. Forinstance, because PCZ also has a bisphenol linkage, we can expect degradation to occur atthe bond indicated in Figure 5.2 in similar fashion to PCA. Furthermore, it would not besurprising to see monomeric units of bisphenol Z come free following a hydrolysis reaction.Nauka et al. from Hewlett Packard studied photoreceptors made from polycarbonateA [26]. Upon stressing, they observed the formation of two new peaks in the aliphaticC-H stretching region. These were identified as methylene stretching features which grewin intensity as the original methyl stretching bands decreased. In addition, they alsoobserved a weakening of nearby aromatic C-H stretching signals [26]. These new featuresdisappeared upon cleaning with IPA. They explained these changes as arising from thecracking of aromatic carbon rings which subsequently reformed into aliphatic CH2 groups[26]. New molecules, separated from the parent polymer, form a thin layer on the surfaceof the photoreceptor and are easily removed by cleaning. This restores the spectrum backto its original state.The side chain of polycarbonate Z is a cyclohexane ring as opposed to two methylgroups in polycarbonate A. Therefore, many of the accepted rearrangements and degra-425.1. Chemical DegradationFigure 5.2: Possible Location of Degradation in Bisphenol Z Polycarbonate. The indicatedbond is weak in a similar fashion to that of bisphenol A polycarbonate.dation pathways taking place in polycarbonate A may be hindered in polycarbonate Z.Interestingly, Rivaton et al. studied a polymer extremely similar to polycarbonate Z. Theyexperimented with trimethylcyclohexane-polycarbonate which differs from polycarbonateZ only by the three methyl groups added onto the cyclohexane side chain. They sub-jected their polymer to thermo-oxidation and tested for stability using IR and UV-Visspectroscopy [27]. They saw an increase over time in the carbonyl and hydroxyl regions intheir IR spectra. They proposed a radical cleavage of the side chain ring due to oxidation.This left a hanging alkyl chain which would then be subject to photo-oxidation to producea variety of ketones and carboxylic acids [27].Following a different path, Steen et al. studied degradation of organic polymers (poly-sulfone, polyethersulfone, and polyethylene) in the presence of H2O plasma by opticalemission spectroscopy, mass spectrometry and X-ray photoelectron spectroscopy. Theysuggest an oxidation of the polymer surface with the OH radical playing the biggest role[28]. In a similar study, Knopf et al. studied the effect of ionized NO3 on a saturatedhydrocarbon surface. They observed slow but gradual volatilization of the surface and theformation of oxidized groups including ketones, aldehydes, and carboxylic groups. Theyproposed a mechanism in which NO3 radicals abstracted a hydrogen off the hydrocarbon,which they proceeded to oxidize further [29].Our FTIR spectra shows us changes in the C-H stretching region after cycling pho-toreceptors to an end of life state. More specifically, these changes occur in the aliphaticstretching region whereas the aromatic C-H stretching peaks seem unchanged. Like Naukaet al., we observe the formation of two new peaks at approximately 2850 cm−1 and 2920435.1. Chemical Degradationcm−1, both of which fall at slightly lower wavenumber compared to the methylene bandsalready present. The formation of these two new peaks could be attributed to a set ofmethylene stretches different from the ones originally present. Destruction of the cyclohex-ane ring leaves a different side chain which could cause this shift in wavenumber. However,unlike Nauka et al. who saw a decrease in aromatic C-H stretching, we see no evidence ofthis in our spectra. Additionally, our IR spectra shows no change in the carbonyl stretch-ing region unlike those seen in Rivaton’s studies, indicating a lack of oxidation. Our IPAcleaning results suggests that degradation products also form a thin loose layer that iseasily removed.If we examine the mass spectrum of pure PCZ in Figure 4.7 again, we see ions atcharacteristic masses 265, 237, 223, and 209 amu. All of our stressed photoreceptors alsoshow these peaks. The mass of monomeric bisphenol Z happens to be 265 amu and itspresence in all our samples suggests that ion impact readily removes monomeric bisphenolZ.Figure 5.3 details a possible hydrolysis reaction at the carbonate atom leading to therelease of monomeric bisphenol Z. From there, we also see a loss of 28, 42, and 56 amu,respectively which correspond to multiples of 14. This points to a degradation pathway inwhich initial cleavage of the weak side chain bond indicated in Figure 5.2 leaves a hangingalkyl chain. This chain could then undergo radical facilitated cleavage to release units of(CH2)n. Examples of such reactions are given in Figure 5.4 and Figure 5.5.It is also important to note that the ratios of the PCZ peaks in the mass spectrumdiffer depending on the toner used. The control photoreceptor and pure polycarbonatesample show relatively similar ratios in which the highest fragment peak is at 209, followedby 223 and finally 237. However, in the stressed photoreceptors, we observe an increase inthe intensity of the 223 peak. This is much more pronounced in the stressed photorecep-tor running with toner A. This may indicate that plasma stressing somehow favours theformation of product 223, with toner B mitigating this effect to a large degree.As mentioned earlier, the two toners differ in the size of zinc stearate beads. Zincstearate, also known as zinc octadecanoate, is a zinc salt of stearic acid and serves as acommon industrial lubricant. The fatty acid component gives it a hydrophobic property;which is of particular interest to us. XRCC work suggests that toner B works better thantoner A in the following way. The new zinc stearate beads are smaller. They are easilycaught between the surface of the photoreceptor and the cleaning blade. As the cleaningblade runs over the photoreceptor surface, the blade crushes the zinc stearate to form athin protective layer on the surface of the photoreceptor. The larger beads of toner A gets445.1. Chemical DegradationFigure 5.3: Degradation Mechanism 1. A proposed hydrolysis mechanism involving degra-dation at the carbonyl carbon.removed by the cleaning blade rather than crushed. This diminishes the protective layerwhen compared to the printer using toner B. As proposed above in Figure 5.3, hydrolysisplays an important role in the degradation of our binder polymer. A thick zinc stearatelayer could possibly act as a hydrophobic barrier to repel water from the surface of thepolymer thus protecting it against hydrolysis. This could lead to fewer free hanging reactiveends and ultimately, less degradation products in general when toner B is used. Our massspectra confirms the presence of zinc, leading us to believe that a zinc stearate layer isindeed formed. However, further testing will be required to determine whether toner Bactually deposits a larger quantity of zinc on the photoreceptor surface.455.1. Chemical DegradationFigure 5.4: Degradation Mechanism 2. A proposed mechanism involving homolytic bondcleavage at the cyclohexane ring to give products of mass 237 and 209.Figure 5.5: Degradation Mechanism 3. A proposed mechanism involving homolytic bondcleavage and hydrogen rearrangement at the cyclohexane ring to give a product of mass223.465.1. Chemical DegradationBased on clues from IR and SIMS, we propose a degradation mechanism involving twolocations of bond cleavage. Cleavage at the carbonyl centre generates monomeric bisphenolZ molecules and cleavage at the side chain cyclohexane ring forms a variety of degradationproducts. Due to the mass range of our SIMS scan, we may not see larger fragments butit is reasonable that these two methods of degradation may occur together or separately.It is also quite possible that several degraded fragments could come together to form across-linked species. Indeed, in our mass spectra, we observe the formation of some peaksfor which we have no identity. An example of this would be the recurring peak at mass293. More work will be required to determine the nature of these products. It will also beinteresting to perform some emission spectroscopy and XPS experiments to determine ifwe see results similar to those of Knopf and Steen [29][28]. So far, our FTIR results showlittle signs of oxidation, but this process cannot be entirely ruled out as volatilization ofdegradation products is also possible.475.2. Physical Degradation5.2 Physical DegradationOur contact AFM images offer little information in terms of chemical degradation butprovides some interesting insight on the physical condition of our samples after stress-ing. While we believe chemical degradation to be a major driving force behind eventualphotoreceptor failure, the significance of mechanical wear cannot be ignored. For instance,changes in the physical morphology may cause uneven charge buildup by influencing chargemigration distances. In extreme cases, photoreceptor material can become thin enough toallow contact between the charging roller and the metal substrate, leading to obvious printdefects on the final product. Nauka et al. performed an experiment in which they used aDektak profiler to map the topology of a 500 µm by 500 µm piece of photoreceptor [30].They observed a relatively flat surface after stressing that gave way to regular grooves afterwashing with solvent. They attributed this change to the removal of a loose 150 - 200 nmthick layer composed of degradation products.Surprisingly, our AFM images illustrate a slightly different situation. Both our stressedphotoreceptors exhibit deep grooves before IPA washing. These occur regularly every fewmicrons. However, the surface topology produced using the two toners appear extremelydifferent. In toner A samples, these grooves span 200 nm - 400 nm in total amplitudewhereas the grooves in toner B samples have a magnitude of only 50 nm - 100 nm. In-cidentally, these regularly occurring grooves appear on a frequency similar to that of adiffraction grating. By diffracting incoming light, these grooves could lead to a blurryimage.After cleaning with IPA, these grooves slightly shrink in magnitude, although they donot disappear. Furthermore, while the surface of the stressed photoreceptor is quite roughbefore cleaning, IPA treatment seems to remove much of this roughness to leave an overallsmoother surface. Our AFM results suggest that, similar to Nauka et al., degradationproducts form a loose layer on the surface of the photoreceptor that is easily removable.While untested, it is fathomable that this surface modification could have an effect on printquality.Another interesting point lies in the difference in the toner samples. With the differencein the two toners being zinc stearate bead size, it is obvious that zinc stearate once againplays a central role. Our AFM results here further supports the hypothesis of a lubricatingprotective zinc stearate layer; a thicker layer may reduce friction between the photoreceptorand other rotating parts, thus reducing the magnitude of physical wear.48Chapter 6Closing RemarksAfter bringing consumer grade xerographic photoreceptors to end-of-life states in standardXerox machines, we observed surface changes using static SIMS, ATR-FTIR, and AFM.We found all three major chemicals making up our photoreceptor (PTFE, PCZ, and TPD)in samples prior to and after stressing. While the state of PTFE and TPD remainedrelatively constant in control and stressed samples, we observed differences in PCZ. In themass spectrum, changes in relative intensities among peaks associated with PCZ were seen.The amount of the ion fragment with a mass of 223 amu increased in stressed samples whencompared to other PCZ fragment peaks. This effect was more pronounced in photoreceptorsstressed using toner A which had larger zinc stearate beads. We also found signs of PCZchange in our FTIR spectra. Stressed samples showed the formation of two new bands inthe aliphatic methylene stretch region. This points to chemical degradation occurring inthe cyclohexane group of the molecule. Furthermore, these peaks were restored to a statesimilar to that of a new photoreceptor upon light cleaning with isopropyl alcohol. Theseresults lead us to believe that chemical degradation in xerographic photoreceptors consistsmainly of polycarbonate Z cleavage at the cyclohexane ring as well as hydrolysis at thecarbonyl carbon to form an easily removable layer of degradation products.Our stress tests using two different toners highlighted the importance of zinc stearate.In most cases, the photoreceptor utilizing toner B showed a state more similar to that ofthe control than the photoreceptor stressed with toner A. This was especially apparent inour AFM images in which we saw a relatively flat surface on one stressed photoreceptorand large deep grooves in the other stressed photoreceptor. It appears that zinc stearatelikely protects the photoreceptor from physical damage as well as possibly also shieldingPCZ from degradation by acting as a hydrophobic shield.Although some progress has been made in determining the nature of degradation in ourxerographic photoreceptors, much work remains. Our methods of photoreceptor stressingseem unable to control the numerous variables present in the printing cycle. This is ap-parent in our failure to obtain significant results in our test page analysis. By creatinga more controlled stressing environment, we may be able to properly gauge the extent of49Chapter 6. Closing Remarkschemical degradation by measuring the output characteristics of printed products. Ideally,we would be able to identify and correlate areas of print defect to physically/chemicallymodified areas on our photoreceptor. Additionally, the BCR stressing apparatus did notperform properly. With improvements, this fixture should eliminate many of the variablespresent in a regular printer. This would also eliminate toner as a variable and allow us tostudy the significance of toner/zinc stearate in more detail. With regard to zinc stearate,depth profiling experiments with SIMS may yield interesting information regarding thethickness of the apparent protective zinc stearate layer. Evidence points to degradationat the cyclohexane ring but we cannot say with certainty what our products are. Subse-quent work would strive to isolate and purify degradation products. This would allow usto employ other analytical technologies such as NMR for structural determination.50Bibliography[1] Charles B. Duke, Jaan Noolandi, and Thieret Tracy. The surface science of xerography.Surface Science, 500(1):1005–1023, 2002.[2] Tech. rep. Worldwide printer and mfp market overview and forecast, 2009-2013: Con-fronting a challenging economy. Technical report, Lyra Research Inc., 2009.[3] Paul M. Borsenberger and David S. Weiss. Organic Photoreceptors for Imaging Sys-tems. Marcel Dekker, Inc., 1993.[4] David S. Weiss and Martin Abkowitz. Advances in organic photoconductor technology.Chemical Reviews, 110(1):479–526, 2010.[5] Damodar M. Pai and B. E. Springett. Physics of electrophotography. Reviews ofModern Physics, 65(1):163–211, 1993.[6] C.K.H. Wong, Y.C. Chan, Y.W. Lam, D.P. Webb, K.M. Leung, and D.S. Chiu. Themodification of electrophotographic and mechanical properties of organic photocon-ductors by ultra-violet irradiation. Journal of Electronic Materials, 25(9):1451–1457,1996.[7] Yasuhiko Shirota and Hiroshi Kageyama. Charge carrier transporting molecular ma-terials and their applications in devices. Chemical Reviews, 107:953–1010, 2007.[8] Jong Dae Lee and Hong Bae Kim. Xerographic properties of metal/metal-free phthalo-cyanine composites in a double-layered photoconductor. Korean Journal of ChemicalEngineering, 26(3):673–678, 2009.[9] E. Montrimas, S. Tauraitiene, and A. Tauraitis. Latent image formation mechanismsin As2Se3 electrophotographic layers. W. F. Berg and K. Hauffe, Eds., New York:Walter de Gruyter, 1972.[10] Luc Van Vaeck, Annemie Adriaens, and Renaat Gijbels. Static secondary ion massspectrometry: (s-sims) part 1: methodology and structural interpretation. Mass Spec-tromety Reviews, 18(1):1–47, 1999.51Bibliography[11] John S. Fletcher and John C. Vickerman. Secondary ion mass spectrometry: character-izing complex samples in two and three dimensions. Analytical Chemistry, 85(2):610–639, 2013.[12] Pierre Giacomo. The michelson interferometer. Mikrochimica acta, 3:19–31, 1987.[13] C. Berthomieu and R. Hienerwadel. 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Tables of molecular vibrational frequencies consolidated vol-ume i. National Standard Reference Data System, 39:1–167, 1972.[21] A. Davis and J. H. Golden. Degradation of polycarbonates iii. viscometric study ofthermally-induced chain scission. Die Makromolekulare Chemie, 78(1):16–23, 1964.[22] Bok Nam Jang and Charles A. Wilkie. The thermal degradation of bisphenol a poly-carbonate in air. Thermochimica Acta, 426:73–84, 2005.[23] I. C. McNeill and A. Rincon. Degradation studies of some polyesters and polycarbon-ates - 8. bisphenol a polycarbonate. Polymer Degradation and stability, 31:163–180,1991.52[24] Marjolein Diepens and Pieter Gijsman. Photodegradation of bisphenol a polycarbon-ate. Polymer Degradation and Stability, 92:397–406, 2007.[25] Yu Seung Kim, Jinlian Yang, Sheng Wang, Ajit K. Banthia, and James E. Mc-Grath. Surface and wear behavior of bis-(4-hydroxyphenyl) cyclohexane (bis-z)polycarbonate/polycarbonate-polydimethylsiloxane block copolymer alloys. Polymer,43:7207–7217, 2002.[26] K. Nauka, Seongsik Chang, and Hou T. Ng. Interactions between organic photocon-ductor and plasma discharge within an electrophotographic environment. Journal ofVacuum Science and Technology A, 27(3):566–571, 2009.[27] A. Rivaton, B. Mailhot, J. Soulestin, H. Varghese, and J.L. Gardette. Compari-son of the photochemical and thermal degradation of bispenol-a polycarbonate andtrimethylcyclohexane-polycarbonate. Polyer Degradation and Stability, 75:17–33,2002.[28] Michelle L. Steen, Carmen I. Butoi, and Ellen R Fisher. Identification of gas-phasereactive species and chemical mechanisms occurring at plasma-polymer surface inter-faces. Langmuir, 2001.[29] D. A. Knopf, J. Mak, S. Gross, and A. K. Bertram. Does atmospheric processing ofsaturated hydrocarbon surfaces by no3 lead to volatilization? Geophysical ResearchLetters, 33(17), 2006.[30] K. Nauka, Seongsik Chang, and Hou T. Ng. Surface modification of an organic pho-toconductor in an electrophotographic charging environment. Journal of ImagingScience and Technology, 54(5):050304–1–050304–5, Sep. - Oct. 2010.53Appendix AAppendix AThe following are some examples of test page analysis done on half tone test prints. Analysiswas done in collaboration with Mr. Pouria TalebiFard from Computer Science at UBC.Density measured the ratio of blue pixels to white pixels in a fixed square enclosing thehalftone dot. Radii measurements compared the radius of each dot with the average dotradius of entire page. Dot size was defined as distance from the centre where pixel intensityfell below a certain threshold.Figure A.1: Analysis of a half tone test page showing variations in density for each dot.Figure A.2: Analysis of a half tone test page showing variations in dot radii.54

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