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UBC Theses and Dissertations

Rheology and processing of biodegradable poly(epsilon -caprolactone) polyesters and their blends with polylactides Noroozi, Nazbanoo

Abstract

The solution rheological properties and melt viscoelastic behaviour of a number of commercial and newly synthesized PCLs with different molecular characteristics was investigated using both rotational and extensional rheometry. The variation of zero-shear viscosity and relaxation spectrum with molecular weight were found to be in agreement with theories for linear polymers. The classic Wagner model was found to represent the rheology of all PCL polymers quite well. In addition, the PCL processing instabilities were studied by capillary extrusion. Sharkskin and gross melt fracture was observed for the high molecular weight PCL at different shear rates. The onset of melt fracture occurred at 0.2 MPa at temperatures higher than 115°C. Moreover, addition of 0.5 wt% of a polylactide (PLA) into the PCL eliminated or delayed the onset of melt fracture to higher shear rates. The thermodynamics and rheological behavior of PCL/PLA blends has been studied in an attempt to find ways to improve the mechanical properties of PCL. The effects of shear and heating rates on the determination of the phase separation boundary of the PCL/PLA blend system were studied in detail. The lower critical solution temperature (LCST) phase boundary for this system is shifted based on the frequency, heating rate and concentration of the components. Additionally, higher molecular weight amorphous PLA leads to phase separation boundary shifted to lower temperatures. Differential scanning calorimetry (DSC) thermograms of PCL/PLA blends exhibit separate melting peaks, which are indicative of immiscible structure at all compositions in agreement with the phase diagram. Scanning electron microscopy (SEM) images have shown droplet morphology of PCL into PLA matrix up to 40wt% of PCL. Above this concentration the co-continuous morphology starts to appear, which becomes again droplet morphology for blends with PCL concentration above 60wt%. The viscoelastic studies in the phase separated region have shown the enhancement of the elastic modulus of blends at small frequencies, which is a signature behavior of immiscible systems due to the presence of interface and interfacial tension contribution to the stress. Emulsion models were found to be successful in the prediction of viscoelastic behavior at the compositions which corresponds to droplet morphology.

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