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The ultraviolet absorption of stretched and unstretched GR-S latex films Mayo, Eleanor Grace


This work on the ultraviolet absorption of stretched and unstretched Type 3 GR-S latex films, was undertaken on the suggestion of Dr. H. D. Smith and Dr. E. Guth, with the expectation that the ultraviolet absorption could give information on the arrangement of the molecules in the stretched state as compared with the unstretched latex. Previous investigators have found that styrene has an absorption band with maximum absorption at 2850 angstroms. Since styrene is one of the constituents of GR-S latex, it was supposed that this absorption band would also appear in any spectral analysis of the latex. To find the nature of this band and its behaviour upon stretching the sample was one of the objectives of this research. The type of spectrograph used for this work was a Hilger E496.303 with a wavelength scale, and Spekker photometer attachment. Eastman Type II-F spectroscopic plates were used for all readings. A tungsten steel spark with about 25,000 volts across the electrodes was used as a source for all plates. The unstretched films were prepared by coating a pane of glass with a ten percent solution of zinc chloride, and when dry, the glass was coated with the latex. After drying, a very thin film of rubber was deposited on the glass which could be peeled off as needed. By this method ultraviolet transparent films of initial thickness 0.020 cm. were obtained. These films were stretched into the shape of a spherical bubble by means of nitrogen gas. By calculating the surface areas of the stretched samples and assuming Poisson's Ratio to be 1/2 for rubber, the thicknesses of the stretched samples could be calculated. A narrow absorption band was found for latex of thickness 0.00260 cm. in the region of 2850 angstroms which did not appear to shift with further stretch. A slight broadening effect upon stretching might have been present but it would not amount to more than 5 or 10 angstroms in either direction. From this it was concluded that the absorption centres of the styrene molecules remained unaffected during stretching. Absorption coefficients were also calculated by Lambert's Law on the assumption that the total loss of radiation was due to absorption. It was found that a decrease in thickness was followed by an increase in the absorption coefficient for a constant wavelength. To obtain the true absorption coefficients, the coefficients computed, by Lambert's Law must, be corrected for surface reflection and body scattering. This may affect the values of the coefficients but not the position of the band. The Increase in scattering with a decrease in thickness is suggested to be due to the existence of microcrystals when the latex is under stress.

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