XI 9. PHOTOELASTIC PROPERTIES OF RUBBERS 9.1. Refractive index and polarizability 174 9.2. Optical properties of long-chain molecules 175 9.3. The Gaussian network 178 9.4. The effect of swelling 182 9.5. The non-Gaussian network 182 9.6. Measurement of birefringence 186 9.7. Investigations on natural rubber 189 9.8. The effect of the degree of cross-linking 195 9.9. Polyethylene 198 9.10. Optical properties of the monomer unit 202 9.11. The equivalent random link 204 9.12. The effect of swelling on stress-optical coefficient 206 9.13. Temperature dependence of optical anisotropy 209

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In order to study the generation of charge on fibrous materials, an apparatus was constructed in which a fiber is held fixed while a second is rubbed across it under controlled mechanical and ambient conditions. It was found that the reproducibility was usually within ± 5% when the same two fibers were rubbed repeatedly; that the charge generated was dependent on the manner in which the materials were rubbed; and that the magnitude of the charge generated was directly proportional to the length of material rubbed and to the normal force between the fibers (although in some cases a limiting maximum value was reached), but was independent of the apparent area of contact between the fibers and of the tension on the fibers. The effects produced by changes in velocity are more complicated to describe. Charge transfer was found to be independent of velocity when insulators (except Teflon) were rubbed together. For metals on insulators other than Teflon, the charge generated was found to increase linearly with velocity until a limiting value was reached, and then to remain constant. When Teflon and metals were rubbed together, the charge increased linearly with velocity without reaching a maximum. When Teflon was rubbed with insulators, the charge increased linearly with velocity in some cases but remained constant in others. A study of the dependence of the sign and amount of charge transferred on the nature of the materials rubbed was undertaken, and a triboelectric series was established. For metals on insulators, the amount of charge generated was found to be related to the work function of the metal and the position of the insulator in the triboelectric series. For insulators rubbed on insulators, the amount of charge transferred appeared to be independent of the positions of the insulators in the series.

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