| Abstract: | Stress-optical coefficients have been determined for crosslinked samples of polyethylen (PE) and polystyrene (PS) at high temperatures, i.e., in the rubberlike state, and when swollen in a variety of liquids. For PE, swelling liquids with long straight molecules gave large values of optical anisotropy whereas liquids with more symmetrical molecules gave minimum values, as found previously for cis- polyisoprene and trans-polyisoprene. This solvent effect is attributed to short-range orientational order in molecularly asymmetric media. Sizes of the equivalent random link for unperturbed molecules of these three polymers were deduced from the minimum values of optical anisotropy. Measures of shape asymmetry were also obtained by matching the optical anisotropy of samples when unswollen with that observed when swollen with a liquid of known molecular asymmetry. Reasonable agreement was found to hold between the two methods. In contrast, the optical anisotropy of swollen PS was found to be substantially independent of the swelling liquid. The apparent absence of a molecular ordering effect in this case is attributed to the bulky nature of the PS molecule. A marked reduction in optical anisotropy on swelling is ascribed to increased phenyl group motion. |
