The stress-optical coefficient and amorphous order

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Abstract
The calculated stress-optical coefficients obtained using the rotational isomer model and the principle of additivity of bond polarizability tensors give values of the segment polarizability anisotropy, Δ TSin reasonable agreement with experimental values for swollen rubber networks. Furthermore, these values agree well with those obtained from streaming birefringence studies. Estimates of the effect of ordering in the amorphous state upon Δ TS lead to values appreciably in excess of those found experimentally. The effect of swelling solvent on Δ TS can be interpreted in terms of its role in separating chains from each other and in itself being oriented by the polymer chain. This model accounts for the observation that Δ TS increases with increasing anisotropy of the swelling solvent. The value of Δ TS found for networks swollen with an isotropic solvent is approximately, but not exactly, equal to the intrinsic anisotropy of the segment, Δ TS found for undiluted networks arises in part from mutual orientation of segments by their neighbors. An additional cause comes from the anisotropy of the internal field arising from local order. All of these effects of the internal field arising from local order. All of these effects are interpretable in terms of relatively short-range interactions and do not require the relatively long-range ordering of a type proposed in some recent theories.