Stress–Optical Coefficient of Polyethylene Networks

The stress–optical coefficient of polyethylene networks is calculated on the basis of a theory previously developed, and results are compared with experimental data recently reported by Gent and Vickroy. Observed temperature coefficients of $\mathrm{\Delta \Gamma }$ , the optical anisotropy of Kuhn's random link (an equivalent to the stress–optical coefficient) for unswollen samples, are much larger than those calculated using acceptable values of $E_G$ , the energy of the gauche conformation, relative to that of the trans. It is concluded that observed temperature coefficients should include some contributions other than those implied in the theory, i.e., those arising from the conformational change with temperature. Data on samples swollen in decalin (about half those for unswollen samples) are better fitted with the theory, yielding values of the effective C–C bond anisotropy, ${\mathrm{\Delta \alpha }}_e{\mathrm{\hspace{0.17em}=\hspace{0.17em}(\alpha }}_1{\mathrm{\hspace{0.17em}-\hspace{0.17em}\alpha }}_2{)}_{\mathrm{C–C}}{\text{\hspace{0.17em}−\hspace{0.17em}2(α}}_1{\mathrm{\hspace{0.17em}-\hspace{0.17em}\alpha }}_2{)}_{\mathrm{C–H}}$ , in satisfactory agreement with those from other sources. Here, ${\alpha }_1{\mathrm{\hspace{0.17em}-\hspace{0.17em}\alpha }}_2$ is the anisotropic bond polarizability. The short‐range orientational order among polymer‐chain segments in concentrated solutions and in the molten state is proposed to be a mechanism responsible for the abnormally large $\mathrm{\Delta \Gamma }$ and its temperature coefficient observed for unswollen samples. The author's previous view supporting Denbigh's bond polarizabilities is abandoned. The possibility of negative ${\mathrm{\Delta \alpha }}_e$ , recently suggested by Murrell and Musgrave, is denied.