Stress-Optical Coefficient of Poly-(dimethylsiloxane) Networks

An expression in matrix form for the optical anisotropy ΔΓ of Kuhn's random link (an equivalent to the stress-optical coefficient) of two-repeat polymers is derived, based on the additivity principle of bond polarizabilities and the rotational-isomeric-state approximation. The interdependence of neighboring-bond rotations is taken into account. Stress-optical data of poly-(dimethylsiloxane) networks are analyzed by assuming rotational states to be three in number, i.e., trans (T), gauche (G), and gauche prime (G′). It is found that semiquantitative agreement is achieved by assuming EG>0 and Δαe>0. Here EG is the energy of the gauche conformation relative to that of the trans, and Δαe=(α1−α2)Si–O−(α1−α2)C–Si+(α1−α2)C–H is the ``effective'' anisotropy of the Si–O bond. A harmonized choice of values of θ(G) the internal-rotational angle for the gauche conformation, and ω1, the supplement to ∠SiOSi, is needed for interpretation of two items of ΔΓ/Δαe and 〈r2〉/nb2, the latter being the characteristic ratio for the unperturbed dimension. Definite conclusion could not be obtained about to what extent the GG′ conformations for neighboring bonds are suppressed.