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Stress strain isotherms are presented for poly (dimethylsiloxane) networks of different degrees of crosslinking, swollen to various extents by a diluent. Results are interpreted in terms of the equation f*v21/3V/(α−α−2)=2C1+2C2α−1, where ƒ* is the tension per unit initial area (unswollen) exhibited by a network when swollen to a polymer volume fraction v2 and elongated to the relative length α, 2C1 is a parameter of the network structure established by the molecular theory of rubber elasticity, and 2C2 is an additional parameter found to give a better representation of experimental data. Values of C2 obtained for networks are found to decrease as the degree of crosslinking is decreased, and the degree of swelling is increased.

According to the molecular theory of rubber elasticity, the stress τ is related to the relative length a in simple extension according to τ=kT(ν/V)〈α〉2(α−1/α2), where (ν/V)〈α〉2 is a parameter of the network structure. Improved agreement with experiment has been claimed through use of the relation τ=(2C1+2C2/α)(α−1/α2) which contains the additional parameter C2. In order to explore factors affecting this theoretically inexplicable term, stress-elongation measurements were performed on networks prepared from natural rubber, Butyl rubber, poly-(dimethylsiloxane), poly-(ethyl acrylate), and poly(methyl methacrylate) under a wide range of experimental conditions. Results may be summarized as follows: (1) the C2 term offers improved agreement over the extension phase of an elongation cycle, but not for the subsequent retraction phase; (2) apparent values of C2 (determined on extension) vary widely from one polymer to another, increasing with hysteresis and with proximity to the glass transition temperature; (3) they are insensitive to the degree of crosslinking; (4) C2 is markedly reduced by permanently swelling the network with a diluent, or by swelling and deswelling the sample at each tension ; (5) incorporation of diluent prior to establishing a network by crosslinking with gamma radiation likewise lowers C2, the value characteristic of the unswollen polymer being restored upon removal of the diluent. The results demonstrate that the controversial C2 term arises from the difficulty of attaining elastic equilibrium; under ideal experimental conditions the value of this parameter becomes negligible. An alleged limitation of the theory of rubber elasticity is thus refuted.

The thermal expansivity α=V−1(∂V/∂T)p and thermal pressure coefficient γ=(∂p/∂T)v for poly(dimethylsiloxane) (PDMS), mol wt ≈ 105, have been determined accurately over the temperature ranges 20–207 and 24–161°, respectively. Characteristic parameters ν*, T*, and p* calculated from these results are compared with those for other polymers and for low molecular weight liquids. For PDMS, α is much larger and γ much smaller than for any other polymer. The characteristic pressure p* appears to be a more reliable index of the intermolecular energy than the cohesive energy density for this purpose. It is approximately the same for a polymer as for corresponding low molecular weight liquids. For PDMS and HMDS (hexamethyldisiloxane) p* assumes exceptionally low values of 341 and 358 J cm−3, respectively, which are duplicated only by the fluorocarbons (p* ≈ 360 J cm−3).