Viscoelastic dispersion of polydimethyl siloxane in the rubberlike plateau zone

Abstract

The viscoelastic properties of six stocks of polydimethyl siloxane, with molecular weights ranging from 0.41 to 4.9 × 10 6, have been studied by dynamic and creep methods, between −49° and 75°C, and from a maximum frequency of 600 cycles/sec. to a maximum time of 36 days. The methods included the Fitzgerald transducer, Morrisson-DeWitt forced oscillation torsion pendulum, Plazek freely oscillating torsion pendulum, and creep and creep recovery in torsion and in simple shear. The temperature dependence of the viscosity of the stock of lowest molecular weight, between −21° and 141°C., corresponded to an apparent heat of activation for flow ( ΔH η ) of 3.65 kcal. All dynamic measurements at different temperatures were successfully superposed by the method of reduced variables with shift factors calculated from the viscosity temperature dependence; all measurements were presumably too far above T g for applicability of the WLF equation. The measurements encompassed only the plateau and terminal zones of the viscoelastic time scale; because of the very high chain mobility, the transition between rubberlike and glasslike consistency was not entered at the highest frequency and the lowest temperature. At −49°, incipient crystallization apparently occurred. For the two stocks of lowest molecular weight, measurements were extended through the terminal zone to steady-state flow, and the results appeared to be normal for linear polymers of high molecular weight. By an indirect calculation involving the terminal relaxation times, the logarithm of the monomeric friction coefficient was estimated to be −7.3 at 25°C. For the other stocks ( M w = 2.2 to 4.9 × 10 6) no perceptible flow was observed at low stresses, even though their complete solubility demonstrated absence of cross-linking. Instead, their creep at long times followed the Andrade equation, J(t) = J A + βt 1 3 . The intercept J A decreased somewhat with increasing molecular weight; so did β except for one discrepancy attributed to a broad molecular weight distribution. At higher stresses, however, a flow contribution was observed which began after a critical deformation had been exceeded; this deformation was independent of the stress magnitude and for M w = 3.2 × 10 6 corresponded to a shear strain of 22% in the sample. The effective viscosity for this delayed flow was far higher than that extrapolated from the relation between η and M w at lower molecular weights, and it decreased sharply with increasing stress. The results suggest the presence of a quasi-permanent network intermediate in character between chemically cross-linked and entanglement networks, the linkages of which yield when a critical strain has been exceeded.