Thermal Cross-Linking of Poly(Vinyl Methyl Ether). I. Effect of Cross-Linking Process on the Viscoelastic Properties

Thermal cross-linking of poly(vinyl methyl ether) (PVME) in the absence of cross-linking agent, was detected Theologically. The linear viscoelastic properties of PVME were found to be greatly changed by the onset of the cross-linking process. The viscoelastic material functions, such as dynamic shear moduli, G′ and G″, complex shear viscosity, η*, and loss tangent, tan δ, were found to be sensitive to the structure changes during the cross-linking process and the formation of a three-dimensional polymer network. At the onset temperature of the cross-linking process, an abrupt increase in G′, G″, and η* (several orders of magnitude) during dynamic temperature ramps (2°C/min heating rate) was observed with some frequency dependence. The temperature dependence of tan δ was found to be frequency independent at the gel-point, T_gel, that is, the crossover in tan δ regardless of the value of frequency can be taken as an accurate method for determination of T_gel. The coincidence of G′ and G″ at the gel-point cannot be considered a general method for evaluation of T_gel due to its high frequency dependence, that is, T_gel determined from the crossover of G′ and G″ in the dynamic temperature ramp at 1 rad/sec is about 20°C less than at 100 rad/sec. Furthermore, a dramatic increase in η₀ above the minimum ("v" shape) was observed at T = T_gel in agreement with the value obtained from tan δ vs. T (190°C). The time-temperature-superposition principle was found to be valid only for temperatures lower than the T_gel (190°C); the principle failed at T ≥ 190°C. This was clearly seen in the low-frequency region as a deviation from the terminal slope in the G′ curve. Similar behavior was observed in the modified Cole-Cole analyses (G″ vs. G′, that is, the curves start to deviate at 190°C.