Thermal Cross‐Linking of Poly(Vinyl Methyl Ether). II. Rheological Behavior at the Gel Point

Abstract

Isothermal time evolution measurements at different constant temperatures (170°C, 180°C, and 190°C) over a wide range of frequency for the thermal cross‐linking process of poly(vinyl methyl ether), PVME, have been investigated rheologically. At the onset of cross‐linking (t onset) the elastic storage modulus, G′, increases abruptly. The magnitude of the elevation in G′ and the value of t onset were found to be temperature‐dependent. Similar behavior was observed for both the viscous loss modulus, G″ and the complex dynamic viscosity, η*; however, the value of G″ shows a very low sensitivity to the cross‐linking process compared to G′ and η* at the same experimental conditions. The gel point, t gel, was evaluated from the point of intersection in plots of tan δ vs. curing time for different constant shear frequencies. At the gel point tan δ is no longer frequency‐dependent, and all curves cross‐over, indicating the validity of the Winter–Chambon criterion. The value of t gel obtained from the coincidence of G′ and G″ was about 10 min longer than that determined from tan δ vs. t, indicating that the crossover of G′ and G″ is not be considered as a general method for evaluation of t gel. The value of the apparent activation energy of gelation determined from the temperature dependence of t gel was 74 kJ mol−1 in good agreement with literature values for other different systems. At the gel point G′ and G″ showed a power law with shear frequency, i.e., G′ ∼ G″ ∼ ω n with critical exponents equal to 0.64 and 0.75, respectively, in close agreement with the percolation theory (n = 2/3). The zero shear viscosity, η0, and the equilibrium shear modulus, G eq, can also be expressed in power low scaling functions with the relative distance from the gel point, ϵ i.e., η0 ∼ ϵ−k and G eq ∼ ϵ z with k = 1.3 and z = 2.4 in good agreement with the predicted values based on the percolation theory.