Uniaxial and Biaxial Extension Behavior of a Lightly Cross-Linked PMMA Melt at Constant Strain Rates

Abstract

For a lightly cross-linked poly (methyl methacrylate) (PMMA) with a gel fraction of 0.65, experimental studies are made on swelling, dynamic viscoelasticity, and uniaxial and biaxial extension at constant strain rates. The values of average molecular weight between cross-links, Mc, (or between a cross-link and a trapped entanglement) are determined by four methods; swelling, the equilibrium modulus, the Mooney-Rivlin plot and the Young's modulus. These values are in the same order but somewhat smaller values are obtained from the latter two methods. The density of weakly attached or untrapped entanglement strands is evaluated from the plateau modulus of (G' −Geq), where G' and G eq are the storage and equilibrium moduli. The entanglement molecular weight, Me, evaluated by this method is certainly smaller than Mc, and is slightly larger than Me of uncross-linked atactic PMMA melt. In uniaxial extension, the stress growth coefficient shows stronger strain-hardening than that of uncross-linked PMMA melts with very high molecular weight component. On the other hand, in biaxial extension, the stress growth coefficient exhibits weak strain-softening followed by small upturn.