Uniaxial Extension Behavior of Cross-Linked Poly (methyl methacrylate)s with Various Degrees of Cross-Linking

Abstract

Cross-linked poly(methyl methacrylate)s with various degrees of cross-linking were prepared, and their swelling, dynamic viscoelasticity and uniaxial extension behavior was investigated. For each sample, the values of molecular weight M_c between cross-links (and trapped entanglements) evaluated from the equilibrium modulus G_eq and the Mooney-Rivlin constants (C ₁+C₂) are comparable, while M_c evaluated from the gel fraction is somewhat higher than those values. It is suggested from the data of excess storage modulus (G'−G_eq) and the Mooney-Rivlin constant C₂ that untrapped entanglement density decreases slightly while trapped entanglement density increases slightly with increasing degree of cross-linking. The strain-hardening tendency at constant strain rates becomes weaker with increasing cross-linking degree, and the strain-hardening does not appear when M _c approaches the entanglement molecular weight M_e. It is suggested for cross-linked polymers with rather high cross-linking density that chain breakage will occur before chain strands between cross-links stretch enough to result in the strain-hardening.