Properties of dimethacrylate copolymers of varying crosslink density

Homopolymers and copolymers of tetrafunctional oligo-ethylene dimethacrylate monomers of widely varying chain lengths are investigated by thermal and fracture techniques. Differential scanning calorimetry of catalysed monomer shows that the polymerization process is inhomogeneous. Quantitative analysis of these results suggest that the mechanism of cure is complex and may be ascribed to complex copolymerization, partial phase separation or a form of interpenetrating network. Dynamic mechanical spectra of the copolymers reveals that their mechanical properties vary monotonically between those of the constituent homopolymers. In some systems, double-peak behaviour is noted in the glass transition region. This is ascribed to possible phase separation. Short rod fracture, a fracture toughness technique suitable for glassy polymers, is also used on a copolymer system to obtain information about the copolymer morphology. The transition from 'stick-slip' to a smoother fracture mode with higher concentrations of the longer, flexible monomer is accompanied by a concomitant decrease in fracture toughness. This behaviour is explained in terms of the inhomogeneity of both homo- and copolymer dimethacrylate systems and failure occurring through points of weakness in the system, the regions of lower crosslink density.