DEFORMATION AND TOUGHNESS OF POLYMERIC SYSTEMS .3. INFLUENCE OF CROSS-LINK DENSITY

In part 1 of this series the phenomenon of a critical ligament thickne ss (ID(c)) below which brittle polymers become ductile was investigate d for polystyrene (PS). Using the thermoplastic polystyrene-poly(2-6-d imethyl-1,4-phenylene ether) (PS-PPE) model system, it was demonstrate d in part 2 of this series that the absolute value of ID(c) as well as the maximum toughness (i.e. maximum strain to break) was dependent on the network density of the polymer used. In this study the toughness and ID(c) of crosslinked thermosetting polymers were investigated usin g epoxides based on the diglycidyl ether of bisphenol A as a model sys tem. The crosslink density (nu(c)) is varied between values comparable with (nu(c) = 9 x 10(25) chains m-3), up to values much higher than ( nu(c) = 235 x 10(25) chains m-3), the entanglement density in the ther moplastic PS-PPE system. The maximum macroscopic toughness proportiona l to the strain to break (lambda(macr)) or given by the slow-speed fra cture toughness (G(Ic)) and the notched high-speed tensile toughness ( G(h)) of core-shell rubber-modified epoxides uniquely increases with a n increasing molecular weight between crosslinks (M(c)). Only by using extreme testing conditions (notched high-speed impact testing), could the ID(c) of a limited range of epoxides be determined: 0.21 mum (nu( c) = 9 x 10(25) chains m-3) less-than-or-equal-to ID(c) less-than-or-e qual-to 0.29 mum (nu(c) = 14 x 10(25) chains m-3). Both the experiment ally determined values of ID(c) and the toughness of the epoxides comp are well with the values determined for the entangled thermoplastic PS -PPE model system in the same range of network densities, elucidating the principal similarity of the influence of entanglements and crossli nks on the deformation processes. Good agreement was observed between the experimentally determined values of ID(c) of the epoxides and the values predicted by the simple model introduced in part 2 of this seri es.