ULTIMATE TOUGHNESS OF AMORPHOUS POLYMERS

The deformation and toughness of amorphous glassy polymers is discusse d in terms of both the molecular network structure and the microscopic structure at length scales of 50-300 nm. Two model systems were used: polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) blends (PS-PPE; wh ere PS possesses a low entanglement density and PPE a relatively high entanglement density) and epoxides based on diglycidyl ether of bisphe nol A (DGEBA) with crosslink densities comparable with up to values mu ch higher than the thermoplastic model system. The microscopic structu re was controlled by the addition of different amounts of non-adhering core-shell-rubber particles. Toughness is mainly determined by the ma ximum macroscopic draw ratio since the yield stress of most polymers a pproximately is identical (50-80 MPa). It is shown that the theoretica l maximum draw ratio, derived from the maximum (entanglement or crossl ink) network deformation, is obtained macroscopically when the charact eristic length scale of the microstructure of the material is below a certain dimension; i.e. the critical matrix ligament thickness between added non-adhering rubbery particles ('holes'). The value of the crit ical matrix ligament thickness (ID(c)) uniquely depends on the molecul ar structure: at an increasing network density, ID(c) increases indepe ndent of the nature of the network structure (entanglements or crossli nks). A simple model is presented based on an energy criterion to acco unt for the phenomenon of a critical ligament thickness and to describ e its strain-rate and temperature dependency.