Ultimate toughness of amorphous polymers

The deformation and toughness of amorphous glassy polymers is discussed 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; where PS possesses a low entanglement density and PPE a relatively high entanglement density) and epoxides based on diglycidyl ether of bisphenol A (DGEBA) with crosslink densities comparable with up to values much higher than the thermoplastic model system. The microscopic structure was controlled by the addition of different amounts of non‐adhering core‐shell‐rubber particles. Toughness is mainly determined by the maximum macroscopic draw ratio since the yield stress of most polymers approximately is identical (50–80 MPa). It is shown that the theoretical maximum draw ratio, derived from the maximum (entanglement or crosslink) network deformation, is obtained macroscopically when the characteristic 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 critical matrix ligament thickness (ID c ) uniquely depends on the molecular structure: at an increasing network density, ID c increases independent of the nature of the network structure (entanglements or crosslinks). A simple model is presented based on an energy criterion to account for the phenomenon of a critical ligament thickness and to describe its strain‐rate and temperature dependency.