The influence of network density and material critical thickness on the ultimate toughness of polymers

The deformation and toughness of amorphous polymers is discussed in terms of their mol. network structure and morphol. Both neat and (nonadhering) core-shell-rubber modified thermoplastics and thermosets are analyzed. The thermoplastic model system consists of miscible blends of polystyrene (I) and poly(2,6-dimethyl-1,4-phenylene ether) in different vol. ratios (consequently with different entanglement densities). The thermosetting system is based on epoxides with various degrees of crosslink d. Toughness is mainly detd. by the max. macroscopic strain at break since the yield stress of all polymers is approx. const. (50-80 MPa). The theor. max. draw ratio can be derived from the max. (entanglement or crosslink) network deformation. Brittle polymers, like I, suffer from catastrophic localization of strain and macroscopically show a strain to break far below this theor. max. However, below a certain dimension of the microstructure, expressed by the crit. matrix ligament thickness between added nonadhering core-shell rubbery particles (holes), the max. network extension can be reached on a macroscopic level. The crit. thickness depends on the mol. structure: with increasing network d. the value of the crit. ligament thickness increases from 0.05 mm for I (high mol. wt. between entanglements: Me = 19.1 kg mole-1) via 0.18 mm for the I blends contg. 40 wt. I (Me = 6.7 kg mole-1) to 0.3 mm for an epoxide having a mol. wt. between crosslinks, Mc, of approx. 4.4 kg mole-1