Deformation and toughness of polymeric systems: 2. Influence of entanglement density

Abstract

In part 1 of this series, the concept of a critical material thickness was introduced and demonstrated experimentally using polystyrene (PS) as a test material. Below the critical thickness, brittle polymers become ductile. The value of the critical thickness is material-dependent and related to the entanglement density. The dependence of the critical thickness on the entanglement density was investigated using the miscible system polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) (PS-PPE). PS possesses a low entanglement density and PPE a high entanglement density, and the system's entanglement density can be varied depending on the relative ratio of PS and PPE in the mixture. Equivalent to the experimental procedure developed in our previous paper, the thickness was set by either changing the PS-PPE layer thickness in stratified PS-PPE/PE tapes (polyethylene (PE) is present to separate the PS-PPE layers) or by adjusting the volume fraction of non-adhering core-shell rubbery particles in the PS-PPE blend, i.e. the ligament thickness. The experimentally determined critical thickness ( ID c) proved to increase continuously from 0.06 μm for PS-PPE 80–20 to 0.18 μm for PS-PPE 40–60 blends. This compares well with the value of 0.05 μm found for pure PS. Under the (moderate) testing conditions used, the PS-PPE 20–80 blend was always tough. The maximum macroscopic strain to break ( λ macr) of the PS-PPE blends correlated with the theoretical value ( λ max) based on stretching the entanglement network to its full extension. The transition from a macroscopic, brittle-to-ductile deformation behaviour is associated with a change in type of deformation mechanism from void formation (e.g. crazing) to shearing, except for the PS-PPE 20–80 blend, which always deforms by shear deformation. A simple model based on an energy criterion could explain the occurrence of a critical material ligament thickness as well as its dependence on the entanglement density.