DEFORMATION AND TOUGHNESS OF POLYMERIC SYSTEMS .1. THE CONCEPT OF A CRITICAL THICKNESS

A relationship between the macroscopic toughness, the intrinsic networ k (entanglement and/or crosslink) density and the relative thickness o f polymeric systems, is presented. Toughness of amorphous, glassy poly mers is mainly determined by the strain to break, since the yield stre ss generally only varies between 50 and 80 MPa. It was found that the strain to break strongly depends on the absolute thickness of the spec imen or, equivalently, the local thickness within the (micro) structur e of the material. Only below a certain critical thickness can the int rinsic strain at break of a polymer be reached. The absolute value of this critical thickness and the intrinsic strain at break of a polymer are both determined by the network density. In this paper polystyrene (PS), a polymer that is generally considered to be very brittle, was investigated with respect to the influence of absolute thickness on it s strain to break. For thin isotropic tapes of PS it was demonstrated that this critical thickness is below 1 mum. Based on experiments with macroscopically 'thick' PS samples (3 mm), which are made locally thi n by the introduction of small non-adhering rubbery particles ('holes' ), we could identify that the critical thickness is 0.05 mum for PS.