Entanglement swelling in polymer glasses

A polymer system in which the chains are much longer than the entanglement molecular weight is well entangled. When a glassy polymer film composed of such chains is uniaxially strained, deformations called crazes may be formed. It is well established that the study of crazes can reveal much about the nature of entanglements. Here, we present results of crazing experiments in which well entangled polystyrene networks have been diluted with various weight fractions of polystyrene with molecular weights in the vicinity of the entanglement molecular weight. Upon dilution, the systems assume an effective reduction in the entanglement density which is a function of both the weight fraction and molecular weight of the small chains. A model which combines simple ``chain packing'' with ``binary contact'' ideas is proposed. The model is found to quantitatively describe measurements in systems with two and three molecular weight components, and can easily be extended to polydisperse systems.