Kinetic model for tensile deformation of polymers. Part IV: Effect of polydispersity

A previously introduced molecular model for tensile deformation of solid, flexible chain polymers is used to study the effect of the molecular weight distribution on the strain at break, or maximum draw ratio. The parameters in the model are chosen to represent melt-crystallized linear polyethylene. We focus, in particular, on the relation between the maximum draw ratio and two distribution characteristics: theM _t molecular weight average, first introduced by Graessley, and the polydispersity ratioM _z /M _w . For a log-normal molecular weight distribution, an increase in polydispersity at constantM _t leads to a broadening of the optimum rate (or temperature) window for achieving maximum elongation, but is accompanied, however, by a substantial decrease in the maximum attainable draw ratio. Studies on the deformation of systems having a bimodal molecular weight distribution indicate that blends made ofequal weight fractions of long and short chains exhibit an unexpectedly high elongation at break. These results are explained in terms of the model and possible technological implications are discussed.Dedicated to Professor Hans-Henning Kausch on the occasion of his 60th birthday.