NONLINEAR SHEAR AND EXTENSIONAL RHEOLOGY OF LONG-CHAIN RANDOMLY BRANCHED POLYBUTADIENE

We present nonlinear shear and uniaxial extensional measurements on a series of polybutadienes with varying amounts of long-chain, random br anching. Startup of steady shear experiments is used to evaluate the d amping function of the melts. The damping function is found to show a trend toward decreased dependency on strain with increasing branching content. Interior chains, which are believed to be responsible for cha nging the damping function, are calculated to comprise less than 3 wt % of the melt. Extensional measurements are used to investigate the ro le of branching in strain hardening. We show that samples with increas ed branch contents do show larger deviations of the transient Trouton ratio from the linear viscoelastic limit of three. However, we also sh ow that the extensional data can be fit using parameters determined so lely by the shear measurements. Furthermore, we show that the changes in the damping function seen in shear have little impact on extensiona l behavior. The extensional behavior of the melt is found to be most a ffected by changes in the relaxation spectra which can result from bot h branching and increases in the high end of the molecular weight dist ribution. This statement runs contrary to the often expressed view tha t strain hardening behavior in extension is exclusively produced by br anching. (C) 1998 The Society of Rheology. [S0148-6055(98)00806-2].