Effect of Polydispersity on the Properties of 1,4-Polybutadiene

As is well established for rubbery polymers and to a lesser extent for plastics, the molecular weight and its distribution exert significant effects on many properties. In this work polydispersity was attained by mixing various monodisperse 1,4-polybutadienes (PB), so the chain length distribution was discontinuous, rather than merely broad. For these materials, to a good approximation, the M_w is the control parameter for the zero-shear viscosity; however, consideration of the MWD is necessary to quantitatively predict η₀ for polymers with broad MWD. An empirical relation that includes M_n most accurately describes the η₀ data for the polydisperse materials herein. At higher shear rates the decrease of the viscosity from the Newtonian value is greater for polydisperse samples, and the transition to power-law behavior occurs over a broader range of higher frequencies. In the power-law regime, the shear rate dependence is essentially independent of MWD. Interestingly, mono- and polydisperse samples can be distinguished by their different relationship of phase angle to complex modulus (van Gurp plots), with no significant difference in the behavior among the polydisperse PB.

MWD affects substantially the gelation of the PB. The only method of gel point determination that was applicable for any MWD was from measurements of the soluble fraction of samples cured beyond t_gel. The gel point is achieved for lower degree of crosslinking as MWD increases, with the number of crosslinks per chain necessary for formation of a gel varying from unity for monodisperse PB to a value as low as 0.16 for M_w/M_n ~ 5.