Molecular weight dependence of shear-induced structures in nematic semiflexible polymer solutions

The role of molecular weight in shear-induced structure transitions, such as striations during shear startup and banding during relaxation, was probed through small-angle light scattering and shear microscopy. The characteristic texture relaxation time for banding, τs, is molecular weight dependent. The minimum molecular weight for these shear-induced complex texture transitions to occur is estimated from τs as 27 000 for a 37 wt % poly(n-hexyl isocyanate) in 1,1,2,2-tetrachloroethane solution at room temperature. τs is independent of applied shear rate, implying that it is a material property. Thus, both of these structure transitions appear to occur only after shearing above a threshold Deborah number, γτs|c, below which viscous forces would suppress these structure transitions. Both the strain dependence of the structure transitions during shear startup and the preshear rate dependence of the banding phenomenon exhibit little or no molecular weight dependence. Increasing molecular weight does appear t...