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

摘要

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, τ_s, is molecular weight dependent. The minimum molecular weight for these shear-induced complex texture transitions to occur is estimated from τ_s as 27 000 for a 37 wt% poly(n-hexyl isocyanate) in 1,1,2,2-tetrachloroethane solution at room temperature. τ_s 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, γτ_s|c, below which viscous forces would suppress these structure transitions. Both the strain dependence of the structure transition during shear startup and the preshear rate dependence of the banding phenomenon exhibit little or no molecular weight dependence. Increasing molecular weight does appear to decrease the characteristic band correlation length and slow the decay rate of the bands.