Rheology of semiflexible polymers
Abstract (Summary)The effect of flexibility and branching on the rheology of wormlike polymers in different concentration regimes was investigated. The goal was to understand flexibility's role in the aggregation behavior and dynamics of wormlike star polymers in isotropic solutions as well as how flexibility controls phase behavior and the response of shear-induced structures in nematics. Two shear response modes were detected for semidilute solutions of 3-arm star PHIC. The fast mode is attributed to the alignment of free chains, with the degree of alignment increasing with molecular weight. Unusually, no linear regime was observed for the lowest molecular weight sample (M W = 44,000), with the fast mode birefringence response scaling as [Special characters omitted.] . This scaling exponent approaches unity with increasing molecular weight. The slow mode response diminished with molecular weight. Static and dynamic light scattering at and below c*, suggests this mode is due to aggregates of size decreasing with molecular weight. This is inconsistent with aggregation caused by solvent quality. For a particular sample, the aggregate radius of gyration appears independent of concentration, though the aggregate number and hence degree of scattering does increase with concentration. It is possible that van der Waals type interactions cause this aggregation and that these forces are increasingly inhibited as the star arms become less rodlike. The role of molecular weight and hence flexibility in shear induced structure transitions, such as striations during shear startup and banding during relaxation, was probed through small angle light scattering. Striations during shear startup and banding during relaxation are found to occur only above a molecular weight independent critical characteristic shear rate, [Special characters omitted.] above which elastic forces are able to overcome viscous forces which tend to inhibit these structure transitions. Ã?s , the characteristic texture relaxation time for banding, is molecular weight dependent. They were not seen for the lowest molecular weight (where the flexibility, L/Ã?Â± Ã¢Â¼ 1.3) since a minimum flexibility (and hence chain elasticity) is needed for these texture changes to occur. From Ã?s , the minimum molecular weight for these shear induced complex texture transitions is estimated as 27,000 (L/Ã?Â± Ã¢Â¼ 2.1) for 37 wt% poly(n -hexyl isocyanate) in 1,1,2,2-tetrachloroethane. The strain dependence of the structure transitions during shear startup and the preshear rate dependence of the banding phenomenon are thought to be functions of the persistence length of the polymer chains rather than flexibility since they show no molecular weight dependence. The characteristic lengthscale for banding does decrease with molecular weight. Increasing molecular weight and hence flexibility also lowers the decay rate of the bands since the rotational and translational cooperative diffusivities are lower.
School Location:USA - Massachusetts
Source Type:Master's Thesis
Date of Publication:01/01/2000