Odd-even effect of flexible spacer length on flow-induced isotropic-to-nematic transition in segmented thermotropic polymers

The dependence of the flow-induced isotropic-to-nematic (I-N) phase transition phenomenon on the spacer length is investigated using the well-behaved thermotropic polymers, PSHQn, with n representing the number of methylene groups as flexible spacer in the main chain. Using optical microrheology methods and mechanical rheological methods, the critical shear rate ((γ) over circle (cr)) required for the onset of nematic-phase formation is measured at temperatures above the quiescent I-N transition temperature (T-IN(0)). In particular, flow-induced birefringence is measured for the PSHQn polymers of varying spacer lengths as functions of both temperature and shear rate. As shear rate increases for a fixed temperature higher than the T-IN(0), a clear transition from low-level birefringence (0.0-0.005) to nematic-level birefringence (>0.01) is observed for most PSHQn polymers. Interestingly, the,, for nematic-phase formation is significantly larger for even-numbered PSHQn compared to odd-numbered PSHQn. This odd-even effect is explained by a significant difference in steady shear viscosity (and terminal relaxation time) at fixed temperature above T-IN. A critical dimensionless group, critical Deborah number De(cr) = (γ) over circle (cr)tau with tau being the terminal relaxation time of polymer, is introduced to explain the experimental observations.