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 (γ̇_cr) required for the onset of nematic-phase formation is measured at temperatures above the quiescent I-N transition temperature (T_IN^o). 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^o, 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 γ̇_cr 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^o. A critical dimensionless group, critical Deborah number De_cr=γ̇_crτ with τ being the terminal relaxation time of polymer, is introduced to explain the experimental observations.