Effect of Flexible Spacer Length on the Rheology of Side-Chain Liquid-Crystalline Polymers

Abstract

The effect of flexible spacer length on the rheology of side-chain liquid-crystalline polymers (SCLCPs) was investigated. For the study, a liquid-crystalline monomer, [n-[(4-cyanobiphenyl)oxy]alkyl]carboxylic acid (nCN-COOH) with varying methylene groups (n = 5, 7, and 11), was grafted onto a nearly monodisperse hydroxylated polyisoprene, yielding SCLCPs, poly{[n-[(4-cyanobiphenyl)oxy]alkyl]carbonyl}oxyisoprenes (PI-nCN). Differential scanning calorimetry has shown that the clearing temperature (Tcl) of PI-nCN increases with increasing n. Polarizing optical microscopy has shown that PI-5CN has a nematic mesophase, while both PI-7CN and PI-11CN have a smectic mesophase. The linear dynamic viscoelasticity, steady shear flow, and transient/intermittent shear flows of PI-nCN were investigated. It was found that flexible spacer length has a profound influence on the rheological responses of PI-nCN. The following observations have been made. (i) At temperatures below Tcl the complex viscosity (|η*|) of PI-5CN exhibits a very weak frequency dependence, but the frequency dependence of |η*| for PI-nCN becomes progressively stronger as the number of methylene spacer groups increases from 5 to 7 and to 11. (ii) The steady-state shear viscosity (η) and first normal stress difference (N1) of PI-nCN decrease as the number of methylene spacer groups increases from 5 to 11. (iii) The η of PI-5CN decreases steadily with increasing temperature without any abnormality as the polymer transforms from the nematic phase to the isotropic phase, whereas the η of both PI-7CN and PI-11CN decreases gradually at T < Tcl and then begins to drop precipitously at Tcl. (iv) Upon start-up of shear flow, very large overshoot peaks of first normal stress difference growth ( ) and shear stress growth (σ+) are observed, the extent of which is found to decrease as the number of methylene spacer groups in PI-nCN increases from 5 to 11. (v) The recovery time upon cessation of steady shear flow is found to increase as the number of methylene spacer groups in PI-nCN increases from 5 to 11.