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

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 ( n CN-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- n CN). Differential scanning calorimetry has shown that the clearing temperature ( T cl ) of PI- n CN 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- n CN were investigated. It was found that flexible spacer length has a profound influence on the rheological responses of PI- n CN. The following observations have been made. (i) At temperatures below T cl the complex viscosity (|η*|) of PI-5CN exhibits a very weak frequency dependence, but the frequency dependence of |η*| for PI- n CN 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 ( N 1 ) of PI- n CN 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 < T cl and then begins to drop precipitously at T cl . (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- n CN 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- n CN increases from 5 to 11.