Structure and rheology of twin liquid crystalline polymers

Abstract

Addition of short mesogenic segments at the ends of flexible chains alters the mechanical properties by orders of magnitude. Twin liquid crystalline polymers (TLCPs) were synthesized from 4-[(4′-alkoxybenzoyl)oxy]-benzoyl chloride and α,ω-dihydroxy-telechelic polytetrahydrofuran (PTHF) of different molecular weights. With increasing temperature, four equilibrium states of these TLCPs, i.e. crystalline state, phase separated state with mesogenic domains in isotropic PTHF matrix, phase separated amorphous state, and single phase isotropic state, have been observed by dynamic mechanical measurements, differential scanning calorimetry and polarizing microscopy. In the phase separated state, mesogenic domains function as physical crosslinks which give rise to unusually high viscoelastic properties at small strains. Disturbing this state by large amplitude shear resulted in very pronounced shear thinning and slow recovery of structure. At increased temperature, the mesogenic domains become isotropic and their effect as physical crosslinks was significantly reduced, as shown by lower viscoelasticity and weak shear thinning. In the single phase isotropic state above the coexistence temperature T s, the TLCPs behaved like a common homopolymer of low molecular weight. T s decreased as the weight ratio of PTHF spacer increased in the experimental range (50–82% PTHF).