On the molecular orientation and viscoelastic behaviour of liquid crystalline polymers: the influence of macromolecular architecture

The influence of macromolecular architecture on the Aom-induced orientation of main-chain thermotropic Liquid crystalline polymers (LCPs) is investiga ted using in situ wide-angle X-ray scattering. In order to get more insight into the interrelationship between microscopic and rheological behaviour, the viscoelastic properties of the nematic melts were also studied. The LCP s studied are wholly aromatics, composed only of mesogenic units, and semif lexibles, which consist of mesogenic units separated by alkyl spacers. Rheo -X-ray scattering evidenced the detrimental influence of the flexible space rs on the orientation process. The wholly aromatic LCPs readily orient alon g the how direction, even under modest shear flows (<(<gamma>) over dot> = 0.1 S-1). The semiflexible LCPs, however, display lower levels of orientati on than the wholly aromatics, and the quality of the orientation worsens as the length of the flexible spacer increases. The rheological characterizat ion shows that, like conventional flexible-chain polymers, the thermotropic LCPs exhibit a linear viscoelastic (LVE) regime. Moreover, dynamic measure ments, within the LVE regime, suggest a level of elasticity in the nematic melts. As the length of the alkyl spacer increases, the rheological behavio ur is more akin to that displayed by common flexible molecular chains. The poor shear orientation exhibited by the semiflexible LCPs is then associate d with a molecular network formed by the less-than-rigid molecular chains. Furthermore, the steady shear viscosity is always smaller than the dynamic viscosity, i.e. the Cox-Merz rule does not hold. This is due to the fact th at steady shear induces molecular orientation, whereas oscillatory shear do es not. The relaxation of orientation after steady shear showed that the mo lecular orientation relaxes after hundreds of seconds. The sheer stress, ho wever, relaxes within only a few seconds. Strikingly, the rate of molecular orientation relaxation was slower for the semiflexible than for the wholly aromatic LCPs.