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 investigated 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 LCPs studied are wholly aromatics, composed only of mesogenic units, and semiflexibles, which consist of mesogenic units separated by alkyl spacers. Rheo-X-ray scattering evidenced the detrimental influence of the flexible spacers on the orientation process. The wholly aromatic LCPs readily orient along the how direction, even under modest shear flows (<(<gamma>) over dot> = 0.1 S-1). The semiflexible LCPs, however, display lower levels of orientation than the wholly aromatics, and the quality of the orientation worsens as the length of the flexible spacer increases. The rheological characterization shows that, like conventional flexible-chain polymers, the thermotropic LCPs exhibit a linear viscoelastic (LVE) regime. Moreover, dynamic measurements, within the LVE regime, suggest a level of elasticity in the nematic melts. As the length of the alkyl spacer increases, the rheological behaviour is more akin to that displayed by common flexible molecular chains. The poor shear orientation exhibited by the semiflexible LCPs is then associated 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 that steady shear induces molecular orientation, whereas oscillatory shear does not. The relaxation of orientation after steady shear showed that the molecular orientation relaxes after hundreds of seconds. The sheer stress, however, relaxes within only a few seconds. Strikingly, the rate of molecular orientation relaxation was slower for the semiflexible than for the wholly aromatic LCPs.