Dynamics and shear alignment behavior of a model thermotropic liquid crystalline polymer

The primary concern of this thesis is therefore the flow behavior of thermotropic liquid crystalline polymers (LCPs), with an emphasis on how flow influences orientation and morphology and how this depends on molecular structure. For this purpose we synthesized a model thermotropic LCP selected for its chemical stability, wide nematic range and optical transparency. This mainchain LCP, designated as DHMS-7,9, has alternating mesogen and spacer structure with dihydroxy-α-methylstilbene as mesogen and two different lengths of alkyl spacers (-C7H14- and -C9H18-). A range of molecular weights were prepared to probe the effects of chain flexibility (ratio of chain length of persistence length). Synthesis was scaled up to provide adequate quantities for physical studies (rheology, rheoconoscopy and rheo-WAXS).; To identify the effect of chain flexibility on the dynamics of this LCP, the rotational viscosity and shear viscosity were measured as functions of molecular weight. Both viscosities showed weaker sensitivity to molecular weight above a characteristic molecular weight, suggesting a crossover to semiflexible character at high molecular weight.; Rheology and shear orientation behavior of DHMS-7,9 are markedly different from that of nematic lyotropic LCPs. Synchrotron WAXS measurements in steady shear show that molecular orientation is relatively high and nearly independent of shear rate. In transient shear during flow inception, flow reversal, and step up/down shear rate, neither shear stress nor orientation parameter shows multiple oscillations.; An interesting feature of DHMS-7,9 is the existence of a mysterious liquid crystalline phase—Phase X. The flow behavior of Phase X is completely different from that of the nematic phase. A striking flip of the orientation from the flow direction to the vorticity direction occurs below a critical shear rate. This orientational flipping is reversible in response to step changes of temperature and/or shear rate. In addition, we found that oscillatory shear flow also induces a similar type of orientational flipping. Examination of the linear viscoelastic properties as a function of orientation in Phase X suggests rheological similarity to layered fluids. (Abstract shortened by UMI.)...