Dynamics and Shear Alignment Behavior of a Model Thermotropic Liquid Crystalline Polymer

Citation

Zhou, Weijun (2001) Dynamics and Shear Alignment Behavior of a Model Thermotropic Liquid Crystalline Polymer. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/dv2d-w145. https://resolver.caltech.edu/CaltechETD:etd-08292008-110129

Abstract

Although liquid crystalline polymers (LCPs) emerged as important engineering materials in the early 1970s, the current level of understanding still falls short of allowing deliberate manipulation of macroscopic orientation, leading to poor control of morphology and material properties. The flow behavior of rod-like LCP solutions (lyotropic) are fairly well understood, yet little progress has been made on LCP melts (thermotropic) because of the formidable experimental difficulties with commercial thermotropes. A simple extension of the knowledge obtained from rod-like LCP solutions to thermotropic LCPs is unlikely to hold due to the enhanced molecular flexibility and intimate molecular contact in LCP melts.

The primary concern of this thesis is therefore the flow behavior of thermotropic 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 main-chain LCP, designated as DHMS-7,9, has alternating mesogen and spacer structure with dihydroxy-[alpha]-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, rheo-conoscopy and rheo-WAXS).

The director response of a monodomain during shear flow is followed by in situ optical conoscopy using a custom-made shear cell. We observe that the director rotates opposite to the vorticity in shear for DHMS-7,9 using planar monodomain samples, demonstrating conclusively that it is flow aligning throughout its nematic temperature range. Director rotation is solely a function of applied strain, independent of shear rate, showing that the Leslie-Ericksen theory is applicable to polymeric nematics for shear rates that are low relative to their molecular relaxation. Comparisons of the observed tumbling parameter of DHMS-7,9 with predictions from available molecular models lead us to infer that molecular flexibility produces shear alignment for this class of thermotropic LCPs. 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. Thus, both steady and transient responses of DHMS-7,9 are characteristic of flow-aligning liquid crystals, in contrast to tumbling rod-like LCPs, which show complex shear rate dependence in steady shear and oscillatory response to these transient flows.

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.

Thesis Files