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The processing temperature of thermotropic liquid crystalline polymers (LCPs) can be reduced by increasing the flexibility of the backbone, simultaneously decreasing the relaxation time and resulting in a decrease in the mechanical properties in the processed polymer. Here, we report on the development of thermotropic LCPs with low processing temperatures, an increased relaxation time, and excellent mechanical performance via a versatile two-step synthesis. These LCPs can be processed at temperatures below 200 degrees C into injection-molded parts which have a mechanical performance comparable to that of commercially available grades with Young's modulus and a tensile strength over 12.5 GPa and 200 MPa, respectively. These LCPs are synthesized using carboxylic acid-terminated thermotropic prepolymers that are produced via a standard acidolysis polycondensation at 220 degrees C, a temperature that allows incorporation of monomers having lower thermal stability. Subsequently, the prepolymers are subjected to a chain-extension reaction with bis(2-oxazoline)s in a reactive extrusion step. The facile two-step method provides excellent control over the chemical composition, molecular weight, and chain architecture of the resulting polymer. As the molecular weight of the LCPs increases, they display interesting and uncharacteristic viscoelastic behavior. The viscoelastic response of the LCPs changes from that of an unentangled melt, typically observed in rigid LCPs, toward a physically constrained melt indicated by the appearance of a crossover point with an increase in molar mass. Such a response is normally not observed in commercial thermotropic LCPs even with an increase in molar mass. We attribute the change in the viscoelastic response to the chain-extension reaction. This fundamental change in the state of the nematic melt greatly increases the relaxation timescales of both the interchain orientation and the nematic texture, providing further evidence toward their interrelation. The change in the relaxation mechanism of the nematic phase, with increasing molar mass, is followed by time-resolved X-ray diffraction and optical microscopy.

Thermoplastic composites based on thermotropic liquid crystalline polymer (LCP) materials are interesting candidates for reinforced composite application due to their promising mechanical performance and potential for recyclability. In combination with a societal push toward the more sustainable use of materials, these properties warrant new interest in this class of composites. Though numerous studies have been performed in the past, a coherent set of design rules for LCP design for the generation of injection-molded reinforced thermoplastic composites is not yet available, likely due to the complex interplay between LCP and matrix components. In this study, we report on the processing of poly(L-lactide) with two different LCPs, at relatively low processing temperatures. The study focuses on critical parameters for the morphological development and mechanical performance of LCP-reinforced composites. The influence of blend composition and the processing conditions, on the mechanical response of the composites, is investigated using rheology, wide-angle X-ray diffraction, mechanical analysis, and microscopy techniques. The study conclusively demonstrates that both the matrix viscosity and viscosity ratio between the dispersed and matrix phase, determine the deformation and breakup of the dispersed LCP droplets during extrusion. In addition, the thermal dependence of the viscosity ratio appears to be a critical parameter for the composite performance after injection molding. For example, during injection molding, stretching and molecular orientation of the LCP phase into highly oriented fibrils are prevented when the viscosity ratio increases rapidly upon cooling. In contrast, melt drawing proves to be a more effective processing route as the extensional flow field stabilizes elongated droplets, independent of the viscosity ratio. Overall, these findings provide valuable insights in the morphological development of LCP-reinforced blends, highlighting the importance of the development of viscoelastic properties as a function of temperature, and provide guidelines for the design of new LCP polymers and their thermoplastic composites.

Water-based cellulose cholesteric liquid crystalline phases at rest can undergo structural changes induced by shear flow. This reflects on the deuterium spectra recorded when the system is investigated by rheo-nuclear magnetic resonance (rheo-NMR) techniques. In this work, the model system hydroxypropylcellulose (HPC)+water is revisited using rheo-NMR to clarify unsettled points regarding its behavior under shear and in relaxation. The NMR spectra allow the identification of five different stable ordering states, within shear and relaxation, which are well integrated in a mesoscopic picture of the system’s structural evolution under shear and relaxation. This picture emerging from the large body of studies available for this system by other experimental techniques, accounts well for the NMR data and is in good agreement with the three distinct regions of steady shear flow recognized for some lyotropic LC polymers. Shear rates in between 0.1 and 1.0 s–1 where investigated using a Taylor–Couette flow and de...