Effect of Shear Rate on the Orientation and Relaxation of a Vanillic Acid Based Liquid Crystalline Polymer

Abstract

In this study, we report on the visco-elastic response during start-up and cessation of shear of a novel bio-based liquid crystal polymer. The ensuing morphological changes are analyzed at different length scales by in-situ polarized optical microscopy and wide-angle X-ray diffraction. Upon inception of shear, the polydomain texture is initially stretched, at larger strain break up processes become increasingly important, and eventually a steady state texture is obtained. The shear stress response showed good coherence between optical and rheo-X-ray data. The evolution of the orientation parameter coincides with the evolution of the texture: the order parameter increases as the texture stretches, drops slightly in the break up regime, and reaches a constant value in the plateau regime. The relaxation of the shear stress and the polydomain texture showed two distinct processes with different timescales: The first is fast contraction of the stretched domain texture; the second is the slow coalescence of the polydomain texture. The timescale of the orientation parameter’s relaxation matched with that of the slow coalescence process. All processes were found to scale with shear rate in the tested regime. These observations can have far reaching implications for the processing of liquid crystal polymers as they indicate that increased shear rates during processing can correspond to an increased relaxation rate of the orientation parameter and, therefore, a decrease in anisotropy and material properties after cooling.

Effects of Poly(ethylene-co-glycidyl methacrylate) on the Microstructure, Thermal, Rheological, and Mechanical Properties of Thermotropic Liquid Crystalline Polyester Blends

“…The shear storage modulus ( G ’), shear loss modulus ( G ”), loss tangent (tan ™), and complex viscosity ( η *) of neat TLCP, neat PEGMA, and TLCP/PEGMA blends at 300 °C were measured as a function of the angular frequency, as can be seen in Figure 9 . The shear storage moduli, shear loss moduli, and complex viscosities of neat TLCP at 300 °C were much lower than those of neat PEGMA ( Figure 9 A–C), which is due to the facile alignment of TLCP with a stiff chain backbone with the applied shear [ 5 ], compared to PEGMA with a high chain flexibility and entanglement in a melt state. The overall shear moduli and complex viscosities of the blends with 1–10 wt% PEGMA loadings are quite consistent with those of neat TLCP, although they increase slightly when increasing the PEGMA content in the blends.…”

“…Thermotropic liquid crystalline polyesters (TLCPs)—aromatic polyesters exhibiting liquid crystallinity in a molten state—are used in a variety of industrial sectors because of their excellent thermal stability, dimensional stability, mechanical performance, and chemical resistance [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ]. Since TLCPs can be processed easily due to their low viscosity as a result of their ordered melt, an extremely high molecular orientation can be attained in fibers, films, and injection molded parts.…”

Controlling Processing, Morphology, and Mechanical Performance in Blends of Polylactide and Thermotropic Polyesters

“…As a result, LCP melts subjected to a higher shear stress display enhanced interchain relaxation rates. 63,64…”

Fiber formation and properties of polyester/lignin blends

“…The chemical structure of the copolyesters explored as matrix for lignin was chosen considering earlier research concepts for aromatic copolyesters as well as recent reports on thermotropic LCP with biobased suberic acid units into account . It was presumed that copolyesters without aliphatic content will be more suitable to generate fibers with high carbon content and regular structure than semiaromatic copolyesters.…”