Flow-induced structure in a thermotropic liquid crystalline polymer as studied by SANS

Abstract

Small‐angle neutron scattering is utilized to determine the flow induced alignment of a model thermotropic liquid crystalline polymer (LCP) as a function of shear rate and temperature. The results demonstrate that the flow‐induced structures in thermotropic liquid crystalline polymers have similarities and differences to those in lyotropic liquid crystalline polymer solutions. The shear rate dependence of the alignment shows that the flow‐induced alignment correlates very well to the viscosity behavior of the LCP in the shear thinning regime, while temperature variation results in a change in the extent of alignment within the nematic phase. Relaxation results also demonstrate that the flow‐induced alignment remains essentially unchanged for up to an hour after the shear field has been removed. Last, there exists a regime at low shear rate and low temperature where alignment of the LCP molecule perpendicular to the applied shear flow is stable. These results provide important experimental evidence of the molecular level changes that occur in a thermotropic liquid crystalline polymer during flow, which can be utilized to develop theoretical models and more efficiently process thermotropic polymers. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3017–3023, 1998

Dynamics and Shear Orientation Behavior of a Main-Chain Thermotropic Liquid Crystalline Polymer

“…The most striking feature in the x-phase is the observation that two distinctive orientation states can be achieved during steady shear, depending on the applied shear rate. The transition between the parallel and perpendicular orientations observed by in situ WAXD agrees with neutron-scattering observations of chain anisotropy in DHMS-7,9 by Dadmun and co-workers . This type of orientational flipping has been reported in two other liquid crystalline polymers: a concentrated solution of PBG and an HBA-HNA thermotropic copolyester (Vectra A900). , In both of these systems, like DHMS-7,9, this phenomenon occurred at the lower limit of the nematic temperature range.…”

Electric‐field‐induced molecular alignment of side‐chain liquid‐crystalline polyacetylenes containing biphenyl mesogens

“…It is well known that the molecular alignment of liquid-crystalline polymers can be achieved within the temperature range of the liquid-crystalline phase with a magnetic field, an electric field, surface shearing, and a stretching effect and then be preserved by the simple cooling of the polymer to its glassy or crystalline state. [1][2][3][4][5] In the literature, the reports on external-field-induced molecular orientation are mainly focused on main-chain liquid-crystalline polymers and sidechain liquid-crystalline polymers with soft back-bones. 6 -8 Side-chain liquid-crystalline polymers with stiff backbones (e.g., side-chain liquid-crystalline polyacetylenes, polypyrroles, and polythiophenes) have attracted much interest from scientists in recent years.…”