Rheo-Optical Evidence of a Flow-Induced Isotropic-Nematic Transition in a Thermotropic Liquid-Crystalline Polymer

A direct rheo-optical characterization of the flow-induced isotropic-nematic (I-N) transition in a semiflexible thermotropic liquid-crystalline polymer (TLCP) was investigated, using a specially constructed apparatus enabling in-situ optical microscopic observations at elevated temperatures, along with cone-and-plate rheometry. For the investigation, an aromatic polyester, poly[(phenylenesulfonyl)p-phenylene 1,10-decamethylenebis(4-oxybenzoate)] (PSHQ10), was synthesized via solution polymerization. Above the equilibrium isotropic-nematic transition temperature for this polymer, T ) 170.5 °C, application of steady-state shear flow above a certain critical value of shear rate, γ̆c, produces a firstorder I-N transition, with γ̆c increasing with temperature. This transition is evidenced by the formation of elongated nematic (birefringent) domains in the isotropic matrix, accompanied by a drastic decrease in shear viscosity (η). Remarkably, the nematic domains that form for γ̆ > γ̆c are optically uniform under cross-polarized optical microscopy; i.e., they are apparently free of disclinations (defects), typical of textured TLCPs. The flow-induced I-N transition in PSHQ10 is found to be reversible; i.e., upon cessation of shear flow, the domains melt to the original isotropic phase and the dynamic moduli rise toward the pretransition values. The observed flow-induced I-N transition may find important applications, such as envisaging new routes for processing TLCPs with better mechanical properties and helping to understand bioprocesses such as silk thread spinning.