Extensional Rheological Properties of Poly-γ-benzyl-l-glutamate Solutions across the Isotropic-Cholesteric Liquid Crystalline Phase Transition

Abstract

Connecting macromolecular properties and microstructure to macroscopic flow behavior and rheological response is of fundamental interest to scientists and formulators interested in the interplay of structure, properties, processability, and application. Here, we characterize the macromolecular properties and flow behavior of poly(γ-benzyl-l-glutamate) (PBLG) solutions that appear to form cholesteric liquid crystalline phases above certain polymer concentrations. Solutions of PBLG with two molecular weights of 90,000 and 150,000 g/mol were characterized with light scattering and optical microscopy. PBLG is rodlike in 1,2-dicholoroethane (DCE) solutions and forms cholesteric liquid crystalline (LC) phases at higher concentrations with the transition depending on the overall contour length of the molecule. Using dripping-onto-substrate (DoS) rheometry, we characterize the pinching dynamics and extensional rheology response of PBLG solutions, including concentrations that appear liquid crystalline. The radius evolution data for all solutions display a power law (PL) response characteristic of shear-thinning fluids. However, after the onset of the LC phase within solutions, the PL response is followed by an elastocapillary response with an exponential decay in the filament radius that provides measurement of extensional relaxation time. Increasing the polymer concentration deeper into the liquid crystalline phase leads to a significant increase in extensional relaxation time. Imaging pinching necks in polarized light enables the visualization of the changes in neck shape, size, and birefringence as markers of underlying macromolecular stretching, alignment, liquid crystallinity, and dynamics. This work presents a novel chapter in the rheology of ordered, liquid crystalline solutions and provides new insights into the extensional flows of PBLG and polymeric rigid rods.