Processing and Characterization of Cellulose Triacetate Films from Isotropic and Liquid Crystalline Solutions

Abstract

Cellulose triacetate (CTA) films have been prepared by extrusion of various liquid crystalline and isotropic CTA solutions in a mixture of dichloroacetic acid (DCA) and formic acid (FA) through an annular die. The extrudate was either given a uniaxial drawdown into a coagulation bath or biaxially deformed via expansion with recirculating coagulant to the inside of the extrudate. Thus solutions with CTA concentrations ranging from 23 to 30% were extruded as uniaxially‐ and biaxially‐oriented films. A survey of solvent systems to produce liquid crystalline CTA showed DCA or DCA/FA to be the only reasonable choices from among the solvents tried. Rheological characterization showed that CTA/DCA/FA could be processed in the 23 to 30% concentration range. A maximum in the viscosity vs. concentration curve, plastic flow stresses, and viscosity‐shear rate behavior typical of liquid crystalline systems were observed. At 60°C, the system reverts to isotropic and provides for some comparisons between isotropic and anisotropic behavior. The CTA films were characterized by small angle x‐ray scattering (SAXS), mass density, scanning electron microscopy (SEM), permeation studies, differential scanning calorimetry (DSC), wide angle x‐ray diffraction (WAXS), and birefringence. The latter two methods were used to characterize the molecular orientation produced with various processing conditions. The volume fraction of microvoids was computed by SAXS invariant analysis and found to vary from 0.01 to 0.18%. This is low compared to results from mass density measurements. Density and SAXS measurements indicate a void content decreasing with decreasing solution concentration and heat treatment. SEM locates micron‐size voids in the cross‐section of films produced from high solution concentrations. Fibrillar structure was observed for “peeled” uniaxial films. This is increased with an increase in solution concentration. Biaxial films could not be peeled.