A study and comparison of polymer films formed from isotropic and liquid crystalline solutions

Various cellulose triacetate (CTA) and poly (phenylene terephthalamide) (PPTA) films have been prepared by extrusion of isotropic and anisotropic solutions through an annular die. The extrudate was given a uniaxial drawdown into a coagulation bath or biaxially expanded using a lubricated mandrel and a coagulant recycle blow up process.

The films were characterized by Small Angle X-ray Scattering (SAXS), mass density, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Small Angle Light Scattering (SALS), Wide Angle X-ray Scattering (WAXS), and birefringence. The latter two methods were used to characterize the molecular orientation produced with processing conditions. Films extruded without drawdown exhibit some polymer chain orientation in the machine direction. This is increased by uniaxially drawing down films. Films produced using an oil coated mandrel and a coagulant recycle blow-up process show approximately equal biaxial orientation. Density and SAXS measurements indicate a void content increasing with increasing solution concentration. The volume fraction of I voids was computed from the invariant analysis of SAXS and from mass density measurements. The void volume fraction increased with increased solution concentration and decreased with increased drawdown ratio. Fibrillar structure was observed for "peeled" uniaxial films. This is increased with an increase in solution concentration. Biaxial films did not peel, apparently due to biaxial orientation developed in them. SEM locates micron size voids in the cross-sections of films produced from high solution concentrations. SAXS indicates much smaller void sizes which are roughly prolate ellipsoids for uniaxial films and oblate ellipsoids for the biaxially oriented films. Various techniques are used to estimate average void size and void size distribution. The mechanical properties of films were also measured. The uniaxial films exhibit highly anisotropic mechanical properties, while the biaxial films show fairly high modulus and tensile strength in all directions. Heat treatment under tension contributed greatly to enhancement of orientation and mechanical properties. Annealing also resulted in a significant decrease in voids and increased surface smoothness. A general comparison has been made between CTA films and PPTA films. The mechanical properties of these films are also compared to some commonly available commercial films.