Characterization of the Rheological, Optical, and Flow-Induced Structural Features of Polymer Liquid Crystals

Two related sets of experiments were conducted to explore the interaction between supramolecular structures, material properties, and flow-induced stresses in selected lyotropic and thermotropic polymers. The first set of experiments focused on the quiescent and rheological characterization of polymers. The objective was to develop a flow-structure model based on the general phenomenological features of the liquid crystalline state. The second set of experiments concerned the characterization of the crystal structure, crystalline orientation, and tensile properties of a model thermotropic polymer, melt spun under varying conditions of extrusion rate, extrusion temperature, drawdown ratio, and die exit conditions. The objective was to determine the most effective process conditions for enhancing both orientation and tensile properties.

The principal materials examined were hydroxypropyl celluose (HPC), ethyl cellulose (EC), poly(p-benzamide) (PBA), poly(p-oxybenzoate-co-ethylene terephthalate) (POB/PET 60/40). HPC, EC and PBA formed lyotropic solutions. EC and POB/PET 60/40 formed thermotropic melts.

Quiescent characterization revealed the presence of birefringent 'polydomain' structures at certain polymer concentrations or melt temperatures. Rheological studies revealed the presence of 'domain flow' stresses associated with these structures. The magnitude of this stress was associated with the 'domain loading' effect of a densely packed polydomain structure. The shear stress and first normal stress difference (N₁) both were generally represented by a shear rate dependent, three region response curve. The low shear region exhibited a plateau associated with the plastic flow behavior. The moderate shear region was quasi-linear and tended to have a slope less than one. For the shear stress the slope represented the degree of shear thinning. For N₁ the slope represented the degree of plastic flow suppression. A stress saturation plateau was found at high shear rates.

The crystalline orientation and tensile properties of POB/PET 60/40 were most improved by extrusion temperatures exceeding the melt transition of the POB rich phase at 255°C. The crystalline orientation was limited to a maximum value of 0.75 for all process conditions. Under certain conditions, tensile strength and modulus were improved by employing in isothermal die exit chamber.