EFFECT OF PROCESSING HISTORY ON THE MORPHOLOGY AND PROPERTIES OF POLYPROPYLENE THERMOTROPIC LIQUID-CRYSTALLINE POLYMER BLENDS/

Preparation, morphology, and mechanical properties were studied of ble nds of a thermotropic liquid crystalline polymer (TLCP) with two diffe rent grades of polypropylene, one with and one without overlap in proc essing temperatures, using two different blending methods. The highly viscous grade (PP-1) was of sufficient thermal stability to be blended with the TLCP (Vectra A950) in a single-screw extruder with an Egan m ixing section on the screw. The low viscous grade (PP-2) could not be processed at the same temperature as the TLCP because of degradation. Its blends were, therefore, prepared by a special coextrusion techniqu e, i.e. feeding the two components from two separate extruders to a Ro ss static mixer. In both methods drawing of the extrudate is necessary to obtain satisfactory mechanical properties. The PP-1/TLCP blends ha d to be extruded twice in order to obtain proper mixing. The morpholog y of these blends ranges from a pronounced skin-core morphology at low extrudate draw ratio (DR = 3) to a high-aspect ratio fiber/matrix mor phology at high draw ratio (DR = 15). The morphology of the PP-2/TLCP blends was always a high-aspect ratio fiber/matrix morphology even at low draw ratios. The TLCP fibers were generated in this coextrusion pr ocess under conditions where the viscosity of the dispersed phase was higher than the viscosity of the matrix. Breakup experiments demonstra te that fibers of a thickness of approximately 1 mu m disintegrate int o droplets within a few seconds at temperatures above the melting poin t of the TLCP. This is probably the cause of the skin-core morphology obtained with single-screw extrusion. Tensile modulus and strength of all blends increase with extrudate draw ratio. The deformation of the TLCP phase in the drawn blends is less than affine, probably because o f slip between the phases. The moduli of the PP-1/TLCP blends as a fun ction of the draw ratio can be described well by a modified Halpin-Tsa i equation taking into account both changes in aspect ratio and molecu lar orientation of the TLCP fibers. The level of reinforcement in the PP-2/TLCP blends is lower than expected, probably because of the low t emperature of drawing. This demonstrates a limitation of the coextrusi on process: blending at temperatures that are too low reduces mechanic al properties.