Fibril formation in in situ composites of a thermotropic liquid crystalline polymer in a thermoplastic matrix

Blends of a thermotropic liquid crystalline polymer, Vectra B950, and a matrix of poly (phenylene ether) and a polystyrene were extruded through a capillary designed to yield a converging flow with a constant elongational rate of strain. Fibril formation of the Vectra phase, initially dispersed as spherical droplets, was studied as a function of strain rate and blend composition. It was found that for blends with low Vectra contents, i.e., small Vectra particles, or below a minimum strain rate, no fibril formation takes place. These phenomena could be explained by the balance between interfacial tension and shear stress in the flow field. The critical conditions for fibril formation, estimated from these experiments, are comparable to the literature data. Furthermore, the elongational viscosity of the blends was calculated from the pressure gradient over the capillary. The elongational viscosity was found to decrease with Vectra content, to observe power-law behavior, and to be three orders of magnitude larger than the shear viscosity of the blends. © 1993 John Wiley & Sons, Inc.