In Situ Composites Based on Thermotropic and Flexible Polymers
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IN SITU COMPOSITES BASED ON THERMOTROPIC AND FLEXIBLE POLYMERS GUIDO CREVECOEUR AND GABRIEL GROENINCKX Catholic University of Leuven, Laboratory for Macromolecular Structural Chemistry, Celestijnenlaan 200 F, B 3030 Leuven, Belgium ABSTRACT Blends of a thermotropic liquid crystalline polymer (TLCP) and a thermoplastic matrix were compounded. Upon subsequent injection moulding and spinning, the TLCP was deformed into fine fibrils in the matrix, giving in-situ reinforcement. Especially after spinning, the composite fibres contain fibrils with very high aspect ratio, and exhibit mechanical properties in accordance with simple composite models for modulus and strength. INTRODUCTION In-situ composites consisting of a thermotropic liquid crystalline polymer (TLCP) and a thermoplastic matrix material can be regarded as an intermediate between conventional (short) fibre reinforced composites and molecular composites. All three of these materials have in common that the aspect ratio of the reinforcing species is of great importance with respect to the final mechanical properties of the product. In short fibre composites the length of the fibres is limited by the processing method used, e.g. extrusion or injection moulding. A high aspect ratio can be obtained in molecular composites, where one or a few stiff single macromolecules are dispersed on a molecular level. Due to their intrinsic immiscibility with flexible polymers, the rigid macromolecules, typically being lyotropic liquid crystalline polymers, have to be dispersed in the thermoplastic matrix by casting from dilute solutions. Indeed, dispersion on a nanometer scale and outstanding mechanical properties can be obtained in such systems [1]. However, the solvent step provides an extra complication in industrial processing. Therefore, a reasonable alternative seems to be blending a TLCP with an engineering polymer in the melt, so that dispersion, elongation and orientation of the TLCP take place in one processing step [2-6]. An additional advantage is that, because the TLCP acts as a lubricant, the melt viscosity of the blend, contrary to those of short fibre reinforced composites, can be lowered as much as one or two orders of magnitude [7-10]. The present paper is concerned with blends of a commercial thermotropic polyester-amide, Vectra B 950, in a matrix consisting of a (miscible) mixture of polystyrene (PS) and poly-2,6-dimethyl-1,4-phenylene ether (PPE). EXPERIMENTAL Vectra B 950, which is believed to consist of 58 mole-% hydroxy naphthoic acid, 21 mole-% terephthalic acid and 21 mole-% 4 aminophenol, was purchased from Hoechst Celanese. Blends containing 70 weight-% PPE and 30 weight-% PS were kindly supplied by General Electric Plastics Europe. Vectra B 950 melts at 280*C, where it shows liquid crystalline behaviour. PPE and PS are known to be miscible over the entire composition range; the as received PPE/PS blend exhibits a single T at 177°C. Blending was performed on a Berstorfl corotating twin screw extruder at 320'C. Strands with a draw ratio (DR) of appro
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