Preparation and tensile mechanical properties of unidirectional liquid crystalline single-polymer composites

Abstract The present study is focused on the preparation and characterization of new single-polymer composites based on liquid-crystalline fibres. Composites were obtained with two commercial wholly aromatic polyester liquid-crystalline fibres (Vectran® M and Vectran® HS) having the same chemical composition but markedly different physical properties. In particular, single-fibre tensile tests performed on Vectran® M and HS fibres evidenced moduli of 83.7 and 88.8 GPa and Weibull scale parameters of 1309 and 3374 MPa, respectively. Moreover, on the basis of DSC analysis considerably different melting signal resulted for Vectran® M and HS fibres, with peaks located at 276 and 315 °C, respectively. Unidirectional composite preforms with 50 vol% of Vectran® M and 50 vol% of Vectran® HS fibres were produced by filament winding. In the subsequent consolidation stage the pressure and temperature conditions were properly selected in order that one component (Vectran® M) formed a continuous matrix while the other one (Vectran® HS) retained its fibrous form and most of the original mechanical properties. In this study, the effects of the consolidation pressure (in the range from 1.8 up to 8.8 MPa) and temperature (from 260 up to 285 °C) on the composite properties were investigated by density measurements, microscopic observations of the cross-sections, and mechanical tensile tests. At a consolidation temperature of 275 °C, a minimum void content value and an optimum in the tensile properties were detected for a consolidation pressure of 4.4 MPa. At this consolidation pressure, the effect of the consolidation temperature was to increase the composites density and to improve the matrix-related tensile properties, such as the transversal modulus (from 0.5 to 1.1 GPa) and the transversal strength (from 1.3 to 3.5 MPa). On the other hand, the longitudinal tensile modulus was practically independent of the consolidation temperature in the range from 260 up to 280 °C with an average value of about 58 GPa, while it markedly decreased at 285 °C. Concurrently, the longitudinal tensile strength decreased from 920 to 480 MPa as the consolidation temperature rose.