Flexural and interlaminar mechanical properties of unidirectional liquid crystalline single-polymer composites

Abstract New single-polymer composites were obtained by combining two types of wholly aromatic copolyester liquid crystalline fibres having melting points differing by about 40 °C. Vectran® M and HS fibres were co-wounded on an open square metal frame in order to obtain a unidirectional preform with an equal volume fraction of both fibres. In a second stage, the preform was consolidated under pressure at temperatures ranging from 260 up to 285 °C. Under the selected processing conditions, 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 the longitudinal direction, the bending modulus resulted to be substantially independent of the consolidation temperature (Tc) with an average value of about 39 GPa, while the maximum stress sustained in the outer layer increased from 157 MPa, at Tc = 260 up to 259 MPa, at Tc = 285 °C. Concurrently, all the matrix related properties such as the transversal bending modulus, the transversal maximum stress in the outer layer, and the interlaminar shear strength (ILSS) markedly improved as the consolidation temperature rose. In particular, both the maximum transversal bending stress and ILSS values increased by a factor of 2.8 as Tc increased from 260 up to 285 °C. Moreover, the consolidation temperature resulted to slightly affect the Charpy impact resistance of the investigated composites that, up to Tc = 280 °C, oscillated around an average value of 79 kJ/m2. On the other hand, the consolidation temperature markedly influenced the energy absorbing mechanisms. In fact, the ductility index showed a maximum value of 9 at Tc = 265 °C and then it monotonously decreased down to a minimum value of 4.5 at Tc = 285 °C.