Importance of Viscosity Control for Recyclable Reinforced Thermoplastic Composites

Thermoplastic composites consisting of a liquid crystalline polymer (LCP) and poly(lactide) (PLA) have the potential to combine good mechanical performance with recyclability and are therefore interesting as strong and sustainable composite materials. The viscoelastic behavior of both the LCP and the PLA is of great importance for the performance of these composites, as they determine the LCP morphology in the composite and play a crucial role in preventing the loss of mechanical performance upon recycling. Though the effect of the matrix viscosity is well-documented in literature, well-controlled systems where the LCP viscosity is tailored are not reported. Therefore, four LCPs, with the same chemical backbone but different molecular weights, are used to produce reinforced LCP-PLA composites. The differences in viscosity of the LCPs and viscosity ratio between the dispersed phase and the matrix of the blends are evident in the resultant composite morphology: in all cases fibrils are formed; however, the diameter increases considerably as the viscosity ratio increases for the higher molar mass LCPs. The fibril diameter ranges from several hundred nanometer to a few micrometer. A typical layered structure in the injection molded composites is observed, where the layer-thickness is influenced by the LCP viscosity. The LCPs are found to effectively reinforce the PLLA matrix, increasing the Young's modulus by 60% and the maximum stress by 40% for the composite containing 30 wt % of the most viscous LCP. Remarkably, this did not result in an increase in brittleness, effectively increasing the toughness of the composite compared to pure PLLA. The feasible reprocessability of this composite is confirmed, by subjecting it to three reprocessing cycles. The relaxation of the LCPs orientation upon heating is measured via in situ WAXD. We compare the relaxation in an amorphous PLA matrix and in a semicrystalline PLLA matrix with that of the pure LCPs. The matrix viscosity is found to strongly influence the relaxation. For example, in a low viscous amorphous matrix relaxation of the LCP fibrils into droplets dominates the process, whereas a semicrystalline matrix helps in maintaining the fibril morphology and intermolecular orientation of the LCP. In the latter case, the LCPs relax via contraction and coalescence of the polydomain texture and maintains a significant degree of orientation until the PLLA crystals melt and the matrix viscosity decreases. The insights gained in this study on the role of the LCP viscosity on the morphology and performance of thermoplastic composites, as well as the relaxation of LCPs in a matrix, will aid progression toward sustainable and reprocessable LCP reinforced thermoplastic composites.