Ternary Polymer Blends Involving Self-Reinforcing Liquid Crystalline Polymer Fibrils Containing Carbon Nanotubes Reinforcement

Liquid crystalline polymers (LCPs) are a versatile class of material, which have attracted interest from both academia and industry, due to their excellent mechanical properties, thermal endurance, and chemical stability. Due to the low melt-viscosity of LCPs, they can reduce the melt-viscosity of the polymer blend, while reinforcing them owing to the high modulus associated with the rigid aspect of the LCP backbone. Since they possess the ability to produce an elongated, fibrillar phase in blends with other engineering thermoplastics, these mixtures have been called ‘self-reinforcing polymer blends.' The incorporation of carbon nanotubes not only enhances the compatibility between the LCP and thermoplastic but also improves the electrical conductivity and mechanical properties of the blend system. The present report addresses the preparation and properties of LCP/MWCNTs composites and PA66/ABS/LCP blends with MWCNTs. The samples were prepared by melt-mixing in a twin-screw microcompounder. The nanocomposites were prepared by varying the nanotube content from 0 to 5 wt%. The ternary blends were prepared with 95 wt% of (50/50 wt/wt PA66/ABS) and 5 wt% of (LCP+MWCNTs), in which the MWCNTs content was varied from 0 to 5 wt%.
LCP/MWCNTs composites exhibited strong shear-thinning behaviour and higher complex viscosity when compared with the neat LCP, due to the strong interactions between the MWCNTs and LCP. This interaction would have strongly influenced the relaxation behaviour of the polymer chains in the case of the LCP nanocomposites. The extent of the increase in the complex viscosity, storage modulus, and loss modulus was more pronounced in the low-frequency region. The decrease in the slope of G’ and G” with the incorporation of MWCNTs was attributed to the transition from ‘liquid-like’ behaviour to ‘solid-like’ behaviour due to the formation of ‘network-like’ structures comprising of ‘polymer-nanotube’ and ‘nanotube-nanotube’ interactions. The dispersion state of MWCNTs in the LCP/MWCNTs composites was investigated using scanning electron microscopic (SEM) and transmission electron microscopic (TEM) investigations. Uniform dispersion of MWCNTs was observed in the LCP matrix. Thermal stability of the LCP/MWCNTs composites was improved with the incorporation of the nanotubes, which might be due to the physical barrier effect of the nanotubes. The glass transition temperature and the storage modulus showed an increase with the increase in the MWCNTs content, which may be due to the restriction of molecular motion due to the nanotubes. Nanoindentation analysis revealed a significant increase in the modulus and hardness values of the LCP/MWCNTs composites with the incorporation of MWCNTs.
The blends containing MWCNTs exhibited shear-thinning behaviour. The extent of the increase in the complex viscosity, storage modulus and loss modulus with the incorporation of MWCNTs was more pronounced in the low-frequency region. The state of dispersion of MWCNTs in the ternary blends was studied using SEM and TEM investigations. The neat polymer blends exhibited a ‘matrix-dispersed’ droplet type morphology with the ABS phase forming the droplets. However, blends containing MWCNTs exhibited a ‘co-continuous’ morphology, and a finer morphology was observed with the increase in the MWCNTs concentration. This observation was supported by the X-ray microscopic analysis which revealed that the ligament thickness of the PA66 phase and also ABS phase was decreased with the incorporation of MWCNTs. MWCNTs were initially localized in the PA66 phase, and with the increase in the nanotube content, MWCNTs were observed at the interface and also in the ABS phase. Non-isothermal crystallisation studies were conducted to study the crystalline morphology of PA66 phase in the presence of the nanotubes. The bulk crystallisation temperature and the degree of crystallinity of the PA66 phase were increased with increase in MWCNTs concentration. The glass transition temperature of the SAN phase of the ABS showed an increase with the increase in the MWCNTs concentration, which otherwise suggests the presence of MWCNTs even in the ABS phase. Incorporation of MWCNTs resulted in an improvement of the resistance to plastic deformation as revealed by the increase in hardness and Young’s modulus from nanoindentation analysis. The ternary blends were characterised through bulk electrical conductivity measurements to investigate the 3D ‘network-like’ structure of the nanotubes, which exhibited a percolation threshold of 1-2 wt%.