The objective of this thesis is to develop a high performance thermotropic liquid crystalline copolyesters (TLCP)/multi-walled carbon nanotubes (MWNTs) composites fiber. The TLCP is composed of multi-domains. The boundary of these domains has no preferred orientation, so called defects. These defects limit the molecular orientation during the fiber spinning process. Hence, how to reduce the defects is crucial for the preparation of high performance TLCP/MWNT composites fiber. In this thesis, we proposed one novel method to reduce the defect density of the TLCPs by the incorporation of small amount of MWNTs. The abundant benzene group in the TLCP backbone may interact with the MWNTs via π-π stacking to form core-shell structure. Because the dimension of MWNTs is much larger than the TLCP...[ Read more ]

The objective of this thesis is to develop a high performance thermotropic liquid crystalline copolyesters (TLCP)/multi-walled carbon nanotubes (MWNTs) composites fiber. The TLCP is composed of multi-domains. The boundary of these domains has no preferred orientation, so called defects. These defects limit the molecular orientation during the fiber spinning process. Hence, how to reduce the defects is crucial for the preparation of high performance TLCP/MWNT composites fiber. In this thesis, we proposed one novel method to reduce the defect density of the TLCPs by the incorporation of small amount of MWNTs. The abundant benzene group in the TLCP backbone may interact with the MWNTs via π-π stacking to form core-shell structure. Because the dimension of MWNTs is much larger than the TLCP molecules, it will help reduce the defect density of the TLCPs.

The TLCP/MWNT composites were prepared via in situ melt polymerization. A two-stage polycondensation procedure was employed in our process, where carboxylic acid modified MWNTs were dispersed in acetic anhydride via ultrasonication prior to being charged to the prepolymerization reactor together with monomers and catalysts for esterification reaction at 130 °C-200 °C. The esterified mixture was then fed into a polycondensation reactor at 280 °C‒320 °C. In this way, fully exfoliated MWNTs are dispersed in the TLCP matrix at concentrations up to 0.3 wt%. Systematic studies show that well dispersed MWNTs acted as “pseudo nucleation sites” for the nematic ordering in the adjacent TLCP melt. Thus the undrawn fibers show a smaller core region and higher overall orientational order. Consequently, the addition of MWNTs is not only effective in improving the mechanical stiffness but also toughness of the composites. For example, the 0.3 wt% TLCP/MWNT composite shows a 62%, 135% and 145% increase in Young’s modulus, tensile strength and toughness, respectively, in comparison with the pure TLCP.

The highly oriented TLCP/MWNT composite fiber was prepared via the spinning process. Due to the higher overall orientational order of the composite, Young’s modulus and tensile strength of the composite fiber are 38% and 32%, respectively higher than that of the pure TLCP fiber. Thereafter, heat treatment is employed to further improve the tensile strength of the fibers. After being annealed at 260 °C for 48 h, the composites fiber shows tensile strength up to 2.16 GPa, which is 29% higher than the pure TLCP fiber.

Keywords: thermotropic liquid crystalline copolyesters (TLCP), multi-walled carbon nanotubes (MWNTs), in situ polymerization, fiber spinning, heat treatment