Characterizations of Nanocomposites of Liquid Crystalline Polymers

Abstract

Nanocomposites of three thermotropic liquid crystalline polymers (TLCPs) with organoclay were prepared. The first TLCP, poly(2-ethoxyhydroquinone-2-bromoterephthaloyl), EHBT, consists of wholly aromatic ester type mesogenic units containing an ethoxy side group, and the second poly(oxybiphenyleneoxy-2,5-dihexyloxyterephthaloyl) (OBDT) is an aromatic polyester TLCP having alkoxy side groups on the terephthaloyl moiety. The last TLCP polyazomethine (PAM) consists of diad aromatic azomethine type mesogenic units. An EHBT with an alkoxy side-group was synthesized from 2-ethoxyhydroquinone and 2-bromoterephthalic acid. Nanocomposites of EHBT with Cloisite 25A (C25A) as an organoclay were prepared by the melting intercalation method above the melt temperature (T_m) of the TLCP. Liquid crystallinity, morphology, and thermo-mechanical behaviors were examined with increasing organoclay content from 0 to 6 wt%. Liquid crystallinity of the C25A/EHBT hybrids was observed when organoclay content was up to 6 wt%. Regardless of the clay content in the hybrids, the C25A in EHBT was highly dispersed in a nanometer scale. The hybrids (0–6 wt% C25A/EHBT) were processed for fiber spinning to examine their tensile properties. Ultimate strength and initial modulus of the EHBT hybrids increased with increasing clay content and the maximum values of the mechanical properties were obtained from the hybrid containing 6 wt% of the organoclay. A TLCP (OBDT)/organoclay nanocomposite was synthesized via in-situ intercalation polycondensation of diethyl-2,5-dihexyloxyterephthalic acid and 4,4′-biphenol in the presence of organically modified montmorillonite (MMT). The organoclay, C18–MMT, was prepared by the ion exchange of Na⁺–MMT with octadecylamine chloride (C18–Cl⁻). OBDT/C18–MMT nanocomposites were prepared to examine the variations of the thermal properties, morphology, and liquid crystalline phases of the nanocomposites with clay content in the range 0–7 wt%. It was found that the addition of only a small amount of organoclay was sufficient to improve the thermal behavior of the OBDT hybrids, with maximum enhancement being observed at 1 wt% C18–MMT. Nanocomposites of PAM with the organoclay C12-MMT were also synthesized by using the in-situ interlayer polymerization method. The variations with organoclay content of the thermal properties, morphology, and liquid crystalline mesophases of the hybrids were determined for concentrations from 0 to 9 wt% C12-MMT. The wide-angle X-ray diffraction (XRD) analysis and transmission electron microscope (TEM) micrographs show that the levels of nanosize dispersion can be controlled by varying the C12-MMT content. The clay particles are better dispersed in the matrix polymer at low clay contents than at high clay contents. With the exception of the glass transition temperature (T_g), the maximum enhancement in the thermal properties was found to arise at an organoclay content of 1 wt%. Further, the PAM hybrids were shown to exhibit a nematic liquid crystalline phase for organoclay contents in the range 0–9 wt%.