Synthesis Of Thermotropic Polymers And LCP/MMT Nanocomposites By In-situ Actylation

Thermotropic poly(butylene terephthalate) (PBT)/p-hydroxybenzoic acid (PHB) copolyesters (designated as PBT/PHB) and PBT/p-aminobenzoic acid (ABA) copolyesteramides (designated as PBT/ABA) were synthesized in the melting state by in situ acetylation with PBT and PHB or ABA, respectively. At the same time, the liquid crystalline copolyester (LCP)/montmorillonite (MMT) nanocomposites were synthesized with 40 mol% PBT, 60 mol% PHB in the presence of organoclay by in situ acetylation. Their structure and properties were characterized by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermogravimetry (TGA), polarized optical microscopy (POM) with hot stage, atomic force microscopy (AFM) and small angle X-ray diffraction (SAXD). Based on the experimental results, some definitive conclusions can be drawn as the following:1) Compared with the pre-acetylation process, the in-situ acetylation method is useful to restrain from the forming of long PHB or ABA blocking chains in these copolymers. PBT/PHB copolyesters containing 20-80mol% PHB, and PBT/ABA copolyesteramides containing 15-40 mol% ABA show the nematic liquid crystalline behavior. And they all have two glass transition temperatures, which are related to PBT-rich phase, PHB-rich or ABA-rich phase in the copolymers. It is worth-mentioned that Tg1 and Tg2 of the copolymer synthesized by in situ acetylation are lower than those of the copolymer synthesized by pre-acetylation. With the incorporation of the mesogenic units (PHB or ABA segments), the thermal decomposition temperature and char residue of the PBT/PHB copolyesters and the PBT/ABA copolyesteramides are greatly increased, thereby their thermal stabilities are improved. At the same content of the mesogenic units, the thermal stability of PBT/ABA copolyesteramide is better than that of PBT/PHB copolyester.2) During the in situ acetylation and melting intercalation condensation polymerization of PBT and PHB in the presence of organoclay, LCP macromolecules entered the galleries of MMT, and fully-exfoliated LCP/MMT nanocomposites are formed, and show the nematic liquid crystalline behavior. The incorporation of the exfoliated MMT layers throughout the LCP matrix results in very fine but imperfect schlieren texture, and the disinclination becomes blurred. Under the induction of LCP, the exfoliated MMT layers can be oriented along the force field. Moreover, the LCP/MMT nanocomposites have two melting points and two crystallization temperatures, which are related to the melting and crystallization of the PBT-rich and PHB-rich domains in LCP macromolecules. Due to the strong interaction between LCP and exfoliated MMT layers, the exfoliated MMT layers restrain the motion activity and orderly packing of LCP, and destroy the packing regularity of LCP chains, these results lead to the decreasing in the melting points, crystallization temperatures and the degree of the crystallinity of LCP in LCP/MMT nanocomposites. In the other hand, the exfoliated MMT layers act as a heterogeneous nucleating agent, enhance the crystallization rate of PBT-rich phases in LCP/MMT nanocomposites. However, the effect of the exfoliated MMT layers is small on the crystallization rate of PHB-rich phases in LCP/MMT nanocomposites. In addition, the incorporation of the exfoliated MMT layers obviously increases the char residue of LCP/MMT nanocomposites. These results are very important to further improve the mechanical, barrier, flame-retardant and ablative properties of polymer/MMT nanocomposites.