Synthesis and characterization of low-melting thermotropic liquid crystalline poly(ester-imide)s.

Two new series of low-melting liquid crystalline poly(ester-imide)s, (PEIM(s)) containing biphenyldiimide mesogen with a high aspect ratio was prepared by the polycondensation of ll-(3,3',4,4'-biphenyltetracarboxydiimido)-bis undecanoic acid with various aliphatic diols and methyl esters of hydroquinone and 4,4'-biphenol by melt polymerization. The mesogenic diimide diacid was prepared by reacting 3,3',4,4'- biphenyltetracarboxylic dianhydride with 11 -amino undecanoic acid using the method of Augl and Wrasidlo. The synthesized polymers were characterized by Gel Permeation Chromatography (GPC), IR Spectroscopy, Dilute Solution Viscosity, Thermogravimetry (TGA), Differential Scanning Calorimetry (DSC) and Polarizing Light Microscopy (PLM). The data obtained from GPC indicated the presence of a wide distribution of molecular weights. The infrared spectra of the polymers showed multiple peaks due to the vibrational coupling of the imide carbonyl groups. The inherent viscosities ranged from 0.5 g/dL to 1.65 g/dL. Thermogravimetric analysis showed that these PEIM(s) have high thermal stability in both oxidizing and inert atmosphere. DSC and PLM studies indicated that these poly(ester-imide)s are semi-crystalline and the degree of crystallinity can be increased by annealing above glass transition. They showed all major thermal transitions. The DSC curves of these polymers showed crystal ordering exotherm(s), glass transition and melting endotherm(s). The glass transition temperature of the polymers decreased with increase in the number of methylene units. The melting point of the even polymers decreased from 136 C (n = 4) to 122 C (n = 12) and interestingly an opposite trend was noticed for 'odd' polymers. The melting point of the 'odd' PEIM(s) were lower than the 'even' ones. These polymers showed a definite odd-even effect in melting point. An additional melting endotherm was observed in the DSC heating curves obtained after annealing the samples at the crystal ordering temperature. This indicated that the title polymers exhibited an enantiotropic liquid crystalline behavior. The optical micrographs of the 'even' PEIM(s) observed between crossed polarizers after annealing indicated the presence of an ordered smectic phase below Tm2 and a less ordered smectic phase around Tm2. Formation of droplets near the clearing temperature followed by the formation of a coarse texture at a lower temperature in the case of 'odd' PEIM(s) are indicative of the presence of a nematic phase, below the isotropic transition temperature. Poly(esterimide) s containing aromatic diols showed higher transition temperatures than those containing aliphatic diols. Multiple endotherms were evident in the DSC thermograms of poly(ester-imide)s containing aromatic diols. The optical micrographs of the aromatic polymers taken at temperatures around Tm were comparable to the fine scale versions of the Schlieren texture. PLM studies and analysis of the cooling traces revealed that poly(ester-imide)s containing aliphatic diols have slow crystallization kinetics. The 'odd' PEIM(s) can be quench cooled from the isotropic melt to the isotropic glass. The rate of crystallization was found to be much faster in the case of polymers containing aromatic diols. The even and the aromatic poly(ester-imide)s were found to be more crystalline than the 'odd' ones. The poly(ester-imide) derived from aliphatic diol n = 7 showed unusual results in the DSC and PLM studies which is explained as a result of crosslinking or branching due to side reactions.