Thermotropic Polyesters with Flexible Spacers Bearing Ether Bonds in Asymmetric Position Pablo Bello, Antonio Bello,* Evaristo Riande, and Nicholas J. Heaton Instituto de Ciencia y Tecnologı ´a de Polı ´meros (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain Received July 14, 2000; Revised Manuscript Received September 29, 2000 ABSTRACT: The synthesis of several polyesters from p,p′-bibenzoic acid and diols bearing ether bonds in asymmetric positions is described. The polyesters develop smectic mesophases, the type of which depends on the chemical structure of the spacer. The ether groups and the asymmetry of the spacers stabilize the liquid crystalline order in these polymers. Glass transition and isotropization temperatures, enthalpies, and entropies of the thermotropic polyesters are reported. Comparison of the melting entropies with the conformational entropies of the chains suggests a great disorder in the mesophases of thermotropic polyesters. For some polyesters, the stretched polymer chains do not follow the direction of the fiber axis, an unusual phenomenon that has already been described for some polybibenzoates with aliphatic separators. The influence of the strain rate on the anomalous flow is discussed. Introduction Several papers have been published showing the ability of polyesters derived from bibenzoic acid to form thermotropic liquid crystals. 1-5 The mesogenic units separated by flexible spacers develop smectic meso- phases whose nature depends on the structure and the even-odd character of the spacers. Polybibenzoates with all methylenic groups in the linear flexible backbone form unstable mesophases that eventually evolve to three-dimensional crystalline structures. Bidimensional f tridimensional order transitions in these thermotro- pic polyesters are usually fast, and the thermodynamic behavior is monotropic. Both the smectic mesophase- crystal transformation rate and the thermal properties depend on the stiffness of the molecular chains. The stiffness is governed by the number of conformations permitted to the chains, which can be modified by inserting bulky side groups and/or ether groups along the paraffinic spacer. The information at hand suggests 6 that, as a result of substituting methylenic groups by a ether linkages in the paraffinic spacer, the thermal transitions of thermotropic polyesters are shifted to lower tempera- tures. Moreover, bidimensional f tridimensional order transitions are totally or partially inhibited, the meso- phases are considerably stabilized, and the thermody- namic behavior becomes enantiotropic. Since the steric requirements for the formation of mesophases are less restrictive than for the develop- ment of three-dimensional order, copolymerization usu- ally favors mesophase structures. Therefore, the use of diols bearing ether bonds in asymmetric positions gives rise to random copolymeric structures along the poly- ester backbone that favor the stabilization of the mes- ophases. The purpose of this work is to consider how the asymmetric nature of the spacer affects the develop- ment of mesophases in thermotropic polyesters. With this aim, a series of four diols possessing ether linkages asymmetrically positioned were synthesized by addition of propylene glycol, ethylene glycol, diethylene glycol, and 2-methyltrimethylene glycol to the cyclic trimeth- ylene oxide. The chemical structures of these ether- diols are shown in Figure 1. In this work, the polycondensation of bibenzoic acid and the ether-diols is reported, emphasizing both the characteristics of the mesophases and the phase transi- tions of the resulting polyesters. Three of these polyes- ters show anomalous orientation when they are stretched at room temperature, similar to that reported for a few other thermotropic polymers subjected to either shear- ing 7,8 or drawing from the melt. 9-11 Special attention is devoted to the influence of the drawing rate on the relative orientations of the fiber and the smectic layers of the polyester PTMTB whose structure is shown in Figure 3. Finally, the conformational entropy of the polyesters is calculated and compared with the entropy involved in the mesophase f isotropic melt transition. Experimental Part Materials. Ethylene glycol, diethylene glycol, 1,2-pro- panediol, and 2-methylpropanediol were distilled prior to their use. 4,4′-Biphenildicarboxylic acid (Aldrich) was crystallized from ethanol. Oxetane (Aldrich) was also purified by distilla- tion from several sodium mirrors in a vacuum line. Ether-Diols Synthesis. The ether-diols used as flexible spacers were prepared under nitrogen atmosphere or vacuum, by adding oxetane to an excess of the corresponding diol, using sulfuric acid as catalyst. In a typical reaction, 0.1 mol of sulfuric acid was slowly dissolved in approximately 1.5 mol of the glycol of interest contained in a round-bottom flask cooled with water. To this solution was added 0.15 mol of purified Figure 1. Oxyethylenes synthesized by oxetane with ethylene glycol (I), diethylene glycol (II), 1,2-propanediol (IIIa and IIIb, 65-35%), and 2-methyl-1,3-propanediol (IV). 181 Macromolecules 2001, 34, 181-186 10.1021/ma001225x CCC: $20.00 © 2001 American Chemical Society Published on Web 12/15/2000