New Wholly-Aromatic Thermotropic Polyesters with Controlled Flexibility
- PDF / 1,725,171 Bytes
- 6 Pages / 417.6 x 639 pts Page_size
- 40 Downloads / 152 Views
Abstract In this work, we present the synthesis and characterization of a new series of whollyaromatic copolyesters derived from the condensation of various weight fractions of 4,4'-(ophenylenedioxy)dibenzoic acid (OPDB) and substituted terephthalic acid (BTA) with 2phenylhydroquinone (PHQ). The Higashi method, involving tosyl chloride and pyridine as solvent, was employed to yield polymer with significant molecular weight. These polymers are intended to enable accessible clearing transition and to control the balance of stiffness and toughness in melt-spun fibers systematically. We report the synthetic details along with characterization of quiescent phase behavior and morphology. Introduction Aromatic copolyesters which are para-linked are known to feature thermotropic behavior in which heating the semicrystalline solid results in melting to a mesomorphic phase, often nematic, giving rise to desirable molding characteristics, mechanical properties, and solvent resistance. Often, their high modulus and strength come at the expense of low toughness, manifested as failure strains less than 5%. In addition, such polymers feature nematic-isotropic transition temperatures in excess of thermal decomposition, eliminating the potential benefit of traversing the isotropic-nematic phase transition during processing. Since thermotropic liquid crystallinity in polymers was first discovered in the 1970's, thermotropic LCPs have been predominantly aromatic polyesters. The two major commercial materials are wholly aromatic polyesters made from diacids (AA) and diols (BB), as well as from AB-type monomers, such as 2-hydroxy-6-naphthoic acid. These wholly-aromatic, para-linked, polyesters have found primarily as molding resins with mechanical properties approaching those of polymer matrix composites. In addition, fiber and film products are beginning to emerge. Despite the success of thermotropic LCP technology, there are several significant challenges to be addressed. The first of these is the large disparity in mechanical properties between molding (30 ksi strength) and fiber spun (400 ksi strength) articles. Second is the extreme sensitivity of mechanical properties to the conditions of melt processing, such as extrusion speeds, temperature profile, and die design. We hypothesize that both of these problems can be addressed by designing LCPs characterized by accessible nematic-isotropic transition temperatures leading to two effects. First, the nematic defect history can be "erased" to obtain a reproducible starting morphology for processing. Second, the flow-induced isotropicnematic transition' can be exploited to obtain outstanding orientational order and mechanical properties. Previous approaches to lowering the nematic-isotropic clearing temperature have involved primarily the use of flexible spacers, as summarized in the review chapter by Sirigu.2 Alternatively, the incorporation of non-linear, aromatic comonomers has enabled the reduction of nematic-isotropic clearing transition temperatures below the decomposition temper
Data Loading...
