Aromatic liquid-crystalline polyesters comprising a 2,5-thiophene unit synthesized and studied by the thin-film polymeri
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Yan Wang Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, and Department of Chemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
Tai-Shung Chunga) Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
Suat Hong Goh Department of Chemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 (Received 24 January 2003; accepted 19 March 2003)
A series of novel liquid-crystalline polymer (LCP) systems, including a heterocyclic 2,5-thiophene unit, were synthesized and studied using a modified thin-film polymerization method as part of a continuing investigation of molecular structural effects on liquid crystallinity. The monomers used in the reaction included p-acetoxybenzoic acid (ABA)/2,6-acetoxynaphtholic acid (ANA), 4,4⬘-biphenol (BP)/4,4⬘-diacetoxybiphenyl (DABP)/p-acetoxyacetanilide (AAA), and 2,5-thiophenedicarboxylic acid (TDA). Polarizing light microscopy and Fourier transform infrared spectra were used to study, in situ record the evolution of morphological change in the thin-film polymerization process, and confirm the occurrence of polymerization. Experimental results indicated that monomer structure and composition as well as temperature influence the formation of the liquid crystalline (LC) phase greatly and revealed that 2,5-thiophene structure is a viable mesogenic core unit. It is more effective than isophthalic acid (IA) in assisting in the LC formation as polymers containing TDA have a better developed LC phase and lower critical ABA content than those with IA. Stripe texture was observed in the ANA/DABP/TDA system, possibly due to the structural characteristics and matching of the monomers. The decrease of isotropic round areas in the LC phase and the annihilation between two pairs of defects with reaction time were also studied.
I. INTRODUCTION
Liquid-crystalline polymers (LCPs) have received a great deal of attention from both industry and academia during the past two decades. The field of LCPs has grown substantially, and today more than seven companies have thermotropic LCP products. Comprehensive progress reviews on LCP research and development have been carried out by both academia and industry.1–10 Thermotropic main-chain LCPs have been intensively studied over the past two decades primarily to extend the range of physical properties of high-performance polymers. Continued a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 7, Jul 2003
interest in refining the molecular structural prerequisites for liquid crystallinity still increases these years. Most commercially important thermotropic LCPs are a subset of polyesters, which retain long-range molecular orientational order in their melts. The mesogenic core is the primitive structural unit having the requisite anisotropic excluded-volume interactions and attractive interactions to establish long-range intermolecular order i
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