Abstract
Two series of thermotropic liquid-crystalline polymers (TLCPs) were synthesized by reacting various dialkoxy terephthalate units with hydroquinone (HQ) and 2,6-naphthalene diol (Naph). The dialkoxy terephthalate moieties used in this study include 2,5-diethoxyterephthalate, 2,5-dibutoxyterephthalate, and 2,5-dihexyloxy-terephthalate. All the TLCPs synthesized in this study formed nematic phases. The molecular motions according to the length of the dialkoxy side groups in the TLCPs were evaluated by 13C cross-polarization/magic angle spinning nuclear magnetic resonance spectroscopy. The thermal properties and molecular dynamics of the TLCPs are found to be affected by the length of the dialkoxy side group and the aromatic diol unit in the main chain. Further, the thermal behaviors, liquid crystalline mesophases, and degree of crystallinity of the two series of TLCPs, i.e., HQ- and Naph-TLCPs, are compared.
Highlights
Liquid crystals (LCs) are materials that exhibit partially ordered states, with their molecular order lying between those of three-dimensionally ordered crystals and disordered or isotropic liquids
These results suggest that introducing various introducing various alkoxy substituents into rigid rod‐type main‐chain thermotropic liquid-crystalline polymers (TLCPs) could diminish their alkoxy main-chain could diminish at their thermal properties and thermalsubstituents properties into and rigid even rod-type the problems such asTLCPs their decomposition high temperatures during even problems such asbe their decomposition at high temperatures during their processing can their the processing can solved
T1ρ values indicate a higher rigidity for main chains in HQ-TLCPs and naphthalene diol (Naph)-TLCPs, which is ascribed to strong interfacial interactions between the alkoxyl groups and main chains
Summary
Liquid crystals (LCs) are materials that exhibit partially ordered states, with their molecular order lying between those of three-dimensionally ordered crystals and disordered or isotropic liquids. Thermotropic liquid crystalline polymers (TLCPs) represent a class of high-performance polymers that have excellent thermo-mechanical properties combined with a low density, resulting in excellent specific properties [5,6] This unique feature of TLCPs arises from the rigidity of the polymer backbone and/or the pendant groups on the main polymer chain in symmetric rigid sequences that are classified as mesogens [7,8,9]. Griffin and Kricheldorf [24,25] used kinked monomers such as m-hydroxybenzoic acid (m-HBA) or various diols; these researchers lowered the melting temperatures of TLCPs by including flexible alkyl groups into the polymer main chain. The spin-lattice relaxation time, T1ρ s, in the rotating frame for each carbon of the dialkoxy side group and main chain in the HQ- and Naph-TLCPs was measured to understand the molecular dynamics.
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