Microstructure and viscoelasticity in thermotropic copolyesters: the influence of monomer concentration |
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Authors: | Angel Romo-Uribe Maraolina Domínguez-Díaz Maria Eugenia Romero-Guzmán Leonardo Rejón Rubén Saldivar-Guerrero |
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Institution: | (1) Laboratorio de Nanopolímeros y Coloides, Instituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Av. Universidad s/n, Cuernavaca, Morelos, C.P. 62210, México;(2) Gerencia de Materiales y Procesos Químicos, Instituto de Investigaciones Eléctricas, Calle Reforma 113, Colonia Palmira, Cuernavaca, Morelos, C.P. 62490, México |
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Abstract: | This research investigates the influence of monomer composition on the thermal transitions, mi crostructure, and viscoelastic
properties of thermotro pic liquid crystalline polymers (LCPs) based on 1, 4-hydroxy-benzoic acid (B) and 2,6-hydroxy-naphthoic
acid (N). Compositions B–N of 1:1, 1:3, 3:1, and 2:1 mol% were studied. The solid-to-nematic transition T
s→n was greatly influenced by the monomer composition, ranging from 230 to 280 °C. However, the decomposition temperature T
dec was independent of composition, all the materials degrading at T
dec = 509 °C. Polarized optical microscopy showed a threaded texture in the nematic phase. Fiber X-ray diffraction showed aperiodic
meridional maxima for all LCPs, the number of maxima depended on composition. B–N 3:1 and 1:3 mol% favor the orthorhombic
crystallographic phase whereas the 1:1 and 2:1 mol% crystallized in a pseudohexagonal phase. Dynamic time sweeps showed that
the nematic phases are stable at the temperatures studied. Strain sweeps showed the existence of a linear viscoelastic (LVE)
regime. The LVE properties ranged from predominantly viscous (G′ < G″) for the 1:1 mol% composition to predominantly elastic (G′ > G″) and with a well-defined rubber-like regime for the 1:3 mol% composition, resembling the rheology of flexible polymer melts.
The persistence length q appears to be the parameter driving the rheological behavior.
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Keywords: | Thermotropic LCP Copolyesters Optical microscopy X-ray scattering Rheology |
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