Reactive Processing Route to Thermotropic Polyesters with a Low Processing Temperature and Enhanced Relaxation Time

G.W. de Kort*, S. Saidi, D. Hermida-Merino, N. Leone, S. Rastogi, C.H.R.M. Wilsens

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

The processing temperature of thermotropic liquid crystalline polymers (LCPs) can be reduced by increasing the flexibility of the backbone, simultaneously decreasing the relaxation time and resulting in a decrease in the mechanical properties in the processed polymer. Here, we report on the development of thermotropic LCPs with low processing temperatures, an increased relaxation time, and excellent mechanical performance via a versatile two-step synthesis. These LCPs can be processed at temperatures below 200 degrees C into injection-molded parts which have a mechanical performance comparable to that of commercially available grades with Young's modulus and a tensile strength over 12.5 GPa and 200 MPa, respectively. These LCPs are synthesized using carboxylic acid-terminated thermotropic prepolymers that are produced via a standard acidolysis polycondensation at 220 degrees C, a temperature that allows incorporation of monomers having lower thermal stability. Subsequently, the prepolymers are subjected to a chain-extension reaction with bis(2-oxazoline)s in a reactive extrusion step. The facile two-step method provides excellent control over the chemical composition, molecular weight, and chain architecture of the resulting polymer. As the molecular weight of the LCPs increases, they display interesting and uncharacteristic viscoelastic behavior. The viscoelastic response of the LCPs changes from that of an unentangled melt, typically observed in rigid LCPs, toward a physically constrained melt indicated by the appearance of a crossover point with an increase in molar mass. Such a response is normally not observed in commercial thermotropic LCPs even with an increase in molar mass. We attribute the change in the viscoelastic response to the chain-extension reaction. This fundamental change in the state of the nematic melt greatly increases the relaxation timescales of both the interchain orientation and the nematic texture, providing further evidence toward their interrelation. The change in the relaxation mechanism of the nematic phase, with increasing molar mass, is followed by time-resolved X-ray diffraction and optical microscopy.
Original languageEnglish
Pages (from-to)1401-1413
Number of pages13
JournalMacromolecules
Volume54
Issue number3
DOIs
Publication statusPublished - 9 Feb 2021

Keywords

  • CYCLIC IMINO ETHERS
  • HYDROXYBENZOIC ACID
  • LINEAR VISCOELASTIC BEHAVIOR
  • LIQUID-CRYSTALLINE POLYMER
  • MICROSTRUCTURE
  • MOLECULAR-ORIENTATION
  • RHEOLOGICAL BEHAVIOR
  • SHEAR
  • TRANSIENT
  • VANILLIC ACID

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