Orientation dynamics in commercial thermotropic liquid crystalline polymers in transient shear flows

Stanley Rendon, Wesley Roth Burghardt*, Robert A. Bubeck

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

13 Scopus citations

Abstract

In situ X-ray scattering measurements of molecular orientation under shear are reported for two commercial thermotropic liquid crystalline polymers (TLCPs), Vectra A950® and Vectra B950®. Transient shear flow protocols (reversals, step changes, and flow cessation) are used to investigate the underlying director dynamics. Synchrotron X-ray scattering in conjunction with a high-speed area detector provides sufficient time resolution to limit the total time spent in the melt during testing, whereas a redesigned X-ray capable shear cell provides a more robust platform for working with TLCP melts at high temperatures. The transient orientation response upon flow inception or flow reversal does not provide definitive signatures of either tumbling or shear alignment. However, the observation of clear transient responses to step increases or step decreases in shear rate contrasts with expectations and experience with shear-aligning nematics and suggests that these polymers are of the tumbling class. Finally, these two polymers show opposite trends in orientation following flow cessation, which appears to correlate with the evolution of dynamic modulus during relaxation. Specifically, Vectra B shows an increase in orientation upon flow cessation, an observation that can only be rationalized by the assumption of tumbling dynamics in shear. Together with prior observations of commercial LCP melts in channel flows, these results suggest that this class of materials, as a rule, exhibits director tumbling.

Original languageEnglish (US)
Pages (from-to)945-956
Number of pages12
JournalRheologica Acta
Volume46
Issue number7
DOIs
StatePublished - Aug 1 2007

Keywords

  • Liquid crystalline polymer
  • Molecular orientation
  • Thermotropic LCP
  • Transient shear flow
  • X-ray scattering

ASJC Scopus subject areas

  • Mechanics of Materials
  • Computational Mechanics

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