Morphological and rheological responses to shear start-up and flow reversal of thermotropic liquid-crystalline polymers

Patrick T. Mather, Hong G. Jeon, Chang Dae Han, Sukky Chang

Research output: Contribution to journalArticle

39 Scopus citations

Abstract

Novel rheo-optical methods have been used to directly observe morphology evolution, during shear start-up and reversal flow, in semiflexible main-chain thermotropic liquid-crystalline polymers (TLCPs). Using a specially designed microrheometer allowing for simultaneous transient optical and mechanical observations, we observed band formation upon reversal of flow direction. It was seen that this band formation causes asymmetric light diffraction in HV small-angle light scattering mode, indicating a tilted arrangement of the regularly spaced bands. Also conducted were shear start-up and flow reversal experiments using a cone-and-plate rheometer under the same thermal and deformation histories as those in rheo-optical experiments for polymers of differing spacer lengths at equal temperature difference below the nematic-to-isotropic transition temperature (TNI). It was observed that, during both shear startup and flow reversal, the first normal stress difference N1+(γ, t) exhibits a large overshoot followed by an oscillatory decay, while shear stress σ+(γ, t) exhibits a large overshoot followed by a monotonic decay. It was found that the higher the applied shear rate, the larger the overshoot of N1+(γ, t) and σ+(γ, t), and the longer the persistence of oscillations in N1+(γ, t). Similarity was found between the ratio N1+(γ, t)/σ+(γ, t) and flow birefringence Δn+(γ, t) during shear start-up and flow reversal of the TLCPs investigated in this study. Further, we found that the ratio σ+(t,γ)/σ scales with γt but the ratio N1+(γ, t)/N1 does not, where σ denotes shear stress at steady state and N1 denotes first normal stress difference at steady state.

Original languageEnglish (US)
Pages (from-to)7594-7608
Number of pages15
JournalMacromolecules
Volume33
Issue number20
DOIs
StatePublished - Oct 3 2000

ASJC Scopus subject areas

  • Organic Chemistry
  • Polymers and Plastics
  • Inorganic Chemistry
  • Materials Chemistry

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