Shear Rheology in a Confined Geometry: Polysiloxane Melts

John Van Alsten, Steve Granick

Research output: Contribution to journalArticle

Abstract

The dynamic mechanical shear response was measured of sharp fractions of molten siloxane oligomers, PDMS [poly(dimethylsiloxane)] and PPMS [poly(phenylmethylsiloxane)], confined between single crystals of muscovite mica, at film thicknesses <100 A and a temperature of 23°C. Five conclusions emerge. (1) A liquidlike mechanical response (in which the apparent dynamic viscosity was significantly enhanced over that of the bulk liquid) was clearly distinguished from a yield stress response (in which sliding over the experimental time scale occurred only after a critical yield stress was exceeded). These same features were observed previously for ultrathin films of smaller nonpolar molecules, and, despite quantitative differences in the present systems, the observation appears to be general. (2) The precise film thickness at the onset of the yield stress response, observed at film thicknesses <30-50 A, did not depend on the molecular weight of the PDMS fractions but did depend markedly on details of the history of the experiment. (3) The yield stress increased with measurement time without a discernible change in separation. The times for the yield stress to reach a plateau increased with molecular weight and ranged from approximately 9 min (PDMS, Mn = 890) to approximately 400 min (PDMS, Mn = 6330) and approximately 650 min (PPMS, Mn = 2240). (4) Enhanced viscous response was observed at larger film thickness than for liquids of smaller molecules. The distance dependence of the apparent dynamic viscosity at 0.875 Hz was quantified for one sample (PDMS, Mn = 1670) by measuring the phase shift and amplitude attenuation in sinusoidal oscillation. The apparent dynamic viscosity appeared to diverge with diminishing film thickness. (5) After discussing how the act of shear may affect the structure of the liquid, we conclude that the yield stress rheological response may reflect a metastable, history-dependent state, in which relaxations of trapped chains have become slower than the experimental time scale of minutes to hours.

Original languageEnglish (US)
Pages (from-to)4856-4862
Number of pages7
JournalMacromolecules
Volume23
Issue number22
DOIs
StatePublished - Jan 1 1990

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ASJC Scopus subject areas

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

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