Exceptional property changes in ultrathin films of polycarbonate: Glass temperature, rubbery stiffening, and flow

Paul A. O'Connell, Jian Wang, T. Adeniyi Ishola, Gregory B. McKenna

Research output: Contribution to journalArticle

55 Scopus citations

Abstract

The TTU nanobubble inflation method has been used to determine the creep compliance of ultrathin films of polycarbonate over a thickness range of 3-22 nm and a temperature range from ambient (25 °C) to 112 °C. The compliance from the segmental regime through a stiffened rubbery plateau regime was able to be measured for films of thickness from 4.2 nm and above, and the results show a greatly reduced glass temperature and stiffening of the rubbery regime relative to that of the macroscopic material. In the case of the 3 nm film, we found that even at ambient conditions the system is above the enhanced rubbery plateau and is in the terminal flow regime as evidenced by a slope of unity on a double-logarithmic representation of the compliance versus time. The biaxial viscosity of the film at this temperature is ∼2.7 × 10 11 Pa s. We also observed flow behavior in a 9.1 nm thick film that had been heated to 75 °C, i.e., 10 °C above the reduced glass transition of the thin film. These are the first evidence of polymer flow in the equibiaxial deformation geometry in the nanobubble inflation experiments. Interpretation of the results suggests that the glass transition of the 3 nm film is ∼10 °C below room temperature, which implies a glass transition reduction of 122 °C, which is substantially greater than has been reported previously for any polymer and particularly surprising because the polymer is of relatively low molecular weight, in a regime where one might anticipate a weak dependence of T g on film thickness. We also observe that the plateau stiffening of the polycarbonate, even to 4.2 nm thickness, falls on the extrapolation of the compliance vs thickness behavior of polystyrene and poly(vinyl acetate) observed previously. This implies a stiffening of over 3 orders of magnitude in the nominal rubbery regime. There is also a stiffening of approximately a factor of 2 in the glassy regime for these films. The results are considered in the context of prior work on ultrathin films.

Original languageEnglish
Pages (from-to)2453-2459
Number of pages7
JournalMacromolecules
Volume45
Issue number5
DOIs
StatePublished - Mar 13 2012

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