Size-dependent elastic modulus of ultrathin polymer films in glassy and rubbery states

- Glassy modulus of polymer films is independent of film thickness down to ∼10 nm.
- Rubbery stiffening can be observed for polymer films of thickness less than 100 nm.
- Size-dependent elastic modulus of PVAc films is explained using the Coupling Model.

The stiffness-related properties of glassy and rubbery polymer films can behave differently when confined at the nanometer scale. However, direct measurements of the glassy and rubbery modulus of nanometer-sized polymer films are still limited and the existing models for explaining the thickness dependence of the polymer modulus remain in significant disagreement. By means of nanomechanical mapping with an atomic force microscopy, we report a direct measurement of the glassy and rubbery modulus of nanometer-sized poly(vinyl acetate) (PVAc) films supported by substrates of different moduli: Si and poly(vinyl alcohol). At ∼15 °C below the bulk PVAc glass transition temperature (Tg), the modulus of PVAc films is found to be independent of the thickness as thin as ∼10 nm. In contrast, the modulus of rubbery films measured at ∼20 °C above the Tg significantly increases when the film thickness is reduced to less than 100 nm, irrespective of the substrate modulus. Such an increase becomes more dramatic near the rubbery plateau regime of PVAc films. Among existing models, we demonstrate that the theoretical framework suggesting an increasing separation between the α-relaxation, sub-Rouse modes and Rouse modes induced by nanoconfinement is more appropriate when describing our experimental observation.

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