The polymer glass transition in nanometric films

The glass transition temperature Tg of thin polymer layers in general differs from the bulk value. Large changes in Tg are expected to occur with extremely thin films, of thicknesses of a few nm, when segmental motions are affected by the presence of limiting bulk phases. However, most existing experimental data correspond to thicker films. In order to fill the gap, we have studied ultrathin polymer films (Langmuir layers deposited on water). These films are in the glass state at low temperature, with a finite shear modulus that vanishes well above the glass transition. As expected, significant Tg reductions are detected, the more hydrophobic the polymer, the larger the reduction.