Rubber-to-Liquid Transition in Amorphous Polymer Thin Films

The characterization of the thermomechanical properties of thin polymer films is of critical importance for a wide range of applications, including microelectronics, functional coatings, and biomedical devices. However, reliable measurements at the nanoscale remain challenging due to confinement effects, substrate influence, and limitations of conventional experimental techniques. In this work, we introduce a novel experimental approach to probe the thermomechanical behavior of thin polymer films based on pull-off force measurements performed by using Atomic Force Microscopy (AFM). The evolution of adhesion forces with temperature provides direct insight into thermal transitions and mechanical softening phenomena occurring within confined polymer layers. The proposed method offers high spatial resolution and sensitivity, enabling the investigation of films with thicknesses down to a few tens of nanometers. In addition to the measurement of the glass transition temperature (Tg), a characteristic temperature (Tmax) is identified, at which the maximum pull-off value is observed. The strong link between Tg and Tmax is demonstrated by the effects of chain length and confinement.