Direct observation of desorption of a melt of long polymer chains

Tuning the thermodynamic state of a material has a tremendous impact on its performance. In the case of polymers placed in proximity of a solid wall, this is possible by annealing above the glass transition temperature, T_g, which induces the formation of an adsorbed layer. Whether heating to higher temperatures would result in desorption, thereby reverting the thermodynamic state of the interface, has so far remained elusive, due to the interference of degradation. Here, we employ fast scanning calorimetry, allowing to investigate the thermodynamics of the interface while heating at 10⁴ K s^-1. We show that applying such rate to adsorbed polymer layers permits avoiding degradation and, therefore, we provide clear-cut evidence of desorption of a polymer melt. We found that the enthalpy and temperature of desorption are independent of the annealing temperature, which, in analogy to crystallization/melting, indicates that adsorption/desorption is a first order thermodynamic transition.