Abstract
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, Tg, 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.
Highlights
Tuning the thermodynamic state of a material has a tremendous impact on its performance
Significant interest has grown around adsorbed polymer layers[7], because of a series of striking correlations identified between the number of chains adsorbed and a broad ensemble of materials properties
The physical picture drawn by these studies is that the process of adsorption is thermodynamically driven and, this phenomenon can be depicted as a phase transition of a nonadsorbed “bulk” polymer melt into a tightly adsorbed layer
Summary
Tuning the thermodynamic state of a material has a tremendous impact on its performance. Adsorption of polymers can, take place already at monomer/substrate interactions smaller than kBT16,17 In such case, while adsorption is still reversible at the monomer level, desorption of a whole chain is less likely to occur, because it requires simultaneous detachment of a large number of adsorbed monomers[18,19]. The physical picture drawn by these studies is that the process of adsorption is thermodynamically driven and, this phenomenon can be depicted as a phase transition of a nonadsorbed “bulk” polymer melt into a tightly adsorbed layer. This transformation entails a reduction of the enthalpy and, to a smaller extent, of the entropy. The main reason is that polymers heated at standard rates (~1–10 K min−1) generally undergo degradation before desorption
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