Entropic nanothermodynamic potential from molecular trapping within photon induced nano-voids in photon processed PDMS layers

In any thermodynamic system, including nanosystems, the free energy change during sorption is related to its internal stressing. However, experimental evidence of energy flow between thermodynamic states before and after sorption of a small number of molecules in nanosystems is rare, due to the minute energy flow and sub-nm strain variation. In this work, it is shown that the entropic variation during isothermal and isobaric sorption of gaseous (water or methanol) molecules in an ensemble of photon induced nano-voids within a PDMS matrix is proportional to the number of nano-voids and the entropic nanothermodynamic potential is the outcome of the confinement of translational motion of the adsorbed molecules within the nano-voids. Following irradiation of PDMS with a molecular fluorine laser at 157 nm, white light reflectance spectroscopy establishes the relation between the entropic variation, via the sub-nm strain field, and the external parameters (number of photons and adsorbed molecules). Moreover, the contribution of entropic, internal, chemical and surface energy components in the stressing field is analyzed by nanoindentation and atomic force microscopy. The methodology allows the identification of nanothermodynamic potentials and strain field variations in thin polymeric layers at pJ and sub-nm levels. © 2012 The Royal Society of Chemistry.