Solvent retention in solution-cast films of PMMA: study by dielectric spectroscopy

Conventional PMMA was dissolved in two different solvents (chloroform and toluene). Solid polymeric films were formed after casting of the solutions on a substrate (aluminium) and evaporation of the solvent at room temperature during several days. The dielectric properties of the solid films were studied by dielectric spectroscopy. The evolution of the loss tangent (tan delta) versus temperature was analysed. The results show that the tan delta peak, corresponding to the alpha-transition (glass transition), is shifted towards the lower temperatures, as compared with the bulk polymer obtained by heating press, without any solvent. The magnitude of the shift depends on the type of solvent. These results indicate that, in all cases, solvent molecules are retained and cannot be completely evaporated at room temperature, even after a long period. Solvent molecules are still trapped inside the PMMA films, but their effect on the polymer chains mobility differs in function of the nature of the solvent. Explanations based on acid-base interactions are proposed to try to understand this phenomenon. This work illustrates the interest of dielectric spectroscopy for studying in situ the properties of thin cast polymer films. (C) 1997 Elsevier Science S.A.