Influence of film thickness on the structure and properties of PDMS membrane

Preparation of a high flux poly(dimethylsiloxane) (PDMS) membrane for separation of organics from water by pervaporation has been a challenging task. For this, the membrane should be as thin as possible since the productivity flux is inversely proportional to the membrane thickness. In an attempt to understand the relationship between the membrane structure and flux, the membranes of different thickness varying from about 0.2 to 150 μm were prepared and studied. The neat PDMS membrane prepared by evaporation casting of the 10% (w/w) polymer solution in n-hexane has an average thickness of 150 μm while a porous support was used for preparing the thinner membranes. ATR-IR, SEM, DSC, XRD and Contact angle measurements showed that the thinner membrane had relatively more crystalline domains and less hydrophobic. The nanostructure of the membranes was investigated using small angle neutron scattering (SANS) measurements. While the SANS data of the neat membrane was fitted well with Debye model of polymer chain, the data of coated membranes was fitted with a combination of Debye chain model and some aggregated structure of polymer chains. Among the coated PDMS films, the thinly coated film exhibited relatively a higher amount of the chain aggregates. This might imply that the thinner membrane had a relatively loose structure responsible for low selectivity but high flux in the separation of organic from water by pervaporation.