Drying study of siloxane-PPG nanocomposites

This is a study of the structural transformations occurring in hybrid silox ane-polypropyleneglycol (PPG) nanocomposites, with different PPG molecular weight, along the drying process. The starting materials are wet gels obtai ned by the sol-gel procedure using as precursor the 3-(trietoxysilyl)propyl isocyanate (IsoTrEOS) and polypropylenglycol bis(2-amino-propyl-ether) (NH2 -PPG-NH2). The shrinkage and mass loss measurements were performed using a temperature-controlled chamber at 50 degreesC. The nanostructural evolution of samples during drying was studied in situ by small angle x-ray scatteri ng (SAXS). The experimental results demonstrate that the drying process is highly dependent on the molecular weight of polymer. After the initial dryi ng stage, the progressive emptying of pores leads to the formation of a irr egular drying front in gels prepared from PPG of high molecular weight, lik e 4000 g/mol. As a consequence, an increase of the SAXS intensity due to th e increase of electronic density contrast between siloxane clusters and pol ymeric matrix is observed. For hybrids containing PPG of low molecular weig ht, the pore emptying process is fast, leading to a regular drying front, w ithout isolated nanopockets of solvents. SAXS intensity curves exhibit a ma ximum, which was associated to the existence of spatial correlation of the silica clusters embedded in the polymeric matrix. The spatial correlation i s preserved during drying. These results also reveal that the structural tr ansformation during drying is governed by capillary forces and depends on t he entanglement of polymer chains.