Supercritical fluid processing of sulfonated styrenic tri-block polymers
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The increasing need to replace current materials with new or reprocessed ones that have more specific properties and uses has led to the use of supercritical fluids (SCFs) for polymer reprocessing. Kraton® poly (styrene-ethyleneisobutylene-styrene) SEBS and Kuraray® poly (styrene-isobutylene-styrene) SIBS tri-block copolymers have been processed with supercritical carbon dioxide (SCCO2) at different processing conditions. The effect of temperature (35, 40, and 45°C), pressure (7.5, 16, and 25 MPa), the sulfonation percent (0, 44-46, 88-93 %), and the addition of a co-solvent (acetone or toluene) on the thermal and morphological properties of the polymers was evaluated. Thermal changes have been quantified using a thermogravimetric analyzer (TGA) where the different polymer degradations have been characterized. The most significant effects in the degradation of the polymer were caused by the degree of sulfonation and the cosolvent effect. The degree of sulfonation increased the polymer degradation temperature from 403°C to 416°C for SIBS and from 451°C to 472°C for SEBS. It also affected the amount of volatiles absorbed in the sample and the stability of the polymer. The presence of toluene reduced the amount of volatiles (water) absorbed in the sample. Both co-solvents (acetone and toluene) influenced the pyrolysis of the polymer suggesting intermolecular interactions with the sulfonated and aromatic segments in the polymer. The morphological changes produced by the supercritical fluid processing were evaluated using a scanning electron microscope (SEM) and an atomic force microscope (AFM). An inverse relationship between the polymer roughness and its sulfonation degree was observed. The understanding gained from this research may lead to more selective membranes for numerous applications such as fuel cells or chemical and biological protective clothing.