High Hexane Sorption Capacity of Loosely Crosslinked PDMS Rubbers at Low Temperatures: Macromolecular and Physicochemical Elucidation for VOC Recovery

A series of soft polydimethylsiloxane (PDMS) rubbers was obtained by diluting reaction mixtures of a PDMS macromer and a crosslinker with an organic solvent. The degree of swelling of the softest rubber was 600 wt% when immersed in pure hexane at room temperature. From the Flory–Huggins equations, the free energy change for hexane sorption per 1 g rubber was calculated to be −7.4 J g−1 and the entropy contribution of the polymer chains was 1.6 J g−1 larger than that of the conventional PDMS rubber. The loosely crosslinked PDMS rubber could sorb 776 mg g−1 of hexane at 298 K and 606 mg g−1 at 195 K. The latter value was 4.4 times larger than that of the conventional PDMS rubber. At a low pressure of 23 Pa, the PDMS rubber sorbed a 3500-fold greater amount of hexane at 195 K, as compared with the value at 298 K. This high sorption amount is because the expansion energy of such soft PDMS rubber is small. The sorption heat of the loosely crosslinked PDMS rubber was 30.3 kJ mol−1 when calculated from the Clausius–Clapeyron equation. This value is substantially smaller than that of commercial activated carbon (75 kJ mol−1). Unlike activated carbon, PDMS rubber is insensitive to water and selectively sorbs VOCs even under a saturated humidity. Because of the large swelling potential and small sorption/desorption heat, the loosely crosslinked PDMS rubber will be an indispensable sorbent for the recovery of flammable gasses such as alkane and iso-alkane VOCs in the chemical and oil and gas industries.