*5.2.1 Rubbery siloxane-containing homo-and copolymers*

Application of PDMS based rubbery siloxane membranes for selective removal of hydrocarbons and recovery of various organic components [3, 100–102] is considered. Summary of C3+ hydrocarbon permeation properties of PDMS, POMS and modified rubbery siloxane membranes in pure and mixed gases can be found in **Table 8**.

PDMS has low *Tg* (123°C), flexible chains, and large free volume. The permeability, solubility, and diffusivity of hydrocarbons in PDMS membranes have been studied in previous literature [52, 83], and results show that feed gas compositions and testing conditions significantly affect membrane separation performance. The effect of chemical compositions on the permeation properties of PDMS membrane by introducing substituted groups in both side and main chains was compressively studied by Stern et al*.* [90, 103–105] and summarized in previous reviews [12, 23]. Results show that the substitution of functional groups in the sidegroups of siloxane polymers has the same general effect on polymer permeability and selectivity as the substitution of such groups in the polymer main chains [104] (**Figure 14**). When the size of the side-groups (e.g. –CH3, –C2H5, –C3H7, –C8H17, C10H11–, CH2CH2CF3, –C6H5, etc.) and main chains (e.g. –(CH2)2, (CH2) 2, (CH2)8, *m*-C6H4, *p*-C6H24) increased, the chains become less flexible resulting in the increases in *Tg* (from 123°C to 28°C) and the decreases in permeability, especially for those condensable gases with larger kinetic diameters (e.g. C3H8) [23].

Compared to PDMS with (Me2SiO)*<sup>x</sup>* backbone chain, membranes based on POMS with (OctMeSiO)*<sup>x</sup>* backbone chain exhibited enhanced C3+ hydrocarbon separation performance (C3+/CH4 selectivities) [14, 56, 57, 88, 89]. Schultz et al. [14] reported that POMS was one of two promising polymers for separation of C3+ hydrocarbons among the total 45 different polymers that were tested under C4H10/ CH4 binary gas mixture (97/3 mol%). Since POMS is a soft material, it is susceptible to compression and thus the permeability goes down significantly with increase in the feed pressure as shown in **Figure 15**.

