A polyphosphazene having a glass transition temperature (“Tg”) of approximately −20° C. or less. The polyphosphazene has at least one pendant group attached to a backbone of the polyphosphazene, wherein the pendant group has no halogen atoms. In addition, no aromatic groups are attached to an oxygen atom that is bound to a phosphorus atom of the backbone. The polyphosphazene may have a Tg ranging from approximately −100° C. to approximately −20° C. The polyphosphazene may be selected from the group consisting of poly[bis-3-phenyl-1-propoxy) phosphazene], poly[bis-(2-phenyl-1-ethoxy)phosphazene], poly[bis-(dodecanoxypolyethoxy)phosphazene], and poly[bis-(2-(2-(2-ω-undecylenyloxyethoxy)ethoxy)ethoxy)phosphazene]. The polyphosphazene may be used in a separation membrane to selectively separate individual gases from a gas mixture, such as to separate polar gases from nonpolar gases in the gas mixture.