A composite membrane comprising a polymer material that is selectively permeable to heavy hydrocarbons having three or more carbon atoms over methane, the polymer material comprising a first copolymer having a polysiloxane backbone according to formula: M―O―(A 1) d ―(A 2) p ―(A 3) m ―M in which: each A 1 is a dimethylsiloxyl monomer; each A 2 is independently chosen from substituted methylsiloxyl monomers having formula: each A 3 is independently chosen from internal-network monomers having formula: each Q is a crosslinking site Z 1 to a second copolymer having a polysiloxane backbone according to formula: M―O―(G 1) n ― b ―(G 2) q ―M in which: each G 1 is independently chosen from monomers having formula: where R 7 is chosen from C 1 ―C 10 alkyl or C 6 ―C 10 aryl; and each G 2 is independently chosen from internal-network monomers having formula: where R 4 is chosen from structures having formula: where each y in formula is an integer from 1 to 50; each L is a linking monomer having formula: where Z 2 is a crosslinking site to either the first copolymer or an additional copolymer; the Z 2 of at least one linking monomer L of the second copolymer is a crosslinking site to the first copolymer; subscripts n, b, and q represent molar fractions of randomly arranged monomers of the second copolymer; wherein:: n is from 0.45 to 0.99, b is from 0.01 to 0.5, q is from 0 to 0.05, and n + b + q = 1; or: n is from 0.005 to 0.55, b is from 0.45 to 0.995, q is 0, Z 2 is the crosslinking site Z 1 of the first copolymer, and n + b + q = 1; each R 1 is independently chosen from a C 5 ―C 10 alkyl or phenyl; subscripts d, p, and m represent molar fractions of randomly arranged monomers A 1, A 2, and A 3 in the polysiloxane backbone; d is from 0.30 to 0.79; p is from 0.20 to 0.50; m is from 0.01 to 0.20; d + p + m ≤ 1; and each M in formula and formula is a terminal silyl group. each A 1 is a dimethylsiloxyl monomer; each A 2 is independently chosen from substituted methylsiloxyl monomers having formula: each A 3 is independently chosen from internal-network monomers having formula: each Q is a crosslinking site Z 1 to a second copolymer having a polysiloxane backbone according to formula: M―O―(G 1) n ― b ―(G 2) q ―M in which: each G 1 is independently chosen from monomers having formula: where R 7 is chosen from C 1 ―C 10 alkyl or C 6 ―C 10 aryl; and each G 2 is independently chosen from internal-network monomers having formula: where R 4 is chosen from structures having formula: where each y in formula is an integer from 1 to 50; each L is a linking monomer having formula: where Z 2 is a crosslinking site to either the first copolymer or an additional copolymer; the Z 2 of at least one linking monomer L of the second copolymer is a crosslinking site to the first copolymer; subscripts n, b, and q represent molar fractions of randomly arranged monomers of the second copolymer; wherein:: n is from 0.45 to 0.99, b is from 0.01 to 0.5, q is from 0 to 0.05, and n + b + q = 1; or: n is from 0.005 to 0.55, b is from 0.45 to 0.995, q is 0, Z 2 is the crosslinking site Z 1 of the first copolymer, and n + b + q = 1; each G 1 is independently chosen from monomers having formula: where R 7 is chosen from C 1 ―C 10 alkyl or C 6 ―C 10 aryl; and each G 2 is independently chosen from internal-network monomers having formula: where R 4 is chosen from structures having formula: where each y in formula is an integer from 1 to 50; each L is a linking monomer having formula: where Z 2 is a crosslinking site to either the first copolymer or an additional copolymer; the Z 2 of at least one linking monomer L of the second copolymer is a crosslinking site to the first copolymer; subscripts n, b, and q represent molar fractions of randomly arranged monomers of the second copolymer; wherein:: n is from 0.45 to 0.99, b is from 0.01 to 0.5, q is from 0 to 0.05, and n + b + q = 1; or: n is from 0.005 to 0.55, b is from 0.45 to 0.995, q is 0, Z 2 is the crosslinking site Z 1 of the first copolymer, and n + b + q = 1;: n is from 0.45 to 0.99, b is from 0.01 to 0.5, q is from 0 to 0.05, and n + b + q = 1; or: n is from 0.005 to 0.55, b is from 0.45 to 0.995, q is 0, Z 2 is the crosslinking site Z 1 of the first copolymer, and n + b + q = 1; each R 1 is independently chosen from a C 5 ―C 10 alkyl or phenyl; subscripts d, p, and m represent molar fractions of randomly arranged monomers A 1, A 2, and A 3 in the polysiloxane backbone; d is from 0.30 to 0.79; p is from 0.20 to 0.50; m is from 0.01 to 0.20; d + p + m ≤ 1; and each M in formula and formula is a terminal silyl group.