Asymmetric hollow fiber membranes and methods of making and using the same

A method of making an asymmetric hollow fiber membrane comprising:

• providing a substrate resin and a skin layer resin,
• the substrate resin including a first semi-crystalline thermoplastic polyolefin copolymer derived by polymerizing at most 3 wt. % of linear or branched alpha olefin monomers with from at least 97 wt. % of 4-methyl-1-pentene monomer;
• and
• the skin layer resin including a second semi-crystalline thermoplastic polyolefin copolymer derived by polymerizing at least 2 wt. % of linear or branched alpha olefin monomers with at most 98 wt. % of 4-methyl-1-pentene monomer;
• co-extruding the substrate resin and the skin layer resin to form an asymmetric hollow fiber membrane precursor;
• and
• stretching the asymmetric hollow fiber membrane precursor to form an asymmetric hollow fiber membrane having a skin layer comprised of the skin layer resin overlaying a porous substrate comprised of the substrate resin and comprising a plurality of pores,
• optionally wherein the linear or branched alpha olefin monomers are selected from the group consisting of ethylene, propylene, 1-butene, 1-pentene, isobutylene, vinylcyclohexane, 3-ethyl-1-pentene, 1,3-methyl-1-pentene, cyclobutene, cyclopentane, 2-norbornene, 3-methyl-2-norbornene, 5-methyl-2-norbornene, tetracyclododecene, methyltetracyclododecene, dimethyltetracyclododecenel-hexene, 1-heptene, 1-octene, 1-nonene, 1-decene, 1-undecene, 1-dodecene, 1-tridecene, 1-tetradecane, 1-pentadecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-eicosene, or a combination thereof;
• wherein the amount of 4-methyl-1-pentene monomer used in the first semi-crystalline thermoplastic polyolefin copolymer differs from the amount of 4-methyl-1-pentene monomer used in the second semi-crystalline thermoplastic polyolefin copolymer by at least 1 wt. %.