Microporous polymer membranes and related methods of fabrication are provided. An asymmetric microporous membrane embodiment includes, but is not limited to, a thermoplastic polymer substrate defining a plurality of micropores, the thermoplastic polymer including one or more of polyethylene (PE), polypropylene (PP), polymethylpentene (PMP), and a combination thereof; and a polymethylpentene (PMP) polymer skin positioned on the thermoplastic polymer substrate, wherein when the thermoplastic polymer substrate includes PMP, the PMP polymer skin has a crystallinity that differs from a crystallinity of the PMP in the polymer substrate.