WE CLAIM:
[Claim 1]
A separation membrane comprising a thermoplastic resin, wherein
the separation membrane has voids therein which have a width of 1 nm -1,000 nm,
and
the voids have a tortuosity of 1.0-6.0.
[Claim 2]
The separation membrane according to claim 1 comprising a co-continuous structure that comprises a phase comprising the thermoplastic resin, and the voids.
[Claim 3]
The separation membrane according to claim 1or 2, wherein, in a graph with horizontal axis for wavenumber and vertical axis for intensity, which is obtained by Fourier transformation of a microscopic image photographed in a square visual field with each side having a length 10 times to 100 times the width of the voids of the separation membrane, when a peak half width is set as (a) and a peak maximum wavenumber is set as (b) in the graph, a region of 0 < (a)/(b) <1.5 is included therein.
[Claim 4]
The separation membrane according to any one of claims 1 to 3, which has a membrane permeation flux at 50 kPa and 25℃ of 0.05 m3/m2/h -20 m3/m2/h.
[Claim 5]
The separation membrane according to any one of claims 1 to 4, which has a thickness of 1μm -1,000 μm.
[Claim 6]
The separation membrane according to any one of claims 1 to 5, which has a shape of a hollow fiber.
[Claim 7]

The separation membrane according to claim 6, wherein the shape of a hollow fiber has an outer diameter of 100 μm -5,000 μm.
[Claim 8]
The separation membrane according to any one of claims 1 to 7, wherein the thermoplastic resin comprises at least one compound selected from the group consisting of cellulose esters, polyamides, and polyesters.
[Claim 9]
The separation membrane according to claim 8, wherein the cellulose esters are cellulose acetate propionate and/or cellulose acetate butyrate.
[Claim 10]
The separation membrane according to claim 8, wherein the polyamides are nylon-6 and/or nylon-66.
[Claim 11]
The separation membrane according to claim 8, wherein the polyesters are poly(lactic acid)-based polymers and/or polyethylene terephthalate.
[Claim 12]
A method for producing a separation membrane, comprising:
a resin melting step of melting and kneading 20 wt% to 90 wt% of a thermoplastic resin and 10 wt% to 60 wt% of a structure-forming agent to prepare a molten resin;
a molding step of discharging the molten resin from a discharge spinneret to obtain a membrane-shaped molded product;
a co-continuous structure forming step of subjecting the molten resin or molded product to heat-induced phase separation in a liquid having a temperature of 40-220℃ to form a co-continuous structure, which includes a first phase comprising the thermoplastic resin and a second phase partially compatible with the first phase; and
an eluting step of eluting the second phase from the molded product to form voids, after the co-continuous structure forming step and the molding step.

[Claim 13]
The method for producing a separation membrane according to claim 12, wherein the thermoplastic resin is a cellulose ester.
[Claim 14]
The method for producing a separation membrane according to claim 12 or claim 13, wherein a spinning spinneret is used as the discharge spinneret in the molding step to form a hollow fiber.