Abstract: Polyimide which is obtained by polymerization and consists essentially of recurring structural units of the formula (I): ##STR1## wherein X is a single bond or a hexafluoroisopropylidene group, is processed to a form of pellet, followed by heat-treating to obtain crystallinity of 5% or more, and fed to an extruder to obtain articles.

Abstract: Polyimide which is obtained by polymerization and consists essentially of recurring structural units of the formula (I): ##STR1## wherein X is a single bond or a hexafluoroisopropylidene group, is processed to a form of pellet, followed by heat-treating to obtain crystallinity of 5% or more, and fed to an extruder to obtain articles.

Abstract: A process for producing a porous film having practically sufficient mechanical strengths, good flexibility, uniform fine pores and high moisture vapor permeability, and further an extremely thin porous film, which process comprises blending 30 to 80 parts by weight of an inorganic fine powder having a specific surface area of 15 m.sup.2 /g or less and an average particle size of 0.4 to 4 .mu.m with 20 to 70 parts by weight of a polyolefin resin, followed by melt-molding the resulting blend into a film and then stretching the film to 2 to 7 times the original length at least in the uniaxial direction.

Abstract: A gas-permeable porous film which is prepared by stretching a film made from a mixture containing a polyolefin resin and an inorganic filler at least uniaxially and possesses a rugged pattern on its surface. One process for producing the film comprises forming the aforesaid mixture into a film, providing on the surface of the film a rugged pattern, stretching the resulting film at least uniaxially, and forming on the stretched film thus-formed a rugged pattern by means of a heated emboss roll and simultaneously allowing the stretched film to contract.

Abstract: A gas permeable porous film which is prepared by stretching a film made from a mixture containing a polyolefin resin and an inorganic filler at least uniaxially and possesses a rugged pattern on its surface. One process for producing the film comprises forming the aforesaid mixture into a film, providing on the surface of the film a rugged pattern, stretching the resulting film at least uniaxially, and forming on the stretched film thus-formed a rugged pattern by means of a heated emboss roll and simultaneously allowing the stretched film to contract.

Abstract: A porous film having a good water vapor permeability is obtained with a high productivity by melting a resin composition comprising 100 parts by weight of a polyolefin resin and 50 to 500 parts by weight of barium sulfate having an electric conductivity of 250 .mu.S/cm or less in terms of the electric conductivity of a supernatant formed when the barium sulfate is added to water, and a particle size of 0.1 to 7 .mu.m, making the melt into a film and stretching the film to 2.5 to 14 times the original area at least in the uniaxial direction.