Porous membrane and method for preparing the same

The porous membrane as claimed in claim 6, wherein the amount of the polymer is 5 parts by weight or more relative to 100 parts by weight of the porous membrane substrate.

A porous hydrophilic membrane comprising a porous membrane substrate having at least part of the surface of its pores formed with a polymer radical as a starting point for polymerization by treatment with a low temperature plasma and then grafted with a polymer having a glass transition temperature of not more than 300 K by introducing a gaseous polymerizable monomer under a reduced pressure and non-plasma condition, wherein said porous membrane has pores having a pore size of 0.02 to 10.0 .mu.m, a void volume of 30 to 95% and an elongation at break of 10% or more when drawn monoaxially.

The rubber-like porous composite material comprising:

• a porous membrane substrate comprising a continuous phase composed of a synthetic polymer comprising mainly a polypropylene or a polyvinylidene fluoride, and
• a polymer having rubber elasticity comprising monomer units having an unsaturated bond in their molecule and grafted on at least a part of the surface of the continuous phase in an amount of 20% by weight or more relative to the weight of the porous membrane substrate and which undergoes an elongation set of 10% or less when drawn at an elongation of 50%, wherein said polymer grafted on at least a part of the surface of the continuous phase is subjected to at least one drawing-relaxation treatment and wherein aid pores of the porous composite material have a pore size of 0.02 to 20 .mu.m and a void volume of 20 to 80%.

A porous membrane comprising a porous membrane substrate having at least a part of the surface of its pores grafted with a polymer having a glass transition temperature of not more than 300 K, wherein said porous membrane has pores having a pore size of 0.02 to 10.0 .mu.m, a void volume of 30 to 95% and an elongation at break of 10% or more when drawn monoaxially, wherein said porous membrane is prepared by subjecting the porous membrane substrate to plasma treatment at a low temperature, and introducing a gaseous polymerizable monomer under a pressure of 0.1 to 100.0 Torr to form graft chains made of a polymer having a glass temperature of not more than 300 K, followed by monoaxial drawing at a temperature of the polymer at a draw ratio of 10 to 1000%.

A method for preparing a porous membrane which comprises:

• subjecting a porous membrane substrate to plasma treatment at a low temperature, and
• introducing a gaseous polymerizable monomer under a pressure of 0.1 to 100.0 Torr to form graft chains made of a polymer having a glass transition temperature of not more than 300 K, followed by monoaxial drawing at a temperature more than the glass transition temperature of the polymer at a draw ratio of 10 to 1000%.