Pore initiation technique

A method for initiating the formation of pores in a polymeric material that comprises a thermoplastic composition, the thermoplastic composition comprising a continuous phase in which a polymeric microinclusion additive and a polymeric nanoinclusion additive that are at least partially incompatible with the matrix polymer are dispersed as discrete micro-scale and nano-scale domains, respectively,

• wherein the polymeric microinclusion additive constitutes from about 5 wt. % to about 20 wt. % of the thermoplastic composition,
• wherein the polymeric nanoinclusion additive constitutes from about 0.5 wt. % to about 5 wt. % of the thermoplastic composition, and
• wherein the continuous phase comprises a thermoplastic matrix polymer,
• the method comprising mechanically drawing the polymeric material while the polymeric material is in a solid state, such that the micro-scale domains and nano-scale domains interact to form a porous network therein in which a total pore volume is from about 30% to about 60%, wherein the porous network comprises a plurality of nanopores adjacent to the nano-scale domains and the micro-scale domains and a plurality of micropores, the plurality of nanopores having an average cross-sectional dimension of about 800 nanometers or less and constituting about 40 vol. % to about 90 vol. % of the total pore volume, and wherein the drawn polymeric material has an elongation at break of about 100% to about 600% in a machine direction thereof and about 100% to about 400% in a cross-machine direction thereof and a tensile modulus of from about 100 MPa to about 1000 MPa in the machine and cross-machine directions.