Biaxially stretched porous film

A film that comprises a thermoplastic composition that contains a continuous phase that includes a polyolefin matrix polymer, a polymeric microinclusion additive and a polymeric nanoinclusion additive, wherein the polymeric microinclusion additive and the polymer nanoinclusion additive are dispersed within the continuous phase in the form of discrete domains, wherein the continuous phase constitutes from about 60 wt. % to about 99 wt % of the thermoplastic composition, and wherein the polymeric nanoinclusion additive constitutes from about 0.05 wt % to about 20 wt. % of the composition, based on the weight of the continuous phase;

• wherein the film is biaxially stretched in a machine direction and cross-machine direction to form a porous network in the composition, the porous network comprising a plurality of micropores and a plurality of nanopores, wherein the plurality of micropores and the plurality of nanopores occupy an average volume within a given unit volume of the film of from about 5% to about 80% per cubic centimeter of the film,
• wherein the plurality of nanopores have an axial dimension and a cross-sectional dimension, wherein the axial dimension is larger than the cross-sectional dimension, wherein a maximum cross-sectional dimension of the plurality of nanopores less than 500 nanometers, and wherein the plurality of micropores have a maximum cross-sectional dimension of more than 0.5 micrometers;
• and
• further wherein at least a portion of the nanopores are oriented in the cross-machine direction and at least a portion of the nanopores are oriented in the machine direction, where the axial dimension of the cross-machine direction oriented nanopores generally extends in the cross-machine direction and the cross-sectional dimension of the cross-machine direction oriented nanopores generally extends in the machine direction, and where the axial dimension of the machine-direction oriented nanopores generally extends in the machine direction and the cross-sectional dimension of the machine-direction oriented nanopores generally extends in the cross-machine direction, and
• wherein the portion of cross-machine oriented nanopores have a volume that constitutes from about 10 vol. % to about 90 vol. % of the total pore volume of the film.