A polymeric material having anisotropic properties, such as mechanical properties (e.g., modulus of elasticity), thermal properties, barrier properties (e.g., breathability), and so forth, is provided. The anisotropic properties can be achieved for a single, monolithic polymeric material through selective control over the manner in which the material is formed. For example, one or more zones of the polymeric material can be strained to create a unique network of pores within the strained zone(s). However, zones of the polymeric material that are not subjected to the same degree of deformational strain will not have the same pore volume, and in some cases, may even lack a porous network altogether.

Multi-layer polyolefin films for envelope windows, comprising at least three layers wherein the central layer comprises a propylene homopolymer; the layer of film on the outer part of the envelope, which forms the anti-reflective layer, is based on propylene copolymers in mixture with HDPE, HDPE being comprised between 40 and 50% by weight of the mixture; the inner layer in contact with the contents of the envelope is based on ethylene/propylene/butene copolymers, the film being isotropic in relation to shear strength.