The stress-strain responses of α-iPP and β-iPP are remarkably similar at low temperatures until very large deformations, at which β-iPP strain hardens more rapidly. The compression of β-iPP involves the dramatic transformation of β crystals to α crystals and an overall drop in the crystallinity. This process is favored at high temperature. α-iPP deformation involves the reduction of α crystallinity however at high temperature strain induced crystallization of α may also be occuring. At 25°C (111) planes were preferentially oriented normal to the compressive direction, but at high temperatures (040) planes are preferentially oriented instead, indicating a change in the relative slip system strengths. When α crystals are formed by the β → α transformation the alignment of α planes exhibits the same temperature dependence.
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