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The effect of 975 W microwave irradiation on the crystalline structure of isotactic polypropylene (iPP) surface layer in Al/iPP/Al composite was studied by attenuated total reflection Fourier transform infrared (FTIR) and asymmetry diffraction geometry small-angle x-ray diffraction (XRD). The results show that after about 60 min irradiation, the orientated structure and crystallinity of the iPP surface layer are dropped, crystal transformation (α → β) happens. The iterative peak-fit of XRD curves indicates that the reasons for crystal transformations are: the microwave effect causes the increase of the order level of the crystal lattice, the defaults within the β-crystals are, thus, reduced. D[300] (stands for β-form phase) becomes larger, β-form iPP becomes more perfect. With microscopic distortion coefficient ε[300] and the microscopic stress of β-form becomes less attraction, the microscopic stress of the α-form goes up. This crystal transformation is useful for improving the toughness of iPP in electromagnetic shielding materials. [ABSTRACT FROM AUTHOR]