Two Different Types of Single Crystal Morphologies of the $\gamma $-Phase and Their Conversion in Isotactic Polypropylene

In the past, the crystallographic relationship between the $\gamma $-phase and the $\alpha $-phase in isotactic polypropylene was extensively studied via oligomers of iPP. We attempt to investigate how the crystal morphological changes take place in the $\gamma $-phase using high molecular weight iPP-co-polyethylene samples. Due to the specific epitaxial growth of the $\gamma $-phase on the elongated $\alpha $-phase single crystals, two different morphologies were identified via transmission electron and atomic force microscopies. The first $\gamma $-phase crystal morphology is needle-like. Selective area electron diffraction results showed that their [$\overline 1 10$] or [110] zone axis was parallel to the thin film normal. The growth of this type of epitaxial $\gamma $-phase crystal was due to the stem direction in the initial $\alpha $-phase single crystal being parallel to the thin film normal. The second $\gamma $-phase crystal morphology was flat lamellae. This requires that the initial $\alpha $-phase single crystal had to have a stem orientation tilted away from the thin film normal. Therefore, the sufficient and necessary condition for the $\gamma $-phase morphological conversion from the needle-like crystal to the flat crystal is the change of the stem orientation direction of the initial $\alpha $-phase single crystals.