Understanding Molecular Epitaxial Mechanism of the $\gamma $-form Crystal and Chain Tilt in the $\alpha $-form Single Crystal of Isotactic Polypropylene

We attempt to investigate how the epitaxial domination of the crystal morphologies takes place in the $\gamma $-form of the chain-folded crystals using high molecular weight isotactic polypropylene (i-PP) samples with a controlled number of stereodefects. Two different morphologies were identified \textit{via} transmission electron and atomic force microscopies (TEM and AFM). One is needle-like and the other is ``flat''. Based on the tilted selected area electron diffraction (SAED) results from TEM, the microscopic formation mechanism of the ``needle'' and ``flat'' morphology was discussed and it revealed that in the ``flat'' $\gamma $-form crystal, the initial $\alpha $-form single crystal had to have a stem orientation tilted away from the thin film normal within the ac-plane around the b-axis. Elongated $\alpha _{2}$-form lath-like single crystals were grown from thin film melt at $T_{x}$ = 145 $^{\circ}$C -155 $^{\circ}$C using commercial sample. SAED experimental results show that the stems in these lath-like single crystals were tilted at an unusual 17$^{\circ}$ angle around the $b$-axis. This 17$^{\circ}$-stem tilt in the $\alpha _{2}$-form single crystals favors the ($10\bar {2})$ fold surface and appears to depend upon both conformational and chain folding constraints.