Abstract
Poly(vinylcyclohexane) (PVCH) is a patented polymeric nucleating agent for isotactic polypropylene (iPP) and induces its alpha crystal modification (αiPP). The structural interactions at the root of this activity are investigated by analyzing the transcrystallization of single crystals of αiPP on single crystals of PVCH. Electron microscopy results indicate (i) a quasi-perfect two-dimensional lattice match in the contact plane, with (100)PVCH facing (110)αiPP or (1̄10)αiPP planes, and (ii) a consistent link between the PVCH single crystals' internal structure and the orientation of αiPP overgrowth. The two polymers chain axes are parallel. The 3-fold (iPP) and 4-fold (PVCH) helices share the same chain axis repeat (≈6.5 Å), and repeat distances normal to the chain axis are ≈21.9 Å for (110)αiPP and (100)PVCH planes. In chain axis projection, the PVCH helices are tilted relative to the a and b axes of the tetragonal unit cell. The tilt generates an “asymmetric sawteeth” surface profile that differs for clock- or counterclockwise tilts. Similar surface profiles exist in the (110)αiPP or (1̄10)αiPP contact planes. Matching of the PVCH and iPP sawteeth profiles is selective and induces a single orientation of the αiPP crystal lattice in the overgrowth. The orientation of the αiPP overgrowth thus becomes also a marker of the local clock- or counterclockwise setting of PVCH helices in the growth face.
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