Epitaxial Crystallization and AFM Investigation of a Frustrated Polymer Structure:  Isotactic Poly(propylene), β Phase

The metastable phase of isotactic polypropylene (βiPP) is crystallized epitaxially on two specific nucleating agents: γ-quinacridone and dicyclohexylterephthalamide (DCHT). The resulting thin films are investigated by electron microscopy and atomic force microscopy. Epitaxial crystallization yields a biaxially oriented sample of βiPP, an orientation which cannot be achieved by mechanical means due to the β → α phase transformation on stretching. Electron diffraction indicates that the βiPP (110) plane is the contact face involved in both epitaxies. AFM investigation with methyl group resolution reveals a lateral periodicity of 19 Å in that (110) contact face, which corresponds to the distance between three chains and is a trademark indicator of the frustrated packing of βiPP. AFM further indicates some variability of the surface pattern, suggesting that two different frustrated structures may coexist in the surface layer; this, in turn, suggests some type of surface reconstruction. Structural requirements that efficient βiPP nucleating agents must meet are analyzed.