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

Abstract

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.

The correlation between structure and β-nucleation efficiency of Ba, Sr, Ca and Mg pimelates in isotactic polypropylene

“…Due to the lowest ionic radius of Mg 2+ , the lowest periodicity is expected in the aliphatic zone of the structure of the anhydrous compound, even on distances smaller than 6.5 Å. On the other hand, it is well known that the beta polymorph modification of iPP is not nucleated on such short distances [35,36,54].…”

“…According to these authors, the epitaxial growth of polyolefines proceeds on the nonpolar part of the molecule of the nucleating agent. It should be mentioned that the epitaxy of polymers on inorganic and organic substrates has been studied intensively by many authors [32][33][34][35][36][37][38][39][40][41]. Lotz and Witmann have analyzed heterogeneous nucleation in terms of physical interactions of the polymer and the nucleating agents.…”

“…In their extensive study of the nucleation of the -form of iPP [35] it was found that the matching lattice dimension is about 0.5 nm for most of the different active substrates, including various polymers, salts and other compounds. The interactions between the polymers and solid substrate surfaces (including polymeric surfaces, see for instance [39,40]) have been a subject of extensive research activities [34][35][36][37][38][39][40][41].…”

“…Stocker et al [36] observed the epitaxial growth of β-form iPP on the crystal of a nucleating agent. The results showed that the nucleating agent with about 0.65 nm periodicity and orthogonal geometry of the contact face could induce the β-form iPP crystals.…”

Expression and Amplification of Chirality in Two-Dimensional Molecular Crystals

“…[10] Similar to the frustrated lattice structures observed for crystalline polymers of single helicity, e.g . isotactic poly(propylene), [12] not all helices can be aligned 'in phase'. This shows that, although the packing is mediated by the Cu substrate grid, the 'one-pitch' [7]H helices follow the same sterically controlled packing rules as extended helical molecules in 3D crystals.…”