Epitaxy of Helical Polymers

From a structural standpoint, iPP exists in three different crystal modifications that all rest on the same threefold helical chain conformation. Only the packing of helices differs in these three forms [26]. In addition, iPP displays an original self-epitaxy, analyzed later on, which leads to lamellar branching in its stable a phase, and exists as a crystallographic feature in its highly unusual /phase [27]. 

The ac face that contains only one methyl group is of particular relevance in the context of epitaxial crystallization. Indeed, the methyl groups that line this face generate a lozenge-shaped pattern (as a result of the =100° j8 angle of the cell) of humps about 0.4 nm in 

The p phase of iPP is obtained under normal crystallization conditions as a minor component compared with the a phase. Although the j8 phase is based on the same threefold helical conformation, it differs significantly from the a phase. The unit cell is trigonal and houses three isochiral helices. These helices have differ- 

2 A Case of Self-Epitaxy in Polymers Epitaxy of Isotactic Polypropylene The stable a phase of iPP displays a highly original morphological feature the 

3 Epitaxy of Isotactic Poly (1-butene) Isotactic poly(1-butene) (iPBul) is an archetypical polymorphic polymer with three different structures that differ by the chain conformation, and thus by the unit-cell geometry and symmetry (cf Chapter 2).The three crystal phases could be obtained by epitaxial crystallization on appropriate substrates [48-50]. Most interesting among them are the epitaxy of Form I (trigonal unit cell, threefold helical conformation, racemic phase) and that of Form III (orthorhombic unit cell, fourfold helical geometry, chiral crystal phase). 

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