Helical polymers epitaxy

Epitaxy between Linear and Helical Polymers Epitaxy between linear and helical polymers may be of particular interest when the contact plane of the helical polymer is build up with isochiral helices, and when the chain axis of the deposited linear polymer is oriented parallel to the helical path. This provides a direct means to determine the exposed helical hand of the substrate polymer. The tilted orientation of chain axes when a helical polymer is involved is therefore an unmistakable indicator of specific interactions, that is, of hard epitaxy [11,29,30]. 

Keywords Epitaxy Nudeation and growth processes Helical polymers ... 

Epitaxial crystallization of helical polymers may involve three different features of the polymer chain or lattice. These are (a) the interchain distance (as for stretched out polymers), (b) the chain axis repeat distance, and (c) the interstrand distance - the distance between the exterior paths of two successive turns of the helix. The two former periodicities are normal and parallel to the chain axis direction, and are therefore not usually sensitive to the chirality of the helix (unless the substrate topography is asymmetric and favors a given helical hand). However, the interstrand distance is oblique to the helix axis (it is normal to the orientation of the outer chain path) and therefore has different, symmetric orientations relative to the helix axis for left-handed and right-handed helices (Fig. 2). In other words, epitaxies that involve the interstrand distances are discriminative with respect to helix chirality. This discrimination becomes visible if the crystal structure is based on whole layers of isochiral helices. Such a situation does indeed exist for isotactic poly(l-butene), Form I, that will be considered soon. 

Polymers, which are more frequently studied for epitaxial growth, include polyethylene, polyesters, polycarbonates, and helical polymers (e.g., iPP and iPB). ... 

Preservation of a high density of interactions is ideally achieved when the substrate and deposit have identical periodicities. It is therefore of interest to search for substrates that match known periodicities of the polymer, in a one-dimensional or, better, a two-dimensional relationship. Epitaxy may also exist when the corresponding distances are multiple, for example, one substrate periodicity for two deposit periodicities. Larger multiples (1 for 3, or 2 for 3) appear very unlikely, and have been observed only occasionally in polymer epitaxy. Matching of polymer periodicities with that of the substrate may be rather complex. The simplest case is of course to match an interchain periodicity, as is the case for PE, considered in the next paragraphs. However, in polymers with a helical conformation, the heUx axis is not materialized by a string of atoms. The outer part of the helix interacts more closely with the substrate. In other words, the helical path may be involved in the epitaxy, or more exactly the distance between two successive helical paths, that is between parts of the helix located on its outer part. The orientation of the helix path relative to the helix axis differs for right-handed and left-handed helices (cf Fig. 8.9). Therefore, epitaxial deposition of polymers with helical conformation... 

The second and third illustrations of epitaxial crystallization deal with syndiotactic polypropylene (sPP). Syndiotactic polymers are by design susceptible to forming either right-handed or left-handed helices, and are therefore suitable materials in the present context of helical hand selection. 

Lotz, B. Wittman, J.C. Epitaxy of helical polyolefins polymer blends and polymer-nucleating agent systems. Makromol. Chem. 1984,185, 2043. 

The crystallisation from strained melt as for instance in a blown film or in the jet during fibre spinning produces a row nucleated structure. " Linear nuclei are formed parallel to the strain direction. They contain more or less extended polymer chains. Secondary epitaxial nucleation on the surface of such linear row nuclei produces folded chain lamellae which are oriented perpendicular to the strain (Fig. 6). In such a case the sample exhibits a high uniaxial orientation of chain axes in the strain direction with random orientation of the a- and b-axes perpendicular to it. If the growing lamellae exhibit a helical twist the chain orientation in the strain direction is very soon replaced by the orientation of the axis of maximum growth rate (b-axis in the case of polyethylene) perpendicular to the strain direction and a more random orientation of the remaining two axes (a- and c-axes in the case of polyethylene) with a maximum in the strain direction. Such a row nucleated structure has parallel cylindrical spherulites (cylindrites) as its basic supercrystalline element. 

The crystal structiue of the p form of iPP was only recently described despite the fact that this phase has been known to exist for 40 years. The structure consists of sections of three isochiral helices packed in a trigonal cell. One of the three helices is embedded in a crystallographic environment which is different from the other two, i.e. the structure is frustrated [5]. AFM studies of the (110) plane confirmed this frustrated character. In addition, it seems that in epitaxy the chains in the contact plane of polymer crystals (grown on dicyclohexylterephthalamide) expose rows of 2, 2 and 1 methyl groups. 

Figure 8.8 Schematic illustration of the /phase crystal structure of isotactic polypropylene and its structural filiation with the aiPP homoepitaxy. The threefold helices are represented as triangular bars. The systematic repetition of the self-epitaxial packing illustrated in Figure 8.6 (indicated here with arrows) generates the first and so far the only polymer crystal structure with nonparallel chain axes. Reproduced from Reference [47] with permission.

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). 

In most of these systems, the issue of compatibility may play a considerable role. However, on a structural basis, the essential rules that govern the epitaxial interactions have been established. In great part, the interactions depend on the linear or helical geometry of the polymer chains. 

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