Copolymer, block surface-induced ordering

Lattice Model Carlo simulations of a block copolymer confined between parallel hard walls by Kikuchi and Binder (1993,1994) revealed a complex interplay between film thickness and lamellar period. In the case of commensurate length-scales (f an integral multiple of d), parallel ordering of lamellae was observed. On the other hand, tilted or deformed lamellar structures, or even coexistence of lamellae in different orientations, were found in the case of large incommensurability. Even at temperatures above the bulk ODT, weak order was observed parallel to the surface and the transition from surface-induced order to bulk ordering was found to be gradual. The latter observations are in agreement with the experimental work of Russell and co-workers (Anastasiadis et al. 1989 Menelle et al. 1992) and Foster et al. (1992). 

So the description of surface-induced ordering at the surface of a semi-infinite lamellar block copolymer melt is the gradual unbinding of an interface between a thin ordered layer at the surface and the disordered bulk as %—>%t the distance of this interface from the surface diverges, Eq. (76). At this point, we note two generalizations ... 

This wetting picture (Fig. 12 [6]) of surface-induced ordering in block copolymer melts has been considered recently by Milner and Morse [60]. They considered the transition from the state of weak surface-induced order (Fig. 12a) to the case of strong-surface induced order (Fig. 12b) and pointed out that typically a first-order transition may occur between these states, in analogy to the "prewetting transition" first proposed by Cahn [226] (Fig. 14a). This prewetting-type first-order transition may persist in a thin film (Fig. 14b), but it ends in a triple point where the surface excess ( ) is still finite, of course, since no diver-... 

Thus, although the analogy between wetting phenomena in mixtures and surface-induced ordering in thin block copolymer films is not complete, the analogy does allow to extend the mathematical methods to study wetting phenomena to the present case, at least approximately. In particular, Milner and Morse [60]... 

The same fact is true for surface effects on the dynamics of spinodal decomposition in polymer blends [374,375], dynamics of surface enrichment in blends [367, 375, 376], and, last but not least, for surface effects on block copolymers there one may have surface - induced ordering [377, 379] and interesting competition effects between the lamellar ordering (of wavelength X) and film thickness D in thin block copolymer films [380—388]. These phenomena are outside of our consideration here. 

Surface-induced ordering in a block copolymer that is disordered in the bulk has been detected using neutron reflectometry for styrene/methyl methacrylate block copolymers, in which the styrene block is favoured at the surface and the methacrylate block is favoured at a silicon oxide substrate (Anastasiadis et al. 1989a, b). Figure 6.29 shows how the decay length characterising the oscilla-... 

In this article we focus on the formation of long-range ordered laterally-segregated structiues of PS-fc-PVP block copolymers at interfaces. When a selective solvent is used, micelles form in solution and are subsequently deposited onto the substrate. In contrast, polymer deposition from nonselective solvents is controlled by the polymer-surface interaction and the resulting patterns are surface induced (Surface Induced NanoPATterns, SINPATs). In this brief review, these two borderhne cases as well as intermediate processes are discussed and compared. 

It has been indicated by experiments that there is another way to achieve perpendicular order even for films containing lamellar mesophases. The use of substrate coatings made of random copolymers allows us to neutralize the interactions of the two blocks with the substrate [16,23,25]. This leads to perpendicular orientation of the lamellae in the vicinity of the so-prepared substrate. It is worth noting that this effect is not simply related to some in-commensurabihty between the lamellar period and the film thickness as has been discussed by several authors, see [52], Remarkably, perpendicular orientation at neutral surfaces can persist against other orientations (e.g. induced by the opposite surface) as has been suggested also by experiments [25]. Furthermore, there are indications from computer simifiations (presented below)... 

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