Surface-induced ordering

With diblock copolymers, similar behavior is also observed. One component is enriched at the surface and depending on miscibility and composition a surface-induced ordered lamellar structure normal to the surface may be formed. Recent investigations include poly (urethanes) [111], poly(methoxy poly (ethyleneglycol) methacrylate)/PS [112] and PS/PMMA [113, 114]. In particular the last case has been extensively studied by various techniques including XPS, SIMS, NR and optical interferometry. PS is enriched at the surface depending on blockcopolymer composition and temperature. A well ordered lamellar structure normal to the surface is found under favourable conditions. Another example is shown in Fig. 6 where the enrichment of poly(paramethylstyrene), PMS(H), in a thin film of a di-... 

This area of research is still at its beginning and many aspects are not resolved. This includes in particular the structure and conformation of polymers at an interface as well as the modification of polymer dynamics by the interface. We have given several examples of the potential of surface and interface analytical techniques. They provide information on surface roughness, surface composition, lateral structure, depth profiles, surface-induced order and interfacial mixing of polymers on a molecular and sometimes subnanometer scale. They thus offer a large variety of possible surface and interface studies which will help in the understanding of polymer structure and dynamics as it is modified by the influence... 

Other forces can arise as a result of elastic strain on the growing film, which can be due to a surface-induced ordering in the first few layers that reverts to the bulk liquid structure at larger distances. This elastic energy is stored in intermolecular distances and orientations that are stretched or compressed from the bulk values by the influence of the substrate at short distances [7]. Similar phenomena are well known to occur in the growth of epitaxial layers in metals and semiconductors. 

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). 

For instance, this kind of quasi-immobilisation may result from multiple adsorption of one polymer chain at the fibre surface [210]. As shown by Bitsanis and Pan [211] by means of molecular dynamics (MD) simulations surface-induced ordering of chain segments can be expected which is re-... 

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]... 

Finally, we return to the case H =0,i.e. consider a surface that does not prefer one species as compared to the other. In the framework of the linear theory, we have seen that still surface-induced order occurs if the constant a in the surface term Eq. (48) is negative. As shown already in Fig. 10, this transition can only occur if -a exceeds a critical value -ac, if fluctuations are taken into account. This fact is simply seen considering the response function %n [216]... 

Segregation/order factor Surface induced order/disorder or neither... 

Fig.9. Surface Induced Order at the LI2(100) surface in AB3 model alloy calculated in the FCEM approximation for r=l. p= and 2 correspond to the first and second under-layer, respectively.

Surface-induced ordering and disordering surface melting. 262... 

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. 

The first indication that the surface-induced order in the isotropic phase of confined liquid crystals can be detected by deuteron NMR was the spectrum of selectively deuterated 4 -n-pentyl-4-cyanobiphenyl (5CB) in the spherical cavities of a PDLC material. Far above Sfi C, where the bulk turns into the isotropic phase, the spectral line (a single one) of the PDLC was considerably broader than in the bulk at the same temperature. An even more definite and surprisingly clear evidence of surface-induced order was later obtained from... 

There are basically two possible scenarios for the behavioxir of this prenematic mean field force when the separation between surfaces is reduced, and both depend on the degree of the surface-induced nematic order. For low surface-induced order, the magnitude of the attractive force just increases when approaching the isotropic-nematic phase transition from above. On the other hand, if a surface induces a high degree of LC orientation, the prenematic phase can spontaneously transform into the nematic phase, when the separation is decreased below a certain value. This is the nematic capillary condensation, that is discussed further on in this Chapter. 

Thin surface-induced ordered liquid crystalline layers can also be observed by measuring their birefringence, which is a sort of ellipsometry method, used to analyze the polarization properties of directly transmitted waves. The method has been used in the pioneering linear optics experiments at solid-liquid crystal interfaces [16-24] and later in [25,26]. 

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