Surface Enrichment-Depletion Duality

A conventional understanding of the surface segregation from polymer blends is that the surface should be enriched in the component with lower bare surface free energy fs, regardless of the value of bulk composition This is however true only when (-dfs/d(j))s does not change its sign when surface concentration is varied (see Fig. 14b). For such blends, surface enrichment in the same 

The shape of the obtained bare surface energy derivative (-dfs/d( ))s recalls similar curves obtained for other blends (see Figs. 20 and 22), which are however 

Similar to other blends composed of random olefinic copolymers EX XEEX, here the more branched (more flexible) component h66 (i.e.,with higher x) also has lower bare surface energy fs than its more linear counterpart d52 (Afs 0). However, in contrast with other isomeric blends studied so far, here the more linear polyolefine may also be enriched at the free surface in the case when it constitutes the minority of the mixture. 

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In recent work Jerry and Dutta [176] reanalyzed, with the mean field approach, conditions [8] of the second order wetting transition. They have found that critical wetting transition must be accompanied by a prerequisite phenomenon of an enrichment-depletion duality it is expected that the surface is enriched in the given component when bulk composition ( )M is below a certain value Q and is depleted in the same component for >Q. Such an effect, easily predicted by simple lattice theory [177] and observed in Monte Carlo simulations [178, 179], has been very recently determined by us for a real polymer blend [175] (see Sect. 3.1.2.4). 

The divergent predictions of theories on entropic surface segregation advocating the surface enrichment in more flexible [184, 186, 205, 206], more stiff [200,204,207,209,214], or both types of molecules [183,208] may be compared with experimental results obtained for blends composed of the statistical olefines E EEx. The more branched (i.e., more flexible) component was found preferred at the free surface (with an exception of enrichment-depletion duality described in Sect. 3.1.2.4) but no segregation was observed at silicon or gold interfaces. This leads to the conclusion [120] that the entropic driving force alone cannot be in charge of the enrichment because in such a case the enrichment... 

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