Surface Energies of the Block Copolymers

Average contact angles 9 and the corresponding surface energies Y,vare given [yiv = 27.76 niN/m 3 = 0.0001247 (mVmJ) ].  

Solid films of our block copolymers and their surface behavior have been examined using a variety of techniques. Block copolymers composed of incompatible polymer blocks are known for mesophase formation as a consequence of the microphase separation of the chains. Our fluorinated block copolymers form a microphase-separated structure with a high degree of order. This can easily be visualized by polarization microscopy and SAXS. 

The mechanical properties also depend on the relative block lengths. For instance, Fluoro-PSB-II is a soft, elastic material. Differential scanning calorimetry (DSC) measurements and wide-angle X-ray scattering (WAXS) 

Therefore, at room temperature Fluoro-PSB-II a thermoplastic elastomer with a soft polymer phase (fluorinated block) and a hard phase (PS-block), similar to the parental polystyrene-h-polybutadiene block copolymer. Depending on the relative volume fraction of both components and the continuity of the phases, the resulting bulk material is rubbery or a high-impact solid. 

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In thin films, the lamellae formed hy symmetric block copolymers can orient either parallel or perpendicular to the substrate. A number of possible arrangements of the lamellae are possible, depending on the surface energies of the blocks and that of the substrate, and whether the film is confined at one or both surfaces. These are illustrated in Figure 4. In the case that a different block... 

AB diblock copolymers in the presence of a selective surface can form an adsorbed layer, which is a planar form of aggregation or self-assembly. This is very useful in the manipulation of the surface properties of solid surfaces, especially those that are employed in liquid media. Several situations have been studied both theoretically and experimentally, among them the case of a selective surface but a nonselective solvent [75] which results in swelling of both the anchor and the buoy layers. However, we concentrate on the situation most closely related to the micelle conditions just discussed, namely, adsorption from a selective solvent. Our theoretical discussion is adapted and abbreviated from that of Marques et al. [76], who considered many features not discussed here. They began their analysis from the grand canonical free energy of a block copolymer layer in equilibrium with a reservoir containing soluble block copolymer at chemical potential peK. They also considered the possible effects of micellization in solution on the adsorption process [61]. We assume in this presentation that the anchor layer is in a solvent-free, melt state above Tg. The anchor layer is assumed to be thin and smooth, with a sharp interface between it and the solvent swollen buoy layer. 

Atomic force spectroscopy shows that when only the polymer-substrate surface is modified, parallel orientation of the nanoscopic cylinders is observed in the 137 nm thick film (see Figure 4.34(a)) because of the lower surface energy of the polystyrene block. However, the presence of a random copolymer at both the polymer-substrate and the polymer-air interfaces eliminates the preferential... 

The synthesis of these materials is outlined in Scheme I. Transmission electron microscopy shows that the morphology of nearly equimolar compositions of the siloxane-chloromethylstyrene block copolymers is lamellar, and that the domain structure is in the order of 50-300 A. Microphase separation is confined to domains composed of similar segments and occurs on a scale comparable with the radius of gyration of the polymer chain. Auger electron spectroscopy indicates that the surface of these films is rich in silicon and is followed by a styrene-rich layer. This phenomenon arises from the difference in surface energy of the two phases. The siloxane moiety exhibits a lower surface energy and thus forms the silicon-rich surface layer. 

In addition to the microphase separation phenomenon, in the presence of an interface, the affinity of one of the blocks by the interface influences the flnal rearrangement of the block copolymer at the outmost surface as has been already reported, for instance by Coulon et al. [96] for the case of polystyrene-6-poly (methyl methacrylate) block copolymers (Fig. 5.12). Initially, upon spin coating the block copolymers are rather disordered due to the fast evaporation process. However, upon annealing reorganization occurs and nanometer scale phases rich in each of the components are observed. Finally, the difference in the surface energies of the components forces the orientation of these domains parallel to the surface, with the lower-surface-energy block located at the surface [96-98]. 

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