Thin films microdomain orientation

Yoon J, Lee W, Thomas EL (2006) Highly oriented thin-film microdomain patterns of ultra-high molecular weight block copolymers via directional solidification of a solvent. Adv Mater 18(20) 2691... 

For instance, order-disorder transitions of BCP blends (typically related to macrophase separation processes) in thin films have been studied by Jeong et al. [48]. They used mixtures of PS-b-PMMA having PMMA cylindrical microdomains with homopolymers of either PMMA or poly(ethylene oxide) (PEO) and studied the morphological transitions and the formation of macrophase-separated domains in thin films (Fig. 6.7). Whereas the microdomains in all thin films are oriented perpendicular to the film surface, they found that the miscibility between PMMA homopolymer and PMMA block in thin film was enhanced compared with that in bulk at a given molecular weight of PMMA homopolymer. Equally, the PMMA homopolymer chains in thin films were more localized to the center of PMMA microdomains than in the bulk, which results in a larger increase of the lattice spacing (D) in the former. 

The self-assembly of block polymers, in the bulk, thin film and solution states, produces uniformly sized nanostructured patterns that are very useful for nanofabrication. Optimal utilization of these nanoscopic patterns requires complete spatial and orientational control of the microdomains. However, the microdomains in the bulk state normally have grain sizes in the submicron range and have random orientations. In block copolymer thin films, the natural domain orientations are generally not desirable for nanofabrication. In particular, for composition-asymmetric cylindrical thin films, experimental... 

In block copolymer thin films, the perpendicular orientation of microdomains relative to the substrate cannot be achieved by the shear methods developed in the bulk case. Based on the additional variables (film thickness and surface/interface interactions) in block copolymer thin films, as described in Sect. 2.1.2, three different strategies are generally applied for orienting block copolymer thin films ... 

Processes which employ combinations of these strategies have proved to be much more effective at yielding uniform long-range ordered patterns than single strategies. Table 1 describes the methods used to control the microdomain orientation of a variety of thin film block copolymers [41,42,66-108],... 

Unlike the bulk morphology, block copolymer thin films are often characterized by thickness-dependent highly oriented domains, as a result of surface and interfacial energy minimization [115,116]. For example, in the simplest composition-symmetric (ID lamellae) coil-coil thin films, the overall trend when t>Lo is for the lamellae to be oriented parallel to the plane of the film [115]. Under symmetric boundary conditions, frustration cannot be avoided if t is not commensurate with L0 in a confined film and the lamellar period deviates from the bulk value by compressing the chain conformation [117]. Under asymmetric boundary conditions, an incomplete top layer composed of islands and holes of height Lo forms as in the incommensurate case [118]. However, it has also been observed that microdomains can reorient such that they are perpendicular to the surface [ 119], or they can take mixed orientations to relieve the constraint [66]. 

Fig. 4 Schematic representation of control over block copolymer thin film orientation by directional eutectic solidification. The scheme shows control of polystyrene-b-polyethylene (PS-b-PE) block copolymer microdomain orientation through directional crystallization of benzoic acid (BA), (taken from [95])...

The surface of bulk block copolymer samples has been studied using TEM by Turturro et al. (1995). They report that non-equilibrium structures with lamellar and cylindrical microdomains oriented normal to the free surface can result from solvent casting, with a high evaporation rate. However, slower evaporation of solvent from their PS-PB diblocks resulted in the equilibrium conformation with domains parallel to the free surface. Perpendicular orientation of PS-PB lamellae at the free surface was observed earlier by Henkee et al. (1988) who studied thin films prepared by solvent casting. They observed that a reduction of this orientation occurs in favour of the parallel one on annealing the sample. 

Morkved TL et al (1996) Local control of microdomain orientation in diblock copolymer thin films with electric fields. Science 273 931... 

Table 1 Summary of block copolymers studied in order to control microdomain orientations in thin film states...

Investigations on the self-assembly in thin films of an amorphous P2VP-PS-P2VP triblock copolymer, forming cylindrical microdomains in bulk, showed that the orientation of microdomains due to the P2VP/substrate interactions persisted in the entire film in contrast to the diblock case [14], This was viewed as a result of the formation of an interconnected structure in the triblock coming from the formation of loops within the microdomains. More recently, AFM and SAKS measurements on a PEO-PBO-PEO amorphous-semicrystalline triblock thin film revealed the presence of a semicrystaUine PEO monolayer at the substrate, comprised of unfolded chains, and PBO blocks at the air/polymer siuface in a looped conformation (15]. 

Park, C., De Rosa, C., Thomas, E.L. Large area orientation of block copolymer microdomains in thin films via directional crystallization of a solvent. Macromolecules 34, 2602 (2001)... 

Tang, C., Wu, W., Smilgies, D.-M., Matyjaszewski, K., Kowalewski, T. Robust control of microdomain orientation in thin films of block copolymers by zone casting. J. Am. Chem. Soc. 133, 11802(2011)... 

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