Thin films of block copolymers

Constraining block copolymers onto substrates induces an orientation of the microdomains with respect to the substrate. One block will preferentially wet the substrate, and similarly, one block will be present at the fiee surface. This preferential wetting results in a parallel orientation of the microdomains to the substrate.  

In relatively thick block copolymer films, a mismatch between the film thickness and the bulk lattice spacing can be distributed over many layers. As the film thickness is decreased to a value equal to a few domain periods, the frustration due to the mismatch becomes more significant and can be released by the formation of islands or holes (see Fig. 10).  

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Application of amphiphilic block copolymers for nanoparticle formation has been developed by several research groups. R. Schrock et al. prepared nanoparticles in segregated block copolymers in the sohd state [39] A. Eisenberg et al. used ionomer block copolymers and prepared semiconductor particles (PdS, CdS) [40] M. Moller et al. studied gold colloidals in thin films of block copolymers [41]. M. Antonietti et al. studied noble metal nanoparticle stabilized in block copolymer micelles for the purpose of catalysis [36]. Initial studies were focused on the use of poly(styrene)-folock-poly(4-vinylpyridine) (PS-b-P4VP) copolymers prepared by anionic polymerization and its application for noble metal colloid formation and stabilization in solvents such as toluene, THF or cyclohexane (Fig. 6.4) [42]. 

Thin films of block copolymer melts, and block copolymers adsorbed at the liquid-liquid interface, have been investigated using specular reflectivity (largely neutron reflectivity due to the ability to vary the scattering contrast). Off-specular reflection is, in principle, a powerful method for determining in-plane structure in block copolymer films but is not yet widely used. 

Thin films of block copolymer/homopolymer blends... 

In contrast to swollen homopolymer films, only a limited number of studies on thin films of block copolymers have been reported in which the degree of the film swelling has been directly accessed. In situ SE has been used to evaluate the polymer-solvent interaction parameters [144], to construct phase diagrams of surface structures [49, 51], and to control the mechanism of lamella reorientation in thick swollen films [118, 163, 164], Spectroscopic reflectometry combined with real-time GISAXS has been used to follow structural instabilities in swollen lamella films [165], Recently, it was demonstrated that swelling of diblock copolymer films in organic selective and non-selective solvents follows the same physical behavior as in thin films of homopolymers [119]. 

These results show that the supramolecular structures of thin films of block copolymers can be manipulated by varying the rod-to-coil ratios. Variables such as the polydispersity, the nature of the structures, and their crystallinity can be controlled in this manner. The factors that govern the formation of ordered structures from these copolymers are, however, complex. Important factors include entropy effects associated with the flexible coil segments, crystallization of the rods, and steric considerations. Upon crystallization of the rods, the entropies of the coil blocks may be increasingly compromised as a result of increasing steric repulsion. This may effect the sizes of the aggregates that are formed. The organization of ordered structures can furthermore be controlled by non-specific interactions such... 

Matsen MW (1997) Thin films of block copolymer. J Chem Phys 106 7781-7791... 

In a large part of what we have discussed above, we considered binary polymer mixtures. However, the situation is somewhat different, if instead of polymer blends, thin films of block copolymers are investigated. Due to the molecular connectivity of the different blocks, the inherent length scale is now determined by the size of the molecules. Early experiments focussed on the thin film morphology in symmetric diblock copolymers, where surface interactions tend to orient the block copolymer lamellae parallel to the boundary surfaces. In contrast to most bulk specimens, the planar interfaces lead to the formation... 

Thin films of block copolymers are likely to find many applications as nanostructured materials, due to the ability to tailor nanoscale dot and stripe patterns. Theory for microphase separation in thin films, especially the effect of confinement on structure orientation is now quite advanced. " Models for the effect of confinement on thermodynamics have also been developed, although this aspect has attracted less attention. 

Fig. 29 Long-term stability of holographic gratings in thin films of block copolymers with different azobenzene side-groups...

Methods. Thin films of block copolymers were cast from their cyclohexane solutions at 20°C, and the films were observed under a polarizing microscope. Ultra thin films (50-500 nm) were cast from the same solutions onto sheet meshes at 20 °C and stained by osmium tetroxide vapor they were then observed under a Hitachi HU-11 electron microscope. 

Sidorenko A, Tokarev I, Minko S, Stamm M. 2003. Ordered reactive nanomembranes/ nanotemplates from thin films of block copolymer supramolecular assembly. J Am Chem Soc 125 12211 12216. 

Other expected further studies will likely assess the nanopattems and the mechanisms of their formation, in thin films of block copolymers with more complex architectures. For instance, the phase behavior of star copolymers is increasingly well described [117]. For enlarging the application openings, more advanced substrates will likely be implemented, like the curved surfaces theoretically considered in recent studies [118, 119]. Efforts will be devoted to the generation of reconfigurable and responsive surfaces, which can adapt to surrounding environments [120], as for instance thermoresponsive nanopattemed surfaces [121]. 

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)... 

Hamley, I.W. Ordering in thin films of block copolymers fundamentals to potential applications. Prog. Polym. Sci. 34, 1161-1210 (2009)... 

A. Sidorenko, I. Tokarev, S. Minko, and M. Stamm, "Ordered Reactive Nanomembranes/Nanotemplates from Thin Films of Block Copolymer Supramolecular Assembly, 7ACS. 125,12211-12216 (2003). 

L. Li, et al., CO2 foaming in thin films of block copolymer containing lluorinated blocks. Macromolecules 39 (2006) 4746—4755. 

For a thin film of block copolymer on a chemical pattern, the effect of the surfaces can be described as follows. In the absence of stronger driving forces, the energy of the free surface, Fsurf, will lead the block with the lowest surface energy to populate the surface layer. Fsurf per chain in the film can be expressed as Equation 10.2 [33,38] ... 

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