Block copolymer thin film composition

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... 

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]. 

Summary PDMS-6-PEO short-chain diblock copolymers were prepared via anionic ring-opening polymerization of cyclosiloxanes. Applying this method, various well-defined block copolymers with different compositions were synthesized and their phase behavior was investigated. The polymers predominantly showed lamellar phases in aqueous solutions. At small surfactant concentrations, vesicles were formed, as observed via cryogenic TEM. The aggregates of the diblock copolymers were used for the formation of lamellar thin films, applying the evaporation-induced self-assembly approach. 

We have already mentioned that depending on composition, semicrystalline triblock copolymers can show some conflict between microphase separation and superstructure formation. In fact, one of the controversial aspects is the question whether block copolymers can or cannot exhibit spherulites. This is a relevant question because spherulitic structures greatly affect the ultimate mechanical properties, and the boundaries between adjacent spherulites are often weak points in mechanical performance. Kim et al. [125] studied the competition between crystallization within microphase-separated regions and reorganization into supermolecu-lar spherulites in semicrystalline PS-b-PB-b-PCL triblock copolymers. These authors found that the formation of spherulites is strongly affected by the thickness of the specimen in such a way that thin films crystallize into... 

We are currently initiating three research projects that include (1) the synthesis of reflective liquid crystal/polymer composite films, (2) a study of microphase separation in hyperbranched block copolymers, and (3) the design and synthesis of polar organic thin films, which is the subject of this proposal. (47 words aim for 41 words)... 

The intrinsic 3D interfacial curvature in compositionally asymmetric block copolymers provides extra degrees of freedom for the phase behavior in hexagonally structured microdomains. It is now well established that confinement of a cylinderforming block copolymer to a thickness other than the characteristic structure dimension in bulk, together with surface fields, can cause the microstructure to deviate from that of the corresponding bulk material. Surface structures in Fig. 1 are examples of simulated [57-59] and experimentally observed morphologies [40, 49, 60-62] that are formed in thin films of bulk cylinder-forming block copolymers. 

As already shown above in Section 4.7, the composition of block copolymers can be changed in a systematic manner. Stupp and co-workers [36] have used this flexibility to investigate the ability of thin films of such materials to form... 

Electron Microscopy. Figure 3 shows electron micrographs of ultra-thin sections of film specimens of the three kinds of block copolymers. As can be seen in the figure, TR-41-1647 and TR-41-1648 specimens have a heterogeneous structure in which the polystyrene domains are dispersed within a polybutadiene matrix and are connected to each other to form a swirl-like structure. On the other hand, TR-41-1649 specimen is seen to consist of alternating lamellar domains of the two components. Changes of the domain structure with fractional compositions of styrene and butadiene components are consistent with predictions of the current theories of micro-phase separation (12,13,14,15) for block copolymers cast from such a nearly nonselective solvent as the mixture of THF and methylethylketon (90/10 in volume ratio). 

Mattoussi H., Radzilowski E. H., Dabbousi B. 0., Fogg D. E., Schrock R. R., Thomas E. E., Rubner M. F. and Bawendi M. G. (1999), Composite thin films of CdSe nanocrystals and a surface-passivating/electron-transporting block copolymer correlations between film microstructure by transmission electron microscopy and electroluminescence , J. Appl. Phys. 86, 4390-4399. 

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