Periodic block copolymers

PRISM theory based on the new molecular closures has recently been generalized by David and Schweizer [86] to treat periodic block copolymers. For... 

Block copolymers are molecules composed of two or more distinct monomers chemically bonded in the same chain. We consider the simplest case where there are two types of elementary units A and B. These units are arranged into bonded linear sequences, or blocks, of variable length that are then repeated a variable number of times. To date, only one-component fluids composed of periodic block copolymers where the A and B block lengths are unique have been studied based on PRISM theory. However, random or statistical copolymers where there is quenched chemical or sequence disorder associated with the polymerization process are also of great interest. ... 

When PAN block copolymers are synthesized with the help of such method the reaction is conducted during a period of time (20—30 min) shorter than the induction period of AN polymerization (45 min) in the presence of cerium ions. When the mechanism and the laws governing the reaction of AN copolymerization with PEO and PPO were studied, it was established that the initiation of the block copolymerization proceeds in accordance with the following scheme ... 

Flocculation studies (6) indicated that the mechanism of steric stabilization operates for the PMMA dispersions. The stability of PMMA dispersions was examined further by redispersion of the particles in cyclohexane at 333 K. Above 307 K, cyclohexane is a good solvent for PS and PDMS, and if the PS-PDMS block copolymer was not firmly anchored, desorption of stabilizer by dissolution should occur at 333 K followed by flocculation of the PMMA dispersion. However, little change in dispersion stability was observed over a period of 60 h. Consequently, we may conclude that the PS blocks are firmly anchored within the hard PMMA matrix. However, the indication from neutron scattering of aggregates of PS(D) blocks in PMMA particles may be explained by the observation that two different polymers are often not very compatible on mixing (10) so that the PS(D) blocks are tending to... 

Shrink-resist science/technology development of, 26 391 Shrink-resist treatments, 26 391-393 additive, 26 393 chlorine-based, 26 392 chlorine-free, 26 392-393 Shuiskite, 6 471t Shutdown period, 29 494 Shutdown systems, 20 671-672 Shuttle vectors, 26 482-483 Sialon-bonded silicon carbide, 22 541 Siberian red lead, 6 468 S-iB-S block copolymers, 24 707 SiC-ceramic, 22 525. See also Silicon carbide... 

In addition to the previously mentioned driving forces that determine the bulk state phase behavior of block copolymers, two additional factors play a role in block copolymer thin films the surface/interface energies as well as the interplay between the film thickness t and the natural period, Lo, of the bulk microphase-separated structures [14,41,42], Due to these two additional factors, a very sophisticated picture has emerged from the various theoretical and experimental efforts that have been made in order to describe... 

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]. 

The formation of bottom-up block copolymer patterns within or on top-down substrate patterns is the basis for so-called templated self-assembly processes, in which long-range order and orientation of microdomain patterns can be imposed by a template or guide . These top-down templates can take a variety of forms including periodic thickness profiles and chemically patterned surfaces. 

A graphoepitaxy method has been developed in which a topographic top-down defined pattern on a substrate is used to direct the epitaxial growth in an overlaying block copolymer bottom-up nanostructure by creating a periodic thickness profile (Fig. 5). Fasolka and coworkers [66] employed a faceted silicon substrate, which has sawtooth-profile corrugations in the nanometer... 

The use of block copolymers to form a variety of different nanosized periodic patterns continues to be an active area of research. Whether in bulk, thin film, or solution micelle states, block copolymers present seemingly unlimited opportunities for fabricating and patterning nanostructures. The wealth of microstructures and the tunability of structural dimensions make them a favorable choice for scientists in a variety of research fields. As reviewed here, they can function as nano devices themselves, or act as templates or scaffolds for the fabrication of functional nanopatterns composed of almost all types of materials. However, there are still two obvious areas which require more work control of the long-range 3D nanostructure via more user-friendly processes and the identification of new materials with different functional properties. 

Figure 14 Schematic representation of the microphase separation of block copolymers. The left graph shows atomic-scale details of the phase separation at intermediate temperatures, and the right graph shows a lamellar phase formed by block copolymers at low temperatures. The block copolymers have solid-like properties normal to the lamellae, because of a well-defined periodicity. In the other two directions, the system is isotropic and has fluid-like characteristics. From reference 54.

Block copolymers have peculiar characteristics due to the coexistence of two or several different parts of different chemical compositions within a chain. They can undergo microphase separation transitions from a homogenous phase to a variety of spatially periodic structures [176]. A distinction should be made between star copolymers, where each arm is composed by two or more blocks, and miktoarm polymers, formed by homopolymer arms of different chemical compositions. Floudas et al. [177] recently performed an extensive study of four-... 

By covalent linkage of different types of molecules it is possible to obtain materials with novel properties that are different from those of the parent compounds. Examples of such materials are block-copolymers, soaps, or lipids which can self-assemble into periodic geometries with long-range order. Due to their amphiphilic character, these molecules tend to micellize and to phase-separate on the nanometer scale. By this self-assembly process the fabrication of new na-noscopic devices is possible, such as the micellization of diblock-co-polymers for the organization of nanometer-sized particles of metals or semiconductors [72 - 74]. The micelle formation is a dynamic process, which depends on a number of factors like solvent, temperature, and concentration. Synthesis of micelles which are independent of all of these factors via appropriately functionalized dendrimers which form unimolecular micelles is a straightforward strategy. In... 

---