Block copolymers, self-assembling limit

The Alexander approach can also be applied to discover useful information in melts, such as the block copolymer microphases of Fig. 1D. In this situation the density of chains tethered to the interface is not arbitrary but is dictated by the equilibrium condition of the self-assembly process. In a melt, the chains must fill space at constant density within a single microphase and, in the case of block copolymers, minimize contacts between unlike monomers. A sharp interface results in this limit. The interaction energy per chain can then be related to the energy of this interface and written rather simply as Fin, = ykT(N/Lg), where ykT is the interfacial energy per unit area, q is the number density of chain segments and the term in parentheses is the reciprocal of the number of chains per unit area [49, 50]. The total energy per chain is then ... 

Tailoring block copolymers with three or more distinct type of blocks creates more exciting possibilities of exquisite self-assembly. The possible combination of block sequence, composition, and block molecular weight provides an enormous space for the creation of new morphologies. In multiblock copolymer with selective solvents, the dramatic expansion of parameter space poses both experimental and theoretical challenges. However, there has been very limited systematic research on the phase behavior of triblock copolymers and triblock copolymer-containing selective solvents. In the future an important aspect in the fabrication of nanomaterials by bottom-up approach would be to understand, control, and manipulate the self-assembly of phase-segregated system and to know how the selective solvent present affects the phase behavior and structure offered by amphiphilic block copolymers. 

Block copolymers have been the focus of much interest during the last 30 years because their constituent blocks are generally immiscible, leading to a microphase separation. Since the different blocks are linked together by covalent bonds, the microphase separation is spatially limited and results in self-assembled structures whose characteristic sizes are of the order of a few times the radius of gyration, Rg, of the constituent blocks and thus range from ca. 10 to 100 nm [1],... 

The examples discussed above illustrate the importance of block copolymer chain segment incompatibilities for the phase separation of bulk materials, combined with the ability to perform chemistry within specific nanoscale domains to impose permanence upon those self-assembled nanostructured morphologies. Each is limited, however, to crosslinking of internal domains within the solid-state assemblies in order to create discrete nanoscale objects. To advance the level of control over regioselective crosslinking and offer methodologies that allow for the production of additional unique nanostructured materials, the pre-assembled structures can be produced in solution (Figure 6.4), as isolated islands with reactivity allowed either internally or on the external... 

The properties of ordered structures in block copolymer melts have yet to be fully exploited, but the structural and rheological anisotropy is likely to lead to applications not all of which can be envisaged yet. The precision self-assembly of block copolymers into ordered structures for thin film and interfacial applications has enormous potential. Other applications such as nanoscale templates, membranes and filters could exploit the self-assembly of block copolymers into domains with periods 10-100 nm. The possibilities are limited only by the molecular engineer s imagination. 

With recent discoveries in the self-assembly of block copolymers, a large step is now being taken towards industrial applications. However, the spontaneous process of microphase separation leads to the formation of polycrystalline microdomain arrays consisting of randomly oriented regions, which limits the potential... 

The synthesis of polypeptide hybrid block copolymers is an area that has been under study for three decades. Initially, this field suffered from limitations in the synthesis of the polypeptide components that required excessive sample purification and fractionation to obtain well-defined copolymers. In recent years, vast improvements in NCA polymerizations now allow the synthesis of hybrid block copolymers of controlled dimensions (molecular weight, sequence, composition, and molecular weight distribution). Such well-defined materials will greatly assist in the identification of new self-assembled structures possible using ordered polypeptide segments, as well as yield new materials with a wide range of tunable properties. 

Because of the softness of interactions in block copolymers (here we restrict our consideration to flexible molten blocks above the glass transition temperature), thermal fluctuation in these systems is expected to be significant, especially near the order-disorder transition temperatures (Fredrickson and Helfand, 1987). In addition, the long relaxation times, due to the slowness of the motion of polymers, often lead to metastable and other kinetic states. Thus, full understanding of the self-assembly in block copolymers requires understanding of the nature of fluctuation, metastability, and kinetic pathways for various transitions. Most of this article is focused on theoretical studies of these issues in the simpler AB block copolymers. A key concept that emerges from these studies is the concept of anisotropic fluctuations first, these fluctuations determine the stability limit of an ordered phase second, they are responsible for the emergence of new structures... 

Self-assembly in RTILs is not limited to low molar mass surfactants. For example, He et al. [38 ] have reported about spherical micelles in mixtures of poly( (1,2 -butadiene) -block-ethylene oxide) (PB-PEO) diblock copolymers in the RTIL l-butyl-3-methylimidazolium... 

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