Block copolymers, self-assembling microphase separation

Recently, researchers paid more attention to the guided self-assembly of block copolymer thin films on a patterned surface. The patterned surface means the surface of a constrained situation is chemically or physically modified to form a pattern with specific property and size. A series of exquisite structures are found in the microphase separation of block copolymer under the patterned surface. In the theoretic work of Wu and Dzenis [43], they designed two kinds of patterned surface to direct the block copolymer self-assembly (Fig. 15.7). The self-assembled structures are found strongly influenced by the commensurability of polymer bulk period and pattern period. With mismatched patterns on two surfaces, both MC simulation [44] and SCFT researching [45] predicted the titled lamellae and perforated lamellae structures for symmetric diblock copolymers. Petrus et al. carried out a detailed investigation on the microphase separation of symmetric and asymmetric diblock copolymers confined between two planar surfaces using DPD simulation [46,47]. It is found that various nonbulk nanostructures can be fabricated by the nanopatterns on the surfaces. 

Beyond a characteristic temperature Todt [14,20], diblock copolymers self-assemble into microphase-separated morphologies. The theoretically predicted equilibriiun states of such ordered phases correspond to perfect superlattices with various symmetries depending on the volume ratio of both blocks. In particular, lamellar (synunetric diblocks), cylindrical (asymmetric diblocks), and spherical (strongly asynunetric diblocks) morphologies are expected and have been observed [2], For typical copolymers, the characteristic length scale of these microphases is in the range from about lOnm up to some 100 nm. It is justified to say that in the bulk, microphase-separated block-copolymers represent ordered nano-structures. 

Of these block copolymers, 42a was first synthesized by McCullough and coworkers [102]. We synthesized block copolymer 42b having both hydrophobic and hydrophilic side chains in each segment [155]. The thin films of 42c [157], 42e [158], and 42i [161] showed nanofiber structures after annealing, probably because of microphase separation of the crystalline P3HT segment and the other amorphous segment. The block copolymer 42d is a crystalline-crystalline diblock copolymer, self-assembled into crystalline nanowires in solution. In melt-phase assembly, a microphase-separated lamellar structure with two crystalline domains characteristic... 

It is well known that block copolymers and graft copolymers composed of incompatible sequences form the self-assemblies (the microphase separations). These morphologies of the microphase separation are governed by Molau s law [1] in the solid state. Nowadays, not only the three basic morphologies but also novel morphologies, such as ordered bicontinuous double diamond structure, are reported [2-6]. The applications of the microphase separation are also investigated [7-12]. As one of the applications of the microphase separation of AB diblock copolymers, it is possible to synthesize coreshell type polymer microspheres upon crosslinking the spherical microdomains [13-16]. 

Blockcopolymer microphase separation [9] Depending on the length of chemically different blocks of monomers in a block copolymer, ordered nanostructures can be obtained in bulk samples and thin films. The film morphology can differ significantly from the bulk morphology, but because the structure is determined by the pair-pair interaction of monomers and/or an interface, and it is a thermodynamically stable structure, it is classified as self-assembly. 

Keywords Self-Assembly Blend Block copolymer Microphase separation ... 

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],... 

Figure 6.7 Illustration of multipoint hydrogen bonding based self-assembly (a) hydrogen bond formation between barbituric acid functionalized gold nanoparticles and Hamilton receptor functionalized block copolymers and (b) selective deposition of nanoparticles on a microphase-separated block copolymer film. Reprinted with permission fi om Binder et al. (2005). Copyright 2005 American Chemical Society.

Figure 9.12 (a) AFM image of spin-cast 15d 16d from benzene showing cylindrical nanodomains from the microphase separation of the supramolecular block copolymer and (b) AFM image of spin-cast 15b 16b from benzene showing self-assembled fibers. 

As we have seen, the phase behaviour of block copolymers consisting of flexible polymer coils is remarkably rich. If one of the blocks is rigid, the copolymer would be expected to exhibit even more complex phase behaviour. For example, the rigid block could be mesogenic. This leads to the possibility of self-assembly of structures consisting of domains of liquid crystalline material within a microphase-separated block copolymer superstructure. Diblock copo-... 

Several additional phases are observed experimentally, but are not thermodynamically stable [13]. Moreover, the synthetic nature of the copolymers implies some heterogeneity in the polymer structure and molecular weight distribution. An excellent review has recently been published [97], and the main conclusion is that the polydispersity index (PDI) influences all aspects of the self-assembly. For example, upon an increase of the PDI of one block, the lattice constant of an ordered structure or the size of microphase-separated domains increases, interfacial thickness increases, and phase transitions may be induced. In addition, macrophase-separation may occur as the PDI is increased at certain compositions and segregation strengths. 

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... 

---