Microphase separation of block copolymers

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.

Albalak RJ, Thomas EE (1993) Microphase separation of block copolymer solutions in a flow field. J Polym Sci B Polym Phys 31(1) 37—46... 

Gurovich E (1994) On microphase separation of block-copolymers in an electric-field - 4 universal classes. Macromolecules 27(25) 7339-7362... 

In their statistical model for microphase separation of block copolymers, Leary and Williams (43) proposed the concept of a separation temperature Ts. It is defined as the temperature at which a first-order transition occurs when the domain structure is at equilibrium with a homogeneous melt, i.e.,... 

The microphase separation of block copolymers has motivated many computer simulational studies, see [8,9,15,31,58]. As a key result of these studies deviations from mean field theory [20] have been observed early [9j. 

The microphase separation of block copolymers is sometimes called order-disorder transition (ODT). The self-consistent-field theory (SCFT) provides a mean-field method to calculate various geometric shapes of microdomains. Edwards first introduced the SCFT into polymer systems on the basis of making path integrals along chain conformations (Edwards 1965). Helfand applied it to the mean-field description of immiscible polymer blends on the basis of the Gaussian-chain model... 

Gurovich, E., On microphase separation of block copolymers in an electric field four universal classes. Macromolecules, 27, 7339-7362 (1994) Gurovich, E., Copolymers under a monomer orienting field. Macromolecules, 27, 7063-7066 (1994) Gurovich, E., Why does an electric field align structures in copolymers Phys. Rev, Lett., 74, 482-485 (1995). 

This contribution does also not include physical almost reversible crosslinking due to microphase separation of block copolymers, to strong hydrogen bonding or to ionic interactions or to crystallite formation. 

The behavior of ternary polymer mixtures containing a diblock copolymer with homopolymer and toluene as a function of mixture composition and temperature were investigated to obtain experimental phase diagram for solvent/copolymer/ homopolymer mixture. In order to avoid the complications associated with the microphase separation of block copolymers, the molar mass of block copolymer was kept low in our experiment (Madbouly Wolf, 2002). 

Kadota, S., Aoki, K., Nagano, S. and Seki, T. (2005) Photocontrolled microphase separation of block copolymers in two dimensions , J. Amer. Chem. Soc., 127, 8266-8267. 

The crystalline homopolymers spatially confined in nanocylinders can be prepared using the microphase separation of block copolymers, followed by the photocleavage of block junctions (Fig. 10.4), where o-nitrobenzyl (ONB) groups inserted between different blocks are conveniently used for the photocleavage reaetion [48]. Nojima et al. [44—47] synthesized poly(e-eaprolaetone)-WocA -polystyrene (PCL-ft-PS) diblock copolymers with ONB between PCL and... 

PS blocks. After forming nanocylinders by the microphase separation of block copolymers (Fig. 10.4a), ONB was cleaved using UV irradiation to get PCL homopolymers confined in nanocyhnders surrounded with PS matrices (Fig. 10.4b), where the vitrification of PS blocks prevented macrophase separation between PCL and PS homopolymers after the photocleavage. As a result, they obtained crystalline PCL blocks and PCL homopolymers both confined in identical nanocylinders with varying diameters. Therefore, it was possible to evaluate the difference in crystallization behavior and crystalline morphology between PCL blocks and PCL homopolymers, and also to clarify the characteristics of homopolymer crystallization spatially confined in nanocylinders. 

The CG method not only provides general models for studying a class of block copolymers but also conducts efficient algorithms for simulation. In this chapter, we overview the theoretical and computational approaches toward the simulations of dynamics of microphase separation of block copolymers with the focus on the recent contributions applying Monte Carlo (MC), dissipation particle dynamics... 

Generally, at molecular level, the off-lattice MC simulation considers more detail than lattice MC simulation does, such as bond angle, bond length, and chain flexibility. However, the researchers pay more attention to the morphology than to the molecular details in the field of microphase separation of block copolymers. Furthermore, the lattice MC simulation runs much faster than the off-lattice MC simulation does. Therefore, many MC simulations employed the lattice model to explore the microphase separation of block copolymers. 

The microphase separation of block copolymers in a constrained situation has attracted interest both from experiments and theories. There are several important characters to make this system favorable in fabricating nanoscale features. Firstly, the constrained situation helps confine the dimension of microphase separation into microscale or nanoscale, so the materials formed in such a system spontaneously possess microstructures. Secondly, the constrained situation can suppress the fluctuation of phase separation and help create an ordered pattern at large scale. Thirdly, the surface of the constrained situation can be deliberately modified to direct the microphase separation of block copolymers with attention to make nonbulk structures. 

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. 

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