Self-assembly copolymers

The enthalpy effect of rod packing influences the microstructures at the air- water interface. The copolymer with the long rods, forms a cylindrical structure in the monolayer due to the bigger enthalpy decrease. Copolymer, with the middle-sized rods, will form micellar structure in the monolayer by self - assembly. Copolymer... 

In analogy to lipids, amphiphilic block copolymers, i.e., macromolecules composed of at least one hydrophilic and one hydrophobic, covalently linked, polymer chains can form in aqueous solutions vesicles the so-called polymersomes. Generally, in self-assembling copolymer solutions, a rich diversity of morphologies is possible. An overview of the various factors important for vesicle formation, including copolymer architecture, presence of additives, solvent composition, and temperature, is given in [19]. To illustrate polymersome structures we reproduce from [21] on the top row of Fig. 2 cryo-TEM images of vesicles formed by 1.0 wt % aqueous solution of PEO- -PBD (PEO, polyethylene oxide PBD, polybutadiene) diblock copolymer for three different sizes of the PEO and PBD blocks. 

Recently, bottom-up processes utilizing self-assembly have been developed that could become the next generation of nanostructure processing, as they overcome the problems listed above. In particular, self-assembling copolymers have attracted much attention as potential nanostructuring agents over the past decades and are still subject to ongoing research [3]. The most important near-term application is their use in the microelectronics industry as etch mask with pattern dimensions far below the optical diffraction limit [4]. 

Self-processes are inherent in the self-assembly of copolymers. These are composed of chemically or physically incompatible units along the same macromolecule, such as polar/hydrophobic monomers or rigid/flexible polymer segments, respectively. The most widely investigated self-assembling copolymers are amphiphilic hnear block copolymers. The chemical incompatibility between covalently linked hydrophobic and hydrophilic polymer segments drives the organization of the macromolecules into... 

The direct synthesis of functional, self-assembling copolymers in aqueous media under mild conditions without protection/deprotection chemistry steps, however, remains a current challenge that focuses research efforts on developing stimuli-responsive delivery systems such as micelles... 

A number of experimental fluorescence studies of specific self-assembling copolymer systems will be described in the next chapter. 

Yet another variant of self-assembly relies on the repulsion between blocks of suitably constituted block copolymers, leading to fine-scale patterns of organisation. One very recent description of this approach is by de Rosa et al. (2000). Details of this kind of approach as cultivated at Oak Ridge National Laboratory can also be found on the internet (ORNL 2000). 

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]. 

The distinctive properties of densely tethered chains were first noted by Alexander [7] in 1977. His theoretical analysis concerned the end-adsorption of terminally functionalized polymers on a flat surface. Further elaboration by de Gennes [8] and by Cantor [9] stressed the utility of tethered chains to the description of self-assembled block copolymers. The next important step was taken by Daoud and Cotton [10] in 1982 in a model for star polymers. This model generalizes the... 

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 ... 

AB diblock copolymers in the presence of a selective surface can form an adsorbed layer, which is a planar form of aggregation or self-assembly. This is very useful in the manipulation of the surface properties of solid surfaces, especially those that are employed in liquid media. Several situations have been studied both theoretically and experimentally, among them the case of a selective surface but a nonselective solvent [75] which results in swelling of both the anchor and the buoy layers. However, we concentrate on the situation most closely related to the micelle conditions just discussed, namely, adsorption from a selective solvent. Our theoretical discussion is adapted and abbreviated from that of Marques et al. [76], who considered many features not discussed here. They began their analysis from the grand canonical free energy of a block copolymer layer in equilibrium with a reservoir containing soluble block copolymer at chemical potential peK. They also considered the possible effects of micellization in solution on the adsorption process [61]. We assume in this presentation that the anchor layer is in a solvent-free, melt state above Tg. The anchor layer is assumed to be thin and smooth, with a sharp interface between it and the solvent swollen buoy layer. 

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. 

Minich E.A., Nowak A.P., Deming T.J., and Pochan, D.J. Rod-rod and rod-coil self-assembly and phase behavior of pol3fpeptide diblock copolymers. Polymer, 45, 1951, 2004. 

Bates F.S. and Fredrickson G.H., Block copolymers-designer soft materials, Phys. Today, 52, 32, 1999. Alexandridis P. and Lindman B. (eds.). Amphiphilic Block Copolymers Self-Assembly and Applications, Elsevier, Amsterdam, 2000. 

Jeoung E., Galow T.H., Schotter J., Bal M., Ursache A., Tuominen M.T., Stafford C.M., Russell T.P., and Rotello V.M. Fabrication and characterization of naoelectrode arrays formes via block copolymers self-assembly, Langmuir, 17, 6396, 2001. 

Tian, L. and Hammond, P.T. Comb-dendritic block copolymers as tree-shaped macromolecular amphi-philes for nanoparticle self-assembly, Chem. Mater., 18, 3976, 2006. 

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