Block copolymers thin film structures

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... 

Fig. 10 Schematic representation of the nanoreplication processes from block copolymers, a Growth of high-density nanowires from a nanoporous block copolymer thin film. An asymmetric PS-fc-PMMA diblock copolymer was aligned to form vertical PMMA cylinders under an electric field. After removal of the PMMA minor component, a nanoporous film is formed. By electrodeposition, an array of nanowires can be replicated in the porous template (adapted from [43]). b Hexagonally packed array of aluminum caps generated from rod-coil microporous structures. Deposition of aluminum was achieved on the photooxidized area of the rod-coil honeycomb structure (Taken from [35])...

Lee B, Park I et al (2005) Structural analysis of block copolymer thin films with grazing incidence small-angle X-ray scattering. Macromolecules 38 4311 —4323... 

Watanabe K, Yoshida H, Kamata K, lyoda T. 2005. Direct TEM observation of perpendicularly oriented nanocylinder structure in amphiphilic liquid crystalline block copolymer thin films. Trans Mater Res Soc Jpn 30 377 381. 

In 2007, a team led by Professor E. Thomas of Massachusetts Institute of Technology (MIT) developed a smart gel based on the cephalopod s skin structure. The team used a self-assembling block copolymer thin film made from layers of polystyrene and poly-2-vinyl-pyridine. The thickness of the layers controls the refractive indices and thus the color of the reflected light. The poly-2-vinyl-pyridine layer is designed to alter its thickness in response to stimuli such as pH and salt concentration thus changing the gel s color. 

Polyferrocenylsilane block copolymers in which the blocks are immiscible (which is generally the case) would be expected to self-assemble to form phase-separated organometallic domains in the solid state. Based on the classical behavior of organic block copolymers, thin films of polyferrocene diblock copolymers would be expected to form domains such as spheres, cylinders, double diamonds (or gyro-ids) (or their antistructures), or lamellae (Chapter 1, Section 1.2.5). The preferred domain structure would be expected to be controlled by the ratio of the blocks, their degree of immiscibility (as defined by the Flory-Hu ins interaction parameter x), and the overall molecular weight of the block copolymer [159]. 

Figure 23.10 SEM images of nanosheet structures that appeared after the CO2 process. Depending on the initial morphology, the films of the same block copolymers show different nanosheet structures. Cross-sectional views of nanosheet thin films cast from (a) TFT and (b) HFB after the CO2 process at a saturation pressure of 8 MPa followed by scratching at liquid nitrogen temperature. The bars indicate 100 nm. (Reprinted with permission from H. Yokoyama, L. Li, C. Dutriez et ai, Horizontally and vertically aligned polymeric nanosheets C02-induced morphological changes of block copolymer thin films, Macromolecules, 41, 8626-8631, 2008. 2008 American Chemical Society.)...

Sivaniah, E., Hayashi, Y. et al. (2005) Sytrrmetric diblock copolymer thin films on rough substrates. Kinetics and structure formation in pure block copolymer thin films. Macromolecules, 38(5), 1837-1849. 

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. 

Foster M D, Sikka M, Singh N, Bates F S, Satija S K and Majkrzak C F (1992) Structure of symmetric polyolefin block copolymer thin films, J Chem Phys 96 8605-8615. 

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. 

She et al. have proposed the use of PS-PLA block copolymers to form structured thin films via self-assembly [170]. By this method, porous PS membranes could be obtained after the degradation of the PLA blocks. This example Illustrates that progress in the field... 

Segura el al. combines Tarazona s WDA DFT for hard-spheres with Wertheim s thermodynamic perturbation theory and has been used in a number of studies of associating fluids in pores and with functionalized walls in the limit of complete association a DFT for polymeric fluids is obtained in this method. Based on these works, Chapman and co-workers have presented the interfacial-SAFT (iSAFT) equation, which is a DFT for polyatomic fluids formulated by considering the polyatomic system as a mixture of associating atomic fluids in the limit of complete association this approach allows the study of the microstructure of chain fluids. Interfacial phenomena in complex mixtures with structured phases, including lipids near surfaces, model lipid bilayers, copolymer thin films and di-block copolymers, have all been studied with the iSAFT approach. 

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. 

Further modification of the above nanostructures is useful for obtaining new functional materials. Thirdly, we apply the dopant-induced laser ablation technique to site-selectively doped thin diblock copolymer films with spheres (sea-island), cylinders (hole-network), and wormlike structures on the nanoscale [19, 20]. When the dye-doped component parts are ablated away by laser light, the films are modified selectively. Concerning the laser ablation of diblock copolymer films, Lengl et al. carried out the excimer laser ablation of diblock copolymer monolayer films, forming spherical micelles loaded with an Au salt to obtain metallic Au nanodots [21]. They used the laser ablation to remove the polymer matrix. In our experiment, however, the laser ablation is used to remove one component of block copolymers. Thereby, we can expect to obtain new functional materials with novel nanostmctures. 

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