Chapter 21
Controlled Pattern Formation in Some Block Copolymer Systems Yoav Tsori
Downloaded by CORNELL UNIV on June 13, 2017 | http://pubs.acs.org Publication Date: November 18, 2003 | doi: 10.1021/bk-2004-0869.ch021
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We discuss several mechanisms useful in control of pattern formation in copolymeric systems. Chemically patterned substrate is shown to induce ordering in confined diblock copolymer melt. The strength and range of this ordering depend on interfacial interactions and surface feature size. Electric field is effective in aligning a sample in a desired direction by the "dielectric mechanism". We explain how this effect can be exploited in certain situations. We consider the new effect of dissociated mobile ions in a melt in electric field. Orienting forces in these non-equilibrium systems are calculated and are found to be important to alignment. Some morphological changes predicted to occur are illustrated.
Many systems in nature show fascinating and complicated pattern formation. Example include diblock copolymer melts (1,2), mixtures of diblock and homopolymers, aqueous solutions of lipids or surfactants, Langmuir monolayers, magnetic garnet films (5), chemical reactions (4), biological systems (5) and convective rolls in heated water (6). The modulated phases in many of these systems result from a competition between short- and long-range forces. Here we consider as a model system confined block copolymers (BCP) with or without electric field. There is a similarity between morphologies in BCPs and the Turing patterns observed in inorganic acidic chlorite-iodide-malonic reactions
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© 2004 American Chemical Society
Pojman and Tran-Cong-Miyata; Nonlinear Dynamics in Polymeric Systems ACS Symposium Series; American Chemical Society: Washington, DC, 2003.
Downloaded by CORNELL UNIV on June 13, 2017 | http://pubs.acs.org Publication Date: November 18, 2003 | doi: 10.1021/bk-2004-0869.ch021
265 (4). In the latter there is long-range inhibition and short-range activation. In the BCP system the competition is between chain stretching and short-range repulsion. BCPs are composed of two or more chemically distinct chains, or blocks, joined by a covalent bond. In the low temperature regime, the macro phaseseparation occurring in most polymers is inhibited because of chain connectivity, giving rise to mesoscopically ordered phases depending on chain length, architecture, etc. The most simple BCP is a linear diblock copolymer comprised of two connected linear chains. The equilibrium phase diagram has been extensively studied and found to consist of a disordered phase (two chains mixed) in the high temperature regime, and Lamella, Cylinder, Gyroid and Sphere phases below the critical temperature. In the next section we discuss first equilibrium systems, and show how morphologies are affected when the melt is put in contact with confining surfaces. Intrinsically non-equilibrium systems in electric fields are considered next, and orientation by electric field is explained. We show that the presence of dissociated ions in the melt can lead to strong aligning forces (accompanied by heating) in AC electric field.
Surface Induced Ordering in a B C P Melt Above O D T
The A/B diblock copolymer melt is characterized by two parameters: f-Nj^N and χΝ, where N is the number of A monomers in a chain of N=N +N monomers, and χ~1/Τ is the so-called Flory parameter characterizing the repulsion between the chains. The bulkfreeenergy of the system A
F„/kT =
jji
A
+h(q
0, Β monomers are preferred; A monomers are preferred if σ
