Advances in Nanostructured Carbons from Block Copolymers

Chapter 21

Advances in Nanostructured Carbons from Block Copolymers Prepared by Controlled Radical Polymerization Techniques 1

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Tomasz Kowalewski , Chuanbing Tang , Michal Kruk , Bruno Dufour , and Krzysztof Matyjaszewski Downloaded by UNIV OF ARIZONA on August 8, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch021

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Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213 Current address: Department of Chemistry, College of Staten Island, City University of New York, 2800 Victory Boulevard, Staten Island, NY 10314

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This chapter summarizes recent progress toward the preparation of nanostructured carbons from well-defined polyacrylonitrile (PAN) copolymers synthesized by controlled/living radical polymerization, particularly atom transfer radical polymerization (ATRP). Nanoscale PAN domains are formed via self-assembly driven by phase separation between PAN and a sacrificial block. PAN domains are then converted into carbon nanostructures by pyrolysis. This approach allows us to prepare thin film and bulk nanostructured carbon with different morphologies and considerable porosities. Control of the long range order in these materials was achieved through a directional casting technique, i.e., zone casting. Routes were developed to prepare highly porous silica and carbon via supramolecular templating using a water soluble PAN block copolymer and by grafting PAN from the surface of the porous silica template.

© 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

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Controlled/living radical polymerizations (CRPs) " based on a fast dynamic equilibrium between dormant and active radical species, are particularly suitable for assuring the relatively simple synthesis of numerous classes of well-defined block copolymers with predetermined molecular weights, narrow molecular weight distributions, and precisely controlled architectures. Herein we describe some recent advances in the application of these techniques to the synthesis of nanostructured carbons based on polyacrylonitrile (PAN) and its copolymers. Interest in nanostructured carbons is driven by their unique chemical, mechanical and electrical properties and their potential applications as gas storage media, catalyst supports, as well as components of nanocomposites, proximal probes, and field effect transistors. PAN represents a particularly interesting case of a segment in phaseseparated block copolymers. Due to the //rtramolecular dipole repulsion from the nitrile groups, PAN macromolecules have a tendency to adopt a stiff, rod-like, irregular helical structure. ' In addition, the wtermolecular dipole attraction packs the helices into laterally ordered domains or "paracrystals", " despite the lack of tacticity that is typically a prerequisite for polymer crystallizability. Much of the interest in PAN and its random copolymers is related to the possibility of their conversion into carbonaceous materials, which found a major application in the manufacturing of high-performance carbon fibers. Recently, we utilized well-established CRP techniques, " including atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP) and reversible addition-fragmentation chain transfer (RAFT), to prepare PAN diblock copolymers. The principles we developed in the synthesis of these PAN diblock copolymers have directed us to prepare other well-defined PAN copolymers with more complex architectures. We have then demonstrated that well-defined PAN block copolymers can be easily fabricated in the form of films, and therefore our route provides an excellent way to prepare nanostructured carbons for the use in thin-film-based devices. " This chapter will describe progress in the development of novel nanostructured materials based on these PAN copolymer systems, ranging from simple diblock copolymers to organic/inorganic hybrids. Well-defined character of copolymers that constitute these materials or serve as their precursors facilitates good control over their nanostructure and morphology. Further opportunities for control of these structures are arising from recent developments in the area of processing techniques, exemplified here by the directional casting technique known as zone casting.

Downloaded by UNIV OF ARIZONA on August 8, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch021

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Nanostructured Carbons from Polyacrylonitrile Block Copolymers One of the recent examples of novel nanostructured materials derived from block copolymers prepared by ATRP are carbons obtained by pyrolysis of block


297 copolymers containing PAN and a sacrificial block (e.g., poly(w-butyl acrylate, PB A) (Scheme l). ' Through self-assembly driven by phase separation between PAN and the sacrificial block, the PAN domains act as nanoscale precursors for the final carbon nanostructure. Upon simple thermal treatment (pyrolysis), PAN domains are converted into nanostructured carbon, whereas the sacrificial phase decomposes and burns away. Preservation of the original morphology of PAN domains is facilitated by thermal stabilization in the oxidative atmosphere, which is then followed by carbonization under inert gas flow at higher temperature. Based on differential scanning calorimetry, the thermal stabilization is completed before the onset of significant thermal decomposition of PBA, which promotes the preservation of nanostructures during thermal transformation. PAN copolymers used in our studies formed a range of "classical" morphologies (Figure 1) such as spherical, cylindrical and lamellar. Following thermal stabilization, these block copolymers were then converted by pyrolysis into carbon spheres,filaments,lamellae and more complex structures.


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Nanostructured Carbons from Block-Copolymer-Derived Precursors Containing a Thermally Stable Sacrificial Phase In order to achieve high porosities, we have designed a new system in which domains of sacrificial blocks of the copolymer are reinforced with inorganic material (see Scheme 2). At first, our approach seems to resemble the synthesis of mesoporous carbon through the inverse replication of mesoporous silica templates. " This established synthetic route usually involves: (i) the preparation of the mesoporous silica template (often using self-assembled organic molecules to template the mesopores; these organics are subsequently burned out or extracted), (ii) infiltration of the pores of the template with carbon precursors (organic compounds), (iii) carbonization of the precursors, and (iv) then removal of the silica template. In our case, steps (i) and (ii) are combined into a single step. It should be noted that another group followed a similar pathway in their attempt to carbonize the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymer surfactant after it has self-assembled with silica into an odered copolymer-silica composite. In the new template system we developed, the sacrificial block of the PAN copolymer is water soluble and the copolymer micelles are used as supramolecular templates for formation of nanostructured silica. Subsequently, the siliceous phase can act as a scaffold that supports the evolving carbon phase during pyrolysis. This scaffold is subsequently etched away, leaving behind the nanoporous carbon (Scheme 2). The synthesis of well-defined poly(ethylene oxide)-6-polyacrylonitrile (PEO-è-PAN) block copolymers was accomplished using ATRP. The PEO-623

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block copolymers consisting of immiscible segments...

—self-assemble into a variety of nanostructures dictated by the relative content of both blocks


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Scheme L Nanostructured carbon by pyrolysis ofselfassembled polyacrylonitrile block copolymer precursors. (Thephase diagram reproducedfrom Bates, F. S.; Fredrickson, G. Physics Today, 1999, 52, 32-38)



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Figure 1 Atomic force microscopy (AFM) height images ofPAN block copolymerfilms:a) spherical (after therma stabilization); c) cylindrical; e) lamellar. AFM height images of corresponding carbonfilmsafter pyrolysis: b) do d)filaments;f) sheets. (ReproducedfromReference 13. Copyright 2002 the American Chemical Society and Reference 20. Copyright 2003 with permissionfromSpringer-Verlag GmbH.)


In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006. 3

carbon nanorods

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nanoporous silica

873K, air '573 K, air 1073 K, N aq. NaOH

surface area 900 m g pore volume 1.9 cm g-

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Scheme 2. Synthesis of nanoporous silicas and carbonsfromself-assembled silica/PEO-b-PAN composites.

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Advances in Nanostructured Carbons from Block Copolymers

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