Polymer Communication Controlling the phase behavior of block copolymers via sequential block growth Arif O. Gozen a , Michelle K. Gaines b, 1 , Mark W. Hamersky c , Panagiotis Maniadis d , Kim Ø. Rasmussen d , Steven D. Smith c , Richard J. Spontak a, b, * a Department of Chemical and Biomolecular Engineering, North Carolina State University, Raleigh, NC 27695, USA b Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695, USA c Miami Valley Innovation Center, The Procter & Gamble Company, Cincinnati, OH 45061, USA d Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA article info Article history: Received 27 July 2010 Received in revised form 29 August 2010 Accepted 3 September 2010 Available online 15 September 2010 Keywords: Block copolymer Phase behavior Mean-field theory abstract Block copolymers remain one of the most extensively investigated classes of polymers due to their abilities to self-organize into various nanostructures and modify polymer/polymer interfaces. Despite fundamental and technological interest in these materials, only a handful of experimental phase diagrams exist due to the laborious task of preparing such diagrams. In this work, two copolymer series are each synthesized from a single macromolecule via sequential living anionic polymerization to yield molecularly asymmetric diblock and triblock copolymers systematically varying in composition. The phase behavior and morphology of these copolymers are experimentally interrogated and quantitatively compared with predictions from mean-field theories, which probe copolymer phase behavior beyond current experimental conditions. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Block copolymers, macromolecules composed of two or more chemically distinct species arranged in long, contiguous sequences that are covalently linked together, remain at the forefront of soft materials research due to their unique surfactant-like attributes [1,2]. Under favorable conditions, they can (i) self-organize into a wide variety of (a)periodic nanostructures and (ii) reduce inter- facial tension along, and thus compatibilize, polymer/polymer interfaces. For these reasons, block copolymers are ubiquitous in a wide range of (nano)technologies that require multifunctionality and/or nanoscale structuring from an organic material [2e6]. The design of systems requiring the use of block copolymers necessarily demands an understanding of the phase behavior of these macro- molecules, and numerous endeavors [7] have sought to provide theoretical frameworks by which to predict block copolymer phase behavior in the absence of scarce experimental phase diagrams [8e10]. Unlike conventional phase diagrams of polymer blends wherein two homopolymers are physically combined, block copolymer phase diagrams require synthesis of a new copolymer with specific block lengths for each composition. Block copolymer phase diagrams are often expressed in a mean-field format with the thermodynamic incompatibility (cN), where c denotes the FloryeHuggins interaction parameter that scales as reciprocal temperature, and N is the number of repeat units along the copolymer backbone. To help overcome the challenges of synthesizing different copolymers to explore, in experimental fashion, the phase behavior of block copolymers, we previously introduced [11] the idea of sequential living anionic polymerization to generate a series of copolymers systematically varying in composition from a single parent macromolecule. The initial objective of this approach was to discern the molecular conditions at which an AB diblock copol- ymer, which consists of two endblocks (or “tails”) constrained by a single, shared junction, began to exhibit the phase and mechan- ical properties more commonly associated with those of an ABA triblock copolymer capable of forming a molecular network by means of midblock bridging [12]. Prior results demonstrated [11,13,14] that, when the second A endblock was sufficiently short, it remained mixed in the B matrix even after microphase separation of the (first) A and B blocks and furthermore served to reduce the interblock incompatibility. As the second A endblock was grown, however, it likewise microphase-separated from the B midblock and co-located with the first A endblock, thereby forcing the mid- block to adopt a bridged or looped conformation (excluding the ill- favored formation of dangling ends [15]). To differentiate the two * Corresponding author. Tel.: þ1 919 417 3554; fax: þ1 919 515 3465. E-mail address: rich_spontak@ncsu.edu (R.J. Spontak). 1 Present address: Electro-Optical Systems Laboratory, Georgia Tech Research Institute, Atlanta, GA 30332, USA. Contents lists available at ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2010.09.006 Polymer 51 (2010) 5304e5308