Templating Conducting Polymers via Self-Assembly of Block Copolymers and Supramolecular Recognition Lynne A. McCullough, Bruno Dufour, Chuanbing Tang, ² Rui Zhang, Tomasz Kowalewski, and Krzysztof Matyjaszewski* Department of Chemistry, Carnegie Mellon UniVersity, Pittsburgh, PennsylVania 15232 ReceiVed August 31, 2007 The success of conjugated polymers as components in anticorrosion coatings, charge-injection layers in organic light- emitting diodes, electromagnetic shielding, plastic circuitry, and biosensors, among others, is due to their high conductivity, low density, and reasonably high processability. 1 Polyaniline (PANI) is particularly attractive because of its easy synthesis, low cost, and high degree of environmental stability in both the doped and undoped states. One drawback is its low solubility, caused by π-stacking of the highly conjugated backbone, resulting in low processability of the final materials. Replacing mineral acid dopants such as HCl or H 2 SO 4 with functionalized organic acids such as 10-camphorsulfonic acid (CSA) and 2-acrylamido-2- methyl-1-propanesulfonic acid (AMPSA) 2 increases the overall solubility by way of the large organic groups attached to the acidic moiety. Doping PANI with CSA and processing from a polar solvent such as m-cresol has resulted in films with conductivities as high as 400 S/cm. 3 Other sulfonic acids are effective as dopants, including polyelectrolytes such as poly- styrenesulfonic acid, 4-6 diesters of sulfophthallic acid, 7 or sulfosuccinic acid, 8 particularly in association with polar solvents such as m-cresol or 2,2-dichloroacetic acid (DCAA). A more difficult problem to address is the highly brittle nature of PANI. One solution to this problem is preparing block copolymers with a conducting polymer segment and a low-T g amorphous polymer segment. Such attempts are most successful with systems like poly(3-hexylthiophene), where the conducting polymer can be functionalized to provide end groups suitable for block extension. 9 These systems have been extended to PANI, though the conductivity of such materials is quite low (10 -5 S/cm). 10,11 Another approach, even more applicable to PANI, involves blending the conjugated polymer with matrix polymers exhibiting the desired mechanical properties. 12 Matrix polymers can include polypropylene, 13 poly(methyl methacry- late), 14 polyurethane, 15 cyanoresins, 16 and poly(butyl methacry- late). 17 However, all these matrices are insulating, and simple blending does not allow the degree of morphology control which would ensure the continuity of the PANI phase necessary for good conductivity. Replacing these homopolymers with block copolymers containing one segment that exhibits an affinity for PANI creates a self-assembling phase-separated system that can serve as a template. The block copolymer composition can be manipulated to control the connectivity of the PANI-containing phase, though conductivity remains low (<1 S/cm) for the styrene-butadiene-styrene system, a result of the weak prefer- ence of PANI for styrene. 18 Herein we report a new approach to the preparation of templated flexible conducting polymer materials which employs block copolymers with both a soft hydrophobic segment and a segment containing a strongly acidic dopant moiety (i.e., poly- (2-acrylamido-2-methyl-1-propanesulfonic acid) (PAMPSA)) that actively interacts with polyaniline or oligoaniline, forming a dopant/conducting polymer complex. Acid-base interaction of PAMPSA with the imine group of the emeraldine base (EB) form of polyaniline, in combination with phase separation between incompatible segments, drives segregation of the conducting polymer into the hydrophilic phase and formation of morphologies assuring high conductivity. Block copolymers were synthesized by a reversible addition fragmentation chain transfer (RAFT) 19-23 process, using cumyl dithiobenzoate (CDB) and AIBN in methanol to prepare PAMPSA macroini- tiators, as polymerizations in methanol were consistently better- controlled than those in DMF. These PAMPSA macroinitiators were then extended with methyl acrylate at 60 °C, as shown in Scheme 1. Chain extension allowed control of the composition and structures of the block copolymers, particularly the forma- tion of a system with the hydrophilic block as a continuous matrix. The use of cumyl dithiobenzoate as the RAFT agent resulted in PAMPSA macroinitiators with relatively low polydispersities (∼1.2) and controlled molecular weight, ranging from ∼4800 to ∼36 000 g/mol, as determined by 1 H NMR end-group analysis (Figure S1 of the Supporting Information). Gel permeation chromatography (GPC) traces were monomodal and cleanly shifted to higher molecular weight with the progress of polymerization (Figure S2). Block extension with acrylate monomers yielded block copolymers with monomodal GPC traces and controlled composition (PAMPSA weight fraction in the range of 10-50 wt % as determined by elemental analysis) and polydispersities below 1.4. These polymers were well-soluble in DCAA, allowing for the preparation of homo- geneous solutions, which significantly facilitated the incorpora- tion of the conducting aniline polymers and oligomers. The basic structure of PANI is a four-ring tetramer which, when it contains two imine and two amine groups, is known as emeraldine base (EB). Each imine nitrogen in this structure is a potential doping site for the acid; thus, the maximum effective dopant concentration is one sulfonic acid group for every two nitrogen atoms. As shown in Scheme 2, the complexes between PANI and PAMPSA block copolymer template were prepared in DCAA by mixing 1 wt % solution of PANI predoped with AMPSA monomer with 5 wt % solution of copolymer. The two solutions were mixed with different doping ratios between PAMPSA and the polyaniline (1:2, 1:4, 1:8, etc.) and were subsequently cast onto precleaned flexible polyethylene sub- strates and dried under vacuum at 75 °C for 24 h. While tetrameric aniline is soluble in DCAA, undoped PANI is not. Direct addition of PANI allowed only small fractions of conducting polymer to complex with PAMPSA: about 1.2 wt % with respect to the PAMPSA block. However, it is well- known that AMPSA monomer-doped PANI exhibits good solubility in DCAA. 2 Therefore, in order to incorporate higher fractions of PANI and obtain good conductivity of the final material, AMPSA-doped PANI was mixed with PAMPSA-b- PMA block copolymers to produce a homogeneous solution. After casting the complexes onto substrates and evaporating the solvent, flexible films with a thickness ranging from 30 to 50 ² Current address: Materials Research Laboratory, University of Cali- fornia, Santa Barbara, CA 93106. 7745 Macromolecules 2007, 40, 7745-7747 10.1021/ma071172k CCC: $37.00 © 2007 American Chemical Society Published on Web 10/04/2007