Solubilization of Polyelectrolytic Hairy-Rod Polyfluorene in Aqueous Solutions of Nonionic Surfactant Matti Knaapila,* ,² La ´ szlo ´ Alma ´ sy, ‡ Vasil M. Garamus, § Christopher Pearson, ⊥ Swapna Pradhan, | Michael C. Petty, ⊥ Ullrich Scherf, | Hugh D. Burrows, # and Andrew P. Monkman ² Department of Physics, UniVersity of Durham, South Road, Durham DH1 3LE, United Kingdom, Research Institute for Solid State Physics and Optics, P.O. Box 49, Budapest-1525, Hungary, GKSS Research Centre, Max-Planck-Strasse 1, D-21502 Geesthacht, Germany, School of Engineering, UniVersity of Durham, South Road, Durham DH1 3LE, United Kingdom, Makromolekulare Chemie und Institut fu ¨r Polymertechnologie, Gauss-Str. 20, Bergische UniVersita ¨t Wuppertal, D-42097 Wuppertal, Germany, and Departamento de Quı ´mica, UniVersidade de Coimbra, P-3004-535 Coimbra, Portugal ReceiVed: October 22, 2005; In Final Form: March 24, 2006 We report on the solubilization, phase behavior, and self-organized colloidal structure of a ternary water- polyfluorene-surfactant (amphiphile) system comprised of polyelectrolytic poly{1,4-phenylene[9,9-bis(4- phenoxybutylsulfonate)]fluorene-2,7-diyl} (PBS-PFP) in nonionic pentaethylene glycol monododecyl ether (C 12 E 5 ) at 20 °C. We show in particular how a high amount (milligrams per milliliter) of polyfluorene can be solubilized by aqueous C 12 E 5 via aggregate formation. The PBS-PFP and C 12 E 5 concentrations of 0.31 × 10 -4 -5 × 10 -4 M and 2.5 × 10 -4 -75 × 10 -4 M, respectively, were used. Under the studied conditions, the photoluminescence (PL), surface tension, static contact angle, and (π-A) isotherm measurements imply that D 2 O-PBS-PFP(C 12 E 5 ) x realizes three phase regimes with an increasing molar ratio of surfactant over monomer unit (x). First, for x e 0.5, the mixture is cloudy. In this regime polymer is only partially dissolved. Second, for 1 e x e 2, the solution is homogeneous. In this regime polymer is dissolved down to the colloidal level. Small-angle neutron scattering (SANS) patterns indicate rigid elongated (polymer-surfactant) aggregates with a diameter of 30 Å and mean length of ∼900 Å. The ratio between contour length and persistence length is less than 3. Third, for x g 4, the solution is homogeneous and there is cooperative binding between polymer and surfactant. Surface tension, contact angle, and surface pressure remain essentially constant with increasing x. A PL spectrum characteristic of single separated polyfluorene molecules is observed. SANS curves show an interference maximum at q ∼ 0.015 Å -1 , indicating an ordered phase. This ordering is suggested to be due to the electrostatic repulsion between polymer molecules adsorbed on or incorporated into the C 12 E 5 aggregates (micelles). On dilution the distance between micelles increases via 3-dimensional packing. In this regime the polymer is potentially dissolved down to the molecular level. We show further that the aggregates (x ) 2) form a floating layer at the air-water interface and can be transferred onto hydrophilic substrates. 1. Introduction There are myriad reasons to develop water soluble π-conju- gated polymers and tailor their phase behavior, structure, and supramolecules in water. Water soluble π-conjugated polymers form an option for interrogating biological substrates. 1,2 The inkjet process and layer-by-layer self-assemblysthe major advantages of polymers over oligomers in light emitting diode (LED) fabricationsbenefit from electroluminescent materials in nonnoxious solvent, harmless for the cartridges. 3 A tuning of the air-water interface of π-conjugated polymers 4 is a base of Langmuir-Blodgett (LB) films 5 having opportunities in sensors. 6 Dissolving π-conjugated “model” polymers in water can also promote general understanding of the self-assembly of π-conjugated hairy-rod polymers. 7 Control of the solubility of π-conjugated polymers is a nontrivial task. Few such polymers can be dissolved in water. A typical strategy to achieve water solution is to introduce neutral or charged hydrophilic functionalities to the terminal position of the polymer backbone. Examples include cationic 8,9 and sulfonated 10 poly(p-phenylene) (PPP) and sulfonated poly- (p-phenylenevinylene) (PPV), poly[5-methoxy-2-(4-sulfoxybu- toxy)-1,4-phenylene-vinylene] (MBL-PPV). 11 Water soluble polyfluorenes (PFs) used in DNA peptide nucleic acid detec- tion 12 belong to this class of materials. However, the water solubility of PFs is difficult to achieve in high concentrations and cosolvents such as methanol are employed to ensure the uniformity of the solutions. 13 Another strategy to assist solubility of π-conjugated polymers is a surfactant layer separating polymer and solvent. The interaction between polymer and surfactant can be due to either hydrophobic-hydrophilic ef- fects 14 or strong physical bonds. 15 Examples include polyelec- trolytic PPV 16 and PF. 14,15,17 This technique can also be used to induce liquid crystallinity as shown for PPP 18,19 and poly- (p-pyridine). 20 * Address correspondence to this author. Phone: +44-191-33-43558. Fax: +44-191-33-43585. E-mail: matti.knaapila@durham.ac.uk. ² Department of Physics, University of Durham. ‡ Research Institute for Solid State Physics and Optics. § GKSS Research Centre. ⊥ School of Engineering, University of Durham. | Bergische Universita ¨t Wuppertal. # Departamento de Quı ´mica, Universidade de Coimbra. 10248 J. Phys. Chem. B 2006, 110, 10248-10257 10.1021/jp0560563 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/06/2006