Solubilization of Polyelectrolytic Hairy-Rod Polyfluorene in Aqueous Solutions of Nonionic Surfactant

Abstract

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 (C12E5) at 20 °C. We show in particular how a high amount (milligrams per milliliter) of polyfluorene can be solubilized by aqueous C12E5 via aggregate formation. The PBS−PFP and C12E5 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 D2O−PBS−PFP(C12E5)x realizes three phase regimes with an increasing molar ratio of surfactant over monomer unit (x). First, for x ≤ 0.5, the mixture is cloudy. In this regime polymer is only partially dissolved. Second, for 1 ≤ x ≤ 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 ≥ 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 C12E5 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.