Nanostructuring of the conjugated polyelectrolyte poly[9,9-bis(4-sulfonylbutoxyphenyl)fluorene-2,7-diyl-2,2′-bithiophene] in liquid crystalline C12E4 in bulk water and aligned thin films

Abstract

We report on the conjugated polyelectrolyte 12 mM poly[9,9-bis(4-sulfonylbutoxyphenyl) fluorene-2,7-diyl-2,2'-bithiophene] (PBS-PF2T) mixed in concentrated aqueous 680 mM tetraethylene glycol monododecyl ether (C12E4) in bulk and thin films. A blue-shift in the fluorescence spectrum demonstrates breakup of PBS-PF2T aggregates in bulk aqueous C12E4. Small-angle X-ray scattering data indicate that this mixture follows a very similar phase behaviour to binary mixtures of a pure surfactant with water, including a micellar phase below about 20 °C, a lamellar phase in between about 20 and 70 °C and a proposed coexistence of water and the liquid surfactant solution above 70 °C. Molecular dynamics simulations reproduce these transitions and suggest that PBS-PF2T is incorporated into the surfactant headgroup region, and is, on average, perpendicular to the alkyl chains. In wet thin films, grazing-incidence small-angle X-ray scattering shows that the phase window for the lamellar phase becomes much narrower, located at about 30-34 °C. Weakly ordered phases exist both below and above these temperatures. These phases are isotropic, but lamellae become aligned in a stacked manner on the surface whether approached from low or high temperatures. Dry films are disordered but can be reversibly ordered and disordered and aligned and misaligned by maintaining the temperature at 30-34 °C and switching relative outside humidity between 32% and 100%.

What conjugated polyelectrolytes tell us about aggregation in polyelectrolyte/surfactant systems

“…I (Supplementary data) shows the final frame of the system showing a distinct separation of phases with the polyelectrolyte side chains exposed to the water environment and with backbones embedded in the surfactant, the surfactant has not aggregated into a particularly diffuse phase. These simulations are fully consistent with observations from SAXS [43].…”

“…However, if the polyelectrolyte has hydrophobic domains, it is suggested that there may be attractive interactions with the nonionic surfactants [103], leading to association. Molecular dynamics simulations in both the micellar [32] and liquid crystalline (this paper and [43]) surfactant concentration regimes demonstrate that these interactions involve both the hydrophobic chain and the ethylene oxide head group of the surfactants. It is possible that similar considerations are valid for other systems involving polyelectrolytes with these surfactants.…”

Molecular Dynamics Study of Self-Assembly of Aqueous Solutions of Poly[9,9-bis(4-Sulfonylbutoxyphenylphenyl) Fluorene-2,7-diyl-2,2’-Bithiophene] (PBS-PF2T) in the Presence of Pentaethylene Glycol Monododecyl Ether (C12E5)

“…These simulations are an expansion on previous studies of the interaction of PBS-PF2T and tetraethylene glycol monodecyl ether (C 12 E 4 ) by Knaapila et al . [27]. These studies included examination of PBS-PF2T mixed into aqueous C 12 E 4 in bulk and thin films.…”

“…PBS-PF2T was selected for both this investigation and the previous study involving C 12 E 4 as an archetypal CPE [27]. Fluorene-bithiophene alternating copolymers, for example poly(9,9-dioctylfluorene- co -bithiophene) are commonly used in molecular electronic devices as charge transport layers [31] while the charged sulfonate groups on the termini of each side chain provide solubility in polar solvents.…”

“…As in previous studies, with these systems we consider a low polymer fraction (12 mM = 0.5% ( v )) but increase the surfactant concentration up to a level where liquid crystal phases are anticipated to start to form (680 mM = 26% ( v )). Previous investigations have demonstrated how PBS-PF2T can be incorporated into a standard surfactant at high mole fraction in bulk and thin films such that the polymer experiences similar optical changes to those in low surfactant fraction, while the surfactant follows its characteristic liquid crystalline behavior [27]. …”