Effects of supersaturation on the structure and properties of poly(9,9-dioctyl fluorene) organogels

Abstract

Rheological and small angle neutron scattering (SANS) measurements are used to characterize the gelation of poly(9,9-dioctyl fluorene) in organic solvents. The effect of supersaturation over the system's structure and properties is quantified. A variable range of solvent compositions and temperatures are used to control supersaturation. The mechanical and structural properties of the gels are characterized under different gelation conditions. Rheological studies reveal a strong dependency between the level of supersaturation of the system and the gelation kinetics. Samples start gelling faster and form stronger gels as the supersaturation of the system increased by lowering the temperature. Fits using the Avrami theory of phase change reveal a systematic variation in the dimensionality of the networks as the driving force for gelation is varied. A lower dimensionality is found for gels formed at lower supersaturation, whereas higher dimensionality typical of bifurcated networks appears as the supersaturation increases. SANS and USANS experiments also provide insight into structural variations occurring over several length scales. A power law dependence appears at low-q (6.0 × 10⁻⁴ < q < 4.0 × 10⁻³ Å⁻¹) showing a higher exponent at low supersaturation indicating the formation of polymer-rich and solvent-rich domains with a well defined interface. Meanwhile, at higher supersaturation, the exponent value systematically decreases indicating that a more intermixed network–solvent system is formed. This is also corroborated with sTEM images for several polymer–solvent systems. The system's conductivity is also probed during gelation but there are no observable differences between the sol and the gel states for any of the systems. This study demonstrates the possibility of controlling network structures in conjugated polymers by changing the driving forces that are used for aggregation so that it may be possible to optimize the structure of organogels for specific applications.