(110d) Tracking Phase Separation Kinetics in Block Copolymer Solutions Using Rheology

Previous work reported on solution-cast membranes for fuel cell applications has revealed a possible connection between polymer morphology and proton conductivity. In the case of solution-cast films, dying conditions, including drying temperature and rate, have been shown to govern the morphological structures, both equilibrium and intermediate, yielded in the final films. It has been shown that while thermodynamic contributions are crucial in predicting final morphology, kinetic factors also play a role when considering phase separation during the removal of solvent as polymer mobility is directly affected by the presence of solvent. It is also well understood that phase separation occurs over a period of time. Thus, the film morphologies will be ultimately determined by the equilibrium morphologies favored at a given concentration and the amount of time that the system is allowed to phase separate at that temperature. It has been proposed that the Avrami equation for crystallization can be used to model phase separation kinetics by monitoring G' and G? as the effects of crystallization and the formation of ordered structures on modulus should be comparable. Based on equilibrium values of the shear modulus, the fraction of the solution that has crystallized can be calculated. When tracked over time, this information can be used to calculate the Avrami exponent, n, and rate constant, z, which are indicative of the dimensions in which nucleation occurs and the rate at which growth of ordered structures occurs, respectively.

Available literature focuses on monitoring the constant concentration kinetics of phase separation following temperature quenches as the disorder-to-order transition occurs. In typical film casting applications, however, polymer solutions are initially cast in dilute form and dried, often times isothermally. The kinetics at the order-order transitions must therefore be understood in order design processes capable of providing the desired final film morphology. It is reasonable to assume that these isothermal, order-order transition kinetics will be distinctly different from non-isothermal order-disorder transition kinetics at the same concentration.

This work concerns the development of methods to monitor phase separation kinetics of block copolymer solutions at order-order transitions using model systems of polystyrene-block-polybutadiene or polystyrene-block-polyisoprene in solvents of varying selectivity. While the ultimate goal of this work is to monitor phase separation while simultaneously removing solvent at various drying rates and temperatures, initial work being performed can be used to gain a better understanding of phase separation in polymer solutions. The first goal is to identify order-disorder transitions and order-order transitions using purely rheological methods. Further, kinetics about the disorder-order transitions are being monitored in order to validate the approach. In doing so, systems of various solvent selectivity, polymer composition, and polymer molecular weight have been utilized. Information gained is being used to describe how these parameters impact the kinetics. Presently, isothermal tracking of phase separation kinetics at various constant concentrations has been performed on SB and SBS solutions. It is observed that the concentration dependences of the Avrami exponent and the rate constant are present. Further, in changing from a diblock to a triblock copolymer, the kinetics are changed drastically as well. At this time, interpretations of the observed trends from a molecular standpoint are being formulated.