Influence of Solvent Removal Rate and Polymer Concentration on Ordering Kinetics of Block Copolymers in Solution
An examination of the ordering process of block copolymer microstructure with respect to concentration was performed. Specifically, the process of solution casting block copolymer films was studied using small-angle X-ray scattering (SAXS). A method for determining the volume fraction of ordered phase in solution as the system dried was developed and used to analyze the solution casting process in several different block copolymer films in the neutral solvent toluene; these polymers include poly(styrene-b-butadiene), poly(styrene-b-isoprene-b-styrene), poly(styrene-b-butadiene-b-styrene), and several poly(methyl methacrylate-b-butyl acrylate-b-methyl methacrylate) polymers with different block fractions. A method was also developed for studying different drying rates of these films at a constant temperature. Temperature quenches of poly(styrene-b-isoprene-b-styrene) were performed to evaluate the effect of concentration on ordering rate.
In all cases studied, an ordering layer was observed where self-assembly was thermodynamically favorable. This layer steadily grew until it reached the bottom substrate, resulting in a two-step ordering process. In the case of the styrene/diene copolymers, a constant polymer concentration was observed in the ordering layer as it grew to encompass the entire film. Kinetic entrapment was observed in the case of the diblock copolymer, as the system with a medium drying rate with respect to the other two experienced faster kinetics than the other two systems. For the two triblock copolymers, it was found that similar kinetics were observed with respect to the ordering layer concentration, largely due to skinning on the surface allowing time for lower sections of the film to order more completely.
In the acrylate copolymers studied, the kinetics were not able to be evaluated with respect to drying rate. This was due to domain compression that cause a disordering of ordered microstructure as solvent was removed. This disordering was attributed to interfacial disruption caused by the compression in the film. In addition, a significant decrease in domain spacing was observed to occur in the vertical direction as a result of compression in that direction and pinning of the film to the substrate in the horizontal direction.
Finally, the Avrami kinetic model was fit to several concentration of styrene/isoprene triblock copolymers as they ordered after a temperature quench. A U-shaped curve was observed in the system, as a result of competition between chain mobility effects and thermodynamic effects that occur as polymer concentration increases away from the CODT. It was found that the Avrami exponent remained constant over all concentrations, and an empirical model was fit to find the various rate constants at each polymer concentration.