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. / Ph. D. / Block copolymers are polymers consisting of two or more separate regions made up of different types of polymer chains. Under favorable conditions, these chains will phase separate into ordered structures, with different components being made up of each block. Because they are attached to each other, these structures are in the size range of 10-100nm. For example, a phase separated styrene/butadiene block copolymer of a particular composition can form cylindrical structures where the cylinders are made up of polystyrene, and the surrounding matrix is made up of polybutadiene. These structures can greatly influence the properties of block copolymers, allowing them to be used for everything from lithography to fuel cell membranes.
A common method for the production of block copolymer films for applications such as fuel cell membranes is solution casting, where a polymer in a solvent is spread on a surface and the solvent is allowed to dry. The rate of this drying is a parameter that is not often taken into account when designing a process, despite the fact that it can have an effect on the resulting structure. Thus, insight into how the ordering of structures in a film during film drying can be used to improve processing of these materials.
Using a computer model to determine the concentration profile of solvent throughout the film, and combining this with x-ray scattering data taken during drying at different rates, it was determined that there was a layer in which ordering could proceed, or ordering layer, that steadily grew as the film dried. This ordering layer continued to grow until it encompassed the entire film. In the diblock (styrene/butadiene) copolymer that was studied, it was found that a medium drying rate produced the fastest ordering. This drying condition balanced the driving force for ordering created by the increased drying rate and the ability of the chains to arrange, which would have been reduced upon faster drying. This effect was not seen in the two triblock copolymers (styrene/butadiene/styrene and styrene/isoprene/styrene). In the triblock copolymers, the ordering rate only depended on bulk ordering layer concentration. This was attributed to the presence of a skin on the surface, which slowed ordering throughout the films. In the case of the acrylate triblocks that were studied, the ordering rate trend could not be determined, as compression in the film due to the removal of solvent caused ordered structures to disorder after they formed.
Finally, a model was fit to the styrene/isoprene/styrene at different solvent concentrations. The different concentrations produced a U-shaped curve with respect to ordering time, resulting again from competition between driving force and the ability of the chains to rearrange.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/85438 |
| Date | 27 April 2017 |
| Creators | Pape, Alicia Richelle |
| Contributors | Chemical Engineering, Martin, Stephen Michael, Baird, Donald G., Davis, Richey M., Marand, Herve |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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