(4s) Modeling Complex Self-Assembled Diblock Polymer Phases in Thin Films

Ordered phases formed from self-assembled soft matter provide platforms for materials engineering on the nanometer length scale, but there remain many unanswered questions about the equilibrium self-assembly of such materials under the conditions that are relevant for applications. These ordered soft materials can, for example, be used as templates for nanolithography, templates for advanced optical materials, and filtration membranes with high order and tunable domain sizes; however, these applications often require thin-film confinement, and the effect of confinement on self-assembly is only well-understood for a small number of simple phases. My Ph.D. thesis work has sought to better understand self-assembly under confinement by narrowing in on a particular system of interest: thin films of the double-gyroid phase in diblock polymers. Block polymers are a versatile class of self-assembling soft matter, with a broad range of chemistries and architectures allowing fine control over the resulting phase and its domain sizes. The phase behavior of block polymers can be accurately characterized using self-consistent field theory (SCFT), making them an ideal model system. In these studies, we used SCFT to predict the equilibrium phase behavior of double-gyroid thin films and explain the mechanisms driving this behavior.

The double-gyroid phase in an AB diblock polymer melt exhibits a complicated morphology containing network-like A-rich and B-rich domains that are co-continuous in all 3 spatial dimensions, separated by an A/B interface with saddle curvature. The first question we considered was that of termination planes: in a bicontinuous morphology, what plane will be most stable against a flat boundary? SCFT results showed that the wetting angle where the A/B interface intersects the film boundary has a preferred value, around 85° when the boundary is nonpreferential. The gyroid distorts to achieve this wetting angle, and the most stable termination plane is that which requires the least distortion. In the case of double gyroid, this is the (422) lattice plane. We then zoomed out to consider the phase behavior more broadly, constructing phase diagrams as a function of film thickness, A-block fraction, and preferential wetting. Results revealed that double gyroid is the stable phase down to very low film thickness, and remains stable against B-preferential boundaries, but is destabilized by A-preferential boundaries. A close inspection of the film morphologies provided insight into the mechanisms driving these results, which are dependent on both the phase and termination planes in the films. Finally, recent work has considered the effect of other polymer properties on the phase behavior, such as conformational asymmetry and polydispersity, and has considered the effect of confinement on other network-like phases.

Research Interests

For a postdoctoral project, I hope to continue working in simulation and theory for materials science, but I am open to a wide variety of projects within this umbrella. The subject areas from my graduate research with which I am most experienced are polymer physics, statistical mechanics, scientific software development, and crystallography/symmetry, so any project involving some or all of these subjects would be of great interest. An ideal project would also have some relevance to sustainability and would involve the use of molecular simulation techniques. Examples of topics that I would love to work on include soft materials for use in electronic devices/batteries, polymer degradation and microplastic formation, and dynamics of polymer processing and recycling.

Select Publications

1. B. R. Magruder & K. D. Dorfman (2024). Theory of Block Polymer Self-Assembly, American Chemical Society. (link)
2. B. R. Magruder, C. J. Ellison, & K. D. Dorfman (2024). "Equilibrium Phase Behavior of Gyroid-Forming Diblock Polymer Thin Films," under review.
3. E. K. McGuinness, B. R. Magruder, K. D. Dorfman, C. J. Ellison, & V. E. Ferry (2024). “Circular Dichroism of Distorted Double Gyroid Thin Film Metamaterials,” under review.
4. B. R. Magruder, D. C. Morse, C. J. Ellison, & K. D. Dorfman (2024). “Boundary Frustration in Double-Gyroid Thin Films,” ACS Macro Letters, 13, 382-388.
5. S.-M. Yang, J. Oh, B. R. Magruder, H. J. Kim, K. D. Dorfman, M. K. Mahanthappa, & C. J. Ellison (2023). “Surface relief terraces in double-gyroid-forming polystyrene-block-polylactide thin films,” Physical Review Materials, 7, 125601.
6. B. R. Magruder, S. J. Park, R. P. Collanton, F. S. Bates, & K. D. Dorfman (2022). “Laves Phase Field in a Diblock Copolymer Alloy,” Macromolecules, 55, 2991-2998.
7. B. R. Magruder & K. D. Dorfman (2021). “The C36 Laves phase in diblock polymer melts,” Soft Matter, 17, 8950-8959.