476173 Harnessing Interfacial Phenomena to Design New Soft Materials
The intersection between colloidal and interfacial sciences has led to the development of self-assembled colloidal arrays which have been profoundly impactful for lithography, responsive materials, energy migration, and bio-sensing. The field now faces the challenge to transition from synthesis and assembly of proof of concept samples to engineering scalable methods that integrate functional colloidal assemblies into materials over large surface areas. In my research program, I aim to pioneer the incorporation of colloidal assemblies into soft materials by directing interfacial polymerization via chemical vapor deposition (CVD) onto colloidal assemblies at fluid interfaces. This approach will strategically leverage trapped states of colloidal assemblies with dynamic polymer growth to independently control the organization and orientation of colloidal monolayers from the polymer matrix for customization of material properties. Here I present previous works on (i) controlling the morphology of polymers deposited onto liquid substrates and (ii) developing a modular synthesis of colloidal Janus particles with diverse chemical compositions which together provide the platform for realizing my vision to harness interfacial phenomena to build colloidal assemblies into soft materials.
CVD methods have been exploited for many years to transform the surface chemistry of structured materials by depositing uniform and conformal polymer coatings that preserve the underlying geometry and surface roughness. The initiated CVD (iCVD) process is unique because it uses free-radical initiators which eliminate precursor fragmentation present in other CVD methods, and produces polymers analogous to solution phase free-radical polymerization. This enables the iCVD process to deposit functional coatings incorporating hydrophobic, hydrophilic, or stimuli-responsive polymers. To extend deposition via iCVD beyond conformal thin films, we explored deposition onto liquids with low vapor pressures (such as silicone oil and ionic liquids) to exploit the dynamic diffusion and wetting at liquid—vapor interfaces. We demonstrated that the deposition can result in the formation of either polymer particles, planar films, or microstructured films at liquid surfaces depending on the spreading coefficient of the polymer on the liquid. In addition, precursors that are soluble in the liquid can absorb into the bulk and polymerize resulting in the formation of polymer—liquid gels, layered polymer composite films, and encapsulating polymer shells. This work elucidates the growth mechanism of polymers on liquid substrates and provides an understanding of how process parameters can be controlled to incorporate chemical functionality into a variety of structures and morphologies for potential applications in optics, sensing, and separations.
Inspired by the diversity of chemical functionality that the iCVD technique provides, we sought to bring the same range of chemical compositions to the fabrication of amphiphilic Janus particles which are considered colloidal analogues to molecular surfactants. The chemistry of Janus particles is the fundamental parameter that controls their behavior in bulk solution and at interfaces. To enable their widespread utilization, scalable methods that allow for the synthesis of Janus particles with diverse chemical compositions and shapes are highly desirable. We developed clickable Janus particles that can be functionalized through thiol-yne click reactions with commercially available thiols which widen the palate of chemical compositions. The extent of functionalization can be used to control the particle morphology, and thus the type of emulsion stabilized, as well as to fabricate composite Janus particles through sequential click reactions. Functionalizing Janus particles through thiol-yne click chemistry provides a fast-reacting, scalable synthesis method for the fabrication of chemically diverse Janus particles. Currently, we are working on applying our ability to tune the chemical composition to study their behaviors at fluid—fluid and solid—solid interfaces.
Chemical engineers provide unique perspectives because they are able to exploit fundamental chemical and physical phenomena within real-world systems to produce and transform materials. This was exemplified in my graduate research which showed that the morphology of polymers deposited onto liquid substrates via iCVD could be controlled by the interplay between intrinsic polymer—liquid interactions and deposition conditions. The iCVD technique is a perfect example of how core chemical engineering principles (transport phenomena, reaction kinetics, thermodynamics, and separations) work together to control the polymer composition, thickness, molecular weight, and morphology. My experience developing the design rules for understanding deposition onto liquids via iCVD has prepared me to teach the core curriculum in chemical engineering, and the utilization of iCVD in my research program will provide a hands-on experience for undergraduate and graduate students to apply concepts learned in the classroom.
As an educator of chemical engineering, I also aim to train students how to merge their backgrounds in the central sciences and transport phenomena to construct efficient chemical processes. I will do this by emphasizing ingenious chemical engineering solutions in our everyday lives (such as decaffeinated coffee, pressure cookers, and time-release pharmaceuticals) throughout lectures and course assignments. These themes will culminate by challenging students to work in teams to redesign a process or material for alternative applications. To further promote the integration between fundamental science and process design, I plan to develop a graduate course on CVD methods that is accessible to students across engineering and the sciences. The purpose of the course will be to demonstrate the capabilities and diverse applications of CVD techniques while promoting discussion to foster collaboration between students with different backgrounds. My goal as a faculty member will be to inspire students to look for engineering solutions at the intersections between fields to optimize process design and product performance.
Selected Publications (15 total, 7 first author, 211 citations):
L. C. Bradley, K. J. Stebe, D. Lee. Clickable Janus Particles. Submitted to J. Am. Chem. Soc. June 2016.
L. C. Bradley, M. Gupta. Microstructured Films Formed on Liquid Substrates via Initiated Chemical Vapor Deposition of Cross-Linked Polymers. Langmuir2015, 31, 7999-8005.
L. C. Bradley, M. Gupta. Copolymerization of 1-Ethyl-3-vinylimidazolium bis(trifluoromethyl-sulfonyl)imide via Initiated Chemical Vapor Deposition. Macromolecules 2014, 47, 6657-6663.
P. D. Haller, L. C. Bradley, M. Gupta. Effect of Surface Tension, Viscosity, and Process Conditions on Polymer Morphology Deposited at the Liquid—Vapor Interface. Langmuir 2013, 29, 11640-11645.
R. J. Frank-Finney,† L. C. Bradley,† M. Gupta. Formation of Polymer—Ionic Liquid Gels Using Vapor Phase Precursors. Macromolecules 2013, 46, 6852−6857. †Authors contributed equally to this work.
L. C. Bradley, M. Gupta. Encapsulation of Ionic Liquids within Polymer Shells via Vapor Phase Deposition. Langmuir 2012, 28, 10276-10280.
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