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Research Interests:

My core areas of interest include polymer science, surface science and metrology, hard-soft material interfaces, and functional composites. Recently, it is important for a material to bring more to an application than its basic properties, like modulus or fracture toughness. Materials can be functional, whether the function is environmental sensing, self-healing, self-assembly, or stimulus response. I aim to create new polymeric and composite materials that take advantage of multiple phases and mechanically activated chemical (“mechanophore”) dopants to create designed functional material components that are resistant to damage and fatigue. My interests align with this aim to make it a promising central focus for me in my future investigations.

One unifying theme of my research has been using model polymer materials to test the bounds of applicability for published structure-property relationships and thermodynamic models. I feel this is important for the health of the polymer science community, as there are strong temptations to claim agreement between data and currently accepted theories. Aside from continuing to perform these deliberate tests of hypotheses, I aim to develop new, improved, and more efficient methods for executing or replicating tests where critical gaps exist between current theories and practice for materials with designed properties.

For example, I have been attracted to the metrology of surfaces due to the wealth of information that can be extracted with high precision and spatial resolution. Although nanometer length scales of colloidal systems can be probed with scattering techniques, I aim to create two-dimensional mimics of interesting colloids and other heterogeneous systems to deeply understand the behavior of the simpler system. The variety of physical and chemical data that can be gathered in that geometry can then be fed back into understanding of the more complex system, helping for example to overcome questions of inhomogeneity in the scattering ensemble.

Also, there is inevitably an extreme gradient in the chemical environment at the interface between hard and soft materials, leading to important macroscopic effects like adhesion, adsorption, and interphase formation. By understanding this region, it is possible to better predict the behavior of composite materials and heterogeneous systems. I aim to explore this combinatorial space to identify novel interfaces or coatings that outperform existing approaches or even have unique new capabilities.

Research Experience:

Currently, I am working in a collaboration between Northwestern University, Owens Corning, and NIST to improve measurement science of the micro- and nanomechanical properties of glass fiber coatings for fiber reinforced polymer composites. We are combining interphase modulus measurements with complementary interfacial shear strength measurements with complimentary fluorescent probe and interphase modulus measurements for the purpose of mimicking larger-scale material tests as well as to better predict macroscopic composite coupon strength and fatigue with multi-scale material models.

Previously, during my NRC postdoc at NIST. I was studying the physics of end-tethered polymer films, at all levels from synthesis to metrology with a variety of techniques available at NIST, from microscopy to neutron scattering, to develop new metrologies for these films. I measured (4) the swelling of dense polymer brushes in response to solvent vapor with X-ray reflectivity, as well as the surface interaction parameters (1) of sub-brush end-tethered chains swollen in solvent with liquid solvent. This project required rigorous thin film synthesis protocols (3) and comparison of optical, X-ray, and neutron scattering effects within the thin film while dry and swollen in solvent.

The focus of my graduate work at CU was on the synthesis, rheology, and applications thermally reversible resins, specifically Diels-Alder networks, as a mendable and recyclable thermoset material. I studied their chemical, mechanical (7), and viscoelastic properties (8), and demonstrated (5) that my model networks required a more advanced theoretical structure-property relationship than is typically applied to polymers containing reversible bonds.

Successful Proposals:

GAANN Fellowship

NRC Postdoctoral Research Associateship at NIST

NCNR neutron beam time requests (4)

Postdoctoral Projects:

Novel Metrology Approaches Utilizing the Swelling Physics of End-Tethered Polymers, under Kate Beers, Materials Science and Engineering Department, National Institute of Standards and Technology.

Multi-Scale Metrology for Visualization and Characterization of Polymer Composite Interphase, under Jeff Gilman, Materials Science and Engineering Department, National Institute of Standards and Technology, and Cate Brinson, Department of Mechanical Engineering, Northwestern University.

PhD Dissertation:

Rheology and Application of Thermoreversible Polymer Networks Based on the Diels-Alder Cycloaddition, under Chris Bowman, Department of Chemical and Biological Engineering, University of Colorado at Boulder

Teaching Interests:

The courses I am best prepared to teach are of course the ones most directly related to my research: General chemistry, reaction kinetics, thermodynamics, numerical methods, statistics, and polymer science. Additionally, I look forward to the opportunity to teach some courses that I enjoyed but have not had a reason to employ directly in my post-graduate work, like process controls, unit operations, and optimal design of experiments. I believe that one does not truly understand a topic until having taught it successfully to a student. Teaching serves as a valuable check of my own assumptions and conclusions, because I have to take the nebulous feelings and intuitions in my mind and render them into a concrete form that is coherent and logical to at least one other human being. As a chemical engineer turned polymer scientist, my teaching is influenced by the need to balance tactile and practical types of understanding with quantitative modeling and data analysis. Personally, mathematical expressions make far more sense to me once I can connect them to a physical process, or at least some kind of visualization. Different students will have different responses to these directions, but I believe in all cases that both parts, practical and quantitative, are necessary for successful learning. The outcomes I expect from teaching the course go beyond fulfillment of the stated syllabus – although that is certainly important! I’ll know I have prepared the students successfully when the students themselves report that they have a mastery of the course material, they feel like the course has been worthwhile, and they are engaged to learn more.

Publications:

(1) Sheridan, R. J.; Orski, S. V.; Jones, R. L.; Satija, S. K.; Beers, K. L. Surface Interaction Parameter Measurement of Solvated Polymers via Model End-Tethered Chains. Macromolecules 2017 (accepted).

(2) Yang, Y.; Urbas, A.; Gonzalez-Bonet, A.; Sheridan, R. J.; Seppala, J. E.; Beers, K. L.; Sun, J. A Composition-Controlled Cross-Linking Resin Network through Rapid Visible-Light Photo-Copolymerization. Polym. Chem. 2016, 7 (31).

(3) Sheridan, R. J.; Orski, S. V.; Muramoto, S.; Stafford, C. M.; Beers, K. L. Ultraviolet/Ozone as a Tool to Control Grafting Density in Surface-Initiated Controlled-Radical Polymerizations via Ablation of Bromine. Langmuir 2016, 32 (32).

(4) Orski, S. V.*; Sheridan, R. J.*; Chan, E. P.; Beers, K. L. Utilizing Vapor Swelling of Surface-Initiated Polymer Brushes to Develop Quantitative Measurements of Brush Thermodynamics and Grafting Density. Polymer 2015, 72. (*equal credit first authors)

(5) Sheridan, R. J.; Bowman, C. N. Understanding the Process of Healing of Thermoreversible Covalent Adaptable Networks. Polym. Chem. 2013, 4 (18).

(6) Chatani, S.; Sheridan, R. J.; Podgórski, M.; Nair, D. P.; Bowman, C. N. Temporal Control of Thiol-Click Chemistry. Chem. Mater. 2013, 25 (19).

(7) Sheridan, R. J.; Bowman, C. N. A Simple Relationship Relating Linear Viscoelastic Properties and Chemical Structure in a Model Diels-Alder Polymer Network. Macromolecules 2012, 45 (18).

(8) Sheridan, R. J.; Adzima, B. J.; Bowman, C. N. Temperature Dependent Stress Relaxation in a Model Diels-Alder Network. Aust. J. Chem. 2011, 64 (8).