Kenneth R. Shull - US grants

Completion of this project will result in the development of gels with exceptional mechanical toughness that also have a self-healing capability. Application areas for these types of materials are quite diverse and range from artificial cartilage to protection systems that rely on the ability of a material to dissipate large amounts of energy under repeated loading conditions. The general design strategy for the synthesis of tough polymer gels is based on recent work with 'double network' gels consisting of a relatively high-modulus primary network, and an independent secondary network with a much lower modulus. In the proposed work the secondary network is based on a self-assembling triblock copolymer gel, and the primary network originates either from additional ionic crosslinking of the secondary network, or from the development of a co-continuous silicate network or array of silica nanoparticles. An additional outcome of the work is the refinement of the nonlinear elastic fracture mechanics analysis needed to provide a general understanding of crack propagation in highly deformable solids. The self-healing capability of the gels originates primarily from the use of non-covalent bonds in the formation of the primary and secondary networks. Additional healing mechanisms are available in the silicate systems because of the formation of covalent bonds that are able to undergo reversible hydrolysis and condensation reactions. Development and understanding of these hybrid silicate/organic gels will impact a broad range of fields in materials science. In order to accomplish these goals the PI and co-PI propose a coherent plan that involves the synthesis of the primary and secondary networks and the use of finite element methods to understand the stress fields in the vicinity of a growing crack. Connections will also be made to analytic models that are more generally accessible to the broader scientific and technical communities interested in the fracture toughness of highly deformable materials.

Non-Technical Summary
Many naturally-occurring materials have mechanical properties that are highly optimized and have not yet been duplicated in synthetic materials. One of the most intriguing properties of some of these naturally-occurring materials is their 'self-healing' capability that enables them to recover their mechanical integrity after they have been damaged. Developing strategies for achieving the balance of properties possessed by these natural materials is one of the aims of this project. The focus is on relatively 'soft' materials, similar to those that make up the soft tissues of plants and animals. Cartilage is an excellent example of a desired combination of materials properties. Cartilage lubricates the joints by presenting a low friction interface between bone surfaces, even when the bones are pressed against one another with very high forces. The design strategy utilized in this project will enable this combination of properties to be obtained in a synthetic material. The work is a collaborative effort involving mechanical modeling, mechanical testing, and the synthesis and processing of new materials. Educational outreach activities are planned with local museums in the Chicago area, beginning with the Chicago Botanic Garden and Shedd Aquarium. These institutions bring expertise in reaching the public at the broadest and most general level. These collaborations will enable the principles being applied in the proposed work to be understood at a qualitative level by people with a natural curiosity about their natural environment.

ARRA STATEMENT

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).

TECHNICAL SUMMARY

A variety of useful functional materials can be produced by adsorption of polymeric molecules, nanoparticles, or a combination of these components to liquid/liquid or liquid/solid interfaces. Applications of these materials require that they have sufficient mechanical strength, which generally originates from their polymeric component. Currently, many of the factors that control the detailed structure and mechanical integrity of these materials are not well understood. This project will develop the field of interfacial mechanics so that the relationships between the processing parameters, structural features and mechanical properties of polymer-based interfacial layers can be understood and controlled. Successful completion of the proposed research will enable interfacial structures to be engineered with the necessary mechanical strength. The general approach involves application of the materials science paradigm that has successfully led to the development of new bulk materials. Well defined model systems will be utilized in conjunction with experimental techniques developed specifically for the investigations of these types of interfacial films. Three types of systems will be investigated: interfaces between oppositely charged polyelectrolyte layers, interfacial assemblies formed by the complexation of oppositely charged species at the interface between two aqueous fluids, and materials undergoing self-assembly or self-organization while confined to two dimensions.

NON-TECHNICAL SUMMARY

The research will result in the development of an important new class of materials with applications in materials and health-related areas. Graduate students will be trained in the relevant scientific techniques in an important, emerging subfield of materials science. The work will also foster international collaboration through joint projects with colleagues in Mexico. An additional initiative entitled "Nature's Solutions to Mechanical Problems" will provide a means for communicating important fundamental concepts in mechanics to a very broad and diverse audience. A primary goal of this portion of the project is to develop a bridge between the scientific community, which is increasingly focused on understanding and mimicking the mechanical behavior of biological systems, and the broader public who are inherently interested in their natural surroundings but are not familiar with the scientific literature in these areas. A secondary goal is to use these examples to develop critical thinking skills, to communicate 'how we know what we know', and to teach both scientists and non-scientists to distinguish between claims that have a rational basis and those that do not. This effort is leveraged by collaborations with local museums in the Chicago area, beginning with the Chicago Botanic Garden and Shedd Aquarium. These institutions bring expertise in reaching the public at the broadest and most general level. Undergraduate students at Northwestern University and high school science teachers in the Chicago area will be involved in this part of the effort under the guidance of the PI.

TECHNICAL SUMMARY:

Investigations of paint coatings in the context of cultural heritage science present a unique set of technical challenges because of the evolution of the properties of materials over time scales that are much too long to be reproduced directly. As a result, data-driven kinetic models of the aging process are needed in order to understand the physical state of aged paints and to develop effective restoration and cleaning strategies. In this collaborative project involving Northwestern University and the Art Institute of Chicago, a kinetic Monte Carlo model of paint curing and aging will be developed, as will experimental systems needed to determine relevant model parameters. The net result will be a set of simulation models that can be viewed as 'virtual' oil-based paint coatings. These 'virtual' coatings will enable the time dependent, structural features of complex, multicomponent paint coatings to be tracked. The models, with their experimentally determined input parameters, represent a physical and chemical knowledge base for oil-based paint coatings that will serve as a platform for addressing a wide range of questions. Specific issues to be addressed concern the curing and aging of systems when subjected to heat, humidity and various cleaning solutions. Kinetic parameters will be determined experimentally with coatings made from well characterized starting materials, using the quartz crystal microbalance (QCM) as the primary experimental tool. Nanoindentation will be used to correlate the high frequency mechanical response obtained with the QCM to complementary measures of the mechanical response and to actual paint samples. While these techniques will be applied directly to oil-based coatings used by artists, the methodology is broadly applicable in a variety of areas, including characterization of high performance protective coatings, and the development of sustainable, bio-derived materials.

NON-TECHNICAL SUMMARY:

This project is a collaborative effort between Northwestern University and the Art Institute of Chicago. State-of-the art scientific tools will be used to study the curing and aging properties of modern oil-based paints and the impact of conservation treatments on them. Although oil-based paints were widely used throughout the 20th century, and in some of the most prominent artworks of that period, their properties are poorly understood. Advancing our knowledge in this area is crucial for preserving the integrity of such artworks for future generations. The results of this research will be used to develop more effective conservation strategies for paintings in collections at the Art Institute and at other museums throughout the world. An educational outreach element linking science and art will also be developed in conjunction with the Art Institute's Department of Museum Education. Outreach offerings will be developed to attract middle school and high school science classes to the museum. These activities will be designed to attract students to science who would otherwise not likely be drawn to science-related programs. Similarly, the connection with the Art Institute will enable undergraduate and graduate students at Northwestern to understand the ways in which scientific concepts can benefit disciplines that lie outside the traditional scientific realm.

The research objective of this grant is to elucidate the mechanisms responsible for the formation of mechanically robust coatings at metal surfaces, arising from the combined action of mechanical and chemical forces. While the methods employed in the project can be applied to a variety of metals, the focus will be on cobalt-based alloys that are of direct relevance to biomedical devices. The scientific questions to be addressed by this work revolve around the nature of mixed metal/organic layers that form at the metal surface, strongly affecting its properties.

Successful completion of this project will result in a greater understanding of alloy materials that are widely used in biomedical implants, including knee and hip replacements. The nature of the mixed metal/organic layer formed at the surface of these materials has been shown to be responsible for many of their favorable properties, but may also play a role in an observed increase in the number of painful and costly device failures. This project will provide the necessary scientific background to develop implants that behave in a more consistent manner. In addition, the collaborative nature of the project brings together physical science undergraduate and graduate students at Northwestern University with medical students at Rush University Medical Center. The project is designed so that several medical students will participate in the project and develop an understanding of scientific issues underlying the use of implant materials that they may be using in future clinical practice. Conversely, engineering undergraduate and graduate students at Northwestern will be informed of the performance requirements and constraints that are relevant to the design and use of medical implant materials.

NON-TECHNICAL SUMMARY:

Polymer gels are flexible, solid materials containing a high proportion of a small-molecule component like water. The aim of the project is to develop polymer gels that are very strong, and that can also be used to produce exceptionally slippery surfaces with ultra-low friction. The possibilities have been illustrated by nature, in the form of certain types of cartilage tissue that are responsible for lubricating load-bearing joints in the body. The availability of synthetic versions of these tissues would transform orthopedic practice, extending the life of hip and knee replacements or even making these replacement procedures unnecessary. In addition, materials with the sought after combination of properties can be used as contact surfaces in a variety of manufacturing settings, enhancing energy efficiency by reducing the energy lost due to frictional heating. Past efforts to produce high-strength, low-friction gels have been limited by the fact that the same factors that enhance strength generally increase the friction as well. Two elements of the project will be undertaken in pursuit of the overall project aim. The first of these is the synthesis and testing of new gels designed to break the connection between gel strength and gel friction. These gels have features in common with their natural counterparts (like cartilage tissue), but can be tuned by altering certain details of the gel structure. The second element of the project is more conceptual, and involves the development of a general design strategy to guide future synthesis of high-strength, low-friction gels.



TECHNICAL SUMMARY:

The goal of the project is to develop high-toughness gels with ultralow friction, and to develop a generalized design strategy for producing these types of materials. The work is based on the synthesis of hybrid gels consisting of a charged, polyelectrolyte network to which multivalent cations have been added. These materials can have an extraordinarily high mechanical toughness, resulting from energy dissipation mechanisms arising from the presence of both weak and strong bonds in the same material. Because the development of a highly lubricious, low-friction material requires that energy dissipation be minimized, it is particularly challenging to maintain material toughness while simultaneously reducing the friction. The working hypothesis for the project is that friction and toughness are controlled by energy dissipation mechanisms that are operative on different time scales. The project is designed to elucidate these effects in a set of model gel systems that enables the relevant time scales to be controlled. The most important time scales are the force-dependent lifetimes of the weaker bonds, and the rate at which these weak bonds are able to reform after they are broken. The experimental model systems are based on gels formed from the controlled assembly of high molecular weight triblock copolymers with a poly(methacrylic acid) (PMAA) midblock and poly(methyl methacrylate) (PMMA) end blocks. Assembly of these polymers results in the formation of polymer gels with the strong bonds consisting of glassy PMMA aggregates. The weak bonds are introduced by complexation of midblock carboxylates with atomic or polymeric cations, with bond formation times and bond lifetimes controlled by the specific cations that are chosen. The tunable and quantifiable nature of the weak and strong bonds in these systems will enable the development of widely applicable design principles for producing high-toughness materials with the desired frictional properties.

Scientific research in cultural heritage is a powerful vehicle for interdisciplinary, integrated and global science training in STEM graduate and undergraduate education and consistently exceeds STEM benchmarks of gender inclusion. This U.S.-Netherlands International Research Experience for Students (IRES) project supports three student cohorts, each consisting of one undergraduate and four graduate students, for research visits to the Netherlands to study the materials, structures and aging of cultural heritage artifacts. The program approaches the analysis of art as a way to provide unique research and educational experiences for U.S. student participants in chemistry, materials science, electrical engineering, and computer science. Led by Principal Investigator, Marc Walton, the cooperative activities with Dutch counterparts are designed to provide access to advanced infrastructure for interdisciplinary cultural heritage science while fostering long-term partnerships between recognized centers of excellence in the U.S. and the Netherlands. The goal is to advance our understanding of change in complex systems where results may lead to improvements in the way we access, interpret, and care for art. Where successful, resulting development of advanced technologies for detection of trace colorants, stand-off chemical sensing, and empirical and predictive models of materials alteration and change, at multiple time scales, can be important drivers for innovation in fields that range from forensics and security to high performance coatings.

The eight-week IRES experience will be centered at the Atelier Building (Ateliergebouw) in Amsterdam, a core facility that brings together the University of Amsterdam, the Rijksmuseum, the scientific research arm of the Cultural Heritage Agency of the Netherlands, the Delft University of Technology, and the Netherlands Forensic Institute. In addition to the Dutch expert mentors from these institutions, collaborations with scientists at major U.S. museums such as The Art Institute of Chicago, The Metropolitan Museum of Art and the National Gallery in Washington, D.C., will ensure timely focus to the IRES students' examination of technical questions related to identifying and characterizing important works of art. With access to cultural heritage scientists who are leaders in their field, the IRES participants are expected to contribute to the central theme of understanding and monitoring change in complex systems, including these areas: 1) microanalytical tools in art and forensics, 2) change and reactivity in complex paint systems, and 3) imaging and data mining. Upon return to the U.S., IRES participants are to present the results of their work at the Gordon Research Seminars on Scientific Methods in Cultural Heritage Research and at professional meetings in the sciences and in conservation. Overall, this IRES will contribute to a globally engaged U.S. science and engineering workforce, capable of excelling in interdisciplinary and international research environments, where science can be transformative.

NON-TECHNICAL SUMMARY:

Polyelectrolytes are large water-soluble molecules that contain electric charges. When water solutions of positively and negatively charged polyelectrolytes are mixed together, complexes are often formed that have either liquid-like or solid-like properties. The ability to tailor these properties has led to their use in a variety of applications, ranging from personal care to industrial waste processing and water treatment. This project is aimed at understanding how the relevant properties of these materials originate from the detailed structure of the components from which they are formed. This understanding will be generated by developing a series of well-characterized model materials systems, and studying their mechanical properties with several experimental techniques. In addition, new processing methods will be developed that enable polyelectrolyte complexes to easily be coated onto different material surfaces. The characterization methods include the use of high frequency sound waves to probe the material response. This technique is widely applicable to a variety of coatings with both protective and aesthetic functions. The project is relevant to membranes for water filtration and includes education and research training of students, broadening participation, and outreach activities.


TECHNICAL SUMMARY:

Polyelectrolyte complexes formed by the interaction of oppositely charged macromolecules are an important class of soft, polymeric materials. These materials are of interest largely because of their mechanical and transport properties. The mechanical properties can span the full spectrum of behaviors from low-viscosity liquids to tough viscoelastic materials to brittle solids, in a manner that can be reversibly controlled through changes in the salt concentration or pH. The primary aim of this project is to understand the factors that control this behavior using well-characterized model systems. A secondary aim is to use this information to develop surface modifications to enhance the performance of membranes used for water purification. The focus of the project is on polyelectrolyte complexes in thin film form, both because of the utility of these materials as surface modifiers, and because the thin film geometry is particularly convenient for the proposed investigations. There are three aspects of these investigations, beginning with new deposition mechanisms based on the electrochemical control of the pH at the surface of interest. The second set of experiments is aimed at mapping out the phase behavior of these materials, including the relationship between equilibrium water content of a film and the salt concentration of the aqueous medium with which it is in contact. The third element of the proposed program is the most extensive, and involves mechanical characterization of the polyelectrolyte complex films. Acoustic methods will be used to characterize the linear viscoelastic properties of these materials on a time-scale of about 60 nanoseconds, approaching the timescale that is accessible by molecular dynamics simulations, bridging the gap between experiment and computational modeling. In addition, the nonlinear properties of these materials will be investigated using creep and fracture experiments designed specifically for investigations of thin films in the hydrated state.

PI: Kenneth Shull (Northwestern University)
co-PIs: Francesca Casadio (Art Institute of Chicago)
Oliver Cossairt (Northwestern University)
Aggelos Katsaggelos (Northwestern University)
Marc Walton (Northwestern University)

Non-Technical Abstract:
Historic art objects provide a collection of materials that have been naturally aged for decades or even centuries. In addition to the intrinsic archival value of these materials, they are also models for understanding property degradation over long periods of time. This project aims to develop computational and experimental tools needed to understand how these changes take place. To accomplish this task a research network has been established between Northwestern University and leaders in cultural heritage science from the Rijksmuseum and the University of Amsterdam in the Netherlands, the National Research Council in Italy, and the Synchrotron Soleil in France. This new infrastructure promises to deliver a significant enhancement of research and education resources (networks, partnership and increased access to facilities and instrumentation) to a diverse group of users. The art objects central to the project provide a series of well-defined case studies for investigating complex materials systems that are both applicable to materials education and push the limits of the existing analytical tools, thus inspiring instrumental innovations across broad sectors of the physical sciences. Further development of these tools will enable art conservators to more effectively make informed decisions about treatments of works of art, and to understand long-term materials degradation more generally. The project will also deliver a significant enhancement of research and education infrastructure by a diverse group of users and will provide meaningful, international research experience to 50 participants, with a strong emphasis on scientists at the beginning of their careers. In addition, the connections between science and art will illustrate the creative aspects of both disciplines to a very broad audience, attracting a more representative cross section of people into science.

Technical Abstract:
The purpose of the proposed program is to probe the properties of heterogeneous material composites at multiple length scales, with a focus on the materials used in creating works of art. The grand challenge is to understand the coupling of material structures from nano- to macro- length scales to the visual appearance, and to use this coupling as a probe of material properties. This coupling will be addressed by incorporating light/matter interactions into computational chemistry approaches, which will also be developed to understand the physical-chemical changes that occur in materials over long periods of time. This information can be used to reconstruct the appearance of an object as originally created, and project the appearance into the future. This methodology is of primary importance to the art conservation community, which has developed advanced research infrastructures in Europe that are rare or nonexistent in the U.S. These European resources are essential for the completion of the project goals, which are to provide a greater understanding of the way in which chemical and physical changes within a material gradually distort its visual perception, and to develop a mechanistic understanding of these alteration pathways. The proposed PIRE project integrates teams from leading cultural heritage science institutes in France, the Netherlands and Italy with their American Counterparts. While the tools will be applicable to modern engineered materials as well, examples from art provide a much broader educational impact. A combination of individual and cohort visits to the three primary international sites will provide students with an international perspective on science and research, while building skills in communicating the role of science in society.