Design and engineering of polymer/macromolecular structures on the 2–100 nm length scale: A personal view on structural research
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HOW POLYMER SCIENCE DEVELOPED TO ENABLE ATOMIC LEVEL ENGINEERING
Polymer science naturally began with the search for experimental evidence establishing the existence of long-chain molecules and for ways to theoretically understand the consequences of long-chain connectivity. Most of the early pioneering research focused on polymer solutions where small changes in factors such as size, shape, and environmental interactions of the polymer can result in large physical property changes. From this foundation, further refinement of synthetic techniques and polymer processing enabled polymers to become both a valuable addition to and a viable alternative for materials such as metals and ceramics in products. As polymers became more pervasive in daily life, it became imperative to determine how to both extend the range of physical properties such tensile strength, impact strength, and elasticity, as well as to exactly tailor these properties for a particular application. Accordingly, as plastics technology developed, it became evident that both ordered polymer structures along with polymer dynamics in different length and time scales were critical for determining polymer properties, and therefore, were essential for establishing structure–property relationships as well as for realizing the full promise of polymeric materials. From this vantage point, the relatively mature science of structural analysis set out to study organized synthetic and natural polymeric materials to determine their crystal structures, morphologies, and crystallinity, to correlate these factors with physical properties. This knowledge, coupled with advanced synthetic techniques, enabled the atomically precise tailoring of structures on the subnanometer length scales to achieve specific material properties. In this way, polymer science has been trying to answer the scientifically inspiring question posed by Richard Feynman: “What could the properties of materials be if we could really arrange the atoms the way we want them?”1 Like any truly important question, it has motivated the work of a generation of scientists, while continually raising new and insightful questions. With his famous query, Feynman has implicitly asked three corollary questions: (1) what material properties hold sufficient untapped potential to deserve society's and science's focused attention and creativity? (2) Is the atomic length scale the one and only critical length scale to achieve superior and precise material properties? (3) How do different arrangements of a set of elements in space affect the relevant material properties?
NEW TECHNOLOGICALLY IMPORTANT STRUCTURES AND LENGTH SCALES
Addressing the first corollary question, the determination of which material properties offer the most promise to society is exceeding complex. A consensus has formed that the ability to cure diseases and heal wounds to enhance both the length and quality of our lives is worth a substantial investment. As a result, a majority of research activities are aimed at the biological and bio-medical fields. These fields are acutely interested in materials with controlled surfaces covering precisely built hierarchical structures on length scales critical to biological processes. Simultaneously, society has also recognized how information and communication technology has increased the efficiency and altered the way we understand and interact with the world at large. As such, there are also significant resources and excitement around photonics, the next revolutionary technological platform upon which computation and communication will be based. These applications require advanced materials with structures on length scales relevant to optical processes whose component materials have high performance in a variety of areas, including electron transport as well as optical linear and nonlinear responses.
Answering the second corollary question from the perspective of a materials scientist, the aforementioned facts indicate that to enable the technologies being envisioned to transform how we will live in the future, we must address how to fabricate a multiplicity of functional, hierarchical structures with characteristic dimensions all the way from the atomic scale through hundreds of nanometers up to the macroscopic scale.
CURRENT WORK TO EXPAND EXPERIMENTALLY AVAILABLE STRUCTURES AND LENGTH SCALES
A significant body of work has been developed on how to precisely arrange materials on greater than atomic length scales. Research on nanometer scale ordered structures has generally taken two approaches. The first approach uses the nanophase separation of block copolymers (in a less controlled fashion, separation of polymer blends, but they are not in thermodynamic equilibrium). These structures are often on interesting length scales (one to tens of nanometers), but there is a limited range of structures available for this method. Additionally, it is difficult to achieve usably large single crystal-like domains of many of these structures without defects or with controlled defects. The second approach has been to design monomers with specific lock and key interactions to self-assemble into targeted ordered structures with intriguing shapes, sizes, and functionalities. Self-organized systems of oligomers/macromers have been pioneered by the work of Stupp2–4 and Percec.5, 6 Furthermore, metal-organics have been shown to construct supramolecular structures with unit cell sizes of around 1 nm in different dimensional spaces via the crystal engineering approach.7–10 On the other end of the size spectrum, Whitesides and coworkers have elegantly shown that self-organization can be used with macroscopic objects on the micrometer scale to create a variety of two- and three-dimensional (2D and 3D) structures.11–13
At present, this leaves a gap in our ability to create the structures at the length scales required to achieve the optimal material performance needed to move the forefront of science and technology to the point where we can truly reap the full benefits of the work invested in nanotechnology. It is my personal view that one of the major goals in structural research should be to bridge that gap and design highly functionalizable polymer/macromolecular building blocks to self-assemble into a diverse range of controlled 2D and 3D structures with unit cells on the length scale of 2–100 nm. Design and construction of these structures using different types of building blocks in this size range will result in new materials that will enable a pervasive new wave of technologies that will permanently alter how we live our lives.
BRIDGING THE GAP—A PATHWAY TO FACE FUTURE TECHNOLOGICAL DEMANDS
The challenges to realize the vision of functional structures on the 2–100 nm scale are many, but they can be roughly broken down into four categories. The first is the designing and engineering of polymer/macromolecular building blocks. To obtain the necessary structures on the desired length scales, rigid-shaped components of a variety of sizes are needed. Specifically, basic building blocks with shapes such as spheres, cylinders, and sheets on this length scale need to be fabricated. Advanced theoretical/computational work will shed light on the optimal shapes and size ratios of the constituent pieces needed to construct complex structures. With that in mind, the most versatile and potentially successful approach to make these building blocks is to use polymers and macromolecules, but polymers are globally “flexible,” thus making shape retention a significant hurdle to their use. Feasible approaches to overcome this difficulty have been proposed. To generate sphere-shaped building blocks, some efficient approaches include utilizing inorganic nanomaterials to form hybrids, the intramolecular crosslinking of dendrimers, the crosslinking of collapsed linear polymers in poor solvents, and others. To generate cylinder-shaped building blocks, linear polymers can be dendronized, phase-separated diblock copolymers can undergo intrablock crosslinking, and the use of stiff laterally-attached side-chain liquid crystalline polymers may also be successful. Flat, scrolled, and helical sheets could be produced by the careful engineering of polymer single crystal surfaces. Angular, “T”, cross, tetrahedral, and other shaped nano-blocks could serve as nodes connecting the building blocks. These nodes could be designed and synthesized utilizing a “molecular library” of cyclic molecular polygons in 2D and 3D, based on metal-ligand connectivity, or miktoarm star polymers with substituents containing different lock and key chemical interactions, which can then be crosslinked after the appropriate associations are made.
The second challenge is to tailor the surface properties of these building blocks to exactly place the specific interactions needed between the building blocks to construct the 2D and 3D nanostructures as well as to provide the desired interactions with the environment. This requires the design and engineering of surface functionalities onto the building blocks. A range of possible interactions providing a number of geometries, directionalities, and strengths exist, including H-bonding, aromatic π-π interactions, nanophase separation, metal connectivities, and covalent bonds (such as crosslinking). The specifically designed surface functionalities will then be directors introducing selectivity and directionality into the self-assembly process, which will control the final structure. Although using these interactions to construct supramolecular structures has been extensively reported, it is still a challenge to achieve specific functionality at predetermined locations on the surfaces of the building blocks.
The third challenge is to actually build the structures with unit cell sizes of 2–100 nm from the functionalized, nanobuilding blocks with single crystal-like domains on the order of micrometers and centimeters. Once the tailored building blocks are available, questions about how these components will assemble and what kinds of ordered structures they will form in 2D and 3D have to be addressed. The assembling process will depend on the thermodynamics and dynamic behavior of the building blocks. The final 2D or 3D ordered structures will be determined by a competition between the close packing of the geometric shapes and the lowest free energy state of the chemical and physical interactions. New theoretical treatment will offer insight into precisely how these factors compete and contribute to the assembly of the final structure.
To help the selectivity and rate of the assembly process, one possible method is to utilize 2D patterned substrates, created via lithography, to epitaxially assemble 3D structures. During the controlled assembly process, the correct stoichiometry and surface functionality of the individual components are necessary to direct the 3D supramolecular structures. These requirements have been proven to be important and successful in the forming of 3D metal-organics supramolecular structures. It has been shown that complicated supramolecular structures may be constructed from those relatively simple building blocks.7–10
Although this approach is intriguing, it is difficult to fabricate ordered structures by mixing the building blocks in either the concentrated or dilute states. To both enhance the rate of self-assembly and to possibly enable the system to explore thermodynamic minima not normally accessible, external force fields can be applied, but substantial care is required to grow single crystal-like structures. To determine how processing affects defect formation and maximizes the correlation length in the assemblies, the formation mechanisms can be monitored in situ by scattering and diffraction experiments as well as be independently assessed by microscopic techniques.
Finally, the fourth challenge is to answer the third corollary question, Feynman's question on the nanoscale, using the building blocks and assembly processes developed to fabricate nanostructured materials with specific biological, mechanical, electric, and/or optical properties. To accomplish this, we need to establish completely new structure–property relationships for ordered structures on these length scales. Correlating macroscopic bulk, surface, and interface properties to chemical and physical structural characteristics requires a variety of techniques recently developed for both the macroscopic and the nanoscales. The performance of these materials will provide feedback for modifying the chemistry and physics of the building blocks and the assembled structures. Through this iterative cycle, the ultimate goal of designing and building specific structures that display a particular set of macroscopic properties can be moved from a societal vision to a scientific reality.
To achieve this objective, we need a deeply multidisciplinary collaborative approach. This will require the integration of cutting-edge experimental and theoretical/computational physics with innovative synthetic designs and techniques in almost unprecedented ways. Although the difficulties are great, the rewards of a tomorrow with new, ubiquitous, and life enhancing technologies that in the future we could never imagine living without make this a worthy effort.
The author is indebted to his colleagues and friends, too numerous to be cited, in the fields of polymer and macromolecular chemistry and physics at The University of Akron as well as other institutions for their valuable ideas and suggestions in the development of the views expressed in this short article. The support of NSF DMR-0203994 and 0516602 is gratefully acknowledged.