In the beginning of the 1960s, Richard Feynman raised the question: ‘What would the properties of materials be if we could really arrange the atoms the way we want them?’ Not only has this question led to the burgeoning of nanoscience, but it has also inspired many scientists to develop “precise” macromolecules (polymers), whose properties and functions are determined by the hierarchical structures assembled in an organised manner across multiple length scales from nano-building blocks.
This is exactly the focus of Dr Stephen Cheng’s work in the Department of Polymer Science at The University of Akron. His primary research is dedicated to the condensed states of polymers, liquid crystals and surfactants, including phase transitions, thermodynamics and kinetics of metastable states, ordered structures and morphologies, surfaces and interfaces in electronic, optical, and advanced functional hybrid materials. His recent focus is on the construction of giant molecules (precisely defined macromolecules) with molecular nanoparticle building blocks and the illustration of the underlying physics for technological applications. These are undoubtedly the key to ‘bottom-up’ nanofabrication technology, offering a distinct scientific understanding of self-assembly and a unique route to the creation of unconventional hierarchical structures and dynamics.
The Holy Grail of polymer science
There are several key aspects which form the core of Dr Cheng’s research. Self-assembly is the process whereby pre-existing components spontaneously form ordered structures and patterns at any scale driven by molecular interactions and thermodynamics. Down to the nanoscale level, self-assembly relies on the structural parameters of each individual molecule. They include shape, charge, functional groups, configuration, and conformation – the elements required to allow molecules to associate with one another to produce sophisticated functional systems. Control of these elements in macromolecules isn’t trivial. In fact, it is the Holy Grail of polymer science to do it precisely.
Macromolecules are divided into two classes: natural macromolecules, such as proteins and nucleic acids, and synthetic polymers, such as common plastics, nylon and Plexiglas. In fact, synthetic polymers typically consist of repeating molecules that are covalently linked (they share electron pairs between their atoms) to one another, and whose properties are determined by their overall molecular weights, polydispersity, chain topology, etc. By contrast, natural macromolecules usually possess precisely defined structures, including molecular weight, sequence, and stereochemistry, to achieve predetermined functions at a level not paralleled by synthetic polymers. In other words, the construction and assembly of macromolecules demand the control of the polymer’s primary chemical structure with molecular precision.
Even though this ‘control’ has been the fundamental topic of research in polymer science for decades, there has been limited success. It hinders our understanding of macromolecules and the use of them to generate systems with supramolecular structures that can efficiently transfer and amplify their individual molecular properties and functions through to macroscopic levels.
A new class of self-assembling materials
A giant molecule, much like its name suggests, is a covalently bonded structure that contains a great number of atoms. Typical examples of such molecules that have the capacity to form giant structures include silicon, silicon dioxide, diamond carbon and graphite. Dr Cheng has added a new branch to the family of giant molecules using selected, precisely functionalised, molecular nanoparticles. These elemental building blocks are called nano-atoms and it is their exact and specified properties (such as their volume, symmetry, and surface functionalities) that Dr Cheng is exploiting in order to construct new prototype giant molecules in a modular, efficient, and precise manner and to afford on-demand hierarchical structures via triggered assemblies. In fact, according to research papers published by his group in recent years, the self-assembly of giant molecules, such as giant polyhedra (e.g., giant tetrahedra), giant Janus particles, and giant surfactants, exhibits unconventional phases and structures typically not found in soft matter. Moreover, their formations and structures are highly sensitive to their primary chemical structures, a trait that is common for small molecules but not conventional macromolecules. Controlled heterogeneity is the key in the design, making giant molecules a new class of self-assembling materials in addition to surfactants, amphiphilic polymers, and dendrimers.
The Materials Genome approach to giant molecules
Dr Cheng’s group has already proposed a modular approach to construct precise macromolecules using functionalised molecular nanoparticles as the fundamental building blocks. One can compare this process to constructing a structure with LEGOTM: the overall properties of this structure are defined by each LEGOTM brick and, ultimately, these giant molecules can be regarded as size-amplified small-molecule analogues. By doing so, more specifically, the group aims to apply the Materials Genome approach to macromolecules. Multiple molecular nanoparticles could be incorporated into a giant molecule with precisely defined composition, sequence and geometry in a series of ‘click’ reactions. Owing to the fact that such ‘click’ reactions are very selective and efficient, the group can then manipulate primary structures of single molecules, even ones with high molecular weights. The subsequent self-assembly will be propelled by various interactions (hydrophilic and hydrophobic) between these nanoclusters and the overall molecular symmetry. Each building block will exhibit distinct heterogeneity, and hence influence the respective self-assembly process. This is exactly what is needed in order to build structures whose properties will be fine-tuned according to the molecules (nano blocks) inserted. Eventually, the final materials would demonstrate the properties that are the phenotypes of the “genes” of corresponding building blocks, much like how the genome defines life in biological systems.
The importance and significance of Dr Cheng’s work in the Department of Polymer Science at the University of Akron, OH, is self-evident as it intends to improve education and to explore the very frontiers in soft materials-based sciences, engineering and technologies. This scientifically innovative research aims to design and synthesise giant molecules with controlled heterogeneities and defined hierarchical structures via precisely arranged nano-building blocks. Ultimately, Dr Cheng’s group envisions the construction and introduction of precise nanostructures that are not only scientifically intriguing but are also technologically relevant, thus triggering a revolution in the field of polymer science toward materials defined by the genes of each component.
Unlike traditional polymers that usually exist as random coils, molecular nanoparticles are precisely defined molecular moieties with rigid conformation. Their shape, functional groups, symmetry, and molecular weight are predetermined and can be modified with molecular precision. Moreover, the diversity of molecular nanoparticles is tremendous. They are found to possess a broad range of compositions, structures, and functions. Therefore, they can serve as unique and versatile building blocks for new macromolecules and materials.
What more is needed in order to effectively succeed in the team’s objective to unravel and introduce functionalised molecular nanoparticles?
First of all, we need to develop methods for the selective functionalisation of molecular nanoparticles. There are usually multiple functional groups on them. Controlling the location and efficiency of functionalisation is the first step in controlling their 3D arrangement and assembly. Second, we need creative ways to covalently bond molecular nanoparticles of distinct features. In other words, we need chemistry that can bridge materials in different categories for hybrid materials with high efficiency. Third, we need to master their interactions so as to manipulate their assembly across all length, time, and energy scales to the structures that we desire.
Heterogeneity at larger length scales: approximately up to what length scales does your group target (molecular weights)?
Heterogeneity at different length scales can be controlled by different means. At larger length scales, traditional “top-down” approaches are very effective in creating heterogeneity within order. At smaller length scales, the “bottom-up” approach works with molecular precision. It is the mesoscale, from 1–100 nm, that poses the most significant challenge to materials scientists. This is exactly the length scales that we are working on and whose structures our materials are good at controlling.
The group has already suggested a means to construct giant molecules – using MNPs. Is this the only approach to generate such supramolecular structures or is it the one you prefer?
This is definitely not the only approach. I would say that this is one of the most effective ways to generate supramolecular structures at 1–100 nm scale. Giant molecules fill the gap between self-assembling small molecules and traditional amphiphilic block copolymers.
Can you name a few specific industrial applications that can take advantage of the introduction of this innovative technology?
IT technology perhaps would benefit the most from our technology in the creation of sub-10nm structures and patterns. Other potential industrial applications include functional membranes (e.g., for water treatment), optoelectronic displays and components, antibody engineering, and advanced diagnostics. I anticipate that the application would be unlimited since this class of materials’ functions are defined by the genes of the virtually limitless types of building blocks.
Dr Cheng’s research focuses on the condensed states in polymers, liquid crystals, surfactants, micelles and hybrid materials. Within this, he looks into the interactions, dynamics and structures of particular materials.
National Science Foundation (NSF)
Prof Wen-Bin Zhang (Peking University), Prof Takuzo Aida (University of Tokyo), and Prof Sharon Glotzer (University of Michigan)
Dr Cheng received his PhD from Rensselaer Polytechnic Institute in 1985. He has been elected to the National Academy of Engineering in US. He currently holds the Frank C. Sullivan Distinguished Research Professor, Robert C. Musson and Trustees Professor at the University of Akron. He has also received numerous awards, including the Presidential Young Investigator Award, John H Dillon Medal and Polymer Physics Prize.
Dr Stephen Cheng
Goodyear Polymer Center
Room 936, The University of Akron