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Future technologies depend on the development of functional material having 1) hierarchical structures spanning multi-length scale down to the molecular level; 2) built-in functionalities, such as biological, optical, electronic, magnetic property; 3) superior selectivity with sensitivity comparable to what found in nature and 4) responsiveness to the external stimuli. These require one to select the right building blocks, understand the principles underpinning the self-assembly, and use these principles to direct the assemblies at various length scales to obtain targeted functional materials. More importantly, we need to develop a versatile methodology to generate new functional materials simply by substituting building blocks, instead of re-building them from scratch. Numerous building blocks have been explored to achieve this end and obtaining functional materials to meet these requirements remains a significant challenge to the soft material community.
Our group focuses on generating hierarchical functional soft materials using synthetic polymers, peptides and proteins, small organic molecules and nanoparticles as building blocks. Each offers unique properties and is complimentary to each other. Natural proteins have complexities in terms of structure and functionaly unmatched by synthetic materials. De novo designed peptides are minimalistic natural proteins that mimic natural protein functionalities but are subject to degradation. Small organic molecules, especially those with optical, electronic and magnetic properties, can be readily synthesized with molecular control to provide built-in functionalities. Nanoparticles, due to their size, exhibit unique properties not seen in macroscopic materials and constitute essential building blocks in the fabrication of nanodevices. It is non-trivial to obtain macroscopic assemblies of either small molecule or nanoparticle at low cost. Polymers with different architectures can be synthesized and are amenable to various processing techniques. Developments in polymer sciences provide guidance to manipulate their assemblies at different length scales. However, generating molecular level assemblies with designer functionalities using polymers alone is challenging. Synergistic assemblies of these selected building blocks clearly have tremendous potential to construct technologically important functional materials. By developing fundamental understanding of the physics of assemblage, our group aims to generate hierarchical structures spanning multi-length scales down to few nanometers with built-in biological, electrical and magnetic functionalities. Currently, our group is engaged in three major research activities to address the challenge of multi-length scale, hierarchical functional assemblies using these building blocks.
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Amphiphlic Polymer/Peptide Conjugates with Side-Conjugation
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Hybrid materials based on peptide-polymer conjugates have been extensively studied for the design of novel functional materials. The fundamental understanding of the effect of the interaction between peptides and polymers on the structural and functional properties of these materials is crucial for their development into technologically relevant materials. We have been investigating the effect of site specific conjugation of polymers with different hydrophobicity on the secondary and tertiary structure of coiled coil helix-bundle peptides. In order to confirm the hypothesis of importance of the polymer hydrophobicity on peptide structure, conjugates of the coiled coil peptide with a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM) have been studied. In case of PNIPAM conjugates, the peptide structure is maintained at room temperature and the rate of peptide unfolding with temperature is greater than that of unmodified peptide. We attribute the larger helicity losses for PNIPAM conjugates to the increase in polymer hydrophobicity with increase in temperature, which leads to unfolding of alpha-helices. The future work involves quantitative evaluation of the thermodynamic and kinetic stability of the peptide-polymer conjugates with polymers of different chemical structure and molecular weights.
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Effect of side-chain conjugation on unfolding rate of polymer/peptide conjugates |
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Directed Assembly of Nanoparticles in Block Copolymer-based Supramolecules in Bulk
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Precise control of the spatial organization of nanoscopic building blocks, for example nanoparticles, over multiple length scales has been a bottleneck in the 'bottom-up' generation of technologically important materials. Only a few methods have been shown to achieve well-ordered nanoparticle assemblies without surface modification. We have developed a simple yet versatile approach to produce hierarchical assembiles of various nanoparticles by using small molecules and block copolymers. By utilizing the small molecules to mediate the interactions between the polymer and the nanoparticles and to modify the chain architecture of one of the diblocks, we can generate organized nanoparticle arrays with controlled inter-particle separation and ordering without the chemical modification of either nanoparticle ligands or block copolymers. The design of the system allows its use with a variety of different small molecules and nanoparticles. At present, we are investigating use of this system to order and align anisotropic nanorod particles, and to incorporate additional functionality into the nanocomposite through the small molecules.
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Self-assembly of functional small molecules in bulk diblock copolymer |
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Directed Assembly of Nanoparticles in Block Copolymer-based Supramolecular Thin Films |
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Controlling nanoparticle assemblies in thin films enables one to exploit their collective properties to generate functional devices such as hybrid photovoltaic cells, capacitors and optical waveguides. This requires precise control over nanoparticle assemblies over multiple length scales with a single particle precision. Moreover, to be compatible with large-scale processing, there is a need to reduce the processing time down to minutes. We have developed a simple and robust approach where hierarchical assemblies of nanoparticles in thin films can be achieved in minutes with control over the macroscopic alignment of nanoparticle assemblies as well as the inter-particle ordering. We investigate the effects of the architecture of the supramolecules, the film thickness, the nanoparticle loading, the size of nanoparticles, and the solvent annealing environment on the self-assembly of the nanoparticles in the nanocomposites. The fundamental studies enable us to gain full understanding of the system and potentially exploit the unique optical and electrical properties of the nanocomposites for novel devices.
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Directed assembly of nanoparticles using a block copolymer based supramolecule |
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Subnanometer Porous Membrane Based on Co-assembly of Cyclic Peptide-Polymer Conjugates in Block Copolymer
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Nature routinely produces membranes containing oriented channels with molecularly defined size, topology and surface chemistry. However achieving this with man-made materials remains a significant challenge. This project focuses on fabricating polymeric membranes containing subnanometer channels that are vertically aligned which make them useful for various applications such as carbon capture, different separation processes in both gaseous phase and solution phase. We use cyclic peptide as our building blocks in which individual cyclic ring stacks to form nanotubes. By conjugating with polymer chains, the cyclic peptide-polymer nanotubes can be well assembled in a polymeric matrix to form vertically aligned channels. Functionality can be introduced in the interior of the channels by changing the cyclic peptide sequence. This allows us the flexibility to fine-tune the performances of these membranes for different purposes. Our research focuses on understanding the fundamental assembly process of the system and investigating various parameters that are crucial for application.
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Self assembly of cyclic peptide nanotubes in block copolymer thin film |
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The Ting Xu Group |
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