Jihoon Choi

Flexible particle array structures by controlling polymer graft architecture.

Surface-initiated atom-transfer radical polymerization is used to synthesize particle brushes with controlled fraction of extended and relaxed conformations of surface-grafted chains. In the semidilute brush limit, the grafting of polymeric ligands is shown to facilitate the formation of ordered yet plastic-compliant particle array structures in which chain entanglements give rise to fracture through a polymer-like crazing process that dramatically increases the toughness and flexibility of the particle assembly.

Toughening fragile matter: mechanical properties of particle solids assembled from polymer-grafted hybrid particles synthesized by ATRP

The effect of polymer-graft modification on the structure formation and mechanical characteristics of inorganic (silica) nanoparticle solids is evaluated as a function of the degree of polymerization of surface-grafted chains. A transition from ‘hard-sphere-like’ to ‘polymer-like’ mechanical characteristics of particle solids is observed for increasing degree of polymerization of grafted chains. The elastic modulus of particle solids increases by about 200% and levels off at intermediate molecular weights of surface-grafted chains, a trend that is rationalized as a consequence of the elastic modulus being determined by dispersion interactions between the polymeric grafts. A pronounced increase (of about one order of magnitude) of the fracture toughness of particle solids is observed as the degree of polymerization of grafted chains exceeds a threshold value that is similar for both polystyrene and poly(methyl methacrylate) grafts. The increased resistance to fracture is interpreted as a consequence of the existence of entanglements between surface-grafted chains that give rise to energy dissipation during fracture through microscopic plastic deformation and craze formation. Within the experimental uncertainty the transition to polymer-like deformation characteristics is captured by a mean field scaling model that interprets the structure of the polymer shell of polymer-grafted particles as effective ‘two-phase’ systems consisting of a stretched inner region and a relaxed outer region. The model is applied to predict the minimum degree of polymerization needed to induce polymer-like mechanical characteristics and thus to establish ‘design criteria’ for the synthesis of polymer-modified particles that are capable of forming mechanically robust and formable particle solid structures.

Silica-Polymethacrylate Hybrid Particles Synthesized Using High-Pressure Atom Transfer Radical Polymerization

Hybrid nanoparticles with a silica core and grafted poly(methyl methacrylate) (PMMA) or poly(n-butyl methacrylate) (PBMA) chains were prepared via activators generated by electron transfer for atom transfer radical polymerization (AGET ATRP) at room temperature under high pressure. Due to enhanced propagation rate constant and reduced termination rate constant for polymerizations conducted under high pressure, the rate of polymerization was increased, while preserving good control over polymerization when compared to ATRP under ambient pressure. Molecular weights of greater than 1 million were obtained. The PMMA and PBMA brushes exhibited semi-diluted or diluted brush architecture with the highest grafting densities ≈0.3 chain·nm(-2).

Effect of Polymer-Graft Modification on the Order Formation in Particle Assembly Structures

The propensity of particle brush materials to form long-ranged ordered assembly structures is shown to sensitively depend on the brush architecture (i.e., the particle radius as well as molecular weight and grafting density of surface-bound chains). In the limit of stretched chain conformations of surface-grafted chains the formation of regular particle array structures is observed and interpreted as a consequence of hard-sphere-type interactions between polymer-grafted particles. As the degree of polymerization of surface-grafted chains increases beyond a threshold value, a reduction of the structural regularity is observed that is rationalized with the increased volume occupied by relaxed polymer segments. The capacity of polymer grafts to increase or decrease order in particle brush assembly structures is interpreted on the basis of a mean-field scaling model, and design criteria are developed to help guide the future synthesis of colloidal systems that are capable of forming mechanically robust yet ordered assembly structures.

Anisotropic Elasticity of Quasi-One- Component Polymer Nanocomposites

The in-plane and out-of-plane elastic properties of thin films of quasi-one-component particle-brush-based nanocomposites are compared to those of classical binary particle-polymer nanocomposite systems with near identical overall composition using Brillouin light scattering. Whereas phonon propagation is found to be independent of the propagation direction for the binary particle/polymer blend systems, a pronounced splitting of the phonon propagation velocity along the in-plane and out-of-plane film direction is observed for particle-brush systems. The anisotropic elastic properties of quasi-one-component particle-brush systems are interpreted as a consequence of substrate-induced order formation into layer-type structures and the associated breaking of the symmetry of the film. The results highlight new opportunities to engineer quasi-one-component nanocomposites with advanced control of structural and physical property characteristics based on the assembly of particle-brush materials.

Excluded Volume Model for the Reduction of Polymer Diffusion into Nanocomposites

An analytic model for the slowing down of polymer chain diffusion in nanocomposites attributable to excluded volume effects is presented. The nanocomposite is modeled as an ensemble of cylinders through which the polymer chains diffuse. The reduction of polymer diffusion in each cylinder is equated with the reduction of diffusion for a sphere through a cylinder. The distribution of cylinder diameters within the ensemble is determined from statistical mechanical theories based on the packing of spherical particles. For low loadings of spherical particles in nanocomposites, this model results in a master curve for the reduced diffusion coefficient. With no adjustable parameters, the model agrees with recent data for tracer diffusion measurements in polymer nanocomposites at low loading.

Flexible Transparent Metal/Polymer Composite Materials Based on Optical Resonant Laminate Structures

Suitable design of periodic metal/polymer composite materials is shown to facilitate resonant tunneling of light at absorbing wavelengths and to provide a means to significantly reduce optical absorption losses in polymer-based metallodielectric composite structures. The conditions for resonant tunneling are established based on the concept of photonic band edge alignment in 1D-periodic systems. For the particular case of a four-layer gold/polystyrene laminate structure, it is shown that the matching of the lower band edge of the 1D-periodic structure with the plasma frequency of the metal component facilitates the increase of optical transmission by about 500% as compared to monolithic film structures of equal total thickness. The effect of sheet thickness on the optical properties of thin metal films is determined and shown to be an important prerequisite for the reliable prediction of resonant metallodielectric structures. The resonant 1D-periodic metal/polymer heterostructures are shown to retain the flexural stability of the polymer matrix and thus could find application as flexible transparent conductors in areas such as plastic electronics.

Controlled morphology of MWCNTs driven by polymer-grafted nanoparticles for enhanced microwave absorption

The development of a lightweight electromagnetic (EM) wave absorber with a wider bandwidth and reflection loss at a low loading is of great interest for applications ranging from conventional electronic devices to specific devices or instruments of the military and aerospace. Although nano-filler (i.e. carbon nanotubes, magnetic nanoparticles, etc.) based polymer nanocomposites (PNCs) have shown great promise in this area of research, typically poor control of their surface modification and dispersion has prevented further development of these materials for application in EM wave absorbers. Here, we introduce and demonstrate a simple and robust platform based on polymer-grafted nanoparticles to facilitate a controlled morphology of nano-fillers, providing a route for strategically designing nanostructured EM wave absorbers with a percolated MWCNT network as well as a controlled NP arrangement. At equal loadings of nano-fillers (1 wt% of MWCNTs), much deeper reflection loss (RL = −26.9 dB at 5.4 GHz) and a wider bandwidth (4.9 GHz for RL < −10 dB) are observed compared to the conventional PNCs. We show that polymer-grafted nanoparticles serve as a matrix for unfunctionalized CNTs and show a much enhanced dispersion of CNTs, providing a novel opportunity for the multi-functional PNCs by combining functions arising from the controlled dispersion of heterogeneous materials (i.e. inorganic nanoparticles and CNTs) in a new type of CNT/NP/polymer nanostructure.

Electron-Irradiated Polymer Brushes for Hollow Carbon Nanosphere Arrays

A thermally stable 2D array of spheres and their morphology control become important for the fabrication of novel nanostructures. Here, a simple method is presented for fabrication of large-area and well-ordered arrays of carbonized polystyrene (PS) hollow spheres with a controlled (close-packed or non-close-packed hexagonal) morphology, prepared by combining the self-assembly of PS-grafted silica nanoparticles, etching, electron irradiation, and subsequent thermal annealing. Fine control in the 2D or 3D nanostructure of carbon materials can open up new opportunities for high-performance nanoscale applications that require an efficient fabrication method for preparation of the porous carbon array.

1D periodic bimetallic superstructures by co-assembly of ternary block copolymer/nanoparticle blends

The formation of hierarchical 1D periodic bimetallic superstructures by co-assembly of ternary block copolymer/particle blend systems is reported. The results point to the relevance of entropy-driven morphology selection processes in enthalpic neutral block copolymer/particle blends that should render the presented approach applicable to a wide range of polymer/particle compositions.