Polymer-grafted Nanoparticles Research Articles

Role of Grafting Mechanism on the Polymer Coverage and Self-Assembly of Hairy Nanoparticles.

It is now well-accepted that controlling the spatial dispersion of nanoparticles (NPs), which can be achieved by grafting them with polymers of different chain lengths and grafting densities, is central to optimizing the thermomechanical properties of the resulting polymer nanocomposites. In general, there are two methods for creating such polymer-grafted NPs: "grafting to" and "grafting from". The conventional wisdom is that the "grafting from" mechanism, where monomer-sized initiator/functional groups are attached to the surface followed by growing the chains, allows for higher polymer grafting densities and hence a more uniform polymer coverage of the NP surface. Here, we perform calculations and instead show that the "grafting to" strategy surprisingly leads to a more uniform polymer coverage of the NP surface at a given grafting density since the brush is formed while respecting the excluded volume constraints of the previously grafted chains. This conclusion is especially clear in the limit of low-to-moderate grafting density. Thus, at a given grafting density, the "grafting to" mechanism leads to an enhanced miscibility of the NPs in the matrix (which has the same chemistry as the grafts) and lower propensity to create self-assembled structures. Another important factor is that the dispersity in the number of grafted chains on the NPs is also smaller in the case of "grafting to" systems, thus leading to better defined materials. These two conclusions imply that the "grafting to" mechanism may provide better control over the NP dispersion state and hence the thermomechanical properties of polymer nanocomposites.

Synthesis of Nanoparticle Copolymer Brushes via Surface-Initiated seATRP

Synthesis of densely grafted (co)polymer brushes, including poly(tert-butyl acrylate) (PtBA), poly(tert-butyl acrylate)-b-poly(styrene) (PtBA-b-PS), and poly(tert-butyl acrylate)-b-poly(butyl acrylate) (PtBA-b-PBA), from 15.8 nm silica nanoparticles (NPs) via a surface-initiated simplified electrochemically mediated atom transfer radical polymerization (SI-seATRP) under constant potential electrolysis conditions is reported for the first time. The rate of polymerization was enhanced by applying more negative potentials, which resulted in an increase in grafting density, up to 0.93 nm–2. Furthermore, temporal control over the polymerization was achieved by adjustment of the applied potential. The polymers showed narrow molecular weight distributions (Đ = 1.20–1.32), and polymer-grafted nanoparticles had excellent size uniformity. Dispersions in suitable solvents were stable without any nanoparticle aggregation. Microstructures observed in thin films of densely block copolymer tethered silica NPs suggested ...

Reversibly Manipulating the Surface Chemistry of Polymeric Nanostructures via a "Grafting To" Approach Mediated by Nucleobase Interactions.

“Grafting to” polymeric nanostructures or surfaces is a simple and versatile approach to achieve functionalization. Herein, we describe the formation of mixed polymer-grafted nanoparticles through a supramolecular “grafting to” method that exploits multiple hydrogen-bonding interactions between the thymine (T)-containing cores of preformed micelles and the complementary nucleobase adenine (A) of added diblock copolymers. To demonstrate this new “grafting to” approach, mixed-corona polymeric nanoparticles with different sizes were prepared by the addition of a series of complementary diblock copolymers containing thermoresponsive poly(N-isopropylacrylamide) (PNIPAM) to a preformed micelle with a different coronal forming block, poly(4-acryloylmorpholine) (PNAM). PNIPAM chains were distributed throughout the corona and facilitated a fast and fully reversible size change of the resulting mixed-corona micelles upon heating. Through the introduction of an environmentally sensitive fluorophore, the reversible changes in nanoparticle size and coronal composition could be easily probed. Furthermore, preparation of mixed-corona micelles also enabled ligands, such as d-mannose, to be concealed and displayed on the micelle surface. This supramolecular “grafting to” approach provides a straightforward route to fabricate highly functionalized mixed polymeric nanostructures or surfaces with potential applications in targeted diagnosis or therapy and responsive surfaces.

Particle localization and hyperuniformity of polymer-grafted nanoparticle materials.

The properties of materials largely reflect the degree and character of the localization of the molecules comprising them so that the study and characterization of particle localization has central significance in both fundamental science and material design. Soft materials are often comprised of deformable molecules and many of their unique properties derive from the distinct nature of particle localization. We study localization in a model material composed of soft particles, hard nanoparticles with grafted layers of polymers, where the molecular characteristics of the grafted layers allow us to "tune" the softness of their interactions. Soft particles are particular interesting because spatial localization can occur such that density fluctuations on large length scales are suppressed, while the material is disordered at intermediate length scales; such materials are called "disordered hyperuniform". We use molecular dynamics simulation to study a liquid composed of polymer-grafted nanoparticles (GNP), which exhibit a reversible self-assembly into dynamic polymeric GNP structures below a temperature threshold, suggesting a liquid-gel transition. We calculate a number of spatial and temporal correlations and we find a significant suppression of density fluctuations upon cooling at large length scales, making these materials promising for the practical fabrication of "hyperuniform" materials.

Using Torsion for Controllable Reconfiguration of Binary Nanoparticle Networks.

Mechanical deformation can potentially provide an effective means of controlling the nanoscale morphology in hybrid materials. The challenge, however, is establishing optimal couplings of the deformation and mechano-responsive components in the material to achieve nanoscopic structural reorganization without causing catastrophic damage. Through computational modeling, we investigate how torsion can be utilized to induce controllable structural changes in networks formed from binary mixtures (A and B) of polymer-grafted nanoparticles (PGNs). The nanoparticles' rigid cores are decorated with a corona of grafted polymers, which contain reactive functional groups at the chain ends. With the overlap of the neighboring coronas, these reactive groups form labile bonds, which can reform after breakage. The labile bond energy between similar PGNs (UAA, UBB) is different than the energy between dissimilar species (UAB). By tailoring the relative values of these bond energies and the boundary conditions acting on the system, the application of a torsional deformation can result in a controllable reconfiguration of the network, leading to intertwining helical structures, or homogeneously mixed nanocomposites. In effect, our mechano-mutable system resembles a "Rubik's cube" material, whose nanostructure, and hence global properties, can be tailored by mechanically twisting the sample.

Surface Segregation and Bulk Aggregation in an Athermal Thin Film of Polymer-Nanoparticle Blends: Strategies of Controlling Phase Behavior.

The phase behavior of an athermal film of a polymer-nanoparticle blend (PNB) driven by depletion attraction is investigated by dissipative particle dynamics for nanospheres and nanocubes. Surface segregation is observed at low nanoparticle concentrations, while bulk aggregation is seen at high concentrations. Surface excess and the aggregation number can be controlled by tuning the nanoparticle concentration. As surface-roughened or polymer-grafted nanoparticles are used, uniform PNBs are acquired due to the lack of depletion. Thus, addition of surface-roughened nanoparticles into PNBs of smooth nanoparticles can be employed to tune the phase characteristics. It is found that bulk aggregation is suppressed for both polymer-nanosphere and polymer-nanocube blends. However, surface segregation is impeded for polymer-nanosphere blend but enhanced for polymer-nanocube blend owing to the distinct influence of the nanoparticle shape on depletion.

Entropy-driven segregation of polymer-grafted nanoparticles under confinement

The modification of nanoparticles with polymer ligands has emerged as a versatile approach to control the interactions and organization of nanoparticles in polymer nanocomposite materials. Besides their technological significance, polymer-grafted nanoparticle (PGNP) dispersions have attracted interest as model systems to understand the role of entropy as a driving force for microstructure formation. For instance, densely and sparsely grafted nanoparticles show distinct dispersion and assembly behaviors within polymer matrices due to the entropy variation associated with conformational changes in brush and matrix chains. Here we demonstrate how this entropy change can be harnessed to drive PGNPs into spatially organized domain structures on submicrometer scale within topographically patterned thin films. This selective segregation of PGNPs is induced by the conformational entropy penalty arising from local perturbations of grafted and matrix chains under confinement. The efficiency of this particle segregation process within patterned mesa-trench films can be tuned by changing the relative entropic confinement effects on grafted and matrix chains. The versatility of topographic patterning, combined with the compatibility with a wide range of nanoparticle and polymeric materials, renders SCPINS (soft-confinement pattern-induced nanoparticle segregation) an attractive method for fabricating nanostructured hybrid films with potential applications in nanomaterial-based technologies.

Polystyrene-Grafted Silica Nanoparticles: Investigating the Molecular Weight Dependence of Glass Transition and Fragility Behavior

Polymer-tethered nanoparticles provide a strategy to improve particle dispersion in polymer nanocomposites and as materials themselves can exhibit self-healing behavior and enhanced mechanical properties. The few studies that previously characterized the glass transition temperature (Tg) behavior of neat polymer-grafted nanoparticles in the absence of a polymer matrix largely focused on average Tg response. We synthesized polystyrene-grafted silica nanoparticles (Si-PS) via ARGET ATRP, achieving the densely grafted state. Using differential scanning calorimetry, we investigated the brush molecular weight (MW) dependence of Tg, Tg breadth, heat capacity jump (ΔCp), and fragility from 12 to 98 kg/mol. Compared with free PS chains of the same MW, brush Tg increases by 1–2 °C, brush Tg breadth remains unchanged within error down to 36 kg/mol and increases by 3–4 °C at brush MWs of 12 and 13 kg/mol, and brush ΔCp and fragility remain unchanged within error down to 52 kg/mol and then decrease with decreasing MW...

Anisotropic Three-Particle Interactions between Spherical Polymer-Grafted Nanoparticles in a Polymer Matrix

Spherical nanoparticles (NPs) uniformly grafted with polymer chains have recently been shown to assemble into anisotropic phases like strings and sheets. Here we investigated the underlying basis for anisotropic interactions between polymer-grafted NPs in a polymer matrix by computing via molecular dynamics simulations the potential of mean force (PMF), and its three-body contribution, for a test NP interacting with a NP-dimer along a set of reaction coordinates differing in their orientation with respect to the dimer axis. The polymer-mediated portions of the PMF and of the three-body contribution were both found to be highly repulsive and anisotropic with the degree of repulsion rising with increasing angular deviation from the dimer axis. The anisotropy was shown to arise from the expulsion of polymer grafts from in between the dimer NPs which leads to a gradient in the graft segmental density around the dimer from its contact point to its poles. This effect produces a concomitant gradient in steric re...

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

A simple and robust platform, based on polymer-grafted nanoparticles to facilitate a controlled morphology of nano-fillers, provides a route for strategically designing nanostructured EM wave absorbers with a percolated MWCNT network as well as a controlled NP arrangement.

Tailoring dielectric properties using designed polymer-grafted ZnO nanoparticles in silicone rubber

Tuning of the dielectric properties of PDMS–ZnO nanocomposites by the control of inter-particle distances and self-assembled NP-morphologies via polymer grafting.

Computer simulation study on the self-assembly of unimodal and bimodal polymer-grafted nanoparticles in a polymer melt.

The controllable distribution of nanoparticles (NPs) in polymer nanocomposites (PNCs) is a challenge in materials science. An important method is grafting chains that are chemically identical to the polymer matrix on NPs. By performing comprehensive molecular dynamics simulations, the self-assembly behavior of polymer-grafted NPs in a polymer matrix is investigated in this study. The relationship between the grafted chain length N, grafting density σ and the NPs' self-assembly morphologies is studied. Phase diagrams of the NP self-assembly structures for both unimodal and bimodal grafted NP systems are constructed on a parameter space, where P is the matrix polymer chain length. NP self-assembly structures of strings, connected/sheet and small clusters are identified in different regions. In order to quantitatively characterize the NP self-assembly morphology, we define a morphological measurement parameter which characterizes the distribution of the Voronoi cell volume of the NPs. Using this parameter, we discuss the influences of both long and short grafted chains on the dispersion of bimodal polymer-grafted NPs in a polymer melt. We find that the short grafted chains can not only shield the NP surface from the polymer matrix but also elongate the long grafted chains into the polymer matrix, therefore favoring a better dispersion of NPs. Our results also indicate that the bimodal grafted NPs will not be fully dispersed until the short grafted chains are dense enough to elongate the long grafted chains, hence forming a wetting NP/matrix interface.

Effects of morphology on the mechanical properties of heterogeneous polymer-grafted nanoparticle networks

The layered binary nanoparticle networks exhibit superior tensile properties and remarkable resilience in comparison with the random binary mixtures.

Assembling and ordering polymer-grafted nanoparticles in three dimensions.

Taking advantage of the aqueous biphasic behavior of polyethylene glycol (PEG)/salts, recent experiments have demonstrated self-assembly and crystallization of PEG-grafted gold nanoparticles (PEG-AuNPs) into tunable two-dimensional (2D) supercrystals by adjusting salt concentration (for instance, K2CO3). In those studies, combined experimental evidence and theoretical analysis have pointed out the possibility that similar strategies can lead to three-dimensional (3D) formation of ordered nanoparticle precipitates. Indeed, a detailed small-angle X-ray scattering (SAXS) study reported herein reveals the spontaneous formation of PEG-AuNPs assemblies in high-concentration salt solutions that exhibit short-range 3D order compatible with fcc symmetry. We argue that the assembly into fcc crystals is driven by partnering nearest-neighbors to minimize an effective surface-tension gradient at the boundary between the polymer shell and the high-salt media. We report SAXS and other results on PEG-AuNPs of various Au core diameters in the range of 10 to 50 nm and analyze them in the framework of brush-polymer theory revealing a systematic prediction of the nearest-neighbor distance in the 3D assemblies.

Using Theory and Simulations To Calculate Effective Interactions in Polymer Nanocomposites with Polymer-Grafted Nanoparticles

Using theory and large-scale simulations, we demonstrate how one can program structure and thermodynamics into polymer-grafted particles filled polymer nanocomposites (PNCs). We simulate varying graft (G) and matrix (M) polymer compositions for varying model graft–matrix bead pairwise interactions, χGM, and calculate structural features and the effective graft–matrix interaction parameter, χGMeff, in the PNC. Varying the graft (G) and matrix (M) polymer compositions provides tunability of morphology (particle dispersion/aggregation) and graft–matrix interpenetration at each χGM. Thermodynamically, for all composites the χGMeff exhibits negative values (effective attraction) at low values of χGM, with a sharp transition to positive values (effective repulsion) at large values of χGM. The sharp transition in χGMeff coincides with the structurally characterized particle dispersion–aggregation transition marked by the onset of upturn in the matrix–matrix structure factor at zero wavenumber. Strikingly, regard...

Stability of Polymer Grafted Nanoparticle Monolayers: Impact of Architecture and Polymer-Substrate Interactions on Dewetting.

The stability of polymer thin films is crucial to a broad range of technologies, including sensors, energy storage, filtration, and lithography. Recently, the demonstration of rapid deposition on solid substrates of ordered monolayers of polymer grafted nanoparticles (PGN) has increased potential for inks to additively manufacture such components. Herein, enhanced stability against dewetting of these self-assembled PGN films (gold nanoparticle functionalized with polystyrene (AuNP-PS)) is discussed in context to linear polystyrene (PS) analogues using high throughput surface gradients: surface energy (20-45 mN/m) and temperature (90-160 °C). PGNs exhibit a lower surface (γp) and interfacial (γsp) energy relative to linear polymers, which results in increased thermal and energetic stability by 10-25 °C and 5-15 mN/m, respectively. This enhanced wetting-dewetting transition is qualitatively consistent with the behavior of star macromolecules and depends on the architecture of the polymer canopy. Increased film stability through canopy architecture expands the manufacturability of thin film hybrids and refines postprocessing conditions to maximize local PGN order.

Molecular Origins of Temperature-Induced Jamming in Self-Suspended Hairy Nanoparticles

Suspensions of solvent-less, polymer-grafted nanoparticles have previously been reported to exhibit enhanced elasticity, glassy dynamics, and jamming as temperature is increased. This so-called thermal jamming behavior is not predicted by current models for soft glassy materials and is opposite to what is normally observed for polymer melts. Here we report results from a detailed study of structural, dynamic, and rheological transitions in solvent-less silica–poly(ethylene glycol) methyl ether (SiO2–PEGME) hybrid particles aimed at understanding the molecular origins of the phenomenon. We find that interdigitated PEGME corona chains enforce coupling between SiO2 cores analogous to cross-links in associating polymer networks leading to elastic behavior at low strains and small deformation rates. Pullout and slippage of interdigitated corona chains is thought to produce yielding of the materials in nonlinear shear flow and stretched exponential relaxation following small amplitude step shear. We show with the help of small-angle X-ray scattering, density functional theoretical calculations, and a constitutive model previously reported for deformation and flow of physically associating polymer networks, that thermal jamming of solvent-less polymer-grafted nanoparticles originates from enhanced cross-linking of the cores produced by increased stretching and interpenetration of particle-tethered polymer molecules to satisfy the space-filling constraint on tethered polymer chains.

Breakdown of the Stokes–Einstein Relation for the Rotational Diffusivity of Polymer Grafted Nanoparticles in Polymer Melts

We report observations of breakdown of the Stokes-Einstein relation for the rotational diffusivity of polymer-grafted spherical nanoparticles in polymer melts. The rotational diffusivity of magnetic nanoparticles coated with poly(ethylene glycol) dispersed in poly(ethylene glycol) melts was determined through dynamic magnetic susceptibility measurements of the collective rotation of the magnetic nanoparticles due to imposed time-varying magnetic torques. These measurements clearly demonstrate the existence of a critical molecular weight for the melt polymer, below which the Stokes-Einstein relation accurately describes the rotational diffusivity of the polymer-grafted nanoparticles and above which the Stokes-Einstein relation ceases to apply. This critical molecular weight was found to correspond to a chain contour length that approximates the hydrodynamic diameter of the nanoparticles.

Self-assembled polymer-grafted nanoparticles for photonic coating applications

Control of a regular arrangement of nanoparticles and their mechanical stability in a form of arrays and films becomes important in various particle-based technologies such as photonic bandgap materials. Here, we present that polymer-grafted nanoparticles (i.e. polystyrene-grafted silica nanospheres), synthesized by surface-initiated atom radical polymerization (SI-ATRP), can achieve ordered yet mechanically robust films by tuning the polymer brush architecture such as molecular weight and grafting density. Evolution of the particle arrangement in the polymer-grafted nanoparticle films is observed and associated with their wavelength at the maximum reflectance as well as their mechanical characteristics. This could open up new opportunities for fabrication of durable and highly ordered colloidal crystal structures such as photonic coating applications.

Integral equation prediction of structure of nanocomposites with polymer-grafted nanoparticles near solid surface

In this work, the inhomogeneous polymer reference interaction site model (PRISM) integral equation theory is extended to investigate the microscopic structure of a new system of nanocomposites with polymer-grafted nanoparticles near solid surface. The chemical and morphologic details of polymers are described by a semiflexible chain model, which can be utilized to capture the intramolecular interactions between different segments. Based on the novel bridge functionals of different segments constructed from the corresponding chemical potentials, the modified hypernetted chain approximation is combined with the inhomogeneous PRISM equation to obtain the detailed density distributions of nanoparticle-polymer blends near substrate. The effects of particle volume fraction, nanoparticle size, attractive interaction strength between different segments and chain stiffness on the density profiles of single tethered polymer nanocomposites (PNCs) near surface are systematically investigated to capture the contribution of the packing and configurational entropies. The end effects of polymer chains and effects of solid wall confinement are also considered. It is found that the particle size and particle volume fraction play the most significant roles in the density profiles of the single tethered PNCs among the present investigated model parameters. The increase of the particle volume fraction leads to a monotonic increasing variation behavior of the density profiles of single tethered nanoparticles as the particle diameter is relatively small. With increasing the nanoparticle size, the intensity of the density profiles of nanoparticles decreases prominently. The detailed packing structure of polymer-nanoparticle blends is also influenced by the polymer chain stiffness and particle-monomer interactions, where these parameters of structure and chemistry can enable the density profiles of the single tethered PNCs to show similar variation trends. The present inhomogeneous PRISM theory can provide a detailed description of packing and conformational entropies on the density profiles of different segments in nanocomposites with polymer-grafted nanoparticles. It could also be easily extended to some real systems with more complicated architecture of polymer-grafted nanoparticles under confinement and anticipated to assist in developing some predictive approaches for design control of PNCs near substrate.