Brush Particles Research Articles

Architecture Controls Phonon Propagation in All-Solid Brush Colloid Metamaterials.

Brillouin light scattering and elastodynamic theory are concurrently used to determine and interpret the hypersonic phonon dispersion relations in brush particle solids as a function of the grafting density with perspectives in optomechanics, heat management, and materials metrology. In the limit of sparse grafting density, the phonon dispersion relations bear similarity to polymer-embedded colloidal assembly structures in which phonon dispersion can be rationalized on the basis of perfect boundary conditions, i.e., isotropic stiffness transitions across the particle interface. In contrast, for dense brush assemblies, more complex dispersion characteristics are observed that imply anisotropic stiffness transition across the particle/polymer interface. This provides direct experimental validation of phonon propagation changes associated with chain conformational transitions in dense particle brush materials. A scaling relation between interface tangential stiffness and crowding of polymer tethers is derived that provides a guideline for chemists to design brush particle materials with tailored phononic dispersion characteristics. The results emphasize the role of interfaces in composite materials systems. Given the fundamental relevance of phonon dispersion to material properties such as thermal transport or mechanical properties, it is also envisioned that the results will spur the development of novel functional hybrid materials.

Copolymer Brush Particle Hybrid Materials with "Recall-and-Repair" Capability.

The effect of sequence structure on the self-healing and shape-memory properties of copolymer-tethered brush particle films was investigated and compared to linear copolymer analogs. Poly(n-butyl acrylate-co-methyl methacrylate), P(BA-co-MMA), and linear and brush analogs with controlled gradient and statistical sequence were synthesized by atom transfer radical polymerization (ATRP). The effect of sequence on self-healing in BA/MMA copolymer brush particle hybrids followed similar trends as for linear analogs. Most rapid restoration of mechanical properties was found for statistical copolymer sequence; an increase of the high Tg (MMA) component provided a path to raise the material's modulus while retaining self-heal ability. Creep testing revealed profound differences between linear and brush systems. While linear copolymers featured substantial viscous deformation when exposed to constant stress in the linear regime, brush analogs displayed minimal permanent deformation and featured shape restoration. The reduction of flow was interpreted to be a consequence of slow cooperative relaxation due to the complex microstructure of brush particle hybrids in which long-range motions are constrained through entanglements and slow-diffusing particle cores. The rubbery-like response imparts BA/MMA copolymer brush material systems concurrent "shape-memory" and "self-heal" capability. This ability to "recall-and-repair" could find application in the design of functional hybrid materials, for example, for soft robotics.

Topologically Induced Heterogeneity in Gradient Copolymer Brush Particle Materials

Topologically Induced Heterogeneity in Gradient Copolymer Brush Particle Materials

Internal Microstructure Dictates Interactions of Polymer-grafted Nanoparticles in Solution.

Understanding the effects of polymer brush architecture on particle interactions in solution is requisite to enable the development of functional materials based on self-assembled polymer-grafted nanoparticles (GNPs). Static and dynamic light scattering of polystyrene-grafted silica particle solutions in toluene reveals that the pair interaction potential, inferred from the second virial coefficient, A2, is strongly affected by the grafting density, σ, and degree of polymerization, N, of tethered chains. In the limit of intermediate σ (∼0.3 to 0.6 nm–2) and high N, A2 is positive and increases with N. This confirms the good solvent conditions and can be qualitatively rationalized on the basis of a pair interaction potential derived for grafted (brush) particles. In contrast, for high σ > 0.6 nm–2 and low N, A2 displays an unexpected reversal to negative values, thus indicating poor solvent conditions. These findings are rationalized by means of a simple analysis based on a coarse-grained brush potential, which balances the attractive core–core interactions and the excluded volume interactions imparted by the polymer grafts. The results suggest that the steric crowding of polymer ligands in dense GNP systems may fundamentally alter the interactions between brush particles in solution and highlight the crucial role of architecture (internal microstructure) on the behavior of hybrid materials. The effect of grafting density also illustrates the opportunity to tailor the physical properties of hybrid materials by altering geometry (or architecture) rather than a variation of the chemical composition.

Polyacrylamide brushes with varied morphologies as a tool for control of the intermolecular interactions within EPDM/MVQ blends

The specific interactions between polyacrylamide brushes composed of polyacrylamide chains tethered to montmorillonite particles (Mt-g-PAm) and the individual components of immiscible ethylene-propylene-diene modified with sorbic acid (EPDM) and methyl vinyl silicone (MVQ) blends were studied. The effect of various lengths of PAm chains and their grafting density on montmorillonite (Mt) surface was investigated.It was determined that, due to interactions generated within the system through carboxyl and amide groups the brush particles were selectively located at the EPDM/MVQ interphase and in the EPDM phase. Furthermore, the longer PAm chains resulted in an increase of the glass transition of EPDM, unification of blend morphology, and the reduction of crystallinity of the MVQ phase. Therefore, precisely designed polymer brushes could be considered as novel functional compatibilizers, that provide a new tool for the future material designing.

Nanosized Organo-Silica Particles with "Built-In" Surface-Initiated Atom Transfer Radical Polymerization Capability as a Platform for Brush Particle Synthesis.

A facile synthetic method was developed to prepare sub-5 nm organo-silica (oSiO2) nanoparticles through the self-condensation of atom transfer radical polymerization (ATRP)-initiator-containing silica precursors. The obtained oSiO2 nanoparticles were characterized by a combination of nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), dynamic light scattering (DLS), and small-angle neutron scattering (SANS). The accessibility of the surface-Br initiating sites was evaluated by the polymerization of poly(methyl methacrylate) (PMMA) ligands from the surface of the oSiO2 nanoparticles using surface-initiated atom transfer radical polymerization (SI-ATRP). The ultrasmall size, tunable composition, and ease of surface modification may render these organo-silica nanoparticle systems with built-in SI-ATRP capability an interesting alternative to conventional silica nanoparticles for functional material design.

Tunable Assembly of Block Copolymer Tethered Particle Brushes by Surface-Initiated Atom Transfer Radical Polymerization.

A strategy to synthesize SiO2-g-PMMA/PMMA-b-PS mono- and bimodal block copolymer particle brushes by surface-initiated atom transfer radical polymerization (SI-ATRP) from silica particles is presented. First, PMMA blocks were prepared by normal ATRP with controlled degree of polymerizations and grafting density. In a second step, the PS block was synthesized through a chain extension using low parts per million of Cu catalyst. Variation of the SiO2-g-PMMA-Br macroinitiator concentration had a pronounced effect on the modality of the chain extension product. In the limit of small concentration, partial termination resulted in bimodal brush architectures, while more uniform brush architectures were observed with increasing concentration of macroinitiator. Brush nanoparticles with bimodal architectures assembled into string-like aggregates that bore a resemblance to structures found in systems comprised of sparse (homopolymer) brush particles. The unexpected effect of modality on structure formation points to opportunities in controlling microstructures in brush particle materials.

Aspect-ratio-dependent interaction of molecular polymer brushes and multicellular tumour spheroids

Molecular polymer brushes allow for independent tailoring of nanoparticle design parameters. Brush particles with altered shape and aspect ratio revealed that particle shape effects may be decoupled from surface chemistry to achieve higher tumour spheroid interaction and penetration.

Thermomechanical Properties and Glass Dynamics of Polymer-Tethered Colloidal Particles and Films.

Polymer-tethered colloidal particles (aka “particle brush materials”) have attracted interest as a platform for innovative material technologies and as a model system to elucidate glass formation in complex structured media. In this contribution, Brillouin light scattering is used to sequentially evaluate the role of brush architecture on the dynamical properties of brush particles in both the individual and assembled (film) state. In the former state, the analysis reveals that brush–brush interactions as well as global chain relaxation sensitively depend on grafting density; i.e., more polymer-like behavior is observed in sparse brush systems. This is interpreted to be a consequence of more extensive chain entanglement. In contrast, the local relaxation of films does not depend on grafting density. The results highlight that relaxation processes in particle brush-based materials span a wider range of time and length scales as compared to linear chain polymers. Differentiation between relaxation on local and global scale is necessary to reveal the influence of molecular structure and connectivity on the aging behavior of these complex systems.

Grafting PMMA Brushes from α-Alumina Nanoparticles via SI-ATRP.

Alumina nanoparticles are widely used as nanofillers for polymer nanocomposites. Among several different polymorphs of alumina, α-alumina has the most desirable combination of physical properties. Hence, the attachment of polymer chains to α-alumina to enhance compatibility in polymeric matrixes is an important goal. However, the chemical inertness and low concentration of surface hydroxyl groups have rendered polymer modification of α-alumina a long-standing challenge. Herein, we report that activation of α-alumina in concentrated or molten NaOH as well as in molten K2S2O7 increased polymer graft density up to 50%, thereby facilitating the synthesis of α-alumina brush particles with uniform grafting density of 0.05 nm(-2) that are readily miscible or dispersible in organic solvents or in chemically compatible polymeric hosts.

Tailoring structure formation and mechanical properties of particle brush solids via homopolymer addition.

Recent progress in the area of surface-initiated controlled radical polymerization (SI-CRP) has enabled the synthesis of polymer-grafted colloids with precise control over the architecture of grafted chains. The resulting 'particle brush materials' are of interest both from a fundamental as well as applied perspective because structural frustrations (associated with the tethering of chains to a curved surface) imply a sensitive dependence of the interactions between brush particles on the architecture of surface-tethered chains that offers new opportunities to design hybrid materials with novel functionalities. An important prerequisite for establishing structure-property relations in particle brush materials is to understand the role of homopolymer impurities that form, for example, by thermal self-initiation. This contribution presents a detailed discussion of the role of homopolymer additives on the structure and mechanical properties of particle brush materials. The results suggest that the dissolution of homopolymer fillers follows a two-step mechanism comprised of the initial segregation of homopolymer to the interstitial regions within the array and the subsequent swelling of the particle brush (depending on the respective degree of polymerization of brush and linear chains). Addition of even small amounts of homopolymer is found to significantly increase the fracture toughness of particle brush assembly structures. The increased resistance to failure could enable the synthesis of robust colloidal crystal type materials that can be processed into complex shapes using 'classical' polymer forming techniques such as molding or extrusion.

Study of the cross contamination effect on post CMP in situ cleaning process

Scaling of gate lengths has led to significant improvement in semiconductor device performance. However, fabrication complexities have also increased and surface defects control becomes very critical for yield enhancement. In particular, chemical mechanical polishing (CMP) process is considered as one of the dirtiest processes to wafer surface contamination. And brush scrubber is the most effective method for post CMP in situ cleaning. Many literatures have reported brush scrubber mechanism and proposed particle removal efficiency based on remaining particles at post brush scrubber cleaning. However, concerns associated with brush to wafer surface contamination have not been addressed properly although real manufacturing has a lot of issues on that effect. This paper discuss cross contamination at brush scrubber process and emphasize that optimum cleaning should consider brush cross contamination in addition to brush particle removal efficiency.

Environmentally responsive self-assembly of mixed poly(tert-butyl acrylate)-polystyrene brush-grafted silica nanoparticles in selective polymer matrices.

Environmentally responsive self-assembly of nearly symmetric mixed poly(tert-butyl acrylate) (PtBA, 22.2 kDa)/polystyrene (PS, 23.4 kDa) brushes grafted onto 67 nm silica nanoparticles in selective homopolymer matrices [PtBA for the grafted PtBA chains and poly(cyclohexyl methacrylate) (PCHMA) for the grafted PS chains] was investigated using both conventional transmission electron microscopy (TEM) and electron tomography (i.e., 3D TEM). A variety of self-assembled phase morphologies were observed for the mixed brushes in selective polymer matrices with different molecular weights, and these can be explained by entropy-driven wet- and dry-brush theories. In a low molecular weight selective matrix, the wet-brush regime was formed with the miscible chains stretching out and the immiscible chains collapsing into isolated domains. In contrast, when the molecular weight of the selective matrix was higher than that of the compatible grafted polymer chains, the dry-brush regime was formed with the mixed brushes exhibiting the unperturbed morphology. In addition to the molecular weight, the size of nanoparticles (or the substrate curvature) was also observed to play an important role. For small particles (core size less than 50 nm), the wet brush-like morphology with a surface-tethered micellar structure was observed. Finally, the wet- and dry-brush regimes also significantly affected the dispersion of mixed brush particles in selective polymer matrices.

High-transparency polymer nanocomposites enabled by polymer-graft modification of particle fillers.

The role of polymeric ligands on the optical transparency of polymer-matrix composites is analyzed by evaluating the effect of surface modification on the scattering cross-section of particle fillers in uniform particle dispersions. For the particular case of poly(styrene-r-acrylonitrile)-grafted silica particles embedded in poly(methyl methacrylate), it is shown that the tethering of polymeric chains with appropriate optical properties (such as to match the effective refractive index of the brush particle to the embedding matrix) facilitates the reduction of the particle scattering cross-section by several orders of magnitude as compared to pristine particle analogues. The conditions for minimizing the scattering cross-section of particle fillers by polymer-graft modification are established on the basis of effective medium as well as core-shell Mie theory and validated against experimental data on uniform liquid and solid particle dispersions. Effective medium theory is demonstrated to provide robust estimates of the "optimum polymer-graft composition" to minimize the scattering cross-section of particle fillers even in the limit of large particle dimensions (comparable to the wavelength of light). The application of polymer-graft modification to the design of large (500 nm diameter) silica particle composites with reduced scattering cross-section is demonstrated.

Double‐Stimuli‐Responsive Spherical Polymer Brushes with a Poly(ionic liquid) Core and a Thermoresponsive Shell

The synthesis of poly(ionic liquid) (PIL) nanoparticles grafted with a poly(N-isopropyl acrylamide) (PNIPAM) brush shell is reported, which shows responsiveness to temperature and ionic strength in an aqueous solution. The PIL nanoparticles are first prepared via aqueous dispersion polymerization of a vinyl imidazolium-based ionic liquid monomer, which is purposely designed to bear a distal atom transfer radical polymerization (ATRP) initiating group attached to the long alkyl chain via esterification reaction. The size of the PIL nanoparticles can be readily tuned from 25 to 120 nm by polymerization at different monomer concentrations. PNIPAM brushes are successfully grafted from the surface of the poly(ionic liquid) nanoparticles via ATRP. The stimuli-responsive behavior of the poly(ionic liquid) nanoparticles grafted with PNIPAM brushes (NP-g-PNIPAM) in aqueous phase is studied in detail. Enhanced colloidal stability of the NP-g-PNIPAM brush particles at high ionic strength compared to pure PIL nanoparticles at room temperature is achieved. Above the lower critical solution temperature (LCST) of PNIPAM, the brush particles remain stable, but a decrease in hydrodynamic radius due to the collapse of the PNIPAM brush onto the PIL nanoparticle surface is observed.

Catalytic activity of nanoalloys from gold and palladium

We present a quantitative study of the catalytic activity of well-defined faceted gold-palladium nanoalloys which are immobilized on cationic spherical polyelectrolyte brushes. The spherical polyelectrolyte brush particles used as carriers for the nanoalloys consist of a solid polystyrene core onto which cationic polyelectrolyte chains of 2-aminoethyl methacrylate are attached. Au/Pd nanoalloy particles with sizes in the range from 1 to 3 nm have been generated which are homogeneously distributed on the surface of the spherical polyelectrolyte brushes. The reduction of 4-nitrophenol has been chosen as a well-controlled model reaction allowing us to determine the catalytic activity of the nanoalloys as a function of the Au/Pd composition. The adsorption behavior was studied by Langmuir-Hinshelwood kinetics. We find a pronounced maximum of the catalytic activity at 75 molar % Au. A comparison of gold, platinum, palladium and gold-palladium alloy nanoparticles is made in terms of Langmuir-Hinshelwood kinetics. Density functional calculations for Au/Pd clusters with up to 38 atoms show that the density of states at the Fermi level increases with increasing Pd content, and that the highest occupied orbitals are associated with Pd atoms. The calculations confirm that small changes in the atomic arrangement can lead to pronounced changes in the particles' electronic properties, indicating that the known importance of surface effects is further enhanced in nanoalloys.

Spherical polymer brushes with vinylimidazolium-type poly(ionic liquid) chains as support for metallic nanoparticles

We present here the synthesis of spherical polyelectrolyte brushes by photo-emulsion polymerization with a solid polystyrene core (diameter ∼ 100 nm) onto which chains of vinylimidazolium-type poly(ionic liquid)s have been densely grafted. The as-synthesized brush particles were employed as nanoreactors for the generation and immobilization of metal nanoparticles. Transmission electron microscopy characterization shows that Au and Pd nanoparticles with diameter of 2.1 ± 0.2 nm and 2.5 ± 0.3 nm are homogeneously embedded inside the PIL brushes, respectively. The study of swelling behavior of the brush particles before and after metal deposition indicates obviously ion-specific effect. The composite particles exhibit a long-term colloidal stability in aqueous solutions as well as in ionic liquids. Catalytic activity of the as-synthesized metal nanocomposite particles is investigated by using the reduction of 4-nitrophenol with NaBH4 as a model reaction, which can be compared directly with reported systems. In addition, it is found that the rate constant kapp of PIL-metal nanocatalyst could be modulated by salt concentration.

Measurement of Friction Surface and Wear Rate between a Carbon Graphite Brush and a Copper Ring

A pin-on-disc surface friction tester designed by the authors was used to measure the surface friction temperature of a carbon graphite brush sliding against a copper ring under the condition of no flow by electric current. Microelectromechanical systems (MEMS) fabrication techniques including sputtering, lithography, and etching processes were utilized to fabricate metal film temperature sensors on the surface of the carbon graphite brush. The true temperature measurement system of the friction surface of the carbon graphite brush was then established. A USB-DAQ system and LabVIEW software were adopted for the data acquisition/management of analog signals. In this research, the relationship between friction temperature and wear rate was investigated using experiment data. Optical and scanning electron microscope (SEM) images of the friction surface of the carbon graphite brush were taken and analyzed to study the effect of temperature on the friction surface. From this experiment, it was found that the debris of a mixture of brush and copper particles softens under high temperature, adheres to the brush surface, and forms flattened areas. These smooth, flattened areas in turn lower the friction coefficient.

Synthesis and Analysis of Zwitterionic Spherical Polyelectrolyte Brushes in Aqueous Solution

We present the synthesis and characterization of spherical polyelectrolyte brush (SPB) particles carrying zwitterionic polyelectrolyte chains. The colloidal particles consist of a divinylbenzene cross-linked poly(styrene) core (PS-co-DVB core) of about 100 nm in diameter onto which linear zwitterionic poly(2-(methacryloyloxy)ethyl dimethyl-(3-sulfopropyl)ammonium hydroxide) (pMEDSAH) chains are chemically grafted via ATRP. ζ-Potential measurements demonstrated that the SPB has an electrophoretic mobility due to the net charge of the PS-co-DVB core particles. There is an increase of the brush thickness L of the zwitterionic brush at high concentrations of sodium chloride at room temperature. Temperature-dependent measurements by dynamic light scattering (DLS) showed that the zwitterionic SPBs swell reversibly with increasing temperature because of the upper critical solution temperature (UCST) of the pMEDSAH chains in water. This effect could be enhanced by the addition of salt. Cryogenic transmission elec...

Formation of Ultrathin Birnessite-Type Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes

A new route of in situ formation and stabilization of ultrathin, needle-like manganese dioxide nanoparticles (MnO2NP) in aqueous solution by using spherical polyelectrolyte brush particles (SPB) is presented. The SPB that act as carrier particles consist of a solid polystyrene core of about 50 nm radius onto which long chains of the positively charged polyelectrolyte poly(trimethyl ammonium ethyl methacrylate chloride) (pTMAEMC) are grafted to yield an overall radius of about 85 nm. Potassium permanganate (KMnO4) is directly reduced within the brush layer of these particles because of the basic environment within this layer. This mechanism appears to limit the size of the MnO2NP to the dimensions of the brush layer. Powder X-ray diffraction, (PXRD), transmission electron microscopy (TEM), and cryogenic transmission electron microscopy (cryoTEM) prove that birnessite-type MnO2NP with a c*-disorder are generated on the SPB without adding any reducing agent. The birnessite nanoparticles have an average lengt...