Brush-grafted Silica Nanoparticles Research Articles

PNIPAM Brushes in Colloidal Photonic Crystals Enable Ex Situ Ethanol Vapor Sensing.

Structural colors are formed by the periodic repetition of nanostructures in a material. Upon reversibly tuning the size or optical properties of the repetitive unit inside a nanostructured material, responsive materials can be made that change color due to external stimuli. This paper presents a simple method to obtain films of ethanol vapor-responsive structural colors based on stacked poly(N-isopropylacrylamide) (PNIPAM)-grafted silica nanoparticles. Our materials show clear, reversible color transitions in the presence of near-saturated ethanol vapor. Moreover, due to the absorption of ethanol in the PNIPAM brushes, relatively long recovery times are observed (∼30 s). Materials based on bare or poly(methyl methacrylate) (PMMA) brush-grafted silica nanoparticles also change color in the presence of ethanol vapor but possess significantly shorter recovery times (∼1 s). Atomic force microscopy reveals that the delayed recovery originates from the ability of PNIPAM brushes to swell in ethanol vapor. This renders the films highly suitable for ex situ ethanol vapor sensing.

Highly stable self-cleaning antireflection coatings from fluoropolymer brush grafted silica nanoparticles

Self-cleaning antireflection coatings have potential applications in outdoor conditions and have been the subject of intense study. However, self-cleaning antireflection coatings are commonly prepared via grafting of small hydrophobic molecules, which are susceptible to detachment from the coatings due to hydrolysis, leading to considerable degradation of the self-cleaning functionality in practical applications. Here, we create fluoropolymer brush grafted silica antireflection coatings that demonstrate long-term self-cleaning performance. Fluoropolymer brush grafted silica nanoparticles were synthesized via surface initiated atom transfer radical polymerization and then coated on glass substrates to form antireflection coatings. The brush grafted coatings exhibited excellent transmission, with nearly 100% transmittance at a desired wavelength. The fluoropolymer brushes rendered the coatings hydrophobic, with water contact angle of 122°. Owing to their hydrophobicity, the brush grafted coatings exhibited excellent self-cleaning functionality. A notable advantage of the brush grafted coatings was their stable self-cleaning functionality. The brush grafted coatings remained their hydrophobicity after immersion in acidic solution, with water contact angle being constant at 122°, which were more stable than the small molecule modified hydrophobic coatings. This work opens a new avenue to create highly stable self-cleaning antireflection coatings that will find applications in harsh conditions.

Friction and adhesion control between adsorbed layers of polyelectrolyte brush-grafted nanoparticles via pH-triggered bridging interactions

HypothesisAdsorption of polyelectrolyte brush-grafted nanoparticles (BGNPs) produces a heterogeneous interface with sub-monolayer surface coverage resulting from lateral electrostatic repulsions that limit packing. As a result, the interaction forces between opposing BGNP layers include an adhesive cross-surface BGNP-substrate bridging force that depends on the interparticle spacing, particle size, and strength of electrostatic interactions. We hypothesize that BGNPs with pH-responsive, annealed polyelectrolyte brushes can undergo controlled changes in surface area coverage through post-adsorption swelling or de-swelling into non-equilibrium layer conformations and that such changes in surface coverage can switch off or switch on particle intercalation, bridging attractions, and enhanced energy dissipation upon sliding. This work aims to characterize the nature of surface forces in heterogeneous BGNP adsorbed layers and to utilize pH-sensitive bridging forces as a mechanism to tune friction and adhesion. ExperimentsColloidal probe atomic force microscopy (CP-AFM) is used to measure normal and lateral forces between negatively charged silica surfaces with adsorbed pH-responsive cationic BGNPs. The BGNPs are poly(2-dimethylaminoethyl methacrylate) brush-grafted silica nanoparticles. Adhesion force and friction analysis is complemented by simultaneous quartz-crystal microbalance and ellipsometry measurements under conditions that render the particles strongly charged and swollen (acidic) or weakly charged and de-swollen (basic). FindingsAdsorbed BGNPs can be swollen or de-swollen via pH rinses, enabling direct control of surface coverage and bridging interactions. Transitions from adhesive bridging contacts with high friction to non-adhesive contacts with low friction forces occur when adsorbed BGNP layers are switched from a de-swollen/weakly charged state to a swollen/highly charged state. The ability to controllably shift the character of normal and lateral forces via coverage-mediated bridging interactions is a unique feature of adsorbed nanoparticulate brush constructs and highlights their potential to condition surfaces with additional functionality compared to dense, planar homopolymer brushes.

Improved Lubricating Performance by Combining Oil-Soluble Hairy Silica Nanoparticles and an Ionic Liquid as an Additive for a Synthetic Base Oil

This article reports on improved lubricating performance by combining oil-soluble poly(lauryl methacrylate) brush-grafted silica nanoparticles (hairy NPs or HNP) and an oil-miscible phosphonium-phosphate ionic liquid (IL) as a friction-reducing additive for a polyalphaolefin (PAO) oil. The HNP was synthesized by surface-initiated reversible addition-fragmentation chain transfer polymerization. At a total concentration of 2% and sufficiently high individual concentrations for HNP and IL in PAO, high-contact stress, ball-on-flat reciprocating tribological tests showed that the friction decreased by up to 23% compared with 2% HNP alone in PAO and by up to 35% compared to the PAO mixed with 2% IL. Scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS) analysis revealed that the tribofilm formed from the PAO containing 1% HNP + 1% IL was enriched with both Si and P, indicating that both hairy NPs and IL were involved in the tribochemical reactions. In addition, the O 1s and Si 2p peaks in the core-level XPS spectra exhibited significant shifts for the mixture of 1% HNP + 1% IL compared to those for 2% HNP, suggesting the possible formation of new covalent bonds. These results indicated that HNP and IL reacted with each other and also with the metal substrate during the rubbing process, which likely strengthened the tribofilm and its bonding with the substrate and thus further improved the lubrication.

Poly(alkyl methacrylate) Brush-Grafted Silica Nanoparticles as Oil Lubricant Additives: Effects of Alkyl Pendant Groups on Oil Dispersibility, Stability, and Lubrication Property.

This article reports on the synthesis of a series of poly(alkyl methacrylate) brush-grafted, 23 nm silica nanoparticles (hairy NPs) and the study of the effect of alkyl pendant length on their use as oil lubricant additives for friction and wear reduction. The hairy NPs were prepared by surface-initiated reversible addition-fragmentation chain transfer polymerization from trithiocarbonate chain transfer agent (CTA)-functionalized silica NPs in the presence of a free CTA. We found that hairy NPs with sufficiently long alkyl pendant groups (containing >8 carbon atoms, such as 12, 13, 16, and 18 in this study) could be readily dispersed in poly(alphaolefin) (PAO), forming clear, homogeneous dispersions, and exhibited excellent stability at low and high temperatures as revealed by visual inspection and dynamic light scattering studies. Whereas poly(n-hexyl methacrylate) hairy NPs cannot be dispersed in PAO under ambient conditions or at 80 °C, interestingly, poly(2-ethylhexyl methacrylate) hairy NPs can be dispersed in PAO at 80 °C but not at room temperature, with a reversible clear-to-cloudy transition observed upon cooling. High-contact-stress ball-on-flat reciprocating sliding tribological tests at 100 °C showed significant reductions in both the coefficient of friction (up to 38%) and wear volume (up to 90% for iron flat) for transparent, homogeneous dispersions of hairy NPs in PAO at a concentration of 1.0 wt % compared with neat PAO. The formation of a load-bearing tribofilm at the rubbing interface was confirmed using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy.

Thermally Reversible Physically Cross-Linked Hybrid Network Hydrogels Formed by Thermosensitive Hairy Nanoparticles.

This Article reports on thermally induced reversible formation of physically cross-linked, three-dimensional network hydrogels from aqueous dispersions of thermosensitive diblock copolymer brush-grafted silica nanoparticles (hairy NPs). The hairy NPs consisted of a silica core, a water-soluble polyelectrolyte inner block of poly(2-(methacryloyloxy)ethyltrimethylammonium iodide), and a thermosensitive poly(methoxydi(ethylene glycol) methacrylate) (PDEGMMA) outer block synthesized by sequential surface-initiated atom transfer radical polymerizations and postpolymerization quaternization of tertiary amine moieties. Moderately concentrated dispersions of these hairy nanoparticles in water underwent thermally induced reversible transitions between flowing liquids to self-supporting gels upon heating. The gelation was driven by the lower critical solution temperature (LCST) transition of the PDEGMMA outer block, which upon heating self-associated into hydrophobic domains acting as physical cross-linking points for the gel network. Rheological studies showed that the sol-gel transition temperature decreased with increasing hairy NP concentration, and the gelation was achieved at concentrations as low as 3 wt %.

Self-organization of homopolymer brush- and mixed homopolymer brush-grafted silica nanoparticles in block copolymers and polymer blends

In this report, hierarchical self-organization of poly(tert-butyl acrylate) (PtBA) brush- and mixed PtBA/polystyrene (PS) brush-grafted silica nanoparticles (NPs) in PtBA-b-PS block copolymer and PtBA/PS polymer blend matrices having different molecular weights was investigated using transmission electron microscopy (TEM). Driven by the entropic interactions between the brushes and the polymer matrices (i.e., wet- and dry-brush effects), various morphologies were observed. First, PtBA brush-grafted 67 nm silica NPs could induce an onion-like structure in a low molecular weight block copolymer matrix because of the wet-brush effect, low particle curvature, and low bending rigidity of the block copolymer lamellae. On the contrary, PtBA brush-grafted 23 nm silica NPs could not induce the onion-like structure, primarily owing to the high particle curvature and high bending penalty of the block copolymer lamellae. Second, the attempt to use ordered block copolymer lamellae to direct microphase separation of mixed homopolymer brushes with a goal of achieving ordered nanopatterns was unsuccessful, likely because the mixed PtBA/PS brushes on NPs underwent microphase separation prior to the ordering of block copolymers during the solution-casting process. Third, in polymer blends, the wet- and dry-brush entropic effects played an important role. In the wet-brush regime, the hairy NPs were uniformly distributed in the preferred polymer phase. In the dry-brush regime, the hairy NPs were driven to the interfaces of immiscible PtBA/PS domains. The knowledge gained on hierarchical self-organization of homopolymer brush- and mixed homopolymer brush-grafted NPs in block copolymer and polymer blend matrices will help us design better polymer nanocomposites with precisely controlled locations of NPs.

In Situ Characterization of Binary Mixed Polymer Brush-Grafted Silica Nanoparticles in Aqueous and Organic Solvents by Cryo-Electron Tomography.

We present an in situ cryo-electron microscopy (cryoEM) study of mixed poly(acrylic acid) (PAA)/polystyrene (PS) brush-grafted 67 nm silica nanoparticles in organic and aqueous solvents. These organic-inorganic nanoparticles are predicted to be environmentally responsive and adopt distinct brush layer morphologies in different solvent environments. Although the self-assembled morphology of mixed PAA/PS brush-grafted particles has been studied previously in a dried state, no direct visualization of microphase separation was achieved in the solvent environment. CryoEM allows the sample to be imaged in situ, that is, in a frozen solvated state, at the resolution of a transmission electron microscope. Cryo-electron tomograms (cryoET) were generated for mixed PAA/PS brush-grafted nanoparticles in both N,N-dimethylformamide (DMF, a nonselective good solvent) and water (a selective solvent for PAA). Different nanostructures for the mixed brushes were observed in these two solvents. Overall, the brush layer is more compact in water, with a thickness of 18 nm, as compared with an extended layer of 27 nm in DMF. In DMF, mixed PAA/PS brushes are observed to form laterally separated microdomains with a ripple wavelength of 13.8 nm. Because of its lower grafting density than that of PAA, PS domains form more or less cylindrical or truncated cone-shaped domains in the PAA matrix. In water, PAA chains are found to form a more complete shell around the nanoparticle to maximize their interaction with water, whereas PS chains collapse into the core of surface-tethered micelles near the silica core. The cryoET results presented here confirm the predicted environmentally responsive nature of PAA/PS mixed brush-grafted nanoparticles. This experimental approach may be useful for the design of future mixed brush-grafted nanoparticles for nano- and biotechnology applications.

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.

Temperature-Induced Transport of Thermosensitive Hairy Hybrid Nanoparticles between Aqueous and Organic Phases

This paper reports on the temperature-induced transport of thermosensitive polymer brush-grafted silica nanoparticles between aqueous and organic phases. Poly(methoxytri(ethylene glycol) methacrylate), a thermosensitive water-soluble polymer with a cloud point of approximately 48 degrees C in H2O, was grown on silica nanoparticles by surface-initiated atom transfer radical polymerization in the presence of a free initiator. These hairy particles were found to quantitatively transfer from aqueous to ethyl acetate phases upon heating at 60 degrees C under the stirring condition. Cooling in an ice/water bath caused the particles to move from ethyl acetate to the aqueous layer. The concentrations of the particles in the original phases during the transport processes were monitored by UV-vis spectrometry. When mutually saturated water and ethyl acetate were used, the transport rates of the particles between the two phases were enhanced. The faster transport was attributed to the solvent phase separation, which produced liquid droplets, dramatically increased the interfacial area, and hence facilitated the transport of the particles. The reversible transfer of the particles between the aqueous and ethyl acetate phases upon heating at 60 degrees C and cooling in an ice/water bath can be repeated consecutively at least 10 times. The hairy particles can also be quantitatively transported from 1-butanol and toluene to H2O by stirring in an ice/water bath. However, only 60% of the particles transferred from water to 1-butanol and no particles to toluene upon heating at 60 degrees C. The reasons are discussed.