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.Keywords: antiwear; colloidal stability; friction reduction; hairy nanoparticles; lubricant additives; oil solubility; surface-initiated RAFT polymerization;

While lower critical solution temperature (LCST)-type thermosensitive nanogels have been intensively studied, the upper critical solution temperature (UCST)-type versions are much less explored. This communication reports a method for the synthesis of zwitterionic UCST nanogels by reversible addition–fragmentation chain transfer (RAFT) polymerization-induced self-assembly in water–organic solvent mixtures. The nanogels were prepared by RAFT polymerization of 3-dimethyl(methacryloyloxyethyl)ammonium propanesulfonate, whose polymer is known to exhibit UCST behavior in water, conducted in ethanol–water mixtures at 70 °C using poly(poly(ethylene glycol) methyl ether methacrylate) as a macro-chain transfer agent (CTA) and a difunctional monomer as cross-linker. At a sufficiently high ethanol content in reaction media, spherical hairy nanogels with a single size distribution were obtained. These nanogels exhibited reversible heating-induced swelling and cooling-induced shrinking, consistent with the expected UCST behavior. The hydrodynamic size, volume changing ratio, and transition temperature of nanogels can be tuned by varying ethanol content in solvent mixtures, molar ratio of monomer-to-macro-CTA, and amount of cross-linker. Hairy nanogels were also successfully synthesized using a water–THF mixture as medium. The use of water–organic solvent mixtures as reaction media allowed for facile incorporation of a hydrophobic fluorescent monomer to make functional UCST nanogels.

Inspired by stimuli-triggered unraveling of the von Willebrand factor from a nonsticky globular to a stretched linear shape with exposure of functional groups in blood clotting, this article reports on the synthesis of thermosensitive binary heterografted linear molecular brushes that exhibit temperature-induced shape changing between extended wormlike and collapsed yet stable globular conformations in water. The molecular brushes are composed of two distinct side chain polymers: thermosensitive poly(ethoxydi(ethylene glycol) acrylate) (PDEGEA), which undergoes a lower critical solution temperature (LCST) transition at 9 °C in water, and poly(ethylene oxide) (PEO), which serves as a stabilizer for the collapsed state. A “grafting to” method was developed to construct molecular brushes by clicking alkyne end-functionalized side chain polymers onto an azide-bearing backbone polymer. While a 1.0 mg/g aqueous solution of PDEGEA homografted molecular brushes turned cloudy upon heating from 0 to 22 °C, at the same concentration the aqueous solution of PEO/PDEGEA binary molecular brushes remained clear, indicating the stabilization of the collapsed state against aggregation by PEO side chains. Atomic force microscopy study revealed a stretched, wormlike morphology of brushes at 0 °C and compact, globular nano-objects at 40 °C. The thermally induced shape changing was exploited to regulate the binding of biotin, which was incorporated into the thermosensitive side chains along with a fluorescent resonance energy transfer (FRET) donor, and Rhodamine B (FRET acceptor)-labeled avidin. FRET study showed that when the molecular brushes changed from the globular to the wormlike state, the binding of biotin and avidin occurred and increased significantly with time.

This article reports on the light-triggered unfolding of single stimuli-responsive linear molecular bottlebrushes, composed of either homografted poly(methoxydi(ethylene glycol) acrylate-co-o-nitrobenzyl acrylate) (P(DEGMA-co-NBA)) or heterografted poly(ethylene oxide) (PEO) and poly(o-nitrobenzyl acrylate) (PNBA) side chains, from compact globular to wormlike shapes in an effort to mimic the triggered unravelling of proteins such as the von Willebrand factor and talin. PDEGMA is a thermosensitive polymer with a lower critical solution temperature (LCST) of 38 °C in water. The hydrophobic o-nitrobenzyl group can be cleaved by UV irradiation, yielding a hydrophilic carboxylic acid group. This increased the LCST or solubility of the corresponding side chains in water, triggering the shape transitions of brush molecules from globular to wormlike. The brushes were synthesized by a “grafting to” method using the copper-catalyzed cycloaddition of alkyne end-functionalized side chain polymers and an azide-bearing backbone polymer. For 0.2 mg g−1 P(DEGMA-co-NBA) molecular brushes in water, a dynamic light scattering study showed that the apparent hydrodynamic size decreased with increasing temperature, with the LCST occurring at 23 °C. Atomic force microscopy revealed that the brush molecules changed from an extended wormlike morphology at 4 °C to compact globular nano-objects upon heating to 30 °C. UV irradiation of the solution at 30 °C for 70 min unfolded the brushes to an extended wormlike conformation. For heterografted PEO/PNBA molecular brushes, the collapsed compact nano-objects in water, stabilized by PEO side chains, were prepared by gradually adding a DMF solution of brushes into water, followed by dialysis to remove DMF. Similarly, upon UV irradiation, the brush molecules changed to wormlike conformations.

A method is reported for enhancing the gelation of doubly thermosensitive hydrophilic linear ABC triblock copolymers in water using thermoresponsive polymer brush-grafted nanoparticles (hairy NPs). A linear ABC triblock copolymer (ABC-Q) composed of a hydrophilic, charged middle block, and two thermosensitive outer blocks with different LCSTs, LCSTA of the lower LCST A block and LCSTC of the higher LCST C block, and two batches of hairy NPs with distinct thermoresponsive properties were prepared. When the temperature was raised from 0 °C to above the LCSTA but below the LCSTC, ABC-Q self-assembled into micelles in water with the lower LCST A block forming the core; further heating to above the LCSTC triggered the collapse of the C block, producing a two-compartment 3-D network micellar hydrogel when the polymer concentration was sufficiently high. Rheological studies showed that adding thermoresponsive hairy NPs with a LCST similar to the LCSTC of ABC-Q led to a significant increase in dynamic storage modulus (G′). For 6 wt % aqueous solutions of ABC-Q, the maximum value of G′ (G′max) increased with increasing amount of hairy NPs; a 45% increase in G′max was observed at the NP-to-polymer mass ratio of 60:100. It is believed that hairy NPs acted as “seeds” to adsorb the collapsed C block of ABC-Q, promoting the formation of bridging chains among micellar cores and NPs and thus enhancing the gelation. In contrast, no benefit was observed when adding hairy NPs with a LCST much higher than LCSTC; the G′max exhibited little change with increasing NP-to-polymer mass ratio. Our explanations for the rheological observations were supported by fluorescence resonance energy transfer studies.

•Hybrid hydrogels with nanoparticles (NPs) initially outside micelles were prepared.•Hybrid polymer hydrogels exhibited a sharp increase in G′ at the LCST of hairy NPs.•The sharp increase of G′ was caused by the collapse of polymer brushes on NPs.•Gel strength depended on the NP-to-ABA copolymer ratio and the LCST of hairy NPs.This article presents a study of the effect of LCST transition of polymer brushes on properties of hybrid micellar network hydrogels of a thermosensitive ABA triblock copolymer and polymer brush-grafted nanoparticles (hairy NPs) with NPs initially residing outside the core of micelles. Four batches of thermosensitive polymer brush-grafted, 20 nm silica NPs with different lower critical solution temperatures (LCSTs) and a thermosensitive ABA triblock copolymer composed of a poly(ethylene oxide) central block and thermosensitive outer blocks (ABA-N) were prepared. The LCSTs of hairy NPs were significantly higher than the critical micellization temperature of ABA-N, resulting in the NPs being initially located outside the core of micelles. The effects of LCST transition of hairy NPs and NP-to-polymer mass ratio on properties of hybrid micellar hydrogels of ABA-N with a concentration of 10 wt% and hairy NPs were investigated by rheological measurements. For all hybrid hydrogels studied in this work, the dynamic storage modulus G′ exhibited a sharp increase on top of a plateau that was forming in the heating ramp at a temperature corresponding to the LCST transition of hairy NPs. This phenomenon was caused by the collapse of the brushes on NPs triggering the absorption of thermosensitive outer blocks of ABA-N molecules in the dangling and loop forms and the reorganization of the 3-D gel network structure, which increased the density of bridging chains and thus the G′. This explanation was supported by the results from fluorescence resonance energy transfer studies. The maximum values of G′ (G′max) of the gels containing higher LCST hairy NPs were noticeably lower compared with the gels with lower LCST hairy NPs and the gel with no NPs. The G′max of all hybrid hydrogels decreased gradually with the increase of the NP-to-polymer ratio.

•Polymethacrylamide homopolymers based on tertiary amine-modified l-alanine are prepared.•The incorporated natural amino acid moiety endow polymers with optical activity.•Monomer structure is critical for polymer to exhibit desired responsive property.•Polymers display LCST in basic pH range and effects of various factors are studied.•Copolymerization is not necessary to obtain multi-responsive polymers.Four poly(N-methacryloyl-l-alanine) homopolymers containing different tertiary amine moieties, dimethylaminoethyl, dimethylaminopropyl, diethylaminoethyl, and diethylaminopropyl, were prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization in an effort to use natural amino acids as building blocks to design stimuli-responsive polymers that display pH-tunable lower critical solution temperature (LCST) transitions in the basic pH range. Monomer structure was found to be critical for the corresponding polymer to exhibit desired stimuli-responsive properties in water. While all four polymethacrylamides showed thermosensitive property in water, only poly(N-methacryloyl-l-alanine 2-(diethylamino)ethylamide) (PMAEE) exhibited LCST behavior in a wide pH range, from 9.0 to 13.0. Other polymers' thermoresponsive properties were found either at very high pH values (e.g., ≥13.0) or in a rather narrow pH range. The effects of pH, polymer molecular weight, polymer concentration, presence of NaCl, and end groups on cloud point of PMAEE in water were investigated; the cloud point decreased with the increase of solution pH and polymer concentration, the addition of NaCl, and the introduction of a more hydrophobic end group but varied little with polymer molecular weight. The incorporation of tertiary amine moieties endowed the polymers with a CO2-responsive property; we demonstrated that the thermosensitive property of PMAEE can be reversibly tuned by bubbling its solution alternately with CO2 and N2 gases.

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 %.

We report in this article that aqueous dispersions of thermosensitive diblock copolymer brush-grafted 17 nm silica nanoparticles (hairy NPs) can undergo in situ, reversible sol–gel transitions in response to temperature changes. The brushes consisted of a thermosensitive poly(methoxydi(ethylene glycol) methacrylate) (PDEGMMA) inner block and a charge-carrying, poly(DEGMMA-co-2-(methacryloyloxy)ethyltrimethylammonium iodide) outer block, which were prepared by a one-pot, surface-initiated atom transfer radical polymerization and subsequent quaternization of tertiary amine moieties in the second block with iodomethane. Above a critical concentration, the aqueous dispersion of hairy NPs with an appropriate block copolymer composition exhibited a reversible transition from a free flowing liquid to a free standing hydrogel upon cooling from elevated temperatures, which was driven by the lower critical solution temperature transition of the thermosensitive inner block of hairy NPs as confirmed by dynamic light scattering study. At the same concentration of hairy NPs, the sol–gel transition temperature was higher when the highly hydrated, charged outer block was longer. The transition temperature decreased with decreasing the concentration of hairy NPs in the dispersion; reversible gelation was achieved with a concentration of hairy NPs in water as low as 5.5 wt%. Interestingly, the LCST transition of the inner thermosensitive PDEGMMA block disappeared and no sol–gel transition was observed in the studied temperature range when the charged outer block was sufficiently long.

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.