Polymer Brush-grafted Nanoparticles Research Articles

Mechanical reinforcement of waterborne latex pressure‐sensitive adhesives with polymer‐grafted nanoparticles

AbstractWaterborne pressure sensitive adhesives (PSAs) consisting of polymer microparticle emulsions (i.e. latex) are more commonly used in commercial applications than solvent‐borne alternatives, as the use of water as a suspension medium provides better consumer safety and reduces environmental impact. However, the lower mechanical performance of waterborne PSAs prevents their use in applications requiring permanent adhesion or strong bonding between substrates. This reduction in mechanical strength is often attributed to void spaces that form during water evaporation and coalescence of the latex particles, and thus a potential strategy to improve PSA strength would be to add filler materials to occupy these voids. Fundamental studies investigating how interfacial interactions between the latex and fillers affect the collective strength of the films would enable better design of adhesive compositions to tailor PSA mechanical properties. Here we report the use of polymer brush‐grafted nanoparticles (PGNPs) as a means of mechanically reinforcing the PSAs, and determine how different aspects of the particle and polymer brush designs enable this improvement in adhesive performance. The PGNPs investigated here are intentionally designed to phase segregate into the aqueous phase of the initial latex suspension, which allows them to both fill free pore volume and also form multivalent supramolecular interactions with the latex particles to form polymer bridges that improve the interconnectivity of the final film. These studies provide insight into potential design strategies for tuning PSA properties with PGNPs, and enable up to 32% improvements to the cohesive strength of the PSAs without the typical deterioration of adhesive strength observed in PSAs using non‐brush‐coated particle fillers.

Reversible Diffusionless Phase Transitions in 3D Nanoparticle Superlattices

Nanocomposite tectons (NCTs), polymer brush-grafted nanoparticles that use supramolecular interactions to drive their assembly, form ordered nanoparticle superlattices (NPSLs) with well-defined unit cell symmetries when thermally annealed. In this work, we demonstrate that appropriate assembly and processing conditions can also enable control over the microstructure of NCT lattices by balancing the enthalpic and entropic factors associated with ligand packing and supramolecular bonding during crystallization. Unary systems of NCTs are assembled via the addition of a small molecule capable of binding to multiple nanoparticle ligands; these NCTs initially form face-centered-cubic (FCC) structures in solvents that are favorable for the particles' polymer brushes. However, the FCC lattices undergo a reversible, diffusionless phase transition to body-centered-cubic (BCC) lattices when transferred to a solvent that induces polymer brush collapse. The BCC superlattices maintain the same crystal habit as the parent FCC phase but exhibit significant transformation twinning similar to that seen in martensitic alloys. This previously unseen diffusionless phase transformation in NPSLs enables unique microstructural features in the resulting assemblies, suggesting that NPSLs could serve as models for the investigation of microstructural evolution in crystalline systems and extend our understanding of NPSLs as atomic material analogues.

Polymer Brush-Grafted Nanoparticles Preferentially Interact with Opsonins and Albumin.

Nanoparticles find increasing applications in life science and biomedicine. The fate of nanoparticles in a biological system is determined by their protein corona, as remodeling of their surface properties through protein adsorption triggers specific recognition such as cell uptake and immune system clearance and nonspecific processes such as aggregation and precipitation. The corona is a result of nanoparticle–protein and protein–protein interactions and is influenced by particle design. The state-of-the-art design of biomedical nanoparticles is the core–shell structure exemplified by superparamagnetic iron oxide nanoparticles (SPIONs) grafted with dense, well-hydrated polymer shells used for biomedical magnetic imaging and therapy. Densely grafted polymer chains form a polymer brush, yielding a highly repulsive barrier to the formation of a protein corona via nonspecific particle–protein interactions. However, recent studies showed that the abundant blood serum protein albumin interacts with dense polymer brush-grafted SPIONs. Herein, we use isothermal titration calorimetry to characterize the nonspecific interactions between human serum albumin, human serum immunoglobulin G, human transferrin, and hen egg lysozyme with monodisperse poly(2-alkyl-2-oxazoline)-grafted SPIONs with different grafting densities and core sizes. These particles show similar protein interactions despite their different “stealth” capabilities in cell culture. The SPIONs resist attractive interactions with lysozymes and transferrins, but they both show a significant exothermic enthalpic and low exothermic entropic interaction with low stoichiometry for albumin and immunoglobulin G. Our results highlight that protein size, flexibility, and charge are important to predict protein corona formation on polymer brush-stabilized nanoparticles.

Enhancing Gelation of Doubly Thermosensitive Hydrophilic ABC Linear Triblock Copolymers in Water by Thermoresponsive Hairy Nanoparticles

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 mod...

Hybrid micellar network hydrogels of thermosensitive ABA triblock copolymer and polymer brush-grafted nanoparticles: Effect of LCST transition of polymer brushes on gel property

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.

Hybrid micellar hydrogels of a thermosensitive ABA triblock copolymer and hairy nanoparticles: effect of spatial location of hairy nanoparticles on gel properties.

This article reports a method for control of spatial location of nanoparticles (NPs) in hybrid micellar hydrogels of a thermosensitive ABA triblock copolymer and polymer brush-grafted NPs (hairy NPs), either inside or outside the core of micelles, and the study of the effect of different locations of NPs on gel properties. Two batches of thermosensitive polymer brush-grafted, 17 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-D) were synthesized. The different locations of NPs were achieved by controlling the LCST of hairy NPs (LCST(NP)) relative to that of the thermosensitive outer blocks of ABA-D (LCST(ABA)). When the LCST(NP) and LCST(ABA) were similar, the NPs resided in the core of micelles upon heating from below the LCST(NP) and LCST(ABA). When the LCST(NP) was significantly higher, the NPs were located outside the core of micelles as confirmed by fluorescent resonance energy transfer. The effects of different locations of hairy NPs and NP-to-polymer mass ratio on properties of hybrid micellar hydrogels formed from aqueous solutions of ABA-D with a concentration of 10 wt % and various amounts of hairy NPs were studied by rheological measurements. The sol-gel transition temperature (T(sol-gel)) and dynamic storage modulus G' of the gels with NPs inside the core of micelles did not change much with increasing the NP-to-polymer mass ratio. In contrast, the T(sol-gel) of gels with NPs in the interstitial space among micelles increased slightly and the G' decreased significantly with the increase of the NP-to-polymer ratio. The hairy NPs in the interstitial space appeared to affect the formation of polymer networks and increase the fraction of polymer loops, resulting in a lower density of bridging chains and thus a lower G'. In addition, for gels with NPs in the interstitial space, a noticeable increase in G' was observed in the heating ramps above 40 °C, which was likely caused by the collapsed hairy NPs adsorbing polymer chains in the dangling and loop forms, increasing the density of bridging chains.

Amphiphilic diblock copolymer gels: the relationship between structure and rheology

Block copolymers dissolved in a selective solvent form micelles that can be regarded as model colloidal dispersions because by tailoring the copolymer composition, the relative size of core and corona can be controlled. This enables the interaction potential to be varied, such that either hard or soft sphere ordering can be observed above the liquid–solid (sol–gel) transition. Here we review recent work on the structure and rheology of micellar phases formed by block copolymers in solution. First, the structure of solid and liquid micellar phases is considered, and its relationship to flow properties of gels and sols is discussed. Then the ordering of hard versus soft spheres is considered in the context of experimental phase diagrams for poly(styrene)–poly(isoprene) in decane and poly(oxyethylene)–poly(oxybutylene) diblock copolymers in water. The use of experiments in the linear viscoelastic regime to locate the gelation (liquid–solid) transition is then reviewed, and the application of measurements in the linear viscoelastic regime to study relaxation phenomena is also outlined. The nonlinear flow behaviour of micellar block copolymer solutions is then considered. In particular the focus is on the development of a yield stress in the solid phase, creep and stress relaxation measurements which throw light on flow mechanisms, and Fourier transform rheology which provides a quantitative measure of nonlinear viscoelasticity under oscillatory shear. Finally, an overview is provided of recent small–angle scattering experiments that have probed the mechanisms of macroscopic alignment of body–centred and face–centred cubic micellar phases subjected to large–amplitude oscillatory shear or steady shear in a Couette cell.