Density effects on collapse, compression, and adhesion of thermoresponsive polymer brushes

Deswelling Mechanisms of PNIPAM Grafted in Nanochannels: A Molecular Dynamics Simulation Study

Porous thermosensitive hydrogels exhibit a more flexible strategy for freshwater capture compared to conventional hydrogels. This study employs molecular dynamics (MD) simulation to investigate the deswelling behavior of poly(N-isopropylacrylamide) (PNIPAM) grafted within the nanochannel, aiming to elucidate the deswelling elimination process at various temperatures. Notably, a distinct phase separation is observed at specific temperatures above the lower solution temperature (LCST). Furthermore, this study takes the effect of heat flux into account, wherein distinct heat fluxes lead to varying levels of phase separation between water and the polymer. Specifically, the number of hydrogen bonds, volume of polymer chains, and density distribution of water molecules are statistically analyzed to reveal the mechanism of phase separation in a thermosensitive hydrogel. These findings provide insight into the accelerated deswelling kinetics of the PNIPAM polymer chain, which has guiding significance for the field of water harvesting by the enhancement of the water release capacity in thermosensitive hydrogels.

Pressure Anisotropy in Polymer Brushes and Its Effects on Wetting

Polymer brushes, coatings consisting of densely grafted macromolecules, experience an intrinsic lateral compressive pressure, originating from chain elasticity and excluded volume interactions. This lateral pressure complicates a proper definition of the interface and, thereby, the determination and interpretation of the interfacial tension and its relation to the wetting behavior of brushes. Here, we study the link among grafting-induced compressive lateral pressure in polymer brushes, interfacial tension, and brush wettability using coarse-grained molecular dynamics simulations. We focus on grafting densities and polymer-liquid affinities such that the polymer and liquid do not tend to mix. For these systems, a central result is that the liquid contact angle is independent of the grafting density, which implies that the grafting-induced lateral compressive pressure in the brush does not influence its wettability. Although the definition of brush interfacial tensions is complicated by the grafting-induced pressure, the difference in the interfacial tension between wet and dry brushes is perfectly well-defined. We confirm explicitly from Young's law that this difference offers an accurate description of the brush wettability. We then explore a method to isolate the grafting-induced contribution to the lateral pressure, assuming the interfacial tension is independent of grafting density. This scenario indeed allows disentanglement of interfacial and grafting effects for a broad range of parameters, except close to the mixing point. We separately discuss the latter case in light of autophobic dewetting.

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.

Surface Forces Characterization of Concentrated PMMA Brush Layers under Applied Load and Shear

Concentrated polymer brushes (CPBs) are known to exhibit excellent lubrication properties. However, the frictional behaviors of CPBs vary, depending on their preparation and operating conditions. In order to understand such complicated properties, it is necessary to determine their structures and correlate them with their properties, during shear motion. In this study, we employed surface forces and resonance shear measurement (RSM) as well as refractive index measurement using fringes of equal chromatic order (FECO) for studying the structure of the CPBs of poly(methyl methacrylate) (PMMA) in toluene. The obtained elastic (ks) and viscous (bs) parameters based on the RSM for the PMMA-PMMA were higher than those obtained for PMMA-silica over the entire distance range. With the increasing shear amplitude on the PMMA-PMMA under an applied load, the bs value first increased and then decreased while the ks value monotonically decreased. These behaviors were consistent with those of the thicker CPBs reported in a previous paper (Soft Matter, 2019). Thus, the dynamics of the CPBs under the applied load and shear were not dependent on the thickness of the polymer brushes in this case. The density distribution of the swollen PMMA brushes along the distance in the thickness direction of the brush layer was estimated by using the measured refractive index values, showing that the fraction of the PMMA brushes in the outer region from the surface (20% in the thickness) was ca. 10%. This lower density region near the surface of the swollen CPBs enabled them to interpenetrate with each other. Changes in the refractive index value under shear were observed, indicating that the interpenetrated PMMA chains were pulled out with increasing shear amplitude. These results demonstrated that broader applications of CPBs are possible by regulating the friction between them under different operating conditions, even for usually lubricious CPBs.

Threshold of Surface Initiator Concentration for Polymer Brush Growth by Surface-Initiated Atom Transfer Radical Polymerization

The surface modification of various materials by grafting functional molecules has attracted much attention from fundamental research to practical applications because of its ability to impart various physical and chemical properties to the surfaces. One promising approach is the use of polymer brushes synthesized by atom transfer radical polymerization (ATRP) from surface-tethered initiators (SIs). In this study, for the purpose of controlling the grafting amounts/densities of polymer brushes, we developed a facile method to precisely regulate SI concentrations of SI layers (SILs) by serial dilution based on a sol-gel method. By simply mixing organosilanes terminated with and without an initiator group ((p-chloromethyl) phenyltrimethoxysilane (CMPTMS) and phenyltrimethoxysilane (PTMS), respectively) with tetraethoxysilane (TEOS), SI concentrations of SILs could be arbitrarily tuned precisely by varying dilution factors of (CMPTMS + PTMS)/CMPTMS (DFs, 1-107). The resulting SILs prepared at different DFs were highly smooth and transparent. X-ray photoelectron spectroscopy (XPS) also confirmed that the SIs were homogeneously distributed at the topmost surface of the SILs and their concentrations were proven to be accurately and precisely controlled from high to extremely low, comparable to theoretical values. Subsequent SI-ATRP in air ("paint-on" SI-ATRP) of two different types of monomers (hydrophobic/nonionic (2,3,4,5,6-pentafluorostyrene) and hydrophilic/ionic (sodium 4-styrenesulfonate)) demonstrated that polymer brushes with different grafting amounts/densities were successfully grafted only from SILs with DFs of 1-104 (theoretical SI concentrations: 3.9 × 10-4 ∼ 3.5 units/nm2), while at DFs of 105 and above (theoretical SI concentrations: <3.9 × 10-5 units/nm2), no sign of polymer brush growth was confirmed by thickness, XPS, and water contact angle data. Therefore, we are the first to gather evidence that the approximate threshold of SI concentration required for "paint-on" SI-ATRP might be on the order of 10-4 ∼ 10-5 units/nm2.

Stable trapping of multiple proteins at physiological conditions using nanoscale chambers with macromolecular gates

The possibility to detect and analyze single or few biological molecules is very important for understanding interactions and reaction mechanisms. Ideally, the molecules should be confined to a nanoscale volume so that the observation time by optical methods can be extended. However, it has proven difficult to develop reliable, non-invasive trapping techniques for biomolecules under physiological conditions. Here we present a platform for long-term tether-free (solution phase) trapping of proteins without exposing them to any field gradient forces. We show that a responsive polymer brush can make solid state nanopores switch between a fully open and a fully closed state with respect to proteins, while always allowing the passage of solvent, ions and small molecules. This makes it possible to trap a very high number of proteins (500-1000) inside nanoscale chambers as small as one attoliter, reaching concentrations up to 60 gL-1. Our method is fully compatible with parallelization by imaging arrays of nanochambers. Additionally, we show that enzymatic cascade reactions can be performed with multiple native enzymes under full nanoscale confinement and steady supply of reactants. This platform will greatly extend the possibilities to optically analyze interactions involving multiple proteins, such as the dynamics of oligomerization events.

Elucidating Interfacial Chain Conformation of Superhydrophilic Polymer Brushes by Vibrational Sum Frequency Generation Spectroscopy

Surface-tethered macromolecules (polymer brushes) are a potent means to modify surfaces with stimuli-responsive properties while avoiding delamination problems. This vibrational sum frequency generation spectroscopy study describes how the conformation of hydrophilic polymer brushes changes in response to environmental conditions, that is, changes in humidity (in air) and upon exposure to liquid water. Three hydrophilic brushes were prepared on silicon oxide surfaces by surface-initiated reversible deactivation radical polymerization of cationic (quaternary ammonium), anionic (sulfonate), and zwitterionic (containing both) monomers. The average tilt angle of methyl groups was analyzed and used to deduce the chain conformations of the polymer brushes. In air, the brush films absorb water and swell with increasing humidity. This is accompanied by the rotation of interfacial polymer chains. The degree of water uptake and chain conformation vary with the nature of the charged hydrophilic moieties. The hydrophilic polymer brush surfaces appear to remain relatively dry except in near-condensation conditions. In water, the quaternary ammonium groups of cationic and zwitterionic brushes are aligned nearly parallel to the surface. The anionic brush chains appear to assume nearly random conformations in water.

Advances in design of polymer brush functionalized inorganic nanomaterials and their applications in biomedical arena

Grafting of polymer brush (assembly of polymer chains tethered to the substrate by one end) is emerging as one of the most viable approach to alter the surface of inorganic nanomaterials. Inorganic nanomaterials despite their intrinsic functional superiority, their applications remain restricted due to their incompatibility with organic or biological moieties vis-à-vis agglomeration issues. To overcome such a shortcoming, polymer brush modified surfaces of inorganic nanomaterials have lately proved to be of immense potential. For example, polymer brush-modified inorganic nanomaterials can act as efficient substrates/platforms in biomedical applications, ranging from drug-delivery to protein-array due to their integrated advantages such as amphiphilicity, stimuli responsiveness, enhanced biocompatibility, and so on. In this review, the current state of the art related to polymer brush-modified inorganic nanomaterials focusing, not only, on their synthetic strategies and applications in biomedical field but also the architectural influence of polymer brushes on the responsiveness properties of modified nanomaterials have comprehensively been discussed and its associated future perspective is also presented. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Investigation of the Effect of Molecular Weight, Density, and Initiator Structure Size on the Repulsive Force between a PNIPAM Polymer Brush and Protein

This paper focuses on the effect of degree of polymerization (N), density ( $\sigma$ ), and pattern size ( $x$ ) on the interaction force between a periodically patterned Poly(N-isopropylacrylamide) (PNIPAM) brush and protein. The hydrophobic interaction, the Van der Waals attractive force, and the steric repulsive force were expressed in terms of $N$ , $\sigma$ , and $x$ . The osmotic constant (k1) and the entropic constant (k2) were determined from the fit of the steric repulsive force to an experimentally obtained force distance curve. The osmotic constant was 0.105, and the entropic constant was 0.255. Using these constants, the steric repulsive force was plotted as a function of the separation distance(s) between the substrate and the protein. The forces were determined at a separation distance equal to 0.3 nm, where L0 is the equilibrium thickness of the PNIPAM brush. At this separation distance, the value of the steric repulsive force was much higher than the value of the sum of the hydrophobic interaction and the Van der Waals attractive force for large degree of polymerization ( $N>100$ ) and density ( $\sigma >0.2$ chains/nm2). However, the repulsive force was comparable to the sum of the hydrophobic interaction and the Van der Waals attractive force for a small degree of polymerization ( $N<100$ ) and density ( $\sigma =0.2$ ). Furthermore, the steric repulsive force was plotted as a function of pattern size $x$ . The plot indicated that the steric repulsive force becomes nearly zero for all degrees of polymerization and density when the value of the initiator structure size was less than 200 nm. In addition to the steric repulsive force, the lateral extension of the chains in the periodically patterned PNIPAM brush was calculated by scaling low and compared with the experimental data taken from previously published literatures. The polymer brush structure was modelled as if the immediate bare substrate is so wide that even a stretched polymer segment cannot reach to the next polymer brush structure. In such models, the value of the lateral extension was equal to the thickness of the homogenous brush. It was independent of the pattern size. However, when the polymer brush structure was modelled as if there is another polymer brush structure at a distance half of the size of the period, the lateral extension was found to be dependent on the size of the initiator structure size due to chain bridging. This was witnessed by the patterning of polymer brushes using the interferometric patterning of PNIPAM brushes and an atomic force microscopy imaging of the polymer brush structures both in air and in water. The polymer brush structure resolution in water was much lower than the resolution in air, which indicates the lateral extension of the polymer chains in water. For such kind of periodic polymer brush structures, the gap between them was calculated, and it was found dependent on the degree of polymerization, density, and initiator structure size.

Grafting density and antifouling properties of poly[N-(2-hydroxypropyl) methacrylamide] brushes prepared by “grafting to” and “grafting from”

Poly(HPMA) brushes prepared by a grafting-from method suppress fouling from blood plasma by an order of magnitude better than the polymer brushes of the same molecular weight prepared by a grafting-to method.

Orthogonal Images Concealed Within a Responsive 6-Dimensional Hypersurface

A photochemical printer, equipped with a digital micromirror device (DMD), leads to the rapid elucidation of the kinetics of the surface-initiated atom-transfer radical photopolymerization of N,N-dimethylacrylamide (DMA) and N-isopropylacrylamide (NIPAM) monomers. This effort reveals conditions where polymer brushes of identical heights can be grown from each monomer. With these data, hidden images are created that appear upon heating the substrate above the lower critical solution temperature (LCST) of polyNIPAM. By introducing a third monomer, methacryloxyethyl thiocarbamoyl rhodamine B, a second, orthogonal image appears upon UV-irradiation. With these studies, it is shown how a new photochemical printer accelerates discovery, creates arbitrary patterns, and addresses long-standing problems in brush polymer and surface chemistry. With this technology in hand a new method is demonstrated to encrypt data within hypersurfaces.

Autonomous Nanoscale Chemomechanical Oscillation on the Self-Oscillating Polymer Brush Surface by Precise Control of Graft Density

As a novel functional surface, a self-oscillating polymer brush that undergoes autonomous, periodic swelling/deswelling during the Belousov-Zhabotinsky (BZ) reaction has been developed. Although extensive research has revealed how the fundamental aspects of the BZ reaction can be regulated based on the surface design of the self-oscillating polymer brush, design strategies for the induction of mechanical oscillation remain unexplored. Herein, we investigated the graft density effects on the phase transition behavior, which is an important design parameter for the mechanical oscillation of the modified polymer. The self-oscillating polymer-modified substrates with controlled graft densities were prepared by immobilizing various compositions of an initiator and a noninitiator followed by surface-initiated atom transfer radical polymerization of the self-oscillating polymer chains. In addition to the characterization of each prepared substrate, atomic force microscopy (AFM) and digital holographic microscopy (DHM) were employed to evaluate the density effects on the static and dynamic surface structures. AFM revealed that equilibrium swelling as well as thermoresponsive behavior is profoundly affected by the graft density. Moreover, using DHM, autonomous mechanical oscillation was captured only on the self-oscillating polymer brush with adequate graft density. Notably, the oscillation amplitude (150 nm) and the period (20 s) in this study were superior to those in a previous report on the self-oscillating polymer modified through the grafting-to method by 10- and 3-fold, respectively. This study presents design guidelines for future applications, such as autonomous transport devices.

Tuning the Thermoresponsive Behavior of Surface-Attached PNIPAM Networks: Varying the Crosslinker Content in SI-ATRP

The synthesis and thermoresponsive properties of surface-attached poly(N-isopropylacrylamide)-co-N,N'-methylene bisacrylamide (PNIPAM-co-MBAM) networks are investigated. The networks are formed via SI-ARGET-ATRP ("grafting-from") on thiol-based initiator-functionalized gold films. This method is reliable, well controlled, fast, and applicable to patterned surfaces (e.g., nanopores) for networks with dry thicknesses >20 nm. Surface-attached PNIPAM-co-MBAM gels are swollen below their volume phase transition temperature but above collapse without complete expulsion of water (retain ∼50 vol %). The swelling/collapse transition is studied using complementary SPR and QCMD techniques. The ratio between swollen and collapsed heights characterizes the thermoresponsive behavior and is shown to not depend on network height but to vary with MBAM content. The higher the proportion of the crosslinker, the lower the magnitude of the phase transition, until all responsiveness is lost at 5 mol % MBAM. The temperature range of the transition is broadened for more crosslinked PNIPAM-co-MBAM gels but remains centered around 32 °C. Upon reswelling, less crosslinked networks display sharp transitions, while for those containing ≥3 mol % MBAM, transitions remain broad. This tunable behavior persists for gels on nanostructured gold surfaces. Investigating PNIPAM-co-MBAM networks on gold plasmonic nanowell arrays is a starting point for expanding their scope as thermo-controlled nanoactuators.

Conformational Stability of Poly (N-Isopropylacrylamide) Anchored on the Surface of Gold Nanoparticles

To verify the temperature sensitive failure of poly (N-isopropylacrylamide) (PNIPAM) anchored on the surface of gold nanoparticles (AuNPs), the UV-Vis spectra with temperature variations of the following aqueous solutions respectively containing AuNPs-PNIPAM, Au-PNIPAM/PNIPAM, PNIPAM, in different media (including salt, ethanol, HCl and cetyltrimethylammoniumbromide (CTAB)), were systematically determined. The results indicated that the UV-Vis spectrum of AuNPs-PNIPAM suspension hardly changed even above the Lower Critical Solution Temperature (LCST) of PNIPAM, but that of Au-PNIPAM/PNIPAM sharply increased only in absorbance intensity. A possible mechanism of the failed temperature sensitivity of PNIPAM anchored on the surface of AuNPs was proposed. Being different from free PNIPAM molecules, a strong interaction exists among PNIPAM molecules anchored on the surface of AuNPs, restraining the change in conformation of PNIPAM. The temperature sensitivity of Au-PNIPAM/PNIPAM originates from the free PNIPAM molecules rather than the anchored PNIPAM one. The changing electrostatic interaction could effectively regulate the aggregation behavior of AuNPs-PNIPAM and enhance its sensitivity to temperature.

Surface-Grafted Polymeric Ionic Liquids with Tunable Morphology via In/Ex Situ Cross-linking Methods

Surface-grafted poly(ionic liquid) (PIL) films were prepared by both in and ex situ cross-linking methods with reversible addition-fragmentation chain transfer (RAFT) polymerization. Cross-linked brushes are more stable than linear brushes without sacrificing the surface functionality and, therefore, have increased potential for applications in biomedicine and materials chemistry. The two methods, in situ via a bifunctional cross-linker and ex situ via thermal cross-linking, were systematically compared on silicon-wafer substrates. Films obtained through in situ cross-linking were superior to films derived from our ex situ cross-linking technique with respect to responsive behavior and controlling the formation of polymer brushes on the surface. Alternatively, more stable layers were obtained by the ex situ cross-linking method using a cross-linker based on Meldrum's acid, where the film structure could be changed from a brush to collapsed film morphologies with an increasing cross-linker ratio.

Swelling of Poly(methyl acrylate) Brushes in Acetone Vapor

Sensor platforms can benefit from the incorporation of polymer brushes since brushes can concentrate the analyte near the sensor surface. Brushes that absorb acetone vapor are of particular interest since acetone is an important marker for biological processes. We present a simple procedure to synthesize acetone-responsive poly(methyl acrylate) brushes. Using spectroscopic ellipsometry, we show that these brushes respond within seconds and swell by more than 30% when exposed to acetone vapor. Moreover, quartz crystal microbalance measurements demonstrate that the brushes can be exploited to increase the acetone detection sensitivity of sensors by more than a factor 6. Surprisingly, we find that the swelling ratio of the brushes in acetone vapor is independent of the grafting density and the degree of polymerization of the polymers in the brush. This is qualitatively different from swelling of the same brushes in liquid environments, where the swelling ratio decreases for increasing grafting densities. Yet, it indicates that the brushes are robust and reproducible candidates for implementation in vapor sensor systems.

Rapid and continuous regulating adhesion strength by mechanical micro-vibration

Controlled tuning of interface adhesion is crucial to a broad range of applications, such as space technology, micro-fabrication, flexible electronics, robotics, and bio-integrated devices. Here, we show a robust and predictable method to continuously regulate interface adhesion by exciting the mechanical micro-vibration in the adhesive system perpendicular to the contact plane. An analytic model reveals the underlying mechanism of adhesion hysteresis and dynamic instability. For a typical PDMS-glass adhesion system, the apparent adhesion strength can be enhanced by 77 times or weakened to 0. Notably, the resulting adhesion switching timescale is comparable to that of geckos (15 ms), and such rapid adhesion switching can be repeated for more than 2 × 107 vibration cycles without any noticeable degradation in the adhesion performance. Our method is independent of surface microstructures and does not require a preload, representing a simple and practical way to design and control surface adhesion in relevant applications.

Using Patchy Particles to Prevent Local Rearrangements in Models of Non-equilibrium Colloidal Gels

Simple models based on isotropic interparticle attractions often fail to capture experimentally observed structures of colloidal gels formed through spinodal decomposition and subsequent arrest: the resulting gels are typically denser and less branched than their experimental counterparts. Here, we simulate gels formed from soft particles with directional attractions ("patchy particles"), designed to inhibit lateral particle rearrangement after aggregation. We directly compare simulated structures with experimental colloidal gels made using soft attractive microgel particles, by employing a "skeletonization" method that reconstructs the three-dimensional backbone from experiment or simulation. We show that including directional attractions with sufficient valency leads to strongly branched structures compared to isotropic models. Furthermore, combining isotropic and directional attractions provides additional control over aggregation kinetics and gel structure. Our results show that the inhibition of lateral particle rearrangements strongly affects the gel topology and is an important effect to consider in computational models of colloidal gels.

Thermo-Responsive Polymer Brushes with Side Graft Chains: Relationship Between Molecular Architecture and Underwater Adherence

During the last few decades, wet adhesives have been developed for applications in various fields. Nonetheless, key questions such as the most suitable polymer architecture as well as the most suitable chemical composition remain open. In this article, we investigate the underwater adhesion properties of novel responsive polymer brushes with side graft chain architecture prepared using “grafting through” approach on flat surfaces. The incorporation in the backbone of thermo-responsive poly(N-isopropylacrylamide) (PNIPAm) allowed us to obtain LCST behavior in the final layers. PNIPAm is co-polymerized with poly(methyl ethylene phosphate) (PMEP), a poloyphosphoester. The final materials are characterized studying the surface-grafted polymer as well as the polymer from the bulk solution, and pure PNIPAm brush is used as reference. PNIPAm-g-PMEP copolymers retain the responsive behavior of PNIPAm: when T > LCST, a clear switching of properties is observed. More specifically, all layers above the critical temperature show collapse of the chains, increased hydrophobicity and variation of the surface charge even if no ionizable groups are present. Secondly, effect of adhesion parameters such as debonding rate and contact time is studied. Thirdly, the reversibility of the adhesive properties is confirmed by performing adhesion cycles. Finally, the adhesive properties of the layers are studied below and above the LCST against hydrophilic and hydrophobic substrates.

Poly(N-isopropylacrylamide) Response to Salt Concentration and Anion Identity: A Brush-on-Brush Study

The stability of poly(N-isopropylacrylamide) (PNIPAM) brush-modified colloidal silica particles was compared to asymmetric and symmetric PNIPAM brush direct force measurements in the presence of 1, 10, and 500 mM aqueous salt solution of KCl, KNO₃, and KSCN between 10 and 45 °C. Dynamic light scattering measurements highlighted subtle variations in the salt-mediated thermoresponse, while atomic force microscopy (AFM) force curves between a bare silica or PNIPAM brush-modified colloid probe and a planar PNIPAM brush elucidated differences in brush interactions. The AFM force curves in the presence of KCl primarily revealed steric interactions between the surfaces, while KNO₃ and KSCN solutions exhibited electrosteric interactions on approach as a function of the chaotropic nature of the ion and the solution concentration. The symmetric PNIPAM brush interaction highlighted significant variations between KCl and KSCN at 1 and 500 mM concentrations, while the approach and retraction force curves were relatively similar at 10 mM concentration. The combination of these techniques enabled the stability of PNIPAM brush-modified colloidal dispersions in the presence of electrolyte to be better understood with specific ion binding and the solution Debye length playing a significant role.

A Kinetic Model of Oligonucleotide-Brush Interactions for the Rational Design of Gene Delivery Vectors

Polymer brushes are attractive candidates for the design of gene delivery vectors as they allow the systematic study of the impact of structural (type, size, and shape of nanomaterials core) and physicochemical parameters (cationic monomer chemistry, brush thickness, and grafting density) on transfection efficiency. However, relatively little is known of their interactions of oligonucleotides. To study such interactions, we use surface plasmon resonance and developed a kinetic model of brush binding and infiltration. We identify the striking impact that brush grafting density and thickness have on oligonucleotide kinetics of infiltration, binding affinity, and maximum loading. Surprisingly, double-stranded RNA molecules are found to load at significantly higher levels compared to DNA molecules of identical sequence (apart from uracils/thymines). Furthermore, analysis of the kinetics of adsorption of these oligonucleotides indicates that the stoichiometry of binding (ratio of amine versus phosphate residues) is close to parity for the uptake of 20 bp double-stranded RNA. Finally, nanoparticles were designed to be used as gene transfection vectors and to quantify if the brush grafting density and thickness significantly impact transfection efficiencies in a small interfering RNA knockdown assay. Therefore, this study demonstrates the rational design of polymer brush-based nanoparticle vectors for efficient delivery of oligonucleotides. The model developed will allow to uncover how the refinement of the physicochemical and structural properties of polymer brushes enable the tuning of RNA binding and allow the systematic study of cationic vectors efficiency for RNA delivery.

Effective Synthesis of Patterned Polymer Brushes with Tailored Multiple Graft Densities

This paper reports an effective method to prepare patterned polymer brushes on surfaces with tailored graft densities. High-density (concentrated), moderate-density (semidiluted), and low-density (diluted) polymer brushes were fabricated in patterned manners, offering defined three-dimensional patterned structures. This method uses a middle/near-UV (≥250 nm) lamp and needs only a short time (≤10 min) to fabricate prepatterns of the initiator, in sharp contrast to the previous high-energy lithography and time-consuming processes. The obtained patterned brush served as a molecular (protein) repellent/adsorptive interface based on a graft-density-dependent size-exclusion effect. This method is facile and accessible to wide ranges of tunable density and pattern shapes, which are attractive for extensive use.

Reversible Formation of Thermoresponsive Binary Particle Gels with Tunable Structural and Mechanical Properties

We investigate the collective behavior of suspended thermoresponsive microgels that expel solvent and subsequently decrease in size upon heating. Using a binary mixture of differently thermoresponsive microgels, we demonstrate how distinctly different gel structures form, depending on the heating profile used. Confocal laser scanning microscopy (CLSM) imaging shows that slow heating ramps yield a core-shell network through sequential gelation, while fast heating ramps yield a random binary network through homogelation. Here, secondary particles are shown to aggregate in a monolayer fashion upon the first gel, which can be qualitatively reproduced through Brownian dynamics simulations using a model based on a temperature-dependent interaction potential incorporating steric repulsion and van der Waals attraction. Through oscillatory rheology it is shown that secondary microgel deposition enhances the structural integrity of the previously formed single species gel, and the final structure exhibits higher elastic and loss moduli than its compositionally identical homogelled counterpart. Furthermore, we demonstrate that aging processes in the scaffold before secondary microgel deposition govern the final structural properties of the bigel, which allows a detailed control over these properties. Our results thus demonstrate how the temperature profile can be used to finely control the structural and mechanical properties of these highly tunable materials.

Making strong polyelectrolyte brushes pH-sensitive by incorporation of gold nanoparticles

Doping polymer brushes with gold nanoparticles (AuNPs) results in composite materials with colorimetric sensor properties. The present paper addresses the effect of electrostatic particle-particle interaction and the effect of the polymer brush type on particle assembly formation within the polymer matrix. The prospect for long-term use as colorimetric sensors is tested. Therefore, two different types of brushes of pH-insensitive polymers, non-ionic poly(N-isopropylacrylamide) (PNIPAM) and cationic poly-[2-(methacryloyloxy)ethyl] trimethylammonium chloride (PMETAC), are studied. After incubation of the non-ionic PNIPAM brush in an aqueous suspension of AuNPs with a pH-sensitive carboxylic acid capping, hydrogen binding led to attachment of the AuNPs, but they were easily detached at high pH due to loss of the hydrogen binding. In contrast, the anionic AuNPs adhere well to cationic PMETAC brushes even after post-treatment at low pH where the charge density of the AuNPs is strongly reduced. Therefore, the PMETAC/AuNP composites were further tested with respect to their stability against pH variations and their impact for colorimetric sensors. Although the neat PMETAC brush is not pH-sensitive, after embedding pH-sensitive AuNPs, the PMETAC/AuNP composite becomes pH-sensitive in a reversible manner. This is detectable by the reversible shift of the plasmon band and the reversible thickness change of the composites by exposing them to different pH.

Polymer on Top: Current Limits and Future Perspectives of Quantitatively Evaluating Surface Grafting

Well-defined polymer strands covalently tethered onto solid substrates determine the properties of the resulting functional interface. Herein, the current approaches to determine quantitative grafting densities are assessed. Based on a brief introduction into the key theories describing polymer brush regimes, a user's guide is provided to estimating maximum chain coverage and-importantly-examine the most frequently employed approaches for determining grafting densities, i.e., dry thickness measurements, gravimetric assessment, and swelling experiments. An estimation of the reliability of these determination methods is provided via carefully evaluating their assumptions and assessing the stability of the underpinning equations. A practical access guide for comparatively and quantitatively evaluating the reliability of a given approach is thus provided, enabling the field to critically judge experimentally determined grafting densities and to avoid the reporting of grafting densities that fall outside the physically realistic parameter space. The assessment is concluded with a perspective on the development of advanced approaches for determination of grafting density, in particular, on single-chain methodologies.

Thermoresponsive poly(glycidyl ether) brushes on gold: Surface engineering parameters and their implication for cell sheet fabrication

Thermoresponsive polymer coatings, optimized for cell adhesion and thermally-triggered cell detachment, allow the fabrication of confluent cell sheets with intact extracellular matrix. However, rational design guidelines for such coatings are rare, since temperature-triggered cell adhesion and detachment from thermoresponsive surfaces are mechanistically not well understood. Herein, we investigated the impact of molecular weight (2, 9, 24kDa), grafting density (0.04-1.4 chains nm^-2), morphology, and roughness of well-characterized thermoresponsive poly(glycidyl ether) brushes on the cell response at 37 and 20°C. NIH 3T3 mouse fibroblasts served as a model cell line for adhesion, proliferation, and cell sheet detachment. The cell response was correlated with serum protein adsorption from cell culture medium containing 10% fetal bovine serum. Intact cell sheets could be harvested from all the studied poly(glycidyl ether) coated surfaces, irrespective of the molecular weight, provided that the morphology of the coating was homogenous and the surface was fully shielded by the hydrated brush. The degree of chain overlap was estimated by the ratio of twice the polymer's Flory radius in a theta solvent to its interchain distance, which should be located in the strongly overlapping brush regime (2 R_f/l>1.4). In contrast, dense PNIPAM (2.5kDa) control monolayers did not induce protein adsorption from cell culture medium at 37°C and, as a result, did not allow a significant cell adhesion. These structural design parameters of functional poly(glycidyl ether) coatings on gold will contribute to future engineering of these thermoresponsive coatings on more common, cell culture relevant substrates.

STATEMENT OF SIGNIFICANCE

Cell sheet engineering as a scaffold-free approach towards tissue engineering resembles a milestone in regenerative medicine. The fabrication of confluent cell sheets maintains the extracellular matrix of cells which serves as the physiological cell scaffold. Thermoresponsive poly(glycidyl ether)s are highly cell-compatible and brushes thereof promote cell adhesion and growth without modification with additional cell adhesive ligands. Thus, a direct correlation of temperature-dependent serum protein adsorption and cell response with surface design parameters such as grafting density and molecular weight became accessible. Hence, surface engineering parameters of well-defined poly(glycidyl ether) monolayers for reproducible cell sheet fabrication have been identified. These design guidelines may also prove beneficial in the development of other brush-like thermoresponsive coatings for cell sheet engineering.

Temperature-Triggered Colloidal Gelation through Well-Defined Grafted Polymeric Surfaces

Sufficiently strong interparticle attractions can lead to aggregation of a colloidal suspension and, at high enough volume fractions, form a mechanically rigid percolating network known as a colloidal gel. We synthesize a model thermo-responsive colloidal system for systematically studying the effect of surface properties, grafting density and chain length, on the particle dynamics within colloidal gels. After inducing an attraction between particles by heating, aggregates undergo thermal fluctuation which we observe and analyze microscopically; the magnitude of the variance in bond angle is larger for lower grafting densities. Macroscopically, a clear increase of the linear mechanical behavior of the gels on both the grafting density and chain length arises, as measured by rheology, which is inversely proportional to the magnitude of local bond angle fluctuations. This colloidal system will allow for further elucidation of the microscopic origins to the complex macroscopic mechanical behavior of colloidal gels including bending modes within the network.

Hofmeister Effect on PNIPAM in Bulk and at an Interface: Surface Partitioning of Weakly Hydrated Anions

The effect of sodium fluoride, sodium trichloroacetate, and sodium thiocyanate on the stability and conformation of poly(N-isopropylacrylamide), in bulk solution and at the gold-aqueous interface, is investigated by differential scanning calorimetry, dynamic light scattering, quartz crystal microbalance, and atomic force microscopy. The results indicate a surface partitioning of the weakly hydrated anions, i.e., thiocyanate and trichloroacetate, and the findings are discussed in terms of anion-induced electrostatic stabilization. Although attractive polymer-ion interactions are suggested for thiocyanate and trichloroacetate, a salting-out effect is found for sodium trichloroacetate. This apparent contradiction is explained by a combination of previously suggested mechanisms for the salting-out effect by weakly hydrated anions.

Grafting density dominant glass transition of dry polystyrene brushes

The effects of the grafting densities (σ_p), molecular weights (M_n) and thicknesses of dry polystyrene (PS) brushes on their glass transition temperature (T) were investigated by ellipsometry. The results show that T strongly depends on the grafting density of the PS brushes. The T of the PS brushes with σ_p > 0.30 increases with decreasing M_n (or brush thickness) and is mainly dominated by entropic effects, in which the grafted chains are highly extended along the film thickness direction resulting in a sharp reduction in configurational entropy. The T of PS brushes with σ_p < 0.30 decreases with decreasing M_n (or brush thickness) which is mainly dominated by surface effects. For intermediate-density brushes (σ_p = 0.30), T becomes independent of M_n or brush thickness. The reason for this grafting density dependence of T is attributed to the transition of the PS brush conformation from mushroom-to-brush.

Minimum free-energy paths for the self-organization of polymer brushes

A methodology to calculate minimum free-energy paths based on the combination of a molecular theory and the improved string method is introduced and applied to study the self-organization of polymer brushes under poor solvent conditions. Polymer brushes in a poor solvent cannot undergo macroscopic phase separation due to the physical constraint imposed by the grafting points; therefore, they microphase separate forming aggregates. Under some conditions, the theory predicts that the homogeneous brush and the aggregates can exist as two different minima of the free energy. The theoretical methodology introduced in this work allows us to predict the minimum free-energy path connecting these two minima as well as the morphology of the system along the path. It is shown that the transition between the homogeneous brush and the aggregates may involve a free-energy barrier or be barrierless depending on the relative stability of the two morphologies and the chain length and grafting density of the polymer. In the case where a free-energy barrier exists, one of the morphologies is a metastable structure and, therefore, the properties of the brush as the quality of the solvent is cycled are expected to display hysteresis. The theory is also applied to study the adhesion/deadhesion transition between two opposing surfaces modified by identical polymer brushes and it is shown that this process may also require surpassing a free-energy barrier.

Pick up, move and release of nanoparticles utilizing co-non-solvency of PNIPAM brushes

A critical complication in handling nanoparticles is the formation of large aggregates when particles are dried e.g. when they need to be transferred from one liquid to another. The particles in these aggregates need to disperse into the destined liquid medium, which has been proven difficult due to the relatively large interfacial interaction forces between nanoparticles. We present a simple method to capture, move and release nanoparticles without the formation of large aggregates. To do so, we employ the co-non-solvency effect of poly(N-isopropylacrylamide) (PNIPAM) brushes in water-ethanol mixtures. In pure water or ethanol, the densely end-anchored macromolecules in the PNIPAM brush stretch and absorb the solvent. We show that under these conditions, the adherence between the PNIPAM brush and a silicon oxide, gold, polystyrene or poly(methyl methacrylate) colloid attached to an atomic force microscopy cantilever is low. In contrast, when the PNIPAM brushes are in a collapsed state in a 30-70 vol% ethanol-water mixture, the adhesion between the brush and the different counter surfaces is high. For potential application, we demonstrate that this difference in adhesion can be utilized to pick up, move and release 900 silicon oxide nanoparticles of diameter 80 nm using only 10 × 10 μm² PNIPAM brush.

The internal structure of PMETAC brush/gold nanoparticle composites: a neutron and X-ray reflectivity study

The amount and distribution of gold nanoparticles within a polymer brush matrix can be changed by altering the electrostatic interaction between particle–particle as well as particle–brush.

Synergistic Effects of Bound Micelles and Temperature on the Flexibility of Thermoresponsive Polymer Brushes

The persistence length is a key parameter for the quantitative interpretation of the flexibility of polymers. We have studied complexes composed of a spherical poly(N-isopropylacrylamide) (PNIPAM) brush and a sodium dodecyl sulfate micelle in an effort to characterize the flexibility of tethered PNIPAM below the lower critical solution temperature T_LCST. An analytical mean-field model is used to describe the persistence length L_p in a broad range of ψ, the number of bound micelles per chain. The persistence length of micelle-constrained PNIPAM is quantitatively correlated with the thermal energy k_BT, electrostatic repulsion f_C, and effective excluded-volume parameter ν_eff. The persistence length per ψ, which depends on T and f_C, is found to scale with a synergistic effect f_C/(ψk_BT). The results reveal that the bound-micelle charges affecting the persistence length are analogous to the fixed charges of polyelectrolytes, though the bound micelles are separated by a large number of neutral monomers. The extension ⟨L⟩ of micelle-constrained PNIPAM decreases as ⟨L⟩ ∼ f_C^-β_F with f_C, where β_F ≈ 0.58-0.8 depending on ψ, but as the universal power law ⟨L⟩ ∼ (f_C/k_BT)^-0.6 with the synergistic effect f_C/(k_BT), irrespective of ψ. In spite of the intricate interplay among the multiple components in the system, the extension scales as a function of ν_eff as ⟨L⟩ ∼ (ν_eff/ψL_p)^-β_V, where β_V ≈ 0.35 for the significant monomer interaction and β_V ≈ 0.2 for the weak or negligible monomer interaction.