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.

Synthesis and Characterization of Stimuli-Responsive Polymer Brushes in Nanofluidic Channels.

Nanochannels with controllable gating behavior are attractive features in a wide range of nanofluidic applications including viral detection, particle sorting, and flow regulation. Here, we use selective sidewall functionalization of nanochannels with a polyelectrolyte brush to investigate the channel gating response to variations in solution pH and ionic strength. The conformational and structural changes of the interfacial brush layer within the channels are interrogated by specular and off-specular neutron reflectometry. Simultaneous fits of the specular and off-specular signals, using a dynamical theory model and a fitting optimization protocol, enable detailed characterization of the brush conformations and corresponding channel geometry under different solution conditions. Our results indicate a collapsed brush state under basic pH, equivalent to an open gate, and an expanded brush state representing a partially closed gate upon decreasing the pH and salt concentration. These findings open new possibilities in noninvasive in situ characterization of tunable nanofluidics and lab-on-chip devices with advanced designs and improved functionality.

Flow-Induced Protein Chain Deformation, Segmental Orientation, and Phase Separation in Native Silk Feedstock.

The ability of many arthropods to spin silk and its many uses bear testament to its importance in Nature. Despite over a century of research, however, the spinning process is still not fully understood. While it is widely accepted that flow and chain alignment may be involved, the link to protein gelation remains obscure. Using combinations of rheology, polarized light imaging, and infrared spectroscopy to probe different length scales, this work explored flow-induced gelation of native silk feedstock from Bombyx mori larvae. Protein chain deformation, orientation, and microphase separation were observed, culminating in the formation of antiparallel ß-sheet structures while the work rate during flow appeared as an important criterion. Moreover, infrared spectroscopy provided direct observations suggesting a loss of protein hydration during flow-induced gelation of fibroin in native silk feedstock, which is consistent with recently reported hypotheses.

Transparent Composite Films Showing Durable Antifogging and Repeatable Self-Healing Properties Based on an Integral Blend Method.

Antifogging coatings for infrastructures and transparent objects have attracted much attention lately from the perspective of safety and visibility. We have developed a one-pot process to fabricate transparent composite films showing long-lasting antifogging and fast repeatable self-healing properties based on an integral blend (IB) method. This method does not require any specific pretreatments of inorganic fillers/particles. Thus, the precursor solutions could be prepared in a single step by simply mixing raw materials, e.g., poly(vinylpyrrolidone) (PVP) having different molecular weights (MWs: 55, 360, and 1300 k), nano-clay particles (NCPs), and amino-terminated organosilane (AOS). In this study, to control the degree of cross-linking between the PVP matrices and NCPs, addition of AOS as a cross-linker to the PVP matrices (weight percentage of AOS to the PVP matrices, α = 0.01-300%) was carefully controlled. Transparency and self-healing abilities/kinetics of the resulting samples were found to be strongly influenced by both the MWs of PVP and α values. Samples spin-coated with the lowest MW of PVP (55 k) and α values of 0.01-1% gave highly transparent and durable antifogging performance. For example, no fogging was observed for 7 days under >80% relative humidity, and scratches about 30 µm in width could be completely self-healed within a few hours. However, samples with α > 10% gave opaque/grayish films that did not show any self-healing abilities because of an increase in cross-linking of the matrices. The optimized precursor solution was also deposited directly onto the glass slides covered with a transparent porous silica nano-framework (SNF) by a spray-coating method. Due to the formation of the hard and superhydrophilic/hygroscopic SNF with a large surface area, durability of antifogging and self-healing properties of the composite films were moderately improved, compared to those on the flat glass slides.

Flow-induced crystallisation of polymers from aqueous solution.

Synthetic polymers are thoroughly embedded in the modern society and their consumption grows annually. Efficient routes to their production and processing have never been more important. In this respect, silk protein fibrillation is superior to conventional polymer processing, not only by achieving outstanding physical properties of materials, such as high tensile strength and toughness, but also improved process energy efficiency. Natural silk solidifies in response to flow of the liquid using conformation-dependent intermolecular interactions to desolvate (denature) protein chains. This mechanism is reproduced here by an aqueous poly(ethylene oxide) (PEO) solution, which solidifies at ambient conditions when subjected to flow. The transition requires that an energy threshold is exceeded by the flow conditions, which disrupts a protective hydration shell around polymer molecules, releasing them from a metastable state into the thermodynamically favoured crystalline state. This mechanism requires vastly lower energy inputs and demonstrates an alternative route for polymer processing.

Large-Scale Formation of Fluorosurfactant-Doped Transparent Nanocomposite Films Showing Durable Antifogging, Oil-Repellent, and Self-healing Properties.

Transparent nanocomposite films with multiple functionalities, such as durable antifogging, dynamic oleophobic, self-healing properties, were successfully prepared by a simple spin- or spray-coating method using aqueous solutions of poly(vinylpyrrolidone) (PVP) and aminopropyl-functionalized nanoclay (AMP-clay) platelets. In this study, anionic/waterborne perfluorooctanesulfonic acid potassium salt (PFOS) was premixed with the aqueous PVP solution to achieve a homogeneous dispersion of PFOS. Due to the addition of PFOS, the resulting nanocomposite film surfaces displayed statically hydrophobic (static water contact angle over 90°) and dynamically oleophobic (5 µL of oil droplets could slide off of the surface at low sliding/substrate tilt angles of less than 10°) behaviors. In spite of our nanocomposite film surface exhibiting a statically hydrophobic nature, the antifogging properties remained unchanged even after being left under high-humidity conditions (over 80% relative humidity) for 3 days. Thanks to both exceptional water-absorbing properties of PVP/AMP-clay matrices and good mobility of PFOS driven by moisture, our oil-repellent nanocomposite films could be repeatedly self-healed even after both severe physical (cutting, scratching, or falling sand abrasion) and chemical (vacuum UV oxidation) damages. Large-scale fabrication of this multifunctional nanocomposite film (30 cm × 30 cm) could also be successfully demonstrated by a spray-coating method based on in situ gel formation.

Perfluorinated compounds are not necessary: pegylated organosilanes can endow good water sliding/removal properties.

From a viewpoint of reducing the burden on both human health and the environment, alternative surface modification techniques for preparing highly water-repellent surfaces without the use of environmentally damaging perfluorocarbons are highly desirable. Among them, the development of hydrophilic surfaces showing superior water sliding/removal properties has been scarcely reported. In this study, we have successfully demonstrated the fabrication of smooth, transparent, and hydrophilic pegylated organosilanes (PEGn-Si, CH3O-(C2H4O)n-C3H6-Si(OCH3)3 where n = 3, 6-9, 9-12)-derived hybrid films showing excellent water sliding/removal properties using a simple sol-gel reaction of PEGn-Si and tetraethoxysilane (TEOS, Si(OC2H5)4). The final static/dynamic surface wetting properties of the samples were found to be significantly influenced by both the PEG chain length and their mixing ratios. The use of PEGn-Si with the longest PEG chain (n = 9-12) was found to be effective for improving water sliding/removal properties. Small volume water droplets (5 µL) on the PEG9-12-Si/TEOS hybrid film (static water contact angle (CA) of ∼40°) at a 90°-inclined surface could slide at an average speed of 3.4 mm/sec without pinning and tailing, which was about twice as fast as that on the PEG6-9-Si/TEOS hybrid film surface (1.5 mm/sec, static water CA of ∼40°), in spite of having similar static hydrophilic nature.

Spatially-Regulated Deposition of Quantum Dots on the Patterned Polymer Brush.

We have demonstrated direct patterning of surface initiator layer (SIL) for Atom Transfer Radical Polymerization (ATRP) using a sol-gel based "ink" containing (p-chloromethyl)phenyltrimethoxysilane and tetraethoxysilane for an inkjet printer. Mechanically/chemically robust and smooth micropatterns of SIL several tens of micrometer in width and 15 nm thickness were directly printed on silicon substrates under mild conditions (open to the atmosphere, at ~28 °C under >60% relative humidity). Subsequent surface-initiated ATRP of glycidyl methacrylate (GMA) under the air atmosphere gave area-selective/stepwise growth of homogeneous polyGMA brushes (pGMA) from the micropatterns of SIL. This area-selective growth of pGMA was also confirmed by a fluorescence microscopy. Because of chemical reactivity of epoxy groups on the grafted pGMA surfaces toward amino-functionalized nanomaterials, CdS/ZnS-alloyed quantum dots were spatially deposited only on the pGMA-covered regions with a complete area-selectivity.

Anti-Fogging/Self-Healing Properties of Clay-Containing Transparent Nanocomposite Thin Films.

Highly transparent antifogging films were successfully prepared on various substrates, including glass slides, silicon, copper and PMMA, by spin-coating a mixture of polyvinylpyrrolidone and aminopropyl-functionalized, nanoscale clay platelets. The resulting films were superhydrophilic and showed more than 90% transmission of visible light, as well as excellent antifogging and self-healing properties.

Continuous, High-Speed, and Efficient Oil/Water Separation using Meshes with Antagonistic Wetting Properties.

We report a novel oil/water separation device, allowing continuous, high-speed, and highly efficient purification of large volumes of oily water. This device uses a pair of hydrophilic/hydrophobic polymer-brush-functionalized stainless steel meshes, which have antagonistic wetting properties, i.e., superoleophobic and superhydrophobic properties, when submerged in the opposite liquid phase. This device can purify large volumes of n-hexadecane/water mixture (∼1000 L) in a continuous process rather than in batches, to high purities (∼99.9% mol/mol) at high flow rates (∼5 mL s(-1) cm(-2)), unlike the oil/water separation meshes reported so far.

Polymer Brush Surfaces Showing Superhydrophobicity and Air-Bubble Repellency in a Variety of Organic Liquids.

Silicon (Si) substrates were modified with polyalkyl methacrylate brushes having different alkyl chain lengths (C(n), where n = 1, 4, 8, and 18) using ARGET-ATRP at ambient temperature without purging the reaction solution of oxygen. The dynamic hydrophobicity of these polymer brush-covered Si surfaces when submerged in a variety of organic solvents (1-butanol, dichloromethane, toluene, n-hexane) depended markedly on the alkyl chain length and to a lesser extent polymer solubility. Long-chain poly(stearyl methacrylate) brushes (C(n) = 18) submerged in toluene showed excellent water-repellant properties, having large advancing/receding contact angles (CAs) of 169°/168° with negligible CA hysteresis (1°). Whereas polymer brushes with short alkyl-chain (C(n) ≤ 4) had significantly worse water drop mobility because of small CAs (as low as 125°/55°) and large CA hysteresis (up to 70°). However, such poor dynamic dewetting behavior of these surfaces was found to significantly improve when water drops impacted onto the surfaces at moderate velocities. Under these conditions, all brush surfaces were able to expel water drops from their surface. In addition, our brush surfaces were also highly repellant toward air bubbles under all conditions, irrespective of C(n) or polymer solubility. These excellent surface properties were found to be vastly superior to the performance of conventional perfluoroalkylsilane-derived surfaces.

An underwater superoleophobic surface that can be activated/deactivated via external triggers.

Poly[(2-dimethylamino)ethyl methacrylate] (pDMAEMA) brush surfaces were prepared using a facile aqueous Activators ReGenerated by Electron Transfer Atom Transfer Radical Polymerization (ARGET-ATRP) protocol at ambient temperature without any need to purge reaction solutions of oxygen. This produced underwater superoleophobic surfaces, which exhibited high advancing (θA, 164-166°) and receding (θR, 153-165°) contact angles (CAs) and low CA hysteresis (1-11°) with a variety of oils. Both in situ spectroscopic ellipsometry and dynamic CA measurements confirmed that pDMAEMA brush surfaces responded to three different external stimuli (pH, ionic strength, and temperature) by changing their thicknesses, degree of hydration, or their chemical composition. Increasing pH resulted in the largest decrease in hydration, followed by increasing temperature, and increasing ionic strength gave the smallest change in hydration. Coincident with these structural changes, stimulus-responsive dynamic dewetting behavior with various oils was observed. Increasing pH or ionic strength drastically reduced the θR values of oil drops and increased CA hysteresis, resulting in a sticky surface on which oil drops were pinned. No noticeable changes in dynamic oleophobicity were observed with increasing temperature. In addition, when oil drops impacted onto the brush surface instead of being gently placed, surfaces did not exhibit stimulus-responsive dewetting properties, being oleophobic under all conditions.

Large-scale and environmentally friendly synthesis of pH-responsive oil-repellent polymer brush surfaces under ambient conditions.

Contrary to conventional ATRP, aqueous A(R)GET-ATRP at ambient temperature without deoxygenating reaction solutions is an extremely facile method to create polymer brushes. Using these techniques, extremely thick poly[2-(dimethylamino)ethyl methacrylate] polymer brushes can be prepared (∼700 nm), or reaction solutions can be low chemical-content, consisting of 99% v/v water. Based on these techniques, we have also developed an easy and inexpensive method, referred to as "paint on"-ATRP, that directly pastes reaction solutions onto various large-scale real-life substrates open to the air. The resulting brush surfaces possess excellent oil-repellent properties, which can be activated or deactivated in response to solution pH.

Why can organic liquids move easily on smooth alkyl-terminated surfaces?

The dynamic dewettability of a smooth alkyl-terminated sol-gel hybrid film surface against 17 probe liquids (polar and nonpolar, with high and low surface tensions) was systematically investigated using contact angle (CA) hysteresis and substrate tilt angle (TA) measurements, in terms of their physicochemical properties such as surface tension, molecular weight/volume, dielectric constant, density, and viscosity. We found that the dynamic dewettability of the hybrid film markedly depended not on the surface tensions but on the dielectric constants of the probe liquids, displaying lower resistance to liquid drop movement with decreasing dielectric constant (ε < 30). Interfacial analysis using the sum-frequency generation (SFG) technique confirmed that the conformation of surface-tethered alkyl chains was markedly altered before and after contact with the different types of probe liquids. When probe liquids with low dielectric constants were in contact with our surface, CH3 groups were preferentially exposed at the solid/liquid interface, leading to a reduction in surface energy. Because of such local changes in surface energy at the three-phase contact line of the probe liquid, the contact line can move continuously from low-surface-energy (solid/liquid) areas to surrounding high-surface-energy (solid/air) areas without pinning. Consequently, the organic probe liquids with low dielectric constants can move easily and roll off when tilted only slightly, independent of the magnitude of CAs, without relying on conventional surface roughening and perfluorination.

Coupling pH-responsive polymer brushes to electricity: switching thickness and creating waves of swelling or collapse.

Electrolysis of water is proposed as a method to couple the pH-responsive behavior of polymer brushes to an electrical stimulus. It is shown that an electrode in close proximity to a pH-responsive polymer brush can change the local solution pH, inducing either swelling or collapse of the polymer brush. By alternating the bias of the voltage applied to the electrode, either acidic or alkaline conditions can be generated, and reproducible cycles of polymer brush swelling and collapse can be achieved. It was found that the length of time which the electrical stimulus is applied to the electrodes can be as short as 10 s and that, once "switched", polymer brushes remain in the switched state for many minutes after the electrical stimulus is turned off. In other experiments, two electrodes were positioned 10 cm apart with a pH-responsive brush in between. Under these conditions waves of either acidic or alkaline solution pH could be generated which caused a coincident wave of polymer brush swelling or collapse. These waves originate from one electrode and travel across the brush surface toward the opposite electrode with a velocity of ~40 µm s(-1).

pH-dependent control of particle motion through surface interactions with patterned polymer brush surfaces.

In this Article, we show that inclined silicon surfaces patterned with poly(methacrylic acid) brushes are able to control the position and movement of 20 µm silica particles, which are propelled across the patterned surface by sedimentation forces. Three different types of behavior were observed depending on the angle between the direction in which a particle sedimented and the orientation of the polymer-brush silicon interface. At small angles, particles were found to sediment to the brush interface and then sediment following the direction of the brush interface. At larger angles, particles sedimented to the interface and then followed the direction of the brush interface, but then after a certain distance changed direction to pass over the interface. At the largest angles where the brush interface was approximately perpendicular to the motion of the particle, particles were found to travel over the interface unperturbed. This behavior was also found to be pH dependent, allowing the formation of pH responsive "gates", which allow particles to pass at low pH but not at high pH. It was also found that if patterned polymer brush surfaces were oriented in the correct way, they were able to control the number of particles present at specific locations.

Importance of particle tracking and calculating the mean-squared displacement in distinguishing nanopropulsion from other processes.

In this paper we show that processes such as Brownian motion, convection, sedimentation, and bacterial contamination can cause small particles to move through liquids in a fashion which may be mistaken as nanopropulsion. It is shown that particle tracking and subsequent statistical analysis is essential to ascertain if small particles actually propel themselves, or if they are propelled by another process. Specifically we find that it is necessary to calculate the mean-squared displacement of particles at both short and long time intervals, to show that the direction of propulsion changes coincident with rotation of the particle by Brownian motion, as this allows nanopropulsion to be differentiated from Brownian motion, convection and sedimentation. We also find that bacteria can attach themselves to particles and cause them to be propelled. This leads to motion which appears very similar to nanopropulsion and cannot be differentiated using particle tracking and therefore find that carefully designed control experiments must be performed. Finally, we suggest an experimental protocol which can be used to investigate the motion of small objects and prove if they move due to nanopropulsion.

Controlling the motion and placement of micrometer-sized metal particles using patterned polymer brush surfaces.

In this paper, we show that silicon surfaces patterned with poly(methacrylic acid) brushes are able to control the Brownian motion of 2-3 µm iron particles, which sediment onto the surface in aqueous solution and experience differences in repulsive force depending upon their position. Differences in repulsion lead to different gravitational potential energies across the surface, which gives bias to the Brownian motion taking place. Three regimes have been identified depending upon the brush height: (i) no control of Brownian motion when the brush height is small, (ii) Brownian motion that is influenced by the polymer brush when the brush 17 height is intermediate, (iii) Brownian motion that is confined by polymer brush barriers when the brush height is greatest. The height of brush found necessary to significantly influence iron particle motion was small at 39 nm or 2% of the particle diameter.