Creating Elastomer And Hydrogel Layers With Spatial Variation Of Crosslink Density

We will discuss methodologies that lead to the production of substrates featuring spatial variation of cross-link density in elastomer and hydrogel layers. First, we will demonstrate how to tune the network density in silicone elastomers spatially and gradually by varying the chemical composition of the base materials as well as by employing UV-based crosslinking. We will then present methods that enable tuning the spatial distribution of cross link densities in surface-bound hydrogel layers by utilizing external ultra-violet and temperature triggers applied in an orthogonal manner. The latter approach will then be generalized to demonstrate how to manufacture polymer networks using commercially-available commodity macromolecules without any need for special chemical synthesis. Please click Additional Files below to see the full abstract

Surface-Anchored Poly( N -isopropylacrylamide) Orthogonal Gradient Networks

We present a versatile synthetic route leading toward generating surface-attached polyacrylamide gels, in which the cross-link density varies continuously and gradually across the substrate in two orthogonal directions. We employ free radical polymerization to synthesize random copolymers comprising ~5% of photoactive methacrylyloxybenzophenone (MABP), ~5% of thermally active styrene sulfonyl azide (SSAz), and ~90% of N-isopropylacrylamide (NIPAAm) units. The presence of MABP and SSAz in the copolymer facilitates control over the cross-link density of the gel in an orthogonal manner using photoactivated and thermally activated cross-linking chemistries, respectively. Spectroscopic ellipsometry is employed to determine the degree of swelling of the gel in water and methanol as a function of position on the substrate. Network swelling varies continuously and gradually across the substrate and is high in regions of low gel fractions and low in regions of high gel fractions

One-pot synthesis of surface anchored network coatings

Abstract is not available

CHAP Enhances Versatility in Colloidal Probe Fabrication

A colloidal probe, comprising a colloidal particle attached to an atomic force microscope cantilever, is employed to measure directly interaction forces between the particle and a surface. It is possible to change or even destroy a particle while attaching it to a cantilever, thus limiting the types of systems to which the colloidal probe technique may be applied. Here we present the Controlled Heating and Alignment Platform (CHAP) for fabricating colloidal probes without altering the original characteristics of the attached particle. The CHAP applies heat directly to the atomic force microscope chip to rapidly and precisely control cantilever temperature. This minimizes particle heating and enables control over the viscosity of thermoplastic adhesive, to prevent it from contaminating the particle surface. 3D-printed components made the CHAP compatible with standard optical microscopes and streamlined the fabrication process while increasing the platforms versatility. Using the CHAP with a thermoplastic wax adhesive, colloidal probes were fabricated using polystyrene and silica particles between 0.7 and 40 m in diameter. We characterized the properties and interactions of the adhesive and particles, as well as the properties of the completed probes, to demonstrate the retention of particle features throughout fabrication. Pull-off tests with CHAPs probes measured adhesive force values in the expected ranges and demonstrated that particles were firmly attached to the cantilevers

Improving Tracer Particle Surface Properties for Wind Tunnel Research

The surface properties of micron size polystyrene latex microspheres (PSLs) modified with quaternary alkylammonium (QA) surfactants were investigated, with a focus on the relationship between surface chemistry and adhesion. These investigations were motivated by the need to develop non-fouling tracer particles for wind tunnel studies. The goals were to relate the work of adhesion between particles and substrates to the type and amount of QA modifier in order to optimize the performance of PSL tracers. Surfactant-free emulsion polymerization (SFEP) can produce PSLs for wind tunnel tracers. Covalentlybound charged groups (derived from the initiator) stabilize PSL surfaces in water. This work used PSLs with anionic surface groups. Previous studies indicated that surface-bound charged groups on PSLs have a significant impact on their interfacial energy. Modifying charged surface groups therefore offers a method to modulate PSL interfacial properties. In this work, PSLs and films were modified by adsorption of QA surfactants

Sonication-enabled rapid production of stable liquid metal nanoparticles grafted with poly(1- octadecene-alt-maleic anhydride) in aqueous solutions

Gallium-based liquid metals are attractive due to their unique combination of metallic and fluidic properties. Liquid metal nanoparticles (LM NPs), produced readily using sonication, find use in soft electronics, drug delivery, and other applications. However, LM NPs in aqueous solutions tend to oxidize and precipitate over time, which hinders their utility in systems that require long-term stability. Here, we introduce a facile route to rapidly produce an aqueous suspension of stable LM NPs within five minutes. We accomplish this by dissolving poly(1-octadecene-alt-maleic anhydride) (POMA) in toluene and mixing with deionized water in the presence of a liquid metal (LM). Sonicating the mixture results in the formation of toluene-POMA emulsions that embed the LM NPs; as the toluene evaporates, POMA coats the particles. Due to the POMA hydrophobic coating, the LM NPs remain stable in biological buffers for at least 60 days without noticeable oxidation, as confirmed by dynamic light scattering and transmission electron microscopy. Further stabilization is achieved by tuning the LM composition. This paper elucidates the stabilization mechanisms. The stable LM NPs possess the potential to advance the use of LM in biomedical applications

Reduction of Wind Tunnel Contamination During Flow Visualization Experiments Using Polystyrene Microspheres

Evaluation of novel methods and materials for seeding tracer particles for particle image velocimetry (PIV) was carried out in the Basic Aerodynamic Research Tunnel (BART) at NASAs Langley Research Center (LaRC). Seeding of polystyrene latex microspheres (PSLs) from ethanol/water suspensions and from the dry state was carried out using custom built seeders. PIV data generated using the novel methods were found to be in general agreement with data collected using the current seeding methods. Techniques for assessing PSL fouling of wind tunnel surfaces were identified and refined. Initial results suggest that dry seeding PSLs may allow comparable data quality to wet seeding while reducing wind tunnel screen fouling. Results also indicate that further developments to the dry seeding system should focus on increasing single particle flux into the wind tunnel. Modifications to PSLs and seeding equipment to achieve this have been identified and are discussed

Polymer Brush/Metal Nanoparticle Hybrids for Optical Sensor Applications: from Self-Assembly to Tailored Functions and Nanoengineering

This review article summarizes the progress of research in the field ofpolymer brush/metal nanoparticle hybrid materials. We will discuss the mutualinfluence of polymer brush matrix and particles. Self-assembly of particles withinpolymer brushes, and ways to control the loading and location of nanoparticlesinside polymer brushes will be described, as well as the possibility to use thebrush templates as nanoreactors to generate metal nanoparticles. The combination of stimuli-responsive polymer brushes and nanoparticles exhibiting surface plasmon resonance, such as gold or silver, enables the design of opticalsensors based on reversible variations of the brush conformation. Sensing devices are capable of detecting avariety of extrinsic variations in their surrounding enviroments. The progress in the development of such optical sensors usingbrush/particle hybrids will be discussed in more detail

Further Insight into the Mechanism of Poly(styrene-co-methyl methacrylate) Microsphere Formation

Polymeric microspheres have been utilized in a broad range of applications ranging from chromatographic separation techniques to analysis of air flow over aerodynamic surfaces. The preparation of microspheres from many different polymer families has consequently been extensively studied using a variety of synthetic approaches. Although there are a variety of methods of synthesis for polymeric microspheres, free-radical initiated emulsion polymerization is one of the most common techniques. In this work, poly(styrene-co-methyl methacrylate) microspheres were synthesized via surfactant-free emulsion polymerization. The effect of comonomer composition and addition time on particle size distribution, particle formation, and particle morphology were investigated. Particles were characterized using dynamic light scattering (DLS) and scanning electron microscopy (SEM) to gain further insight into particle size and size distributions. Reaction kinetics were analyzed alongside of characterization results. A particle formation mechanism for poly(styrene-co-methyl methacrylate) microspheres was proposed based on characterization results and known reaction kinetics

Measuring Work of Adhesion of Polystyrene Microspheres

Particle adhesion is relevant in fields ranging from aerospace and energy to civil engineering and medicine. The functions of aerodynamic surfaces, heat exchangers, solar panels, ventilation systems, and blood vessels are affected by the buildup of particulates on their surfaces. Direct measurement of the adhesive force between a particle and a surface is key to understanding and mitigating particle fouling. Approaches such as the Johnson-Kendall-Roberts (JKR) and Derjaguin-Muller-Toporov (DMT) models offer a first approximation of the forces involved but do not account for non-idealities like roughness or plastic deformation. Experimental measurements of adhesive forces often deviate significantly from predictions. One approach to measure adhesion is the colloidal probe technique, which uses a particle attached to the tip of an atomic force microscope (AFM) cantilever. The particle is touched to a surface and then withdrawn and a pull-off force (FPO) determined by cantilever deflection. FPO can be used to estimate work of adhesion (Wa) and other properties from existing models. We describe a new method for producing colloidal probes using wax as an adhesive to attach micrometer-scale spheres to AFM tips. This method can be used with a range of particles and minimizes the potential for changes to the particle surface chemistry or geometry from exposure to heat, chemicals, radiation, or external forces. Particle attachment to AFM tips is robust and reversible, allowing old particles to be replaced with new ones in a few minutes. Pull-off measurements using polystyrene (PS) particles, pristine and modified with myristyltrimethylammonium bromide (14-TAB), were collected from various substrates to demonstrate the viability of this technique and investigate the impact of particle surface modification