Showing papers by "Michael S. Silverstein published in 2012"

PEO-Based Star Copolymers as Stabilizers for Water-in-Oil or Oil-in- Water Emulsions

Abstract: A series of well-defined poly(ethylene oxide) (PEO)-based star copolymers with controlled number of arms, compactness, and compositions were prepared by atom transfer radical polymerization using a simple “arm-first” method. Tuning the composition of the arms allowed these star polymers to stabilize either oil-in-water or water-in-oil emulsions. Stable oil-in-water emulsions were obtained with star copolymers containing only hydrophilic PEO arms due to their better affinity to the aqueous phase. In contrast, the insertion of a fraction of hydrophobic poly(butyl acrylate) arms into the star structure significantly changed the surfactant behavior of the star copolymers, leading to the formation of stable water-in-oil emulsions. Additionally, in both cases, an extremely low surfactant concentration, <0.01 wt % vs total weight of water and xylene used for emulsion formation, was sufficient to generate stable emulsions.

One-Pot Synthesis of Elastomeric Monoliths Filled with Individually Encapsulated Liquid Droplets

Abstract: The ability to encapsulate and store liquids within discrete, micrometer-scale containers is essential for living organisms and is used by water-storage cells to keep plants healthy, even during a prolonged drought. This paper describes the one-pot synthesis of liquid droplet elastomers (LDEs), a novel materials system with advantageous properties. LDEs are tissue-like elastomeric monoliths that contain around 85% water in the form of individually encapsulated micrometer-scale droplets. The LDE structure is templated within high internal phase Pickering emulsions, emulsions stabilized by nanoparticles that also function as cross-linking centers. The polyhedral shapes of the droplets, which accommodate the high volume fraction of water, are “locked-in” to the LDE during polymerization. The water retention in these unique materials is exceedingly high, and as seen for living systems, the presence of the encapsulated water droplets enhances the resistance to compressive deformation and to ignition upon direc...

One-Pot Emulsion-Templated Synthesis of an Elastomer-Filled Hydrogel Framework

Abstract: Sulfonated polymers are of interest as semipermeable ion-selective materials for membrane applications and as conductive materials for antistatic, electrode, and fuel cell applications. An innovative “one-pot” synthesis route was developed to produce relatively flexible systems containing a cross-linked polyacrylate elastomer within a continuous sulfonated hydrogel network. This synthesis route was based on the formation of a high internal phase emulsion (HIPE) containing an aqueous solution of styrenesulfonate and a cross-linking comonomer in the external phase and containing 2-ethylhexyl acrylate and a cross-linking comonomer in the internal phase. Both emulsifier-stabilized HIPEs and emulsifier-free polymer–nanoparticle-stabilized Pickering HIPEs were investigated using this route. A “three-step” route consisting of sulfonating a cross-linked polystyrene (xPS)-based polyHIPE in sulfuric acid and then polymerizing 2-ethylhexyl acrylate and a cross-linking comonomer within the polyHIPE was used for compa...

Shape memory polymer foams from emulsion templating

Abstract: Shape memory polymers (SMPs) change their shape under a stimulus (thermal, chemical, light) and return from an imposed temporary shape to their permanent, original shape. SMPs usually contain “permanent” domains that determine the permanent shape (chemical or physical crosslinks) and “reversible” domains that determine the temporary shape, usually by heating above a glass transition temperature or a melting point (Tm). Compared to fully dense SMPs, SMP foams can undergo higher temporary deformations and can exhibit higher deformations when they recover. In this paper, SMP foams based upon (meth)acrylates with crystallizable long side-chains were synthesized through emulsion-templating within nanoparticle-stabilized high internal phase Pickering emulsions where the nanoparticles also served as crosslinking centers. The nature of the polymer backbone affected the nature of the crystalline phase for identical side chains. The SMP foams at room temperature maintained the temporary shape (a strain of 0.7) imposed above the Tm and exhibited good recovery upon reheating for all four compression–recovery cycles. While the methacrylate-based SMP exhibited a single-stage recovery, the acrylate-based SMP, with identical side-chains, exhibited a two-stage recovery that can be associated with the existence of two crystalline phases. The recovery behavior was described using Kelvin–Voigt units in series with the dependence of viscosity on temperature described using a WLF-like relationship.

Liquid-retaining elastomeric compositions

Abstract: Disclosed are compositions-of-matter composed of a continuous elastomeric matrix and a liquid; the matrix entrapping the liquid therein in the form of closed-cell droplets dispersed throughout the matrix. The disclosed compositions-of-matter are characterized by a low tensile/compressive modulus and are capable of retaining the liquid for exceedingly long periods of time. Further disclosed are processes for forming the compositions-of-matter and uses thereof.