Michael S. Silverstein

Organic-inorganic networks in foams from high internal phase emulsion polymerizations

Hybrid foams, which combine an inorganic polysilsesquioxane network with an organic polystyrene network, were successfully synthesized using high internal phase emulsions (HIPE). Methacryloxypropyltrimethoxysilane (MPS) was copolymerized radically with styrene (S) and divinylbenzene (DVB) in the organic phase of the emulsion. The hydrolytic condensation of the trimethoxysilyl groups formed an inorganic polysilsesquioxane network, which significantly increased the high temperature modulus and thermal stability of the foam. The hybrid foam had an open-cell structure typical of polymer foams based on HIPE (polyHIPE), with a significantly smaller cell diameter than S/DVB polyHIPEs. In the absence of DVB, there is no crosslinked network to restrict the mobility of MPS and the polysilsesquioxane network is more fully developed. At low MPS contents, the thermal stability of the foams is higher for foams with DVB, reflecting their higher crosslinking density. At high MPS contents, the thermal stability is higher for foams without DVB, reflecting the more fully developed inorganic network.

Porous poly(2-hydroxyethyl methacrylate) hydrogels synthesized within high internal phase emulsions

Hydrogels, such as those based on poly(2-hydroxyethyl methacrylate) (PHEMA), are hydrophilic three dimensional network structures that undergo extensive swelling in water. PolyHIPEs are highly porous, crosslinked polymers typically synthesized within high internal phase emulsions (HIPEs). This research describes materials with enhanced water absorption that combine hydrogel water absorption with capillary action by synthesizing PHEMA-based polyHIPEs within oil-in-water HIPEs. The variation in the ,-methylenebisacrylamide (MBAM) crosslinking comonomer content yields a narrow synthesis window in which water-swollen micro-gel particles phase separate, agglomerate, and form a heterogeneous polyHIPE wall structure with nanoscale porosity. Surprisingly, a hydrogel polyHIPE with a relatively high MBAM content also had the highest surface area and the highest water absorption. Ultimately, it is the influence of the MBAM content on the polymer hydrophilicity and on the porous structure that determines its effects on the properties.

Bicontinuous hydrogel–hydrophobic polymer systems through emulsion templated simultaneous polymerizations

Polymerization in the external phase of a high internal phase emulsion (HIPE) yields an emulsion templated, porous polyHIPE. This work describes the synthesis of novel bicontinuous hydrogel–hydrophobic polymer systems through simultaneous polymerization and cross-linking reactions in the HIPEs external organic-phase (styrene or 2-ethylhexyl acrylate with divinylbenzene) and internal aqueous-phase (acrylamide with N,N-methylenebisacrylamide). The ability to reversibly dry and hydrate these materials demonstrates their potential for biomedical and separation applications. The unexpected and significant reduction in the hydrated polyHIPE modulus with increasing acrylamide content in the aqueous-phase indicated that the polymerizations were not mutually exclusive and that the acrylamide affected the molecular structure of the hydrophobic polymer synthesized in the external phase. The locus of initiation (which can be in the aqueous-phase, organic-phase, or both) was also shown to have a profound affect on the molecular structure, porous structure, and properties.

Cross-linker flexibility in porous crystalline polymers synthesized from long side-chain monomers through emulsion templating

A polyHIPE is a cross-linked, highly porous polymer from the polymerization of monomers and cross-linking comonomers in the external phase of a high internal phase emulsion (HIPE). Crystallizable polyHIPE were synthesized through the copolymerization of stearyl acrylate (A18) with ethylene glycol dimethacrylate (EGDMA). PolyHIPE synthesized with the more flexible EGDMA were compared to those synthesized with the more rigid divinylbenzene. The polyHIPE densities were around 0.11 g cm-3. EGDMA destabilized the HIPE yielding relatively large voids and a structure whose partial closed-cell nature increased with increasing EGDMA content. The EGDMA cross-linked polyHIPE exhibited higher melting peak temperatures and higher crystallinities than the divinylbenzene cross-linked polyHIPE. Organic phase initiation of the EGDMA cross-linked polyHIPE yielded lower melting temperatures, lower crystallinities, and a highly interconnected porous structure that resulted in a lower modulus.

Plasma copolymerization : hexafluoropropylene and a nonpolymerizable gas

Plasma polymerized (PP) fluoropolymer films have been synthesized by the plasma copolymerization of hexafluoropropylene (HFP) and a nonpolymerizable gas. Plasma etching was inhibited by fluorine scavenging and HF formation on the addition of hydrogen and was accelerated on the addition of oxygen. Nitrogen, oxygen, and hydrogen were incorporated into the deposited polymer molecules when added to the HFP plasma. The polar component of surface tension increased on nitrogen addition and the dispersive component on hydrogen addition. The higher surface tension drove the deposition and coalescence of smaller particles from the gas phase polymerization yielding smooth surfaces. The significant drop in breakdown voltage on the addition of nitrogen was attributed to a different conduction mechanism in the relatively polar PP fluoropolymer film.

Organic-inorganic character of plasma-polymerized hexamethyldisiloxane

Plasma polymerization is a useful method for depositing thin films on substrates. The films formed are cross-linked, and their character depends on the plasma polymerization process conditions. The influence of power and monomer flow rate on the character of plasma-polymerized hexamethyldisiloxane (PPHMDSO) was investigated. The deposition rate increased strongly with HMDSO flow rate at low powers, indicating that the plasma is mono-mer-deficient. Increasing the power yielded a rapid drop in deposition rate, which reached a relatively flow rate-independent plateau at high power. At high flow rates and low powers, the similarity between the plasma polymer with its high hydrocarbon concentration and the monomer was greatest. At high powers, the monomer was more intensively fragmented yielding a more inorganic structure rich in silicon and oxygen. Generally, the plasma polymer seems to be a silicon-oxygen network with short hydrocarbon chains that may include hydroxyl and/or carbonyl groups attached to the silicon in the backbone. The inorganic nature of the plasma polymer at high powers and low flow rates is reflected in its relatively high polar component of surface tension.

Shape memory polymer foams from emulsion templating

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.

Microstructure of polyacrylate/polystyrene two-stage latices

Abstract The microstructure of multi-stage latices is investigated in the cold stage of the transmission electron microscope (TEM) through differences in the behaviour of their component polymers when irradiated by an electron beam. Frozen two-stage latices (TSL) of polyacrylates (PA) and polystyrene (PS) are examined in the TEM and in a high resolution scanning electron microscope (SEM). A PS core is revealed in a frozen PS seeded PS/PA TSL in the TEM. Crosslinked PS (XPS) shells are observed in the TEM of a frozen crosslinked PA (XPA) seeded XPA/XPS TSL with 50% PS. The identity of the particles with XPS shells is retained in the moulded material and observed in SEM. The existence of an interpenetrating polymer network (IPN)/microdomain structure is indirectly indicated in XPA seeded XPA/XPS TSL with 25% XPS whose mechanical behaviour also favours an XPA continuum and both intra- and inter-particle XPS microdomains. IPN/microdomain particles and particles with XPS shells in an XPA seeded XPA/XPS TSL with 35% XPS are observed in the TEM and reflected in the structure of the moulded material.

Properties and structure of elastomeric two-stage emulsion interpenetrating networks

Abstract Interpenetrating polymer networks (IPNs) consist of two crosslinked polymers which form a network within a network. Bulk-prepared IPNs are thermosetting and cannot be processed due to the formation of a macroscopic network. Emulsion IPNs, although thermosetting, can be processed as thermoplastic materials. This is due to a special particle-slippage flow mechanism which is practically insensitive to molecular weight. The morphology of the reported latex particles is unique in the sense that very small polystyrene domains are formed by phase-separation of polymerizing styrene added to a seeded flexible polyacrylate latex. Compression or injection moulded specimens of these crosslinked elastomeric materials show significant mechanical properties. Some properties and structural observations using a special electron microscopy technique are described in this article. This method is based on the differential radiation damage to various polymers embedded in ice and can be used as an analytical tool to determine the microstructure of certain multiphase systems.

Novel semi-IPN through vinyl silane polymerization and crosslinking within PVC films

Novel semi-IPN (interpenetrating polymer networks) were synthesized through vinyl silane modification of unplasticized poly(vinyl chloride) (PVC) films using relatively low temperatures, relatively high vinyl silane contents, and several different processing routes. A free-radical initiator was used to promote reaction of the vinyl groups, and an aqueous acetic acid solution was used to promote the methoxysilane hydrolysis and condensation (HC) reactions for siloxane crosslink formation. A gel consisting of silane alone was formed prior to the HC process, indicating the formation of a semi-IPN. The gel content following the HC process far exceeded the silane content, indicating a significant amount of PVC was entrapped by the silane network. This conclusion is supported by the homogeneous molecular structure and morphology of the films.