James B. McClain

Dispersion Polymerizations in Supercritical Carbon Dioxide

Conventional heterogeneous dispersion polymerizations of unsaturated monomers are performed in either aqueous or organic dispersing media with the addition of interfacially active agents to stabilize the colloidal dispersion that forms. Successful stabilization of the polymer colloid during polymerization results in the formation of high molar mass polymers with high rates of polymerization. An environmentally responsible alternative to aqueous and organic dispersing media for heterogeneous dispersion polymerizations is described in which supercritical carbon dioxide (CO2) is used in conjunction with molecularly engineered free radical initiators and amphipathic molecules that are specifically designed to be interfacially active in CO2. Conventional lipophilic monomers, exemplified by methyl methacrylate, can be quantitatively (>90 percent) polymerized heterogeneously to very high degrees of polymerization (>3000) in supercritical CO2 in the presence of an added stabilizer to form kinetically stable dispersions that result in micrometer-sized particles with a narrow size distribution.

Extraction of a hydrophilic compound from water into liquid CO2 using dendritic surfactants

Dendrimers are well defined, highly branched polymers that adopt a roughly spherical, globular shape in solution. Their cores are relatively loosely packed and can trap guest molecules, and by appropriate functionalization of the branch tips the macromolecules can act as unimolecular micelle-like entities. Here we show that dendrimers with a fluorinated shell are soluble in liquid carbon dioxide and can transport CO2-insoluble molecules into this solvent within their cores. Specifically, we demonstrate the extraction of a polar ionic dye, methyl orange, from water into CO2 using these fluorinated dendrimers. This observation suggests possible uses of such macromolecules for the remediation of contaminated water, the extraction of pharmaceutical products from fermentation vessels, the selective encapsulation of drugs for targeted delivery and the transport of reagents for chemical reactions (such as polymerization) in liquid and supercritical CO2 solvents.

Design of Nonionic Surfactants for Supercritical Carbon Dioxide

Interfacially active block copolymer amphiphiles have been synthesized and their self-assembly into micelles in supercritical carbon dioxide (CO2) has been demonstrated with small-angle neutron scattering (SANS). These materials establish the design criteria for molecularly engineered surfactants that can stabilize and disperse otherwise insoluble matter into a CO2 continuous phase. Polystyrene-b-poly(1,1-dihydroperfluorooctyl acrylate) copolymers self-assembled into polydisperse core-shell-type micelles as a result of the disparate solubility characteristics of the different block segments in CO2. These nonionic surfactants for CO2 were shown by SANS to be capable of emulsifying up to 20 percent by weight of a CO2-insoluble hydrocarbon into CO2. This result demonstrates the efficacy of surfactant-modified CO2 in reducing the large volumes of organic and halogenated solvent waste streams released into our environment by solvent-intensive manufacturing and process industries.

Applications of “Dry” Processing in the Microelectronics Industry Using Carbon Dioxide

ABSTRACT Condensed carbon dioxide (CO2) has emerged as a leading enabler of advanced semiconductor manufacturing processes. By exploiting the physical properties of CO2, some of the current challenges encountered in microelectronics processing related to shrinking feature sizes and materials compatibility have been addressed. Furthermore, the potential for reduction of chemicals used in processing is realized. Applications of CO2 in microelectronics operations such as wafer cleaning, spin-coating, development, and stripping of photoresists, drying, low-k film preparation and repair, etching, and metal deposition are discussed.

Enhanced drug delivery capabilities from stents coated with absorbable polymer and crystalline drug.

Current drug eluting stent (DES) technology is not optimized with regard to the pharmacokinetics of drug delivery. A novel, absorbable-coating sirolimus-eluting stent (AC-SES) was evaluated for its capacity to deliver drug more evenly within the intimal area rather than concentrating drug around the stent struts and for its ability to match coating erosion with drug release. The coating consisted of absorbable poly-lactide-co-glycolic acid (PLGA) and crystalline sirolimus deposited by a dry-powder electrostatic process. The AC-SES demonstrated enhanced drug stability under simulated use conditions and consistent drug delivery balanced with coating erosion in a porcine coronary implant model. The initial drug burst was eliminated and drug release was sustained after implantation. The coating was absorbed within 90 days. Following implantation into porcine coronary arteries the AC-SES coating is distributed in the surrounding intimal tissue over the course of several weeks. Computational modeling of drug delivery characteristics demonstrates how distributed coating optimizes the load of drug immediately around each stent strut and extends drug delivery between stent struts. The result was a highly efficient arterial uptake of drug with superior performance to a clinical bare metal stent (BMS). Neointimal thickness (0.17±0.07 mm vs. 0.28±0.11 mm) and area percent stenosis (22±9% vs. 35±12%) were significantly reduced (p<0.05) by the AC-SES compared to the BMS 30 days after stent implantation in an overlap configuration in porcine coronary arteries. Inflammation was significantly reduced in the AC-SES compared to the BMS at both 30 and 90 days after implantation. Biocompatible, rapidly absorbable stent coatings enable the matching of drug release with coating erosion and provide for the controlled migration of coating material into tissue to reduce vicissitudes in drug tissue levels, optimizing efficacy and reducing potential toxicity.

Neutron scattering characterization of homopolymers and graft-copolymer micelles in supercritical carbon dioxide

Abstract Superficial fluids (SCF) are becoming an attractive alternative to the liquid solvents traditionally used as polymerization media [1]. As the synthesis proceeds, a wide range of colloidal aggregates form, but there has hitherto been no way to measure such structures directly. We have applied small-angle neutron scattering (SANS) to characterize such systems, and although SCF polymerizations are carried out at high pressures, the penetrating power of the neutron beam means that typical cell windows are virtually transparent. Systems studied include polymers soluble in CO 2 such as poly(1,1-dihydroperfluorooctyl acrylate) (PFOA), poly(hexafluoropropylene oxide) (PHFPO) and poly(dimethyl siloxane) (PDMS). PFOA has previously [2] been shown to exhibit a positive second virial coefficient ( A 2 ), though for PHFPO, A 2 appears to be close to zero ( 10 4 A 2 ⋍ 0 ± 0.2 cm 3 g −2 mol ). PDMS is soluble on the molecular level only in the limit of dilute solution and seems to form aggregates as the concentration increases (c > 0.01 g cm −3 ). Typical hydrocarbon polymers are insoluble in CO 2 , but it has been found that polymerizations may be accomplished via the use of a stabilizer [3]. For example, styrene has been polymerized in CO 2 by means of a polystyrene-b-PFOA block copolymer surfactant, which forms micelles in CO 2 and is also amenable to SANS characterization. Other amphiphilic surfactant molecules that form micelles include PFOA-g-poly(ethylene oxide) (PFOA-g-PEO) graft copolymers, which swell as the CO 2 medium is saturated with water. These systems have been characterized by SANS, by taking advantage of the different contrast options afforded by substituting D 2 O for H 2 O. This paper illustrates the utility of SANS to measure molecular dimensions, thermodynamic variables, molecular weights, micelle structures etc. in supercritical CO 2 .

Drug Coated Stents

Drug-coated stents appear to be the most promising approach among all interventional strategies to prevent restenosis. These stents both suppress geometric remodeling and inhibit neointimal hyperplasia with a therapeutic agent. Animal studies and recent randomised clinical trials with sirolimus-eluting stents have achieved excellent results in the prevention of restenosis. These stents also have a good safety record and demonstrate a durable clinical benefit for patients at long-term follow-up. This article summarises experimental and clinical experiences with local drug delivery via a stent coating in the prevention of restenosis after coronary angioplasty, outlining the clinicians view of current trends.

The importance of surfactants for polymerizations in carbon dioxide

Publisher Summary This chapter describes the effects of various surfactant and stabilizer systems as they apply to polymerizations in CO 2 . For CO 2 to be an effective continuous phase for polymerizations, heterogeneous reaction systems must necessarily be developed analogous to classical emulsion, inverse emulsion, dispersion, suspension, and precipitation polymerization processes. With some highly reactive monomers, such as acrylic acid and tetrafluoroethylene, free-radical precipitation polymerization can afford polymers with high yields and molecular weight. However, many industrial monomers require the utilization of surfactant stabilized reaction conditions. This chapter discusses the utilization of nonionic—homopolymer, block copolymer, and reactive macromonomer—surfactants consisting of covalently bound CO 2 -philic and CO 2 -phobic segments in the dispersion polymerization of various CO 2 -insoluble polymers. These reactions produce a stabilized polymer colloid in CO 2 solution and dry, free flowing powders after isolation.