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Extramural Research

Final Report: Nonionic Surfactants for Dispersion Polymerizations in Carbon Dioxide

EPA Grant Number: R826115
Title: Nonionic Surfactants for Dispersion Polymerizations in Carbon Dioxide
Investigators: DeSimone, Joseph M.
Institution: University of North Carolina at Chapel Hill
EPA Project Officer: Karn, Barbara
Project Period: November 1, 1997 through October 31, 2000
Project Amount: $370,000
RFA: Technology for a Sustainable Environment (1997)
Research Category: Nanotechnology , Pollution Prevention/Sustainable Development

Description:

Objective:

More than 30 billion pounds of organic and halogenated solvents are used world-wide each year as process aids, cleaning agents, and dispersants, and solvent-intensive industries are considering alternatives that can reduce or eliminate the negative impact that solvent emissions can have on the environment. Recently, the design and synthesis of surfactant molecules that are active in CO2 has enabled a variety f new processes that will reduce hazardous waste production and emission. With this innovation, we have demonstrated that CO2 can be used as a replacement for more hazardous volatile organic compounds (VOCs) and chlorofluorocarbons (CFCs) that are traditionally used as solvents in the polymer industry. The key feature of this work has been the rational design and utilization of surfactants, or soaps, which are amphiphilic and interfacially active in a CO2 continuous phase. It is apparent that the widespread use of CO2 by industries will depend strongly on the design and efficient synthesis of effective surfactants (amphilphilic macromolecule that act as soaps in CO2) for a variety of applications. This pursuit is the main focus of this proposal.

Summary/Accomplishments (Outputs/Outcomes):

Recent efforts to synthesize methyl methacrylate (MMA) via dispersion polymerization in CO2 investigated the influence of helium concentration in CO2 on the resulting poly(methyl methacrylate) (PMMA) particle sizes and distribution. This study was important since many CO2 supply tanks are sold with a helium head pressure. It was found that the presence of 2.4 mol% helium in CO2 increased the PMMA average particle diameters form 1.9 to 2.7 mm, while decreasing particle size distribution from 1.29 to 1.03. Furthermore, the small percentage of helium in the reaction mixture decreases the solvent strength of the medium, as determined by solvatochromic investigations.

The dispersion polymerization of styrene in supercritical CO2 using amphiphilic diblock copolymers has also been studied in detail. Using a surfactant with poly(styrene) (PS) anchoring block and a polydimethylsiloxane (PDMS) soluble block (PS-b-PDMS), spherical PS particles were isolated in the form of a dry, free-flowing powder. The resulting high yield (>90%) of PS was obtained in the form of a stable polymer colloid comprised of submicron-sized particle. The affinity of the amphiphilic diblock copolymers for the PS particle surface was confirmed by interfacial tension measurements in a CO2 continuous phase.

The anchor-to-soluble balance (ASB), or ratio of the two block lengths, of the PS-b-PDMS stabilizer was found to be a crucial factor affecting both the stability of the resulting latex in CO2 and the particle morphology. As expected, both the concentration of monomer and the concentration of stabilizer affected the morphology of the resulting PS particles. Additionally, the temperature and pressure of the reaction mixture were found to effect results-such as average particle diameter and molecular weight-of the PS product.

The preparation of stable dispersions of poly(vinyl acetate) (PVAc) and ethylene-vinyl acetate copolymers in liquid and supercritical CO2 has recently been investigated. Both fluorinated and siloxane-based stabilizers including homopolymers, block copolymers, and reactive macromonomers were employed. The influence of the concentration of stabilizer, stabilizer anchor-soluble balance, and pressure on the resulting colloidal product was explored. In addition, turbidimetry was used successfully to monitor dispersed phase volume fractions, particle sizes, and number densities during the polymerization. The vinyl acetate polymerization stabilized by PDMS homopolymer, vinyl-terminated PDMS macromonomer, or PVAc-b-PDMS all produced collapsed latexes of high yield and high molecular weight polymer, whereas the polymerization stabilized by PVAc-b-PFOA provided stable latexes. Turbidity showed that PFOA and PVAc-b-PFOA with a short anchoring group (PVAc Mn= 4x103 g/mol) inefficiently anchored to the PVAc particles. The PVAc-b-PFOA with the longest blocks (PVAC Mn=3.1 x 104 g/mol; PFOA Mn = 5.4 x 104 g/mol) produced the smallest diameter polymer particles.

We have recently reported the successful dispersion polymerization of acrylonitrile in carbon dioxide using a block copolymer consisting of polystyrene and poly(1,1,-dihydroperfluorooctyl acrylate) as a stabilizer. The dispersion polymerization of sytrene in supercritical CO2 utilizing random copolymers including 1,1-dihydroperfluoroctyl methacrylate (FOMA) and sytrene and 3-[tris(trimethylsilyloxy)silyl]propyl methacrylate (SiMA) was investigated. The resulting high yield of spherical and relatively uniform polystyrene (PS) particles with micron-size range was formed using various amounts of the random copolymers as a stabilizer with good stability until the end of the reaction. Although the copolymers with styrene can be good stabilizers when the incorporated ratio of FOMA is high (>92 w/w %), the copolymers with SiMA can indeed be good stabilizers even when the incorporated ratio of FOMA is as low as 26 w/w %. The particle diameter was shown to be dependent on the percent of FOMA incorporated in the stabilizer and the weight percent of added stabilizer.

The successful cationic dispersion polymerization of styrene in liquid carbon dioxide using amphiphilic block copolymers is reported. Vinyl ether block copolymers consisting of a CO2-philic fluorinated segment and a poly(methyl vinyl ether) anchoring segment were empolyed to stabilize the growing polystyrene (PS) particles. The effect of stabilization in these polymerizations on molar mass, molar mass distribution, and polymer yield was studied as a function of temperature and surfactant composition. Scanning electron microscopy confirmed the formation of PS particles ranging in size fro 300 nm to 1 um in diameter.

The dispersion polymerization of 1-vinyl-2-pyrrolidone in supercritical CO2 has been examined using poly(1,1-dihydroperfluorooctyl acrylate) [PFOA] as a polymeric stabilizer. The polymerizations produced stable latexes that resulted in high yields of polymer with high molecular weights [Mw=3.1x106 g/mol]. Spherical and relatively uniform particles were formed with diameters ranging from 0.56 to 2.89 m while the surfactant concentration was varied from 0.25 to 6 w/v %. Upon increasing the initial concentration of the monomer from 20 to 60 w/v %, the latex stability decreased while the particle size increased. Dispersion polymerizations were also carried out under a series of different pressures (141-351 bar) which resulted in moderate to high yields and particles in the micron-size range. An increase in the final pressure of the system was observed for most cases. Finally, preliminary extraction studies revealed that some stabilization appears to occur by PFOA grafting onto the PVP particles.

The behavior of polymeric surfactant polyvinyl acetate (PVAC)-b- poly(1,1,2,2-tetrahydroperfluorooctyl acrylate) (PTAN) in supercritical carbon dioxide (CO2) was investigated using static and dynamic light scattering. We observed three regions on the phase diagram of the copolymer in supercritical CO2: (1) two-phase region at low CO2 density; (2) solutions of spherical micelles at intermediate CO2 densities; and (3) solutions of unimers (individual copolymer chains) at high CO2 densities. The aggregation number (the number of copolymer chains in a micelle) decrease with an increasing density of supercritical CO2 in region (ii). An increase of the CO2 density corresponds to the improvement of solvent quality for both blocks of the copolymer (PVAC and PTAN). the hydrodynamic radius of micelles and unimers was measured using dynamic light scattering in regions (ii) and (iii), respectively. This light-scattering study is the first one reporting a solvent density-induced transition between spherical micelles at lower supercritical CO2 density and unimers at higher CO2 density. The light-scattering technique appears to be a very powerful tool for the analysis of the carbon dioxide density-induced micellization transition. This phenomenon is unique to supercritical fluids and demonstrates a convenient control over the polymer solubility.

We have illustrated the use of temperature and solvent quality to regulate the degree of assocation of block copolymer amphiphiles in highly compressible supercritical carbon dioxide. Small angle neutron scattering (SANS) has been used to examine the association behavior of a block copolymer containing a CO2-phobic moiety, poly(vinylacetate) and a CO2-philic block, poly(1,1-dihydroperfluoro-octylacryalte). By adjusting the density of the medium through pressure and temperature profiling, the self-assembly can be reversibly controlled from unimers to core-shill spherical micelles and this establishes a critical micelle density (CMD), a phenomenon distinctive of highly compressible fluids, such as supercritical CO2. Mathematical modeling of the data in terms of core-shell micelle structures permits a detailed description of the structure and the degree of swelling (penetration) of the solvent into the different regions of the aggregates throughout this transition.

A block copolymer composed of CO2-phobic polyvinylacetate (PVAc, 10.3 kDa) and a CO2-philic fluorinated octyl acrylate (PFOA, 43.1 kDa) of average effective molecular weight 90.4 kDa has been studied by time of flight small angle neutron scattering (SANS) in supercritical CO2 (sc-CO2) at 65°C [1-3]. A sharp unimer-micelle transition is obtained due to the tuning of the solvating ability of sc-CO2 by profiling pressure, so that the block copolymer, in a semidilute solution, finds sc-CO2 a good solvent at high pressure and a poor solvent at low pressure. At high pressures, the copolymer is in a monomeric state with a random coil structure. However, on lowering the pressure, aggregates are formed with a structure similar to aqueous micelles--the hydrocarbon segments forming the core and the fluorocarbon segments forming the corona of the micelle. This unimer-aggregate transition is driven by the gradual elimination of CO2 molecules solvating the hydrocarbon segments of the polymer. Comparison of these results with related data on the same polymer at different temperatures indicates that the transition is critically related to the density of the solvent. This suggests the definition of a critical micellization density. Mathematical modeling of the data in terms of core-shell micelle structures permits a detailed description of the structure and the degree of swelling (penetration) of the solvent into the different regions of the aggregates throughout this transition.

In parallel with SANS studies of the structure of polymer aggregates in CO2, we have begun to characterize a PS (polysytrene) -b-PFOA copolymer molecular dynamics by medium-resolution quasi-elastic neutron scattering (QENS). Line shapes are dominated by localized diffusive modes and segmental dynamics of the anchored, finite-length PFOA chains. For Q <0.6 Å-1, we obtain effective diffusion coefficients of approximately 0.8 x 10-6 cm2/s. At higher Q, a single component is not sufficient as shown by excess intensity on the flanks. For Q> ³1.5 Å-1, the wings reflect contributions due to a distribution of faster, more localized chain modes.

This PS-b-FOMA copolymer has been utilized as the stabilizer in the dispersion polymerization of 2-hydroxyethyl methacrylate (HEMA) in CO2 continuous phase. HEMA was effectively emulsified in CO2 with the amphiphilic diblock copolymer surfactant. Spherical particles in the submicrometer range were obtained with relatively narrow particle size distributions. The initial pressure and concentrations of stabilizer have substantial effects on the size of the colloidal particles.


Journal Articles on this Report : 15 Displayed | Download in RIS Format

Other project views: All 21 publications 15 publications in selected types All 15 journal articles

Type Citation Project Document Sources
Journal Article Buhler E, Dobrynin AV, DeSimone JM, Rubenstein M. Light scattering study of diblock copolymers in supercritical carbon dioxide CO2 density-induced micellization transition. Macromolecules 1998;31(21):7347-7355. R826115 (1999)
R826115 (Final)
not available
Journal Article Canelas DA, DeSimone JM. Dispersion polymerizations of styrene in carbon dioxide stabilized with poly(styrene-b-dimethylsiloxane). Macromolecules 1997;30:5673-5682. R826115 (1999)
R826115 (Final)
not available
Journal Article Canelas DA, Betts DE, DeSimone JM. Preparation of poly(vinyl acetate) latexes in liquid and supercritical carbon dioxide. Polymer Preparation (American Chemical Society, Division of Polymer Preparation) 1997;38:628-629. R826115 (1999)
R826115 (Final)
not available
Journal Article Canelas DA, Betts DE, DeSimone JM, Yates MZ, Johnston KP. Poly(vinyl acetate) and poly(vinyl acetate-co-ethylene) latexes via dispersion polymerizations in carbon dioxide. Macromolecules 1998;31(20):6794-6805. R826115 (1999)
R826115 (Final)
not available
Journal Article Carson T, Lizotte J, DeSimone JM. Dispersion polymerization of 1-vinyl-2-pyrrolidone in supercritical carbon dioxide. Macromolecules 2000, Volume: 33, Number: 6 (MAR 21), Page: 1917-1920. R826115 (1999)
R826115 (Final)
not available
Journal Article Clark MR, DeSimone JM. Cationic dispersion polymerizations in liquid carbon dioxide. Macromolecules 1997;30:6011-6014. R826115 (1999)
R826115 (Final)
not available
Journal Article Harrison KL, da Raocha SRP, Yates MZ, Johnston KP, Canelas DA, DeSimone JM. Interfacial activity of polymeric surfactants at the polystyrene-carbon dioxide interface. Langmuir 1998;14:6855-6863. R826115 (1999)
R826115 (Final)
not available
Journal Article Hsiao YL, DeSimone JM. Dispersion polymerization of methyl methacrylate in supercritical carbon dioxide: Influence of helium concentration on particle size and particle size distribution. Journal of Polymer Science Part A-Polymer Chemistry 1997;35(10):2009-2013
abstract available  
R826115 (1999)
R826115 (Final)
not available
Journal Article Shiho H, DeSimone JM. Dispersion polymerization of acrylonitrile in supercritical carbon dioxide. Macromolecules 1999;33(5):1565-1569. R826115 (1999)
R826115 (Final)
not available
Journal Article Shiho H, DeSimone JM. Dispersion polymerization of 2-hydroxyethyl methacrylate in supercritical carbon dioxide. Journal of Polymer Science Part A - Polymer Chemistry 2000;38:3783-3790.
abstract available  
R826115 (2000)
R826115 (Final)
not available
Journal Article Shiho H, DeSimone JM. Dispersion polymerization of styrene in supercritical carbon dioxide utilizing random copolymers including fluorinated acrylate for preparing micron-size polystyrene particles. Journal of Polymer Science Part A - Polymer Chemistry 2000;38:1146-1153.
abstract available  
R826115 (Final)
not available
Journal Article Triolo A, Triolo F, Lo Celso F, Betts DE, McClain JB, DeSimone JM, Wignall GD, Triolo R. Critical micellization density: A small-angle-scattering structural study of the monomer-aggregate transition of block copolymers in supercritical CO2. Physical Review e 2000;62(4):5839-5842
abstract available  
R826115 (2000)
R826115 (Final)
not available
Journal Article Triolo F, Triolo A, Triolo R, Londono JD, Wignall GD, McClain JB, Betts DE, Wells S, Samulski ET, DeSimone JM. Critical micelle density for the self-assembly of block copolymer surfactants in supercritical carbon dioxide. Langmuir 2000;16(2):416-421
abstract available  
R826115 (2000)
R826115 (Final)
not available
Journal Article Triolo R, Arrighi V, Triolo A, Migliardo P, Magazu S, McClain JB, Betts D, DeSimone JM, Middendorf HD. QENS from polymer aggregates in supercritical CO2. Physical Review B 2000;276-278, 386-387. R826115 (2000)
R826115 (Final)
not available
Journal Article Triolo R, Triolo A, Triolo F, Steytler DC, Lewis CA, Heenan RK, Wignall GD, DeSimone JM. Structure of diblock copolymers in supercritical carbon dioxide and critical micellization pressure. Physical Review E 2000;61(4):4640-4643. R826115 (2000)
R826115 (Final)
not available
Supplemental Keywords:

carbon dioxide, VOC, CFCs, alternatives, innovative technology, waste reduction, waste minimization, environmentally conscious manufacturing., RFA, Scientific Discipline, Sustainable Industry/Business, cleaner production/pollution prevention, Environmental Chemistry, Sustainable Environment, Technology for Sustainable Environment, Economics and Business, Chemistry and Materials Science, cleaner production, non-ionic surfactants, environmentally benign solvents, dispersion polymerization, emission controls, green process systems, carbon dioxide, plastic, chlorofluorocarbons, pollution prevention, source reduction, Volatile Organic Compounds (VOCs), polymer design, green chemistry, solvents, polymeric coatings

Relevant Websites:

http://www.unc.edu/depts/chemistry/faculty/desimone/resact.htm Exit
http://www.nsfstc.unc.edu Exit
http://www2.ncsu.edu:80/champagne/ Exit

Progress and Final Reports:
Original Abstract
1999 Progress Report
2000 Progress Report

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The perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.

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