Neutron scattering characterization of homopolymers and graft-copolymer micelles in supercritical carbon dioxide
| Autor: | J. D. Londono, G.D. Wignall, Dorian A. Canelas, James B. Mcclain, J. R. Combes, Edward T. Samulski, Roberto Triolo, Joseph M. DeSimone, H.D. Cochran, D. Chillura-Martino, Douglas E. Betts |
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| Rok vydání: | 1996 |
| Předmět: |
chemistry.chemical_classification
Organic Chemistry Hexafluoropropylene oxide Polymer Neutron scattering Micelle Small-angle neutron scattering Supercritical fluid Analytical Chemistry Inorganic Chemistry chemistry.chemical_compound Polymerization chemistry Chemical engineering Polymer chemistry Copolymer Spectroscopy |
| Zdroj: | Journal of Molecular Structure. 383:3-10 |
| ISSN: | 0022-2860 |
| Popis: | 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 . |
| Databáze: | OpenAIRE |
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