Small-Angle Neutron Scattering Study of Temperature-Induced Emulsion Gelation: the Role of Sticky Microgel Particles
In this work, small-angle neutron scattering (SANS) is used to probe the structural transformations that accompany temperature-induced gelation of emulsions stabilized by a temperature-responsive polymer. The latter is poly(NIPAM-co-PEGMa) (N-isopropylacrylamide and poly(ethyleneglycol) methacrylate...
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Published in | Langmuir Vol. 21; no. 15; pp. 6734 - 6741 |
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Main Authors | , |
Format | Journal Article |
Language | English |
Published |
Washington, DC
American Chemical Society
19.07.2005
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Subjects | |
Online Access | Get full text |
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Summary: | In this work, small-angle neutron scattering (SANS) is used to probe the structural transformations that accompany temperature-induced gelation of emulsions stabilized by a temperature-responsive polymer. The latter is poly(NIPAM-co-PEGMa) (N-isopropylacrylamide and poly(ethyleneglycol) methacrylate) and contains 86 mol% NIPAM. Turbidity measurements revealed that poly(NIPAM-co-PEGMa) has a lower critical solution temperature (T LCST) of 36.5 °C in D2O. Aqueous polymer solutions were used to prepare perfluorodecalin-in-water emulsions (average droplet size of 6.9 μm). These emulsions formed gels at 50 °C. SANS measurements were performed on the poly(NIPAM-co-PEGMa) solutions and emulsions as a function of temperature. The emulsion was also prepared using a D2O/H2O mixture containing 72 vol% D2O in order to make scattering from the droplets negligible (on-contrast). The SANS data were analyzed using a combination of Porod and Ornstein−Zernike form factors. The results showed that the correlation length (ξ) of the polymer scaled as ξ ∼ φp -0.68 at 32 °C, where φp is the polymer volume fraction. The ξ value increased for all systems as the temperature increased, which was attributed to a spinodal transition. At temperatures greater than T LCST, the polymer solution changed to a polymer dispersion of poly(NIPAM-co-PEGMa) aggregates. The aggregates have features that are similar to microgel particles. The average size of these particles was estimated as 160−170 nm. The particles are “sticky” and are gel-forming. The on-contrast experiments performed using the emulsion indicated that the interfacial polymer chains condensed to give a relatively thick polymer layer at the perfluorodecalin−water interface at 50 °C. The gelled emulsions appear to consist of perfluorodecalin droplets with an encapsulating layer of collapsed polymer to which sticky microgel particles are adsorbed. The latter act as a “glue” between coated droplets in the emulsion gel. |
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Bibliography: | ark:/67375/TPS-VXZ8ZHN5-7 istex:4D47C38021AE1480A30BE102E41899F57EFF2DF1 ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
ISSN: | 0743-7463 1520-5827 |
DOI: | 10.1021/la050600h |