Preparation, Characterisation and Applications of Poly(2-vinylpyridine) Latex/Microgel

• Main Author: Dupin, Damien
• Published: University of Sheffield 2008
• Subjects: 547.7
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.489657

Emulsion polymerisation of 2-vinylpyridine (2VP) in the presence of divinylbenzene (DVB) cross-linker, a cationic surfactant and monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) macromonomer, at around neutral pH and 60°C afforded near-monodisperse, sterically-stabilised latexes at approximately 10 % solids. Judicious selection of the synthesis parameters enabled the mean latex diameter to be varied from 370 to 970 nm depending on the initiator, polymeric stabiliser and surfactant concentrations. These lightly cross-linked latexes acquired cationic microgel character at low pH, as expected. The critical pH for this latex-to-microgel transition was around pH 4.1 at 1.0 wt. % DVB. The kinetics of swelling of these near-monodisperse, lightly cross-linked poly(2vinylpyridine) [PEGMA-P2VP] latexes was investigated by the pH jump method using a commercial stopped-flow instrument. The characteristic swelling time (tens of milliseconds) correlates linearly with the mean particle diameter, as predicted by the Tanaka equation. However, faster swelling was observed in the presence of added salt. This salt effect was not expected from Tanaka theory. Kinetics of deswelling for the PEGMA-P2VP microgel-to-Iatex transition was also investigated. Slower deswelling time scales of the order of hundreds of seconds were observed due to retarded salt excretion from the microgel. A near-monodisperse styrene-functionalised poly[2-(dimethylamino)ethyl methacrylate] [PDMA] macromonomer was evaluated as a reactive steric stabiliser for the preparation of P2VP latexes via emulsion polymerisation. The solution pH was shown to be a critical parameter for successful syntheses: stable latexes with minimal coagulum were only obtained at (or above) neutral pH. Combined DLS and electrophoretic data indicated that the PDMA-P2VP particles existed in three states depending on the solution pH: swollen microgels were obtained at low pH, non-solvated latex particles with a cationic stabiliser layer were obtained at intermediate pH and flocculated latex particles with neutral PDMA stabiliser chains were obtained at around pH 8.5. A PDMA-P2VP latex of 280 nm was shown to be an efficient pH-responsive Pickering emulsifier for stabilising water-in-1undecanol emulsions. Three near-monodisperse heavily cross-linked PEGMA-P2VP latexes of 380 nm, 640 nm and 820 nm diameter were prepared in tum by emulsion copolymerisation of 2VP and DVB (5.0 wt.%) using PEGMA macromonomer. Each of these latexes proved to be an effective particulate stabiliser for the production of long-lived foams generated by either hand-shaking or using foam columns. Dried foams prepared using the 380 nm latex exhibited interesting optical effects when viewed in reflectance mode. These PEGMAP2VP latex foams can be readily destabilised at low pH due to the latex-to-microgel transition. Finally, solution viscosities of PEGMA-P2VP microgels prepared with lower levels of DVB cross-linker were measured at low pH. Free-standing gels were obtained for aqueous solutions of PEGMA-P2VP particles prepared with 0.16 wt.% of DVB crosslinker at 1.0 w/v % and pH 3.4. However, no gels could be formed in the presence of added salt, even at salt concentrations as low as 0.01 M NaC!. Copolymerisation of 2VP with either methyl acrylate or ethyl acrylate did not alleviate this salt sensitivity.