Study On CO₂-Triggered Reversibly Coagulatable And Redispersible Latexes

Stability is one of the key issues in the study of polymeric latexes. It is great challenging to produce latexes which can be reversibly coagulated and redispersed. There is no such latex existed so far. The latex stability strongly relies on surfactants. The latexes prepared by generic surfactants can not be re-stablized after coagulation. Although a few switchable surfactants have been reported lately, none of them can be applied for emulsion polymerization. Research and development in this area is believed to have important scientific and industrial impact on the progress of emulsion polymerization technology, energy saving and environmental protection.In this study, CO2/N2was chosen to trigger the reversible coagulation and redispersion of latexes synthesized with switchable surfactants. The switchable surfactants were readily bonded on the surface of latex particles, thus making the latex reversibly coagulatable and redispersible. Herein, a CO2-responsive switchable surfactant of (N-amidino)ethyl undec-10-enoate (UEm), a switchable monomer of2-methyl-1-(4-vinylbenzyl)-1,4,5,6-tetrahydropyrimidin (SCm), a switchable reactive surfactant of (N-amidino)dodecyl acrylamide (DAm), and a polymeric surfactant of poly(2-(dimethylamino)ethyl methacrylate)10-b-poly(methyl methacrylate)14(PDM) were synthesized. Emulsion (co)polymerizations of styrene (St) or methyl methacrylate (MMA) with these switchable surfactants were studied. The resulted latexes, which could be reversibly coagulated by trace amount of caustic soda or bubbling N2with heat, and redispersed by CO2bubbling with ultrasonication, were first developed in this study.The following were covered in this thesis:(1) Seven amidine compounds were synthesized. They were N-dodecyl amidine (C12Am), UEm,(N-amidino)ethyl undec-10-enamide (UAm),(N-amidino)hexyl methacrylate (M6Em),(N-amidino)hexyl methacrylamide (M6Am), N,N-dimethyl ethyl amidine (DMEm), and1,2-dimethyl-1,4,5,6-tetrahydropyrimidine (DM6Cm). Conductivity tests showed there was no CO2/N2switchability with UAm and M6Am bearing amide groups due to formation of hydrogen bond with amidine, while other compounds were CO2/N2-responsive with DM6Cm having most significant switchability. The cyclic amidine DM6Cm and2-methyl-2-imidazoline (MeIMD), as well as UAm and M6Am bearing amide groups were more stable without hydrolysis under experimental conditions. Hydrolysis occurred with the acyclic amidines C12Am, UEm, and DMEm.(2) Stable PSt latexes were produced by emulsion polymerization of St using UEm as a switchable surfactant. The surfactant had a good repeatable CO2/N2switchablity. The PSt latexes could be readily coagulated by bubbling N2at45℃, and the coagulated latex particles were redispersible by CO2bubbling with ultrasonication. The washed and dried latex particles could also be reversibly coagulated and re-dispersed.(3) Soap-free emulsion copolymerizations of St with SCm as a switchable comonomer were conducted. Stable PSt, St/butyl acrylate, St/MMA, and St/acrylonitrile latexes were obtained. The comonomer SCm possessed a good repeatable CO2/N2switchablity and outstanding stabilities against heat and hydrolysis. The resulted latexes could be coagulated by trace amount of caustic soda, and the coagulated latex particles were redispersible by CO2bubbling with ultrasonication, as well as the washed and dried latex particles.(4) By using DAm as a switchable reactive surfactant, stable PSt latexes were produced during emulsion polymerization of St. The surfactant DAm was found to have a good repeatable CO2/N2switchablity. The resulted PSt latexes could be coagulated by bubbling N2with heat, and the coagulated latex particles were redispersible by CO2bubbling with ultrasonication. The coagulation and redispersion processes were repeatable by the CO2/N2bubbling. Moreover, the PSt latexes possessed good stability against electrolyte.(5) Emulsion (co)polymerizations of MMA were carried out with an acidified switchable polymeric surfactant PDM. The surfactant was synthesized by RAFT solution polymerization. The PDM had a number-average molecule weight of2360g/mol and PDI of1.10. The resulted PMMA latexes could be coagulated by trace amount of caustic soda, and the coagulated latex particles after washing with deionized water could be redispersed into fresh water to form stable latexes again by CO2bubbling with ultrasonication. The recovered latexes could then be coagulated by N2bubbling with gentle heating. These coagulation and redispersion processes could be repeated with alternatively CO2/N2bubbling.