Fluorinated Acrylate/Acrylates (mini)Emulsion Copolymerization

Acrylate copolymers with fluorinated side chains possess many interesting properties, such as low refractive index,good tribological properties,low surface tensions and the related non-wettability.Making a fluorinated dispersion by classical emulsion polymerization is difficult.This technique relies on monomer transport from the monomer droplets to the growing latexes,and the solubility of fluorinated monomer in water is usually extraordinarily low.This is why classical industrial recipes using fluorinated acrylate monomers add some amounts of an organic solvent to the continuous phase,for instance acetone.The polymerization in miniemulsion does not rely on monomer transport through the water phase,but droplet nucleation of the minidroplets is the predominant initiation mechanism.The problem of polymerization and copolymerization of fluorinated monomers reduces to the question of the preparation of stable miniemulsions,whereas solubility questions have been avoided.Dodecafluoroheptyl methacrylate(DFMA),n-butyl acrylate(BA)and methyl methacrylate(MMA)were copolymerized for preparation of fluorinated latexes through miniemulsion copolymerization without any costabilizer.In this reaction system,extremely hydrophobic DFMA was used as a chemical reagent which made the monomer droplets unimodal distribution.The oligomers of polymer,containing a lot of-CF₂- and -CF₃ segments with low surface energy,was generated quickly and it was proved from ¹H-NMR spectrum which showed DFMA has a high composion in the earlier copolymer.The oligomers with not only extremely hydrophobic but also oleophobic natural capacity would take the same effect of conventional costabilizer on the miniemulsion polymerization to increase the surface of monomer droplets and decrease the monomer droplets size.The monomer droplet or particle size distribution of the semifinished and the final products were determined using LS-230 Coulter particle size analyzer.As the process of polymerization was going on,the smaller monomer droplets or particles would detach from the larger ones.Simultaneously,the particles did not maintain unimodal distribution after the beginning of polymerization.However,as the process of polymerization was going on,the particle size distribution would return to the unimodal distribution again. Between the processes of polymerization,the particle sizes changed from about 400 nm to 80 nm.The reason is the oligomers,firstly generated at the process of polymerization, contains a lot of-CF₂- and -CF₃ segments with low surface energy to make the smaller particles existence stably.There is no differences about the particle size distributions between the final produces initiated by potassium persulfate(KPS,water-soluble initiator) and 2,2'-azobisisobutyronitrile(AIBN,oil-soluble initiator).There were too much differences between the two kinds of initiator,the final produces of this polymerization would exhibit bimodal distributions,if some crosslinking agents were added into the systems to reduce the activities of oligomer chains.On the base of the experimental results, the mechanism of miniemulsion polymerization with in-situ formed costabilizer was presented.On the different initiated systems,there is a large differences on surface energy of polymer which are initiated by AIBN and KPS.Under the same contents of DFMA, different locus of polymerization initiated by various initiator led to different particle formation and molecular chain structure.The polymerization kinetics were studied in this system.There are something difference with classical miniemulsion polymerization.The stageâ…£is not exist which is the manifestations about gel effect in the reaction.That's because the latex particle separation makes the gel effect decrease.And then,the relationships between influencing factors(reaction temperature,concentrations of emulsifier and initiator)and reaction rate of the end of the stageâ… were obtained.The change of molecular weight of polymer with time and conversion were obtained by GPC.The change of polymer chains with time was obtained from the definition of number molecular weight and monomer conversion.The effects of polymerization temperature,initiator concentration,emulsifier concentration on the molecular weight of copolymer were also investigated.And then,through the change contents of BA,the polymers with different T_g were got.The stability emulsions are synthesized by this miniemulsion polymerization.Though change of the species and dosage of the emulsifier and initiator,the effect of the stability of emulsion,conversion and the particle structure were studied.The test result indicate:the latex particle synthesized with the different initiator and emulsifier have much difference between them,and the well distributed core-shell latex particle can received through miniemulsion polymerization.Not only improve the traditional technique of prepared fluoroacrylates emulsion,but also make the produce which were synthesized through new technique much more stabilization when it was reacted and stored.