聚合过程中原位生成助稳定剂的细乳液聚合

以甲基丙烯酸十二氟庚酯(DFMA)、甲基丙烯酸甲酯(MMA)、丙烯酸丁酯(BA)为共聚单体,不加任何传统的助稳定剂进行细乳液聚合.在共聚体系中,由于DFMA在初期反应生成的聚合物中占有较高比例,初期形成的少量低聚物可以起到助稳定剂作用,因此DFMA作为反应单体的同时,又可以原位生成助稳定剂,维持单体液滴或乳胶粒子的稳定,以细乳液聚合的方式进行聚合.分别采用油溶性引发剂(AIBN)和水溶性引发剂(KPS)引发聚合,考察细乳液聚合过程中乳胶粒子粒径的变化规律,粒径由初始时刻的400nm左右减少到80nm左右,最终与使用传统的助稳定剂得到的粒径相当.提出了原位生成助稳定剂的细乳液聚合机理,并使用交联剂验证了提出的原位生成助稳定剂的细乳液聚合机理.

MINIEMULSION POLYMERIZATION WITH in situ FORMED COSTABILIZER

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.But after a while,the monomer droplets coalescenced together and made the monomer droplet size changing from about 130 nm to 400 nm.The oligomers of poly(dodecafluoroheptyl methacrylate),containing a lot of —CF_2— and —CF_3 segments with low surface energy,were generated quickly and it was proved from ~1H-NMR spectrum which showed DFMA has a high composition in the earlier copolymer.The oligomers not only with 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 was 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.On the base of fundamental theory of miniemulsion polymerization,the particle sizes of the latexes with the convensional costabilizer would not change too much.So,the reason is the oligomers,first generated at the process of polymerization,contain a lot of —CF_2— and —CF_3 segments with low surface energy to make the smaller particles existence stably.There is no differences about the particle size distributions between the final products 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 products of this polymerization would exhibit bimodal distributions,if some crosslinking agents were added into the systems to reduce the activities of oligomer chains.One of the peaks was at about 80 nm,equaling to that one without any crosslinking agent,the other one was at about 400 nm,equaling to that one before polymerization.With the same contents of crosslinking agents,the effect on particle size distribution of the final products was much more initiated by AIBN.Because the polymerization was taken place in the center of the particles initiated by AIBN to generate the oligomers,the crosslinking agents would reduce the movement activity of the oligomer and make the oligomers shift to the surfaces of the particle more difficult.However,the effect on particle size distribution of the final products was less initiated by KPS.Because the polymerization initiated by KPS was taken place at the inside surface of the particle,as well as the generated oligomers,the movement of the oligomer was not necessary.On the base of the experimental results,the mechanism of miniemulsion polymerization with in situ formed costabilizer was presented.