Surface modification of calcium carbonate: radical graft polymerization of vinyl monomers onto calcium carbonate surface initiated by azo groups introduced onto the surface

—The preparation of calcium carbonate modified by 12-hydroxystearate groups and the grafting of polymers onto the surface by the polymerization of vinyl monomers initiated by azo groups introduced onto the surface were investigated. The preparation of calcium carbonate modified by 12-hydroxystearate was achieved by the reaction of calcium chloride with sodium carbonate containing a small amount of sodium 12-hydroxystearate. The introduction of azo groups onto calcium carbonate was successfully achieved by the direct condensation of the carboxyl group of 4,4'-azobis(4-cyanopentanoic acid) with 12-hydroxystearate groups on the modified calcium carbonate using N,N'-dicyclohexylcarbodiimide as a condensing agent. It was found that the radical polymerization of vinyl monomers, such as methyl methacrylate (MMA), styrene, and N-vinylcarbazole (NVC), was initiated by azo groups introduced onto the surface, and the corresponding polymers were grafted onto the surface based on the propagation of polymer from the surface: the percentage of grafting of polyMMA, polystyrene, and polyNVC reached 5.7, 9.5 and 3.5%, respectively, at 70°C. The percentage of grafting was found to decrease with decreasing monomer concentration. The wettability of calcium carbonate surface was found to turn from hydrophilic to hydrophobic by the grafting of polymers.     

Radical graft polymerization initiated by azo groups introduced onto the surface of carbon whisker

The radical graft polymerization of vinyl monomers onto carbon whisker, i.e. vapor grown carbon fiber, initiated by azo groups introduced onto the surface has been investigated. The introduction of azo groups onto the surface is achieved by (1) the reaction of 4,4'-azobis(4-cyanopentanoic acid) with isocyanate groups on the surface, which are introduced by the treatment of surface carboxyl and phenolic hydroxyl groups with tolylene-2,4-diisocyanate and (2) the reaction of 2,2'-azobis(2-cyano-n-propanol) with acyl chloride groups on the surface, which are introduced by the treatment of surface phenolic hydroxyl groups with terephthaloyl dichloride. The carbon whiskers having azo groups prepared from the above two methods are abbreviated as CW-Azo 1 and 2, respectively. The azo group content of CW-Azo 1 and 2 is determined to be 0.04 and 0.07 mmol/g, respectively. It is found that the radical polymerization of methyl methacrylate and styrene is initiated by the surface azo groups to give the corresponding polymer-grafted carbon whisker. The polymer-grafted carbon whisker gives a stable colloidal dispersion in good solvents for grafted polymer.     

Radical graft polymerization from carbon whisker initiated by peroxyester groups introduced onto the surface

The radical graft polymerization of vinyl monomers onto carbon whiskers, i.e. vapor grown carbon fibers, initiated by tert-butyl peroxyester groups that are introduced onto the surface was investigated. The introduction of tert-butyl peroxyester groups onto the carbon whisker surface was achieved by the reaction of acyl chloride groups on the surface with tert-butyl hydroperoxide. The carbon whisker having acyl chloride groups was prepared by the following three methods: (1) reaction of surface carboxyl groups with thionyl chloride, (2) reaction of surface phenolic hydroxyl groups with succinyl dichloride, and (3) reaction of surface phenolic hydroxyl groups with phthaloyl dichloride. The carbon whiskers having tert-butyl peroxyester groups prepared from these three methods were abbreviated as CW-POE 1, 2, and 3, respectively. The peroxyester group content of CW-POE 1, 2, and 3 was determined to be 0.06, 0.05, and 0.17 mmol/g, respectively. It was found that the radical polymerization of vinyl monomers such as methyl methacrylate, styrene, and N-vinylcarbazole was successfully initiated in the presence of CW-POE 2 and 3, and the corresponding polymers were grafted onto the surface. However, CW-POE 1 failed to initiate the radical graft polymerization, because surface phenyl radicals formed by the thermal decomposition of the tert-butyl peroxyester groups are stabilized by the aromatic rings of the carbon whisker surface.     

Grafting of polymers onto graphene oxide by cationic and anionic polymerization initiated by the surface-initiating groups

The grafting of polymers onto graphene oxide (GO) was achieved by two process: (1) cationic polymerization initiated by carboxyl (COOH) groups on GO and (2) anionic alternating copolymerization of epoxides with cyclic acid anhydrides initiated by potassium carboxylate (COOK) groups on GO. The cationic polymerizations of isobutyl vinyl ether and N-vinylcarbazole were successfully initiated by COOH groups on GO to give the corresponding polymer-grafted GO. The cationic polymerization was considered to be initiated by proton addition from COOH groups to monomer and propagation of polymer cation proceeds with carboxylate anion as a counter ion. It was found that the corresponding polymer was successfully grafted onto GO based on the termination reaction of growing polymer cation and surface counter carboxylate anion. On the other hand, the anionic ring-opening alternating copolymerization of epoxide and cyclic acid anhydrides were also initiated by COOK groups on GO, which were previously introduced onto GO by the neutralization of COOH groups with KOH. During the anionic ring-opening copolymerization of styrene oxide (SO) with phthalic anhydride (PAn) and maleic anhydride (MAn), the corresponding polyesters, poly(SO-alt-PAn) and poly(SO-alt-MAn), were successfully grafted onto GO, based on the propagation of the polyesters from COOK groups. The grafting of polymers onto GO during the above cationic and anionic polymerizations was confirmed by thermal decomposition gas chromatogram/mass spectrum. The untreated GO in THF was immediately precipitated within 15 min. On the contrary, these polymer-grafted GOs gave stable dispersions in THF and no precipitation of polymer-grafted GOs was observed even after one week.