Silk Graft Copolymerization Under The Controlling Over The Amphiphilic Structure And Reactivity Of Free Radical In Aqueous Phase

After grafting copolymerization with water-soluble monomer, silk fiber not onlyretains its original admirable features (i.e., luster, hygroscopicity.), but also improvesits crease-resistant properties. Nevertheless, the unexpected homo-polymerizationoccurring in the aqueous medium is accompanied with grafting copolymerization,causing the decrease of grafting efficiency (Ge) and hard handling. There are quite alot of studies showing that electrostatic attraction between fibers and free radicalsenhances the formation of silk radicals, and thus leads to an increase in Ge. Beinginspired by these results, it is subsequently questioned that whether othercharacteristics of radicals could be employed to increase the Ge. In this paper, threetypes of macromolecular RAFT agents with designed R groups were synthesized.Their chain transfer reactions with aqueous radicals released R group radicals withdefined structures as well as reactivities. Then, they were engaged to mediate silkgraft copolymerizations with DMAAm (N,N-Dimethylacrylamide). The effects of thecomposition and structure of these agents on the graft copolymerization kinetics andGe were studied. Finally, the properties, for example dying properties, mechanicalstrength and surface morphologies, of the grafted silk fibers were evaluated.Three types of macromolecular RAFT agents which released different R groupradicals, namely, polyacrylic acid-b-polystyrene radical (PAA-b-PSt~●), polyacrylicacid-b-polybutyl acrylate radical (PAA-b-PBA~●) and polystyrene-b-polyacrylic acidradical (PSt-b-PAA~●), were chosen as the transfer agents. During the graftcopolymerizations, the newly born PAA-b-PSt~●in the aqueous phase enhanced theformation of silk macroradicals, consequently accelerated the copolymerization rateand improved monomer grafting efficiency (Ge) greatly. The Ge was less improvedwhen the RAFT agent capable of producing PAA-b-PBA~●were substituted for the one producing PAA-b-PSt~●. In addition, Ge also benefited from an increase in the chainlength of PSt block in PAA-b-PSt~●, as well as PBA block in PAA-b-PBA~●. However,the addition of RAFT agent giving rise to PSt-b-PAA~●had negligible effects on Ge.The reasons were ascribed to both the radical reactivity and capacity of radicaladsorption onto silk fibers.In compared to the properties of grafted silk fibers without using RAFT agents,RAFT reactions imposed little effects on moisture regain, breaking strength,elongation and infrared spectrum of grafted fibers using RAFT agents. However,RAFT agent controlled graft copolymerization produced silk fibers with higherwhiteness, more even distribution of grafted polymer chains and lower K/S values forboth acid and reactive dyes. A further comparion with the original silk fibers revealedthat K/S values of the RAFT agent mediated fibers decreased obviously for reactivedye. Moreover, K/S values for acid dye, the whiteness and breakingforce of fibersdecreased little, but the hygroscopicity improved remarkably.