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Time-resolved small-angle neutron scattering study on soap-free emulsion polymerization

Motokawa, Ryuhei   ; Koizumi, Satoshi; Hashimoto, Takeji; Nakahira, Takayuki*; Annaka, Masahiko*

A soap-free emulsion polymerization method was employed in an aqueous medium in order to synthesize an amphiphilic block copolymer of poly(N-isopropylacrylamide)-block-poly(ethylene glycol) (NE), where Poly(N-isopropylacrylamide) (PNIPA) was polymerized from the radically activated chain ends of poly(ethylene glycol) (PEG). As the polymerization proceeds, PNIPA block chains form micelle cores stabilized by PEG brush chains emanating therefrom. When this polymerization was employed at temperatures equal to or higher than 34$$^{circ}$$C, radical polymerization proceeds living-likely. To elucidate origins of the living nature, we carried out time-resolved ultra-small- and small-angle neutron scattering (USANS and SANS). USANS and SANS revealed that (1) from 15 to 60$$^{circ}$$C, polymerized NE forms micelle in the solution when molecular weight reaches to a critical molecular weight for micelle formation, and (2) as the temperature increases from 34$$^{circ}$$C, water and monomer content in the micelle core becomes less due to a lower critical solution temperature of PNIPA. These results indicate that low diffusivity of chain end radicals, compartmentalized in the solid-like micelle core, plays an important role to lead a quasi-living polymerization behavior.

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Category:Physics, Condensed Matter

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