Formation of mesoglobular phase of PNIPAM-g-PEO copolymer with a high PEO content in dilute solutions

Free radical copolymerization of N-isopropylacrylamide (NIPAM) and poly(ethylene oxide) (PEO) macromonomer (M-w = 5000) end-capped with methacrylate in water at 45 degrees C, higher than PNIPAM's lower critical solution temperature (similar to 32 degrees C), resulted in (PNIPAM-g-PEO) copolymer with 48 wt % of PEO. As expected, the grafting of PEO on the PNIPAM chain backbone was heterogeneous because PEO is hydrophilic and stayed on the periphery of the PNIPAM segments collapsed at the reaction temperature. The copolymer is soluble in water at lower temperatures. When the solution temperature is raised to similar to 34 degrees C, a limited number of the copolymer chains can collapse and associate to form stable narrowly distributed mesoglobules with a collapsed PNIPAM core and a swollen PEO shell, different from either individual single-chain globules or macroscopic precipitation. The formation of such mesoglobules was studied by a combination of static and dynamic laser light scattering. We found that the mesoglobular formation has two transition stages, occurring at similar to 34 and similar to 47 degrees C. The transition at similar to 34 degrees C is related to the contraction of the PNIPAM segment between two grafted PEO chains. The high-temperature transition is attributed to the stretch and collapse of the grafted PEO chains on the periphery. The fast heating generally resulted in smaller mesoglobules, which is due to a competition between intrachain contraction and interchain association as well as the viscoelastic effect.