Folding and unfolding of individual PNIPAM-g-PEO copolymer chains in dilute aqueous solutions

Linear poly(N-isopropylacrylamide) chains grafted with different contents of short poly(ethylene oxide) branching chains (PNIPAM-g-PEO) were prepared by free radical copolymerization of NIPAM and PEO macromonomers (M-w = 5000 g/mol) end-capped with methacrylate in water at 45 ° C higher than the lower critical solution temperature (∼ 32 ° C) of PNIPAM homopolymer. As expected, the copolymerization led to a heterogeneous distribution of PEO on the chain backbone because PNIPAM becomes hydrophobic and fully collapsed at 45 ° C. Both the folding and unfolding of individual PNIPAM-g-PEO chains in extremely dilute aqueous solutions were investigated by a combination of static and dynamic laser light scattering (LLS). Our results showed that each copolymer chain could indeed fold into a globule without any interchain association when the solution was heated from 25 to 50 ° C, resulting in a core-shell nanostructure with a collapsed PNIPAM core and a swollen PEO shell. The folding of the copolymer chains with a higher PEO content involves two transitions; namely, one sharp transition at ∼ 33 ° C and another broader one around ∼ 45 ° C. Such two transitions also occurred in the cooling (unfolding) process. The second transition gradually disappears when we lower the PEO content. A combination of LLS and micro-differential scanning calorimetric results suggests that the first transition is related to the collapse of the PNIPAM segment between two grafted PEO chains. The second one at higher temperatures is related to the steric repulsion-induced stretching and the n-clustering-induced collapse of the hydrophilic PEO chains on the periphery of the collapsed PNIPAM core.