Tracking solvent-induced conformational collapse of periodically grafted amphiphilic polymers using PFG NMR diffusometry

Abstract

Periodically grafted amphiphilic polymers (PGAPs) belong to an interesting class of amphiphilic polymers that carry long hydrophobic alkylene segments along the backbone and periodically located hydrophilic pendant segments, such as polyethylene glycol monomethyl ether (MPEG). In earlier studies, we showed that such PGAPs undergo zigzag folding to segregate the backbone alkylene segments and the pendant MPEG segments; consequently, they organize in the solid state to yield a lamellar morphology with precisely tunable domain sizes. In the present study, two new periodically clickable polyesters were prepared; one bearing dodecyl (C12) segments between adjacent clickable propargyl units and the other carrying longer docosyl (C20) segments. These were clicked with MPEG-azides of different molecular weights, namely 550, 750 and 2000, to generate a series of PGAPs with varying hydrophilic-lyophilic balance (hlb) and, consequently, different folding propensities. The collapse of these PGAPs chains in solution was examined by pulse field gradient nuclear magnetic resonance (PFG NMR) diffusometry studies, in chloroform-methanol mixtures of varying composition; the diffusion constants, estimated from these measurements, underwent a sudden increase at a specific methanol content, which signified the chain-collapse transition. As expected, the collapse occurred at a lower methanol content in the polymers bearing smaller MPEG pendant segments. Additionally, the variation of the relative intensities of the proton peaks belonging to the central hydrophobic segment, that upon collapse becomes more solid-like, also exhibited a sudden drop at roughly the same solvent composition, reconfirming that the collapse occurs to generate a structure with a dense core and solvated shell.