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Cyclic Graft Copolymer Unimolecular Micelles: Effects of Cyclization on Particle Morphology and Thermoresponsive Behavior.

ABSTRACT: The synthesis of cyclic amphiphilic graft copolymers with a hydrophobic polycarbonate backbone and hydrophilic poly(N-acryloylmorpholine) (PNAM) side arms via a combination of ring-opening polymerization (ROP), cyclization via copper-catalyzed azide-alkyne cycloaddition (CuAAC), and reversible addition-fragmentation chain transfer (RAFT) polymerization is reported. The ability of these cyclic graft copolymers to form unimolecular micelles in water is explored using a combination of light scattering, small-angle X-ray scattering (SAXS), and cryogenic transmission electron microscopy (cryoTEM) analyses, where particle size was found to increase with increasing PNAM arm length. Further analysis revealed differences in the solution conformations, loading capabilities, and morphologies of the cyclic graft copolymers in comparison to equivalent linear graft copolymer unimolecular micelle analogues. Furthermore, the cyclic and linear graft copolymers were found to exhibit significantly different cloud point temperatures. This study highlights how subtle changes in polymer architecture (linear graft copolymer versus cyclic graft copolymer) can dramatically influence a polymer's nanostructure and its properties.

SUBMITTER: Williams RJ

PROVIDER: S-EPMC4861350 | biostudies-literature | 2016 Apr

REPOSITORIES: biostudies-literature

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Cyclic Graft Copolymer Unimolecular Micelles: Effects of Cyclization on Particle Morphology and Thermoresponsive Behavior.

Williams Rebecca J RJ Pitto-Barry Anaïs A Kirby Nigel N Dove Andrew P AP O'Reilly Rachel K RK

Macromolecules 20160317 7

The synthesis of cyclic amphiphilic graft copolymers with a hydrophobic polycarbonate backbone and hydrophilic poly(<i>N</i>-acryloylmorpholine) (PNAM) side arms via a combination of ring-opening polymerization (ROP), cyclization via copper-catalyzed azide-alkyne cycloaddition (CuAAC), and reversible addition-fragmentation chain transfer (RAFT) polymerization is reported. The ability of these cyclic graft copolymers to form unimolecular micelles in water is explored using a combination of light ...[more]

PMID: 27175037

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Project description:Reversible addition-fragmentation chain transfer (RAFT) aqueous dispersion polymerization of 2-hydroxypropyl methacrylate was used to prepare three poly(glycerol monomethacrylate) x -poly(2-hydroxypropyl methacrylate) y (denoted G x -H y or PGMA-PHPMA) diblock copolymers, namely G37-H80, G54-H140, and G71-H200. A master phase diagram was used to select each copolymer composition to ensure that a pure worm phase was obtained in each case, as confirmed by transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS) studies. The latter technique indicated a mean worm cross-sectional diameter (or worm width) ranging from 11 to 20 nm as the mean degree of polymerization (DP) of the hydrophobic PHPMA block was increased from 80 to 200. These copolymer worms form soft hydrogels at 20 °C that undergo degelation on cooling. This thermoresponsive behavior was examined using variable temperature DLS, oscillatory rheology, and SAXS. A 10% w/w G37-H80 worm dispersion dissociated to afford an aqueous solution of molecularly dissolved copolymer chains at 2 °C; on returning to ambient temperature, these chains aggregated to form first spheres and then worms, with the original gel strength being recovered. In contrast, the G54-H140 and G71-H200 worms each only formed spheres on cooling to 2 °C, with thermoreversible (de)gelation being observed in the former case. The sphere-to-worm transition for G54-H140 was monitored by variable temperature SAXS: these experiments indicated the gradual formation of longer worms at higher temperature, with a concomitant reduction in the number of spheres, suggesting worm growth via multiple 1D sphere-sphere fusion events. DLS studies indicated that a 0.1% w/w aqueous dispersion of G71-H200 worms underwent an irreversible worm-to-sphere transition on cooling to 2 °C. Furthermore, irreversible degelation over the time scale of the experiment was also observed during rheological studies of a 10% w/w G71-H200 worm dispersion. Shear-induced polarized light imaging (SIPLI) studies revealed qualitatively different thermoreversible behavior for these three copolymer worm dispersions, although worm alignment was observed at a shear rate of 10 s-1 in each case. Subsequently conducting this technique at a lower shear rate of 1 s-1 combined with ultra small-angle x-ray scattering (USAXS) also indicated that worm branching occurred at a certain critical temperature since an upturn in viscosity, distortion in the birefringence, and a characteristic feature in the USAXS pattern were observed. Finally, SIPLI studies indicated that the characteristic relaxation times required for loss of worm alignment after cessation of shear depended markedly on the copolymer molecular weight.

Dataset Information

Cyclic Graft Copolymer Unimolecular Micelles: Effects of Cyclization on Particle Morphology and Thermoresponsive Behavior.

Publications

Cyclic Graft Copolymer Unimolecular Micelles: Effects of Cyclization on Particle Morphology and Thermoresponsive Behavior.

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