| Autor: |
Warren NJ; Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K.; School of Chemical and Process Engineering, University of Leeds, Leeds LS2 9JT, U.K., Derry MJ; Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K., Mykhaylyk OO; Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K., Lovett JR; Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K., Ratcliffe LPD; Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K., Ladmiral V; Ingénierie et Architectures Macromoléculaires, CNRS, UM, ENSCM, Institut Charles Gerhardt UMR 5253, Place Eugène Bataillon, Cedex 5 34095 Montpellier, France., Blanazs A; BASF SE, GMV/P-B001, 67056 Ludwigshafen, Germany., Fielding LA; School of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, U.K., Armes SP; Department of Chemistry, University of Sheffield, Brook Hill, Sheffield S3 7HF, U.K. |
| Abstrakt: |
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 G 37 -H 80 , G 54 -H 140 , and G 71 -H 200 . 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 G 37 -H 80 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 G 54 -H 140 and G 71 -H 200 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 G 54 -H 140 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 G 71 -H 200 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 G 71 -H 200 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.
Competing Interests: The authors declare no competing financial interest. |
