Thermo-responsive Diblock Copolymer Worm Gels in Non-polar Solvents

• Published in: Journal of the American Chemical Society Vol. 136; no. 15; pp. 5790 - 5798
• Main Authors: Fielding, Lee A, Lane, Jacob A, Derry, Matthew J, Mykhaylyk, Oleksandr O, Armes, Steven P
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 16.04.2014
• Subjects: Chain transfer; Copolymers; Dispersions; Gels; Heating; Polymerization; Self assembly; Solvents; Worms
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja501756h

Benzyl methacrylate (BzMA) is polymerized using a poly­(lauryl methacrylate) macromolecular chain transfer agent (PLMA macro-CTA) using reversible addition–fragmentation chain transfer (RAFT) polymerization at 70 °C in n-dodecane. This choice of solvent leads to an efficient dispersion polymerization, with polymerization-induced self-assembly (PISA) occurring via the growing PBzMA block to produce a range of PLMA–PBzMA diblock copolymer nano-objects, including spheres, worms, and vesicles. In the present study, particular attention is paid to the worm phase, which forms soft free-standing gels at 20 °C due to multiple inter-worm contacts. Such worm gels exhibit thermo-responsive behavior: heating above 50 °C causes degelation due to the onset of a worm-to-sphere transition. Degelation occurs because isotropic spheres interact with each other much less efficiently than the highly anisotropic worms. This worm-to-sphere thermal transition is essentially irreversible on heating a dilute solution (0.10% w/w) but is more or less reversible on heating a more concentrated dispersion (20% w/w). The relatively low volatility of n-dodecane facilitates variable-temperature rheological studies, which are consistent with eventual reconstitution of the worm phase on cooling to 20 °C. Variable-temperature 1H NMR studies conducted in d 26-dodecane confirm partial solvation of the PBzMA block at elevated temperature: surface plasticization of the worm cores is invoked to account for the observed change in morphology, because this is sufficient to increase the copolymer curvature and hence induce a worm-to-sphere transition. Small-angle X-ray scattering and TEM are used to investigate the structural changes that occur during the worm-to-sphere-to-worm thermal cycle; experiments conducted at 1.0 and 5.0% w/w demonstrate the concentration-dependent (ir)­reversibility of these morphological transitions.