Despax et al. report the synthesis of triblock copolymers and their application as thermoresponsive hydrogels.
Temperature responsive gelators can benefit a wide range of biomedical applications and typically comprised of triblock copolymers with a central hydrophilic block and terminal blocks that undergo a hydrophilic to hydrophobic transition at a specific temperature. However, typical ABA triblock copolymers obtained from commercially available monomers require concentrations of at least 50-100 g L-1.
Harrisson, Destarac and co-workers have managed to circumvent this by synthesizing high molecular weight triblock copolymers via low temperature reversible addition-fragmentation chain-transfer (RAFT) gel polymerization. The targeted triblock copolymers were based on polydimethylacrylamide (PDMA) as the long central hydrophilic block and poly(N-isopropylacrylamide) (PNIPAM) as the shorter terminal blocks and the gel formation was initially demonstrated via vial-inversion tests.
Two different molecular weight triblock copolymers were tested with the PDMA block varying from 58 kg mol-1 to 421 kg mol-1 showing self-supporting gels at 30 g L-1 and 6 g L-1 concentration respectively, which is already a significant improvement over previously reported materials. As the vial-inversion test is subject to experimental variations, a more objective measure of the effect of the temperature was obtained from the evolution of the storage and loss moduli of aqueous polymer solutions.
For the lower molecular weight polymer, a two-step transition consisting of an initial thickening of the solution at the lower critical solution temperature (LCST) of PNIPAM occurred followed by gel formation at 38–39 °C requiring a minimum concentration of 20 g L-1. For the longer polymer, only the second transition was observed; gel formation occurred at 40-45 °C with a minimum concentration of 4 g L-1. With a storage modulus of only 0.1 Pa however, this gel is likely too soft for practical use.
In an attempt to improve the mechanical properties of the gels, 2-acrylamido-2-methylpropanesulfonic acid was also incorporated (20 mol% of DMA) resulting on the formation of self-supporting gels at 2 g L-1, an order of magnitude improvement over previously-reported ABA copolymers. These results approach the performance obtained from exotic polymers such as polyisocyanopeptides.
Tips/comments directly from the authors:
- High monomer concentrations are helpful to obtain high molecular weights. However, the polymerization of acrylamides is very exothermic so it is important not to exceed 30 wt%.
- As very low initiator concentrations are used, it is important to thoroughly degas all solutions prior to polymerization.
- Take care to exclude any air bubbles from the solution when carrying out rheology measurements.
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Low concentration thermoresponsive hydrogels from readily accessible triblock copolymers
L. Despax, J. Fitremann, M. Destarac and S. Harrisson
Polym. Chem., 2016, 7, 3375-3377
Dr. Athina Anastasaki is a web writer for Polymer Chemistry. She is currently an Elings fellow working alongside Professor Craig Hawker at the University of California, Santa Barbara (UCSB).