Thermoresponsive hydrogels from ABC triblock terpolymers

Abstract

Two-compartment hydrogels, which are three-dimensional networks with two distinguishable hydrophobic domains, have been prepared from aqueous self-assembly of poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEP-b-PEO-b-PNIPAm, PON) triblock terpolymers. The PON terpolymers were synthesized using a combination of anionic and reversible addition-fragmentation chain transfer (RAFT) polymerization. They self-assembled into well-defined micelles with hydrophobic PEP cores surrounded by hydrophilic PEO-PNIPAm coronae at low temperatures and these micelles associated to form larger aggregated structures upon heating above the lower critical solution temperature (LCST) of PNIPAm in dilute aqueous solutions (0.5 and 0.05 wt%). At higher polymer concentrations (1-5 wt%), micellar aggregation manifests itself as gelation on heating due to the non-covalent association of PNIPAm blocks. The separation of micellization and gelation leads to the formation of a two-compartment network with a very high fraction of bridging conformations for the PEO midblocks. Therefore, gelation can be achieved at a much lower concentration, with a much higher modulus at a given polymer concentration and a much sharper sol-gel transition, as compared to poly(N-isopropylacrylamide)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (NON) copolymer hydrogels, in which both looping and bridging conformations are possible. The formation of a micellar network with two discrete PEP and PNIPAm hydrophobic domains in PON hydrogels is verified by cryogenic scanning electron microscopy (cryo-SEM) and cryogenic transmission electron microscopy (cryo-TEM) experiments and is further confirmed by small-angle neutron scattering (SANS) measurements of two PON triblocks with a normal PNIPAm and a deuterated PNIPAm block. This study confirms the assumption that the formation of two-compartment networks in PON terpolymer hydrogels results in better gelation properties compared with NON copolymer hydrogels. In addition to temperature, it is desirable to have other stimuli such as pH to control the polymer self-assembly. Therefore, poly(ethylene-alt-propylene)-b-poly(ethylene oxide)-b-poly(N-isopropylacrylamide-co-acrylic acid) (PO(N/A)) triblock terpolymers in which the PNIPAm block contains a small fraction of AA monomers were prepared to achieve the dual pH- and temperature-sensitive micellar aggregation and gelation in aqueous solutions.Finally, the self-assembly of PON triblock terpolymers in the ionic liquid 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide ([EMI][TFSA]) shows well-defined sol-gel transitions upon cooling with a lower gelation concentration and a higher modulus when compared with NON copolymers, which further confirms that ABC triblock terpolymers can be beneficial for gel formation in comparison to ABA triblock copolymers. Overall, we demonstrated that the rational design of two immiscible, hydrophobic endblocks in ABC triblocks is crucial for the preparation of compartmentized hydrogels with improved gelation properties. These studies will help guide the design and development of new systems with enhanced performance.