Monodisperse Thermoresponsive Microgels with Tunable Volume-Phase Transition Kinetics

Stimuli-sensitive hydrogel microspheres or microgels are polymeric particles that consist of cross-linked three-dimensional networks. They shrink or swell significantly by expelling or absorbing large amounts of water in response to external stimuli, such as changes in temperature, pH, electric or magnetic fields. The chemical composition of the microgel determines the stimulus that can trigger this volume-phase transition. The dramatic response and stimuli-specific behavior makes these materials extremely valuable for numerous applications, including drug delivery, chemical separations, sensors, catalysis, enzyme immobilization, and color-tunable crystals. Several potential applications of these microgels, such as ‘smart’ actuators, on–off switches, and pulse-release, require a short response time. Currently, numerous strategies are employed to speed up the response kinetics of typical thermo-responsive hydrogel microspheres such as poly(N-isopropylacrylamide) (PNIPAM) microgels. One simple strategy is to use small sized microgel particles. Since the characteristic time of gel swelling is proportional to the square of the linear dimension of the hydrogels, this results in a significant decrease in the response time. However, for certain applications, like actuators in a tube of fixed diameter, the size of the microgel cannot be used as a control variable. Another well-known method is to chemically graft linear side chains of the same stimuli-sensitive polymer onto the cross-linked polymer network. The linear side chains, with one free end each, respond to stimulus faster than the cross-linked network, and this leads to a shorter response time for the entire hydrogel as compared with a hydrogel with ungrafted polymer networks. However, the process of chemically grafting linear side chains onto a polymer network is complicated, which limits the widespread use of this technique. Another alternative is to fabricate hydrogels with heterogeneous internal microstructures instead of a homogeneous net-like microstructure. The microgel-like particle clusters with numerous free ends inside the heterogeneous hydrogels can flex without any restrictions, which results in a faster response to stimuli as compared to microgels with a homogeneous microstructure. However, the heterogeneity of the microstructure may also cause some unwanted side effects; for example, the temperature-dependent equilibrium volume-deswelling ratio of PNIPAM hydrogels with a heterogeneous microstructure is much smaller than that of PNIPAM hydrogels with a homogeneous microstructure. In this study, we report a new method to control the volumephase transition kinetics of thermo-sensitive PNIPAM microgels. Our method does not involve any chemical manipulation of PNIPAM networks, nor does it require any change in the size of the microgel particles. The response rate is controlled simply by changing the size and number of spherical voids in-