Rapid Actuation of Thermo-Responsive Polymer Networks:Investigation of the Transition Kinetics

The swelling and collapsing of thermo-responsivepoly(N-isopropylacrylamide)-based polymer (pNIPAAm) networksare investigated in order to reveal the dependency on their kineticsand maximum possible actuation speed. The pNIPAAm-based networkwas attached as thin hydrogelfilm to lithographically prepared goldnanoparticle arrays to exploit their localized surface plasmon resonance(LSPR) for rapid local heating. The same substrate also served forLSPR-based monitoring of the reversible collapsing and swelling of thepNIPAAm network through its pronounced refractive index changes.The obtained data reveal signatures of multiple phases during the volume transition, which are driven by the diffusion of watermolecules into and out of the network structure and by polymer chain re-arrangement. For the micrometer-thick hydrogelfilm in theswollen state, the layer can respond as fast as several milliseconds depending on the strength of the heating optical pulse and on thetuning of the ambient temperature with respect to the lower critical solution temperature of the polymer. Distinct differences in thetime constants of swelling and collapse are observed and attributed to the dependence of the cooperative diffusion coefficient ofpolymer chains on polymer volume fraction. The reported results may provide guidelines for novel miniature actuator designs andmicromachines that take advantages of the non-reciprocal temperature-induced volume transitions in thermo-responsive hydrogelmaterials