Dynamically Hydrogen-Bonded Microphase Separation Enabling Phase Transition in the Gel Composites With Tunable UCST

Thermal-responsive hydrogels have broad applications, yet it is challenging to tune their phase transition temperature over a wide range, especially for UCST-type of hydrogels. Here, we design a polyacrylamide (PAm)-tannic acid (TA) gel composite, which exhibits UCST-type of phase transition with tunable UCST in a wide range. The physical origin of the phase transition, including the evolution of microstructures and dynamics of the gels, has been explored by Very Small Angle Neutron Scattering (VSANS), dynamic light scattering and pulsed-field-gradient NMR, etc. The microphase separation, driven by the reversible association-dissociation of hydrogen bonding between TA and PAm gel strands, governs the UCST-type of phase transition. Dynamical studies show that at higher temperature the collective diffusion coefficient of TA increases fivefold, along with the acceleration of dissociation-association hierarchical relaxations of all the hydrogen bonds in gels due to the weakening of TA-PAm hydrogen bonding. A programmable information encryption/ decryption gel device was designed by tuning UCST. The interplay between microstructure changes during phase transition and macroscopic viscoelastic properties was also investigated. This work provides new physical strategies to design smart gel materials with tunable phase transition temperature. It also has broad applications in optical gel devices and environmental temperature sensors.