Photocontrol of pattern formation and hysteresis loops in polymer gels with host-guest interactions

Pattern formation and reconstruction under external stresses plays an important role in defining functionality of a broad range of soft confined systems. We focus on dynamic control of pattern formation in hydrogels with host-guest interactions. We extend the three-dimensional gel Lattice Spring Model (gLSM) to capture the dynamics of the PNIPAAm hydrogels with pendant azobenzene moieties immersed into alpha-cyclodextrins (alpha-CD) solutions. While trans-azobenzene moieties are recognized and accommodated by the alpha-CD cavities to form inclusion complexes, an exposure to UV light drives the trans-to-cis photoisomerization and dissociation of the complexes. First we demonstrate that the proposed model reproduces volume phase transitions in azobenzene-functionalized hydrogels with host-guest interactions observed in prior experiments. We then use spaciotemporal variations in UV irradiation to control pattern formation in thin hydrogel films under the rigid and soft confinements and to control hysteresis loops. The soft confinement is introduced via illuminating the ridges of the thin samples and thereby introducing localized regions with higher rigidity. We show that controllable feedback mechanisms can be introduced into the networks with host-guest interactions via well-defined illumination patterns.