Controlling Stiffness and Glass Transition Temperature in Azobenzene-Containing Liquid Crystalline Polymer Actuators through Multi-Wavelength Light Exposure

Shape-shifting polymers that respond to body heat are of critical importance for a variety of biomedical applications. Despite this, there is a dearth of materials that possess such smart characteristics. Prior research has incorporated thermal responsiveness into glassy liquid crystalline polymers (LCPs), recognized for their high glass-transition temperatures and rigidity, through the application of the photo-softening effect of azobenzene. Yet, the means of controlling this effect across varying material compositions and photo-programming conditions is not fully understood. This study seeks to bridge this knowledge gap by examining the thermo-mechanical behavior of azo-LCPs under dual-wavelength light exposure. Our results show that when smectic phase azo-LCPs are subjected to a combination of 530 nm and 365 nm light, the modulus decreases in a linear fashion as the ratio of the 365 nm wavelength increases. However, the glass transition temperature ($\mathrm{T}_{\mathrm{g}}$) remains stable at a value dictated by the composition and the liquid crystal phase. We discovered that the isomerization of azobenzene in the Smectic phase effectively reduces $\mathrm{T}_{\mathrm{g}}$, an essential factor for body-temperature actuation performance. Interestingly, we also observed that the nematic phase fails to deliver substantial $\mathrm{T}_{\mathrm{g}}$ variation, even with an increased chromophore concentration. To delve further, we experimentally explored the body temperature actuation capability and analyzed it using a light-coupled viscoelastic Bernoulli beam model. This model has provided key insights into the micro-mechanical mechanisms that drive body temperature actuation behavior.