Rapid viscoelastic switching of an ambient temperature range photo-responsive azobenzene side chain liquid crystal polymer

We report the rheological properties of a new azobenzene side chain liquid crystal polymer (SCLCP) with photo-responsive behavior in the range of 0 °C to 50 °C as probed by small amplitude oscillatory shear (SAOS) rheometry with in situ UV irradiation. In the linear viscoelastic (LVE) strain regime, temperature sweeps measured G′ and G″ from 80 °C to −5 °C and identified glass transition temperatures (Tg) at 2.2 °C and 0.3 °C for UV-off and UV-on, respectively. Also in the LVE strain regime, frequency sweeps identified G′ and G″ cross-over frequencies at 5 °C and 0 °C. Most importantly, rapid shear modulus changes of between 7 and 35% were demonstrated at 50 °C, 25 °C, 5 °C, and 0 °C by successively turning the UV light on and off during time sweeps in the LVE strain regime. The elastic (G′) and viscous (G″) shear moduli reversibly decreased by a maximum of 35% and 21%, respectively, with time constants (τ) from 6 s to 18 s as fit by a stretched exponential model. This significant, rapid, and reversible UV-triggered modulus switching at temperatures as low as 0 °C and up to 50 °C is noteworthy because it represents a bulk property change at temperatures that are in the ambient range for several geographic regions and has reasonable time constants across the range. [Display omitted]