In situ Photo-rheology Monitors Viscoelastic Changes in Photo-responsive Polymer Networks

We engineer polymers to advance materials for human health and planetary health. We investigate how molecular design influences material properties such as mechanical strength, stimuli-responsive behavior, and recyclability.

In polymer engineering, mechanical tests are used to understand material strength and durability. Integrating mechanical tests with chemical or optical measurements allows us to study molecular features underlying these properties.

We hope to reduce the barrier to entry for researchers who want to use photo-rheology. We describe how we designed our instrument and how others can design their own system.

Institute photo-rheology measures a material's mechanical response during photostimulation. This approach allows us to track materials as they are formed or as they change in response to light.

Our studies shed light on the design of photoresponsive polymer networks, which invite new questions about tuning their mechanical properties. For example, how might we formulate recyclable networks?

[Instructor] To begin, install the upper Peltier plate and geometry on the rheometer. Next, install the lower optics plate to enable uniform irradiation of samples during photo-rheology measurements. Use an optics plate that is transparent to the wavelengths of interest to allow light transmission through the plate. Wipe the lower optics plate clean and dry. Then center the light source under the transparent plate and secure it using posts, holders, or other appropriate supports. Now place a photodiode power sensor connected to an optical power meter on top of the transparent plate with the sensor facing downward toward the light source. If necessary, take measures to prevent exposure to light. Then turn on the light and measure the light intensity using the power meter. If required, adjust the light intensity using the light source driver. Mark the positions of the light source support to allow reproducible placement for future experiments. Estimate the required sample volume using the formula for the volume of a cylinder and dispense slightly more to avoid underfilling. For a low-viscosity liquid sample, such as 100 milligrams per milliliter of PEG-anthracene in water, pipette 75 microliters onto the center of the optics plate. Next, lower the rheometer head until the geometry contacts the sample, then pause to avoid forming bubbles. View the sample through the optics plate to check for bubble formation. If bubbles are visible, raise the geometry to break the connection between the plates, then gently rotate and lower the geometry again to reestablish contact. Then gradually lower the geometry until reaching the final experimental gap height. Now gently rotate the geometry to homogenize the liquid sample. To prevent evaporation over short timescales, soak lint-free wipes in water and place them near the sample to create a high-humidity environment around the sample. For longer measurements, seal the sample with an admissible layer, such as mineral oil, to isolate it from the ambient environment. Close the upper Peltier plate jacket to protect the sample from ambient light and temperature changes. Then add additional shielding, like UV-blocking sheets, as needed to protect from harmful stray irradiation. To determine an appropriate strain for dynamic experiments, perform preliminary strain-amplitude sweeps on the sample to identify the linear viscoelastic region. Sweep the strain from 1% to 1000% at a constant frequency of 10 radians per second before and after UVA-induced network formation. Use a 10% strain for this sample to ensure linear viscoelasticity both before and after network formation. Set the sample temperature to 22 degrees Celsius. Preshear the sample for 10 seconds, then allow it to equilibrate for another 60 seconds. Perform a frequency sweep before irradiation, ranging from 100 radians per second to 0.1 radians per second. Use the sweep results to confirm the initial physical state of the sample. Set an oscillation time sweep to span the irradiation process, including buffer periods before and after light exposure. Take measurements for 60 seconds prior to UVA irradiation. Continue for one hour during irradiation and measure for 10 seconds after irradiation. Use 10 radians per second frequency and 10% strain amplitude throughout the sweep. Confirm that the sample temperature is stable during irradiation despite the energy input. Finally, perform a second frequency sweep after irradiation using the same frequency range as before to compare post-irradiation viscoelastic properties with the initial measurements. Before irradiation, the amplitude sweep revealed a strain-independent region across the full range of 1% to 1000%, indicating a broad linear viscoelastic region. After one hour of irradiation, the linear viscoelastic region extended only to 100% strain, beyond which a yielding event occurred. Before irradiation, the frequency sweep showed a slight frequency dependence consistent with a viscoelastic liquid. After one hour of irradiation, the moduli showed minimal dependence on frequency, with elastic modulus exceeding viscous moduli, confirming the material's solid-like behavior. Increasing the polymer concentration from 20 to 100 milligrams per milliliter led to faster crosslinking, as indicated by an earlier crossover between the elastic and viscous moduli. This change also resulted in stiffer networks, reflected by higher plateau values of the elastic modulus.