23rd International Colloquium Tribology - January 2022 65 Reversible Viscosity Tuning using UV-Light Dr. Dominic Linsler Fraunhofer IWM, Mikrotribologie Centrum µTC, Karlsruhe, Germany Fraunhofer Cluster Programmable Materials CPM Corresponding author: dominic.linsler@iwm.fraunhofer.de Chris Gäbert Fraunhofer IAP, Potsdam, Germany Fraunhofer Cluster Programmable Materials CPM Stefan Reinicke Fraunhofer IAP, Potsdam, Germany Fraunhofer Cluster Programmable Materials CPM Theodora Rangova Fraunhofer IWM, Mikrotribologie Centrum µTC, Karlsruhe, Germany Fraunhofer Cluster Programmable Materials CPM Florian Schlüter Fraunhofer IWM, Mikrotribologie Centrum µTC, Karlsruhe, Germany Fraunhofer Cluster Programmable Materials CPM Martin Dienwiebel Fraunhofer IWM, Mikrotribologie Centrum µTC, Karlsruhe, Germany Karlsruhe Institute of Technology KIT, IAM, Karlsruhe, Germany Fraunhofer Cluster Programmable Materials CPM 1. Introduction Viscosity is an intrinsic property of a lubricant that can be changed by various measures. But in a final product, it follows a given temperature dependency. In many cases, the choice of a lubricant or the scope of application is limited by the viscosity. Conversely, a reversibly tunable viscosity allows the expansion of the parameter field of technical applications. Various systems with switchable viscosity are known, besides “smart liquids” with polarizable particles, microgels and responsive micelles, dissolved responsive polymers are described to enable bond formation controlled by an external trigger, thus facilitating programmability. Here, we use anthracene esters as photoresponsive group that allow a reversible chain extension and thus viscosity increase by UV irradiation. Among the stimuli for responsive materials, UV light sources have many advantages: They are non-invasive and commercially available at relatively low cost. Furthermore, parameters like intensity, wavelength, duration and area of exposure allow precise spatiotemporal control over the programming of a material. 2. Materials and Methods We present results of functionalized polydimethylsiloxane (PDMS) and polyalkyleneglycole (PAG) and mixtures of a. PDMS with a non-functionalized silicone oil and b. PAG with water. The polymer chains of both lubricants were functionalized with anthracene moieties at their termini that show an equilibrium reaction between the cross-linked and separated state. The chemical equilibrium can be shifted by the irradiation of UV-light with wavelength of 365 and 254 nm, respectively, see figure 1. Viscosity measurements were performed in a rheometer with a parallel plate setup using a custom made fused silica top plate for UV transparency, see figure 2. 66 23rd International Colloquium Tribology - January 2022 Reversible Viscosity Tuning using UV-Light Figure 1: reversible anthracene photodimerization [1]. Figure 2: setup in the rheometer with the transparant plate and functionalized lubricant under UV irradiation. 3. Results and discussion The parameters for characterization of the tunable viscosity are the absolute range of modulus that changes under light irradiation and the speed of the chain growth. The dilution of functionalized polymers with a liquid of lower molecular weight yields to a faster viscosity change due to a higher mobility of the functionalized chains accompanied by a decrease of the range of possible modulus variation, due to the change of the chain length distribution. This is exemplary shown for PAG diluted with water in different ratios from 2: 3 to 1: 4, see figure 3. All mixtures show an exponential increase of viscosity until the chemical reaction levels off after 14 and approx. 40 minutes, respectively, in the case of higher dilutions. The change in viscosity by light irradiation also significantly modifies the stribeck curve. During several experiments with duration of 5000s in mixed lubrication, no degradation of the lubricant due to the shear in a tribological contact was found, indicating adequate shear stability of the anthracene dimer. Figure 3: Loss modulus over time for various mixtures of PAG with water. Irradiation time with 365nm UV-light of 90 minutes is indicated by the dashed lines. Note the logarithmic scale. 4. Conclusion The analysis of the response of two anthracene-functionalized lubricants to UV-irradiation shows significant viscosity changes by more than two decades considering the loss modulus. The dilution of the functionalized oils yields the expected decrease of the absolute range of viscosity variation and increases the speed of the polymer chain growth. Sufficient shear stability makes the functionalized lubricants interesting for future applications of adaptive viscosity. This work has been funded by the Fraunhofer-Gesellschaft through the Cluster of Excellence “Programmable Materials CPM” References [1] van Damme, Jonas und Du Prez, Filip. Anthracene-containing polymers toward high-end applications. Progress in Polymer Science, 82: 92-119, 2018.