Fast, Local, and In-situ Cure Rheology of Photo-processable Polymers using Atomic Force Microscopy

Callie Higgins, NIST

Abstract: Photo-polymerizable and photo-degradable materials are the focus of extensive research across a diverse variety of fields ranging from additive manufacturing to regenerative medicine. However, a lack of thorough understanding of material mechanical and rheological properties during polymerization/degradation at the relevant exposure intensities and length-scales limits advancements in part performance, voxel-resolution, and throughput. Voxel-relevant in-situ characterization of these photopolymerizable materials is not currently possible due to the volume and sampling-rate constraints of traditional, bulk techniques (e.g., oscillatory rheometry, optical spectroscopy). Atomic Force Microscopy (AFM) affords the requisite spatiotemporal resolution to observe the dynamic kinetics inherent to these materials, but has yet to be adapted to suit the specific needs of in-situ photo-processing, such as using an independent light source to initiate reactions. We demonstrate the use of novel atomic force microscopy techniques to locally characterize the mechanical and rheological properties of photo-processable materials on the relevant reaction kinetic time-scales for materials ranging from hydrated, soft (10s kPa) hydrogels to stiff (10s GPa) additive manufacturing materials. By modifying a photothermal AFM excitation source (405 nm) to operate independently from the AFM scanning controls, we locally polymerized/degraded polymer at different transverse locations from the cantilever tip (0, 1, 5, 10 µm from tip) and with varied illumination power (1- 12 mW). Concurrently, the AFM was operated in various continuous stiffness sensing modalities (i.e., fast force spectroscopy and contact resonance force microscopy) to detect the voxel-scale (