(538e) Pinch-Off Dynamics and Printability of Semi-Flexible and Rigid Rod Polymer Solutions

Understanding and controlling drop formation and liquid transfer is crucial for designing printable photovoltaic and electronic devices using semi-flexible and rigid rod polymer solutions. Applications involving jetting, coating, spraying, atomization and various printing techniques (ink-jet, gravure printing, roll-to-roll) all involve formation of liquid necks that undergo capillary-driven thinning and pinch-off. Stream-wise velocity-gradients associated with extensional flows arise in thinning liquid necks, and as the response of polymer solutions to extensional flows influences both their processability, as well as final orientation and morphology. Characterizing and controlling the extensional viscosity that quantifies the resistance to stream-wise gradients and extensional relaxation time of polymer solutions that determines the pinch-off time are of critical importance for selecting and optimizing semi-flexible and rigid rod polymer solutions used in functional inks. However, due to the low viscosity and low elasticity of most printable inks, the quantitative analysis of pinch-off dynamics and extensional rheology response is beyond the measurement rage of commercially extensional rheometer called Capillary Break-up Extensional Rheometer (CaBER). In this contribution, we employ the recently developed dripping-onto-substrate (DoS) rheometry protocols for visualizing and analyzing capillary-driven thinning and pinch-off dynamics of a columnar neck formed between a nozzle and a sessile drop for typical functional inks including aqueous PSS-PEDOT solutions, aqueous NaPSS solutions and polythiophene solutions in organic solvents. We elucidate the influence of flexibility and extensibility on pinch-off dynamics and processability, emphasizing how printability can be quite distinct even if different polymer solutions exhibit a dynamically similar response in shear rheology characterization.