Volatile dripping onto substrate (vDoS) extensional rheology of viscoelastic polymer solutions

Extensional flow properties of volatile polymer solutions govern many industrially-relevant coating processes, but existing instrumentation lacks the environmental control necessary to control evaporation. To mitigate evaporation during volatile dripping-onto-substrate (vDoS) extensional rheology measurements, we developed a chamber to enclose the sample in an environment saturated with solvent vapor. We validated the vDoS device by measuring a model high molecular weight polyethy-lene oxide (PEO) in various organic solvents both inside and outside of the chamber. Evaporation substantially increased the extensional relaxation time λ E for PEO in volatile solvents like dichloromethane (DCM) and chloroform. PEO/chloroform solutions displayed an over 20-fold increase in λ E due to the formation of an evaporation-induced surface film; evaporation studies confirmed surface features and skin formation reminiscent of buckling instabilities commonly observed during drying in polymer solutions. Finally, the relaxation times of semi-dilute PEO/chloroform solutions were measured with environmental control, where λ E scaled with concentration by the exponent m=0.62. These measurements validate the vDoS device by confirming that chloroform is a good solvent for PEO, with a Flory exponent of ν=0.54. Our results demonstrate the first environmental control of evaporation during DoS extensional rheology, and provide guidelines establishing when environmental control is necessary to obtain accurate rheological parameters.