Non-destructive and Quantitative Viscoelastic-Mapping of Cellulose Nanofibrils Using Low-Total-Force Contact Resonance Force Microscopy (LTF-CRFM)1

Low-total-force contact resonance force microscopy (LTF-CRFM), an atomic force microscopy method, is introduced as a non-destructive means to quantify the local viscoelastic loss tangent (δ) of cellulose nanofibrils (CNFs). The method limits static and dynamic forces during measurement to minimize substrate and geometry effects and to reduce the potential for stress-induced CNF damage. LTF-CRFM uses Brownian motion to achieve the thermally-limited lowest dynamic force, while approaching adhesive pull-off to achieve the low static force. LTF-CRFM measurements were shown to generate analyzable data without evidence of nonlinear artifacts and without damage to the CNF over static forces ranging from 11.6 nN to 84.6 nN. The measured δ of CNFs was 0.015 ± 0.0094, which is the first reported δ measurement of an isolated CNF. Finally, LTF-CRFM successfully mapped δ along the length of CNFs to determine that kink defects along the CNF do not impart a local viscoelastic property change at the length scale of the measurement.