Physical Aging, Composition, and Carbon-Additive Identity as Separable Rheological Controls in Colloidal-Cellulose Nanofibril–Natural-Rubber-Latex Hydrogels

Hydrogels prepared from TEMPO-oxidized cellulose nanofibrils (TCNFs), natural rubber latex (NRL), and carbon additives change their rheological properties during storage, so formulation comparisons made at uncontrolled aging states can be misleading. Here, TCNF–NRL hydrogels containing graphene conductive additive (GCA), graphene nanoplatelets (GNP), multi-walled carbon nanotubes (MWCNT), carboxylated multi-walled carbon nanotubes (MWCNT-COOH), or graphite were examined through four experimental sets that separate the effects of physical aging, total solids content, TCNF/carbon-additive ratio, carbon-additive loading, and additive identity. Oscillatory and steady-shear measurements were described with Cross and generalized Maxwell fits, and oscillatory-steady comparisons were quantified with a Cox–Merz deviation metric. Aging increased the storage modulus and the Cross low-shear viscosity by factors of 1.3 to 1.9 over about two weeks while the spectra remained elastic-dominated. At the late-storage comparison state, composition changed mainly the magnitude of the moduli and of the flow resistance: rankings by storage modulus and by low-shear viscosity agreed with Spearman rank coefficients of 0.89 to 1.00 across the four sets. At fixed base composition, the carbon additive changed the low-shear viscosity over a seven-fold range, from graphite and GCA at the low end to MWCNT at the high end, and the Cox–Merz deviation, expressed as the mean ratio of complex to steady-shear viscosity, ranged from 3.7 (GNP) to 7.3 (MWCNT). The late-storage states used for these comparisons are operational rather than equilibrated, and their ages differ between formulations, from 11 to 35 days, so the additive comparison was repeated at matched ages and the grouping was confirmed. No electrical or microstructural measurements were performed, so additive effects are reported as rheological observations, not as evidence of conductive-network formation.