This article presents the results of capillary break-up extensional rheometer experiments conducted for semidilute solutions of carboxymethylcellulose sodium salt (Na-CMC) with degrees of substitution (DS) ranging from 0.62 to 1.04 in distilled water and propylene glycol (PG)/water mixtures. The partial aggregation of Na-CMC chains with DS<1 observed in aqueous solutions triggers an increase in apparent extensional viscosity and extension of break-up time. The rheological properties of Na-CMC solutions in propylene glycol/water mixture are determined by the solubility of the polymer and the physical crosslinking of chains. The disappearance of the elasto-capillary regime during the filament thinning of Na-CMC solutions with DS<1 in propylene glycol/water mixture was linked to the physical crosslinking of polymer chains. The shape of the extensional viscosity curve for Na-CMC solutions with DS = 1.04 in PG/water mixture was characteristic for semidilute polymer solutions with a low number of entanglements. (c) 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019