The structural and rheological properties of aqueous suspensions of spray-dried cellulose nanocrystals (CNCs) were investigated and compared to those of freeze-dried. The cellulose nanocrystals were obtained from sulfuric acid hydrolysis of wood pulp. Ultrasonication was used to disperse cellulose nanocrystals in Milli-Q water and the power applied during ultrasonication was shown to be the controlling parameter for their dispersion, more than total energy. Dynamic light scattering measurements showed a decrease of the average hydrodynamic diameter down to the same limiting value, i.e. ∼75 nm, for both spray and freeze-dried cellulose nanocrystals. Since the same maximum dispersion state was reached for both CNC types, it indicated that the spray drying process did not limit dispersion, provided that sufficient ultrasonication was provided. Moreover, no desulfation occurred during ultrasonication at ambient temperature. Strong ultrasonication also caused a decrease of intrinsic viscosity, along with an increase in maximum packing concentration. These properties were correlated to agglomerates break-up, which released both ions and water in suspension. The ionic strength increase may lead to a thinner electrostatic double layer surrounding the cellulose nanocrystals, reducing their apparent concentration.

The rheological behavior of cellulose nanocrystals (CNCs) in polar media based on polyethylene glycol (PEG) was investigated from aqueous suspensions to nanocomposites. The aim of this work is to improve our knowledge on the CNC behavior in polymer media and develop rheological indices to characterize the dispersion of nanoparticles in polymer matrices. CNCs were obtained from sulfuric acid hydrolysis of wood pulp and supplied after a spray-or freeze-drying process. Ultrasonication was used to break agglomerates and disperse CNCs in aqueous suspensions before mixing with an aqueous PEG solution at room temperature. The samples were subsequently dried and compression molded. From capillary and oscillatory shear rheology, no adsorption of PEG chains on CNCs could be detected, as many had previously hypothesized. The increase of PEG concentration in aqueous suspension favored the gelation by depletion effect and suggested CNC orientation. Viscoelastic properties and transmission electronic images of PEG/CNC nanocomposites highlighted the formation of a percolated network of CNCs for low concentrations  0.15 vol%. From Shih et al.'s model, a fractal dimension of 2 was obtained for these percolated nanocomposites, suggesting a 2D network of CNCs in the PEG matrix.

Graphene nanoplatelets (GNP) of various sizes were mixed with polypropylene (PP) in an internal mixer to prepare composites. The effects of mixing conditions, GNP size and concentration in composites were investigated. The composites were characterized at different scales, using electron microscopy, X‐ray diffraction, thermogravimetric analysis, and rheometry. It was shown that the PP/GNP composites had to be considered as non‐intercalated and non‐exfoliated microcomposites. However, the thermal, rheological and mechanical properties of the PP/GNP microcomposites were improved with respect to that of the matrix and similar to those of thermoplastic/organoclay or reduced graphene nanocomposites. In the best cases, storage modulus plateau in the glassy domain was increased by 30% and onset of degradation temperature by 40°C. Finally, in the investigated range of this study, the mixing conditions applied in the internal mixer did not affect the structural and rheological properties of the PP/GNP composites. POLYM. ENG. SCI., 58:1937–1944, 2018. © 2017 Society of Plastics Engineers