Effect of Chemical Modification on the Rheology of Aqueous Dispersions of Phytoglycogen Nanoparticles

Abstract

Phytoglycogen (PG) is a natural polysaccharide produced in the form of compact, 44 nm diameter nanoparticles (NPs) in the kernels of sweet corn. Its highly branched, dendritic structure and soft, compressible nature leads to interesting and useful properties that make the particles ideal as unique additives in personal care, nutrition, and biomedical formulations. To enhance the practical use of PG NPs towards these applications, it is often desirable to modify their properties. In this thesis, we consider the effects of three chemical modifications of PG NPs: acid hydrolysis, the covalent attachment of positively charged glycidyltrimethylammonium chloride (GTAC) chemical groups and the covalent attachment of negatively charged, hydrophobic octenyl succinic anhydride (OSA) chains to the particle surface. We have performed rheology measurements to investigate the effects of these chemical modifications on the flow properties and interactions between PG NPs in water as a function of the effective volume fraction of the particles, focusing on steady shear rheology measurements of the zero-shear viscosity and oscillatory measurements of the storage 𝐺′ and loss 𝐺" moduli. The rheology of acid hydrolyzed PG NPs was consistent with softer particles relative to native PG NPs, as indicated by a smaller fragility index 𝑚 value and enhanced interpenetration between the particles that led to highly dissipative viscoelastic behaviour with an experimentally accessible 𝛼-relaxation time. Dispersions of GTAC-modified PG NPs were significantly more viscous than those of native PG NPs, showing a much steeper increase in viscosity with increasing concentration 𝐶, and exhibited a more pronounced solid-like viscoelastic response. GTAC-modified PG dispersions were sensitive to the addition of salts and showed a significant reduction in viscosity with added NaCl. The behaviour of OSA-modified PG NPs was dependent on the degree of substitution (DS) of OSA: for low DS values, the rheology of OSA-modified PG NPs was dominated by electrostatic interactions. For higher DS values, hydrophobic interactions were more significant, and we observed evidence for clustering of the particles when NaCl was added to the dispersions. These results highlight the high degree of tunability of PG NPs and suggest promising new applications for the particles.