Complexation and network formation in a suspension of oppositely charged cellulose nanocrystals and poly(allylamine)

The structure formation, determined by tunable electrostatic interactions, in systems consisting of negatively charged cellulose nanocrystals (CNC) and a positively charged flexible weak polyelectrolyte, poly(allylamine) (PAAm), is discussed. Water dispersions, contained 3 wt.% CNC and 0.0364 wt.% PAAm of high molecular weight (65 kDa) that corresponds to the ionizable group stoichiometric ratio of system components, were examined. Variation in the dispersion pH allowed controlling the PAAm and CNC ionization level and hence the electrostatic interactions of the system constituents. For samples with high charge density, steady state viscosity measurements demonstrated cluster formation which consist of several CNC rods connected (due to ionic coupling) by PAAm tie macromolecules. Under certain conditions, these clusters (micro-gels) start to grow, merge and finally form a global network providing system elasticity. Oscillatory shear deformation experiments revealed a strong dependence of the storage and loss moduli on the PAAm ionization level. For high ionization degree of the system components, a strong network was formed but only after sonication, i.e. as a result of an activation; whereas for a low PAAm ionization level, the formed network was weak but required no activation.