Positional Correlation Length-Induced Morphological Transformation of Interpolyelectrolyte Complexes (IPECs) Made of Polysaccharides: The Role of Molar Charge Ratio

By combination of oppositely charged polyelectrolytes, one can form interpolyelectrolyte complexes (IPECs) that may serve, for example, as water-based delivery systems. Such macromolecular IPECs may exhibit broad size and shape distributions; thus, quantitative structural analysis is often prone to misinterpretation. By means of small-angle neutron scattering (SANS), we present a systematic analysis of the internal macromolecular structures of interpolyelectrolyte complexes (IPECs) in a quantitative fashion. As a model system, we study polyelectrolyte mixtures of anionic biosourced sodium carboxymethyl cellulose (NaCMC) and cationic synthetic poly-(diallyldimethylammonium chloride) (PDADMAC). By integrating model-independent and model-dependent approaches, we reveal a morphological transformation from larger globular aggregates to smaller, multimodal lumpy structures with increasing molar charge ratio, i.e., upon approaching charge equimolarity. At the same time, the aggregates undergo significant compaction, driven by local structuring and a reduction in the characteristic correlation length of charge distribution, and they become increasingly anisometric. Additionally, the complexes retain a substantial amount of solvation water, which is gradually released as the charge ratio approaches equimolarity.