Aqueous Processing of Mechanically Robust, Dense Films from Carboxymethyl Cellulose-Based Coacervates

This study fabricates free-standing films from the natural polyanion carboxymethyl cellulose (CMC) paired with the synthetic polycation poly-(diallyldimethylammonium chloride) (PDADMAC) using all-aqueous processing. First, the ability of CMC and PDADMAC to form polyelectrolyte complexes (PECs) in the presence of sodium chloride (NaCl) or potassium bromide (KBr) was screened by using turbidity measurements. This revealed that an excess of polycations was required to induce coacervation in CMC/PDADMAC in the presence of either salt, which amassed a net positive charge in the coacervates. The optimized PEC conditions for casting were corroborated via rheological testing, which indicated higher coacervate sensitivity to KBr than to NaCl. Next, blade casting was used to spread the coacervates into a thin layer, which were processed into films using aqueous phase separation (APS). The morphology, thermal properties, and mechanical performance of the films were evaluated by using scanning electron microscopy (SEM), dynamic mechanical analysis (DMA), and tensile testing, respectively. SEM revealed that the films were dense, with Young's moduli ranging from 436 to 975 MPa. Higher salt concentrations in the coacervates yielded films with fractured and unstable surfaces, which resulted in a significant decrease in mechanical strength, though the films were still robust with Young's moduli ranging from 110 to 330 MPa. With the knowledge that our CMC/PDADMAC films were manufactured with an excess of PDADMAC, we hypothesized that their cationic nature would result in intrinsic antimicrobial properties via contact killing. Indeed, all CMC/PDADMAC films inactivated ∼50% of the Escherichia coli and Staphylococcus aureus, supporting that the manufactured films present free cationic groups available to kill microbes or further chemical modifications. We suggest that these high-strength biopolymer-based films produced via sustainable all-aqueous processing hold potential for use as membranes, packaging materials, and high-touch coatings.