The effect of electric charge density of polyacrylamide (PAM) on properties of PAM/cellulose nanofibril composite films

Poly(acrylamides) (PAMs) with various anionic and cationic charge densities from -1.64 to +0.98 meq/g but similar molecular weights were synthesized through radical polymerization. One of the aqueous PAM solutions and an aqueous dispersion of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibril (TOCN) were mixed at various PAM/TOCN weight ratios, and self-standing PAM/TOCN composite films were prepared by casting and drying. Because nonionic and cationic PAM molecules may have had some attractive interactions with anionic TOCN elements, the visual appearances and particle sizes of the mixtures varied depending on the charge densities of PAMs. In particular, PAM molecules with electric charge densities with +0.54 and +0.98 meq/g formed polyelectrolyte complexes with anionic TOCN elements, resulting in formation of large agglomerates in the aqueous mixtures. The PAM/TOCN composite films at weight ratios of 10/90 and 25/75 had higher Young's moduli and tensile strengths than those of 100 % TOCN film, when PAMs with charge densities from -1.64 to +0.54 were used. The PAM/TOCN composite films at the weight ratio of 10/90 had elongations at break similar to that of 100 % TOCN film, resulting in higher work of fracture values or ductile properties, whereas the PAM/TOCN composite films at the weight ratio of 25/75 were rather brittle.