Mechanical properties of polyelectrolyte complex films based on polyvinylamine and carboxymethyl cellulose

Homogeneous polyelectrolyte complex films were cast from mixtures of poly(vinylamine-co-vinylformamide) and carboxymethyl cellulose (CMC) in 50% formic acid aqueous solution with compositions varying between CMC blended with pure polyvinylamine (PVAm) and CMC blended with pure poly(N-vinylformamide) (PNVF). The tensile strength and tensile modulus of the complex films were measured as functions of polymer ratio, molecular weight, function group content, and water content. PVAm addition lowered the strength and modulus of dry CMC whereas PNVF did not; intermediate PVAm-PNVF copolymers gave intermediate results. Mechanical strength decreased with increasing molecular weight of CMC and carboxyl content, but was not affected by molecular weight of PVAm. The mechanical properties of all the films decreased with increasing water content. CMC: PVAm films were swollen gels in water, whereas CMC: PNVF films dissolved in water. Heating did not improve the mechanical strength, whereas cross-linking with 5% glutaraldehyde doubled the strength. It was proposed that hydrogen bonding was the predominant intermolecular force responsible for the strength of dry film blends whereas ionic bonds between CMC-carboxyl and PVAm-ammonium ions were responsible for the integrity of water swollen polyelectrolyte films.