|It is important to synthesize gels with lower toxicity and higher processibility to widen their applications. In this regard, the monomer N-vinyl formamide has an advantage over its widely used isomer acrylamide. A novel hydrogel, poly (N-vinyl formamide) (PNVF) was synthesized and its properties were measured when swollen to equilibrium in water. This gel was synthesized through free radical copolymerization/crosslinking with (N-vinyl formamido) ethyl ether (NVEE) initiated by VAZO-44 at 50˚C. Transparent gels of reproducible, uniform properties were obtained using NVEE, apparently due to nearly random copolymerization with NVF, likely due to similarity in the polarity if their propagating radicals.
These gels were characterized by varying monomer concentration from 10% to 25%. In addition to an increase in monomer concentration, crosslinker concentration was also varied from 1% to 3% on a mass basis, to study the effect on swelling degree and mechanical properties. Mechanical properties were calculated by conducting tensile tests to measure fracture stress, fracture strain, Young's modulus (E) and shear modulus (G). Young's modulus of PNVF gels increased from 138 kPa to 609 kPa, shear modulus increased from 49 to 212 kPa, fracture stress increased from 17 to 66 kPa where as the fracture strain decreased from 14.8 to 11.1 with an increase in crosslinking density. Other parameters like solubility parameter of the polymer and crosslink density were also calculated using established theories. The crosslink density was calculated from the shear modulus using the phantom network model of rubber elasticity and was on the order of 10-4mol/cm3. Combining this with measurements of the swelling degree, and Flory- Huggins solubility parameter (χ) of these polymers, was calculated to be ~0.42. PNVF gel properties were found to be generally comparable to polyacrylamide, suggesting that PNVF gels crosslinked with NVEE could substitute for PAAm gels in applications.
PNVF hybrid gels were also synthesized by adding PNVF nanogels to PNVF single network. It was hypothesized that nanogels will improve fracture properties of these gels. Addition of PNVF nanogels to PNVF macrogels, however, did not have a significant effect on the fracture properties in the limited ranges tested.