A novel hydrogel, crosslinked poly (N-vinyl formamide) (PNVF), was synthesized and its physical properties were comprehensively evaluated when swollen to equilibrium in water. N-Vinyl formamide (NVF) is a water soluble monomer that has been produced on the commercial scale by several major chemical companies in the US, Europe and Japan though with limited commercial success to date. NVF is an isomer of the technologically important hydrogel-forming monomer acrylamide, but NVF has a number of advantages over acrylamide, including lower toxicity and that it is more easily processed because NVF monomer is a liquid at room temperature. Another useful feature of PNVF gel is that it can be hydrolyzed in acid or base to form a cationic hydrogel based on polyvinylamine, which has potential utility in drug delivery and chemical separations. Its synthesis as a molded hydrogel has not been previously reported.
PNVF gel was synthesized through free radical copolymerization/cross-linking with a novel crosslinker, initiated by the thermally-activated VAZO-44 at 50˚C. Transparent gels of reproducible, uniform properties were obtained. The monomer concentration was varied from 10 to 30 wt% in water and the crosslinker concentration was varied from 0.5% to 5% on a mass basis, to study the effect on swelling degree and mechanical properties; however the crosslinker concentration was compared against PAAm on a mole basis to accurately compare the properties of the two different crosslinkers with different molecular weights. PNVF swelling is up to twice that PAAm for a given formulation. From swelling and compression measurements, PNVF gels were found to be more hydrophilic than PAAm gels such that Flory-Huggins solubility parameters were found to be Χ=0.35Φ2+0.47 for PNVF and χ=0.26Φ2+0.49 for PAAm, where Φ2 = polymer volume fraction. When correlated against Φ2, similarity of mechanical properties suggest that these PNVF gels have a network structure to comparable PAAm gels. The fracture strains of both gels declined with Φ2 by the same linear function while fracture stresses were all around 600 kPa regardless of gel formulation. The shear modulus for PNVF scales with Φ22.0 and Φ22.5 for PAAm, consistent good solvent behavior for both gels, with PNVF somewhat more hydrophilic. PNVF gel properties were found to be generally comparable to polyacrylamide, suggesting that PNVF gels crosslinked with a novel crosslinker could substitute for PAAm gels. We are also developing strategies for making interpenetrating networks and nanostructured gels for the improvement of fracture properties of this class of hydrogels, since poor ultimate properties limit the utility of such materials in applications as diverse as tissue engineering, biomedical devices, and separations media.
This work was supported by NSF DMR 0805264.
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