Abstract
Tough hydrogels were made by hydrolysis of a neutral interpenetrating network (IPN) of poly (N-vinyl formamide) PNVF and polyacrylamide (PAAm) networks to form an IPN of polyvinylamine (PVAm) and poly (acrylic acid) (PAAc) capable of intermolecular ionic complexation. Single network (SN) PAAm and SN PNVF have similar chemical structures, parameters and physical properties. The hypothesis was that starting with neutral IPN networks of isomeric monomers that hydrolyze to comparable extents under similar conditions would lead to formation of networks with minimal phase separation and maximize potential for charge–charge interactions of the networks. Sequential IPNs of both PNVF/PAAm and PAAm/PNVF were synthesized and were optically transparent, an indication of homogeneity at submicron length scales. Both IPNs were hydrolyzed in base to form PVAm/PAAc and PAAc/PVAm IPNs. These underwent ~5-fold or greater decrease in swelling at intermediate pH values (3–6), consistent with the hypothesis of intermolecular charge complexation, and as hypothesized, the globally neutral, charge-complexed gel states showed substantial increases in failure properties upon compression, including an order of magnitude increases in toughness when compared to their unhydrolyzed states or the swollen states at high or low pH values. There was no loss of mechanical performance upon repeated compression over 95% strain.
Highlights
Hydrogels can be crosslinked by either irreversible or reversible physical interactions [1,2,3]
The polyelectrolyte interpenetrating network (IPN) hydrogels were synthesized from a sequential IPN of two neutral networks
Based on the change of mass it was found that the composition of the networks was 48:52 for PAA/PNVF hydrogels and 55:45 for PNVF/PAAm
Summary
Hydrogels can be crosslinked by either irreversible (covalent bonds) or reversible physical interactions (e.g., van der Waals interactions, ionic interactions, dipole-dipole interactions, and hydrogen bonding) [1,2,3] Their ability to contain over 95% water content is a physical property that is desirable in many applications, in the biomedical and pharmaceutical fields [4]. When the DN is put under stress, the strain energy is dissipated by fracture of the covalent bonds in the brittle network and transfer of the stress to the ductile second network. This work’s goal was to create a high charge density interpenetrating network of oppositely charged polyelectrolytes in which the ionic interactions would provide energy-dissipation mechanisms capable of self-healing [7,12,13,14]
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