A novel interpenetrating network (IPN) hydrogel with ultrahigh compressive strength and fracture strain has been prepared using the copolymer of 2-acrylamide-2-methylpropane sulfonic acid (AMPS) and acrylamide (AM) [P(AMPS-co-AM)] or N-isopropylacrylamide (NIPAM) [P(AMPS-co-NIPAM)] as the primary network and polyacrylamide (PAM) as the secondary network. The as-prepared IPN hydrogel of P(AMPS-co-AM)/PAM has a significantly high compressive strength (91.8 MPa), which is 4 times greater than that of the common PAMPS/PAM IPN hydrogel as well as the compressively strongest hydrogel reported in the literature. The P(AMPS-co-AM)/PAM IPN hydrogel is tough enough not to fracture even when the compressive strain reaches 98%. Synchrotron radiation small-angle X-ray scattering (SAXS) analysis has indicated that the presence of an AM comonomer changes the size of the physically cross-linked domains in the IPN hydrogel, which may partially account for its unique mechanical properties. This study has presented the compressively strongest hydrogel reported to date and also provided a novel and feasible method to prepare the highly strong and tough hydrogel.
An interpenetrating network (IPN) hydrogel with a highly enhanced elongation-at-break has been prepared using poly(ethylene glycol) (PEG)-swollen poly(2-acrylamide-2-methylpropane sulfonic acid) (PAMPS) as the first network and polyacrylamide (PAM) as the second network. The new IPN hydrogel of PAMPS-PEG/PAM has remarkably high elongation-at-break ($2100%) in tensile deformation, which is 4 times larger than common PAMPS/PAM IPN hydrogel and PAMPS/PAM-PEG IPN hydrogels synthesized by incorporating PEG into the second network. The microstructures of single network (SN) and IPN hydrogels were investigated by synchrotron radiation small-angle X-ray scattering (SAXS). SAXS results were analyzed by Guinier, Ornstein-Zernike (OZ), and generalized Ornstein-Zernike (GOZ) models. It was found that the incorporated PEG increases the size of cross-linked domains and decreases the fractal dimension of domains. The toughening mechanism of PEG on IPN hydrogel was discussed. It is proposed that the largely enhanced toughness of PAMPS-PEG/PAM IPN hydrogel is due to the increased size of physical cross-linked domains, hydrogen bonding between the first and second network, and the increased pull-out resistance of PAM chains under deformation.
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