Preparation And Structure Characterization Of Interpenetrating Network Hydrogel With Modified First Network

A series of modified ploy(2-acrylamide-2-methylpropane sulfonic acid) (PAMPS)/poly(acrylamide) (PAM) interpenetrating network (IPN) hydrogels with highly enhanced mechanical properties were prepared through adding poly(ethylene glycol) (PEG) or poly(ethylene oxide) (PEO) in the first network, or using copolymers of AMPS as the first network. First, ductile PAMPS-PEG/PAM hydrogel and strengthened PAMPS-PEO/PAM IPN hydrogels were synthesized. The effects of hydrophilic polymers on the mechanical properties of IPN hydrogels were studied. Microstructures of IPN hydrogels were investigated by scanning electron microscope (SEM) and small-angle X-ray scattering (SAXS). Toughen mechanism of IPN hydrogels was proposed. A new IPN hydrogel with ultrahigh compressive strength was prepared using the copolymer of AMPS and AM as the first network. The effect of comonomer on swelling ratio and mechanical properties of IPN hydrogels was studied. Microstructures of single network (SN) and IPN hydrogels were investigated by confocal laser scanning microscopy (CLSM), SEM and SAXS. Strengthen and toughen mechanism of IPN hydrogels was proposed.The IPN hydrogel of PAMPS-PEG/PAM had remarkable high elongation-at-break (～2100%) in tensile deformation, which was 4 times larger than common PAMPS/PAM IPN hydrogel; the corresponded fracture energy was 1.5 times larger than that of PAMPS/PAM IPN hydrogel. The IPN hydrogel with PEG incorporated in the second PAM network was also prepared for comparison:it was found that toughness of IPN hydrogels would be enhanced only inducing PEG in the first network. The PAMPS-PEO/PAM IPN hydrogel also had largely enhanced compressive properties; it became brittle when PEO amount was higher than 6%.Microstructures of PAMPS-PEG/PAM and PAMPS-PEO/PAM IPN hydrogels were investigated by SEM and SAXS. SEM results showed that lamellar-like structures of PEO chains reinforced the first network. Presence of physical cross-linked domains in the IPN hydrogel was discovered. Power Law, general Ornstein-Zernike (GOZ) equation, Guinier equation, Debye-Bueche equation and Ornstein-Zernike (OZ) equation were used to analyze the structure of cross-linked domains and interaction between polymers in hydrogel. Incorporation of PEG into the first network enlarged the size of cross-linked domains, decreased fractal dimension of cross-linked domains and enhanced the interaction between the first and second networks. Increasing PEO molecular weight or PEO amount could enlarge the size of cross-linked domains and changed the conformation of second network. Toughen mechanism of IPN hydrogels was proposed.P(AMPS-co-AM)/PAM IPN hydrogel had a compressive strength as high as 91.8 MPa, which was much higher than that reported in literatures. The IPN hydrogel did not fracture under a compressive strain of 98%. Mechanical properties of IPN hydrogels with comonomer containing ionogenic side chains were weaker, while that of IPN hydrogels with comonomer containing hydrophobic side chains were enhanced.The microstructure structures of P(AMPS-co-AM)/PAM IPN hydrogels and SN hydrogels were investigated by the SAXS, SEM and CLSM. The pore walls of P(AMPS-co-AM) SN hydrogel were thicker and more uniformly distributed than that of PAMPS SN hydrogel. Power Law, Guinier equation, Debye-Bueche equation and OZ equation were used to analyze microstructural changes of IPN hydrogels. The results showed that when AM content in the first network increase, the size of cross-linked domains first increases then decreases, the first network become softer and more evenly distributed, interaction between first and second networks is enhanced. Strenghen mechasim of IPN hydrogel was proposed. The PAM free polymers in second network were pulled out from the random coils and became Gaussian chains under large deformation, PAM chains and the first network clusters formed physical cross-linkers. Comonomer containing ionogenic side chains increases heterogeneity of the first network, compact the cross-linked domains and weakened interaction between the first and second networks. Comonomer containing hydrophobic side chains loosened and enlarged the cross-linked domains, increased interaction between the first and second networks of IPN hydrogels.