Characterization and structure of poly(ethylene glycol)/poly(acrylic acid) interpenetrating polymer network hydrogels [electronic resource]
Keywords: Chemical Engineering

Hydrogels are hydrophilic polymer networks that can imbibe large amounts of water, but are insoluble due to the presence of network cross-links. Hydrogels are attractive materials for a number of applications including soft tissue replacement, three-dimensional cell scaffolds, and drug delivery vehicles. The high water content of hydrogels is essential in order to obtain a high rate of diffusion of critical cell nutrients, waste products, and/or active drug molecules. Despite their attractive diffusion characteristics, the applications of high water content hydrogels are limited by their extremely fragile nature. This work examines the mechanical properties and molecular-level structure of poly(ethylene glycol)/poly(acrylic acid) interpenetrating polymer networks (PEG/PAA IPNs), which have both a high water content (80 -- 90 wt %) and an enhanced initial modulus and fracture stress compared to PEG single networks. The mechanical properties were characterized primarily by compressive measurements and interpreted in terms of the initial modulus, strain at break, and stress at break. Small-angle X-ray and neutron scattering (SAXS/SANS) were used to probe the molecular-level structure of PEG/PAA IPNs. Ultimately, a mechanism of strength enhancement is proposed based on the structure and swelling properties of the IPNs. SAXS measurements revealed that photopolymerized end-linked PEG single networks, which serve as the first network in PEG/PAA IPNs, have a weakly ordered structure due to the uniform molecular weight between dense cross-link junctions. The position of the correlation peak was used to determine the average distance between cross-link junctions or the average PEG chain end-to-end distance within the network. This distance ranged from approximately 6 to 16 nm, dependent upon both the polymer volume fraction and the molecular weight of the PEG macromonomers. SANS measurements with contrast matching allowed the individual structures of the PEG and PAA networks within the IPN to be probed. When PAA initially interpenetrated the PEG network at low pH, the PEG network structure was largely undisturbed and was similar to that of a PEG single network. As the pH increased to physiological pH 7.4, the PAA network became ionized and swelled significantly. This swelling disrupted the weak ordering of the PEG network and forced the PEG chains to expand. At physiological pH, the PEG chains within the IPN were extended to 45 - 70 % of their maximum achievable length. Near these high extension ratios, the force required to further strain the PEG chains increased due to the entropic effects of finite chain extensibility. This led to a three-fold increase in both initial modulus and fracture stress for PEG/PAA IPNs at pH 7.4 compared to PEG single networks. The enhanced mechanical properties of PEG/PAA IPN hydrogels broaden their use to a number of applications requiring both high water content and robust mechanical properties. One such application is for a fully synthetic artificial cornea, wherein both a high rate of nutrient diffusion and robust mechanical properties are required. The mechanism of strength enhancement employed in PEG/PAA IPNs may also be applied to other swollen polymer network systems where an enhanced modulus and fracture stress are desired without an increased solids content.