SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF HYDROGELS FROM RENEWABLE RESOURCES BASED ON SOY PROTEIN MACROMER AND POLY(ACRYLIC ACID)

Wenjia Song

Polymeric materials from renewable resources are promising replacements for synthetic polymers based on petrochemistry. Superabsorbent hydrogels are used in consumer products such as baby diapers and food packaging, and in agriculture and horticulture as soil conditioners. Renewability and biodegradability are desirable characteristics for superabsorbent hydrogels, especially when the production and consumption occur at large scales and the disintegration after use is needed. A method of preparing superabsorbent hydrogels consisting of soy protein (SP) and poly(acrylic acid) (PAA) was developed based on the copolymerization of neutralized acrylic acid (AA) monomer with SP macromer. The SP macromer was synthesized by functionalization of the primary amine groups in SP using methacrylic anhydride (MAh), which introduced polymerizable methacrylamide functionalities to SP. The use of SP macromer alone resulted in crosslinked hydrogels by the SP acting as a macro-crosslinker. In some cases, to further adjust the crosslink density, a small-molecular crosslinker, N,N'-methylenebisacrylamide (MBA), were also used. The SP macromers were characterized by ultraviolet–visible (UV-vis) spectrophotometry and proton nuclear magnetic resonance (1H NMR). The use of SP macromer improved mechanical properties and reduced the equilibrium swelling ratio of the hydrogels due to improved crosslinking. The effects of the reaction conditions, i.e., the polymerization temperature, the molar ratio of the reducing agent to the initiator, and the ratio of the initiator to the monomer, were studied. Compression stress-strain behavior, swelling, gel content, composition, and absorbency under load (AUL) were measured. All of these properties were found to depend significantly on the reaction conditions. The effects of crosslink density and SP content on the structure and properties were investigated. The compression data were interpreted using rubber elasticity theory to obtain the crosslink density. As the crosslink density was increased, the modulus of the hydrogels increased, and the equilibrium swelling decreased. A maximal AUL was found at an intermediate level of crosslink density. The potential application of the SP-PAA hydrogels in agriculture was explored. Soil biodegradation studies revealed that the SP portion of the hydrogels was biodegradable. Germination study and field trial demonstrated the compatibility of the SP-PAA hydrogels with plants.

SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF HYDROGELS FROM RENEWABLE RESOURCES BASED ON SOY PROTEIN MACROMER AND POLY(ACRYLIC ACID)

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