Preparation of composite hydrogel materials for agricultural applications

In the present thesis, a series of composite hydrogels from dimethyl acrylamide (DMAAm), maleic acid (MA) and starch were synthesized. Methylenebisacrylamide was used as a cross-linker during preparation of composite hydrogels. The suitability of those composite hydrogels for agricultural purposes were investigated. Composite hydrogels were synthesized via a redox polymerization technique and the surface of the synthesized composite hydrogels was modified using acids and bases. Characterization of the synthesized composite hydrogels were studied. Their swelling behaviors were conducted in pure, well, tap, dam water, and under different pH values. The structural characterizations were carried out using Fourier Transform Infrared Spectrophotometer and Thermogravimetric Analyzer. It was observed that the equilibrium swelling values of composite hydrogels increased with the addition of MA and Starch to the polymerization reactions. The highest equilibrium swelling value was measured for p(DMAAm-co-MA-co-Starch)/NaOH composite hydrogel. Synthesized composite hydrogels were also tested as absorbents for the absorption of urea and sulfate ions, which are nutrient sources for plants. Absorption experiments were carried out at different concentrations of urea and sulfate ions, different pH values, and different temperatures (oC). The absorption performances of composite hydrogels were investigated. It was observed that synthesized composite hydrogels were good absorbents for urea and sulfate ions. After the absorption studies, the release behaviors and release kinetics of composite hydrogels were investigated at different pH values, and under well and tap water environments. The results of analyzes at different pH and different aquatic media have shown that the cumulative ions (for urea and sulfate ion) release of the composite hydrogels varies between 10 % and 100 %, and the release occurs slowly. The release kinetics of composite hydrogels were also computed with Zero Degree Kinetic Model, First Degree Kinetic Model, Higuchi Model, Korsemeyer-Peppas Model. It was evaluated that Korsemeyer-Peppas Model was the best fitted model. In the last phase of the study, the suitability of composite hydrogels loaded with urea for agricultural purposes were invesitgatied. Based on the results of the swelling, absorption, and release experiments, three types of urea-loaded composite hydrogels (p(DMAAm-co-MA-co-Starch), p(DMAAm-co-MA-co-Starch)/HCl, and p(DMAAm-co-MA-co-Starch)/NaOH) were deemed to have potential in that regard, and their effects on the growth rate of lettuces were studied. During the investigation, plant size, stem diameter, number of leaves, and chlorophyll and color values were monitored to measure the effects of composite hydrogels on the development of the plants. In addition, drought stress was applied to the plants in the last two months of the study and the response of the plants was observed. The data and photographs obtained at the end of the study indicate that the synthesized composite hydrogels were not toxic to the soil and plant structure. The plants could easily acquire and keep the moisture and fertilizers as they needed, and at the same time, they could release them over a long time and controlled manner.