Synthesis And Characterization Of Organic/Inorganic Nanocomposite Hydrogels

Organic/inorganic nanocomposite hydrogels, as a new type of hydrogel, have both stability of inorganic compounds and the functionality of organic compounds. The research on nanocomposite hydrogel covers organic chemistry, inorganic chemistry, material, polymer and other interdisciplinary subjects, and becomes the intensive topic. Compared with conventional organic hydrogels, organic/inorganic nanocomposite hydrogels can improve the swelling, salt tolerance and mechanical strength of hydrogels, so they are expected to have a application in the fields of medicine, sanitary materials, agriculture, forestry and etc.Instead of cationic surfactant as an intercalating agent, we choose cationic polyacrylamide to intercalate into bentonite forming cation polyacrylamide/bentonite nanocomposite under a large dosage much more than cationic exchange capacity of bentonite through both monomer intercalative polymerization and polymer solution intercalation. Then amphoteric nanocomposite hydrogels with interpenetrating polymer network (IPN) or semi-IPN structure are obtained by subsequent crosslinking graft polymerization of acrylic acid onto starch. The resulting hydrogels show good swelling capacity and mechanical strength. The strength of hydrogel prepared by monomer intercalative polymerization is 2 times of the one by polymer solution intercalation, and reaches 28.9 KPa under a high water content of 99.6%, while the maximum swelling is achieved at 1010 g/g. Some synthesis conditions, including initiator amount, crosslinker amount, content of acrylic acid, bentonite content, and etc, are investigated with respect to the swelling capacity and temperature in the polymerization system. The chemical composition and structure are observed by FTIR, XRD and TEM while a quantitative analysis is given by Kjeldahl method and molecular weight measurement. This gives a good explanation for the swelling behaviors, as well as the difference on the swelling and compressive stress of amphoteric nanocomposite hydrogels by two methods. Urea is used as pore producer to improve the swelling speed of nanocomposite hydrogels instead of traditional carbonate. A porous hydrogel can be produced by decomposition of urea solution into CO₂ and NH₃ at high temperature. As a result, it avoids the special demands for the pH of solution, pore-producing time and gelation time when the carbonate is used as a pore producer, so the process becomes easily feasible. In addition, the swelling capacity and swelling speed are obviously improved. The swelling increases by one time while the swelling speed increases by about 40 times compared with the nonporous hydrogels. Some factors, such as urea amount, heating velocity, washing style and dry method are studied in termed of the swelling capacity. The swelling kinetic is also discussed.Based on satisfactory intercalation of cationic polyelectrolyte into bentonite interlayers and contribution to the good swelling and hydrogel strength, polydiallyldimethylammonium chloride is chosen as another intercalating agent due to satisfactory hydrophility, high charge density and easy synthesis method. Polydiallyldimethylammonium chloride/bentonite nanocomposites are prepared by polymer intercalation. Polymerization conditions are investigated, including polymerization temperature, initiator dose and monomer concentration in respect to the molecular weight and intercalation. FTIR, XRD and TEM are used to characterize the chemical structure and intercalation. Based on the successful exfoliation, amphoteric nanocomposite hydrogels are formed by crosslinking polymerization of acrylic acid onto starch. Some synthesis conditions, including initiator amount, crosslinker amount, polymerization temperature, content of acrylic acid, and content of polydiallyldimethylammonium chloride, are investigated in terms of the swelling capacity. In addition, pH and salt sensitivity, as well as the hydrogel strength of amphoteric nanocomposit hydrogel are measured. The results confirm that amphoteric IPN structure and nanocomposite make a big contribution to the swelling capacity and mechanical strength. The hydrogel strength increases by one time more than the traditional hydrogels and reaches 28.2 KPa under a water content of 99.2%. In addition, the high swelling can be kept in a wide pH range.Inorganic silica is used to fabricate organic/inorganic hybrid hydrogels adopting the similar design above. Silica network is obtained by using the soluble 3-aminopropyltriethoxysilane and sodium silicate as silica precursor through sol-gel technology, while organic polymer network is formed by the crosslinking polymerization of acrylic acid. As a result, the polyacrylic acid/silica nanocomposite hydrogel are formed with double network structure. The hydrogel is treated by HF treatment, and then the distribution of SiO₂ is observed by TEM. The results show SiO₂ particles are distributed in the polymer network in a size of about 100 ran when 3-aminopropyltriethoxysilane is used. The hydrogel exhibits good compressive stress and high swelling capacity. The hydrogel strength reaches 59.0 KPa under a water content of 99.1% and the maximum swelling is 1084.7 g/g, The hydrogel is stretchy even in the fully swollen state. Differently, a core-shell nanocomposite hydrogel with PAA outer shell and SiO₂ inner core is formed when sodium silicate is used. The resulting nanocomposite hydrogel shows distinguished salt tolerance and high hardness. The hydrogel strength reaches 45.6 KPa when the water content is 99.1%. The swelling shows a decrease of 10% below, while the traditional organic hydrogels show a decrease of 25% when the concentration of NaCl solution increases from 0.9% to 1.8%.