Development Of Polyaspartate Materials: Design, Synthesis, Characterization And Applications

Polyaspartate (PAA) is typical of environmentally friendly chemicals and the macromolecule with a biodegradable polypeptide structure. Developing various kinds of polyaspartate (PAA) and extending their application have attracted a great deal of worldwide attention, in order to put directly them to a good use or substitute harmful substances in some key fields. It is of great theoretical and practical significance to focus on synthesis of polyaspartate (PAA) materials with an excellent heavy-metal-removing performance, investigation of relation of structure-activity, and their application to some Chinese herbal medicines. The main contents, conclusions and results are as follows:1. Polysuccinimide, precursor of polyaspartate, was synthesized by suspension polycondensation of L-aspartic acid dispersed in a medium. The effects of catalyst,temperature,time and particle size of L-aspartic acid on molecular weight ( M w) and its distribution were investigated deliberately.The experimental results revealed that particle size of L-aspartic acid and temperature are the major factors affecting the molecular weight of the product. Polysuccinimide, having a weight average molecular weight of 20 000³0 000 and M w/ M ndistribution of 1.3¹.5, was obtained under appropriate conditions. The results also showed that H3PO4 as the catalyst can prevent–NH2 group from side reactions,accelerate the rate of polymerization and increase M w, but an excess of catalyst results in closing end-group, and so blocks the increase in M w.2. Taking inhibition of calcium in aqueous solution as a probe, conversion of polysuccinimide to polyaspartate by alkaline hydrolysis was investigated by means of orthogonal experimental design. The effects of temperature, time, concentration and amount of NaOH on the scale inhibition capacity and molecular weight ( M w) of the hydrolysis products were taken into account. The experimental results indicated that the reaction temperature is the key factor affecting the molecular weight of the polymer, and the kinetics of the hydrolysis reaction was deduced to be a 2nd order cascade type reaction. Approximate calculation values of the activation energy of the hydrolysis reaction of succinimide unit in polysuccinimide and amide unit in polyaspartate were 28 mol/kJ and 40 mol/kJ, respectively.3. Scale inhibition performance test demonstrated that for the purpose of preparinginhibitor of calcium,no matter what starting substance was, such as L-aspartic acid, DL-aspartic acid, or maleic anhydride, polyaspartate with excellent scale inhibition the scale inhibition can be available under appropriate conditions. Polyaspartates from different raw materials are similar in inhibition performance, and competitive with polyacrylic acid-based water treatment chemicals. The appropriate polycondensation conditions of L-aspartic acid by means of suspension polymerization were 0.02⁰.03 molH3PO4/mol L-aspartic acid , 220℃and 1.5 h, with a 90⁹5 % scale inhibition and light colour. The appropriate conditions of the hydrolysis of polysuccinimide determined by means of an orthogonal experimental design were 2 mol/L NaOH (in water), 0.45⁰.48 g NaOH / g polysuccinimide, 50⁹0℃and 30 min. Polyaspartate plays the scale inhibition role of increasing dissolvability by complexing, distorting crystal lattice and dispersing aggregate.4. Pb²⁺-binding behavior of linear polyaspartate, the primary structure of polyaspartate material, was evaluated by potentiometer determination. It was demonstrated that the linear polymer exhibits higher Pb²⁺ uptake capacity (about 1.0 g Pb²⁺ /g,based on amount of polysuccinimide) and lower Pb²⁺ equilibrium concentration (<0.25 mg Pb²⁺/L). It was noticeable that the uptake is still high even in the range of lower concentration of Pb²⁺. The equilibrium sorption data for Pb²⁺ on polyaspartate fitted well with Freundlich and Langmuir models in spite of its dissolving in water.5. Crosslinked polyaspartate hydrogels were prepared by crosslinking polysuccinimide, having a weight average molecular weight of 10 000³0 000, with diamine used as cross-linking agents. Microwave technique was adopted in the process. Effects of reactant molar ratio, temperature or heating mode and reaction time on crosslinking were discussed. The experimental results showed that reaction time was shortened remarkably and temperature had little effect on the reaction by use of microwave technique. Polyaspartate hydrogel also exhibits to be an effective agent for removal of Pb²⁺, and its Pb²⁺ uptake varied with concentration of Pb²⁺ in solution, pH of media, and so on. Temperature had little effect on its binding performance, and only a short period for reaching equilibrium was required. Citrate and acetates were preferably desorbing agents. By means of infrared spectra (IR), it is inferred that polyaspartate hydrogel bound Pb²⁺ by both ion exchange mechanism and chelating mechanism6. A clean process using water as the solvent media was explored for the preparation of Polyaspartate hydrogel, and meanwhile Ca2+ template was adopted to improve thestrength and uptake capacity of gel. The experimental results showed that the gel having better strength and uptake capacity was obtained, with an uptake capacity of about 4.0 mmol per gram of dry gel.7. A novel and green polymer with similar structure with peptidepolysaccharide, the copolymer of ployaspartate and chitosan, was developed by the copolymerization of chitosan and ployaspartate with the side amino-group. The copolymer can be used for the packed bed due to its lower expansion performance in the presence of water. Several types of the copolymer, such as granule, membrane and thiol-type, can be prepared with different methods. The appropriate conditions for the granule type copolymer were experimentally obtained: chitosan: ployaspartate with the side amino-group (g/g)=1:15, 0.5¹.0 ml 50% glutaraldehyde / g (ployaspartate with the side amino group), The addition of appropriate pore-forming agents in the copolymerization of chitosan and ployaspartate gave a bifunctional ultrafiltration membrane of ion exchange and adsorption with porosity. When the copolymer reacts with thioglycollic acid, a new copolymer containing 2% (wt.) thiol content can be obtained. The presence of thiol group may be helpful to the removal of organic heavy metal ions, such as CH3Hg+ etc.8. The researches on the function of polyaspartate material for heavy-metal removal and on the relation between structure and its activity were conducted systematically. It was verified that polyaspartate is a more excellent Pb²⁺-binding agent by comparison with some polyaspartamide derivatives having different side chains in that it possesses higher Pb²⁺ uptake and lower Pb²⁺ equilibrium concentration. It was suitable for application to both higher and lower concentration of heavy metal ions. The experimental results also demonstrated that crosslinked polyaspartate hydrogel is superior to polyacrylic acid-based resins, poly styrene-based chelating resins and chitosan. The infrared spectra (IR) and X-ray photoelectron spectra (XPS) revealed that polyaspartate hydrogel binds Pb²⁺ by both ion exchange mechanism and chelating mechanism. Along with the equilibrium sorption data for Pb²⁺ on polyaspartate and those from IR, it was deduced that the complex is formed between Pb²⁺ and polyaspartate hydrogel and the coordination number of Pb²⁺ is 4. Meanwhile, using Ca²⁺ as a reference, the experimental results showed that Cd²⁺ and Hg²⁺ were bound to polyaspartate hydrogel by the same mechanism as Pb²⁺.9. Polyaspartate materials were introduced into some Chinese herbal medicines to remove heavy metal in them. The performance was tested for their heavy metal-binding behavior in some chosen herbal solutions with a higher heavy metal contents.Polyaspartate materials were demonstrated to be the effective agents for the removal of Pb²⁺, Cd²⁺ and Hg²⁺ from the aqueous solution of glycyrrhizin, angelica or gynostemma pentaphyllum. The concentration of the solutions, or solid content in herbal medicine solution, had an influence on their performance. Except for Hg²⁺, Polyaspartate structure was found to be ineffective for the removal of mercury in other form. Polyaspartate and chitosan copolymer with thiol group exhibited an excellent elimination of lead, cadmium and mercury.10. The biodegradation of various ployaspartates was systematically investigated using several testing methods, such as BOD/COD, TOC, growth grade rating, loss of weight, scanning electron microscope (SEM), elemental analysis and infrared spectrum analysis (FT-IR). The results indicated that TOC is an appropriate method to evaluate biodegradation of water-soluble ployaspartate due to the simplicity of operation, reliability and excellent repeatability of data. According to standard OECD 201B, water-soluble ployaspartate belongs to the ready-biodegradable polymer. Several methods were also suggested to evaluate the biodegradation of water-insoluble ployaspartate. They are growth grade rating with the aid of SEM, loss of weight with the help of elemental analysis and FT-IR of residue. It was found that the biodegradability of copolymer of chitosan and ployaspartate is better than that of polyaspartate hydrogel, and that both the copolymer and polyaspartate hydrogel have a better biodegradability than the commercial polyacrylic acid-based ion-exchange resin 110 (gel type).