Study On Preparation And Properties Of Poly Lactic Acid/Montmorillonite Nanocomposites Synthesized Under Microwave Irradiation

The exploration of degradable polymer, namely poly lactic acid(PLA), using renewable resources as raw materials and the application of green techonology have become one of the most effective solutions in polymer fields for petroleum-based plastic waste problem, as well as an important way to realize sustainable development. In order to solve the existing defects such as the high energy consumption, complicated process, inhomogeneous dispersibility of nano-particles and the environmental safety problem of the catalysts in synthesizing nanocomposites, we investigated the microwave-assisted synthesis of poly lactic acid using solid acid, not only as catalyst but also nanometer additives. And the PLA/MMT nanocomposites, which obtain different performances to meet various industry requirements were prepared.Microwave technology was firstly introduced to prepare PLA through melt polycondensation using SO₄^2-/Al₂O₃ solid super acid as the catalyst in order to test the catalytic activation of the solid acid with different acid strength. The effects of certain factors such as reaction temperature and time, vacuum, and catalyst amount on average molecular weight of PLA under microwave irradiation were discussed, and the technological conditions were optimized. On this basis, the molecular weight, structure and thermal stability of PLA sythensized with different catalyst system were compared. The different effects between microwave heating and traditional heating were evaluated on aspects of reaction time, energy consumption and molecular weight of PLA. The mechanism of the synthesis of PLA by melt polycondensation with SO₄^2-/Al₂O₃ solid super acid as catalyst under microwave irradiation was speculated thereafter.PLA/MMT nanocomposite was then prepared by means of microwave-assisted in situ melt polycondensation with direct addition of nanosized sodium-montmorillonite (Na-MMT). The reaction conditions were optimized in terms of mechanical properties. The thermal properties and distribution of montmorillonite were characterized by TG-DSC, SEM and XRD. The results showed that the molecular weight, mechanical and thermal properties of PLA/MMT nanocomposites were greatly improved. The reaction mechanism of the microwave- assisted synthesis of exfoliated PLA/MMT by in situ melt polycondensation doping Na-MMT was studied according to the surface composition of MMT characterized by FT-IR.Acidic MMT (H-MMT) was prepared by activating Na-MMT with acid under microwave irradiation based on the high catalytic activation of solid super acid. The acidity and catalytic activity (tested in terms of ester yield of n-butyl lactate) were significantly improved. After that, PLA/MMT nanocomposite was synthesized by in situ melt polycondensation under microwave irradiation with H-MMT compounded with SnCl₂ as catalyst and nanometer additives. The results showed that H-MMT existed in an exfoliated form in the polymer matrix and had good catalytic effects on elevating the molecular weight of the PLA based nanocomposite, and the mechanical and thermal properties of nanocomposite were improved.The chemical reaction kinetics of melt polycondensation of lactic acid under microwave irradiation of different catalystic system was studied, and kinetics equation was demonstrated. By comparison of the kinetic parameters of different catalyst system under the optimum reaction temperature, it is confirmed that H-MMT/SnCl₂ compounded system showed certain advantages on properties of catalytic synthsized product, rate of catalytic reaction and energy consumption, and was believed to be a good catalyst system.Using home-made lactide as raw material, organically modified montmorillonite(OMMT) as the nanosized additives, and polyethylene glycol(PEG) as the plasticizer, PLA/MMT/PEG nanocomposite was prepared by microwave assisted in situ ring-opening polymerization. Influences of different OMMT, microwave conditions and plastisizer on mechanical properties were studied systematically. The plasticized PLA/MMT showed better tensile strength and elongation at break. Thermogravimetric analysis(TGA) was introduced to study influences of different modification methods on thermal decomposition temperature. The analyse of scanning electron microscope(SEM) confirmed that the addition of OMMT and PEG improved toughness of PLA. Besides, the intercalation and exfoliation behavior of PLA in the OMMT had been investigated by X-ray diffraction(XRD) and transmitting electron microscope(TEM).The experiments of soil landfill and enzymatic degradation experiments were carried out to study the degradability of the PLA based nanocomposites. In the experiment of soil landfill, the changes of weight loss and viscosity molecular weight versus time were investigated, the configuration of PLA-based nanocomposites before and after biodegradation were observed through SEM, and the effect of the addition of nano-particles and plasticizer on the degradation processes of PLA-based nanocomposites were discussed. In the experiment of enzymatic degradation, the degradation processes of PLA-based nanocomposites expressed by different enzymatic reaction kinetics models were analyzed and compared, and the dynamic equations of enzyme hydrolysis were established.The results showed that the montmorillonite was an effective nano-additive to improve the mechanical and thermal characteristics of the PLA/MMT nanocomposites. Besides it could also act as a catalyst after acidic modification. The application of microwave heating method can accelerate the reaction rate, shorten reaction time and reduce energy consumption significantly. Microwave-assisted synthesis of PLA/MMT nanocomposites doping montmorillonite is proved to be a fast ,efficient and green synthesis method.