Microwave Assisted Polymerization of Lactide and In Situ Preparation of Poly (Lactic Acid) - Clay Nanocomposites

Abstract

Poly(lactic acid), PLA is made from renewable agricultural products and is readily degradable. Since PLA can be synthesized from lactic acid, which is a product of fermented starch, and can be degraded into nontoxic end-products thus it is an environment friendly material. This thesis embodies the subject matter resulting out of this study and is arranged in five chapters. A brief introduction, historical background and applications of PLA have been described first. A brief account of the advantages of using microwave for the synthesis of PLA has been included. The mode of heating (conventional and microwave heating) for the synthesis of PLA has been compared. The importance of lactic acid, the starting material, used in the synthesis of polylactide has also been discussed. The important method of synthesizing PLA is the ring-opening polymerization technique which is more useful than polycondensation for the production of high molar mass PLA. Polymerization kinetics of microwave-assisted synthesis of PLA has also been discussed. Microwave heating has been used for the synthesis of PLA from l-lactide and the ring-opening polymerization method has been chosen for the present work. Relevant literature for the polycondensation method and ring-opening method used in microwave oven has been reviewed. In-situ preparation of PLA-clay nanocomposites under microwave irradiation has also been reviewed. Two types of microwave heating system: domestic microwave and monomode microwave have been used to synthesize PLA and its clay nanocomposites in the present work. Studies on the effects of different parameters: polymerization temperature, polymerization time, monomer to initiator ratio, nature of initiator, effect of clay etc. which affect the polymerization of l-lactide to give PLA and its clay nanocomposites have been studied and discussed. The in-situ ring-opening polymerization of l-lactide to synthesize PLA/Clay nanocomposites by the addition of different types of clays have been successfully carried out under microwave irradiation. In this part of the work, effects of clay loading have been studied and nanocomposite products have been examined with respect to their morphology and degree of nanoclay dispersion. The present study reports for the first time the degradation of the polymer in chloroform solvent when the solution is left overnight before SEC characterization, resulting in a lower molar mass PLA. To carry out ring-opening polymerization using microwave irradiation for synthesizing PLA, three different experimental set-ups were used for the polymerization of lactide and to prepare its clay nanocomposites. The first set-up was used for the synthesis of PLA in a domestic microwave oven and a second set-up was used in a monomode microwave oven. The third set-up was used for the synthesis of PLA/Clay nanocomposites in the monomode microwave oven. Here, an ultrasonic probe was used for proper dispersion of clay in the solvent. Synthesis of PLA was carried out using initiators like stannous octoate (SnOct2), dibutyltin dimethoxide (DBTM) and one co-initiator, dimethyl aminopyridine (DMAP). The reason for selecting these initiators was that bulky groups attached to the metal atom would provide the steric hindrance around metal atom during ring-opening polymerization reaction. A new catalyst DBTM and co-initiator DMAP have been used for the first time in microwave assisted synthesis of PLA. PLA has been successfully synthesized in domestic microwave oven with molar mass above 50,000. Different molar ratios of monomer and initiator have been used for the synthesis of PLA in monomode microwave. In monomode microwave the ring-opening polymerization time was reduced to few mins from several hours reported for conventional heating. Size exclusion chromatography (SEC) was used to determine the molar mass of synthesized PLA. The molar mass of PLA obtained using SnOct2 was above one lakh. The initial increase in the molar mass with an increase in [Mo]/[Io] ratio, followed by a decrease at higher [Mo]/[Io] ratio can be explained as follows: as [Mo]/[Io] ratio increases, the relative number of initiator molecules decreases. So with an increase in [Mo]/[Io] ratio there is reduction in the number of chains onto which given monomer molecules can distribute themselves. Thus, the molar mass increases with an increase in [Mo]/[Io] ratio. In few cases, with further increase in [Mo]/[Io] ratio, the molar mass decreases. It is possible that at very low initiator concentration the effect of impurities on termination of a few growing chains may become the controlling factor with regard to molar mass. Additionally, it is known that during conventional polymerization of PLA at higher reaction times and at higher temperature, back-biting and transesterification reactions become significant leading to lowering of the average molar masses. In case of DBTM, PLA with low molar mass of a few thousands has been obtained as DBTM is known to be an effective transesterification catalyst and also known to cause ‘back-biting’ degradation. DMAP is a better nucleophile and good esterification catalyst; hence it prevents the back-biting reaction. Hence use of DMAP as a co-initiator with SnOct2 was expected to enhance the polymerization rate and much higher molar mass of PLA. In fact high molar mass PLA has been obtained only in case of SnOct2 as the initiator and DMAP as the co-initiator. Nano-dispersion of clay was achieved by using an ultrasonic probe. Ultrasonication provides tremendous amount of energy to the clay layers, so that they get separated to a larger extent. It is known that the addition of clay can efficiently improve the mechanical and barrier properties provided that they are well dispersed in the matrix and form an exfoliated structure. The present work showed that the incorporation of organically modified clay into PLA enhances thermal degradation temperature and hence markedly increases the thermal stability of the resulting PLA/Clay nanocomposites. The effect of clay loading was of much significance with respect to the polymerization yield. It showed negative effect on the polymerization yield. The formation of exfoliated structures was confirmed by X-ray diffraction studies and transmission electron microscopy. The kinetics of PLA synthesis in a microwave reactor is very different from its conventional synthesis where it has been reported that the rate of reaction is first order with respect to both monomer concentration and initiator concentration. The results indicate that the microwave-assisted ROP is accelerated not only by the microwave-induced temperature conditions (thermal microwave effect) but also by the microwaves themselves, that is, the non-thermal microwave effect.