| High molecular weight Poly (lactic acid) (PLA) polymers were prepared by researchers of Dupont Company in 1954, for many years, these polymers were regarded as useless compounds because of their sensitivity to heat and water which precluded thermal processing based on extrusion or injection molding. This situation ended in the 1960s when advantage was taken of their sensitivity to water to make artifical degradable PLA sutures, aimed at replacing denatured collagen. PLA has been currently recognized as a suitable material for medical treatment, agriculture and packaging, due to its several preferable properties, such as biocompatibility, mechanical strength, degradability and absorbability.PLA has been produced by ring-opening polymerization of lactide or direct polycondensation of lactic acid. The direct polycondensation of lactic acid is commonly known as a process unable to obtain a high molecular weight polymer having useful properties; an equilibrium between free acids, water, and polyesters along with the formation of lactide, was explained to prevent the desired reaction to get a sufficiently high molecular weight polymer. High molecular weight PLA has been usually prepared by a multi-step process, including the isolation of lactide (a cyclic dimmer of lactic acid) followed by a ring-opening polymerization.It is well known that many properties of polymeric materials can be improved by increasing the molecular weight of step-growth polymers such as polyamides and polyesters, and that one of the routes to high molecular weight has been through solid-state polymerization (SSP). This is achieved by heating these polymers in an inert atmosphere to a temperature well above their glass transition, but below that at which they melt. Solid-state polymerization can produce a substantial increase in the degree of polymerization of the polymer,while the material still retains its solid shape.Two kinds of lactic acid prepolymer were synthesized by both direct polycondensation and chain extending methods, based on their different synthesis mechanism. The polymerization conditions such as polymerization temperature, polymerization time, vacuum etc were discussed too.Studies on the possibility of the solid-state polymerization (SSP) process to increase the average molecular weight of poly (1-lactic acid)(PLLA) were carried out. Some factors influencing the SSP process were initially examined, such as the mode of preparation of the PLLA prepolymer, polymerization temperature and time, and the nitrogen gas flow rate applied in the reaction vessel. The results show that relative high molecular weight products can be obtained by using prepolymer under a high SSP temperature and with a long reaction time under the hot nitrogen gas flow. Changes of PLLA's crystallinity and thermal stability are obvious, though that of prepolymer after chain extending is slight.A new model, based on the lactide ring-opening polymerization kinetics, includes lactide ring-opening polymerization and PLLA's thermal degradation reaction was employed to predict the monomer conversion of lactide, molecular weight and molecular weight distribution index in the polymerization process. The moment equations, which obtained by Z-transform technique, were solved by Runge-Kutta method. Comparison with the published data, the simulation results show that this model is more sufficient and reliable.The polymer's thermal stability affects directly its application and further molding process, higher process temperature decreases its molecular weight, the process becomes more difficult. A mathematical model to describe the molecular weight and polydispersity index(Q) in PLLA heat treatment has been developed. Based on the random chain scission mechanism, effects of temperature and time on the molecular weight and polydispersity index are included in this model. It incorporates the degradation and recombination reaction of PLLA heat treatment, while taking into account the equal probabilityassumption. The developments of molecul...
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