Melt Polycondensation Of L-lactic Acid And The Crystallization And Melting Behavior Of PLLA

Poly (L-lactic acid) (PLLA) is a typical green polymer since it is produced from renewable resources, corn starch, and can be degraded to lactic acid and finally to carbon dioxide and water under compost condition after usage. PLLA is nontoxic, bioabsorbable and biocompatible. It also has good mechanical properties and can be processed by traditional processing methods. Therefore, it has found wide applications not only in biomedical fields as drug delivery carriers, surgical implants and tissue engineering scaffolds, but also in industrial and agricultural fields as general plastics. Because of its bright prospects, PLLA has attracted much attention from both industrial and academic fields.In this study, first, synthetic approaches of PLLA have been summarized and reviewed, from which we arrive at a conclusion that melt/solid polycondensation is one of the most competitive method to synthesize PLLA. Then, based on this viewpoint, melt polycondensations of LLA catalyzed by the well-known SnCl₂·2H2O/TSA and new 1,3-dialkylimidazolium salts catalysts have been investigated in detail. Last, crystallization and melting behaviors of the resulting PLLA have been studied. The research results will constitute foundations for further melt/solid polycondensation of PLLA.Modified melt polycondensation of L-lactic acid (LLA) was conducted with a binary SnCl₂·2H2O/TSA catalyst system to synthesize high molecular weight poly (L-lactic acid) (PLLA). The effect of the catalyst amount, reaction temperature and time on the molecular weight and yield of PLLA had been investigated. PLLA with weight-average molecular weight of about 100,000 was successfully synthesized at a yield of 70% with 0.4 wt% catalysts at 180 ℃ and under 300Pa for 20 hours. The microstructure and thermal properties of PLLA have been characterized by FT-IR, ¹H NMR and DSC, respectively. The resulting PLLAs possess good film-forming and fiber-forming properties, but the color is still not satisfactory.1,3-Dialkylimidazolium salts, the most commonly used ionic liquids (ILs), were used as a new type of single-component metal-free catalysts for the first time in the meltpolycondensation of L-lactic acid (LLA), and poly (L-lactic acid) (PLLA) with moderate molecular weight (over 20,000) but high optical purity (89-95%), high crystallinity (40-55%) and a little discoloration was successfully synthesized at high yield (over 70%). The polymers were characterized with GPC, 13C NMR, DSC and polarimeter. A series of 1,3-dialkylimidazolium salts with different anions and different substituents on the imidazolium cation have been examined. It is found that both less bulky substituent and less nuleophilic anion are conducive to the catalytic activity and thus, a possible catalytic mechanism is proposed and the experimental results have been interpreted. All the 1,3-dialkylimidazolium salts promoted the melt polycondensation of LLA, among which l-ethyl-3-methyl-imidazolium acetate (emimAc), l-ethyl-3-methyl-imidazolium bromide (emimBr), 1,3-diethylimidazolium lactate (eeimLLA) and l-butyl-3-methyl- imidazolium chloride (bmimCl) seem to be the most effective ones. Although being inferior to the well-known binary catalyst system SnCl2-2H2O/TSA in terms of molecular weight increasing, the present catalysts are better with regard to PLLA yield and comparable or better in terms of preventing discoloration and racemization. Combining with the nature of no residual metal contamination in the final products, 1,3-dialkylimidazolium salts may be promising catalysts for synthesizing metal-free PLLA with high quality, especially in a melt/solid polycondensation process.Differential scanning calorimetry (DSC) and polarizing microscope were utilized to characterize the crystallization and melting behaviors of PLLA with different molecular weights, namely, sample A with Mw of 26,000 and sample B with Mw of 63,000.In the DSC study, it is found that the cold isothermal crystallization behavior is much different from the hot isothermal crystallization behavior. In the hot isothermal crystallization process, the crystallization peaks were very weak, but in the cold isothermal crystallization process, the crystallization peaks were clearly observed. When the samples gone through hot or cold isothermal crystallization were reheated, no crystallization peak but double melting peaks, a low-temperature melting peak (L peak) and a high-temperature melting peak (H peak), were observed. Especially, triple melting peaks were observed for the PLLA sample B crystallized at 105°C. The L peak temperature (Tm(L)) increased linearly with logarithm of the isothermal crystallizationtemperature while the H peak temperature (rm(H)) kept constant. Melt-recrystallization model was used to explain the multiple melting behavior of PLLA. The crystallization kinetics indicates that the Avrami exponentials of PLLA were between 2 and 3, and the spherulites grow in a two-dimensional manner with a homogeneous nucleation mode.Formation of PLLA spherulite can be observed with polar microscope in both hot and cold crystallization manners. In the cold crystallization manner, the nucleation rate is very rapid to form numerous small crystals contacting with each other in few minutes. Therefore, the spherulite growth rates were determined only in hot crystallization manner. In hot isothermal crystallization of PLLA, the spherulite radii all increase linearly with time. For sample A, a maximum spherulite growth rate of 9.3 um/min was reached at 105°C and most perfect crystals were observed at 95 °C. And for sample B, the maximum spherulite growth rate of 14.2 um/min was reached at 115°C and the most perfect crystals were observed 110°C. The spherulite growth rates can also be obtained by non-isothermal crystallization manner, and the results thus obtained agree well with that determined by isothermal hot crystallization.Hoffman-Lauritzen equation was used to fit the data of crystallization rates. The nucleation constants of sample A and sample B are estimated to be 6.35* 105 K2 and 3.35*105 K2, respectively, which suggest that the higher the molecular weight, the more difficult to nucleate. In addition, it was observed that heating the sample gone through hot isothermal crystallization induced formation of small crystals in the amorphous region, leading to the double distributions of the crystals in the polymer. This maybe can account for the triple melting peaks in the following heating scan.It is found that the crystallinity of after hot isothermal crystallization is lower than that after cold isothermal crystallization in the temperature range investigated. However, the perfection of the crystals is not the case. The perfection of the crystals after cold crystallization is inferior to that after hot crystallization in the range of 90-110°C, the same with that after hot crystallization at the temperature of 110°C and superior to that after hot crystallization in the range of 110-135°C. This is supported by both DSC measurement and polarizing microscope observation.