Stereoblock poly(lactic acid) (sb-PLA), consisting of poly(L-lactic acid) (PLLA) and poly(D-lactic acid) (PDLA) in a blocky sequence, can successfully be synthesized by solid-state polycondensation of a stereocomplexed mixture of PLLA and PDLA. First, the melt polyconden-sation of L- and D-lactic acids is conducted to obtain PLLA and PDLA with medium molecular weights. Then, both polymers are melt-blended to easily form the stereocomplex. The resulting stereocomplexed mixture (melt-blend) is subjected to solid-state polycondensation for chain extension. The molecular weight (Mw) of the resultant sb-PLA is strongly affected by the lactide/oligomer content in the melt-blend, which is determined by the melt-blending conditions, because it is directly correlated with the polymer crystallinity of the polycondensation products. 相似文献
The step-growth polymerization of L-lactic acid in solution was studied in this work. In order to attain a polymer with high molecular weight, the water formed during the polymerization must be continuously removed. The use of organic solvents with high boiling point, drying agents and reduced pressure led to poly(lactic acid) (PLA) with high molecular weight, directly from the monomer. Tin (II) chloride dihydrate, SnCl2.2H2O, was the best of the catalysts tested as it allowed achieving PLA with a molecular weight close to 80 000 g.mol−1. However, the stereoregurarity control is a severe problem in PLA synthesis by step-growth due to transesterification reactions, which lead to an inversion of the conformation and a decrease of the optical purity of the polymer. Specific rotation measurements were used in this work and showed to be a powerful technique to evaluate the racemization extent. The thermal stability of the PLA samples was evaluated by DSC which exhibits a thermal behaviour similar to the commercial Polylactide. 相似文献
Poly(lactic acid)(PLA)is one of the most important bio-plastics,and chemical modification of the already-polymerized poly(lactic acid)chains may enable optimization of its material properties and expand its application areas.In this study,we demonstrated that poly(lactic acid)can be readily dissolved in acrylic acid at room temperature,and acrylic acid can be graft-polymerized onto poly(lactic acid)chains in solution with the help of photoinitiator benzophenone under 254 nm ultraviolet(UV)irradiation.Similar photo-grafting polymerization of acrylic acid(PAA)has only been studied before in the surface modification of polymer films.The graft ratio could be controlled by various reaction parameters,including irradiation time,benzophenone content,and monomer/polymer ratios.This photo-grafting reaction resulted in high graft ratio(graft ratio PAA/PLA up to 180%)without formation of homopolymers of acrylic acid.When the PAA/PLA graft ratio was higher than 100%,the resulting PLA-g-PAA polymer was found dispersible in water.The pros and cons of the photo-grafting reaction were also discussed. 相似文献
The ring-opening polymerization of L -lactide (LA) has been initiated by aluminum triflate (trifluoromethanesulfonate) in air using a simple glass tube at 100 °C without desiccation steps and stirring. It was found that the molecular weight of poly(lactic acid) (PLA) was increased by the addition of an alcohol as an initiator to the reaction mixture. The highest number averaged molecular weight, molecular weight distribution, and recovery of the obtained PLA at 100 °C for 6 h were 18,200, 1.20, and 73%, respectively. With the addition of a small percentage of alcohol and a long reaction time of the polymerization method with the re-addition of LA, PLA (ca. 80 wt%) with a higher molecular weight (ca. 30,000) initiated by the added alcohol was produced with PLA (ca. 20 wt%) with a lower molecular weight (ca. 2,000) initiated by impurities such as water, which exist in a monomer, initiator, or catalyst. 相似文献
Summary: In this paper, unsaturated groups were introduced into poly(lactic acid) (PLA) for fabricating PLA‐based nanospheres with carbon‐carbon double bonds as functional groups. The morphology dependencies on fabrication conditions, including the fabrication time as well as the stirring rate, were also investigated.
SEM‐EDX image of poly(lactic acid) diacrylate nanospheres after exposure to osmium tetroxide vapor for 14 h. 相似文献
PLA nanocomposites based on organically modified montmorillonites at 5% w/w loading were prepared by melt blending using an internal mixer and then degraded in a commercial compost. The addition of nanoclays was found to increase the PLA degradation rate, especially for the highest dispersed clay in the polymer matrix. Biodegradation by microorganisms isolated from the compost showed the bacterium Bacillus licheniformis as one of the responsible for PLA biodegradation in compost. It was also found that clays can influence the polymer bacterial degradation depending on their chemical structure and affinity of the bacterium towards the clay. 相似文献