| Poly（lact ic acid） （PLA） is a biodegradable, biocompatible, nontoxicaliphatic polyester derived from renewable resources, which has beenexpected to be substitutes of conventional plastics. However, broadsubstitution of PLA for conventional plastics is thwarted by its some defectssuch as high cost, poor heat resistance, slow crystallization rate and brittlebehavior under impact loads.In the synthesis methods of PLLA, direct polycondensation of lactic acidis the least expensive route, but it is difficult to obtain high molecular weightPLA. To improve the properties of PLLA and reduce the cost of production,we have improved on the traditional direct polycondensation and developed anew progress to synthesize high molecular weight and valuable PLLA, inwhich the poly（L-lactic acid）（PLA） was synthesized through the meltpolycondensation of L-lactic acid with decompression step by step. To improve the properties of PLLA , PLLA-PEG blends were prepared bymethod of solvent casting. The works were summarized below:1. During melt polycondensation with decompression step by step, the effectof the reaction condition, such as reaction time, temperature and catalyst’sconcentration, on the molecular weight of PLLA products was studied.The results indicated that the melt polycondensation with decompressionstep by step was an effective method to prepare relative high molecularweight PLLA, and the molecular weight of PLLA reached a maximumvalue under an optimum reaction condition as following, LA wasesterified for 7 hours, and reducing the pressure to 40-70Pa for 7 hours,then polycondensation for 45 hours and the whole reaction was carried outat 180℃and catalyst’s concentration was 5wt%. FTIR（Fourier transforminfrared）, 1H NMR（nuclear magnetic resonance） and 13C NMR were usedto characterize the structure of PLLA, and the results showed that PLLAwas synthesized successfully by melt polycondensation withdecompression step by step.2. The Differential Scanning Calorimetry（DSC） spectrum exhibited the glasstransition temperature （Tg）, melt temperature （Tm）, melt enthalpy （△Hm）and crystallinity of PLLA. The polarized optical microscopy （POM） wasused to observe the crystalline morphology of PLLA and the resultsshowed that PLLA exhibited a spherulitic morphology. Thermaldecomposition kinetics analysis from Thermogravimertric Analysis （TGA） data indicated that one-order kinetics was followed for PLLA.3. The blend film of PLLA/PEG400 was obtained by solvent casting. TheDSC exhibited that the Tg and Tmdecreased with the increasing PEG400content, while△Hm and crystallinity increased. Mechanical properties ofblends were also studied and the results showed that thePLLA/PEG400blends because very flexible when thePLLA/PEG400=70/30, the elongation at break of the material was up to170％. The contact angle measurements showed that blends werehydrophilic and the degradation test indicated the degradable rate ofblends was faster than that of PLLA. |
PDF Full Text Request