Preparation Of Poly (L-Lactic Acid) By Melt Polycondensation And The Effect Of γ-Radiation On It

Polylactic acid（PLA） has been more and more emphasized as a biodegradable material.It has two common polymerization methods:the ring opening and direct polycondensation.Although higher relative molecular weight of PLA is easily obtained by the ring opening polymerization,the process is more complicated.While the direct polycondensation is simple,but it is more difficult to get PLA with higher relative molecular weight.In order to improve relative molecular weight and radiation stability of PLA,in this work,Poly（L-Lactic acid）（PLLA） was synthesized via melt condensation polymerizing with L-lactic acid as raw material,and the effect of reaction time,reaction temperature,type and amount of catalysts on the relative molecular weight of the PLLA had been investigated firstly;then PLLA-TMPTA （trimethylolpropane trisacrylate） systems with the synthetic PLLA as raw material and TMPTA as a sensitizer were prepared by melt blending with extruder and reactive still（This method hasn’t been reported,to our knowledge） respectively.By ⁶⁰Co-γray with lower radiation doses（D）,The PLLA-TMPTA systems were radiated（D≤25kGy, which is necessary for the food radiation sterilization and to be a common dose for medical supplies of radiation sterilization） and the effect of theγ-radiation on the relative molecular weight and properties of the PLLA was investigated.The main results are as follows:（1） At improving relative molecular weight of the PLLA in the experimental extent,SnCl₂·2H₂O had a better catalytic result than Sn（Oct）₂,single used as the catalyst.The viscosity-average molecular weight（（?）η） of PLLA synthesized was about 22,400 under the condition with 0.5wt%SnCl₂·2H₂O at 160℃for 19 hours. While at the same reaction temperature and time,the（?）ηof PLLA was about 27,500 with the composite catalyst SnCl₂·2H₂O/Sn（Oct）₂（0.3wt%SnCl₂·2H₂O,0.2wt% Sn（Oct）₂）.（2） In the experimental radiation dose（0-25kGy）,（?）ηof the PLLA decreased with increasing the dose,and the（?）ηdecreased about 23%of the unradiation PLLA at 25kGy.（3） With theγ-radiation treatment,（?）ηwas increased at first and then decreased with the increase of radiation dose at both the melt blending PLLA-TMPTA with the extruder and reactive still.Compared with reactive still,melting-blending technique had a stronger effect of the sensibilization of TMPTA to PLLA in increasing the relative molecular weight of the PLLA and the proper condition was:2wt%TMPTA in the PLLA-TMPTA system,melt-extrud blending temperature at 120℃and radiation dose at 15kGy.Through proper radiation and technological condition,the average molecular weight can be improved observably.（4） Gel Permeation Chromatography（GPC） analysis showed that the maximum value of weight-average molecular weight（（?）w） of the PLLA obtained by melt polycondensation was 32,300,while it reached 46,900 with adding TMPTA andγ-radiation treatment.The polydispersity index of PLLA was less than 2.（5） The Infrared Spectrum（IR） analysis results demonstrated that the structure of polymerisate was in keeping with PLA and it wasn’t remarkable changed byγ-radiation.While according to thermo gravimetric analysis（TG）,the thermal decomposition temperature of PLLA was increased.（6） The thermal gravimetric analysis（DSC）,x-ray diffraction（XRD） results shown that PLLA synthesized in this paper was non-crystal polymers,so were polymers which treated byγ-radiation.