Study On The Branching Modification Of Polylactic Acid

Polylactic acid(PLA),also known as polylactide,is a high molecular polymer that is synthesized from lactic acid(LA).It has good environmental friendliness,biocompatibility,processability,and energy economy.However,it is difficult to process the film and foam because of its low melt strength.Which limits its application in the blow molding,thermoforming,and foaming etc.It has been reported that the introduction of long-chain branching structures(LCB)into the PLA backbone has been shown to improve the melt rheological properties and processability of PLA significantly.At present,there are mainly two methods have been devised to produce LCBPLA: reactor method and post-reactor method.The transesterification reaction in the post-reactor process is a type of reaction that is widely present in the melt processing of polyester polymers.However,the transesterification reaction between PLA and small molecule polyfunctional ester monomers is rarely reported.In this paper,LCBPLA was prepared by melt branch modification of PLA in the presence of the environment-friendly nano-ZnO etc.as the accelerator and three-functional monomer.Infrared spectroscopy(FTIR),triple detector gel permeation chromatography(GPC),nuclear magnetic resonance(1H NMR),rotational rheometry,capillary rheometry,differential scanning calorimetry(DSC),X-ray diffraction,transmission polarization microscopy(POM),scanning electron microscopy(SEM)and other detection methods,were used to investigate the reaction mechanism and various properties of LCBPPLA specifically.The main contents of the study include the following three parts:1.Melt-branching modification of linear polylactic acid(PLA)was carried out in the presence of nano-zinc oxide(nano-ZnO)as the accelerant and tri-functional monomer pentaerythritol triacrylate(PETA)as the branching centre.Results showed that PLA with long branching chains without gel was obtained and the reaction mechanism of the melt branching reaction is transesterification reaction.The addition of nano-ZnO could promote the branching reaction of PLA and PETA obviously,and in addition,the molecular weight distribution became wider compared with the virgin linear PLA.A shoulder generated owing to the introduction of long branches in the molecular weight distribution curve.The branching degree of modified PLAs decreased with the increase of accelerant content.When the accelerator content is 0.2 phr,the modified PLA sample exhibited lower phase angle and star branching characteristics and more apparent strain hardening behavior.After this melt branching reaction,the melt strength of PLA was enhanced from 0.02 N to 0.11 N and its melt possessed obvious strain hardening.Differential scanning calorimetry(DSC)results revealed that faster crystallization rate and lower crystallization onset temperature in LCBPLA than that of PLA,which is due to the presence of LCB,that increases the entanglement between the molecular chains and promoting the crystallization of the system.Although the introduction of long-chain branches destroyed the regularity of the molecular chain,its nucleation effect promoted the crystallization of PLA greatly,resulting in the increase of crystallinity and the decrease of spherulites growth rate.The modified PLAs are the mixtures of linear chains and branched macromolecules,because a double melting peak can be seen in the DSC curve,and the branching parts whose completeness was poor would melt first.X-ray diffraction(XRD)analysis showed that the crystal form of this LCBPLA did not change compared with linear PLA while the completeness of crystal was decreased.Thermal platform polarized optical microscopy(POM)analysis exhibited that faster nucleation rate in LCBPLA than that of PLA,because it was easy to nucleate for LCBPLA due to the entanglements of the branched macromolecules.2.The LCBPLA was promoted by different accelerators in the reaction of PETA and PLA system.A variety of rheological methods such as complex viscosity curve,Cole-Cole plot and Han plot were uses to distinguish between PLA and LCBPLA effectively.Compared with PLA,the melt strength of LCBPLA was improved significantly,and the addition of nano-SnO2 promoter was increased from 0.02 N to 0.16 N for pure PLA.Branched PLA samples had a lower crystallization onset temperature,a faster crystallization rate,and a higher degree of crystallinity than linear pure PLA.Modified PLA was a mixture of branched and linear structures.The results of POM showed that the sphaerocrystals of LCBPLA were refined and have a higher nucleation density.The order of the promoting activity of various nano-accelerants for PETA and PLA reaction systems was: nano-SnO2,nano-ZnO,nano-Ti O2,nano-Fe2O3,nano-Al2O3.3.After testing the mechanical properties of the polylactic acid and the branched samples,it was found that the tensile strength and impact strength of the long-chain branched modified PLA were respectively increased by 6.75% and 17.01% compared with the linear pure PLA.However the bending strength was decreased by 6.91%.In addition,the Vicat softening temperature of the long-chain branched modified PLA was increased by 2.6? compared to the linear pure PLA.The dynamic mechanical properties test showed that the storage modulus and loss modulus of the long-chain branch modified PLA were higher than those of the linear pure PLA,and the temperature dependence of the branched sample was more severe than that of the linear pure PLA.The result of extrusion foaming showed that the long-chain branched modified PLA had more cells than the linear pure PLA,and the cells' s size was smaller and the distribution was more uniform.The result of injection foaming showed that compared with the linear pure PLA,the cells formed by the long-chain branched PLA rarely had the "bulk" phenomenon,and the cells' s density was larger,the size was smaller,and the distribution was more uniform.