The Crystallization, Phase Separation Behavior And Properties Of Polylactic Acid Blends

Polylactide(PLA),a kind of biodegradable and bio-based macromolecule,has great potentials to replace petroleum-based polymers.PLA has good biocompatibility,mechanical properties and has been widely used in the field of medicine and packaging materials.In order to expand the application of PLA,chemical modification,physical modification and other blending methods have been used to improve the performance of PLA.To extand the application of PLA,crystallization and phase separation in PLA blends should be studied.In this thesis,the following three aspects are investigated: formation,growth process and molecular structure of PLA stereocomplex mesophase;phase separation process of PLA/ionic liquid blends by dielectric spectroscopy;thermally induced phase separation and permeability of the PLA/ionic liquid blends.The results are listed as following:(1)Stereocomplex mesophase is disclosed by annealing of 1:1poly(L-lactide)(PLLA)/poly(D-lactide)(PDLA)blends just above glass transition temperature.Its structure is analogue to that of stereocomplex crystals,as a result of weak molecular interactions between PLLA and PDLA chains.In the limited temperature range formation of stereocomplex mesophase is enhanced with annealing temperatures;and its growth is well illustrated by Avrami formula with an index of about 3.Upon heating stereocomplex mesophase begins to melt at temperatures above 80?,followed by re-crystallization into homocrystals.The stereocomplex mesophase retards homocrystallization to some extent,since individual PLLA and PDLA chains need to be released from stereocomplex mesophase.(2)The presence of an ionic liquid in the plasticized PLA contributes to enhancement of dielectric permittivity due to its electrical conductivity.Phase separation,induced by thermal annealing and mechanical stretching,respectively,canfurther enhance dielectric permittivity.It offers opportunities to monitor phase separation in the ionic liquid plasticized PLA by dielectric spectroscopy.Phase separation is increased with the extent of crystallization in the PLA matrix upon thermal annealing,so does dielectric permittivity.Formation of mesophase and crystals is strain dependent during mechanical stretching of the plasticized PLA.Correspondingly,dielectric permittivity and phase separation also show a dependence on the strain.(3)The phase separation process and moisture permeability of PLA/ionic liquid blends were studied.The PLA/ionic liquid blended membranes were prepared by annealing at 30? and thermally induced phase separation at different temperature,respectively.With aid of the SEM measurements,it's found that the extent of the crystallization induced phase separation increases with the increase of the ionic liquid content after annealing at 30?.At the same time water vapor became better,and the best water vapor transmission rate was 864.1g/m2*24h of the sample of 60 wt% ionic liquid.In the film of thermally induced phase separation at different temperature,the lower the heat treatment temperature,the higher water vapor transmission rate was.And the best water vapor transmission rate was 882.2g/m2*24h of the sample with 40wt% ionic liquid treated at 80?.