Control And Study Of Crystallization Behavior Of Biodegradable PLA

Polylactide(PLA), a kind of biodegradable material, is synthesized by renewable materials and has good biocompatibility property. It has been widely used in the fields of packing and medical application. However, low crystallization rate usually makes PLA amorphous state, which restricts the application of PLA. This study adopts three methods to modify the crystallization process of PLA to enhance PLA crystallization rate. Major results are as follows, which provides theoretical supports for the extended application of PLA.(1) Above glass transition temperature, stretching of amorphous PLA can bring about strain-induced crystallization, and this is mainly related to stretching rate and temperature.It is demonstrated in the study that, under the same stretching conditions, thermal annealing of a given polylactide before stretching is effective to facilitate strain-induced crystallization. The local order, which is generated through thermal annealing, contributes as physical cross-links that effectively reduces molecular relaxation and thus accelerates the occurrence of strain-induced crystallization.(2) It remains a huge task to get high degree of stereocomplexation in enantiomeric polylactides with high molecular weight. It is demonstrated here that gelation of enantiomeric polylactides in a nonvolatile ionic liquid is an effective way to overcome this dilemma. The ionic liquid facilitates the mobility and mutual diffusion of chains, and thus eliminates the kinetic barrier for stereocomplex formation to a large extent. It is responsible for the enhancement in stereocomplexation rate of enantiomeric polylactides with high molecular weight.(3) Crystallization kinetics and crystal morphology of PLA stereocomplex formedthrough melting re-crystallization of homocrystals is further studied. Small homocrystals with the α’-form and large ones with the α-form are produced through annealing of amorphous 1:1 poly(L-lactide)/poly(D-lactide) blends at 80 and 120 °C, respectively.Through melting re-crystallization process, small homocrystals with the α’-form is more conducive to facilitating the formation of stereocomplex than large ones with the α-form.In addition, rod-like stereocomplex crystals are produced through melting of small homocrystals with α’-form at adopted crystallization temperatures. While plate-like or spherulitic stereocomplex crystals are generated by melting of large homocrystals with theα-form. The difference in the crystal morphology of stereocomplex is correlated with the variation of nucleation density induced by annealing and crystallization temperatures.