Effects Of Poly(Lactic Acid) Stereocomplex On The Crystallization Behavior

Poly lactic acid is a polycrystalline polymer.L-lactic acid and D-lactic acid can form stereocomplex crystals(SC)when co-existing.In this paper,the crystallization behavior of PDLA/PLDLA blends are studied.PLDLA in PDLA/PLDLA blends are PLA random copolymers containing L-lactic acid of 89%,83%and 50%,respectively.The crystallization kinetics and crystal structure of the PDLA/PLDLA blends are characterized by polarized optical microscope(POM),differential scanning calorimetry(DSC),and wide angle X-ray diffraction(WAXD)to study the effects of SC on the crystallization kinetics.The semi-crystallization time(t_1/2)of the blends are shortened,which indicates that the overall crystallization rates are accelerated.Due to the nucleate promoting effect of SC,the number of spherulites in the field of view significantly increases.However,according to POM observation the radial spherulite growth rates G decrease in PDLA/PLDLA blends due to the presence of SC.SC may acts as obstacle and limites the diffusion of PDLA segments.The following thermodynamic analysis shows that the equilibrium melting point and zero growth temperature of PDLA in PDLA/PLDLA blends are not affected by the generation of SC,and the existence of SC leads to an increase in crystallization temperature during non-isothermal crystallization process.By adjusting the proportion of PLDLA component in PDLA/PLDLA blends,opposite trend on the kinetic behavior appears related to difference of PLDLA molecular chain structure.Further studies show that the SC crystallinity of the PLDLA/PDLA blends and the spherulite density in the field of view change with time obey totally different tendency.The stereocomplex crystallization is a progressive optimization process of local ordered structure,which eventually merges into a highly ordered crystal structure.