PLA Crystallization Behavior and Nucleating Agents

PLA has been ciurently received a lot of attention for commodity applications because of environmental degradability and sustainable biomass resources. The crystal structure, crystallization behavior and even nucleating agents can greatly determine the mechanical and thermal properties of PLA. 

PLA has four optical enantiomers PLLA, PDLA, PDLLA and meso- PLhK. The PLLA is most commonly used that has three crystal structures a, P and y. 

Lotza et al. [214] found that a-phase PLLA was formed when growing epitaxially on hexamethylbenzene (HMB) at higher temperature of 155X1 , which had the same unit-cell parameters as that observed by Santis and Kovacs [201]. While in lower temperature of 140 °C, the y-phase PLLA was formed, the structure of which is close to P-phase observed by Brizzolara et al. [220], the difference is that two anti-parallel helices are packed in an orthorhombic unit-cell with parameters of a=0.995nm, = 0.625 nm, c = 0.88nm. The structure of 

PLLA is semi-crystalline polymer. The glass transition temperature of PLLA is 50-60°C, the crystallization temperature is 90-140°C, and the melting temperature 

As for isothermal crystallization of PLLA, the spherulite growth rate is determined by crystallization temperature and molecular weight. As the crystallization temperature rises, the spherulite growdi rate first increases and then decreases. As the molecular weight of PLLA increases, the spherulite growth rate decreases. The maximum of spherulite growth rate occurs at 120°C and isothermal overall crystallization rate occurs at 105°C [221]. 

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