Crystallization of poly(lactic acid) nucleated with the sorbitol TBPMN

We report on the crystallization of poly(lactic acid) (PLA) films in the presence of a sorbitol-based nucleating agent; 1,2,3-tridesoxy-4,6:5,7-bis-O-[(4-propylphenyl) methylene]-nonitol (TBPMN). Dispersion of the nucleating agent was performed by solution-mixing and melt-blending. Crystallization behavior was studied by differential scanning calorimetry (DSC). The structure and morphology were characterized by wide-angle X-ray scattering (WAXS), optical imaging, and atomic force microscopy (AFM). A significant impact of the nucleating agent was observed at or above a threshold concentration of 2 wt%, which was assigned to the solubility limit of TBPMN. The degree of crystallinity reached up to 39.6 % with 2 wt% TBPMN. An influence of the dispersion method was observed on the peak temperature of crystallization (T-cp). While T-cp decreased slightly for solution-mixed films, a sigmoidal trend was noted for melt-blended samples. Under isothermal conditions at 100 degrees C, the crystallization half-time was lowered from 6.5 min to 1.0 min. Avrami analysis pointed to the formation of 2-dimensional or 3-dimensional crystalline domains; WAXS data revealed alpha- and alpha'-crystals, depending on the dispersing method. AFM imaging showed a nanosized fibrillar network of the nucleator within the PLA matrix, demonstrating that TBPMN acted as a soluble self-assembly nucleator.