Layered metal phosphonate reinforced poly(L-lactide) composites with a highly enhanced crystallization rate

Layered metal phosphonate, zinc phenylphosphonate (PPZn), reinforced poly(L-lactide) (PLLA) composites were fabricated by a melt-mixing technique. The nonisothermal and isothermal crystallization, melting behavior, spherulite morphology, crystalline structure, and static and dynamic mechanical properties of the PLLA/PPZn composites were investigated. PPZn shows excellent nucleating effects on PLLA crystallization. With incorporation of 0.02% PPZn, PLLA can finish crystallization under cooling at 10 degrees C/min. The crystallization rate of PLLA further increases with increasing PPZn concentration. Upon the addition of 15% PPZn, the crystallization half-times of a PLLA/PPZn composite decrease from 28.0 to 0.33 min at 130 degrees C, and from 60.2 to 1.4 min at 140 degrees C, compared to the neat PLLA. With the presence of PPZn, the nuclei number of PLLA increases and the spherulite size reduces significantly. Through analysis of the crystal structures of PLLA and PPZn, it was proposed that the nucleation mechanism of the PLLA/PPZn system is epitaxial nucleation. The incorporation of PPZn has no discernible effect on the crystalline structure of PLLA. Moreover, PPZn has good reinforcement effects on the PLLA matrix. The tensile strength of the composite is enhanced with the addition of a relatively small amount of PPZn (<5%). The tensile and storage moduli of composites increase with increasing PPZn loadings, and they respectively improve by 28% and 34% with the incorporation of a 15% PPZn filler, as compared to the neat PLLA.