Zhichao Liang

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.

Uracil as nucleating agent for bacterial poly[(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)] copolymers.

In this study, uracil has been introduced as the nucleating agent (NA) for bacterially synthesized poly[(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)] (PHBHHx) copolymers with HHx content of 5, 10, 18 mol-%, and poly(3-hydroxybutyrate) (PHB) homopolymer for the first time. Its effect was compared with the conventional NA of PHB, that is, boron nitride (BN), and two other naturally occurring pyrimidine derivatives, i.e., thymine and cytosine. The effects of uracil on the crystallization kinetics, melting behavior, spherulite morphology, and crystalline structure of PHBHHx and PHB were investigated by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray diffraction (WAXD). Uracil and BN exhibit the comparable nucleation efficiency on the crystallization of PHB, whereas uracil shows much more effective nucleation ability than BN for PHBHHx copolymers. With incorporation of 1 wt.-% uracil, PHBHHx with 0-10 mol-% HHx units can finish crystallization upon cooling at 10 degrees C x min(-1). The crystallization half-times (t(1/2)) of all the PHB and PHBHHx samples decrease significantly with presence of uracil. The crystallization rate of polymers further enhances with increase in uracil concentration. With addition of 1 wt.-% uracil, the t(1/2) value of PHBHHx with 10 mol-% HHx units melt-crystallizing at 80 degrees C decreases to approximately 4.0% of the neat polymer, and the nucleation density increases by 3-4 orders of magnitude. The incorporation of uracil has no discernable effect on the crystalline structure of PHBHHx, as evidenced by WAXD results. It was proposed that the nucleation mechanism of the uracil/PHBHHx (or PHB) system might be the epitaxial nucleation.