Comparative Study Of Crystallization Kinetics Of Poly(Lactic Acid) Nucleated By Zinc Phenylphosphonate And Multiamide

Poly(lactic acid)(PLA) is a biodegradable plastics manufactured at a large industrial scale, with growing applications in the fields of films, bottles and fibers. However, one of the main features of PLA is intrinsic slow crystallization rate, resulting in amorphous state under practical processing, which limits its wider applications in the fields where thermal stability is required. The addition of nucleating agent is quite effective in enhancing the crystallization of PLA. It was reported that zinc phenylphosphonate (PPZn) and multiamide (TMC) were among the most excellent nucleating agents of PLA. In this thesis, we intend to perform a comparative study on the crystallization kinetics of PLA nucleated by PPZn and TMC, based on the following two considerations:first, the related study on crystallization kinetics is the key to realize the relationship between processing and properties of the materials; second, PPZn is a kind of inorganic metal salt, which will not change itself upon cooling process, whereas TMC is a kind of organic compound, which can be dissolved in PLA melt and self-organized into nuclei upon cooling, therefore, it is expected that PPZn and TMC will have different effects on the crystallization of PLA. The main results of this thesis are as follows:(1) The study of crystallization kinetics from the melt. PLA/PPZn and PLA/TMC samples were prepared by melt-mixing. The non-isothermal and isothermal crystallization behaviors of samples were investigated by differential scanning calorimetry (DSC). It was found that, at a given cooling rate, the crystallization peak temperatures of PLA/PPZn and PLA/TMC were much higher than that of neat PLA, moreover, the value of PLA/PPZn was higher than that of PLA/TMC; whereas under isothermal crystallization, the crystallization rate of PLA/TMC was higher than that of PLA/PPZn, and the trend was more apparent at higher crystallization temperature. It is probable that under non-isothermal crystallization, the TMC in PLA melt has not enough time to self-organized into nuclei, which has negative effect on its nucleation effective; whereas under isothermal crystallization, the TMC in PLA melt has enough time to self-organized into nuclei, presenting excellent nucleation effect. On the other hand, the non-isothermal crystallization kinetics of the specimens were analyzed by the Avrami, Tobin, and Ozawa models. The activation energies of the materials were also determined by the Kissinger method. It was found that the Avrami, Tobin and Ozawa models described the experimental data fairly well. The values of activation energies of PLA/PPZn and PLA/TMC were higher than that of neat PLA, indicating that the addition of PPZn and TMC hindered the transformation of molecular segments from PLA melts to the crystal growth surface, which leaded to the increase of activation energy values.(2) The study of cold-crystallization kinetics from the glassy state. The non-isothermal and isothermal cold-crystallization behaviors of samples were investigated by DSC. It was found that the addition of PPZn or TMC enhanced significantly both non-isothermal and isothermal cold-crystallization of PLA. Moreover, the nucleating effect of PPZn was better than that of TMC. The non-isothermal cold-crystallization kinetics of the specimens were also analyzed by the Avrami, Tobin, and Ozawa models. The activation energies of the materials were also determined by the Kissinger method. It was found that the Avrami and the Tobin described the experimental data fairly well, while the Ozawa model failed to describe the experimental results. The values of cold-crystallization activation energies of PLA/PPZn and PLA/TMC were higher than that of neat PLA.