镧配合物对等规聚丙烯等温结晶性能的影响

利用Avrami方程和Lauritzen-Hoffman结晶动力学理论研究了一种镧配合物LaC对等规聚丙烯(iPP) 等温结晶行为的影响. 差示扫描量热法(DSC)和X射线衍射技术(XRD)研究表明, LaC的加入并未改变iPP的结晶形态, 但LaC的存在能提高体系的结晶度并显著加速iPP的结晶过程. 在130 ℃进行等温结晶时, 含0.5%LaC(质量分数)的iPP与纯iPP相比, Avrami指数n值无显著差异, 但前者总的结晶速率常数k值比后者提高约4倍,而半结晶时间t1/2值减少到后者的62%, Avrami方程分析结果表明LaC的存在主要起到增加晶核的作用. 利用Lauritzen-Hoffman结晶动力学理论, 通过对iPP在121 ℃、124 ℃、127 ℃和130 ℃下等温结晶的数据进行分析可知, 加入0.5%的LaC后, 体系的成核常数kg从纯PP的3.3×105 K2增加到PP/LaC的3.8×105 K2, 而结晶生长时大分子在垂直于分子链方向折叠的界面自由能σe从纯PP的0.223 J·m-2降低到PP/LaC的0.154 J·m-2, 表明LaC在iPP结晶过程中不仅起到增加晶核的作用, 同时使大分子链更易排入晶格, 即起到促进结晶成长的作用.

Effect of Lanthanum Complex on Isothermal Crystallization Kinetics of Isotatic Polypropylene

The influence of a lanthanum complex (LaC) on the crystalline characteristics of isotactic polypropylene (iPP) has been investigated using X-ray diffractometer (XRD) and differential scanning calorimeter (DSC). The results of XRD measurements and the melting behavior examined by DSC show that the additive leads to an increase in crystallinity, whereas no obvious change is observed on the crystal modifications. The kinetics of isothermal crystallization of pure iPP (PP) and iPP containing 0.5% of the additive (PP/LaC) have been studied based on the analysis of the DSC experimental data by Avrami equation and Lauritzen-Hoffman theory. The results of Avrami equation analysis reveal that the additive accelerates nucleation and dramatically increases the overall rate of crystallization. The half crystallization period (t1/2) of isothermal crystallization at 130 ℃ decreases from 12.8 min for PP to 7.9 min for PP/LaC. On the other hand, the analysis of Lauritzen-Hoffman theory reveals that the introduction of LaC not only increases nucleus number and accelerates nucleation, but also accelerates the growth rate of crystallization. The nucleation constant (kg) increases from 3.3×105 for PP to 3.8×105 K2 for PP/LaC, while the surface free energy of the developing crystals(σe) decreases from 0.223 J·m-2 to 0.154 J·m-2. All these results suggest that LaC in this system considerably promote crystallization and act as an effective nucleating agent.