接枝咪唑阳离子对聚乳酸结晶行为影响研究

Effect of Grafted Imidazolium Cations on Crystallization Behavior of Polylactide

聚乳酸(PLA)受到伽马射线辐照发生降解,加入1-烯丙基-3-甲基咪唑氯盐(AMIMCl)与PLA发生接枝反应,可缓解其降解,得到咪唑阳离子接枝聚乳酸(PLA-IM).核磁共振(NMR)和凝胶渗透色谱(GPC)测试结果表明和直接辐照样品(PLA-S)相比,PLA-IM端羟基含量显著下降,分子量为PLA-S的3倍.利用偏光显微镜(POM)和示差扫描量热仪(DSC)研究PLA-IM等温结晶行为,发现其结晶时仍呈现球晶形貌,晶体生长速度变化相对较小,主要仍受分子量的影响,在相同结晶温度时,PLA-S的晶体生长速度最快,其次是PLAIM,而PLA-L最慢.咪唑阳离子接枝使PLA-IM成核密度显著提高,致使结晶动力学加快,当结晶温度≥125℃时,PLA-IM的半结晶时间小于低分子量的PLA-S.成核能力的提高是导致PLA-IM结晶动力学加速的主要原因.与共混物中离子液体降低PLA成核作用不同,PLA链上接枝阳离子后,离子相互作用有利于成核时高分子链段发生局部有序排列,促进晶核数目的提高.

Polylactide （PLA） underwent molecular degradation when it was exposed to gamma ray radiation, the degradation was inhibited somehow in the presence of 1-allyl 3-methylimidazolium chloride （AMIMC1）. The grafting reaction of AMIMC1 on PLA chains occurred during the irradiation to obtain imidazolium cations grafted PLA （PLA-IM）. The molecular masses and the contents of end groups of the resultant PLA samples were measured by gel permeation chromatography （GPC） and nuclear magnetic resonance （1H-NMR）, respectively. Compared with the PLA sample irradiated without AMIMC1 （PLA-S）, PLA-IM possessed less hydroxyl end groups and higher molecular mass （3 times as high as that of PLA-S）. The nucleation and growth of crystalline during isothermal crystallization process were observed using polarized optical microscopy （POM）. The results revealed that PLA-IM possessed spherulitic morphology which was same as other PLA samples,and the growth rate of the spherulites was mainly dependent on the molecular mass of PLA samples, demonstrating highest growth rate for PLA-S, followed by PLA-IM and lowest for PLA-L at the same crystallization temperature. However, different from PLA-L and PLA-S, PLA-IM showed increased nucleation density in the whole experimental temperature range due to the presence of grafted imidazolium cations on PLA chains. Isothermal crystallization kinetics for the PLA samples was then investigated by differential scanning calorimetry （DSC） and analyzed using Avrami equation. The results showed that the crystallization rate of PLA-IM had improved obviously because of its higher nucleation density. When the isothermal crystallization temperature was higher than 125 ℃, the half crystallization time of PLA-IM was shorter than that of PLA-S with lower molecular mass. Different from the retardant effect of ionic liquids （ILs） on PLA nucleation in PLA/ILs blends,the ionic interaction between the imidazolium cations grafted on PLA chains facilitates the orderly arrangement of PLA chains and formation of nuclei in the crystallization process, which contributes to the acceleration of crystallization kinetics.