不同旋光结构聚乳酸体系冷结晶行为及其影响因素研究
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摘要
聚乳酸具有良好的生物相容性、可生物降解性以及良好的力学性能，在包装、农业、生物医学材料等领域得到了广泛的应用。不同旋光性的乳酸单体分子合成的聚乳酸有左旋聚乳酸(PLLA)、右旋聚乳酸(PDLA)和消旋聚乳酸(PDLLA)三种。不同旋光结构聚乳酸的结晶能力不同。对结晶性高分子而言，结晶能力的大小、结晶度和晶体形态是影响高分子材料制品性能的重要因素。因此本文系统地研究了不同旋光结构聚乳酸体系的冷结晶行为。主要研究结果如下：
(1)对比研究了物理老化和玻璃化温度(Tg)以上退火的PLLA样品的冷结晶行为。研究结果表明，物理老化和Tg以上退火PLLA的冷结晶能力都得到了增强，并且Tg以上退火样品的增强效果更明显。形态学观察发现，物理老化PLLA中的成核密高于Tg以上退火样品，但结晶动力学数据却显示物理老化PLLA的晶体生长速率有显著的下降。这是因为PLLA样品在物理老化过程中受限的链段运动能力在冷结晶过程没有完全回复，使得PLLA的晶体生长速率下降。而Tg以上退火对晶体的生长速率没有影响，成核密度的增加是结晶加快的唯一原因。
(2)研究了热处理对左旋聚乳酸及其与消旋聚乳酸1:1共混物的冷结晶行为的影响。结果表明，热处理后PLLA冷结晶速率的加快，是通过增强成核来实现的。冷却速率越慢，成核效应越显著，PLLA的结晶速率越快。相比于PLLA，PLLA/PDLLA共混物成核能力的变化对冷却速率不敏感。这与非结晶性PDLLA阻碍了热处理过程中局部有序结构的形成有关。
(3)研究了右旋聚乳酸与左旋聚乳酸共混物的冷结晶行为，探讨了高度有序PLA立体复合物的形成机理。研究发现PLLA/PDLA共混物的冷结晶温度越低，形成的PLA立体复合物的熔点反而更高。这是因为冷结晶温度越低，形成的PLA立体复合物的有序度越低。在低温冷结晶形成的无序PLA立体复合物，在高温会通过重要的固-固相转变进行结构重排，从而得到高度有序的PLA立体复合物，具有较高的熔点。而在高温结晶形成的有序PLA立体复合物，基本不进行结构重排，熔点相对较低。
Polylactides with good biocompatibility, biodegradability and mechanical properties,have been widely applied in packaging, agriculture and biomedical field. The synthesis ofthe lactic acid with different chiral structure yield three kinds of Polylactidehomopolymers(poly(L-lactide)(PLLA), poly(D-lactide)(PDLA) and poly(DL-lactide)(PDLLA)). The cold crystallization behavior of Polylactide is related to the chiral structrue.For the crystalline polymers, the crystallization ability, degree of crystallinity and crystalmorphology have a great influence on the performance of polymer. In this paper, the coldcrystallization behavior of PLLA, PLLA/PDLLA and PLLA/PDLA blends have beeninvestigated. Major results as follows:
(1) A comparative study of cold crystallization behavior in poly (L-lactide)(PLLA)annealed below and just above glass transition temperature (Tg) has been conducted. Theresults show that annealing benefits the cold crystallization process, which becomessignificant in PLLA annealed just above Tg. Surprisingly, morphological observationreveals high density nuclei in PLLA annealed below Tg, contrary to its relatively slowcrystallization kinetics. This unusual crystallization behavior in physically aged PLLAarises from the retarded crystal growth rate because of incomplete recovery of reducedsegmental mobility above Tg. In contrast, annealing just above Tg has little influence onthe crystal growth rate, and the increased nucleation density alone accounts for theaccelerated crystallization rate.
(2) Effect of thermal treatment on the cold crystallization of PLLA and PLLA/PDLLAblend have been studied. It has been evidenced that the acceleration in the coldcrystallization of PLLA arises from the enhanced nucleation after the thermal treatment.The slower is the cooling rate, the higher is the nucleation effect. As Compared to thePLLA, the change of nucleation ability with respect to cooling rate becomes lesssignificant in the PLLA/PDLLA blends, resulted from that amorphous PDLLA prevents the formation of local order during thermal treatment.
(3) The cold crystallization behavior of the PDLA/PLLA blends and the formationmechanism of highly ordered PLA stereocomplex have been investigated. Coldcrystallization of poly (L-lactide)/poly (D-lactide) blends at low temperatures results in theformation of disordered stereocomplex. Upon heating disordered stereocomplex enduressignificant reorganization into highly compact one before dominant melting via solid-solidtransition, which yields extremely high melting point. In contrast, ordered stereocomplexgenerated at high temperatures exhibits little reorganization, and is melted directly attemperatures lower than that of reorganized stereocomplex.
(1)对比研究了物理老化和玻璃化温度(Tg)以上退火的PLLA样品的冷结晶行为。研究结果表明，物理老化和Tg以上退火PLLA的冷结晶能力都得到了增强，并且Tg以上退火样品的增强效果更明显。形态学观察发现，物理老化PLLA中的成核密高于Tg以上退火样品，但结晶动力学数据却显示物理老化PLLA的晶体生长速率有显著的下降。这是因为PLLA样品在物理老化过程中受限的链段运动能力在冷结晶过程没有完全回复，使得PLLA的晶体生长速率下降。而Tg以上退火对晶体的生长速率没有影响，成核密度的增加是结晶加快的唯一原因。
(2)研究了热处理对左旋聚乳酸及其与消旋聚乳酸1:1共混物的冷结晶行为的影响。结果表明，热处理后PLLA冷结晶速率的加快，是通过增强成核来实现的。冷却速率越慢，成核效应越显著，PLLA的结晶速率越快。相比于PLLA，PLLA/PDLLA共混物成核能力的变化对冷却速率不敏感。这与非结晶性PDLLA阻碍了热处理过程中局部有序结构的形成有关。
(3)研究了右旋聚乳酸与左旋聚乳酸共混物的冷结晶行为，探讨了高度有序PLA立体复合物的形成机理。研究发现PLLA/PDLA共混物的冷结晶温度越低，形成的PLA立体复合物的熔点反而更高。这是因为冷结晶温度越低，形成的PLA立体复合物的有序度越低。在低温冷结晶形成的无序PLA立体复合物，在高温会通过重要的固-固相转变进行结构重排，从而得到高度有序的PLA立体复合物，具有较高的熔点。而在高温结晶形成的有序PLA立体复合物，基本不进行结构重排，熔点相对较低。
Polylactides with good biocompatibility, biodegradability and mechanical properties,have been widely applied in packaging, agriculture and biomedical field. The synthesis ofthe lactic acid with different chiral structure yield three kinds of Polylactidehomopolymers(poly(L-lactide)(PLLA), poly(D-lactide)(PDLA) and poly(DL-lactide)(PDLLA)). The cold crystallization behavior of Polylactide is related to the chiral structrue.For the crystalline polymers, the crystallization ability, degree of crystallinity and crystalmorphology have a great influence on the performance of polymer. In this paper, the coldcrystallization behavior of PLLA, PLLA/PDLLA and PLLA/PDLA blends have beeninvestigated. Major results as follows:
(1) A comparative study of cold crystallization behavior in poly (L-lactide)(PLLA)annealed below and just above glass transition temperature (Tg) has been conducted. Theresults show that annealing benefits the cold crystallization process, which becomessignificant in PLLA annealed just above Tg. Surprisingly, morphological observationreveals high density nuclei in PLLA annealed below Tg, contrary to its relatively slowcrystallization kinetics. This unusual crystallization behavior in physically aged PLLAarises from the retarded crystal growth rate because of incomplete recovery of reducedsegmental mobility above Tg. In contrast, annealing just above Tg has little influence onthe crystal growth rate, and the increased nucleation density alone accounts for theaccelerated crystallization rate.
(2) Effect of thermal treatment on the cold crystallization of PLLA and PLLA/PDLLAblend have been studied. It has been evidenced that the acceleration in the coldcrystallization of PLLA arises from the enhanced nucleation after the thermal treatment.The slower is the cooling rate, the higher is the nucleation effect. As Compared to thePLLA, the change of nucleation ability with respect to cooling rate becomes lesssignificant in the PLLA/PDLLA blends, resulted from that amorphous PDLLA prevents the formation of local order during thermal treatment.
(3) The cold crystallization behavior of the PDLA/PLLA blends and the formationmechanism of highly ordered PLA stereocomplex have been investigated. Coldcrystallization of poly (L-lactide)/poly (D-lactide) blends at low temperatures results in theformation of disordered stereocomplex. Upon heating disordered stereocomplex enduressignificant reorganization into highly compact one before dominant melting via solid-solidtransition, which yields extremely high melting point. In contrast, ordered stereocomplexgenerated at high temperatures exhibits little reorganization, and is melted directly attemperatures lower than that of reorganized stereocomplex.
引文
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