半结晶性聚合物剪切场下的结晶动力学及结晶形貌研究

发布时间：2018-05-10 12:44

本文选题：剪切场 + 结晶动力学　；参考：《中国科学技术大学》2017年博士论文

【摘要】：聚合物在实际加工过程中必然受到各种剪切场的作用,导致聚合物结晶动力学及结晶形貌与静态条件下的巨大差异,从而严重影响聚合物制品的最终性能。为使目前的剪切场结晶理论与实际加工过程更好的结合,有必要将研究对象扩展到具有应用价值的聚合物材料体系中。本工作通过选取除了纯线性聚合物之外的体系为研究对象,主要通过流变法和偏光显微镜搭配剪切热台系统研究不同聚合物体系中长链支化、预存在结构、熔体-基底界面、共混物等在剪切场作用下结晶动力学和结晶形貌演变过程。主要研究内容包括以下四个方面:1.通过在线性聚乳酸(linear PLA)中加入三官能度单体,三羟甲基丙烷三丙烯酸酯(TMPTA),在利用伽马射线辐照的办法,制备了一系列具有不同支化度的长链支化聚乳酸(LCB PLA)样品。动态频率扫描测试表明,LCB PLA样品的松弛时间谱被拓宽,尤其是在最长松弛时间区域。将结晶中的LCB PLA模型化为类悬浮体系,可以获得半结晶时间。对于LCB PLA,结晶速率的加速效应会在一定的剪切时间下达到饱和,且LCB PLA的支化程度越高,饱和剪切时间则越低。通过流变得到的空间填充率利用Avrami方程计算成核密度,也出现了成核密度随剪切时间饱和的现象。这种足够长剪切时间下的饱和现象与偏光显微镜观察到的结晶形貌从球晶到取向晶型的转变有着对应的关系。LCB PLA取向晶体形貌的形成机理与拉伸后的长链支化分子链网络被抑制的松弛特性有关。不同支化度的LCB PLA结晶速率的大幅提高和结晶形貌的转变对于提高PLA的加工效率,以及对PLA高性能制品的生产提供了潜在的理论指导。2.从形态学角度研究了预存在球晶和剪切场对iPP的结晶行为的共同作用。对已存在球晶的iPP熔体施加剪切场后,结晶形貌发生了从球晶到另外两种独特形貌的转变。这两种形貌的产生与球晶在熔体-基底表面不同的相互作用有关。较低剪切温度时,球晶与基底有很好的依附作用,导致球晶周围剪切强度比本体内高,且固定在球晶表面的分子链自由端更容易发生高度的伸展和取向,因此形成了密集堆砌的晶核聚集体为中心结构。而提高剪切温度,球晶的高程度熔融导致其在剪切中发生与熔体-基底的相对移动,诱导了基底表面大量较为疏散的异相成核,且生长为为微小球晶为聚集体的中心结构。随着结晶时间的增加,在该两种中心结构的外沿发生了片晶的大量生长,最终形成了两种不同结构的柱形形貌。该工作对聚合物在加工中所面临的熔体-基底作用,预存在晶体或其他添加的粒子等对聚合物结晶形貌的影响具有一定的指导意义。3.主要从形态学角度研究了聚乳酸/聚己内酯(PLA/PCL)共混物这一典型的生物可降解共混物体系在剪切场下的结晶行为。首先使用DSC研究了其基本的热性能,证实了 PCL相能显著提升PLA的冷结晶行为。流变研究了PLA/PCL共混物基本的粘弹性行为,表征了该体系PCL相界面的松弛效应对粘弹性行为的影响。静态结晶行为表明PCL的加入对PLA的成核和晶体生长并未产生较大影响。然而施加剪切场之后,发现随着剪切强度的提高或结晶温度的降低,PLA/PCL共混物发生了从球晶形貌到取向的串珠状形貌的转变。虽然不同剪切条件下PCL相均发生了取向,然而PLA取向结构的形成则主要是由PLA的成核和PCL相在剪切后的松弛相互竞争从而导致PLA在PCL相界面出诱导成核所影响。虽然取向排列的串珠状形貌并非shish-kebab,但是对加工该生物可降解共混物材料或其他共混体系的聚合物具有一定的指导意义。4.研究了不同螺杆转速制备的左旋聚乳酸/右旋聚乳酸/乙烯-甲基丙烯酸缩水甘油酯共聚物(PLLA/PDLA/EGMA)共混物在剪切场下的结晶行为。首先使用DSC研究了其基本的热性能,证实了螺杆转速的提高能显著提升EGMA对PLA的冷结晶行为的促进作用。流变研究了 PLLA/PDLA/EGMA共混物基本的粘弹性行为,表征了该体系SC晶体和EGMA相界面的松弛效应对粘弹性行为的影响。施加剪切场之后,发现不同螺杆转速制备的PLLA/PDLA/EGMA共混物形成了不同的结晶形貌。较低转速下,结晶形貌为分散均匀的小尺寸球晶,而当转速提高后,结晶形貌转变为不均一的团状晶体甚至为云雾状的晶体形貌。该现象受SC晶体,EGMA相区尺寸和EGMA与基体的界面作用共同控制。本论文主要创新点:1)探索了长链支化聚乳酸中不同的支化程度对剪切场下结晶动力学和结晶形貌的影响,以及通过流变手段将结晶动力学的变化与结晶形貌建立了联系,为该生物可降解材料的加工以及应用范围的拓展提供了理论支持。2)强调了聚合物熔体中预存在晶体对剪切场下结晶行为的影响,以及聚合物-基底界面所扮演的角色。3)对聚乳酸不相容共混物的剪切结晶行为进行了系统研究,发现了不相容共混物在剪切场下形成了区别于shish-kebab的独特的取向晶体形貌,为共混物的加工过程中的形态结构演变提供了直观的了解。
[Abstract]:In actual processing, polymers are bound to be affected by various shear fields, leading to the huge difference between the crystallization kinetics and the crystalline morphology and static conditions of the polymer, which seriously affects the final properties of the polymer products. In this work, the system of polymer materials which is of applied value has been developed. By choosing a system other than pure linear polymer as the research object, the long chain branching, the pre existence structure, the melt base interface, the blends, etc. are studied by the rheological method and the polarizing microscope with the shear thermal platform system. The main contents include four aspects as follows: 1. by adding three functional monomers, three hydroxymethyl propane three acrylate (TMPTA) in linear polylactic acid (linear PLA), a series of long chain branched polylactic acid with different branching degrees was prepared by gamma ray irradiation. (LCB PLA) samples. The dynamic frequency scanning test shows that the relaxation time spectrum of the LCB PLA sample is widened, especially in the longest relaxation time region. The LCB PLA model in the crystallization is transformed into a class suspension system, and the semi crystallization time can be obtained. For LCB PLA, the accelerating effect of crystallization rate should be saturated at a certain shear time and LCB PLA. The higher the branching degree is, the lower the saturated shear time is, the nucleation density is calculated by the Avrami equation, and the nucleation density is saturated with the shear time. The saturation phenomenon in the long enough shear time and the transformation of the crystalline morphology observed by the polarizing microscope from spherulite to orientation crystal are observed. The formation mechanism of the corresponding.LCB PLA oriented crystal morphology is related to the relaxation characteristics of the long chain branched molecular chain network after stretching. The significant increase in the crystallization rate of LCB PLA and the transformation of the crystalline morphology of the different branching degrees provide a potential reason for improving the processing efficiency of PLA and the production of PLA high performance products. The common effect of pre existence of spherulites and shear fields on the crystallization behavior of iPP was studied from the morphological point of view. After applying the shear field to the iPP melts of the existing spherulites, the crystallization morphology changed from spherulites to the other two unique morphologies. The production of the two morphologies was different from that of the spherulites on the melt substrate surface. When the shear temperature is lower, the spherulite and the substrate have a good dependence on the substrate. The shear strength around the spherulites is higher than that in the body, and the free ends of the molecular chain fixed on the surface of the spherulite are more prone to high extension and orientation. Therefore, the core aggregates of dense stacked nuclei are formed as the central structure. The shear temperature and the high degree of spherulites are improved. Melting leads to the relative movement of the melt and substrate in the shear, which induces a large number of dispersed hetero nucleates on the surface of the substrate, and grows to be the central structure of the small spherulites. With the increase of the crystallization time, the growth of the two kinds of central structures in the outer edge of the central structure has resulted in the formation of two different junctions. This work has a certain guiding significance for the influence of melt basement, pre existence of crystals or other particles on the crystalline morphology of polymers in the process of processing..3. mainly studies the biodegradable blend of polylactic acid / polyhexyllactone (PLA/ PCL) blends from the morphological point of view. The crystallization behavior of an object under the shear field. First, the basic thermal properties were studied by DSC. It was confirmed that the PCL phase could significantly improve the cold crystallization behavior of PLA. The rheological behavior of the PLA/PCL blends was studied by rheology. The effect of the relaxation effect on the PCL phase of the system on the viscoelastic behavior was characterized. The static crystallization behavior indicated that the PCL was the static crystallization behavior. The addition has no significant effect on the nucleation and crystal growth of PLA. However, after the shear field was applied, it was found that with the increase of shear strength or the decrease of crystallization temperature, the PLA/PCL blends changed from spherulite morphology to orientation. Although the orientation of the PCL phase occurred under different shear conditions, the PLA orientation structure was observed. The formation is mainly caused by the nucleation of PLA and the relaxation of the PCL phase after shear, resulting in the effect of PLA on the induced nucleation of the PCL phase interface. Although the bead like morphology of the orientation arrangement is not shish-kebab, it has a certain guiding significance for the processing of the biodegradable blends or other blends. The crystallization behavior of the blends of L-polylactic polylactic acid / polylactic lactic acid / ethylene glycidyl methacrylate copolymer (PLLA/PDLA/EGMA) prepared by different screw speeds was studied under the shear field. First, the basic thermal properties were studied by using DSC. It was proved that the improvement of screw speed could significantly enhance the cold crystallization behavior of EGMA to PLA. The basic viscoelastic behavior of PLLA/PDLA/EGMA blends was studied by rheology. The effect of relaxation effect on the interface between SC crystal and EGMA phase on the viscoelastic behavior was characterized. After applying the shear field, it was found that the PLLA/PDLA/EGMA blends prepared at different screw rotational speeds formed different crystalline morphology. When the rotational speed is increased, the crystalline morphology changes into uneven mass and even cloud like crystal morphology. This phenomenon is controlled by the interface of SC crystal, EGMA phase size and EGMA with the matrix. The main innovation of this paper is: 1) the different branching degree of the long chain branched polylactic acid was explored. The influence of crystallization kinetics and crystal morphology under the tangent field, and the relationship between the change of crystallization kinetics and the crystalline morphology by rheological means, which provide theoretical support for the processing of the biodegradable material and the extension of its application range,.2) emphasized the effect of the pre existence of the crystals on the crystallization behavior under the shear field. As well as the role of the polymer substrate interface (.3), the shear crystallization behavior of poly (lactic acid) incompatible blends was systematically studied. It was found that the incompatible blends formed a unique orientation crystal morphology different from shish-kebab under the shear field, providing a visual understanding of the morphological structure evolution of the blends during the process of processing.

【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：O631.13

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