【作者】 王勇；

【导师】 傅强；

【作者基本信息】 四川大学 ， 高分子化学与物理， 2004， 博士

【摘要】 研究高分子成型加工中的形态控制问题,在成型过程中引入特殊加工方法,改变聚合物的凝聚态结构,从而改变其性能是优化材料性能和用途的经济而实用的方法,是目前国际上高分子科学研究的热点之一,已成为我国高分子材料科学与工程的一个重要研究课题。本论文采用在注射成型过程中引入剪切应力场的作用,使聚合物及其共混物在往复剪切应力作用下逐渐冷却固化成型,通过剪切应力的作用控制基体和分散相在成型过程中的形态发展,相容与相分离、相转变、取向和结晶等凝聚态结构,并且通过熔体冷却固化将应力对共混物微观形态结构的影响冻结下来,进而研究微观形态结构变化对共混物材料宏观力学性能的影响。通过上述方法制备了几种聚烯烃共混物,系统研究了剪切应力作用下聚烯烃及其共混物取向和结晶,相容与相分离、相转变等形态结构的变化,以及共混体系性能的变化。本文还详细研究了剪切应力诱导橡胶粒子形变和取向,及其共混物 PP/EPDM 在冲击断裂过程中的断裂行为。主要研究结果如下:(1)研究了剪切对注射成型制品各向异性的影响。发现在剪切应力作用下,熔体经注塑机注射入模具型腔后,熔体遇到处于室温的模具,皮层首先快速冷却固化;随后施加的剪切应力推动熔体在型腔中往复流动,受到持续的剪切应力作用,熔体逐渐冷却固化成型,应力对形态结构的影响一并冻结下来,即形成剪切层;剪切停止后,试样进一步冷却,应力的影响可以通过聚合物分子链的松弛得以消除,形成明显的芯层结构。后续的实验证明,剪切作用下固化成型的动态试样不同区域表现出不同的形态结构。(2)对聚烯烃(HDPE、LDPE、LLDPE、PP 等)及其共混物(HDPE/LLDPE、HDPE/LDPE、PP/LLDPE 等)的研究表明,剪切应力作用下固化成型的聚合物及其共混物的拉伸强度、杨氏模量、缺口冲击强度等都有不同程度的提高,获 1<WP=9>四川大学博士学位论文得了具有高拉伸强度和高冲击强度的超级聚烯烃共混物。HDPE 和 PP 的拉伸强度增加比较明显,而 LDPE 和 LLDPE 拉伸强度增加较弱,这主要是由于不同的聚合物在相同的剪切应力作用下分子链结构的差异导致其运动能力不同,从而导致不同的取向结果。2d-WAXD 研究结果表明,在相同的剪切应力条件下,HDPE 和 PP 具有明显的取向行为,其取向度较大,而 LLDPE 和 LDPE 的取向较弱。分散相对基体取向的影响不同,研究发现,在共混物 HDPE/LDPE 中,分散相 LDPE 的增加,基体 HDPE 的取向程度减弱。而 PP/LLDPE 共混物中,分散相 LLDPE 的增加,PP 的取向程度先增加而后降低。在 PP/EC 共混物中也发现加入低含量的 EC,可以显著改善基体的取向程度,从而改善共混物的拉伸性能。具有较低粘度和较好流动性的分散相的加入改善了共混物的熔体流动性是造成基体取向程度增加的主要原因。进一步证明,拉伸强度主要与剪切层的面积大小有关,即与共混物在熔体冷却过程中的取向程度的大小有关;而冲击强度主要与剪切层和芯层有关。在冲击断裂过程中,动态样品不同区域表现出不同的断裂模式,皮层和芯层产生的大量银纹和材料的拔出吸收大量能量,为典型的韧性断裂;而剪切层断面平整光滑,为脆性断裂模式。(3)采用半晶性聚合物研究了剪切应力对聚合物结晶形态和片晶取向的影响,DSC、SEM、2d-SAXD 等结果表明在应力作用下,共混物不同区域形成不同的结晶结构。HDPE 在应力作用下形成较多的 shish-kebab 结构,表现出力学性能的显著增加;PP 在皮层以 shish 结构为主,剪切层中形成 shish-kebab 结构,而芯层以 kebab 结构为主。共混物 PP/LLDPE 中,PP 在不同区域都形成 shish-kebab结构,而 LLDPE 除了生成片晶取向方向垂直于剪切流动方向以外,还沿剪切流动方向发生一定角度的偏转(450～500),这可能与片晶中晶块在剪切作用下沿流动方向滑移有关。剪切作用下,聚合物所形成的 shish 伸直链晶体,在熔融重结晶过程中,诱导基体在较低的过冷度下结晶。EC 的加入使 PP 在相同剪切条件下生成更多的 shish 伸直链晶体。(4)橡胶增韧塑料的研究由来已久,并在此基础上提出了许多增韧理论和判据,其中最著名的就是 Wu 氏增韧理论,认为临界基体层厚度Tc 是橡胶增韧塑料的唯一判据,即当橡胶粒子间距T <Tc ,共混物表现出韧性断裂;当T >Tc ,共混物表现为脆性断裂;当T =Tc 时,共混物发生脆韧转变。但需要注意的是, 2<WP=10>四川大学博士学位论文Wu 氏增韧理论最重要的一个前提就是作为分散相的橡胶粒子在基体中呈球形简立方分布。因此为了进一步研究橡胶在塑料中的增韧行为,本文选择 EPDM作为分散相,PP 作为基体,在注射成型过程中施加动态剪切应力场,控制基体的取向、分散相的形变和取向、以及共混体系相分离行为。实验表明,随着橡胶含量的增加,其形变和取向的程度增加,当这种形变达到一定程度时,共混物的韧性明显降低,体系发生明显的韧-脆转变行为。通过对基体层厚度的测量和计算表明,当共混物中橡胶粒子发生形变和取向时,Wu 氏增韧理论的临界基体层厚度作为共混物韧性行为的判据不再适用,此时,当T <Tc 时,共混物表现出明显的韧性降低更多还原

【Abstract】 The morphology controlling of polymer blends during the process is one of thehottest research topics in polymer science and engineering at present. The aim ofmorphology controlling is to achieve the desired structure via special ways thusimprove the properties of polymer blends. In this work, polyolefin and their blendswere cooled and solidified under the shear stress condition via the introduction ofshear stress field during the injection process. The morphology, miscibility and phaseseparation, phase inversion, orientation and crystallization of polymer blends weregreatly affected under the effect of shear stress fields. The alteration of morphologywas kept by the way of solidification of melt blends. And then, the correlationbetween the morphology and the mechanical properties could be established.The main results are: 1.The effect of shear stress on anisotropic of injection-molded samples was studied.After the melt was injected into the mold, the specimen was forced to moverepeatedly in a chamber by two pistons that moved reversibly with the samefrequency as the solidification progressively occurred from the mold wall to themolding core part. During the cooling of the sample, three different structures wereformed, namely, the skin, the shear layer and the core. The further experimentssuggested that the morphologies among the three layers were different. 2. The study of the polyolefin (such as HDPE, LDPE, LLDPE and PP) and theirblends (such as HDPE/LLDPE, HDPE/LLDPE and PP/LLDPE) suggested that thetensile strength, the elastic module and the Izod notched impact strength could beimproved at the different degree, depending on the molecular architecture and phasemorphology. And the super polyolefin blends, which had higher tensile properties 5<WP=13>四川大学博士学位论文and higher impact properties, were obtained. 2d-WAXD was adopted to study theeffect of shear stress on orientation of the polymers and their blends. The resultsshowed that different polymer had the different orientation behavior in the sameshear condition, and the orientation of HDPE and PP was easier than that of LLDPEand LDPE. On the other hand, in the blends, the effect of dispersed phase on theorientation of matrix was different. In HDPE/LDPE blends, the degree of orientationof HDPE was decreased with the increasing of LDPE content, while in PP/LLDPEblends, the degree of orientation of PP was increased firstly and then was decreasedwith the increasing of LLDPE content. In PP/EC blends, the orientation ability of PPwas improved greatly when a few EC was added into PP. Further experimentsshowed that the tensile strength was related to the area of shear layer, which furtherproved that the orientation of polymers is a key to improve the mechanical properties.On the other hand, the impact strength was related to the shear layer and the corelayer. The skin and the core showed a typical ductile-fracture mode since the muchcraze was formed and some materials were pulled out during the fracture propagation.While for the shear layer, the fractured surface was very smooth, which is the typicalcharacter of brittle-fracture mode. 3. The effect of shear stress on crystallization and lamellar orientation was studiedin the work. The results of DSC, SEM and 2d-SAXD proved that different crystalstructures were formed in the different zones of the prepared samples. Much moreshish-kebab structure was formed in HDPE and the mechanical properties wereproved greatly. While in PP, shish structure formed in the skin, shish-kebab structureformed in the shear layer and kebab structure formed in the core zone. But inPP/LLDPE blends, PP formed shish-kebab structure in each different zones, while avery unique crystal morphology and lamellar orientation of LLDPE were obtained,with the lamellar stack oriented either perpendicular or 45-500 away from the shearflow direction. And the further study suggested that shish structure formed in th更多还原