【作者】 于逢源；

【导师】 张洪斌；

【作者基本信息】 上海交通大学 ， 高分子物理与化学， 2009， 博士

【摘要】 聚合物材料在加工成型过程中,要经历复杂的热历史与流场作用,材料的流变特性及内部结构发生了改变,这很大程度上影响了材料的最终性能。对于结晶聚合物,在成型流场中可以明显观察到流动诱导结晶过程。加工成型过程中的很多现象都与流动诱导结晶相关,如高速熔融纺丝中的颈缩现象。不同于静态结晶过程,研究发现流动对结晶的影响主要体现在结晶速率(主要是成核速率)和结晶形态上。因此研究流场下聚合物结晶行为,建立材料内部结构与宏观性能的对应关系一直是聚合物加工领域重要研究内容之一。本文采用等规聚丙烯(PP)和高密度聚乙烯(HDPE)作为主要研究对象,以流变学作为主要研究手段,通过观察聚烯烃在剪切流场作用下的结晶行为,研究了其流动诱导结晶行为与粘弹特性之间的对应关系,重点分析并总结了流场强度对聚烯烃结晶动力学的影响。同时,对分子结构参数和共混组份等因素对聚烯烃流动诱导结晶行为的影响也详细地进行了研究,并通过在现有流动诱导结晶理论模型中引入新的一类模型—构象张量模型,对流场中聚合物流动诱导成核数目以及诱导时间等参数进行了理论预测。通过进一步改进两相流动诱导结晶模型,将流场中聚乙烯结晶与熔体粘弹特性相关联,讨论了材料内部结构变化与流变特性之间的关系。主要研究内容及结论简述如下:(1)对于纯PP:采用流变仪小幅振荡剪切模式,研究了PP静态等温结晶过程中的液固相转变过程,测定了其相变过程中的模量、相角、法向应力等参数的变化过程,并根据“反向淬火”法和物理凝胶理论测定了PP的物理凝胶点,发现随结晶度的上升,体系储能模量G’逐渐偏离末端区行为,存在更长的松弛时间;通过热台偏光显微镜对PP特定温度下的球晶数目进行统计,并根据Hoffman的成核理论对不同剪切速率下的晶体生长进行了理论分析;研究了非线性大应变对PP结晶的影响,并借助傅里叶变换手段,对PP结晶过程的波形进行变换,结果表明,PP显现出流动诱导结晶现象,非线性大形变条件下的振荡剪切场可以显著地加速PP结晶过程。(2)从分子结构对聚烯烃流动诱导结晶行为影响的角度出发,研究了具有长支链结构聚丙烯(LCB PP)的线性粘弹行为及其流动诱导结晶过程:利用Han图、Cole-Cole图和松弛谱等方法深入研究LCB结构与流变性能之间的关系。研究结果表明,将长支链引入线性PP中会显著改变PP流变行为,导致PP熔体显示出更长的松弛行为。实验中采用了两种剪切模式:稳态剪切和预剪切模式。通过观察熔体粘度和模量的变化,分别测定了LCB PP结晶诱导时间和半结晶时间。实验结果显示,随枝化链含量上升,LCB PP结晶行为对流场的响应变得更加敏感,长支链的引入加速了PP成核动力学。长支链对PP成核速率的促进作用来自于两个方面:首先,长支链结构有利于PP的成核过程;其次,长支链的存在使得PP的结晶对流场的响应更快,剪切过程中即已能形成大量晶核。对经过预剪切后结晶完全的LCB PP样品进行广角X射线衍射(WAXD)测试表明,在剪切速率为0.1s-1条件下,尽管对LCB PP样品施加了不同时间的预剪切,但是LCB PP的晶型并没有发生明显的变化。流场对LCB PP结晶的影响只存在于促进其结晶动力学方面,而对其晶型的影响不大。(3)系统研究了PP/PEOc共混体系的流变学性质及其流动诱导结晶行为:Cole-Cole图和松弛谱等方法可以明显反映PP和PEOc两种聚合物以及两相界面的松弛过程,通过考察两种聚合物不同配比对共混物流动诱导结晶的影响,发现Cole-Cole方法对反映结晶现象敏感;在剪切速率大于0.2s-1时,相同剪切速率下的共混物粘度突跃时间较纯PP更短,PEOc相的存在增强了流场对PP结晶速率的影响;结合构象张量本构方程,对Coppola单相流动诱导结晶模型进行了改进,所得的方程能在较宽的剪切速率范围内,对两相体系中的PP流动诱导结晶行为进行预测,并发现体系特征松弛时间λ随PEOc含量的增加而增长。(4)结晶聚合物在流场作用下的成核动力学过程和静态条件下的成核动力学过程具有明显区别。因而本文在构象张量的基础上构建了流动诱导结晶模型,对预剪切作用下的结晶过程也进行了理论探讨。基于对预测剪切作用下应力过冲行为和体系自由能变化过程的描述,发现在流场作用下,熔体自由能变化是导致晶核数目增加的主要推动力,自由能变化越大则成核速率越快,当剪切速率较高时,体系自由能变化对PP成核过程起到了支配作用;计算了不同预剪切速率和预剪切时间下的半结晶时间t1 /2随剪切速率的变化曲线,发现和现有实验数据能很好地吻合,理论和实验数据都显示t1 /2随剪切流动开始即大幅下降,但不随预剪切时间的延长而变化。特别是,计算结果表明, t1 /2随剪切速率变化存在两个转折点。第一个转折点在弱剪切区,这时熔体自由能处于平衡值。第二转折点对应着高剪切区,这是应力过冲和自由能过冲引起的。两个转折点的存在都反映了流场作用下结晶动力学的加速过程,也反映了流场变化和强度对PP成核过程具有重要影响。(5)根据非平衡热力学,利用Avrami动力学方程和汉密尔顿泊松括号理论,推导了能够预测高密度聚乙烯(HDPE)结晶过程的连续性流动诱导结晶模型,并利用该模型,对稳态剪切和预剪切两种剪切模式下的粘度和模量进行了理论模拟。结果表明,理论值能与测得的实验数据相吻合,所得模型并能描述体系结晶度的变化过程;理论计算和实验结果都显示,流场可以显著加速HDPE的结晶过程,但是流场强度对HDPE结晶的促进作用有限,具有“饱和现象”,当剪切速率足够大时,流场对HDPE结晶动力学的加速程度减小;理论预测结果还表明,体系处于结晶诱导期时,结晶度较低,但在诱导期后期,结晶度的些微变化将导致体系粘度和模量的急剧上升。此外,模拟结果显示,拉伸流较剪切流对HDPE的流动诱导结晶促进作用更加显著。本论文的主要创新点归纳如下:1.基于流场中聚合物体系自由能变化,建立了一个考虑大分子链取向与拉伸对于结晶固化过程影响的动力学理论模型,并利用该模型预测了不同组成PP/POEc共混物的流动诱导结晶过程。由于流动成核过程实验不易表征,本文将构建的构象张量模型和结晶成核速率方程联立,理论模拟了PP的成核过程,首次提出流场作用下PP成核数目存在两种不同增长方式,应力过冲现象对成核具有重要影响,并发现PP半结晶时间随剪切速率变化存在两个转折点,已有实验数据与理论预测能很好吻合。2.发展了两相流动诱导结晶微结构流变模型,对HDPE过冷熔体流动诱导结晶过程进行了理论研究,很好地预测了HDPE粘度以及模量随时间的变化过程,在理论上首次详细地讨论了HDPE体系粘度发生拐点变化时所对应的结晶度变化问题。3.利用旋转流变仪对聚合物熔体施加大应变正弦振荡剪切流场,研究了聚合物相变过程中的非线性粘弹性,并借助傅里叶变换手段研究了聚合物相变过程中应力与应变之间、应变与相变之间的关系。系统研究了长支链结构对PP的流动诱导结晶影响,并采用广角X射线衍射仪(WXAD)分析了不同剪切速率下LCB PP等温结晶的晶型变化。更多还原

【Abstract】 During the processing process, the polymeric materials experience complex effects of heat and flow, thus the rheological feature and internal structure of the material will be changed, which will greatly affect the final properties of the products. As for the semicrystal polymer, their melts will show an evident flow-induced crystallization behavior under a flow field. Many experimental phenomenons can be related to the flow-induced crystallization of polymeric materials, for example, the necking process of the high-speed fiber spinning process. Different from the quiescent crystallization, the research results show that the effect of flow on the polymer crystallization mainly lies on the acceleration of the nucleation kinetics and change of the crystalline morphology. Thus, study on the flow-induced crystallization and establish the relationship between the macropscopic property of the material and its internal mesoscopic structure are now one of the most important research topics in the polymer processing field.In this thesis, isotactic polypropylene (PP) and high-density polyethylene (HDPE) were selected as the investigated subject with the rheological methods. Through determing the flow-induced crystallization behavior, the relationship of the rheological behavior and crystallization behavior of polyolefin was studied. The theoretical analysis was focused on the effect of flow strength on the crystallization parameters of polyolefin. At the same time, the effect of the molecular structure parameters and the content ratio of the polymer blends on the flow-induced crystallization behavior also be studied in detail. To predict the nucleation number and nucleation induction time, a new coupled solving system, i.e. velocity-pressure-conformation tensor formula, was introduced into the nucleation rate equation. Furthermore, a two-phase model was modified to describe the relationship between the polyethylene’s crystallization and viscoelastic properties of the polymer melts. The simulation work was also furthered into the internal structure of material. The main research work and conclusion are introduced as follows:(1) Studies on the liquid-solid transition during the isothermal crystallization of the commercial iPP were carried out with the small amplitude oscillation determination method. The evolutions of the modulus, phase angle and normal force were determined at the same time. According to Winter’s physical gelation theory, we employed a new method, namely,“inverse quench”method, to characterize the gel point during the crystallization process of polypropylene. The frequency sweep results showed that the curve of G’ deviated the terminal zone behavior of the linear PP as the crystallinity increases, which indicated the formation of a long term relaxation process in the melts. Furthermore, the evolution of the spherical number with time was counted by a polarized light microscopy and simulated by Hoffman nucleation rate equation. Besides the small amplitude oscillation, the different treatments of the finite large amplitude oscillation shear were also performed on the undercooling PP melts. The Fourier transformation method was selected to analysis the waveform during the crystallization of PP melts under the large amplitude oscillation shear.(2) In order to study the rheological properties of a long chain branched isotactic polypropylene (LCB PP) polymers and the effects of its molecular architecture on the crystallization kinetics, self-made LCB PPs were isothermal crystallized under different shear rates. The relationship between the rheological feature and the molecular structure was analyzed through plotting the figures of Han, Cole-Cole and relaxation time spectrum. The results showed that the long branch chain could change the rheological behavior of the samples evidently and result in a longer relaxation process. Two shear modes, including steady shear and preshear treatment, were performed on the melts by rheometer. The crystallization rate was investigated by characterizing the induction time and the half crystallization time. The results showed that the LCB PP samples became more and more sensitive to the flow fields with the content of the branched component increasing; moreover, long branched chains also accelerated the nucleation kinetic of polymer samples. The positive effect of long branched chains on nucleation rate originates from two aspects: firstly, the long branched chains act as a nucleation agent to promote the formation of nuclei; secondly, long branched chain make the FIC process of polymer melts became more sensitive to shear flow and form more nuclei after the finish the preshear treatment. In additional, the crystal form of these LCB PPs after the preshear treatment was investigated by WAXD. The results showed that, even if the different shear time of shear treatment was performed on the polymer melts, there was no evident change being observed at a relative small shear rate 0.1s-1. Namely, the shear flow seems to just promote the crystallization ability of LCB PPs and don’t affect the crystal form of LCB PPs.(3) The researcher often encounters the processing of the binary polymer mixtures to obtain satisfactory mechanical properties in practical applications. Therefore, it is also important to verify the relationship between the kinetic and rheological parameters in the flow-induced crystallization of thermoplastic polymers. The shear-induced crystallization of pure PP and PP/PPEOc blends has been studied in this thesis. In the dynamic measurements, the PP/PEOc blends showed a long characteristic time, which was a second relaxation mechanism in the blends, corresponding to droplet-matrix structure. Through observation of normalized viscosity, it has been found that, similar to the pure PP, the blends also showed the sensitivity to the change of shear rate and the crystallization induction time of blends also decreased as the increasing of shear rate. The presence of high viscosity of elastomer PEOc trend to promote the chain of iPP oriented in the shear flow field, thus acting as nucleation promoter in the blends. In addition, a new microstructure rheological model based on the conformation tensor will be proposed to describe the flow-induced crystallization behavior of PP/POE blending system. With conformation tensor equation, we estimated the value of free energy change, which was induced by flow field. Then we predicted the induced time of PP/POE system with Coppla’s FIC model. The results showed a good agreement with experiments, even though our blend was not a homogeneous system. This result might help us to comprehend the FIC process in a more complex system. (4) The acceleration of nucleation kinetics manifests that there is a significant difference between flow-induced crystallization (FIC) and quiescent crystallization in polymer melting. In this thesis, the effect of pre-shear flow on the subsequent crystallization process was investigated and a FIC model based on the conformation tensor incorporating the pre-shear effect was proposed. The model is capable of predicting the overshoot of the stress and the flow-induced free energy change of the polymeric system at high pre-shear rates. Under the condition of flow, the increase in the activated nuclei number was contributed by the flow-induced free energy change, which showed an overwhelming effect on the nuclei formation during the pre-shear process at high shear rates. The half crystallization time ( t1 /2) of PP as functions of pre-shear rate and pre-shear time at different crystallization temperature was predicted and compared with the experiment data. A good agreement between the model predictions and experimental data was found. Both numerical and experimental results showed that t1 /2 of PP decreased dramatically when the flow started but leveled off at long times. It was found that two transformation stages in t1 /2 existed within a wide range of shear rates. For the first stage where the melting polymer experienced a relatively weak shear flow, the acceleration of crystallization kinetics was mainly contributed by the steady value of free energy change while in the second stage for high shear rates, strong overshoot in flow-induced free energy change occurred and the crystallization kinetics was thus significantly enhanced. Therefore, the overshoots in stress and flow-induced free energy change reflected an important role of flow on the primary nucleation especially when the flow was strong enough.(5)In the thesis, the isothermal flow-induced crystallization (FIC) of high density polyethylene (HDPE) under a simple shear flow was investigated. To determine the viscosity and modulus of the crystallization process, two experimental modes, including steady shear and preshear treatment, were performed on the polymer melt. Based on the non-equilibrium thermodynamic theory, the FIC process of HDPE was predicted through the modification of a continuum FIC model. The theoretical predictions of the evolution of both the viscosity in steady shear flow and the complex modulus under preshear treatment were essentially related to the crystallinity of HDPE, in agreement with the experimental findings. Both experimental and predicted results showed that the applied flow field could accelerate the crystallization kinetics of HDPE significantly. However, the effect of the intensity of shear flow on the crystallization of HDPE was finite, showing a saturation phenomenon, namely, the accelerated degree of crystallization tending to level off when the shear rate was large enough. In additional, it was found that the predicted crystallinity of HDPE was very low in induction period either in steady shear flow or by preshear treatment. The theoretical calculated results indicted that the melt viscosity was sensitive to the crystallinity. Moreover, the results also showed that the FIC process of HDPE melt illustrated a more evident acceleration in the extensional flow field than in the shear flow field.Therefore, the main innovations of the present research work are listed as follows:1. Considering the free energy change of polymer melt under the flow, a new flow-induced crystallization model was developed incorporating the effect of orientation and stretch of polymer chains. The predicted induction time of crystallization process of the PP/PEOc blend under the shear flow agreed with the experimental data well. Through introducing the conformation tensor equation into the nucleation rate equation, a system of simultaneous equations was developed to predict the PP’s crystallization process. The predicted results showed that the evolution curves of half crystallization time with shear rate have two inflexions indicating there was the different crystallization mechanism as the change of shear rate.2. The McHugh’s work was extended. Based on their theory, a modified two-phase microstructure rheological model was developed to predict the flow-induced crystallization of the undercooling polymer melts. The model could predict the evolutions of the viscosity and modulus with time under the flow. The predicted results showed a good agreement with the experimental data. It was first time to theoretical discuss the relationship between inflexion point of viscosity curve and crystallinity.3. Study on the nonlinear rheology during the polymer phase transition by using the rotation rheometer. It was the first time that, through the Fourier transition, the relationships between the stress and strain, and the strain and phase transition were analyzed. The rheological properties of a long chain branched isotactic polypropylene (LCB PP) polymers and the effects of its molecular architecture on the crystallization kinetics were also determined by the rheometer. Subsequently, the complete crystal samples were determined by WAXD. There was no apparent crystalline form change in the observation.更多还原