非线性大应变振荡剪切流场中PP结晶行为的傅立叶转变流变学研究

通过对等规聚丙烯(iPP)过冷熔体施加不同应变的正弦振荡剪切场,研究了大应变振荡剪切流场作用下,等温iPP的流动诱导结晶过程.研究结果表明,施加大应变振荡剪切流场可以明显加速iPP的结晶动力学,且施加的应变越大可以促使iPP结晶动力学加速现象越明显.通过对iPP熔体的应力波形进行傅立叶变换分析发现,由于结晶会引起熔体内部非线性行为改变,傅里叶变化峰值的3倍基频峰值I3与基频峰值I1之比I3/I1可以敏感反映iPP的流动诱导结晶过程.当流变仪施加的应变幅度从10%提高到30%时,I3/I1比值发生突跃的时间则从2500 s迅速减小为600 s.     

流场作用对聚丙烯结晶成核过程和晶体生长过程动力学的影响

通过改进基于构象张量构建的流动诱导结晶理论模型,考虑了流场对结晶生长的影响,对比研究了流场作用对聚丙烯(PP)结晶过程中成核与晶体生长两阶段速率的不同影响程度.理论研究表明,虽然相比成核过程,流场诱导作用对PP晶体生长速率的加速程度有限,但要更为精确地描述流动引起的整个结晶动力学过程,特别是在剪切速率较高且熔体结晶温度较低的情况下,考虑流场对结晶生长速率的影响是十分必要的.研究结果还显示,由于分子链的松弛作用,PP在较低温度下的晶体生长速率受剪切流场的影响要大于较高温度下的影响.本文理论预测结果与实验现象和实验数据的很好吻合亦说明模型改进的合理性和可行性.     

剪切流场诱导iPP等温结晶行为的研究

利用流变学方法研究了剪切诱导等规聚丙烯(iPP)的等温结晶行为.在稳态剪切流场作用下,结晶初期的法向压应力和粘度基本为一定值,一定时间后,会出现增大并迅速上升的现象.在高剪切速率下,与粘度相比较,法向压应力突变的时间要早;剪切速率减小,二者趋于一致;当剪切速率很低时,法向压应力的值很小且超出了仪器的量程,只有粘度的值是可信的.针对该现象提出了一种新的结晶速率表征法,即利用法向压应力和粘度二者中较早的突变时间表征结晶诱导时间(ton).诱导应变γ=.γ.ton,q是决定剪切影响结晶行为的一个无量纲参数,可以将不同温度下的结晶诱导时间曲线组合成一条主曲线.     

振荡剪切对聚乳酸熔体分子链解缠结的研究

利用哈克旋转流变仪在一定的温度和压力条件下对聚乳酸(PLA)熔体施加应变为正弦变化的振荡剪切场,从而对PLA分子链进行高效的降黏解缠.结果表明,当温度为190°C、样品厚度为1.5 mm时,PLA熔体在振动频率和剪切应变分别为3.5 Hz和50%处呈现最佳的降黏效果,其黏度相比于未经处理的熔体黏度下降了3~4个数量级.同时,凝胶渗透色谱(GPC)测试结果表明经过振荡剪切处理的PLA其分子量基本不变,表明熔体黏度的大幅下降是由PLA分子链有效解缠结导致而非分子链降解.示差扫描量热仪(DSC)结果表明,解缠效果最佳的PLA样品相比于未经处理的样品呈现出更低的玻璃化转变温度和较高的结晶度,进一步验证了熔体解缠的效果.不仅如此,我们研究了不同退火时间(1~30 min)和温度(180~200°C)对PLA分子链重新恢复缠结的影响,发现随着退火温度和时间的增加,PLA在120°C下等温结晶的半结晶时间不断增加,并向未经解缠处理的PLA样品的半结晶时间不断靠近,表明振荡剪切导致的解缠能在低温度下保持较长时间,在高温下快速复缠.     

分级聚丙烯的取向结晶过程

聚丙烯熔体在剪切或应变应力作用下 ,分子链发生取向形成伸直链纤维晶 ,这些先取向形成的纤维晶成为其后结晶的晶核 .这种线形排列的特殊自晶核被称作排核 ( Row nuclei) [1] .排核诱导的结晶温度高于异相核和均相核 .折叠链片晶在排核上附生生长 ,形成具有柱状对称性的超分子结构 ,称为柱状晶 ( Cylindrite) [2 ,3] .聚合物的分子量 ,剪切温度和剪切速度等因素对柱状晶的生成有很大影响 [4 ,5] .本文选用不同级分的聚丙烯样品 ,利用高分子 (特别是取向结晶 )的记忆效应 [6,7] ,研究了剪切后薄膜试样在熔融重结晶过程中柱状晶的形成和发展…     

剪切作用下纳米粘土对LDPE熔体粘弹响应的影响

以LDPE/EVA/纳米粘土复合体系为研究模型,考察了剪切作用下,分散良好的纳米粘土对聚合物基体熔体稳态及瞬态粘弹响应的影响.发现剪切作用下,纳米粘土增加了聚合物熔体粘弹特性对剪切速率、剪切应变及剪切作用史的依赖性,改变了相应的依赖关系.稳态剪切时,纳米粘土的加入使体系第一法向应力差(N1)在低剪切速率区变为负值,而在高剪切速率区N1与粘土的含量无关;同时就瞬态剪切应力及N1的应变依赖关系而言,复合体系明显不同于聚合物基体;预剪切对聚合物基体瞬态粘弹响应几乎没有影响,而当纳米粘土的加入量大于3wt%后,与未经预剪切的样品相比较,经预剪切的复合体系的瞬态剪切应力值、应力过冲程度以及稳态剪切应力值均明显下降,且预剪切前后复合体系达到稳态时其瞬态剪切应力差值随纳米粘土含量的增高而线性增加.此外,纳米粘土的添加对聚合物熔体受剪切作用的非线性粘弹响应存在影响.复合体系熔体呈现特异非线性粘弹响应,其缘由被认为是由于纳米粘土在聚合物基体中剥离分散,或聚合物分子链插层于粘土片层间,形成局部有序结构,受剪切作用而排列取向.     

快压法制备非晶聚醚醚酮的应变诱导结晶行为

利用快速增压和淬火2种方法分别制备了非晶聚醚醚酮(PEEK), 并利用二维广角X射线衍射(WAXD)研究了2种非晶样品在不同拉伸温度(T_d)和不同应变速率(v)条件下的结晶行为. 结果表明, 在相同拉伸温度及应变速率条件下, 快压样品的临界结晶应变明显低于淬火样品; 随着拉伸温度和应变速率的升高, 2种样品的临界结晶应变均逐渐降低; 在相同应变条件下, 拉伸温度对PEEK材料结晶度的影响很大, 而应变速率对其影响较小.     

剪切对iPP/PEcO共混物结晶行为的影响

对剪切场作用下的全同聚丙烯/弹性体乙烯-辛烯共聚物(iPP/PEcO)的共混物结晶行为进行研究, 结果表明, 剪切使得iPP球晶密度增加, 微晶和片晶均发生取向, 且片晶取向明显; 片晶取向度随共混物中PEcO含量的增加而增大, 而微晶取向度随PEcO含量的增加而减小; 强剪切诱导出现纤维状结晶形态. 利用同步辐射(SAXS)技术对共混物在剪切场下的等温结晶行为进行研究, 结果表明, 随结晶的进行长周期呈现先减少而后固定的趋势; 高剪切速率缩短了结晶诱导时间, 加快了共混物中结晶部分的结晶动力学过程.     

聚丙烯熔体流动中结构化的流变学研究

<正> 各种流场对高分子流体流变性质的不同影响及其分子机理的探索是当今流变学中引入注目的课题。本文提供一个有兴趣的例子:聚丙烯熔体在入口收敛流中发生结构化,又在随后的毛细管切流动中消除。 全同立构聚丙烯在约210℃以下挤出会发生熔体流变性质和加工性质的特征性变化,一些作者不恰当地将流动曲线和粘度-温度关系的反常笼统归结为切力诱导结晶和熔化时剩余的有序结构,要弄清这一问题应进行更细致的实验,尤其应分析入口收敛流场和毛细管中剪切场起的不同作用。     

聚苯乙烯/蒙脱土纳米复合材料中的剪切诱导有序结构(Ⅰ)——广角X射线衍射与透射电镜研究

聚合物/层状硅酸盐(PLS)纳米复合材料是近10余年来迅速发展起来的的交叉学科.由于其具有常规复合材料所没有的结构、形态以及较常规聚合物基复合材料更优异的性能等而引起人们的广泛关注[1].以往文献主要报道PLS纳米复合材料的制备与性能表征,如尼龙-6/蒙脱土[2]、PET/蒙脱土[3]和硅橡胶/蒙脱土[4]等.对于熔融加工过程中粘土片层及高分子的取向和结构研究很少.Kojima等[5]发现并研究了尼龙-6/蒙脱土纳米复合材料中的熔融剪切诱导取向结构,其X射线衍射与透射电镜(TEM)结果均表明,粘土片层沿熔体流动方向平行取向,但片层间距不等,因此为一平行取向但无序的结构.对于PLS纳米复合材料中的剪切诱导有序结构尚未见报道.     

Overshoots in stress and free energy change during the flow-induced crystallization of polymeric melt in shear flow

<正>The effect of pre-shear flow on the subsequent crystallization process of polymeric melt was investigated and a flow-induced crystallization(FIC) model based on the conformation tensor incorporating the pre-shear effect was proposed. The model is capable of predicting the overshoot phenomena 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(f_(1/2)) of polypropylene(PP) as functions of pre-shear rate and pre-shear time at different crystallization temperatures was predicted and compared with the experiment data.Both numerical and experimental results showed that t_(1/2) of PP decreased dramatically when the flow started but leveled off at long times.It was found that two transformation stages in t_(1/2) 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.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.     

The influence of thermoelastomers on the crystallization behavior of isotactic polypropylene under shear

The crystallization behaviors of isotactic polypropylene (iPP) and its blends with thermoelastomers have been investigated with in situ X‐ray scattering and optic microscopy. At quiescent condition, the crystallization kinetics of iPP is not affected by the presence of elastomers; while determined by the viscosity, the differences are observed on sheared samples. With a fixed shear strain, the crystallization rate increases with increasing the shear rate. The fraction of oriented lamellar crystals in blends is higher than that in pure iPP sample, while the percentage of β phase is reduced by the presence of the elastomers. On the basis of experimental results, no direct correlation among the fraction of oriented lamellae, the percentage of β phase, and growth rate can be deduced. The evolution of the fraction of oriented lamellae supports that shear field promotes nucleation rather than growth process. Shear flow induces the formation of nuclei not only with preferring orientation but also with random orientation. The total density of nuclei, which determines the crystallization kinetics, does not control the ratio between nuclei with and without preferring orientation, which determines the fraction of oriented lamellae. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1188–1198, 2006     

A Unified Thermodynamic Model of Flow-induced Crystallization of Polymer

We propose a unified thermodynamic model of flow-induced crystallization of polymer (uFIC),which incorporates not only the conformational entropy reduction but also the contributions of flow-induced chain orientation,the interaction of ordered segments,and the free energy of crystal nucleus and crystal morphology.Specifically,it clarifies the determining parameters of the critical crystal nucleus size,and is able to account for the acceleration of nucleation,the emergence of precursor,different crystal morphologies and structures induced by flow.Based on the nucleation barrier under flow,we analyze at which condition precursor may occur and how flow affects the competition among different crystal forms such as orthorhombic and hexagonal phases of polyethylene.According to the uFIC model,the different crystal morphologies and structures in the flow-temperature space have been clarified,which give a good agreement with experiments of FIC.     

Micro-Rheological Modeling of Flow-Induced Crystallization in Mixed Shear/Extensional Flows

Flow Induced Crystallization (FIC) is the common term to indicate the acceleration in polymer crystallization kinetics due to the action of flow. When modeling FIC, two major challenges are encountered. On the one hand, the model must be able to produce quantitative reliable results, while correctly describing the coupling between the intrinsic (quiescent) crystallization kinetics and the rheological response of the polymer. On the other hand, the model must be able to describe the complex kinematics taking place in real industrial processes. In this paper, we present the predictions of a recently proposed model for FIC in the case of a mixed flow, where both shear and extensional components are present at the same time. In particular, the effects of the overall flow intensity and of relative weight between shear and extension on the enhancement in nucleation rate are presented and discussed. Some guidelines for future development are also proposed.     

Coupling between kinetics and rheological parameters in the flow-induced crystallization of thermoplastic polymers

Flow Induced Crystallization (FIC) is the common term to indicate the acceleration in polymer crystallization kinetics due to the action of flow. FIC is expected to be the result of the coupling between the intrinsic (quiescent) crystallization kinetics and the rheological response of the polymer. The choice of a suitable rheological model, therefore, is a crucial requirement for a successful FIC model. Recent work of our group^[1] has demonstrated that the Doi-Edwards rheological model (DE), based on the concept of chain reptation, can be easily incorporated into classical crystallization models to successful predict the enhancement in nucleation rate under the action of a steady shear flow. In this paper, the interaction between the rheological parameters of the DE model and the crystallization kinetics parameters is investigated in more details. In particular, the effect of the crystallization temperature, which acts on both the polymer relaxation time and the free energy jump between liquid and solid phase, is determined and discussed.     

Experimental study of flow-induced crystallization in the blends of isotactic polypropylene and poly(ethylene-co-octene)

The crystallization behavior of the blends of isotactic polypropylene (iPP) and poly(ethylene-co-octene) (PEOc) under quiescent condition and shear flow were studied by differential scanning calorimetry (DSC) and rheology, respectively. The DSC curves of the iPP phase in the blends showed similar crystallization exothermic peaks to that of pure iPP itself, indicating that the addition of PEOc up to a percentage of 30 in weight almost had no influence on the crystallization behavior of iPP at quiescent condition. The rheological results of isothermal flow-induced crystallization (FIC) of iPP in the blends showed that the crystallization kinetics of iPP was enhanced with the increase of shear rate, similar to that of pure iPP, but the presence of PEOc enhanced the effect of shear on the crystallization kinetics of iPP significantly in the cases of shear rates larger than 0.2 s⁻¹, which was due to that PEOc played an important role to promote the nucleation of iPP. The rheological results also implied that the characteristic relaxation times of blends were longer than that of pure iPP during the FIC process, indicating a different relaxation mechanism which might be related to the occurrence of interface relaxation and chain relaxation of the PEOc phase in the blends.     

Polyethylene fiber formation in tubular flow. I. Observations of growth pattern and fiber properties

The process of seeded growth of fibrillar polyethylene crystals has been studied in a tubular flow geometry for 0.01-wt % solutions of a high-molecular-weight polyethylene in xylene. The transformation sequence has been followed visually by using polarized-light illumination in conjunction with a video camera. Data are presented to show that transformation is initiated by the formation of a concentrated, unoriented, amorphous precursor fiber within which oriented birefringent crystals subsequently grow in consequence of the stresses transmitted by the flowing solution. Time constants for the precursor formation, birefringence initiation, and completion of birefringence were measured as functions of temperature and flow rate over a range of growth conditions. Wide-angle x-ray diffraction, overall birefringence, and optical hot-stage melting data were also obtained on the grown fibers. The net result of these observations is to conclude that fibrillar crystal growth during flow is always preceded by the formation of a liquidlike phase transformation which produces the concentrated, unoriented precursor. Subsequent orientation is in consequence of stress-induced crystallization with overall fiber orientation showing an increase with solution flow rate at a fixed temperature and a decrease with temperature at a fixed flow rate. At higher temperatures and lower flow rates, birefringence develops in an oscillatory fashion, indicating a remelting process possibly due to slippage of trapped chain entanglements formed by flow. A discussion is given of the implications of these observations for the understanding of flow-induced structure development, phase transformation, and oriented crystallization; this is expanded upon in a companion paper, Part II.     

Extensional flow-induced crystallization of a polyethylene melt

Measurements of flow-induced orientation and crystallization have been made on a high-density polyethylene melt (HDPE) undergoing a planar extensional flow in a four-roll mill. The HDPE was suspended as a cylindrical droplet at the flow stagnation point in a linear low density polyethylene (LLDPE) carrier phase. Deformation and crystallization of the HDPE droplet phase were monitored using video imaging in conjunction with measurement of the birefringence and dichroism to quantify the in-situ transformation kinetics. Planar deformation rates along the symmetry axis of the molten HDPE phase were on the order of 0.03 s^?1. Measurements of the initial transformation rate following flow cessation at 131.5°C and 133.2°C show a dependence on initial amorphous phase orientation and the total Hencky strain achieved during flow. The flow-induced crystallization rate is enhanced over the quiescent transformation rate by orders of magnitude, however, the dependence on temperature is less dramatic than expected for a nucleation-controlled growth mechanism. Analysis demonstrates that the melting point elevation model cannot account either qualitatively or quantitatively for the phenomena observed, suggesting that alternative explanations for the strong orientation dependence of the transformation rate are needed.     

Crystallization of a polypropylene terpolymer made by a Ziegler-Natta catalyst: formation of gamma-phase

The appearance of the gamma phase of a polypropylene-containing terpolymer prepared with a Ziegler-Natta catalyst was analyzed, and a novel strategy to make a polypropylene terpolymer which contains a high content of the gamma phase was experimentally sought. Wide-angle X-ray scattering and differential scanning calorimetry measurements have been conducted to investigate gamma-phase formation. Using a mixed flow of both elongational and shear components (a foaming process), we could introduce a lot of nuclei in the melt by flow-induced crystallization (FIC). The large number of nuclei accounting for the structural disorder during crystallite formation was ascribed to be the reason for gamma-phase formation in the terpolymer crystallization. A uniform distribution of defects in the melt crystallization was the key to obtaining a high content of the gamma phase.     

Phenomenological analysis of elongational flow birefringence in semidilute solutions of hydroxypropylcellulose

The relaxation time of a polymer chain in an elongational flow field was investigated for hydroxypropylcellulose (HPC) semidilute solution systems by two methods: phenomenological analysis of elongational flow-induced birefringence, and dynamic light scattering (DLS) and rheological measurements. To understand the relaxation time of an entangled semiflexible polymer solution in an elongational flow field, scaling analysis of the elongational flow-induced birefringence curve was performed. The results of both temperature and concentration scaling analyses showed that birefringence curves at different temperatures and at several HPC concentrations were described well by a universal birefringence–strain rate curve. This scaling behavior was compared with the "fuzzy cylinder" model. The critical strain rate corresponded to the correlation time of the slow relaxation mode determined by DLS measurement and the relaxation spectrum obtained by dynamic viscoelasticity measurement. The elongational flow-induced birefringence observed in an HPC semidilute solution was concluded to be attributed to the orientation of the HPC segment in the entangled molecular system, because the dominant relaxation mode is found to be the concentration fluctuation of an entangled molecular cluster in a quiescent state.