剪切均匀性对流动诱导等规聚丙烯结晶的影响

固定应变和最终应变速率,采用瞬时和缓慢2种电机加速方式对样品施加剪切,研究了流场加载模式对样品流变和结晶行为的影响.实验结果显示缓慢加速能够消除剪切过程中流场的非均匀性,使样品取向度增加,提高流场对聚合物熔体的作用效果.同时,流动诱导结晶对于加速时间有依赖性.对于速率为17.7 s-1的剪切,加速时间为1 s时,熔体流动均匀且流动诱导的晶体取向最强,短加速时间(0.5 s)和长加速时间(1.5 s)样品的流动诱导结晶效果都弱于加速时间为1 s的样品.但是,对于不同剪切速率,其对应的最优加速时间不同.对于流动诱导结晶来说,加速时间应当作为一个重要参数来考虑,其背后的真实物理含义还需要进一步研究来说明.     

流变技术在高分子表征中的应用：拉伸流变测试

流变测试是与高分子加工和应用相关的重要表征手段.尤其在纺丝、发泡、吹膜、吹塑等以拉伸流场占主导的高分子加工工艺中，高分子的拉伸流变行为对材料聚集态结构的形成起了决定性的作用，进而影响了产品最终的使役性能.本综述第一部分介绍了拉伸流场的定义、拉伸流场的分类以及各自的特征；第二部分概述了目前商业拉伸流变仪的种类，并围绕其基本原理、测试技巧以及优缺点等内容展开讲解；第三部分介绍了拉伸流变的测试模式，并举例说明了各个模式的优势；第四部分列出了在拉伸测试中存在的常见问题，并给出了相应的解决方案和建议；最后对拉伸流变与其他表征联用技术的发展趋势进行了总结和展望.     

高分子复杂体系的结构与流变行为

流变学测试对非均相体系的结构变化具有敏感响应,被认为是表征多相/多组分聚合物体系结构与性能极为有效的方法.本文主要依据作者及其合作者的工作,对近年来围绕非均相体系形态结构与流变响应所开展研究的最新结果进行了总结和评述,涉及LCST型高分子共混体系的相形态与黏弹松弛、嵌段共聚物的微结构与线性/非线性黏弹行为、聚烯烃剪切诱导结晶时间尺度与流变响应、填充聚合物体系的结构性能和流变行为.对多相/多组分高分子共混体系形态结构演变的特征流变响应的充分认识,将有助于优化非均相体系的形态结构与最终力学性能.     

壳聚糖浓溶液流变性质研究──分子量的依赖性

壳聚糖浓溶液流变性质研究──分子量的依赖性王伟，徐德时（中国科学院长春应用化学研究所高分子物理开放实验室长春１３００２２）关键词壳聚糖，浓溶液，粘度，零剪切粘度，分子量作者曾研究了一些外部条件对壳聚糖浓溶液流变学性质的影响，如浓度、温度、溶剂性质、外...     

氧化石墨烯对聚乙烯醇/硼酸水凝胶流变性能的影响

研究了氧化石墨烯(GO)对聚乙烯醇(PVA)/硼酸(borate)水凝胶线性及非线性流变性能的影响。 通过扫描电子显微镜、硼谱核磁共振波谱以及流变研究了水凝胶的流变性能。 结果表明,GO质量浓度在稀溶液区时,GO片层与PVA链间通过硼酸根离子形成了具有弹性活性的缔合点,有效地提高了水凝胶的平台模量、松弛时间和零切粘度;当GO进一步增加到亚浓溶液区,部分的交联剂被GO的团聚体捕获并处于非弹性活性的缔合状态,处于有效缔合状态的交联剂变少,导致平台模量、松弛时间和零切粘度降低。 稳态剪切测试下,样品在剪切增稠区的粘度增加随着GO添加量的增加明显加强,这与剪切场下取向的氧化石墨烯片层参与网络结构的重排有关。     

流变学法研究交联聚合物凝胶的抗剪切性能

流变学法研究交联聚合物凝胶的抗剪切性能;交联聚合物;凝胶;屈服应力;流变学方法     

几种典型含氟树脂流变行为研究概述

主要对乙烯-四氟乙烯共聚物(ETFE)、聚全氟乙丙烯(FEP)、聚偏氟乙烯(PVDF)、全氟磺酸树脂(PFSA)和全氟羧酸树脂(PFCR)这几种典型含氟树脂的加工流变行为和线性粘弹性进行了综述。FEP高速挤出时稳定流动区非常窄,并且进行模型拟合时发现其剪切流动行为符合Carreau模型。ETFE为热流变简单高分子,其剪切粘度、拉伸粘度、挤出胀大和动态模量等流变函数均可进行时温叠加。在低剪切速率下,PVDF熔体表现出牛顿流体行为;在高剪切速率下,PVDF熔体呈现出剪切稀化现象。PFSA和PFCR熔体的表观黏度随剪切速率、温度的升高而降低,流动曲线显示该熔体属于假塑性流体。     

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。     

PLLA在稳态剪切场下的非等温结晶研究

利用光学流变显微镜(ORM)在线研究剪切场下左旋聚乳酸(PLLA)非等温结晶过程,采用差示扫描量热法(DSC)和广角X射线衍射(WAXD)研究了剪切后样品的热行为和结晶性能。实验结果表明,施加剪切作用不改变PLLA的晶型,但大大加快了结晶过程,提高了起始温度。PLLA在剪切场下的结晶形态主要受剪切速率的影响,当剪切速率高于临界剪切速率(10s-1)时,PLLA由球晶向串晶转变。随着剪切速率增大,串晶的数量变多变窄,结晶更完善,结晶度增大。与剪切速率相比,剪切时间对PLLA的非等温结晶影响影响较小。     

不同结构驱油聚合物的界面剪切流变性质

利用双锥法研究了油田现场用超高分子量部分水解聚丙烯酰胺(PHPAM)和疏水改性聚丙烯酰胺(HMPAM)溶液与航空煤油间的界面剪切流变性质,考察了时间、应变幅度和剪切频率对不同浓度PHPAM和HMPAM溶液界面剪切流变参数的影响.结果表明,只有在适宜的剪切频率条件下,流变数据才能反映界面膜的结构信息.HMPAM分子具有界面活性,能吸附在界面上,其界面膜的强度随时间变化逐渐增强,且在高浓度时以黏性为主;PHPAM分子不具有界面活性,其剪切流变参数没有时间依赖性,界面层以弹性为主.HMPAM能通过疏水作用形成界面网络结构,界面膜的剪切复合模量明显高于PHPAM界面层.HMPAM界面层中网络结构在剪切形变作用下的破坏与重组这一慢弛豫过程是其强度较高的原因.     

Shear banding in semidilute entangled polymer solutions

Shear banding in semidilute polymer solutions and other soft materials is one of the most intensely debated topics in current rheology. By means of rheo-optical experiments, physical modeling, and numerical simulations, researchers have started to develop a more thorough understanding of this flow instability within the past few years. Nevertheless, much effort is still required to identify the exact microscopic mechanisms leading to shear band formation and to clarify whether the phenomenon is universal for polymers. For this purpose, basic rheological characteristics, such as the appearance of a stress overshoot during start-up of a simple shear flow, have to be revisited and better related to the dynamics of the polymeric network.     

The Morphology of Liquid-Crystalline Polymers and the Possible Consequences for their Rheological Behaviour

Abstract

Polymer liquid crystals can occur as polydomain materials where the domain size may be tens of microns. While the material within each domain may be characterized by a common order parameter, the directors of the domains can be more or less randomly distributed. Since the transition from polydomain to monodomain material only involves the removal of grain boundaries and the alignment of directors, the free energy change must necessarily be small. Such a transition can readily be achieved, therefore, by the action of any external field: electrical, magnetic, stress or surface. In this work optical photomicrographs of polymeric liquid crystals with widely varying and in some cases well controlled morphologies are presented. Probable dependence of rheological behaviour on morphology is also discussed. Such dependence is expected to be considerable under certain conditions. Due to experimental and sample limitations, however, direct correlations of rheology and morphology are sparse. Morphological consequences for the rheology of liquid-crystalline materials can be exemplified by the following possibilities. In contrast to the case of isotropic melts, wall effects can be non-negligible. Zero shear rate rheological parameters are not expected to be uniquely defined quantities since the domain sizes are large and the director may not be effectively averaged over typical sample dimensions. Non-zero shear-rate measurements of rheological parameters is effected by the propensity of: (1) individual domain directors to align under the influence of a stress field and (2) flow alignment to dominate surface-induced alignment above some critical shear rate. The effects might be manifested by a non-newtonian regime as well as yield stress behaviour and thixotropy. The kinetics of relaxation from mono- to poly-domain material has implications for the dynamic response and rheological hysterises of the material.     

The Morphology of Liquid-Crystalline Polymers and the Possible Consequences for their Rheological Behaviour

Polymer liquid crystals can occur as polydomain materials where the domain size may be tens of microns. While the material within each domain may be characterized by a common order parameter, the directors of the domains can be more or less randomly distributed. Since the transition from polydomain to monodomain material only involves the removal of grain boundaries and the alignment of directors, the free energy change must necessarily be small. Such a transition can readily be achieved, therefore, by the action of any external field: electrical, magnetic, stress or surface. In this work optical photomicrographs of polymeric liquid crystals with widely varying and in some cases well controlled morphologies are presented. Probable dependence of rheological behaviour on morphology is also discussed. Such dependence is expected to be considerable under certain conditions. Due to experimental and sample limitations, however, direct correlations of rheology and morphology are sparse. Morphological consequences for the rheology of liquid-crystalline materials can be exemplified by the following possibilities. In contrast to the case of isotropic melts, wall effects can be non-negligible. Zero shear rate rheological parameters are not expected to be uniquely defined quantities since the domain sizes are large and the director may not be effectively averaged over typical sample dimensions. Non-zero shear-rate measurements of rheological parameters is effected by the propensity of: (1) individual domain directors to align under the influence of a stress field and (2) flow alignment to dominate surface-induced alignment above some critical shear rate. The effects might be manifested by a non-newtonian regime as well as yield stress behaviour and thixotropy. The kinetics of relaxation from mono- to poly-domain material has implications for the dynamic response and rheological hysterises of the material.     

Shear and NMR experiments of materials with flow behaviour depending on the deformation history

Oxide ceramic masses react to simple shearing with hardening (peptisation: increase in the shear stress with the shear deformation). In the present study the correlation between the increase in the shear stress and the porosity, agglomeration processes and the type of flow are analysed. For this purpose oxide ceramic masses are tested in a shear device especially developed for pastes and analysed by rheometric experiments, NMR methods and particle size analysis. The results support the hypothesis that structural changes (hardening, increase in the mean porosity) of the material during the peptisation mainly depend on the magnitude and not on the kind of the energy input and thus of the type of flow. The fraction of bound (more generally, the immobilised) water increases with the shear displacement. Also crushing of primary particles could be observed. Both the crushing of solid particles causing an increased solid surface and the formation of a three-dimensional gel structure are microscopic effects capable of resulting in the binding or retaining water. On a macroscopic scale these phenomena cause hardening. Magnetic resonance imaging visualises flow-induced agglomerates, which form owing to the shear flow and increase the porosity averaged over the whole sample. After the shear experiment rolls of paste can be seen which indicate that the general assumption of a plane shear flow in the shear device is not warrantable. Received: 19 July 2001 Accepted: 25 October 2001     

The spatial inhomogeneity of rodlike polymeric systems subject to simple shear flow

A novel simulation approach has been developed by the combination of Brownian dynamics and Lebwohl-Lasher nematogen potential, by which we have made preliminary investigation on the spatial inhomogeneity of nematic rodlike polymeric system with planar surface anchoring and subject to simple shear flow. Some rheological behaviors such as the damped oscillation of stresses with time or the negative first normal stress difference at moderate shear rate find good agreement with previous experimental observations reported in literature. The distribution function of rod orientation takes on “breathe-like” time-evolution and is verified not Gaussian-featured. The layered director, order parameter, shear stress and first normal stress difference have also been calculated by statistics, and most importantly, all of these statistic quantities show spatial distribution in different layers and corresponding distribution functions are time-dependent. Therefore, our simulation implies the complicated pattern formation and pattern evolution in this system. It is, hence, very meaningful to further study the spatial inhomogeneity of rodlike polymers under shear flow extensively both in theory and in experiment.     

Unusual contributions of molecular architecture to rheology and flow birefringence in hyperbranched polystyrene melts

With the increase in sophisticated synthesis methods, it appears that polymer architecture may be a tunable property. Therefore, the role of architecture in rheological and processing properties has received renewed attention, mainly because of dendrimer synthesis and metallocene‐catalyst technology. Linear polymers and hyperbranched polymers represent two ends of branching complexity. Some previous studies have suggested that hyperbranched polymers may behave like unentangled polymers, whereas others have proposed that they exhibit the properties of soft colloids. In an effort to compare the responses of linear and hyperbranched polymers, we synthesized starlike hyperbranched polystyrenes (HBPSs) of various branch lengths and numbers of branches. The HBPSs used in this study were unentangled or weakly entangled, allowing us to study the effect of branch density more readily. Two linear polystyrene (L‐PS) melts and two HBPSs were studied. Using a custom‐built rheooptical apparatus, we characterized the rheology and flow birefringence of these materials. To our knowledge, these are the first flow birefringence measurements on highly branched polymer melts. Our results suggest that the flow behavior of HBPS is significantly different from that of L‐PS: (1) HBPS shows nonterminal behavior in the low‐frequency rheological response; (2) when the stress‐optical rule (SOR) holds, the stress‐optical coefficient of HBPS is much lower than those of analogous linear polymers; and (3) when the branch density is high and the branch length is sufficiently low, the SOR fails for these homopolymer melts. A significant increase in the birefringence for a given amount of stress in the low‐frequency region suggests that there may be a soft core in these materials due to the strong preferential radial orientation of chain segments near the center of a molecule versus those near the periphery. The predominantly elastic response of the soft structures may be responsible for the enhanced form birefringence. Our preliminary results indicate that these materials may exhibit both polymeric and soft‐colloid natures. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2562–2571, 2001     

Rheology of concentrated carbon nanotube suspensions

The rheological properties of non-Brownian carbon nanotube suspensions are measured over a range of nanotube volume fractions spanning the transition from semidilute to concentrated. The polymer-stabilized nanotubes are "sticky" and form a quiescent elastic network with a well-defined shear modulus and yield stress that both depend strongly on nanotube volume fraction with different but related critical exponents. We compare controlled-strain-rate and controlled-stress measurements of yielding in shear flow, and we study the effect of slow periodic stress reversal on yielding and the arrest of flow. Our measurements support a universal scaling of both the linear viscoelastic and steady-shear viscometric response. The former allows us to extract the elastic shear modulus of semidilute nanotube networks for values that are near or below the resolution limit of the rheometers used, while the latter provides a similar extrapolation of the yield stress. A simple scaling argument is used to model the dependence of yield stress and elastic modulus on concentration.     

Fiber motion and rheology of suspensions with uniform fiber orientation

One of the main goals in the studies of fiber suspensions is the prediction of fiber orientation in a short fiber composite part, using the processing variables, mold geometry, and material characteristics. The rheological properties of the fiber suspensions are strongly associated with the fiber orientation distribution. The understanding of the relations between the fiber structure in the suspension and its rheological properties is a key step in the design and implementation of processing operations. The fiber motion in shear flow is analyzed in this article. The study is focused on the relation between fiber orientation and rheological properties for a suspension with uniform (delta function) fiber orientation distribution in a Newtonian fluid. The study shows that the rheological properties of the suspension, measured during the start up of steady shear flow, can be used to determine the fiber orientation in the sample. The first normal stress coefficient is the property to measure in order to determine whether or not the suspension has a random fiber orientation. Any of the shear flow transient rheological properties can be used to determine the fiber initial orientation. It was found that the normal stress coefficients can show negative or positive values depending on the fiber orientation. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1788–1799, 2000     

Birefringence banding in a micellar solution or the complexity of heterogeneous flows

We have investigated the shear flow behavior of a classical viscoelastic equimolar wormlike micellar system made of cetyltrimethylammonium bromide and potassium bromide in the semidilute regime using mechanical and optical measurements. The experimental flow curve of this surfactant solution exhibits, above a critical shear rate, a well-defined stress plateau, characteristic of a flow of the shear-banding type. We first focus on the rheological and rheo-optical transient response of the sample after the sudden start-up of flow. The time-dependent stress profiles are strongly similar to those observed on various other systems with the occurrence of an overshoot at short times followed by a stretched exponential relaxation toward the steady state on a long time scale. This behavior is then correlated to the temporal evolution of the birefringence intensity and the extinction angle; the latter exhibits an undershoot just after the inception of the flow. Using direct visualizations of the sheared sample and spatially resolved flow birefringence across the gap of the Couette cell, we have been able to highlight a peculiar banding structure made up of three distinct regions: two layers of homogeneous but strongly differing orientations located against the walls, separated by a mixed layer, the width of which can reach half of the gap as a function of the effective applied shear rate. The induced structures contained in the band adjacent to the inner moving cylinder are found to be almost fully aligned along the flow direction. The relative proportions of each region are derived from the orientation profiles and compared to the predictions of the lever rule. The results suggest that orientation bands and shear bands are not linked in an obvious way, and the observed band structure can finally be interpreted as the coexistence either of three distinct "phases" or of only two homogeneous phases separated by an interface which can be broad, or thin and fluctuating.