Measurement of transient behavior of the thermal diffusivity of flowing polymer melt

Molecular orientation of polymer molecules created by shear force due to the flow of the polymer melt in a duct rapidly relaxes after the change in the shear rate. This relaxation causes the changes in anisotropic behavior of mechanical. thermal, and optical properties which have strong effect in precision forming of casted polymer products. Among these properties, thermal diffusivity is one of the most difficult properties to measure by the conventional techniques. The present paper describes and discusses the subsecond measurement of relaxation characteristics in thermal dilfusivity anisotropy of flowing polymer melt in conjunction with measurements of some typical flow characteristics. The method used was the forced Rayleigh scattering method, an optical method which was developed and modified by the authors' group for applying to oriented polymer materials. The measuring lime of I ms can trace the relaxation occurred in the order of several seconds. Measured results of thermal diffusivity anisotropy were compared with transient characteristics of the now and quantitatively showed a good agreement with estimated behavior of oriented polymer molecules.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–39, 1995, Köln, Germany.     

Recent development and applications of an optical method for measurements of thermophysical properties

The experimental determination of thermophysical properties has been greatly improved by the introduction of laser technology. The laser beam is used for sensing and also for heating (or exciting) the specimen. The advantage of using a laser beam is most strongly felt in the measurement of the thermal conductivity or the thermal diffusivity, which are some of the most difficult properties to measure. Interesting features of new techniques for investigating various aspects of thermal conductivity in fluids and solids are reviewed. An optical method, the so-called forced Rayleigh scattering method, or the laser-induced optical-grating method, has been developed and used extensively by the present author's group. The method is a high-speed remote-sensing method which can also quantitatively detect anisotropy, namely, direction dependence of heat conduction in the material. It was used for determination of the thermal diffusivity and its anisotropic behavior for high-temperature materials such as molten salts, liquid crystals, extended polymer samples, and flowing polymer melts under shear. Interesting applications of the method were demonstrated also for thermal diffusivity mapping and microscale measurement.Invited paper presented at the Twelfth symposium on Thermophysical Properties, June 19–24, 1994, Boulder, Colorado, U.S.A.     

Measurements of the Thermal Conductivity and Thermal Diffusivity of Polymer Melts with the Short-Hot-Wire Method

In this paper, the thermal conductivity and thermal diffusivity of four kinds of polymer melts were measured by using the transient short-hot-wire method. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a wire with the same length-to-diameter ratio and boundary conditions as those in the actual experiments. The present method is particularly suitable for measurements of molten polymers where natural convection effects can be ignored due to their high viscosities. The results have shown that the present method can be used to measure the thermal conductivity and thermal diffusivity of molten polymers within uncertainties of 3 and 6%, respectively. Further, the thermal conductivity and thermal diffusivity of solidified samples were also measured and discussed.     

Real-Time Sensing of the Thermal Diffusivity for Dynamic Control of Anisotropic Heat Conduction of Liquid Crystals

Molecular orientational order can be used to characterize the anisotropic behavior in mechanical, optical, and thermophysical properties. The creation of appropriate molecular orientation has the potential for producing a novel material or thermal switching device, which can control anisotropic heat conduction. Liquid crystals, which are widely used in display elements, have anisotropy not only in their optical, but also in their thermophysical properties, under given molecular orientational alignment conditions; this material can be a variable device with anisotropic heat conduction by controlling the molecular alignment. In the present study, a real-time sensing system for thermal diffusivity using the forced Rayleigh scattering (FRS) method was developed to investigate the transient behavior in the thermal anisotropy of nematic liquid crystals. This technique can be used to measure the in-plane thermal diffusivity perpendicular to the transient thermal grating created by interfering pulsed laser beams, and the thermal anisotropy of the sample can be determined using this non-contact method. The present FRS system can provide continuous measurements of the thermal diffusivity with subsecond time resolution, allowing evaluation of the dynamic behavior of anisotropy in the thermal diffusivity even during a transient process. In this article, the anisotropy of the in-plane thermal diffusivity of 4-4′-pentyl-4-biphenylcarbonitrile (5CB) with molecular alignment induced by either a rubbed substrate or an electric field has been measured. Also, the time evolution of the anisotropic thermal diffusivity in real-time under a dynamically controlled external electric field has been measured. The experimental results demonstrate the capability of dynamic anisotropic control of heat conduction by molecular alignment variations.     

Anisotropic Thermal Diffusivity Measurements in Deforming Polymers and the Stress-Thermal Rule

In recent years, both experimental and theoretical research on energy transport in deforming polymeric materials has increased. Theoretical results indicate that the thermal conductivity in such systems is anisotropic, and support, analogous to the well-known stress-optic rule, the validity of a stress-thermal rule where the thermal conductivity and stress tensors are linearly related. In this study a method to measure the thermal diffusivity in deforming polymers has been developed. The method is based on an optical technique known as forced Rayleigh scattering. This sensitive and non invasive technique is shown to be capable of quantitative measurements of anisotropic thermal diffusivity in both static and dynamic (relaxing) polymers subjected to deformations. Results have been obtained for a polymer melt in step-shear strain flows and a cross-linked elastomer in uniaxial extension. Thermal diffusivity data are complemented by measurements of stress and birefringence so that evaluations of the stress-optic and stress-thermal rules can be made. Results show that the thermal diffusivity is enhanced in the flow (or stretch) direction compared to the equilibrium value and that the stress-thermal rule is valid for the modest deformations achieved in this study.     

Effect of screw rotating speed on polymer melt temperature profiles in twin screw extruder

The effect of screw rotating speed on two-dimensional temperature profiles of flowing polypropylene melt was investigated in the barrel of a counter-rotating twin screw extruder using a designed experimental apparatus and a thermocouple temperature sensing device, the experimental apparatus being connected to a high speed data logger and a computer. The flow patterns of the polymer melt in the barrel of the extruder were also revealed. The changes in melt temperature profiles with extruding time were discussed in terms of flow patterns of the polymer melt during the flow, the increase in melt temperature being closely associated with total flow length of the melt, and shear heating and heat conduction effects.     

注塑充模聚合物流动形态与力学行为分子机制研究

以聚甲基丙烯酸甲酯(Polymeric methyl methaerylate,PMMA)为实验材料,基于分子动力学模拟实验研究了注塑成型聚合物充模流动与力学行为的分子机制.构建包含10条聚合度为20的无规PMMA分子链所构成的链团模型,基于能量最小化与SA算法实现了体系能量初始化;基于周期性边界,引入COMPASS从头算分子力场及Velocity-Verlet算法,实现了PMMA胞元在恒温平面流场中的流态与力学行为模拟实验.结果表明,PMMA充模与形变过程首先需要克服包含体系内能、分子链松弛与解缠在内的“活化能”,且存在时间和应力阈值,前者体现了瞬时加载内能协调效应,后者对应于高剪切力作用下分子松弛与解缠现象.体系C原子回转半径分布表明剪切力的作用使得高分子沿流场方向取向排布,剪切力越大则取向越明显,剪切力过大则分子链将断裂而弹性恢复.MSD结果揭示了熔态聚合物充模流动的实质是大分子链定向迁移和取向排布协调运动的结果,且进一步验证了“活化能”的存在,克服这一制约之后大分子链的迁移效应才变得明显,且迁移速率随剪切应力的增大呈非线性增大变化.     

气辅挤出成型中两相流界面的稳定性

使用南昌大学制备的气辅挤出成型设备,研究了气体压力和流量对熔体/气体两相流界面稳定性的影响,通过实验得出,实现气辅挤出的条件是口模内注气点处气体压力大于等于熔体的压力,而pmelt≈pgas是实现稳定气辅挤出的前提条件;实验还表明控制气体流量对实现稳定挤出非常重要,当气体流量为0.4 m3/s时,熔体以稳定层流形式挤出,气体流量增大到2 m3/s时,气体产生紊流并强迫挤出物以波纹状进行抖动挤出。     

Epoxy resin-based nanocomposite films with highly oriented BN nanosheets prepared using a nanosecond-pulse electric field

Facile orientation of boron nitride (BN) with high anisotropy in epoxy resin-based nanocomposite films was performed in a polyepoxide matrix using a nanosecond-pulse electric field to generate a high electric flux. Control of the BN anisotropy was achieved in the polymer without damaging the composite films or requiring surface modification of the BN. The degree of BN orientation perpendicular to the nanocomposite film plane, which was parallel to the electric flux, could be controlled by applying the nanosecond pulse for different lengths of time before cross-linking. The resulting composite films with oriented BN nanosheets manifested improved thermal diffusivity compared to a composite prepared without orientation.     

Measurement of the Thermal Diffusivity of an Anisotropic Graphite Sheet Using a Laser-Heating AC Calorimetric Method

The thermal diffusivity of a graphite sheet having an extremely high anisotropy has been measured by a laser heating AC calorimetric method in the temperature range from 30 to 350 K. This graphite sheet has characteristics of high thermal diffusivity and high anisotropy, and it is only 100 m thick. Thus, it is difficult to apply the conventional AC technique. Therefore, we propose a simultaneous measurement method for the in-plane and out-of-plane thermal diffusivities, by analyzing the three-dimensional heat conduction process, which contains the effects of anisotropy and thermal wave reflections. This method was verified by checking with thermal diffusivity measurements of isotropic materials such as stainless steel and pure copper and was then applied to the anisotropic thermal diffusivity measurement of the graphite sheet.     

纵向超声波辅助微注塑方法

微注塑过程中,聚合物熔体在微小腔体中流动时充模阻力比常规注塑大,这影响了熔体填充效果,同时热量损失的不均衡性和不确定性容易导致注塑精度不高．提出了纵向超声波辅助微注塑方法,并对超声波振动对聚合物熔体的作用机理进行了探讨,分析了超声换能器结构对应力、振幅和响应频率的影响．基于对微注塑过程的模拟结果,开发了纵向超声波辅助微注塑装置．通过在微注塑过程中纵向超声波对熔体的能量作用降低熔体黏度,改善了熔体流动和充填性能．为了验证超声波辅助微注塑的效果,进行了菲涅尔透镜实际注塑实验．实验结果表明,相同的注塑工艺条件下,超声辅助微注塑过程中聚合物熔体的充填性能提高了6．91％．     

Synthesis,processing sand properties of thermotropic liquid-crystal polymers

Thermotropic liquid-crystal polymers for high-performance applications are typically based on wholly aromatic polyester and polyamide architectures. The linear character of the aromtic monomers produces polymer chains with stiff, extended conformations. As a result, the chains organize themselves into a nematic melt and readily orient in response to processing flow fields. The close coupling between chain orientation and flow fields produces both a rich variety of materials and a high degree of controllability of their structure and properties (such as the tensile strength, elastic modulus and coefficient of thermal expansion). In this paper, we describe synthetic efforts to develop liquid-crystal polymer structures and the relationships between chain molecular structure, processing, and properties of this class of polymers in two distinct processing situations: Injection molding and fiber formation. The special importance of the orientational flow field in developing high orientation and excellent mechanical properties will be highlighted.     

NPL Initiatives in the Development of Thermal Properties Reference Materials

The present paper describes the status of a number of studies involving NPL which are directed towards development of internationally accepted reference materials to serve different requirements. These include: (1) thermal diffusivity and thermal conductivity measurements on candidate molten metal reference materials; (2) co-operative flash diffusivity measurements on glassy carbon, a dense fine grain alumina, and other ceramics; (3) inter-European studies, one using different steady-state and transient methods to evaluate the properties of a high temperature ceramic and the other using the guarded hot plate method to produce a high temperature thermal insulation reference; (4) a six-laboratory European program to certify a replacement European thermal insulation reference material for use at room temperature; and (5) an intercomparison between standards organizations in five countries on the current reference materials supplied in each country or geographical area. Brief mention is made concerning other on-going and planned work together with recommendations for materials to address additional needs.     

Fibre formation in injection-moulded polyethylene-sodium lignosulphonate blends and its relationship to the morphological zones

In injection-moulded parts, the degree of orientation (anisotropy) varies over the cross-section. Normally, such parts exhibit a layered structure, with a biaxially oriented surface layer (stretching of the flow front during mould filling), a highly oriented shear zone, and a core region with a relatively low orientation. This paper describes a novel method to show the different morphological zones by studying the occurrence and structure of fibres formed during injection moulding of certain incompatible blends, in the present case a mixture of high-density polyethylene and a technical lignosulphonate grade (Wanin S). The polyethylene fibres formed during processing of this composite were ribbon-shaped in the surface layer (biaxial orientation), while they had a normal appearance in the shear zone. Only relatively few fibres were found in the core region. A qualitative agreement was found when the fibre character in the various layers was compared with orientation data from thermal shrinkage measurements, as obtained with pure polyethylene samples (test bars).     

An instrument for online measurement of structural orientation in translucent polymer sheets

In many applications of polymer materials, a high tensile strength is required. There is a definite correlation between the degree of orientation of molecular chains achieved in the orientation process and the tensile strength obtained. In this work, we describe design and fabrication details of a device, ‘Laser based polymer orientation grader’, for online measurement of the degree of orientation to obtain a high consistent tensile strength as output. This instrument has been designed making skilful use of the optical anisotropic property of the oriented polymer strip. The principle is based on the fact that the spatial distribution of laser light passing through the translucent polymer sheet depends on the degree of orientation of long chain molecules in the sheet. The method has potential online application in monitoring and controlling of anisotropy in manufacturing processes.     

Depth-Resolved Surface Thermophysical Property Measurement by Laser-Produced Plasmas

A new laser-based method for real-time in situ measurement of thermophysical properties of materials has been developed. It entails production by a high-power laser pulse of a plasma plume from the surface of a condensed-phase specimen and simultaneous measurement of a material's response to the excitation. The specimen may be a solid or in a molten state at high temperatures. It has been shown that the thermal diffusivity can be determined, for instance, from the mass loss due to laser excitation. In one implementation the mass loss is determined from the impulse imparted on the surface by the ablated matter which is measured by an impulse transducer. In this paper, we present a new spectroscopic method for measurement of the mass loss, facilitating in situ non-contact measurement of the thermal diffusivity for the first time. An implementation of this method is described, whereby the thermal diffusivity of a complex layered surface is determined as a function of depth with resolutions as small as 13 nm.     

剪切流场对塑料发泡成核行为的影响

本文着重讨论了剪切流场对成核行为的影响,对宏观规律进行微观分析。通过剪切流场对高聚物熔体中分子取向排列的影响,在熔体中形成大量均匀分布的成核点,这不仅为开发优质泡体提供了理论依据,也是为国外正在加紧开发的微孔发泡技术提供参考。     

Nanometer-scale flow of molten polyethylene from a heated atomic force microscope tip

We investigate the nanometer-scale flow of molten polyethylene from a heated atomic force microscope (AFM) cantilever tip during thermal dip-pen nanolithography (tDPN). Polymer nanostructures were written for cantilever tip temperatures and substrate temperatures controlled over the range 100-260?°C and while the tip was either moving with speed 0.5-2.0 μm s(-1) or stationary and heated for 0.1-100 s. We find that polymer flow depends on surface capillary forces and not on shear between tip and substrate. The polymer mass flow rate is sensitive to the temperature-dependent polymer viscosity. The polymer flow is governed by thermal Marangoni forces and non-equilibrium wetting dynamics caused by a solidification front within the feature.     

纤维增强聚合物熔体的纤维取向和流场应力分析

针对纤维增强聚合物基复合材料的成型流动过程, 基于宏观流场、 介观纤维取向和微观聚合物大分子链三尺度信息耦合的多尺度模型, 使用有限体积法和有限差分法相结合的数值算法, 分析了纤维增强聚合物熔体在收缩流腔中的流动行为, 得到了其纤维取向和应力分布, 并讨论了纤维存在对聚合物熔体流场应力的影响。结果表明: 当剪切运动占优时, 纤维呈现周期旋转取向; 而拉伸运动占优时, 纤维沿单轴拉伸取向。同时, 由于纤维的周期旋转, 纤维增强聚合物熔体法向应力差的空间分布呈现出随时间逐步发展的拟序涡结构。