超拉伸聚乙烯的弹性模量和导热性能

为了揭示聚合物分子链伸展、取向的本征特性，发展了两个新的测量方法和实验装置，用于研究拉伸比高达２００的超拉伸聚乙烯凝胶的弹性性能、传热性能和聚合物结构的关系．应用激光脉冲热致超声法给出材料拉伸方向和横向杨氏模量，应用激光脉冲光热辐射法给出拉伸方向，横向和厚度方向的导热系数．随拉伸比λ的增加，轴向杨氏模量急剧的增加，而横向的仅有少许减小．导热系数具有相似的特性．本文发现当λ＝２００时，这种拉伸取向聚乙烯的轴向模量可达钢的８０％，而导热系数甚至可达２倍，直至成为热的良导体，这是由于在高拉伸比时形成了相当数量的伸展分子链构成的针状晶体———晶桥．本文提出晶桥作为短纤维分散相的取向聚合物的结构模型，对于超拉伸聚乙烯的上述特性可以进行统一描述和定量化分析，和实验结果很好符合．     

Thermal expansion coefficient and bulk modulus of polyethylene closed‐cell foams

A regular Kelvin foam model was used to predict the linear thermal expansion coefficient and bulk modulus of crosslinked, closed‐cell, low‐density polyethylene (LDPE) foams from the polymer and gas properties. The materials used for the experimental measurements were crosslinked, had a uniform cell size, and were nearly isotropic. Young's modulus of biaxially oriented polyethylene was used for modeling the cell faces. The model underestimated the foam linear thermal expansion coefficient because it assumed that the cell faces were flat. However, scanning electron microscopy showed that some cell faces were crumpled as a result of foam processing. The measured bulk modulus, which was considerably smaller than the theoretical value, was used to estimate the linear thermal expansion coefficient of the LDPE foams. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3741–3749, 2004     

Hardness and Young's modulus of transcrystalline polypropylene by Vickers and Knoop microindentation

A study of the anisotropic microhardness and Young's modulus of transcrystalline isotactic polypropylene grown from the surface of high modulus carbon fibers is described. Static microindentation experiments were performed with Knoop and Vickers tips. The Young's moduli of the transcrystalline region were estimated from Knoop microindentation data by using a method recently developed in our laboratory. Data for the different lamellar directions were generated using the Knoop tip, which is sensitive to material anisotropy. We found that the hardness and Young's modulus of the transcrystalline layer are higher by up to 30% when the longer diagonal of the probing Knoop tip is perpendicular to the transcrystalline growth direction, compared to when the diagonal is parallel to that direction. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 523–530, 1999     

Thermal conductivity of high strength polyethylene fiber in low temperature

High strength polyethylene fiber (Toyobo, Dyneema® fiber, hereinafter abbreviated to DF) used as reinforcement of fiber‐reinforced plastics for cryogenic use has a high thermal conductivity. To understand the thermal conductivity of DF, the relation between fiber structure and thermal conductivity of several kinds of polyethylene fibers having different modulus from 15 to 134 GPa (hereinafter abbreviated to DFs) was investigated. The mechanical series‐parallel model composed of crystal and amorphous was applied to DFs for thermal conductivity. This mechanical model was obtained by crystallinity and crystal orientation angle measured by solid state NMR and X‐ray. Thermal conductivity of DF in fiber direction was dominated by that of the continuous crystal region. The thermal conductivity of the continuous crystal part estimated by the mechanical model increases from 16 to 900 mw/cmK by the increasing temperature from 10 to 150K, and thermal diffusivity of the continuous crystal part was estimated to about 100 mm²/s, which is almost temperature independent. The phonon mean free path of the continuous crystal region of DF obtained by thermal diffusivity is almost temperature independent and its value about 200 Å. With the aforementioned, the mechanical series‐parallel model composed of crystal and amorphous regions could be applied to DFs for thermal conductivity. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1495–1503, 2005     

Morphological study on poly-p-phenylenebenzobisoxazole (PBO) fiber

Morphological survey on new PBO fiber (Zylon®) was conducted by X-ray and transmission electron microscopic studies. Crystal size, orientation of the crystal, fibrils, microvoids, and fine structure were discussed. It was found that the molecule in the fiber showed high orientation (more than 0.99 in Hermann's orientation function for heat-treated fiber) and relatively small crystal sizes in the longitudinal (160 Å) and the transverse (110 Å) directions. Crystal modulus estimated by extrapolation to perfect orientation on the plot of the fiber modulus as a function of fiber orientation (Northolt's method) shows discrepancy from the crystal modulus directly obtained by X-ray scattering. This discrepancy means that the Northolt's model is insufficient to describe the Young's modulus of PBO fiber. Microvoids elongated to the fiber direction were examined by small-angle X-ray scattering and transmission electron microscopic methods. The diameter of the microvoids was 20 Å to 30 Å and the fiber had a very thin microvoids-free layer (0.2 μm). Preferential orientation of the a-axis of crystal in the fiber was also confirmed. Summarizing these results, a structure model of the PBO fiber was proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 39–48, 1998     

Thermal conductivity and thermal expansivity of thermotropic liquid crystalline polymers

The thermal conductivity and thermal expansivity of a thermotropic liquid crystalline copolyesteramide with draw ratio λ from 1.3 to 15 have been measured parallel and perpendicular to the draw direction from 120 to 430 K. The sharp rise in the axial thermal conductivity K_par; and the drastic drop in the axial expansivity α_∥ at low λ, and the saturation of these two quantities at λ > 4 arise from the corresponding increase in the degree of chain orientation revealed by wide-angle x-ray diffraction. In the transverse direction, the thermal conductivity and expansivity exhibit the opposite trends but the changes are relatively small. The draw ratio dependences of the thermal conductivity and expansivity agree reasonably with the predictions of the aggregate model. At high orientation, K_par; of the copolyesteramide is slightly higher than that of polypropylene but one order of magnitude lower than that of polyethylene. In common with other highly oriented polymers such as the lyotropic liquid crystalline polymer, Kevlar 49, and flexible chain polymer, polyethylene, α_par; of the copolyesteramide is negative, with a room temperature value differing from those of Kevlar 49 and polyethylene by less than 50%. Both the axial and transverse expansivity show transitions at about 390 and 270 K, which are associated with large-scale segmental motions of the chains and local motions of the naphthalene units, respectively. ©1995 John Wiley & Sons, Inc.     

Anisotropic elastic moduli and Poisson's ratios of a poly(ethylene terephthalate) film

Expressions for the directional dependence of Young's modulus and Poisson's ratio were derived for a general material under plane‐stress conditions. Experiments with a laser extensometer to measure the Young's modulus and Poisson's ratio directly by a tension test are described, and the results are compared with the theoretical expressions. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 260–266, 2004     

An analysis of deformation process on poly‐p‐phenylenebenzobisoxazole fiber and a structural study of the new high‐modulus type PBO HM+ fiber

This study is concerned with fiber structure of new high‐modulus type PBO fiber. Crystal modulus and molecular orientation change with stress was surveyed. Standard‐modulus type PBO (AS) fiber has hysteresis effect to applied stress while high‐modulus type PBO (HM) fiber shows reversible change. In order to raise actual PBO fiber modulus higher, nonaqueous coagulation process was adopted with conventional heat treatment. The fiber (HM+) so made gives 360 GPa in the Young's modulus and an absence of small‐angle X‐ray scattering pattern that is characteristic for aqueous‐coagulated PBO fiber with heat treatment (Zylon™ HM). The crystal structure form and crystal size for the HM+ fiber are the same as those of the HM fiber. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1605–1611, 2000     

Theoretical modeling of the relationship between Young's modulus and formulation variables for segmented polyurethanes

We describe a new modeling approach to prediction of Young's modulus of segmented polyurethanes. This approach combines micromechanical models with thermodynamic considerations based on the theory of block copolymers. The resulting model predicts both the equilibrium morphology and the “ideal” Young's modulus of a segmented polyurethane polymer as a function of its formulation (hard segment chemical structure, hard segment weight fraction, soft segment equivalent weight) and temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2123–2135, 2007     

In situ measurement of the thermal expansion behavior of benzocyclobutene films

In situ measurement techniques suitable for determination of the coefficient of thermal expansion (CTE) in thin, spin‐cast polymer films in both the in‐plane and through‐plane directions are presented. An examination of the thermal expansion behavior of cyclotene thin films has been performed. In particular, the effect of film thickness on the in‐plane and through‐plane CTE and in‐plane Young's modulus of spin‐coated cyclotene films was examined. It is shown that the mechanical response of in situ cyclotene films can be adequately described by isotropic film properties. It was also demonstrated that there is no thickness dependence on the free‐standing mechanical properties or on the resulting through‐plane thermal strain in an in situ film. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 311–321, 1999     

Tensile properties of ultradrawn polyethylene

High-density polyethylene filaments prepared by a solid-state deformation in an Instron capillary rheometer show unusually high crystal orientation, chain extension, axial modulus, and ultimate tensile strength. The Young's modulus and ultimate tensile strength have been determined from stress–strain curves. Gripping of this high modulus polyethylene has been a problem heretofore, but the measurement of ultimate tensile strength has now been made feasible by a special gripping procedure. Tensile moduli show an increase with sample preparation temperature and pressure. Values as high as 6.7 × 10¹¹ dyne/cm² are obtained from samples extruded at 134°C and 2400 atm and tested at a strain rate of 3.3 × 10^?4 sec^?1. The effect of strain rate and frequency on modulus has also been evaluated by a combination of stress–strain data and dynamic tension plus sonic measurements over nine decades of time.     

Molecular dynamics calculation to clarify the relationship between structure and mechanical properties of polymer crystals: the case of orthorhombic polyethylene

The molecular dynamics (MD) technique was used to calculate the temperature dependence of the structure, molecular motion, and mechanical property of the orthorhombic polyethylene (PE) crystal. The potential functional parameters reported by Karasawa et al. (J Phys Chem, 95 (1991) 2260) were refined further so that the vibrational frequencies of infrared and Raman bands, measured by us at ultra-low temperatures for the normal and fully deuterated PE, could be reproduced well. The flip-flop motion around the chain axis and the torsional motion of the skeletal chains were found to start above ca. 350 K and increase the amplitude of these motions progressively. Coupling these two types of chain motion resulted in a steep increase of the thermal vibration parameters or the mean-square-displacements of carbon and hydrogen atoms, corresponding well with the X-ray data. The lattice constants and the related linear thermal expansion coefficients were also found to be in good agreement with the observed data. The calculated Young's modulus along the chain axis decreased gradually with the increasing temperature: 330 GPa at 0 K to 280 GPa at room temperature. The latter was in good agreement with the value of 280–305 GPa evaluated from the Raman measurement of the longitudinal acoustic mode. Young's modulus was found to relate intimately with the chain contraction caused by the skeletal torsional motion. Only 0.3% contraction of the chain resulted in the reduction of the modulus by ca. 35%. A similar behavior was also seen in the trigonal polyoxymethylene and nylon 6 α forms.     

Mechanical properties and structure of the zone‐drawn poly(L‐lactic acid) fibers

The zone‐drawing (ZD) method was applied three times to the melt‐spun poly(L ‐lactic acid) (PLLA) fibers of low molecular weight (M_v = 13,100) at different temperatures under various tensions. The mechanical properties and superstructure of the ZD fibers were investigated. The resulting ZD‐3 fiber had a draw ratio of 10.5, birefringence of 37.31 × 10⁻³, and crystallinity of 37%, while an orientation factor of crystallites remarkably increased to 0.985 by the ZD‐1. The Young's modulus and tensile strength of the ZD‐3 fiber respectively attained 9.1 GPa and 275 MPa, and the dynamic storage modulus was 10.4 GPa at room temperature. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 991–996, 1999     

Dislocation approach to the plastic deformation of semicrystalline polymers: Kinetic aspects for polyethylene and polypropylene*

The plasticity of semicrystalline polymers is analyzed in the framework of Young's dislocation model under the assumption of nucleation of screw dislocations from the lateral surface of the crystalline lamellae. It is proposed that the driving force for the nucleation and propagation across the crystal width of these screw dislocations relies on chain twist defects that migrate along the chains stems and allow a step‐by‐step translation of the stems through the crystal thickness. Such defects are identified as thermally activated conformational defects responsible for the so‐called crystalline relaxation. Dislocation kinetic equations are derived. Plastic flow rates attainable by dislocation motion in polyethylene and polypropylene are assessed with frequency–temperature data of the crystalline relaxation. Comparisons are made with experimental strain rates that enable homogeneous plastic deformation. In addition to temperature, the crystal lamellar thickness, which is a basic factor of the plastic flow stress in Young's dislocation model, is a major factor in dislocation kinetics through its influence on chain twist activation. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 593–601, 2002; DOI 10.1002/polb.10118     

Micromechanical characterization of the interphase layer in semi‐crystalline polyethylene

The interphase layer in semi‐crystalline polyethylene is the least known constituent, compared to the amorphous and crystalline phases, in terms of mechanical properties. In this study, the Monte Carlo molecular simulation results for the interlamellar domain (i.e. amorphous+ interphases), reported in (Macromolecules 2006, 39, 439–447) are employed. The amorphous elastic properties are adopted from the literature and then two distinct micromechanical homogenization approaches are utilized to dissociate the interphase stiffness from that of the interlamellar region. The results of the two micromechanical approaches match perfectly. Interestingly, the dissociated interphase stiffness lacks the common feature of positive definiteness, which is attributed to its nature as a transitional domain between two coexisting phases. The sensitivity analyses reveal that this property is insensitive to the non‐orthotropic components of the interlamellar stiffness and the uncertainties existing in the interlamellar and amorphous stiffnesses. Finally, using the dissociated interphase stiffness, its effective Young's modulus is calculated, which compares well with the effective interlamellar Young's modulus for highly crystalline polyethylene, reported in an experimental study. This satisfactory agreement along with the identical results produced by the two micromechanical approaches confirms the validity of the new information about the interphase elastic properties in addition to making the proposed dissociation methodology quite reliable when applied to similar problems. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013 , 51, 1228–1243     

Hydrostatic extrusion of linear polyethylene: Effects of extrusion temperature and polymer grade

The hydrostatic extrusion behavior of linear polyethylene has been examined for two homopolymers of very different molecular weight characteristics and for a copolymer. Good unflawed extrudates could be obtained in all cases, and the extrusion behavior at a fixed temperature correlated well with the melt flow index. Although the maximum values of axial Young's modulus obtainable from the higher molecular weight homopolymer and the copolymer were lower than those possible for the lower molecular weight homopolymer, such materials do show improvements in creep behavior which could be advantageous. The effect of temperature on the extrusion behavior is discussed; the results suggest that for each grade of polymer there is an optimum temperature for effective extrusion, i.e., extrusion which gives optimum modulus enhancement. Finally, the melting behavior and the temperature dependence of the axial Young's moduli of the extrudates are considered in terms of our present knowledge of the structure of these high modulus materials.     

Effects of humidity on Young's modulus in poly(methyl methacrylate)

Tensile tests on poly (methyl methacrylate) (PMMA) were conducted to clarify the effects of humidity and strain rate on tensile properties, particularly Young's modulus. Prior to the tensile tests, specimens were kept under various humidity conditions at 293 K, which were the same as the test conditions, for a few months to adjust the sorbed water content in the specimens. The tensile tests were performed under each humidity condition at three different strain rates (approximately 1.4 × 10^?3, 1.4 × 10^?4, and 1.4 × 10^?5 s^?1). Stress‐strain curves changed with humidity and strain rate. Young's moduli were also measured at small applied stresses (below 6.7 MPa) under various humidity conditions at 293 K. Young's modulus decreases linearly with increasing humidity and a decreasing logarithm of strain rate. These results suggest that Young's modulus of PMMA can be expressed as a function of two independent parameters that are humidity and strain rate. A constitutive equation for Young's modulus of PMMA was proposed. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 460–465, 2002; DOI 10.1002/polb.10107     

Ultrasonic measurements of the elastic moduli of ultradrawn polyoxymethylene

We have employed an ultrasonic method to measure from ?40 to 60°C the five independent elastic moduli C₁₁, C₁₃, C₃₃, C₄₄, and C₆₆ of polyoxymethylene with draw ratio λ from 1 to 26 prepared by continuous drawing under microwave heating. The elastic moduli are controlled by three major factors: molecular orientation in the crystalline regions, fraction of noncrystalline taut tie molecules, and void content. The steep rise in the axial extensional modulus C₃₃ and axial Young's modulus E₀ with increasing draw ratio results from the alignment of chains in the crystalline blocks and an increase in the number of disordered taut tie molecules. Below the γ relaxation (located at 0°C at our measurement frequency of 10 MHz), these two factors also give rise to a slight decrease in the transverse extensional modulus C₁₁, Young's modulus E₉₀ and shear modulus C₆₆. At high temperature where the amorphous regions have very low modulus, the stiffening effect of taut tie molecules becomes dominant, leading to an increase in all moduli as λ increases from 1 to 10. At higher λ the void fraction increases appreciably, causing small decreases in E₉₀, C₁₁, and C₆₆ at all temperatures.     

Porosity‐dependent Young's modulus of membranes from polyetherether ketone

The porosity‐dependent Young's modulus for PEEK membranes was determined and the data compared to several empirical and semiempirical equations often applied to porous systems. The Spriggs equation, Wang's approximation, Sudduth's equation, and the foam modulus‐density relationship were all tested against the data. The relatively wide range of porosities tested in these experiments shows the Spriggs equation to be inadequate to fitting the data, especially above 50% porosity where the Young's modulus decreases rapidly. Wang's approximation to second order fitted the data well, and the porosity‐modulus relations had non‐negative coefficients as required and consistent with the ceramic data obtained by others. The data also fitted Sudduth's equations, usually applied to sintered ceramics, but equivalently good fits were obtained with nonunique fitting parameters. The foam modulus‐density relationship fitted the data for foamlike membranes but fitted less well to nonfoam morphology membranes. Finally, the data were used to determine the range of porosities and hollow fiber dimensions necessary for microfiltration and composite membrane application. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1168–1174, 2003     

Effect of the comonomer content on the mechanical parameters and microhardness values in poly(ethylene‐co‐1‐octadecene) synthesized by a metallocene catalyst

Stress–strain and microhardness measurements were carried out on a series of copolymers of ethylene and 1‐octadecene with different comonomer contents in the corresponding homopolymer of ethylene, synthesized with a metallocene catalyst. The different mechanical properties, deduced from the stress–strain curves (Young's modulus, yield stress, deformation at break, and energy to break) are interpreted in terms of the crystallinity and molecular weight of the samples because these two characteristics show considerable variations with the comonomer content. The microhardness values are explained in terms of these properties, and they are also correlated with Young's moduli and yield stresses deduced from the stress–strain curves. Linear relations are found between microhardness and yield stress and between the logarithm of the microhardness and the logarithm of the elastic modulus. The properties deduced from these lines are compared with literature values. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 277–285, 2001