Influence of extrusion ratio on the tensile properties of ultradrawn polyethylene

The tensile properties have been evaluated for high-density solid-state polyethylene extruded to different extrusion (draw) ratios. The results are compared with measured and theoretical values on this and other polymers. An extrusion (draw) ratio and a deformation gradient are defined and discussed. The content of extended tie molecules in extruded high-density polyethylene was calculated from a model and modulus data.     

Processing of semicrystalline polymers by high-stress extrusion

A study has been conducted on the solid-state extrusion of three semicrystalline polymers:poly-propylene (PP), poly(vinylidene fluoride) (PVDF), and high-density polyethylene (HDPE). HDPE has been extruded in continuous lengths with area reductions up to 25× at temperatures substantially below the melting region. Such extrusion has been identified as a solid-state process, since measurements of the temperature of the polymer during extrusion indicate the absence of significant heating due to deformation. In contrast, continuous lengths of PP and PVDF could not be obtained substantially below their melting temperatures, indicating that crystallization during extrusion is an important process for these polymers. Under severe extrusion conditions (low temperatures, high area reductions. etc.), all three polymers failed within the tapered region of the extrusion die. Two modes of failure have been identified, brittle fracture and, surprisingly, necking. Grid-line distortion patterns and a highly simplified upper-bound plasticity analysis both indicate that shear deformations are a major factor during high-stress extrusion.     

A microcalorimetric study of crystallizable polymers subjected to severe plastic deformation

Differential scanning calorimetry has been used to investigate the evolution of the structural and thermophysical parameters of a number of semicrystalline polymers (high-density polyethylene, polyamide-6, polyoxymethylene) that is initiated by severe plastic deformation imposed through equal-channel multiple-angle extrusion. Thermograms of the deformed polymers have been found to exhibit an additional high-temperature melting peak. It has been shown that the onset, maximum, and end temperatures for both melting peaks increase with the strain buildup. The degrees of crystallinity and the thicknesses of crystallites increase as well. The magnitude of the effects is determined by the deformation route selected. It has been revealed that conformational transitions due to the formation of “double-triple” folds in macromolecular chains can occur during the course of equal-channel multiple-angle extrusion.     

Tribological properties of an antifriction polymer modified by severe plastic deformation

The possibility of applying severe plastic deformation technologies to improving the tribological characteristics of semicrystalline antifriction polymers was studied. By the example of Nylon 6 subjected to equal-channel multiple angular extrusion, it was shown that severe plastic deformation favors a significant (more than three orders of magnitude) increase in the wear resistance of polymer and a 15–20% decrease in its friction coefficient. There are also increases in the maximum allowable contact pressure and temperature in the friction zone, at which the wear and the friction coefficients are stable.     

General principles of polymer processing modeling

Specific rheological and thermal properties of molten polymers and their consequences on the numerical simulation of forming processes are first reviewed. The strong coupling between flow kinematics and material temperature is pointed out. According to geometrical specificities, to the type of deformation the material is submitted to and to thermal conditions, continuum mechanics and energy balance equations can be simplified and more or less sophisticated rheological equations can be used. Finally, these considerations are applied to the modeling of steady extrusion and fiber spinning and of unsteady molding processes.     

Processing,structure and properties of oriented polymers

The fabrication techniques now available for the production of highly oriented polymers are reviewed. These techniques include tensile drawing from both melt-spun and gel-spun polymers, extrusion under pressure from the melt, and hydrostatic extrusion, ram extrusion and die drawing in the solid phase. In addition, lyotropic and thermotropic liquid crystalline polymers offer new routes to very stiff and strong polymers. Following the review of processing methods, an account is given of low strain mechanical behaviour and its relationship to structure, thermal properties (including thermal conductivity and thermal expansion behaviour) and barrier properties (permeability to liquids and gases and solubility).     

高分子熔体和浓溶液流变性能的研究

基于多重缠结网络结构模型和高分子链上缠结点在流动中可进行动态解缠和再缠结的多重蠕动机理,用统计力学和动力学相结合的方法,分别计算出了缠结链组的末端距分布函数;处于缠结状态下高分子链构象统计分布函数;受力下聚合物熔体粘弹性形变自由能和解除外力下高分子挤出体可回复性粘弹性形变自由能,提出了高分子挤出体可回复形变的粘弹性分子理论。推导出的高分子熔体的回忆函数、简单剪切流下的本构方程和物料函数,并采用一种新的方法测定出物料的四种参数： η0、 GN0、 n′和 a。对于高分子挤出体,可回复性粘弹性形变由快速弹性形变和慢速粘弹性形变两者组成,当把两种形变量的复合结构参数－分子链的反式构象分数引入两种形变自由能表达式后,就从理论上得到了可回复形变量同挤出胀大比间的定量表达式,从而建立起一个具有分子链结构参数的新的挤出胀大比方程,可回复形变量同挤出条件（温度、挤出速率和量以及口模长径比不同的挤出机）和树脂结构特征（分子量及分布）的关系式以及在特殊情形下的简化表达式,并用几种高分子熔融体系的挤出胀大比和可回复性形变量的实验数据对理论进行验证,理论方程同实验数据较好的符合。     

Liposome formation of selectively-polymerized diene-containing phospholipids and their postpolymerization

Small and large unilamellar liposomes composed of 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) are prepared by sonication and extrusion, respectively. They are polymerized with water-insoluble radical initiator, azobis(isobutyronitrile) (AIBN) which can selectively polymerize diene groups in 1-acyl chains of the lipids. Polymerized liposomes are freeze-dried to obtain the polymerized liposome powder. There are two methods to redisperse lyophilized liposomes into water. The extrusion is an effective method to disperse them because the energy at extrusion is necessary only for redispersion, whereas the excess energy at sonication gives damage on liposome structure. There is no difference in stability between polymerized liposomes before and after redispersion with extrusion. DODPC polymers, obtained from free radical-initiated polymerization with AIBN, are linear and have polymerizable diene groups in 2-acyl chains. The liposome powder is therefore soluble in organic solvents. Reconstruction of polymerized liposomes is performed with lipid polymers having low or high molecular weight. The lipid polymers having high molecular weight provide stable large unilamellar liposomes by ethanol injection, but unstable small unilamellar liposomes are formed by sonication. The liposomes reconstructed from lipid polymers having low molecular weight are unstable regardless of their size. After reconstruction of liposomes selectively polymerized by AIBN, diene groups in 2-acyl chains are polymerized by water-soluble radical initiator or UV-irradiation to yield highly crosslinked structure. Their stability is improved remarkably by this postpolymerization.     

Grundzüge der Deformationsprozesse in Metallen

The physical principles of our present understanding of the elastic, the anelastic, and the plastic deformation of metals are discussed. The following specific cases are considered in details.

1. Elastic deformation due to one of the following processes. Energy elasticity, vibrational entropy elasticity, anomalies in the elastic behaviour resulting from lattice instability or ferromagnetism. This behaviour is compared with the entropy and energy elasticity of polymers.
2. Anelastic deformation due to one of the following mechanisms. Difussion of solute atoms and/or point defects, dislocation relaxation, dislocation resonance interaction between dislocations and other lattice defects, migration of grain boundaries, phase boundaries or domain walls.
3. Plastic deformation of pure metals by cooperative shear, dislocation motion, mechanical twinning, martensitic transformations and by diffusion. The effect of alloying on these processes is discussed in the case of solid solutions, ordered alloys and precipitation hardened materials. The dominant deformation modes of polymers are different from those of metals because metals are formed by atoms with relatively isotropic interatomic forces, whereas polymers generally consist of long chains characterized by a strong anisotropy in chemical bonding. This difference makes the deformation stress of polymers and metals depend differently on temperature, pressure, and deformation rate. An other characteristic difference between the deformation of polymers and metals is that metals deform plastically, whereas a large fraction of the so-called plastic deformation of polymers is really anelastic.

     

Strain-induced fibrillation of glassy polymers

Literature data on structural rearrangements taking place in amorphous glassy polymers upon their plastic deformation are analyzed. This deformation is shown to be primarily accompanied by polymer self-dispersion into fibrillar aggregates composed of oriented macromolecules with a diameter of 1—10 nm. The above structural rearrangements proceed independently of the deformation mode of polymers (cold drawing, crazing, or shear banding of polymers under the conditions of uniaxial drawing or uniaxial compression). Principal characteristics of the formed fibrils and the conditions providing their development are considered. Information on the properties of the fibrillated glassy polymers is presented, and the pathways of their possible practical application are highlighted.     

Direct pyrolysis in the mass spectrometer of aromatic polysulfonates and polythiosulfonates

The thermal degradation mechanism of three aromatic polysulfonates and polythiosulfonates was investigated by direct pyrolysis in the ion source of a mass spectrometer. Thermal degradation reactions were followed directly by this method by detecting the thermal and electron impact induced fragments. The results obtained have provided evidence that sulfur dioxide extrusion from the polymer backbone takes place in these polymers above 300°C. The synthesis and molecular characterization of the polymers studied are reported in the text.     

Deformation of semicrystalline polymers via crystal–crystal phase transition

A classification is given of flexible, semicrystalline polymers based on the early stage deformation behavior in the solid state. The criteria of each category are discussed with experimental evidence from the literature on 17 polymers. The central aim of this classification is to point out the possibility of ductility induced during deformation. The understanding of the induced ductility in a given semicrystalline polymer suggests a systematic route to optimize solid-state deformation processes for achieving high draw ratios.     

Electron emission during deformation of polymers

Electron emission was detected during deformation of both carbochain and heterochain polymers in vacuum. It was found that the features of emission are similar to those observed in molecular scissions under drawing of unoriented and oriented polymers. This fact indicates that there is a relationship between the fracture process and electron emission under deformation of polymers. This relationship is also obvious from the experiments with interruption of loading when electron emission during the repeated loading does not begin immediately at the moment of load application, but can be recorded only at the degree of deformation which is higher than that reached during the first loading. The interconnection between deformational electron emission and molecular scissions allowed visualization of the fracture process in the subsurface layers of polymers using an electron-optical convertor which gives a mechanoemission image of a stretched sample. It is supposed that the deformation-induced electron emission of polymers is caused by ionization of stressed macromolecules resulting from tunnel transitions of electrons into deep traps. During deformation, the traps are destroyed and a part of electrons escapes in vacuum.     

反应性挤出加工制备无卤阻燃高分子材料

聚合物反应性加工集聚合物加工与化学反应为一体,以聚合物加工装置为反应器,通过聚合物加工过程中的化学反应形成新物质和新结构,实现高分子材料的高性能化和功能化,是高分子材料科学的研究前沿之一.本文简要介绍了我们研究小组近年来采用反应性挤出加工制备高性能无卤阻燃高分子材料方面的研究进展.利用反应性挤出加工剪切力强、温度可控以及易于传质传热的特点实现了常规方法难以合成的高黏阻燃剂三聚氰胺磷酸盐季戊四醇酯（MPP）和三聚氰胺氰尿酸（MCA）的高效合成,制备了综合性能优良的聚丙烯/MPP、尼龙6/MCA等无卤阻燃高分子材料.研究所涉及的化学和物理方法,为聚合物无卤阻燃提供了高效、经济、环保和易于工业化的新技术,并拓宽了聚合物反应性加工的应用领域.     

Deformation structures in solid-state extruded polyethylene

Deformation structures resembling kink bands have previously been reported in a number of oriented semicrystalline polymers which have undergone various modes of deformation. In the present work, such structures have been observed and studied in solid-state extruded polyethylene which has been processed to give a biaxial, “single crystal” texture. Deformation of this material by bending followed by unbending has been observed to lead to shear during the bending stage and to void formation during the unbending stage. The kink bands which form during this treatment exhibit a single morphology regardless of the axis of bending so long as the direction of compression during bending is parallel to the original extrusion direction. Besides intracrystalline slip, which is known to contribute at least in part to the process of kink band formation, mechanisms involving interlamellar slip and interfibrillar slip are also considered. These mechanisms are considered in terms of three distinct experimental observations: the relationship between the kink boundary and the x-ray long period, the process of void formation during unbending, and the single characteristic morphology of the kink bands.     

Toughness Enhancement of Nanostructured Amorphous and Semicrystalline Polymers

An overview is given of different micromechanical deformation processes leading to an enhancement of toughness in heterophase polymers. The well-known mechanism of rubber or particle toughening of semicrystalline polymers was studied in HDPE and PP blends. In particular, the micromechanical processes in the semicrystalline polymer strands between modifier particles were investigated in detail, revealing processes of separation, yielding, breaking and twisting of lamellae. These processes are compared with lamellae forming amorphous SBS block copolymers with alternating soft (polybutadiene) and hard (polystyrene) layers. Depending on the deformation direction, the mechanism of thin layer yielding or chevron formation appears. In both polymeric systems, the initial stage of deformation is characterized by a plastic yielding of the soft phase with a reorganization of the hard (glassy or crystalline) lamellae. The second stage is determined by the alignment of the hard phase towards the deformation direction and the plastic yielding. Detailed comparison of these similar mechanisms in very different polymers with similar nanostructured morphology should help to improve toughening of amorphous as well as semicrystalline polymers.     

Mechanisms of anelastic deformation in solid polymers: Solidlike and liquidlike processes

Several aspects of anelastic deformation of glassy polymers that cannot be explained in terms of existing theories are considered. Resemblance in the stress-strain response for solids of various natures and structures, including semicrystalline and glassy polymers, organic and inorganic solids, and low-molecular-mass and high-molecular-mass compounds, is analyzed. It was pointed out that the phenomena of the yield peak, strain softening, strain concentration (localization) in narrow shear bands, and transient effects are characteristic of the plastic deformation of any solid. The same is true for differences in the kinetics and mechanism of deformation at low (T _def < 0.7T _g) and high deformation temperatures (T _def > 0.7T _g). The mechanism of plastic deformation is discussed in detail for glassy polymers; at microscopic and nanoscale levels, plastic deformation proceeds via two stages: initial nucleation of small-scale shear transformations and their further coalescence. This coalescence leads to the advance of the shear front in the sample and to the nucleation and displacement of classical shear bands. The heat of plastic deformation is released out at the coalescence of shear transformations. It was assumed that shear transformations are responsible for the development and evolution of the yield peak in glassy polymers, strain softening, and other phenomena. The proposed mechanism of deformation in glasses fully agrees with the results of thermodynamic measurements and other experimental data reported in the literature. Computer simulation makes it possible to visualize the scenario of nucleation and evolution of shear transformations at the atomic level.     

Preparation of reference materials for the determination of RoHS-relevant flame retardants in styrenic polymers

Reference materials for the analysis of polybrominated diphenyl ethers, polybrominated biphenyls and other common brominated flame retardants (FR) in styrenic polymers were prepared to suit the demands of actual restriction of the use of certain hazardous substances in electrical and electronic equipment analytics. Three methods of preparation were employed, viz. pellet forming, dissolution/vaporisation and extrusion, whereby extrusion proved to be the most suitable method. For extrusion, three procedures of pre-mixing were investigated: the polymers were either mixed with FR powder, FR solutions or FR concentrates that were taken from waste industrial polymers. The latter procedure proved to be most appropriate in terms of analyte concentration, predictability and recovery. The homogeneity of the samples, as well as the chemical and thermal long-term stabilities, was investigated. The result was an optimised method to prepare a suitable reference material for laboratory use. Figure   Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Structural phenomena in polymers arising at low temperatures and by the action of high forces

New structural phenomena which can be produced in polymers at low temperatures or by the action of high forces are described and discussed. Experimental evidence supports the argument that the deformation of polymers can develop not only as a result of conformational changes of the macromolecules proper but also by transformation of more complex structural formations. The consequence of this phenomenon is the possibility of large deformations far below the glass-transition temperature in a crystalline polymer with well-developed supermolecular structure. This type of deformation takes place without molecular orientation. Another phenomenon discussed is the sharp change of supermolecular structure in crystalline polymers caused by the action of a shock wave. These effects ought to be connected with an energetic rather than entropic deformation mechanism because the transformations occur at a supermolecular level. Thus, there can be two extreme types of deformation processes: the well-known conformation changes that occur at a molecular level, and the deformation of supermolecular structures. Examples of the pure form of the latter type of mechanism obtained under extreme conditions are given.