Modeling of deformation behavior and strain-induced crystallization in poly(ethylene terephthalate) above the glass transition temperature

A constitutive model for large deformation stress–strain behavior and strain-induced crystallization in poly(ethylene terephthalate), at temperatures above the glass transition temperature, is proposed. In this model, the intermolecular resistance is treated in a composite framework where the crystalline and amorphous phases are considered as two separate resistances coupled through two different analog representations leading to the upper and the lower bound approaches. The crystallization rate is expressed following a non-isothermal phenomenological expression based on the modified Avrami equation. Our predicted results are compared to existing experimental results and good agreement is found.     

Stress-orientation-strain relationships in non-crystalline polymers

The approach to non-crystalline polymer deformation set out in Part 1 [1], involving the resolution of deformation into orientational and non-orientational components, is employed as the foundation of a simple deformation model applicable to the rubbery state. The analysis is performed with reference to an assemblage of mobile orienting units of idealized ellipsoidal shape, free to undergo orientational and extensional motion but subject to constraint arising from both intramolecular and intermolecular sources. Stress-strain-orientation relationships are calculated and compared to alternative theories and to experiment, with particular reference to poly(methyl methacrylate) and poly(ethylene terephthalate).     

Interactions of a liquid crystalline polymer with polycarbonate and poly(ethylene terephthalate)

Thermal behaviour of blends of a liquid crystalline copoly(ester amide) (Vectra B950) with two isotropic polymers has been studied by differential scanning calorimetry. One of the isotropic polymers is an amorphous polymer – polycarbonate, the other is a semi-crystalline polymer – poly(ethylene terephthalate). It was found that the glass transition temperature of polycarbonate decreases with increasing Vectra concentration in the blend, suggesting a partial miscibility between the Vectra liquid crystalline polymer (LCP) and polycarbonate. The miscibility is enhanced through heat treatment at elevated temperatures presumably due to a transesterification reaction. Moreover, the presence of the amorphous poly- carbonate hinders the crystallization of the liquid crystalline polymer in the blends. It was also observed that heat treatment of the Vectra LCP and poly(ethylene terephthalate) blends causes a loss in crystallinity and shifts in transition temperatures of poly(ethylene terephthalate), indicating that exchange reactions occur between Vectra B950 and poly(ethylene terephthalate). Based on these results, a new strategy, in situ compatibilization, is proposed to improve the interfacial adhesion between an LCP and an isotropic polymer. This revised version was published online in November 2006 with corrections to the Cover Date.     

Micromechanical modelling of the viscoelastic behaviour of an amorphous poly(ethylene)terephthalate (PET) reinforced by spherical glass beads

Homogenization micromechanical models are applied to predict the linear viscoelastic properties of an amorphous poly(ethylene)terephthalate (PET) composite in the range of glass transition by using the elastic-viscoelastic superposition principle defined by Hashin [1]. An amorphous PET is reinforced by glass beads and is submitted to dynamic mechanical tests at temperatures surrounding its quasi-static glass transition temperature. The viscoelastic properties of the matrix and the elastic properties of the beads are measured experimentally. The micromechanical models predictions of the linear viscoelastic behaviour in the glassy state are acceptable. In the rubbery state, the beads seem to reduce the molecular mobility of the matrix driving to a large change in the viscoelastic properties of the materials. Thus, this paper aims to emphasize that classical homogenization micromechanical models, which depend only on the constituent behaviour, shape and distribution, cannot predict this change in the linear viscoelastic behaviour of the beads/PET composites.     

Micromechanical properties in lamellar heterophase polymer systems

Several nanoanalytical techniques based on electron and atomic force microscopy were used to analyse the micromechanical deformation mechanisms in different nanostructured lamellae forming heterogeneous polymers: in (semicrystalline) -modified isotactic polypropylenes and (amorphous) lamellar styrene/butadiene block copolymers. It was found that the deformation processes in these two entirely different classes of materials are governed by fundamentally similar mechanisms due to similar dimension and arrangement of the nanostructures. The basic mechanism shows two steps: The initial step is determined by a plastic deformation of the soft (amorphous or rubbery) phase with a reorganisation of the hard (crystalline or glassy) lamellae and their orientation towards the deformation direction. The second step is characterised by the intense plastic deformation (yielding) of the hard lamellae up to elongations of several 100%.     

热致液晶共聚酯酰胺／PET共混物的相容性和形态结构

用溶解度参数预测热致液晶共聚酯酰胺（ＰＥＡ）与聚对苯二甲酸乙二酯（ＰＥＴ）共混物的相容性。采用偏光显微镜、扫描电镜研究其形态结构。结果表明：ＰＥＡ／ＰＥＴ共混物虽然在理论上是热力学相容的，但因ＰＥＡ的热致液晶性，从液晶有序相变为各向同性存在热效应，致使理论预测与实验结果产生偏差。ＰＥＡ／ＰＥＴ共混物呈两相结构，当ＰＥＡ含量少时，两相之间具有一定的相容性。共混物熔体在剪切力作用下，液晶相形成取向微纤     

Optimizing workholding efficiency through fixture function simulation

In the past two decades, simulating workpiece–fixture interactions has remarkably drawn the attention of academia. However, many aspects of the workpiece–fixture system, specifically workpiece–fixture contact condition, still enjoy research effort. For the efficient use of Automated Fixture Design (AFD) systems, verification of their outputs is required. This paper presents a workholding simulation approach that considers assessment of AFD system outputs regarding different workholding parameters. The effect of friction forces on workholding and part rigid body stability is considered. Besides, determining deformation and stress in the workpiece, locators and clamps are modelled and their elastic deformation and stress distribution are computed. A procedure based on the application of contact elements in finite-element method is developed to quantify the status of contact between the workholding elements and the workpiece. Clamping tightness is modelled and its effect on the workpiece–fixture system performance is quantified. The central component of the verification tool is the use of nonlinear finite-element analysis to model the frictional interactions in workpiece–fixture system.     

A micromechanical model taking into account the contribution of α- and γ-crystalline phases in the stiffening of polyamide 6-clay nanocomposites: A closed-formulation including the crystal symmetry

A clay-induced crystal transformation has been widely pointed out in semi-crystalline polymer–clay nanocomposites, from the α-form to the γ-form in the particular case of polyamide 6 (PA6). The proposition of a predictive model taking explicitly into account the polymer crystalline structure is still needed for a reliable prediction of the structure–property relationship and for a better understanding of the reinforcement mechanism in such systems. The aim of this paper is to present an approach issued from the continuum-based micromechanical framework using self-consistency condition to predict the overall stiffness of PA6-clay nanocomposites. Besides the effect of clay particle characteristics, the micromechanical model introduces the contribution of α- and γ-form crystals in the overall stiffness by considering the PA6 matrix as a heterogeneous medium containing distinct amorphous and crystalline phases. A closed-formulation of the micromechanical model is derived using the Walpole spectral decomposition of the stiffness tensors for the two monoclinic crystalline phases. Two possible representations of the microstructure are considered: the first one considers that all the phases are independent and the second one that the γ-crystalline phase constitutes an interphase region around clay particles. The micromechanical model using the two morphological representations is used to predict the overall stiffness of PA6 nanocomposites reinforced with montmorillonite clay (adjusted from 1 wt% up to 20 wt%) for which the polymer crystalline structure was characterized by infrared spectroscopy and calorimetry. The respective role of clay particles and of two crystalline species in the stiffening of exfoliated and intercalated PA6-clay nanocomposites is discussed with respect to the micromechanical model.     

Diamond-like carbon,a barrier coating for polymers used in packaging applications

Water and oxygen permeability measurements on two polymers, poly(ethylene terephthalate) and polypropylene, are presented as a function of diamond-like carbon coating thickness. Results show that reliable and reproducible coatings can be achieved on poly(ethylene terephthalate) such that levels of permeability are about 1 cc/m²/day for oxygen and 1.5 g/m²/day for water vapour, comparable to the levels for silicon oxides and aluminium coatings used in the packaging industry. The advantages conferred by diamond-like carbon over aluminium is primarily that of retaining optical transparency in the thickness of films used in this work (20 nm). The advantages of diamond-like carbon over silicon oxides is related to its intrinsic flexibility. Other advantages over other barrier films (e.g. polyvinylidine chloride) and coating technologies is the ability to recycle the used product. The permeability of diamond-like carbon-coated polypropylene to oxygen is in the range of 200 cc/m²/day, again comparable to results obtained with the other coatings. The optimum film thickness for poly(ethylene terephthalate) to minimize permeability was 20 nm. Atomic force microscopy revealed agglomerated structures (possibly graphitic) with the underlying substrate appearing smoother than the starting material. In comparison, polypropylene exhibited increased surface roughness under the same coating conditions.     

Fracture modelling of WC-Co hardmetals using crystal plasticity theory and the Gurson model

The behaviour of edge cracks under Mode I loading in the WC–Co material system is studied using the finite element method (FEM). This work focuses on ductile failure mechanisms in the Co binder. A micromechanical approach is taken whereby Co layers are modelled explicitly. An embedding technique is employed. Crystal plasticity theory and J₂ flow theory are used to represent plastic deformation in Co ligaments. Areas of high hydrostatic stress, triaxiality and accumulated slip or effective plastic strain are identified within the binder material. The Gurson model is used to model crack growth in the Co ligaments. Fracture resistance curves are obtained giving a relationship between macroscopic material behaviour and microscopic failure mechanisms. Factors effecting the crack growth in single and multiple ligaments are identified.     

The dependence of tear behaviour on the microstructure of biaxially drawn polyester film

The structure-property relationship between a biaxially oriented film from poly(ethylene terephthalate) and its fracture behaviour measured using the Trouser Tear method, has been explored. X-ray diffraction (XRD) was used to characterise the orientation distribution of crystalline and non-crystalline material in the plane of the film and compared with the fracture energy, G _c measured in four directions during tearing. The fracture energy averaged over the four directions ranged between 12 and 25 kJ m^–2, and was found to correlate closely to the draw ratio during manufacture and therefore the degree of molecular orientation. However the individual values of G _c displayed a further level of complexity.The expected anisotropic character of the fracture energy was found to change systematically as a function of position across the original width of manufactured film. This feature compared well with the underlying, crystalline orientation distribution and provided strong evidence that under the mode III deformation of the tear test, the fracture mechanism involves the amorphous-crystallite surface boundary.Further support for this mechanism was provided by a simple model which, based on this assumption was shown to predict reliably, the anisotropic character of the film.     

聚对苯二甲酸乙二酯预取向丝收缩机理研究

采用SS－1型收缩－声速取向联合测试仪和DSC（差式扫描量热）对涤纶预取向丝（POY）纤维的升温热收缩过程进行了研究。探索涤纶POY纤维的收缩机理。试验结果表明，POY纤维的热收缩可以分为5个转变温度区间，在不同的温度区间以不同的收缩机理为主，低温时是非晶区大分子边的解取向作用，在冷结晶温度附近，纤维中发生的结晶过程和晶区的结构重排作用，限制了纤维的收缩。外应力对纤维收缩过程中的解取向和结晶行为有一定的影响，外应力的增加有利于纤维冷结晶过程的进行，纤维的收缩率减小。     

1,4环己二甲醇改性聚对苯二甲酸乙二酯

聚对苯二甲酸乙二酯 (PET)是由对苯二甲酸 (TPA) (或对苯二甲酸二甲酯 (DMT) )与乙二醇 (EG)缩聚而成的。如果在PET的生产中用 1,4环己二甲醇 (CHDM )部分取代EG ,经共缩聚所得聚酯即为CHDM改性PET的ET/CT共聚酯。本文概述了ET/CT共聚酯的研发现状、制备、性能和应用     

Micromechanical modelling of oval particulates subjected to bi-axial compression

One of the questions that still remain unanswered among researchers dealing with granular materials is how far the particle shape affects the micro-macroscopic features of granular assemblies under mechanical loading. The latest advances made with particle instrumentation allow us to capture realistic particle shapes and size distribution of powders to a fair degree of accuracy at different length scales. Industrial applications often require information on the micromechanical behaviour of granular assemblies having different particle shapes and varying surface characteristics, which still remains largely unanswered. Traditionally, simulations based on discrete element method (DEM) idealise the shape of individual particles as either circular or spherical. In the present investigation, we analyse the influence of particle shape on the shear deformation characteristics of two dimensional granular assemblies using DEM. We prepared the assemblies having nearly an identical initial packing fraction (dense), but with different basic shapes of the individual particles: (a) oval and (b) circular for comparison purposes. The granular assemblies were subjected to bi-axial compression test. We present the evolution of macroscopic strength parameters and microscopic structural/topological parameters during mechanical loading. We show that the micromechanical properties of granular systems are significantly influenced by the shape of the individual particles constituting the granular assemblies.     

Progressive failure of triaxial woven fabric (TWF) composites with open holes

A computational approach to the investigation of crack evolution and interaction effects of microcracks and particles on the overall behavior of particle-reinforced brittle composites (PRBCs) is presented. To account for interactions of microcracks and particles, and their effects on the overall mechanical behavior, approximate solutions of a micromechanical model considering second-order, ensemble-volume averaged perturbations are employed. By combining the micromechanical framework with a fracture-mechanics based damage model, an evolutionary damage model of PRBCs is subsequently developed and the evolutionary damage model is implemented into a finite element code. The proposed computational damage model is exercised from benchmark examples on PRBCs to illustrate the capability of the proposed damage models for predicting the progressive crack evolution in PRBCs.     

二氧化钛催化光降解聚对苯二甲酸乙二酯的研究

利用原位聚合法合成PET/TiO2复合材料.通过分析PET/TiO2复合薄膜在紫外光降解过程中的质量和表面形态的变化情况,研究了锐钛矿型二氧化钛对PET光氧化降解性能的影响.结果说明,在试验范围内,在253.9nm的紫外光照射的光降解实验中,锐钛矿型TiO2的含量越高,PET/TiO2复合薄膜的质量损失率就越高;同时,紫外光照射试样288h之后,由SEM照片发现复合材料表面损伤程度也随着TiO2含量的升高而越发严重.这说明,锐钛矿型TiO2对PET的光氧化降解具有催化作用,而且含量越高催化作用就越强.     

Micromechanics of spatially uniform heterogeneous media: A critical review and emerging approaches

Outside of the classical microstructural detail-free estimates of effective moduli, micromechanical analyses of macroscopically uniform heterogeneous media may be grouped into two categories based on different geometric representations of material microstructure. Analysis of periodic materials is based on the repeating unit cell (RUC) concept and the associated periodic boundary conditions. This contrasts with analysis of statistically homogeneous materials based on the representative volume element (RVE) concept and the associated homogeneous boundary conditions. In this paper, using the above classification framework we provide a critical review of the various micromechanical approaches that had evolved along different paths, and outline recent emerging trends. We begin with the basic framework for the solution of micromechanics problems independent of microstructural representation, and then clarify the often confused RVE and RUC concepts. Next, we describe classical models, including the available RVE-based models, and critically examine their limitations. This is followed by discussion of models based on the concept of microstructural periodicity. In the final part, two recent unit cell-based models, which continue to evolve, are outlined. First, a homogenization technique called finite-volume direct averaging micromechanics theory is presented as a viable and easily implemented alternative to the mainstream finite-element based asymptotic homogenization of unit cells. The recent incorporation of parametric mapping into this approach has made it competitive with the finite-element method. Then, the latest work based on locally-exact solutions of unit cell problems is described. In this approach, the interior unit cell problem is solved exactly using the elasticity approach. The exterior problem is tackled with a new variational principle that successfully overcomes the non-separable nature of the overall unit cell problem.     

Structural changes during photodegradation of poly(ethylene terephthalate)

An investigation has been conducted into the effects of photodegradation on the structure of poly(ethylene terephthalate) (PET). Films, with and without ultraviolet absorbers and prepared by biaxial orientation after extrusion, have been exposed in the laboratory for periods of up to 1020 hours. The samples were investigated by differential scanning calorimetry (DSC), X-ray diffraction and size exclusion chromatography. The appearance of a cold crystallization peak during DSC heating scans was noted for exposed samples and this was considered to be a result of released molecules in the amorphous region that could rearrange into a crystalline phase. From X-ray analysis, a loss of crystalline orientation was observed after degradation and an interpretation was given based on relaxation in the mesophase region. In samples containing the photostabilizer additive the magnitude of changes in structure was lower, possibly due to segregation effects during film production making the non-crystalline region relatively immune to degradation effects.