Saïd Elkoun

Plastic deformation of polyethylene and ethylene copolymers: Part I Homogeneous crystal slip and molecular mobility

The plastic behaviour of polyethylene and ethylene copolymers is studied under uniaxial tensile testing in parallel with the viscoelastic properties. Homogeneous plastic deformation is shown to take place at temperatures above the crystalline mechanical relaxation. The activation of homogeneous crystal slip is discussed in relation to the crystal lamella thickness and the molecular mobility of the crystalline chain stems. The thermally activated process of nucleation and propagation of screw dislocations that is proposed for the mechanism of the homogeneous crystal slip relies on the generation of 180° chain twists in the crystal stems of the sheared crystals. This kind of conformational chain defect is the basic link between the plastic and the viscoelastic properties of the materials. Homogeneous crystal slip can take place as long as the applied strain rate is consistent with the strain rate affordable by the screw dislocation propagation. The dependence on draw temperature of the crystal thickness in the fibre structure is ascribed to the stress-induced activation of 180° chain twists which allows an adjustment of the crystal thickness to the temperature of the experiment faster than an annealing treatment will.

Transfer matrix method applied to the parallel assembly of sound absorbing materials

The transfer matrix method (TMM) is used conventionally to predict the acoustic properties of laterally infinite homogeneous layers assembled in series to form a multilayer. In this work, a parallel assembly process of transfer matrices is used to model heterogeneous materials such as patchworks, acoustic mosaics, or a collection of acoustic elements in parallel. In this method, it is assumed that each parallel element can be modeled by a 2 × 2 transfer matrix, and no diffusion exists between elements. The resulting transfer matrix of the parallel assembly is also a 2 × 2 matrix that can be assembled in series with the classical TMM. The method is validated by comparison with finite element (FE) simulations and acoustical tube measurements on different parallel/series configurations at normal and oblique incidence. The comparisons are in terms of sound absorption coefficient and transmission loss on experimental and simulated data and published data, notably published data on a parallel array of resonators. From these comparisons, the limitations of the method are discussed. Finally, applications to three-dimensional geometries are studied, where the geometries are discretized as in a FE concept. Compared to FE simulations, the extended TMM yields similar results with a trivial computation time.

Plastic deformation of polyethylene and ethylene copolymers. Part II Heterogeneous crystal slip and strain-induced phase change

The plastic behaviour of polyethylene and ethylene copolymers is studied under uniaxial tensile testing with particular attention to the development of plastic instability. Heterogeneous crystal slip is suggested to take place when homogeneous crystal slip either is not allowed at the temperature and strain rate of the experiment or is exhausted owing to extension of the chain folds. The chain unfolding concomitant to the fragmentation of the crystalline lamellae is suspected to have a low strain hardening that is responsible for the phenomenon. Partial screw dislocations with a shorter Burgers vector than in the case of homogeneous slip are proposed to become operative because of the activation of 90 ° chain twists in the crystalline stems. Dissociation of dislocations into partials involves stacking faults in the orthorhombic structure that may turn into monoclinic structure through a martensitic-like transformation. Crystal slip is likely to concentrate in these faulty regions owing to the reduced molecular interactions and lower density. Two types of correspondence of the transformed monoclinic phase with the parent orthorhombic structure are observed. The modification of the chain-folding macroconformation as a function of the crystallinity of the materials is suspected to influence the transformation shear mode.

In Situ Synthesis and Characterization of Silver/Polymer Nanocomposites by Thermal Cationic Polymerization Processes at Room Temperature: Initiating Systems Based on Organosilanes and Starch Nanocrystals

New methods for the preparation of silver nanoparticles/polymer nanocomposite materials by thermal cationic polymerization of ε-caprolactone (ε-CL) or α-pinene oxide (α-PO) at room temperature (RT) and under air were developed. The new initiating systems were based on silanes (Si), starch nanocrystals (StN) and metal salts. Excellent polymerization profiles were revealed. It was shown that silver nanoparticles (Ag(0) NPs) were in situ formed and that the addition of StN improves the polymerization efficiency. The as-synthesized nanocomposite materials contained spherical nanoparticles homogeneously dispersed in the polymer matrices. Polymers and nanoparticles were characterized by gel permeation chromatography (GPC), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and UV-vis spectroscopy. A coherent picture of the involved chemical mechanisms is presented.

Comparison between parallel transfer matrix method and admittance sum method

A transfer matrix method to predict absorption coefficient and transmission loss of parallel assemblies of materials which can be expressed by a 2 × 2 transfer matrix was published recently. However, the usual method based on the sum of admittances is largely used to predict also surface admittance of parallel assemblies. This paper aims to highlight differences between both methods through three examples on a parallel assembly backed by (1) a rigid wall, (2) an air cavity, and (3) an anechoic termination.

Prediction of acoustic properties of parallel assemblies by means of transfer matrix method

The Transfer Matrix Method (TMM) is used conventionally to predict the acoustic properties of laterally infinite homogeneous layers assembled in series to form a multilayer. In this work, a parallel assembly process of transfer matrices is used to model heterogeneous materials such as patchworks, acoustic mosaics, or a collection of acoustic elements in parallel. In this method, it is assumed that each parallel element can be modeled by a 2x2 transfer matrix, and no diffusion exists between elements. The method is validated by comparison with finite element method (FEM). Then, an overview of the possibilities, such as the combination of series and parallel matrices, the sound absorption coefficient and the transmission loss of a parallel array of resonators or three-dimensional geometries is presented and discussed.

Importance of the PMMA viscoelastic rheology on the reduction of the leakage risk during osteoporotic bone augmentation: A numerical leakage model through a porous media

Osteoporotic fractures poses one of the most problematic health issues that affects millions of people by weakening their bones (Osteoporosis). Polymethylmethacrylate (PMMA) cement is usually used to augment the bone and stabilize the fractures. Despite the benefit of using PMMA, it might cause a leakage where the cement undesirably access the surrounding tissues or vessels and lead to a serious complications. Consequently, it is important to study the leakage phenomenon and associated geometric and operation interactions. Although the experimental leakage models have been reported in many studies, a representative numerical leakage model is not exist. Therefore, the objectives of the present paper are to: (a) to develop and validate a representative numerical leakage model; and (b) to investigate numerically and analytically the importance of the rheological parameters (viscosity and relaxation time) on the cement flow to reduce the risk of leakage. ANSYS Polyflow was utilized to implement a 2D numerical leakage model to study the interaction of complex rheological parameters of the cement with the operational and geometrical structure of the representative porous media. In this model, the cement (represented by the upper-convected Maxwell model) flows from the entrance (tip of an 8 gauge cannula) through a porous media with a leakage path (blood vessels) toward the output (Bottom side). The verified and validated numerical leakage model showed the importance of the elastic and viscous part of the cement to control the uniformity of the distributed cement and augmentation pressure, respectively. Moreover, increasing the flow rate can lead to reduce the risk of leakage since the elastic effect will increase. Geometrical parameters of the porous media has a minor effect on changing the elasticity and subsequently on the uniformity of the distributed cement. In conclusion, Cement rheological parameters are found to be the most influential parameters to reduce the risk of leakage by controlling the uniformity of the distributed cement and the augmentation pressure.

Thermo-physical properties of synthetic mucus for the study of airway clearance

In this article, dynamic viscosity, surface tension, density, heat capacity and thermal conductivity, of a bronchial mucus simulant proposed by Zahm et al., Eur Respir J 1991; 4: 311-315 were experiementally determined. This simulant is mainly composed of a galactomannan gum and a scleroglucan. It was shown that thermophysical properties of synthetic mucus are dependant of scleroglucan concentrations. More importantly and for some scleroglucan concentrations, the syntetic mucus, exhibits, somehow, comparable thermophysical properties to real bronchial mucus. An insight on the microstructure of this simulant is proposed and the different properties enounced previously have been measured for various scleroglucan concentrations and over a certain range of operating temperatures. This synthetic mucus is found to mimic well the rheological behavior and the surface tension of real mucus for different pathologies. Density and thermal properties have been measured for the first time.

Isolation of cellulose-II nanospheres from flax stems and their physical and morphological properties

In this study, cellulose-II nanospheres (CNS) were extracted from flax fibers and analyzed to understand the crystalline, functional and morphological properties by means of X-ray Diffraction (X-RD), Fourier Transform Infrared (FT-IR) and Scanning Electron Microscopy (SEM). FT-IR and SEM results indicate the effective removal of extractives, lignin and hemicellulose. X-RD results clearly show the transformation from cellulose-I to cellulose-II during the mercerization process. Further, the resulting cellulose fibers were treated with sulfuric acid in order to obtain cellulose nanospheres (CNS). The morphology was measured by SEM, Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). The size distribution and the surface charge of CNS were investigated by Small-Angle X-ray Scattering (SAXS) and Nanosizer. The results indicate a size distribution of CNS between 20 and 90nm moderately dispersed. Finally, the transversal elastic modulus of CNS-II was determined by using AFM, and results reveal the range varying from 6 to 25GPa.

Degradation characteristics of new bio-resin based-fiber-reinforced polymers for external rehabilitation of structures

Sustainability has recently become a key issue in the design and manufacture of products, due to dwindling oil reserves and increased environmental awareness. Therefore, bio-sourced resins are suggested to be used as an alternative in order to reduce the environmental impact of composite production. This paper presents the mechanical and physico-chemical characterization and the environmental degradation evaluation of furan resin-based glass fiber reinforced polymer (bio-sourced glass fiber reinforced polymer) for structural retrofitting applications in comparison with those of an equivalent thermoset-based glass fiber reinforced polymer (petro-sourced glass fiber reinforced polymer). Epoxy resin was used as a representative for petroleum-derived synthetic thermoset. To conduct this preliminary study, the following steps were taken: (1) prepare composites made from furan and epoxy resins, (2) characterize the mechanical and physico-chemical properties of the composites, (3) study the moisture absorption, and finally, (4) evaluate the degradation of both composites when subjected to alkaline solutions which was simulated to leached concrete pore solution. The experimental results show that the use of furan resin as polymer matrix in glass fiber reinforced polymer leads to an increase in the moisture absorption and a significant decrease in the degradation in alkaline solution.