Abdellah Ajji

Influence of elastic properties on drop deformation and breakup in shear flow

In this article we report experimental results on the deformation and the critical breakup conditions of a single drop in a medium under simple shear flow. The role played by both drop and matrix elasticities is quantified by using constant viscosity elastic (Boger) fluids. The experiments were conducted using two transparent parallel disks mounted on a R-18 Weissenberg rheogoniometer. The critical shear rate was determined by imposing successive small changes in shear rate from lower to higher values until the drop breakup was observed. The results show remarkable differences in the mode of deformation and breakup for Newtonian and elastic fluid systems. It is also found that the drop resistance to deformation and breakup increases with increasing elasticity ratio. The contribution of the drop and matrix elasticities is quantified by using an empirical relation established between the drop deformation and the capillary number, Ca. The critical breakup conditions, such as a dimensionless breakup time, tb*...

Core−Shell Structured PEO-Chitosan Nanofibers by Coaxial Electrospinning

Core-shell structured PEO-chitosan nanofibers have been produced using a coaxial electrospinning setup. PEO and chitosan solutions, both in an aqueous acetic acid solvent, were used as the inner (core) and outer (shell) layer, respectively. Uniform-sized defect-free nanofibers of 150-190 nm diameter were produced. In addition, hollow nanofibers could be obtained subsequent to PEO washing of the membranes. The core-shell nanostructure and existence of chitosan on the shell layer were confirmed by TEM images obtained before and after washing the PEO content with water. The presence of chitosan on the surface of the composite nanofibers was further supported by XPS studies. The chitosan and PEO compositions in the nanofibrous mats were determined by TGA analysis, which were similar to their ratio in the feed solutions. The local compositional homogeneity of the membranes and the efficiency of the washing step to remove PEO were also verified by FTIR. In addition, DSC and XRD were used to characterize the crystalline structure and morphology of the co-electrospun nonwoven mats. The prepared coaxial nanofibers (hollow and solid) have several potential applications due to the presence of chitosan on their outer surfaces.

Orientation and structure of drawn poly(ethylene terephthalate)

Abstract Drawing of poly(ethylene terephthalate) (PET) films was performed from the amorphous state at different drawing rates and at 80°C. Crystallinity of the films was determined by thermal analysis and orientation of the different phases was determined by birefringence, Fourier transform infra-red ( FT i.r.) specular reflection and wide angle X-ray diffraction. The mechanical properties of the oriented films were studied by dynamic mechanical analysis. FT i.r. and X-ray results showed large orientation of the different phases ( trans conformers and crystalline phase), in contrast with refractive index measurements which showed unusually low birefringence results. The observed orientation was particularly high for draw ratios (λ) higher than 3. The orientation contribution of the non-crystalline phase was determined by combining FT i.r. and X-ray results. The trans -conformer contribution to the non-crystalline phase is shown to be very high for the high λ for which crystalline orientation was observed. A structural model consisting of a crystalline phase, a mesomorphic amorphous phase (constituted of trans conformers) and a purely amorphous phase (constituted solely of gauche conformers) is a possible representation of the observed results. Finally, the tensile modulus increased continuously with the overall orientation.

Rheological properties of copolymer modified polyethylene/polystyrene blends

In this study, the effect of hydrogenated styrene butadiene diblock copolymers on the rheological properties of high‐density polyethylene (HDPE)/high impact polystyrene (HIPS) blends is investigated. The blends were prepared in the melt state at 180 °C for 5 min in a batch mixer. The results show that the rheological properties of the blends were very sensitive to the copolymer concentration present in the blend mainly in the low frequency region. At low copolymer content (e.g., 1 wt. %), an important decrease of the zero‐shear viscosity (η0) was observed in the case of HIPS rich blends. For the HDPE rich blends, this decrease was not observed when modified with the pure diblock copolymer. As the copolymer content increases, and depending on the copolymer structure, an increase or a further decrease of η0 was obtained. This behavior was interpreted as due to the change in the copolymer state in the blend, i.e., saturation of the interface and micelles formation. Predictions of an emulsion model of two viscoelastic liquids including an interfacial tension (σ) contribution to the elastic properties were close to the experiments for the 90/10 and 20/80 HDPE/HIPS unmodified blends, but unsatisfactory in the other cases. According to the general features of polymer suspensions and to the predictions of this model, it is concluded that phase interactions as produced by the addition of the copolymer are very important. This aspect must be included in any rheological model for copolymer modified immiscible blends.

Influence of elastic properties on drop deformation in elongational flow

We report experimental results on the deformation of a single drop suspended in a medium under uniaxial elongational flow along the central axis of a converging conical channel made of Plexiglas. Both the drop and the continuous phases consist of constant viscosity elastic fluids, so-called Boger fluids. This study reveals several interesting features about the role played by both the drop and matrix elasticities on the drop deformability. In a given matrix fluid, the drop deformation decreases as its elasticity increases. For a given drop fluid, the matrix elasticity has the opposite effect: the drop deformation increases with increasing matrix elasticity. An empirical relation between the drop and matrix deformations is established as a function of the drop and matrix characteristic elastic times.

Characterization of Surface Orientation in Poly(Ethylene Terephthalate) by Front-Surface Reflection Infrared Spectroscopy

The use of front-surface specular reflection FT-IR spectroscopy to characterize surface orientation in thick samples of poly(ethylene terephthalate) (PET) has been investigated. It has been shown that, even for samples whose surface uniformity is less than perfect, absorption index spectra of excellent quality are obtained from the Kramers-Kronig transformation. These spectra provide detailed information, both qualitative and quantitative, on the molecular conformation and orientation of the polymer. Of particular interest are the strongly absorbing bands such as the carbonyl and ester peaks, which are generally saturated in transmission spectra. The reflection technique opens up the possibility of obtaining new information on conformational changes and orientation in PET through a detailed study of these peaks.

Color-changing and color-tunable photonic bandgap fiber textiles

We present the fabrication and use of plastic Photonic Band Gap Bragg fibers in photonic textiles for applications in interactive cloths, sensing fabrics, signage and art. In their cross section Bragg fibers feature periodic sequence of layers of two distinct plastics. Under ambient illumination the fibers appear colored due to optical interference in their microstructure. Importantly, no dyes or colorants are used in fabrication of such fibers, thus making the fibers resistant to color fading. Additionally, Bragg fibers guide light in the low refractive index core by photonic bandgap effect, while uniformly emitting a portion of guided color without the need of mechanical perturbations such as surface corrugation or microbending, thus making such fibers mechanically superior to the standard light emitting fibers. Intensity of side emission is controlled by varying the number of layers in a Bragg reflector. Under white light illumination, emitted color is very stable over time as it is defined by the fiber geometry rather than by spectral content of the light source. Moreover, Bragg fibers can be designed to reflect one color when side illuminated, and to emit another color while transmitting the light. By controlling the relative intensities of the ambient and guided light the overall fiber color can be varied, thus enabling passive color changing textiles. Additionally, by stretching a PBG Bragg fiber, its guided and reflected colors change proportionally to the amount of stretching, thus enabling visually interactive and sensing textiles responsive to the mechanical influence. Finally, we argue that plastic Bragg fibers offer economical solution demanded by textile applications.

Differentiation of neuronal stem cells into motor neurons using electrospun poly-L-lactic acid/gelatin scaffold

Neural stem cells (NSCs) provide promising therapeutic potential for cell replacement therapy in spinal cord injury (SCI). However, high increases of cell viability and poor control of cell differentiation remain major obstacles. In this study, we have developed a non-woven material made of co-electrospun fibers of poly L-lactic acid and gelatin with a degradation rate and mechanical properties similar to peripheral nerve tissue and investigated their effect on cell survival and differentiation into motor neuronal lineages through the controlled release of retinoic acid (RA) and purmorphamine. Engineered Neural Stem-Like Cells (NSLCs) seeded on these fibers, with and without the instructive cues, differentiated into β-III-tubulin, HB-9, Islet-1, and choactase-positive motor neurons by immunostaining, in response to the release of the biomolecules. In addition, the bioactive material not only enhanced the differentiation into motor neuronal lineages but also promoted neurite outgrowth. This study elucidated that a combination of electrospun fiber scaffolds, neural stem cells, and controlled delivery of instructive cues could lead to the development of a better strategy for peripheral nerve injury repair.

Processing-structure-properties relationship of multilayer films. 1. Structure characterization

Abstract Low density polyethylene (LDPE)/poly(ethylene terephthalate) (PET) multilayer films with and without a reactive tie layer were prepared by blowing process. The layers contents were fixed and the parameters evaluated were blow-up ratio (BUR), draw-down ratio (DDR) and frost-line height (FL). Their effect on the structure was studied. No crystallinity for the PET layer was induced by the blowing process. The orientation of the multilayer films was characterized by birefringence and infrared spectroscopy (FTIR). The measured birefringence showed a negligible orientation for PET layer, and the LDPE layer oriented mainly along the machine direction. FTIR characterization also showed a negligible orientation for PET layer, and for LDPE layer, the crystalline a-axis tended toward machine direction, crystalline b-axis located in transverse–normal plane with no orientation for the amorphous phase. The lamellar arrangement of LDPE layer with respect to machine direction was obtained by dissolving the amorphous phase in solution of potassium permanganate in a mixture of equal volumes of orthophosphoric and sulfuric acid. The crystallization of oriented LDPE melts produced a morphology consisting of rows of lamellar crystals aligned parallel to the machine direction at higher draw-down ratio. It was not always of the row-nucleated type, the exact morphology patterns greatly depended on the distribution of the crystalline a-axis and b-axis orientation, which in turn was dependent on DDR, BUR and frost-line height.

Study of polypropylene morphology obtained from blown and cast film processes : Initial morphology requirements for making porous membrane by stretching

Four different polypropylene (PP) resins are extruded using the tubular blown and cast film processes. The resin morphology is observed by SEM and the effect of extrusion processing variables on the morphology is investigated. Melt rheological experiments are also carried out to characterize the polymer melts. It has been found that the molecular weight distribution and the chain structure as well as the processing conditions have important effects on the morphology. Efforts have been focused on developing a row-nucleated morphology from PPs through the control of processing conditions. The possibility of generating a porous membrane from the initial row-nucleated morphology using a stretching technique is evaluated. It has been found that the initial lamellae arrangement of the precursor films and the stretching conditions play a significant role in obtaining a porous structure.