Serife Akkoyun

Depuis quelques années, la microplasturgie est un secteur en plein développement. Cependant, le comportement rhéologique des matériaux polymères dans des géométries très minces (dimension inférieure à 100 µm) n’est pas bien caractérisé. Peu de travaux ont été entrepris à ce sujet, en particulier en ce qui concerne les écoulements de Poiseuille qui sont pourtant les plus représentatifs des conditions de mise en œuvre usuelles. Ainsi, ce travail a pour but la mise au point d’une méthode expérimentale permettant d’obtenir des données pertinentes afin de caractériser de façon approfondie le comportement des matériaux polymères en écoulement de Poiseuille dans des géométries micrométriques. Afin de décrire au mieux la physique de tels écoulements, nous avons également cherché à les simuler numériquement, soit en utilisant des lois de comportement classiques, soit à l’aide de modèles se référant à la dynamique moléculaire. Pour atteindre ces objectifs, une filière à fente plate instrumentée avec des capteurs de pression et température, d’entrefer variant entre 50 et 200µm, a été conçue afin d’effectuer des mesures à l’aide d’un rhéomètre capillaire. Ce dispositif a été validé en confrontant les mesures à celles obtenues par d’autres méthodes (rhéométrie capillaire en filière classique et rhéométrie dynamique). Le glissement à la paroi a également été étudié, selon la méthode de Mooney. La simulation numérique de l’écoulement a d’abord été réalisée à l’aide de POLYFLOW®. L’effet de la pression sur la géométrie ainsi que sur le matériau polymère a été étudié. Puis, l’écoulement a également été simulé sous MATLAB® en utilisant des lois constitutives de type moléculaire basées sur le modèle du tube de Doï-Edwards ainsi que sur le concept de « Molecular Stress Function » introduit par Wagner pour rendre compte des effets d’orientation des molécules (variation du diamètre du tube) dans le champ de contraintes. L’écart constaté entre ces calculs et les résultats expérimentaux est expliqué et discuté à la lumière des simulations sous POLYFLOW®. Il modifie les perspectives d’étude de ce type d’écoulements.

Aggregation-enhanced plasmonic coupling improves nonlinear absorption and optical limiting in PMMA/AgNP thin films compared with nanofibers.

Silver nanowires (AgNWs) effectively enhance the nonlinear absorption (NA) properties of polyvinylpyrrolidone (PVP) nanofibers and significantly improve their optical limiting performance in the visible region.

In this work, the optical limiting performance of titanium dioxide (TiO₂) particle-filled polyvinylpyrrolidone (PVP) composite nanofibers produced by electrospinning was investigated. Effectively filled PVP composite nanofibers with TiO₂ were observed in the backscattering scanning electron microscopy (SEM) images. A reduced band gap energy of the composite nanofibers was determined with an increasing amount of TiO₂. Increasing localized defect states were obtained with increasing TiO₂ particles in PVP. Furthermore, open-aperture Z-scan experiments at 532 nm were conducted at different intensities to investigate the composite nanofibers’ nonlinear absorption (NA) characteristics. The NA signal was obtained for all composite nanofibers, and their main NA mechanism was 2PA. The 2PA coefficient and the NA coefficient values of the pure PVP nanofibers increased with the increase in TiO₂ filler in the PVP nanofibers. This increment indicated the contribution of absorption originating from localized defects inside the NA. Stronger optical limiting performance with a lower onset optical limiting threshold was observed for higher TiO₂-filled PVP composite nanofibers. TiO₂ composite nanofibers could be an effective optical limiter in the visible wavelength range due to their strong optical limiting ability, low onset optical limiting threshold, and high optical damage threshold.

The utilization of 2-level factorial design has been extensive in the literature to observe the relationship between parameters and responses. Among the subjects open for exploration, the process of nanofiber creation stands out as an intriguing avenue to explore the correlations that emerge between variables and outcomes. The primary objective of the study is to establish the relationships between the parameters of electrospinning of polyamide 6 (PA6) solutions to obtain desired nanofiber diameters by response surface method (RSM) and two level full factorial design. The investigation hones in on four critical parameters related to the electrospinning of PA6 solutions. These parameters encompass factors like solution concentration, applied voltage, distance between the spinneret and the collector, and the flow rate of the solution. Employing a two-level factorial design, these parameters are methodically manipulated at two distinct levels each to systematically unravel their individual and collective impacts on nanofiber diameter outcomes. To understand the relationship between electrospinning process and these factors, these kind of experimental studies gives us much accurate results.

In this work, vanadium pentoxide (V₂O₅) nanoparticles-filled electrospun polyvinylpyrrolidone (PVP) nanofibers were investigated systematically at various nanofiller weight percentages (8 and 10 wt%) and input intensities to reveal the effective optical limiting feature in the visible spectrum. XRD analysis demonstrated the purity of the produced V₂O₅ nanoparticles. According to SEM findings, V₂O₅ nanoparticles were effectively integrated into the PVP nanofibers. Two distinct absorption bands were observed at around 400 and 217 nm. These bands were related to PVP and V₂O₅ nanoparticles in linear absorption measurements, respectively. Moreover, an increased Urbach energy value was obtained with an increase in V₂O₅ nanofiller content within PVP. Open-aperture Z-scan measurements were taken at 532 nm considering the band gap energy of the V₂O₅ nanofillers in PVP composite nanofibers. In 8 wt% V₂O₅ nanofilled PVP nanofibers, one-photon absorption (OPA) was the main nonlinear absorption (NA) mechanism, and the defect states of the V₂O₅ nanoparticles had no contribution to NA. On the other hand, sequential two-photon absorption was the main NA mechanism, and the defect states of the nanoparticles caused more efficient NA behavior in 10 wt% V₂O₅ nanofilled PVP nanofibers. The effective optical limiting behavior was obtained for this composite nanofiber with lower limiting threshold as 1.49 × 10^–5 J/cm². The V₂O₅ nanofilled PVP nanofibers presented strong potential optical limiters in the visible wavelength region. This was attributed to their high linear transmittance at low input intensities and their robust NA behavior at higher input intensities.