archives@tulane.edu / 1 / Azeem Farinmade

archives@tulane.edu / 1 / Olakunle Francis Ojo

archives@tulane.edu / 1 / Olakunle Francis Ojo

Parmi les biopolymères, le poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) fait l’objet d’un grand intérêt de la part des chercheurs et des industriels. Cependant, sa sensibilité thermique et son comportement mécanique fragile restreint son utilisation dans certaines applications. Ainsi pour améliorer ses propriétés, deux grandes stratégies ont été suivies au cours de cette thèse. La première consiste à incorporer une argile de type halloysite (HNT), issue du gisement de Djebel Debbagh à Guelma (Algérie). À cet effet, des nanocomposites PHBV/HNT ont été élaborés par voie fondue. Les résultats de la microscopie électronique à balayage (MEB) et STEM ont montré une distribution relativement homogène de l’HNT avec la présence de larges agrégats. En conséquence et dans le but, d’améliorer la dispersion de ces nanotubes, il a été nécessaire de procéder à la modification des interfaces polymère-argile, soit par la modification chimique de l’halloysite, soit par l’incorporation d’un compatibilisant de type PHBV-g-MA dans le système binaire. Les résultats obtenus mettent en évidence la coexistence d’agrégats et de nanotubes individualisés. La seconde approche consiste à mélanger le PHBV avec un autre biopolymère comme le polybutylène succinate (PBS). Celui-ci a été choisi pour sa bonne stabilité thermique et ses bonnes propriétés mécaniques. Des systèmes hybrides ont été préparés par voie "fondue" en incorporant l’HNT et le PHBV-g-MA comme compatibilisant. L’étude révèle à travers le MEB que l’ajout de 5% en masse de PHBV-g-MA améliore la morphologie du mélange PHBV/PBS 80/20 qui se traduit par une diminution de la taille des nodules de PBS. L’ajout de 5% en masse de l’HNT dans le mélange favorise aussi la diminution de la taille des nodules de PBS. Cependant, la combinaison du PHBV-g-MA et de l’HNT limite l’effet émulsifiant de l’agent compatibilisant dû à l’agrégation de l’HNT. Les résultats de DSC et d’ATG montrent que le PHBV-g-MA n’a aucun effet sur les propriétés et la stabilité thermiques du mélange PHBV/PBS. Toutefois, la présence de l’HNT joue un rôle positif dans la diminution du pic de dégagement de chaleur (HRR). Les propriétés mécaniques du mélange ternaire PHBV/PBS/HNT avec ou sans compatibilisant sont comparables à celles du mélange pur PHBV/PBS 80/20.Une dernière partie des travaux a été menée sur le recyclage thermomécanique à travers une évaluation des effets du nombre de cycles d'excursion répétés sur le PHBV, le PBS, les nanocomposites PHBV/HNT et PBS/HNT, le mélange binaire PHBV/PBS 80/20 et ternaire PHBV/PBS 80/20+HNT avec et sans compatibilisant. Les résultats de cette étude ont montré que la recyclabilité de ces systèmes est possible du fait que la nanostructure du matériau recyclé soit améliorée et que les propriétés thermiques et mécaniques ne sont pas affectées après 5 cycles d’extrusion. / Among biopolymers, the poly (hydroxybutyrate-Co-hydroxyvalerate) (PHBV) has attracted the attention of researchers and industry. However, its thermal sensitivity and its fragility limited its use for some applications. Thus, to improve its properties, two great strategies were considering during this thesis. The first consists in incorporating halloysite (HNT), type of clay, collected from Djebel Debbagh in Guelma (Algeria). For this purpose, nanocomposites PHBV/HNT were prepared by melt compounding. The results of scanning electron microscopy (SEM) and STEM showed a relatively homogeneous distribution of the HNT with the presence of large aggregates. Consequently and in the aim to improve the dispersion of these nanotubes, it was necessary to carry out the modification of interfaces polymer-clay, either by the chemical modification of halloysite, or by the incorporation of compatibilizer like PHBV-g -MA in the binary system. The results obtained highlight the coexistence of individualized and aggregated nanotubes.The second approach consists in mixing the PHBV with another biopolymère like polybutylene succinate (PBS). This one was selected for its good thermal stability and its good mechanical properties. Hybrid systems were prepared by melt compounding by incorporating HNT and PHBV-g-MA as compatibilizers. The SEM analysis reveals that the addition of 5wt.% of PHBV-g-MA improves morphology of PHBV/PBS 80/20 blend inducing a reduction in the size of PBS nodules. The addition of 5wt.% of the HNT in the blend favorites also the reduction in the size of PBS nodules. However, the combination of PHBV-g-MA and the HNT limits the emulsifying effect of the compatibilizer due to the aggregation of the HNT. DSC analysis and TGA show that PHBV-g-MA has no effect on the thermal properties and the thermal stability PHBV/PBS blend. However, the presence of the HNT plays a positive role in the reduction in the peak of heat release rate (HRR). The mechanical properties of ternary mixture PHBV/PBS/HNT with or without compatibilisant are comparable with those of PHBV/PBS 80/20 pure blend.Another study came supplemented this work from thesis while being focused on the thermomechanical recycling of the PHBV, the PBS, nanocomposites PHBV/HNT and PBS/HNT, the PHBV/PBS 80/20 binary blend and PHBV/PBS 80/20+ HNT ternary blend with and without compatibilization. The results of this study showed that the recyclability of these systems is possible owing to the fact that the nanostructure of recycled material is improved and that the thermal and mechanical properties are not affected after 5 cycles of extrusion.

Thin film Nanocomposite (TFN) membranes are a relatively new class of high-performance semipermeable membranes for Reverse Osmosis (RO) applications. Large scale applications of TFN membranes have not been achieved yet due to the high production cost of the nanoparticles, agglomeration of the nanoparticles in the thin polyamide matrix of the membrane, and leaching out of typically toxic inorganic nanoparticles into the downstream. In this work, these challenges are addressed by incorporation of two different nanofillers: Cellulose NanoCrystals (CNC), and surface functionalized Halloysite NanoTubes (HNT). Amine groups, carboxylic acid groups, and the first generation of poly(amidoamine) (PAMAM) dendrimers were used for functionalization of the HNT. CNC and HNT are environmentally friendly, low/non-toxic, abundant, and inexpensive nanoparticles with a unique size, and chemical properties. TFN membranes were synthesized via in situ interfacial polymerization of m-phenylenediamine (MPD) with trimesoyl chloride (TMC) and the nanoparticles. The control Thin Film Composite (TFC) membranes, and CNC and HNT based TFN membranes were characterized by Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Fourier Transform Infrared spectroscopy (FTIR) and contact angle measurements. The antifouling capacity of CNC based membranes was investigated with a solution of Bovine Serum Albumin (BSA) as the fouling agent. Also, the leachability of the HNT from the membranes was examined by shaking the membranes in a batch incubator for 48 h, and then tracing the leached out HNT using Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Separation characteristics of the membranes were studied by desalination of synthetic brackish water with a cross flow RO filtration system. It was revealed that incorporation of functionalized HNT enhanced the permeate flux without sacrificing the salt rejection (99.1 % ± 0.1 %). Also, incorporation of 0.1% (w/v) CNC doubled the permeate flux (from 30 to 63 L/m2.h at 20 bar) without compromising the salt rejection (97.8%). At the same time, leaching out of HNT from the TFN membranes was decreased as a result of the HNT functionalization and formation of covalent bonds with the TMC. Also, antifouling properties of the CNC-TFN membranes were 11% improved in comparison with control TFC membrane.

The primary objectives of this research are: (1) use a hierarchical structure to study electrode materials for next-generation lithium-ion batteries (LIBs) and (2) understand the fundamentals and utility of solid polymer electrolytes (SPEs) with the addition of halloysite nanotubes (HNTs) for battery applications. Understanding the fundamental principles of electrode and electrolyte materials allows for the development of high-performance LIBs. The contributions of this dissertation are described below. Encapsulated Si-VACNT Electrodes. Two hurdles prevent Si-based electrodes from mass production. First, bulk Si undergoes volume expansion up to 300%. Second, a solid-electrolyte interphase (SEI) forms between the interface of the electrolyte and electrode, which consumes battery capacity and creates more resistance at the interface. Si volume changes were overcome by depositing silicon on vertically-aligned carbon nanotubes (VACNTs). Encapsulating the entire Si-VACNT electrode surface with carbon was used to mitigate SEI formation. Although SEI formation was reduced by the encapsulation layer, capacity fade was still observed for encapsulated electrodes, indicating that SEI formation was not the primary factor affecting capacity fade. Additionally, the impact of the encapsulation layer on Li transport was examined. Two different transport directions and length scales were relevant””(1) radial transport of Li in/out of each Si-coated nanotube (~40 nm diameter) and (2) Li transport along the length of the nanotubes (~10 µm height). Experimental results indicated that the height of the Si-VACNT electrodes did not limit Li transport, even though that height was orders of magnitude greater than the diameter of the tubes. Simulation and experimental data indicated that time constant for Li diffusion into silicon was slow, even though the diffusion distance was short relative to the tube height. Other factors such as diffusion-induced stress likely had a significant impact on diffusion through the thin silicon layer. Solid Polymer Electrolytes. A thorough understanding of the relationships between physical, transport, and electrochemical properties was studied. HNT addition to polyethylene oxide (PEO) electrolytes not only improved the physical properties, such as reduction of the crystallinity of PEO, but also enhanced transport properties like the salt diffusivity. The processing steps were important for achieving enhanced properties. Moreover, HNTs were found to stabilize the interfacial properties of the SPE films during cycling. Specifically, HNT-containing SPE films were successfully cycled at room temperature, which may have important implications for SPE-based batteries.

Indiana University-Purdue University Indianapolis (IUPUI) / The biodegradation of the hybrid layer of dental restorations is due in part to the degradation of the demineralized collagen by matrix metalloproteinases (MMPs). During the bonding procedure, phosphoric acid/acidic primers activate MMPs that degrade denuded type I collagen. As a result, the hybrid layer loses its integrity overtime, leading to the failure of the resin composite restoration. This study aimed to evaluate doxycycline (DOX) for its effects on preventing the degradation of the hybrid layer through the modification of the dental adhesive with aluminosilicate clay nanotubes (HNT) loaded with doxycycline. Doxycycline was encapsulated into HNT at three distinct concentrations (10%, 20% and 30% DOX, w/v). The increases in the concentration of doxycycline significantly increased the amount of doxycycline that was encapsulated into HNT and the drug loading into the HNT. Conversely, the encapsulation efficiency was significantly decreased with the increases in concentration of doxycycline. The modified adhesives were fabricated by incorporation of DOX-encapsulated HNT into a commercially available dental adhesive (Adper Scotchbond Multi-Purpose, SBMP). The degree of conversion (DC), Knoop microhardness, doxycycline release profiles, the biological activity (antibacterial and anti-MMP activity), and cytocompatibility of the modified adhesives were investigated. There were no statistically significant differences (p > 0.05) in DC and Knoop microhardness compared to the control (SBMP). None of the adhesive eluates was cytotoxic to the human dental pulp stem cells. Although higher concentrations of doxycycline led to a higher release of doxycycline from the modified adhesives, the differences were not significant (p = 0.259) among the groups (10%, 20% and 30% DOX). A significant growth inhibition of S. mutans and L. casei by direct contact illustrated successful encapsulation of doxycycline into the modified adhesives. Doxycycline released from the modified adhesives did not inhibit the growth of both cariogenic bacteria but inhibited MMP-1 activity. The results suggested that subantimicrobial levels of doxycycline were gradually released. The immediate microtensile bond strengths were not significantly different from those of the control (SBMP), suggesting no negative effect of doxycycline on dentin bonding (only 10% DOX were investigated). The long-term resin-dentin bond durability should be evaluated.

Biobased all-cellulose composites (ACCs), in which the matrix and the reinforcement are made out of the same material, have gained a noticeable increased attention in recent years. Their successful application would solve the commonly faced challenges with natural fiber composites regarding their chemical antipathy between the hydrophilic fiber and the usually hydrophobic polymer matrix, while still keeping the advantages of being environmental friendly. Moreover, the use of man-made continuous regenerated cellulose fibers for this purpose could result in unidirectional all-cellulose composites with excellent mechanical properties. In this study, a new processing technique for unidirectional all-cellulose composites, reinforced with continuous regenerated cellulose nanocomposite fibers, has been developed, where the fibers are wound directly after the coagulation bath and then welded together while still being swelled in order to form all-cellulose composite sheets without the need of adding any additional solvent or chemicals. Scanning electron microscopy and tensile testing were used to investigate and compare the microstructure and mechanical properties, of a reference material without nanoreinforced fibers and two variants reinforced with 2 % cellulose nanocrystals (CNCs) and 2 % halloysite nanotubes (HNTs). Analysis revealed that transparent all-cellulose composites with a high compaction degree and minimal warpage during shrinkage, showing high mechanical properties could be made. However, the addition of nanoreinforcements did not lead to any improvements.