Design of assemblies based on polymer-coated quantum dots and organic dye

During last years, photophysical properties of complexes of semiconductor quantum dots (QDs) with organic dyes have attracted increasing interest. The development of different assemblies based on QDs and organic dyes allows to increase the range of QDs applications, which include imaging, biological sensing and electronic devices.1 Some studies demonstrate energy transfer between QDs and organic dye in assemblies.2 However, for electronic devices purposes, a polymeric matrix is required to enhance QDs photostability. Thus, in order to attach the QDs to the polymer surface it is necessary to chemically modify the polymer to induce electronic charges and stabilize the QDs in the polymer. The present work aims to investigate the design of assemblies based on polymer-coated QDs and an integrated acceptor organic dye. Polymethylmethacrylate (PMMA) and polycarbonate (PC) were used as polymeric matrices, and nile red as acceptor. Additionally, a PMMA matrix modified with 2-mercaptoethylamine is used to improve the attachment between both the donor (QDs) and the acceptor (nile red), as well as to induce a covalent bond between the modified PMMA and the QDs. An enhancement of the energy transfer efficiency by using the modified PMMA is expected and the resulting assembly can be applied for energy harvesting.

Color change using HSB color system of dental resin composites immersed in different common Amazon region beverages

The purpose of this study is to evaluate in vitro the color stability of composite resins when exposed to beverages with high coloring contents from the Amazon region. 240 samples from four different composite brands (Natural Look, Z350, 4Seasons and Opallis) of hue A3 were fabricated using an acrylic matrix. The samples were stored in distilled water at 37ºC for 24 hours. The initial color (T0) was registered using a Canon EOS Rebel XTi 10 mp camera, and then the samples were divided into four groups (n=15): G1 (coffee), G2 (açaí juice), G3 (energetic guaraná) and G4 (control - distilled water). The samples were exposed to solutions of DES (6hs) and RE (18hs) and placed in a double boiler under constant agitation, at 37ºC for 30 days. The samples were immersed in the coloring solutions for 15 minutes daily. After 7, 15 and 30 days, new photographic registers were made (T1, T2 and T3). The images were analyzed using Corel PHOTO-PAINT 12 software to identify the colors through the HSB system. The Kruskal-Wallis and t tests (p<0.05) demonstrated significant differences in color (hue, saturation and brightness). The results revealed that none of the tested composites showed color stability when exposed to coloring solutions, and that the Amazon region beverages (açaí juice and energetic guaraná) showed to be less coloring than coffee.

Optimization of magnetoelectric composites based on electroactive polymers for energy harvesting and sensor application

Tese de Doutoramento em Engenharia de Materiais.

Estudo da ligação entre componentes estruturais de GFRP (glass fiber reinforced polymer) e componentes estruturais em betão

Dissertação de mestrado integrado em Engenharia Civil

Enzymatic hydrophobic modification of jute fibers via grafting to reinforce composites

Horseradish peroxidase (HRP)/H2O2 system catalyzes the free-radical polymerization of aromatic compounds such as lignins and gallate esters. In this work, dodecyl gallate (DG) was grafted onto the surfaces of lignin-rich jute fabrics by HRP-mediated oxidative polymerization with an aim to enhance the hydrophobicity of the fibers. The DG-grafted jute fibers and reaction products of their model compounds were characterized by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). The results clearly indicated the grafting of DG to the jute fiber by HRP. Furthermore, the hydrophobicity of jute fabrics was determined by measuring the wetting time and static contact angle. Compared to the control sample, the wetting time and static contact angle of the grated fabrics changed from ~1 s to 1 h and from ~0° to 123.68°, respectively. This clearly proved that the hydrophobicity of jute fabrics improved considerably. Conditions of the HRP-catalyzed DG-grafting reactions were optimized in terms of the DG content of modified jute fabrics. Moreover, the results of breaking strength and elongation of DG-grafted jute/ polypropylene (PP) composites demonstrated improved reinforcement of the composite due to enzymatic hydrophobic modification of jute fibers.

Thermoplastic Starch-based Composites Reinforced with Rape Fibers: Water Uptake and Thermomechanical Properties

Fully biodegradable composite materials were obtained through reinforcement of a commercially available thermoplastic starch (TPS) matrix with rapeseed fibers (RSF). The influence of reinforcement content on the water sorption capacity, as well as thermal and thermo-mechanical properties of composites were evaluated. Even though the hydrophilic character of natural fibers tends to favor the absorption of water, results demonstrated that the incorporation of RSF did not have a significant effect on the water uptake of the composites. DSC experiments showed that fibers restricted the mobility of the starch macromolecules from the TPS matrix, hence reducing their capacity to crystallize. The viscoelastic behaviour of TPS was also affected, and reinforced materials presented lower viscous deformation and recovery capacity. In addition, the elasticity of materials was considerably diminished when increasing fiber content, as evidenced in the TMA and DMTA measurements

Combined Electrostatic and Covalent Polymer Networks for Cell Microencapsulation

Two types of hydrogel microspheres have been developed. Fast ionotropic gelation of sodium alginate (Na-alg) in the presence of calcium ions was combined with slow covalent cross-linking of poly(ethylene glycol) (PEG) derivatives. For the first type, the fast obtainable Ca-alg hydrogel served as spherical matrix for the simultaneously occurring covalent cross-linking of multi-arm PEG derivative. A two-component interpenetrating network was formed in one step upon extruding the mixture of the two polymers into the gelation bath. For the second type, heterobifunctional PEG was grafted onto Na-alg prior to gelation. Upon extrusion of the polymer solution into the gelation bath, fast Ca-alg formation ensured the spherical shape and was accompanied by cross-linker-free covalent cross-linking of the PEG side chains. Thus, one-component hydrogel microspheres resulted. We present the physical properties of the hydrogel microspheres and demonstrate the feasibility of cell microencapsulation for both types of polymer networks.

High-Performance-Tensile-Strength Alpha-Grass Reinforced Starch-Based Fully Biodegradable Composites

Though there has been a great deal of work concerning the development of natural fibers in reinforced starch-based composites, there is still more to be done. In general, cellulose fibers have lower strength than glass fibers; however, their specific strength is not far from that of fiberglass. In this work, alpha-fibers were obtained from alpha-grass through a mild cooking process. The fibers were used to reinforce a starch-based biopolymer. Composites including 5 to 35% (w/w) alpha-grass fibers in their formulation were prepared, tested, and subsequently compared with those of wood- and fiberglass-reinforced polypropylene (PP). The term “high-performance” refers to the tensile strength of the studied composites and is mainly due to a good interphase, a good dispersion of the fibers inside the matrix, and a good aspect ratio. The tensile strength of the composites showed a linear evolution for fiber contents up to 35% (w/w). The strain at break of the composites decreased with the fiber content and showed the stiffening effects of the reinforcement. The prepared composites showed high mechanical properties, even approaching those of glass fiber reinforced composites

Research on the suitability of organosolv semi-chemical triticale fibers as reinforcement for recycled hdpe composites

The main objective of this research was to study the feasibility of incorporating organosolv semi-chemical triticale fibers as the reinforcing element in recycled high density polyethylene (HDPE). In the first step, triticale fibers were characterized in terms of chemical composition and compared with other biomass species (wheat, rye, softwood, and hardwood). Then, organosolv semi-chemical triticale fibers were prepared by the ethanolamine process. These fibers were characterized in terms of its yield, kappa number, fiber length/diameter ratio, fines, and viscosity; the obtained results were compared with those of eucalypt kraft pulp. In the second step, the prepared fibers were examined as a reinforcing element for recycled HDPE composites. Coupled and non-coupled HDPE composites were prepared and tested for tensile properties. Results showed that with the addition of the coupling agent maleated polyethylene (MAPE), the tensile properties of composites were significantly improved, as compared to non-coupled samples and the plain matrix. Furthermore, the influence of MAPE on the interfacial shear strength (IFSS) was studied. The contributions of both fibers and matrix to the composite strength were also studied. This was possible by the use of a numerical iterative method based on the Bowyer-Bader and Kelly-Tyson equations

Non-resorbable glass fibre-reinforced composite with porous surface as bone substitute material: Experimental studies in vitro and in vivo focused on bone-implant interface

The development of load-bearing osseous implant with desired mechanical and surface properties in order to promote incorporation with bone and to eliminate risk of bone resorption and implant failure is a very challenging task. Bone formation and resoption processes depend on the mechanical environment. Certain stress/strain conditions are required to promote new bone growth and to prevent bone mass loss. Conventional metallic implants with high stiffness carry most of the load and the surrounding bone becomes virtually unloaded and inactive. Fibre-reinforced composites offer an interesting alternative to metallic implants, because their mechanical properties can be tailored to be equal to those of bone, by the careful selection of matrix polymer, type of fibres, fibre volume fraction, orientation and length. Successful load transfer at bone-implant interface requires proper fixation between the bone and implant. One promising method to promote fixation is to prepare implants with porous surface. Bone ingrowth into porous surface structure stabilises the system and improves clinical success of the implant. The experimental part of this work was focused on polymethyl methacrylate (PMMA) -based composites with dense load-bearing core and porous surface. Three-dimensionally randomly orientated chopped glass fibres were used to reinforce the composite. A method to fabricate those composites was developed by a solvent treatment technique and some characterisations concerning the functionality of the surface structure were made in vitro and in vivo. Scanning electron microscope observations revealed that the pore size and interconnective porous architecture of the surface layer of the fibre-reinforced composite (FRC) could be optimal for bone ingrowth. Microhardness measurements showed that the solvent treatment did not have an effect on the mechanical properties of the load-bearing core. A push-out test, using dental stone as a bone model material, revealed that short glass fibre-reinforced porous surface layer is strong enough to carry load. Unreacted monomers can cause the chemical necrosis of the tissue, but the levels of leachable resisidual monomers were considerably lower than those found in chemically cured fibre-reinforced dentures and in modified acrylic bone cements. Animal experiments proved that surface porous FRC implant can enhance fixation between bone and FRC. New bone ingrowth into the pores was detected and strong interlocking between bone and the implant was achieved.

Rigid Rod Polymer Fillers in Acrylic Denture and Dental Adhesive Resin Systems

Polymeric materials have been used in dental applications for decades. Adhesion of polymeric materials to each other and to the tooth substrate is essential to their successful use. The aim of this series of studies was two-folded. First, to improve adhesion of poly(paraphenylene) based rigid rod polymer (RRP) to other dental polymers, and secondly, to evaluate the usability of a new dentin primer system based on RRP fillers. Poly(paraphenylene) based RRP would be a tempting material for dental applications because of its good mechanical properties. To be used in dental applications, reliable adhesion between RRP and other dental polymers is required. In this series of studies, the adhesion of RRP to denture base polymer and the mechanical properties of RRP-denture base polymer-material combination were evaluated. Also adhesion of BisGMA-TEGDMA-resin to RRP was determined. Different surface treatments were tested to improve the adhesion of BisGMA-TEGDMA-resin to RRP. Results were based on three-point bending testing, Vickers surface hardness test and scanning electron microscope analysis (SEM), which showed that no reliable adhesion between RRP and denture base polymer was formed. Addition of RRP filler to denture base polymer increased surface hardness and flexural modulus but flexural strength decreased. Results from the shear bond strength test and SEM revealed that adhesion between resin and RRP was possible to improve by surface treatment with dichloromethane (DCM) based primer and a new kind of adhesive surface can be designed. The current dentin bonding agents have good immediate bond strength, but in long term the bond strength may decrease due to the detrimental effect of water and perhaps by matrix metalloproteinases. This leads to problems in longevity of restorations. Current bonding agents use organic monomers. In this series of studies, RRP filled dentin primer was tested in order to decrease the water sorption of the monomer system of the primers. The properties of new dentin primer system were evaluated in vitro by comparing it to commercial etch and rinse adhesive system. The results from the contact angle measurements and SEM showed that experimental primer with RRP reinforcement provided similar resin infiltration to dentin collagen and formed the resin-dentin interface as the control primer. Microtensile bond strength test and SEM revealed that in short term water storing, RRP increased bond strength and primer with BMEP-monomer (bis[2-(methacryloyloxy)-ethyl]phosphate) and high solvent concentration provided comparable bonding properties to the commercial control primers. In long term water storing, the high solvent-monomer concentration of the experimental primers decreased bond strength. However, in low solvent-monomer concentration groups, the long-term water storing did not decrease the bond strength despite the existence of hydrophilic monomers which were used in the system. These studies demonstrated that new dentin primer system reached the mechanical properties of current traditional etch and rinse adhesive system in short time water storing. Improved properties can be achieved by further modifications of the monomer system. Studies of the adhesion of RRP to other polymers suggest that adhesion between RRP and other dental polymers is possible to obtain by certain surface treatments.

Feasibility study of metal to polymer hybrid joining

The increasing demand for lightweight components has led to a huge exploitation of non-metallic materials such as polymers, fibers and elastomers in industrial and manufacturing processes. Recent trends towards cost effectiveness, weight reduction and production flexibility in industrial production and manufacturing processes has led to a growing interest in hybrid components where two or more dissimilar materials coexist to achieving specifically optimized characteristics. The importance of this research is to serve as a bridge to understanding the theories behind various joining techniques and the adaptation of the process for metal to polymer hybrid joints. Moreso, it helps companies to select the most productive and yet economical joining process for realization of lightweight metal to polymer hybrid components. This thesis is a literature review analyzing various materials that has been published on various joining methods for metal to polymer hybrid joints on the feasibility and eventual realization of the joint between these dissimilar materials. This study is aimed at theoretically evaluating the feasibility of joining processes between metal and plastic components by exploiting exhaustively joining and welding sources.

Unidirectional fiber-reinforced composite as an oral implant abutment material: Experimental studies of E-glass fiber/BisGMA-TEGDMA polymer in vitro

Fiber-reinforced composites (FRCs) are a new group of non-metallic biomaterials showing a growing popularity in many dental and medical applications. As an oral implant material, FRC is biocompatible in bone tissue environment. Soft tissue integration to FRC polymer material is unclear. This series of in vitro studies aimed at evaluating unidirectional E-glass FRC polymer in terms of mechanical, chemical, and biological properties in an attempt to develop a new non-metallic oral implant abutment alternative. Two different types of substrates were investigated: (a) Plain polymer (BisGMA 50%–TEGDMA 50%) and (b) Unidirectional FRC. The mechanical behavior of high fiber-density FRCs was assessed using a three-point bending test. Surface characterization was performed using scanning electron and spinning disk confocal microscopes. The surface wettability/energy was determined using sessile drop method. The blood response, including blood-clotting ability and platelet morphology was evaluated. Human gingival fibroblast cell responses - adhesion kinetics, adhesion strength, and proliferation activity - were studied in cell culture environment using routine test conditions. A novel tissue culture method was developed and used to evaluate porcine gingival tissue graft attachment and growth on the experimental composite implants. The analysis of the mechanical properties showed that there is a direct proportionality in the relationship between E-glass fiber volume fraction and toughness, modulus of elasticity, and load bearing capacity; however, flexural strength did not show significant improvement when high fiber-density FRC is used. FRCs showed moderate hydrophilic properties owing to the presence of exposed glass fibers on the polymer surface. Blood-clotting time was shorter on FRC substrates than on plain polymer. The FRC substrates also showed higher platelet activation state than plain polymer substrates. Fibroblast cell adhesion strength and proliferation rate were highly pronounced on FRCs. A tissue culture study revealed that gingival epithelium and connective tissue established an immediate close contact with both plain polymer and FRC implants. However, FRC seemed to guide epithelial migration outwards from the tissue/implant interface. Due to the anisotropic and hydrophilic nature of FRC, it can be concluded that this material enhances biological events related with soft tissue integration on oral implant surface.

A fucan from the brown seaweed Spatoglossum schröederi inhibits Chinese hamster ovary cell adhesion to several extracellular matrix proteins

Fucans, a family of sulfated polysaccharides present in brown seaweed, have several biological activities. Their use as drugs would offer the advantage of no potential risk of contamination with viruses or particles such as prions. A fucan prepared from Spatoglossum schröederi was tested as a possible inhibitor of cell-matrix interactions using wild-type Chinese hamster ovary cells (CHO-K1) and the mutant type deficient in xylosyltransferase (CHO-745). The effect of this polymer on adhesion properties with specific extracellular matrix components was studied using several matrix proteins as substrates for cell attachment. Treatment with the polymer inhibited the adhesion of fibronectin to both CHO-K1 (2 x 10(5))()and CHO-745 (2 x 10(5) and 5 x 10(5)) cells. No effect was detected with laminin, using the two cell types. On the other hand, adhesion to vitronectin was inhibited in CHO-K1 cells and adhesion to type I collagen was inhibited in CHO-745 cells. In spite of this inhibition, the fucan did not affect either cell proliferation or cell cycle. These results demonstrate that this polymer is a new anti-adhesive compound with potential pharmacological applications.

Viologen based electroactive polymers and composites for durable electrochromic systems

Electrochromism, the phenomenon of reversible color change induced by a small electric charge, forms the basis for operation of several devices including mirrors, displays and smart windows. Although, the history of electrochromism dates back to the 19th century, only the last quarter of the 20th century has its considerable scientific and technological impact. The commercial applications of electrochromics (ECs) are rather limited, besides top selling EC anti-glare mirrors by Gentex Corporation and airplane windows by Boeing, which made a huge commercial success and exposed the potential of EC materials for future glass industry. It is evident from their patents that viologens (salts of 4,4ʹ-bipyridilium) were the major active EC component for most of these marketed devices, signifying the motivation of this thesis focusing on EC viologens. Among the family of electrochromes, viologens have been utilized in electrochromic devices (ECDs) for a while, due to its intensely colored radical cation formation induced by applying a small cathodic potential. Viologens can be synthesized as oligomer or in the polymeric form or as functionality to conjugated polymers. In this thesis, polyviologens (PVs) were synthesized starting from cyanopyridinium (CNP) based monomer precursors. Reductive coupling of cross-connected cyano groups yields viologen and polyviologen under successive electropolymerization using for example the cyclic voltammetry (CV) technique. For further development, a polyviologen-graphene composite system was fabricated, focusing at the stability of the PV electrochrome without sacrificing its excellent EC properties. High electrical conductivity, high surface area offered by graphene sheets together with its non-covalent interactions and synergism with PV significantly improved the electrochrome durability in the composite matrix. The work thereby continued in developing a CNP functionalized thiophene derivative and its copolymer for possible utilization of viologen in the copolymer blend. Furthermore, the viologen functionalized thiophene derivative was synthesized and electropolymerized in order to explore enhancement in the EC contrast and overall EC performance. The findings suggest that such electroactive viologen/polyviologen systems and their nanostructured composite films as well as viologen functionalized conjugated polymers, can be potentially applied as an active EC material in future ECDs aiming at durable device performances.