Maria L. Auad

Liquid rubber modified vinyl ester resins: fracture and mechanical behavior

Abstract The fracture and mechanical behavior of vinyl ester resins (DVER) cured with styrene (S) and modified with two different liquid rubbers has been determined and related to the microstructure of the resulting modified thermosets. Carboxyl terminated poly(butadiene-co-acrylonitrile) (CTBN), a common toughening agent for epoxy resins, is an almost unreactive rubber with the DVER and S comonomers. During crosslinking the system undergoes a phase separation mechanism similar to that occurring in unsaturated polyester resins (UPE) modified with a low profile additive (LPA), such as polyvinyl acetate (PVAc). This process leads to materials, which exhibit a sharp drop in density at high CTBN concentrations (≥10% by weight) and to the development of a co-continuous microstructure in these materials. This feature is consistent with a maximum in fracture toughness as a function of the additive (CTBN) content, followed by a rapid deterioration in toughness at higher concentrations. On the other hand, the use of a reactive rubber, vinyl terminated poly(butadiene-co-acrylonitrile, VTBN, as the additive leads to a different morphology consisting on rubber inclusions in the thermoset matrix. This structure gradually reduces the fracture and mechanical performance of the resins modified with increasing concentration of reactive elastomer.

Effects of surface functionalization on the surface phage coverage and the subsequent performance of phage-immobilized magnetoelastic biosensors

One of the important applications for which phage-immobilized magnetoelastic (ME) biosensors are being developed is the wireless, on-site detection of pathogenic bacteria for food safety and bio-security. Until now, such biosensors have been constructed by immobilizing a landscape phage probe on gold-coated ME resonators via physical adsorption. Although the physical adsorption method is simple, the immobilization stability and surface coverage of phage probes on differently functionalized sensor surfaces need to be evaluated as a potential way to enhance the detection capabilities of the biosensors. As a model study, a filamentous fd-tet phage that specifically binds streptavidin was adsorbed on either bare or surface-functionalized gold-coated ME resonators. The surface functionalization was performed through the formation of three self-assembled monolayers with a different terminator, based on the sulfur-gold chemistry: AC (activated carboxy-terminated), ALD (aldehyde-terminated), and MT (methyl-terminated). The results, obtained by atomic force microscopy, showed that surface functionalization has a large effect on the surface phage coverage (46.8%, 49.4%, 4.2%, and 5.2% for bare, AC-, ALD-, and MT-functionalized resonators, respectively). In addition, a direct correlation of the observed surface phage coverage with the quantity of subsequently captured streptavidin-coated microbeads was found by scanning electron microscopy and by resonance frequency measurements of the biosensors. The differences in surface phage coverage on the differently functionalized surfaces may then be used to pattern the phage probe layer onto desired parts of the sensor surface to enhance the detection capabilities of ME biosensors.

Epoxy-based divinyl ester resin/styrene copolymers: Composition dependence of the mechanical and thermal properties

Epoxy-based divinyl ester resins (DVER) were obtained by reacting diglycidyl ether of bisphenol A (DGEBA) with methacrylic acid (MA) and characterized by FTIR and 1H-NMR spectroscopies and gel permeation chromatography (GPC). The densities and viscosities of the DVER in styrene (S) solutions were measured at different temperatures, 25, 40, and 60°C and compositions, 3.4 to 100% by weight of styrene. Dynamic mechanical measurements (DMA) and differential scanning calorimetry (DSC) were used to determine the glass transition temperatures of the homopolymers and the DVER/S copolymers: 20, 40, 60, and 80% by weight of styrene. The values obtained are in the range limited by the homopolymers glass transition, 100°C for polystyrene and 173°C for the cured DVER. The data were well fitted if two contributions to the glass transition are taken into account: the “linear copolymer” contribution (Fox eq.) and the “crosslinking” contribution (Nielsen model). Uniaxial static compression tests were carried out to determine the modulus, yield stress, and ultimate stress in samples with different compositions. All the mentioned properties decrease with an increase in the styrene concentration in the final copolymer. It was found that the volumetric contraction during curing increases with styrene concentration.

Preparation of alginate–chitosan fibers with potential biomedical applications

The preparation of alginate-chitosan fibers, through wet spinning technique, as well as the study of their properties as a function of chitosans molecular weight and retention time in the coagulation bath, is presented and discussed in this work. Scanning electron microscopy (SEM) revealed that the fibers presented irregular and rough surfaces, with a grooved and heavily striated morphology distributed throughout the structure. Dynamic mechanical analysis (DMA) showed that, with the exception of elongation at break, the incorporation of chitosan into the fibers improved their tensile properties. The in vitro release profile of sulfathiazole as a function of chitosans molecular weight indicated that the fibers are viable carriers of drugs. Kinetic models showed that the release of the model drug is first-order, and the release mechanism is governed by the Korsmeyer-Peppas model. Likewise, fibers loaded with sulfathiazole showed excellent inhibition of Escherichia coli growth after an incubation time of 24h at 37 °C.

Mechanical Behavior of Hybrid Composite Phenolic Foam

Hybrid composite phenolic foams are reinforced with chopped glass and aramid fibers in varied proportions. The mechanical properties are measured and compared with those of foams reinforced with only aramid and glass fibers. The compression and shear properties of the hybrid reinforced foams are also compared with those of commercial polyurethane foams. The reinforced hybrid phenolic foams exhibit greater resistance to cracking and are significantly stiffer and stronger than foams with only glass and Nomex® fibers. In general, the mechanical properties of reinforced hybrid phenolic foams are comparable to that of commercial polyurethane foam of equivalent density. The experimentally observed compressive properties (compression modulus) of reinforced phenolic foam with different fiber loading have been compared with existing theories of reinforcement. Composite models such as parallel, series, Halpin—Tsai, and the Hirsch model have been evaluated to fit the experimental data. The findings presented here, coupled with earlier results, demonstrate the potential use of hybrid composite foams as a low-cost engineering material that is tough, strong, and fire retardant.

Rubber modified vinyl ester resins of different molecular weights

The morphology, as well as the related fracture and mechanical behavior of vinyl ester resins (DVER) of different molecular weights cured with styrene (S) and modified with two different liquid rubbers are presented and discussed. The liquid rubbers are: carboxyl terminated poly(butadiene-co-acrylonitrile) (CTBN), a common toughening agent for epoxy resins, and an almost unreactive rubber with the DVER and S comonomers, and a reactive rubber (vinyl terminated poly(butadiene-co-acrylonitrile), (VTBN). The initial miscibility of the modified systems and the reactivity of the rubber determine the final morphology of the material. This morphology will correspond to a continuous main phase (rich in the DVER-S copolymer) with simple rubber rich inclusions (as in the epoxy-rubber systems) or with inclusions with a complex internal structure, where phase separation occurs as in the low profile modified unsaturated polyester resins. The morphologies developed are strongly dependent on the resin molecular weight as well as on the elastomer added. In spite of the initially higher compatibility of the S-DVER-CTBN system with respect to the S-DVER-VTBN system, the reactivity of the vinyl-ended elastomer leads to a much finer distribution of the elastomeric phase. In particular, the low molecular weight resin cured with S and modified with 10% of CTBN leads to a cocontinuous structure with microvoids that generates a material of low density and poor mechanical and fracture properties. On the other hand, the use of VTBN as additive leads to a more compact morphology, with gradual reduction of the mechanical performance of the modified resins and improved fracture behavior.

Seed-Mediated growth of gold nanorods: limits of length to diameter ratio control

The effects of the seed reaction conditions on the two-step seed-mediated growth of gold nanorods and the effect of gold and reducing agent content in the growth solution were evaluated. Results indicate that the reaction conditions used to produce the seeds have a significant impact on the aspect ratio of the gold nanorods produced. Increasing the concentration of gold or the reaction temperature in the seed production step results in lower length to diameter (aspect ratio) gold rods. In addition, the amount of prepared seed added to the growth solution impacts the rod aspect ratio, with increasing amounts of seed reducing the aspect ratio. The effects of reducing agent, ascorbic acid (AA), and gold content of the growth solution on the aspect ratio of the produced rods are strongly interrelated. There exists a minimum ascorbic acid to gold concentration below which rods will not form; however, increasing the ratio above this minimum results in shorter rods being formed. Characterization of nanorod growth is performed by UV-vis-NIR spectrophotometry and transmission electron microscopy (TEM).

Quasibinary and quasiternary styrene, dimethacrylate resin, and CTBN (or VTBN) liquid rubber systems: phase diagrams, interaction parameters and cured materials morphologies

Abstract Four binary and two ternary experimental cloud point curves (CPC) for systems formulated with styrene (S), a dimethacrylate resin (DMAR), and poly(butadiene-co-acrylonitrile) liquid rubbers terminated in vinyl (VTBN) or carboxyl groups (CTBN), are presented. Measured CPCs of the binaries DMAR–CTBN, DMAR–VTBN and S–DMAR show upper critical solution temperature (UCST), while CPCs of the ternary S–DMAR–rubbers show liquid–liquid partial miscibility on the DMAR–rubber binary sides. A Flory–Huggins (F–H) thermodynamic analysis of the three measured binary cloud-point data allows the calculation of the respective interaction parameters and their temperature dependence. Interaction parameters for the other two binaries, S–CTBN (VTBN), were selected as the set of values that produced calculated spinodals, for the S–DMAR–rubbers ternaries, tangent to the respective experimental cloud-point diagrams at the critical point. The thermodynamic analysis was done taking into account the molecular polydispersity of CTBN, VTBN and DMAR components. Final morphologies of CTBN modified cured materials, obtained with two different molecular weight DMAR, were well correlated with the predicted miscibility for both systems.

Preparation and Characterization of Epoxy Resin Cross-Linked with High Wood Pyrolysis Bio-Oil Substitution by Acetone Pretreatment

The use of cost effective solvents may be necessary to store wood pyrolysis bio-oil in order to stabilize and control its viscosity, but this part of the production system has not been explored. Conversely, any rise in viscosity during storage, that would occur without a solvent, will add variance to the production system and render it cost ineffective. The purpose of this study was to modify bio-oil with a common solvent and then react the bio-oil with an epoxy for bonding of wood without any loss in properties. The acetone pretreatment of the bio-oil/epoxy mixture was found to improve the cross-linking potential and substitution rate based on its mechanical, chemical, and thermal properties. Specifically, the bio-oil was blended with epoxy resin at weight ratios ranging from 2:1 to 1:5 and were then cured. A higher bio-oil substitution rate was found to lower the shear bond strength of the bio-oil/epoxy resins. However, when an acetone pretreatment was used, it was possible to replace the bio-oil by as much as 50% while satisfying usage requirements. Extraction of the bio-oil/epoxy mixture with four different solvents demonstrated an improvement in cross-linking after acetone pretreatment. ATR-FTIR analysis confirmed that the polymer achieved a higher cross-linked structure. DSC and TGA curves showed improved thermal stability with the addition of the acetone pretreatment. UV-Vis characterization showed that some functional groups of the bio-oil to epoxy system were unreacted. Finally, when the resin mixture was utilized to bond wood, the acetone pretreatment coupled with precise tuning of the bio-oil:epoxy ratio was an effective method to control cross-linking while ensuring acceptable bond strength.

The effect of ethanol on hydroxyl and carbonyl groups in biopolyol produced by hydrothermal liquefaction of loblolly pine: (31)P-NMR and (19)F-NMR analysis.

The goal of this study was to investigate the role of ethanol and temperature on the hydroxyl and carbonyl groups in biopolyol produced from hydrothermal liquefaction of loblolly pine (Pinus spp.) carried out at 250, 300, 350 and 390°C for 30min. Water and water/ethanol mixture (1/1, wt/wt) were used as liquefying solvent in the HTL experiments. HTL in water and water/ethanol is donated as W-HTL and W/E-HTL, respectively. It was found that 300°C and water/ethanol solvent was the optimum liquefaction temperature and solvent, yielding up to 68.1wt.% bio-oil and 2.4wt.% solid residue. (31)P-NMR analysis showed that biopolyol produced by W-HTL was rich in phenolic OH while W/E-HTL produced more aliphatic OH rich biopolyols. Moreover, biopolyols with higher hydroxyl concentration were produced by W/E-HTL. Carbonyl groups were analyzed by (19)F-NMR, which showed that ethanol reduced the concentration of carbonyl groups.