Rheological study of the curing kinetics of epoxy–phenol novolac resin

The curing reaction of an epoxy–phenolic resin under different conditions was monitored using rheological measurements. The evolution of viscoelastic properties, such as storage modulus, G′, and loss modulus, G″, was recorded. Several experiments were performed to confidently compare the rheological data obtained under varied curing conditions of temperature, catalyst concentration, and reactive ratios. The values of G′ measured at the end of the reactions (at maximum conversion) were independent of the frequency and temperature of the tests in the range of high temperatures investigated. The overall curing process was described by a second‐order phenomenological rheokinetic equation based on the model of Kamal. The effects of the epoxy‐to‐phenolic ratio as well as the curing temperature and the catalyst concentration were also investigated. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4430–4439, 2006     

Study of nanoreinforced shape memory polymers processed by casting and extrusion

Multi‐walled carbon nanotubes (CNTs) and cellulose nanofibers (CNFs) reinforced shape memory polyurethane (PU) composite fibers and films have been fabricated via extrusion and casting methods. Cellulose nanofibers were obtained through acid hydrolysis of microcrystalline cellulose. This treatment aided in achieving stable suspensions of cellulose crystals in dimethylformamide (DMF), for subsequent incorporation into the shape memory matrix. CNTs were covalent functionalized with carboxyl groups (CNT‐COOH) and 4,4′‐methylenebis (phenylisocyanate) (MDI) (CNT‐MDI) to improve the dispersion efficiency between the CNT and the polyurethane. Significant improvement in tensile modulus and strength were achieved by incorporating both fillers up to 1 wt% without sacrificing the elongation at break. Electron microscopy was used to investigate the degree of dispersion and fracture surfaces of the composite fibers and films. The effects of the filler (type and concentration) on the degree of crystallinity and thermal properties of the hard and soft segments that form the PU sample were studied by calorimetry. Overall, results indicated that the homogeneous dispersion of nanotubes and cellulose throughout the PU matrix and the strong interfacial adhesion between nanotubes and/or cellulose and the matrix are responsible for the enhancement of mechanical and shape memory properties of the composites. POLYM. COMPOS., 2011. © 2011 Society of Plastics Engineers     

Physico‐Mechanical Properties of Biodegradable Starch Nanocomposites

Nanocomposites of cassava starch reinforced with waxy starch nanocrystals were prepared. They showed a 380% increase of the rubbery storage modulus (at 50 °C) and a 40% decrease in the water vapor permeability. X‐ray spectra show that the composite was more amorphous than the neat matrix, which was attributed to higher equilibrium water content in the composites. TGA confirmed this result and its thermal derivative suggested the formation of hydrogen bonding between glycerol and the nanocrystals. The reinforcing effect of starch nanocrystals was attributed to strong filler/matrix interactions due to the hydrogen bonding. The decrease of the permeability suggests that the nanocrystals were well dispersed, with few filler/filler interactions.

     

Composites made from a soybean oil biopolyurethane and cellulose nanocrystals

Cellulose nanocrystals (CNC) obtained by acidic hydrolysis from microcrystalline cellulose were dispersed in a biopolyurethane matrix to prepare composite films. The polyurethane was prepared from a hydroxylated soybean oil (SO‐OH) and a polymeric diphenyldiisocyanate (pMDI), using a organotin compound as the catalyst. The composite films contained different concentrations of nanocelullose, without any macroscopic aggregates in all cases. Thermal, tensile and dynamic mechanical properties of the films were determined for all the samples. In particular, it was observed that the glass transition temperature of the nanocomposites slightly increased with the concentration of the cellulose nanocrystals. The nanocomposite with 1 wt% of nanocellulose showed the highest tensile strength of the series. POLYM. ENG. SCI., 58:125–132, 2018. © 2017 Society of Plastics Engineers     

Curing kinetics of epoxy‐urethane copolymers

The cure of the epoxy resin diglycidyl ether of bisphenol A (Araldyt GY9527) with a mixture of cycloaliphatic amines (Distraltec) was studied, and the focus was on the effect of the copolymerization with a commercial polyurethane (PU) elastomer (Desmocap 12). A simplified phenomenological model was proposed to represent the copolymerization reaction. It considered the effect of the temperature and the concentration of the elastomer on the reaction rate, and it was simple enough to be included in models of processing conditions. A nonlinear regression analysis of the experimental conversion data obtained from differential scanning calorimetry was utilized to find the best fitting parameters to Kamal's equation for the chemically controlled part of the reaction (short times) under isothermal and constant heating‐rate conditions. The Rabinowitch approach together with the Addam–Gibbs theory was utilized to introduce the effect of diffusion control at the end of the reaction on the overall constant for the reaction rate. The Di Benedetto equation was used to predict the conversion at which vitrification takes place for each run. Experimental results for conversions higher than this critical conversion were utilized to obtain information about the diffusion kinetic constant using a nonlinear regression analysis as previously. The overall model obtained was used to calculate a calorimetric conversion and reaction rate as functions of time, which was in excellent agreement with the experimental results. The addition of PU elastomers affected the values of the activation energies of the chemically and diffusion controlled parts of the reaction, as well as the final conversion reached by the epoxy–amine system. The proposed model allowed prediction of all the observed features using parameters that were independent of the temperature of the curing reaction. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1771–1779, 2001     

Growth Temperature and Salinity Impact Fatty Acid Composition and Degree of Unsaturation in Peanut-Nodulating Rhizobia

Growth and survival of bacteria depend on homeostasis of membrane lipids, and the capacity to adjust lipid composition to adapt to various environmental stresses. Membrane fluidity is regulated in part by the ratio of unsaturated to saturated fatty acids present in membrane lipids. Here, we studied the effects of high growth temperature and salinity (NaCl) stress, separately or in combination, on fatty acids composition and de novo synthesis in two peanut-nodulating Bradyrhizobium strains (fast-growing TAL1000 and slow-growing SEMIA6144). Both strains contained the fatty acids palmitic, stearic, and cis-vaccenic + oleic. TAL1000 also contained eicosatrienoic acid and cyclopropane fatty acid. The most striking change, in both strains, was a decreased percentage of cis-vaccenic + oleic (≥80% for TAL1000), and an associated increase in saturated fatty acids, under high growth temperature or combined conditions. Cyclopropane fatty acid was significantly increased in TAL1000 under the above conditions. De novo synthesis of fatty acids was shifted to the synthesis of a higher proportion of saturated fatty acids under all tested conditions, but to a lesser degree for SEMIA6144 compared to TAL1000. The major adaptive response of these rhizobial strains to increased temperature and salinity was an altered degree of fatty acid unsaturation, to maintain the normal physical state of membrane lipids.     

Proteomic Analysis Identifies Molecular Players and Biological Processes Specific to SARS-CoV-2 Exposure in Endothelial Cells

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has been responsible for the severe pandemic of acute respiratory disease, coronavirus disease 2019 (COVID-19), experienced in the 21st century. The clinical manifestations range from mild symptoms to abnormal blood coagulation and severe respiratory failure. In severe cases, COVID-19 manifests as a thromboinflammatory disease. Damage to the vascular compartment caused by SARS-CoV-2 has been linked to thrombosis, triggered by an enhanced immune response. The molecular mechanisms underlying endothelial activation have not been fully elucidated. We aimed to identify the proteins correlated to the molecular response of human umbilical vein endothelial cells (HUVECs) after exposure to SARS-CoV-2, which might help to unravel the molecular mechanisms of endothelium activation in COVID-19. In this direction, we exposed HUVECs to SARS-CoV-2 and analyzed the expression of specific cellular receptors, and changes in the proteome of HUVECs at different time points. We identified that HUVECs exhibit non-productive infection without cytopathic effects, in addition to the lack of expression of specific cell receptors known to be essential for SARS-CoV-2 entry into cells. We highlighted the enrichment of the protein SUMOylation pathway and the increase in SUMO2, which was confirmed by orthogonal assays. In conclusion, proteomic analysis revealed that the exposure to SARS-CoV-2 induced oxidative stress and changes in protein abundance and pathways enrichment that resembled endothelial dysfunction.