Temperature Dependence of the Back-Stress in Shear for Glassy Polycarbonate

Summary: Back-stress is the equilibrium stress and represents conditions under which relaxation events in the material stop and the material can carry an applied load indefinitely without a change in strain. In most models for glassy polymers, back-stress plays a central role since relaxation in materials is closely related to the distance of the current conditions from equilibrium. A number of these models that are commonly used for modeling glassy polymers use a modeling structure similar to large deformation plasticity. The flow rule for the plastic strain in these models are directly connected to the “over-stress,” a properly invariant difference between the stress and the back-stress. The importance of correctly evaluating the back-stress to use in these models is clear. For this class of models, the authors have recently developed a method for directly calculating the back-stress under shear deformations. This method is based on evaluating the slope of the stress-strain response under conditions of similar elastic and plastic strain, but different strain rates. Since plastic flow goes to zero at equilibrium, the back-stress can be found by locating points of zero plastic strain rate. Using the proposed method, the back-stress in glassy polycarbonate has been evaluated under shear in isothermal tests going from room temperature to 120 °C, just below the glass transition temperature for polycarbonate. The proposed method provided a full map of the back-stress for polycarbonate over a large range of shear strain and temperature.     

The effect of process and structural parameters on the stability,thermo-mechanical and thermal degradation of polymers with hydrocarbon skeleton containing PE,PP, PS,PVC, NR,PBR and SBR

Journal of Thermal Analysis and Calorimetry - Many studies have been done on the stability, thermo-mechanical degradation and pyrolysis of polymers with hydrocarbon skeleton. According to the main...     

Theory of a Two-Dimensional Rotating Wigner Cluster

JETP Letters - A two-dimensional Wigner cluster (2DWC) with the number of electrons up to 200 in a parabolic potential well has been studied numerically and analytically. It has been shown that the...     

The use of cubic B‐spline scaling functions for solving the one‐dimensional hyperbolic equation with a nonlocal conservation condition

Problems for parabolic partial differential equations with nonlocal boundary conditions have been studied in many articles, but boundary value problems for hyperbolic partial differential equations have so far remained nearly uninvestigated. In this article a numerical technique is presented for the solution of a nonclassical problem for the one‐dimensional wave equation. This method uses the cubic B‐spline scaling functions. Some numerical results are reported to support our study. © 2007 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq, 2007     

Dissociation Behavior of <Emphasis Type="SmallCaps">l</Emphasis>(+)-Lactic Acid in Aqueous Solutions of (1-Alkyl-4-methylpyridinium bromide + Poly (ethyleneglycol)) at <Emphasis Type="Italic">T</Emphasis> = (288.15–318.15) K

The effect of 1-alkyl-4-methylpyridinium based ionic liquids on the conductivity behavior of l(+)-lactic acid (LaH) was studied in Poly(ethylene glycol) (PEG) aqueous solutions. The molar conductivities of LaH in the aqueous solutions of PEG, (PEG + 1-hexyl-4-methylpyridinium bromide) and (PEG + 1-octyl-4-methylpyridinium bromide) were measured over the temperature ranges of 288.15–318.15 K. The molar conductivity data were analyzed by applying the Quint–Viallard (QV) conductivity equation to determine the limiting molar conductivities (Λ ⁰) and dissociation constants (\( K_{\text{D}} \)). The results show that the values of limiting molar conductivity increased as the temperature increased, which indicates that the dissociation process is endothermic. The \( K_{\text{D}} \) values were also used to calculate the dissociation standard thermodynamic functions (\( \Delta G_{\text{D}}^{0} \), \( \Delta S_{\text{D}}^{0} \) and \( \Delta H_{\text{D}}^{0} \)). The results revealed that the dissociation process of LaH is controlled by entropy at all temperatures.     

Computational studies on the doped graphene quantum dots as potential carriers in drug delivery systems for isoniazid drug

In this study, the application of graphene quantum dots (GQDs) and doped GQDs as potential carriers for the delivery of isoniazid (Iso) drug has been investigated, using density functional theory (DFT) calculations. For this purpose, the hexa-peri-hexabenzocoronene (as a GQD model) and its BN-, BP-, AlN-, and AlP-doped (C₃₆X₃Y₃H₁₈ where X = B, Al and Y = N, P) forms were selected. Our results indicated that the adsorption energies of isoniazid on doped GQDs were more negative than that of pure GQD. Moreover, the calculations showed that adsorption of isoniazid on AlN- and AlP-doped GQDs was thermodynamically favorable. The dipole moments of BP-, AlN-, and AlP-doped GQDs were much greater (5.799, 1.860, and 3.312 D, respectively) than that of pristine GQD (0 D). The AlN-Iso and AlP-Iso complexes had small energy gaps, low chemical potentials, and low global hardnesses, which were appropriate for their attachments to the target site. The nature of interactions was analyzed by the quantum theory of atoms in molecules (QTAIM) and noncovalent interaction (NCI) analyses. Overall, the results confirmed that the AlN- and AlP-doped GQDs could be used as potential carriers for drug delivery application.     

Waste-grass-mediated green synthesis of silver nanoparticles and evaluation of their anticancer,antifungal and antibacterial activity

Grass waste was used for transform an inexpensive waste into health. Silver nanoparticles (AgNPs) have been synthesized using waste material (dried grass). The average size of silver nanoparticles observed in transmission electron images was estimated to be about 15?nm. The anticancer, antifungal and antibacterial effect of AgNPs were studied in vitro. The minimum inhibitory concentration of AgNPs against Pseudomonas aeruginosa and Acinetobacter baumannii was calculated about 3?µg/ml. The highest level of inhibitory effect of AgNPs against Fusarium solani was close to 90% at a concentration of 20?μg/ml of AgNPs. An inhibitory effect on the cancer cell growth is reach, by increasing the concentration of AgNPs to 5?µg/ml; the cancer cells’ survival decreases about 30%. Western results showed that the expression of Cyclin D1 protein of MCF-7 cell line decreased after treatment with the effective concentration of AgNPs.     

Magnetic molecularly imprinted polymer nanoparticles for dispersive micro solid-phase extraction and determination of buprenorphine in human urine samples by HPLC-FL

A simple, sensitive and accessible while reliable method was developed and validated for quantitation of buprenorphine (Bup) as a highly lipophilic drug in human urine samples. The proposed method is based on a rapid and easy dispersive micro solid-phase extraction procedure using magnetic molecularly imprinted polymer nanoparticles (MMIPNPs), magnetite (Fe₃O₄) cores surrounded by polyamidoamine and Bup as template, followed by high-performance liquid chromatographic analysis with fluorescence detector (HPLC-FL). The prepared MMIPNP adsorbent was characterized by different techniques. Transmission electron microscopy images show that the Fe₃O₄ core nanoparticles are well enwrapped by MIP layers. In determination process, Bup could be quantitatively extracted using MMIPNPs and then can be easily desorbed by mixture of sodium hydroxide (0.1 mol/L) and methanol (1:9, v/v) solution before injection to HPLC. The relative recoveries of Bup were found to be 97.4–100.3%, and the linear dynamic range was within the ranges of 1–1000 ng/mL with R² of 0.9998. Remarkably high quality of 0.21 and 0.71 ng/mL was obtained as the limit of detection and limit of quantification, respectively. The developed method was successfully applied for determination of Bup in infected human urine samples.     

Synthesis and Characterization of New Polymer Systems Containing 4‐Silylmethylstyrene Units

A series of terpolymers containing silyl pendant groups were prepared by free radical cross‐linking copolymerization. Et₃Si and HMe₂Si were covalently linked with 4‐vinylbenzyl and abbreviated as TESiMSt and DMSiMSt, respectively. Et₃Si was covalently linked with 2‐hydroxyethyl methacrylate (HEMA). The silyl‐linked HEMA are abbreviated as TESiEMA. Free radical terpolymerization of the methacrylic acid (MAA) with different molar ratios of organosilyl monomers was carried out at 60–70 °C. The compositions of the polymers were determined by FT‐IR spectroscopy and ¹H‐NMR. The glass transition temperature (Tg) of the polymers was determined calorimetrically. The study of DSC curves showed that incorporation of monomers with cyclic units in polymer chains increases the rigidity of terpolymers and the Tg value is subsequently increased.     

Quantitative assay of plasma homocysteine thiolactone by gas chromatography/mass spectrometry

Enzymatic cyclization of homocysteine forms a reactive thiolactone that may play an important role in its cardiovascular toxicity, but reliable quantitation of the free thiolactone metabolite in physiological fluids has not been reported. We have therefore used a highly selective gas chromatography/mass spectrometry (GC/MS) technique combined with the sensitivity of negative chemical ionization (NCI) to develop a quantitative method for the detection of homocysteine thiolactone (HcyTL) in plasma. To improve accuracy the deuterated isomer d(4)-HcyTL was synthesized and added to plasma as internal standard. The plasma was then treated with silica solid-phase extraction and derivatized with heptafluorobutyric anhydride. The derivative was analyzed by GC/MS in NCI mode with methane as the reagent gas and quantified by analyzing for the HcyTL ion [M(-)[bond]HF] and its d(4)-HcyTL counterpart in single-ion monitoring mode. The calibration curve showed a dynamic linear range up to 40 nmol/L. Within-day precision (n = 20, nominal concentration 5.2 nmol/L) was 0.96% and between-day precision was 3.9%, with a detection limit of 1.7 nmol/L and quantification limit of 5.2 nmol/L. Two human plasma samples had HcyTL concentrations of 18 and 25 nmol/L. This facile method for quantitation of homocysteine thiolactone opens the way for more detailed clinical studies of its potential role in homocysteine-induced arteriosclerosis and vaso-occlusive disease.