Automatic Computerized Analysis of Multi-Exponential Decay Curves

Abstract

A prototype method is described which automatically analyzes exponential decay curves by using nonlinear regression analysis and deviation pattern recognition. Lifetimes and preexponential factors are computed for the proper model. The program successfully classified simulated data consisting of one, two, or three exponentials when noise followed a Poisson distribution. However, for real luminescence decay data, systematic instrumental noise of higher frequency tended to obscure low frequency deviations from incorrect models in smoothed deviation plots. This factor complicated classification of multi-exponential data, although the program easily distinguished between single and multiple exponential behavior.     

Strain stiffening and negative normal stress in alginate hydrogels

Alginate hydrogels are polysaccharide biopolymer networks widely useful in biomedical and food applications. Here, we report nonlinear mechanical responses of ionically crosslinked alginate hydrogels captured using large amplitude oscillatory shear experiments. Gelation was performed in situ in a rheometer and the rheological investigations on these samples captured the strain‐stiffening behavior for these gels as a function of oscillatory strain. In addition, negative normal stress was observed, which has not been reported earlier for any polysaccharide networks. The magnitude of negative normal stress increases with the applied strain amplitude and can exceed that of the shear stress at large‐strain. Fitting a constitutive relationship to the stress‐strain curves reveals that the mode of deformation involves stretching of the alginate chains and bending of both the chains and the junction zones. The contribution of bending increases near saturation of G blocks as Ca²⁺ concentration was increased. The results presented here provide an improved understanding of the deformation behavior of alginate hydrogels and such understanding can be extended to other crosslinked polysaccharide networks. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1767–1775     

Mechanical behavior of a highly oriented polyethylene rod

Dynamic mechanical and single step stress relaxation data are presented for a highly oriented linear polyethylene rod as produced by a high pressure extrusion technique. The results indicate that both major relaxation processes normally observed in unoriented polyethylene are virtually absent in torsion, whereas they are present in flexure. Stress relaxation in torsion data show a very highly nonlinear behavior when compared to similar data for an unoriented polyethylene rod.     

Role of Alkylthio Substituents on Tetrathiafulvalene and 1,3-Dithiole Rings: A Theoretical Study

Abstract

The effect of alkylthio substituents on the redox and nonlinear optical behavior of tetrathiafulvalenes has been studied by Density Functional Theory Calculations.     

Nonlinear actuation of non-prismatic dielectric elastomeric membrane

ABSTRACT

Nowadays, the soft material, dielectric elastomers (DEs) are being used for many engineering applications due to their exceptional properties like high strain rate sensitivity, low power consumption, capable of static force measurement and capability of changing dimensions when subjected to voltage. They have the ability of converting mechanical energy to electrical energy and vice versa. In this article, nonlinear dynamic behavior of viscoelastic tapered DE under mechanical and electromechanical loading is studied. A dynamic model based on standard linear solid model is incorporated for viscoelasticity. Strain energy density of the system is derived from Gent model of hyperelasticity. The tapered DE is analyzed with different values of damping coefficients, ratio of shear moduli of springs, width and height tapers. The effects of tapers greatly put an impact on the dynamic responses of the system. The tapered DEs exhibits damped vibration and weak nonlinearity with an increase in damping force. The dynamic stability of tapered DEs is also studied using phase diagrams, and the results indicated that with an increase in damping force, the dynamic stability changes from a state of aperiodic vibration to quasi-periodic vibration. It is also found that the resonant frequency and peak amplitude reduce with an increase in the damping force.     

Comparison of Linear and Nonlinear Forms of Isotherm Models for Strontium Sorption on a Sodium Bentonite

Sorption on bentonite will play an important role in retarding the migration of radionuclides from a waste repository. Bentonite is characterized by low permeability, water swelling capability and excellent sorption potential for cationic radionuclides. To correctly assess the sorption potential of radionuclides on bentonite is essential for the development of predictive migration models. The sorption isotherm model is usually used to describe the sorption behavior and assess the sorption potential of radionuclides on bentonite. However, there are few studies to investigate the feasibility of isotherm models for the sorption of radionuclides on bentonite. Thus, in this study, we compared the goodness-of-fit of linear and nonlinear forms of two common isotherm models, Langmuir and Freundlich equations. The experimental sorption isotherms of strontium (Sr) on Wyoming bentonite, MX-80, were used for illustration. The results showed that the nonlinear forms of Langmuir and Freundlich isotherm models are more suitable for fitting the experimental sorption isotherms of Sr on MX-80 than are the linear forms. Thus, the nonlinear forms of isotherm models should be primarily adopted to fit experimental isotherms. On the other hand, we also found that the goodness-of-fit of Langmuir model is better than that of Freundlich model. Moreover, based on the theoretical assumptions of Langmuir isotherm model, the parameters M and K _L represent the sorption capacity and affinity, respectively. One can use the values of M and K _L , obtained from fitting the experimental isotherms, to assess the sorption potential of radionuclides in bentonite. Thus, we suggested that the Langmuir isotherm model is more useful for investigating the sorption behavior of radionuclides on bentonite.     

Study of the influence of nanoparticles on the behavior of composite materials

The objective of this work is to study the influence of additives of multi-walled carbon nanotubes (MWCNT) and silica nano-powder (SiO₂) on the mechanical behavior of epoxy resin. Different volume fractions of MWCNT and SiO₂were added. Mechanical characterization by tensile, three-point bending and Charpy tests were carried out. The experimental results show an increase in the mechanical performance of the mixtures (MWCNT ​+ ​epoxy), (SiO₂ ​+ ​epoxy) up to 2% of additive. Beyond this value, a degradation of performance was observed. The addition of MWCNT gives better results when compared to the addition of SiO₂. K_IC-K_CV correlations were made using empirical formulas to estimate the critical stress intensity factor K_IC from the impact energy of the Charpy test.Unfortunately, this estimation does not provided a promising results, but other optimization methods were used to fit these empirical models to the behavior of our nanocomposites for which a good estimate was obtained after fitting these empirical models for SiO₂.     

Rheo-optical properties of polyethylene films in the nonlinear viscoelastic region

A new apparatus was designed to investigate the dynamic viscoelastic properties of solid polymer materials in the nonlinear viscoelastic region. The apparatus was combined with a birefringence apparatus in such a way that birefringence could be measured simultaneously with stress under oscillatory deformation. The nonlinear viscoelastic behavior of bulk-crystallized high-density polyethylene films was examined. Nonlinearity of mechanical properties became evident around 30°C, while optical properties became markedly nonlinear around 50°C. The nonlinearity of viscoelastic properties changes very little when the films are swollen with tetrachloroethane. It is proposed that disruption of lamellae to crystallites in the drawing process is one of the most important causes of the nonlinear behavior of high-density polyethylene films.     

Synthesis and Characterization of Novel Poly(Arylene Ether)s from 1,3‐bis(4‐Hydroxyphenyl) Benzene

Abstract

Several poly(aryl ether)s have been prepared by the condensation of 1,3‐bis(4‐hydroxy phenyl) benzene with different trifluoromethyl activated bis‐fluoro compounds. IR, ¹H and ¹³C NMR, and elemental analyses have established the resulting polymer structures. The properties of the polymers have been evaluated by DSC, TGA, dynamic mechanical analysis (DMA) and stress–strain analysis. The polymers 1a and 1c showed semi‐crystalline behavior as evident by sharp crystalline melting peaks at 299°C and 330°C along with glass transitions at 202°C and 216°C, respectively. The polymers showed very good thermal stability in air, high modulus, and high tensile strength with low elongation at break.     

Development and characterization of biodegradables packaging obtained from biopolymers mixture

The aim of this work is the development and the characterization of biodegradables films obtained from a mixture of soluble starch and sodium alginate. The influence of sodium alginate ratio on the behavior of these films is also studied in order to use them as new formulations to produce food packaging. The obtained films are generally homogeneous, thin, smooth and having a good coherence with no visual defects. The results of the FITR, SEM and XRD tests have shown the existence of a good compatibility, between alginate and starch due to both kind of strong interactions like hydrogen bonds and ionic interactions. Water sorption isotherm, water vapor permeability, and mechanical tests were strongly influenced by the addition of sodium alginate in the different formulations. This influence is caused by the plasticization effect of sodium alginate.     

Tensile yield-stress behavior of poly(vinyl chloride) and polycarbonate in the glass transition region

The yield-stress behavior of two glassy polymers is studied through the glass transition region over a wide range of strain rates. For temperatures below the glass transition temperature, the yield stress behavior could be described as a non-Newtonian flow in agreement with Eyring's theory, if one excepts a narrow range relating to the slowest strain rates. For temperatures above T_g, the yield-stress behavior is still nonlinear but fits the relations based on the concept of free volume.     

Comparison of optical and mechanical limits of linear relaxation behavior in glassy polycarbonate

The decay in birefringence of glassy polycarbonate held at constant extension has been studied at 23°C, in the time-scale range 10–10³ sec, up to about 6% strain. The results show that, under these conditions, the birefringence can be validly expressed as a linear hereditary integral of the strain history up to a relatively high strain level which is about 3.4% for an experimental time-scale of 100 sec. Comparison with previously obtained data on the stress relaxation behavior of the same polymer shows that, other factors remaining constant, mechanical relaxation is linear only up to about 1.1% strain. The earlier onset of mechanical nonlinearity is discussed and it is suggested that the mechanical relaxation spectrum is richer than the optical spectrum in relatively long relaxation times, corresponding to relatively slow molecular motions. It is further suggested that these slow molecular motions are accelerated first as the polymer is extended beyond the limit of linear viscoelastic behavior. The observed nonidentity between strain limits for linear mechanical and linear optical behavior is discussed in the light of current practices in photomechanical stress analysis.     

Investigating the Time Dependent Behavior of Thermoplastic Polymers under Tensile Load

The relationship between the results of the tensile and the stress relaxation tests of polypropylene specimens were analyzed and an attempt was made to find a way to estimate the former from the latter based on the measurements and the theory of linear viscoelasticity. The mechanical response of real polymers are basically of nonlinear character, therefore their behavior patterns do not meet the idealized (linear) ones. Experiments were performed on poly(propylene) (PP) as a test material and the stress relaxation behavior, as well as the linear elastic and linear viscoelastic approximation of the tensile load-time curve were analyzed. To demonstrate the applicability of our idea and to perform the numerical calculations we have chosen a flexible function with three parameters to realize the nonlinear behavior.     

NEW DEVELOPMENTS IN THE DEGRADATION,STABILIZATION, AND SENSITIZATION OF POLY(METHYL METHACRYLATE)

Abstract

Physical and chemical processes, combined under the general term aging, begin immediately after formation of a polymer. Oxygen and other components of the environment -ozone, water, etc.-are involved. Aging is accelerated by a rise in temperature, light, strong ionizing radiations, and mechanical stresses, which are especially rapid under processing conditions for thermoplastic polymers. Poly(methy1 methacrylate) (PMMA) is of particular interest, since at least at temperatures where thermal degradation is important, photolysis occurs by end-group initiation and is accompanied by extensive depolymerization. This polymer has also been photodegraded in air at room temperature by radiation from a low-pressure mercury lamp, and a quantum yield for random chain scission has been determined. Owing to the importance of these points, it is believed that a review of studies of degradation, stabilization, and sensitization of PMMA up to the present date will supply information on the most general problems of behavior of polymeric items during their service lives.     

REVIEW: SPECTROSCOPIC STUDIES OF OXOVANADIUM COORDINATION COMPOUNDS

Abstract

In this review we present selected examples of our studies of oxovanadium(IV) and oxovanadium(V) complexes relevant for the bioinorganic chemistry of vanadium. Some of the investigated complexes are good models for different steps of vanadium metabolism or for a better understanding of the structural and electronic peculiarities of the coordination spheres of these oxocations in biomolecules. The investigated systems include ligands such as nucleotides, carbohydrates, phosphates, amino acids, oxine derivatives, porphine-like cores and other simple organic and inorganic ligands. All these complexes have been systematically investigated by means of vibrational (infrared and Raman) and electronic spectroscopy and, in some cases, also by thermal and electrochemical behavior. The potentialities and possibilities of the spectroscopic methodologies are illustrated and discussed and some general trends, useful for the structural characterization of these and similar systems, are emphasized.     

Formation of Co-continuous Structures in Melt-Mixed Immiscible Polymer Blends

Abstract

Co-continuous structures can be regarded as the coexistence of at least two continuous structures within the same volume. Blends with co-continuous structures may combine the properties of both components in a favorable way, for example, mechanical moduli. This review article deals with the identification, characterization, and properties of co-continuous structures as well as with the development of co-continuous structures during the melt blending process. Co-continuous structures usually can be formed within a composition region about the phase inversion composition, which mainly depends on the viscosity ratio. On the other hand, co-continuous structures can be found independent of composition as intermediate stages during the initial state of morphology development and during phase inversion process in blends in which the component finally forming the dispersed phase forms the matrix in early mixing states. In addition, even at low volume fractions of one component, stable co-continuous morphologies can be created using suitable processing conditions, forming long elongated interconnected structures that do not break up because of the flow. The interfacial tension plays an important role for the stability; a lower interfacial tension leads to broader composition ranges of co-continuous structures. Another factor enhancing the formation and stability of co-continuous structure is melt yield stress of one or both components of blends. In addition, this article reviews the stability of co-continuous structures during further processing and the influence of compatibilization on the structure formation and stability. Subsequently, two models describing the co-continuous composition range are discussed.     

Experimental investigation and modeling of the mechanical behavior of PC/ABS during monotonic and cyclic loading

The purpose of this work is to characterize the mechanical behavior of blends of polycarbonate (PC) and acrylonitrile-butadiene-styrene (ABS) during monotonic and cyclic loading. Compression experiments were performed using a SHIMADZU universal testing machine (10⁻⁴ to 10⁻² s⁻¹) and a split Hopkinson pressure bar (1600–5000 s⁻¹), with, the test temperatures ranging from 293 to 353 K. The influence of the rate and temperature on the deformation of PC/ABS is discussed in detail. Based on the investigation of numerous constitutive models, a phenomenological model called DSGZ was chosen to describe the compression behavior of PC/ABS. This model could not accurately reproduce the deformation of polymers at high strain rates when utilizing the same material coefficients for the low and high strain–rate deformations. In addition, this model was unable to capture the deformation features during unloading and subsequent reloading when adopting the original stress–strain updating algorithm. Hence, some improvements to the model have been implemented to better predict the deformation. Finally, the model predictions are shown to be consistent with the experimental results.     

Solvatochromic Determination of Second-order Polarizabilities of Chromophore-functionalized Dextran and Amylose Derivatives

Linear dextran and helix amylose were covalently bonded with nonlinear optical (NLO) chromophores. The influence of the conformation of the polymer matrix on the NLO behavior of the supramolecular structure has been studied. The second order hyperpolarizability depends not only on the secondary structure of the biopolymer, but also on the position of the chromophore towards the polymeric backbone. Functionalization of NLO-phores with biopolymers led to increased thermo- and photostability.