Uniaxial and biaxial stretching of silicone putty

Two novel experimental methods are used. Vertical uniaxial stretching is obtained by attaching a perspex rod to the lower end of a silicone putty cylinder; the rod then descends into water of constant depth. The stress and rate of extension change little during each test, but the rate of extension may be varied from 0.005 to 0.10 s⁻¹ by modifying the experimental conditions. Biaxial stretching is acchieved by placing a disc of silicone putty across the top of an open glass cylinder which is lightly pressurized. The sample expands as a spherical cap, the height of the centre above the cylinder being timed. The stress in the cap passes through a shallow minimum as it expands (at constant pressure) and the slowly varying rate of biaxial extension may be readily determined. This lies in the range 0.003–0.06 s⁻¹. For low rates of uniaxial or biaxial extension, it is possible to plot the extension against time and to show how the extensional viscosity varies with the strain rate (or principal extension ratio). For high rates of extension, a ‘single point’ determination of the extensional viscosity may be made, with the stress and strain rate averaged at the mid-point of the sample's extension. The temperature is 26.5 ± 1.5 °C. The following is shown under the experimental conditions:(a) the extensional viscosity (uniaxial or biaxial) is in the range 1.0 × 105 to 3.0 × 10⁵ Pa s;(b) for extensional strain rates between 0.01 and 0.04 s⁻¹, the uniaxial and biaxial extensional viscosities are of comparable value;(c) both forms of the extensional viscosity tend to decrease with increased extensional strain rate, the biaxial extensional viscosity falling more rapidly and being higher than the uniaxial viscosity at low strain rates and lower at high strain rates;(d) there are no signs of rupture in uniaxial extension (principal extension ratios up to 1.8 and extensional strain rate up to 0.1 s⁻¹);(e) in biaxial extension, the sample tends to rupture more easily as the strain rate is increased. (The sample fails at the principal extension ratio of 2.0 at an extensional strain rate of 0.02 s⁻¹ and fails at a principal extension ratio of 1.3 at an extensional strain rate of 0.07 s⁻¹.)     

Dependence on strain rate of the nucleation of cracks in polystyrene at 293°K

The processes associated with the deformation and fracture of polystyrene tested in uniaxial tension have been studied over a range of strain rates from 1.4 × 10^?2 to 4.3 × 10^?7 sec^?1 and at constant stresses between 4.1 and 2.9 kg/mm². The effect of strain rate on the surface craze distribution prior to fracture, the fracture stress, the mechanism of nucleation of cracks, and the nature of fracture surfaces associated with slow and fast crack propagation have been determined. The changes in fracture surface appearance have been studied using optical and stereoscan microscopy. The observations are consistent with the model presented in a previous paper. Fracture is preceded by craze formation, cavitation in the craze, coalescence of cavities to form large planar cavities which propagate slowly until a critical stage is reached at which fast crack propagation occurs. The effect of changes of strain rate and material variables on these processes is discussed.     

Compression stress relaxation in carbon black reinforced HNBR at low temperatures

Findings of a study of stress relaxation behaviour of hydrogenated nitrile butadiene rubber (HNBR) at nominal compressive strains up to 0.4 and temperatures above and below the glass transition temperature T_g are reported. Two formulations of a model HNBR with 36% acrylonitrile content and carbon black (CB) loading of 0 and 50 phr were investigated. The relaxation function of HNBR is found to be independent of strain at temperatures right above the T_g or at times longer than 10⁻³ s for the deformations employed. CB imparts higher long-term stiffness and also larger relaxation strength at times longer than 10⁻⁴ s to the HNBR, but it does not affect the relaxation behaviour of the rubber in the time span from 10⁻³ – 10⁴ s. In addition, the relationship between the strain energy function of HNBR and temperature is demonstrated to have a complex concave-downward shape which is affected by two competing contributions of entropy elasticity and the stress relaxation.     

Comparison of the transient stress–strain response of rubber to its linear dynamic behavior

Master curves of the small strain and dynamic shear modulus are compared with the transient mechanical response of rubbers stretched at ambient temperature over a seven‐decade range of strain rates (10^?4 to 10³ s^?1). The experiments were carried out on 1,4‐ and 1,2‐polybutadienes and a styrene–butadiene copolymer. These rubbers have respective glass transition temperatures, T_g, equal to ?93.0, 0.5, and 4.1 °C, so that the room temperature measurements probed the rubbery plateau, the glass transition zone, and the onset of the glassy state. For the 1,4‐polybutadiene, in accord with previous results, strain and strain rate effects were decoupled (additive). For the other two materials, encroachment of the segmental dynamics precluded separation of the effects of strain and rate. These results show that for rubbery polymers near T_g the use of linear dynamic data to predict stresses, strain energies, and other mechanical properties at higher strain rates entails large error. For example, the strain rate associated with an upturn in the modulus due to onset of the glass transition was three orders of magnitude higher for large tensile strains than for linear oscillatory shear strains. © 2011 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys, 2011     

Influence of cetyltrimethylammonium bromide aqueous solutions on the surface strength of quartz glass under conditions of pH variation

A mechanism of the combined influence of solutions of electrolytes with various pHs (in 10^?3 N KCl solution) and cetyltrimethylammonium bromide (CTAB) cationic surfactant on the surface strength of quartz glass in a mechanical contact is studied. A similar cation-active medium is shown to embrittle quartz glass in neutral and alkaline pH ranges. The brittle strength of a near-surface layer of a material decreases owing to adsorption and electrocapillary reduction of its surface energy. The surface strength of the glass decreases to the largest extent at the CTAB critical micellization concentration. At low concentrations (10^?6M), a protective effect of forming chemisorbed CTAB layers prevails that is manifested in the hydrophobization of the surface of quartz glass by CTAB molecules and its screening from direct contact with an electrolyte solution. At the CTAB concentrations that are higher than the CMC, a formed micellar structure of CTAB has a lubricating effect, leading to softer boundary conditions of a frictional contact and a decreased probability of the damage of the near-surface layer of quartz glass. In acidic electrolyte solutions, CTAB cations or molecules did not affect the surface strength of quartz glass.     

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.     

Mechanochemical effect of strain sign at wetting of glass

Contact angles formed by water with the surface of the K-8 optical glass and the cover glass for microscopy under the condition of stress were studied. At bending glass plates, the contact angle is always larger on the convex (stretched) side than on the concave (compressed) side. Thus, the mechanochemical effect of the strain sign at wetting of glass is observed.     

Elastic properties of the (001) face of xenon crystals

In this work we investigated the elastic properties of the (001) face of xenon crystal. The slabs (twodimensional crystals) defined by (001) planes are generated, their structures are optimized and the slabs thermodynamic functions in excess to the crystal bulk calculated. The calculations are based on the Lennard-Jones 6?12 force field, classical elasticity theory and surface thermodynamics. In this work, the number of planes undergoing relaxation is not a priori constrained but it follows from the minimization of the free energy of the slabs and of the bulk, in respect to atomic positions. The value of the surface free energy is calculated as a function of the homogeneous strain of the 2D (001) cell measured relatively to the cell of the stable 3D crystal. At 0 K, when strain is not applied, the specific surface free energy is about 0.064 Jm^-2 and decreases by about 6% at 50 K. The surface stress is positive amounting to 0.010 Jm^-2 at 0 K, and it decreases by about 50% at 50 K. We find that the surface stress can be released by a reorganization of the interatomic distances at the crystal surfaces. The surface excess mean value of the slab elastic constants at 0 K is small (0.012 GPa) and it decreases by about 35% at 50 K. The method proposed can be alternative to molecular dynamics simulations in order to assess the excess surface properties of materials having a complex structure.     

Visualization of deformation-induced structural rearrangements in amorphous polymers

A technique is proposed for decorating amorphous polymers: Before the deformation (shrinkage) of an amorphous polymer, its surface is decorated with a thin metal coating. The subsequent deformation is accompanied by surface structure formation, which makes the processes that occur in the polymer visible. The proposed technique makes it possible to visualize and describe the mechanism of transfer of the polymer from the surface into the bulk and vice versa and to obtain direct information about the direction of the actual local stress. The technique makes it possible to obtain information about the topological heterogeneity of rubber networks, to reveal the features of structural rearrangements that occur during the cold rolling of amorphous polymers, and to describe the phenomenon of self-elongation during annealing of the oriented PET. These microscopic data explain the following features of the structural and mechanical behavior of glassy polymers from a unified viewpoint: stress relaxation in a polymer in the elastic (Hookean) region of the stress-strain curve, an increase in stress in a deformed glassy polymer during its isometric annealing below T _g, the low-temperature shrinkage of a deformed polymer glass in the strain range below its yield point, the storage of internal energy in a deformed glassy polymer in the strain range below the yield point, some anomalies of thermophysical properties, and some other features.     

Thermal decomposition of cycloadducts of vinylnaphthalenes with electron-deficient dienophiles

Light transmittance was measured during heating of thin NaPO₃ glass plates at different heating rates. According to the results, the crystallization of orthorhombic textured NaPO₃ glass proceeds from sample surfaces into their interior due to the foregoing surface nucleation. The glass surface crystallization process resulted in the sigmoidal decrease of the optical transmittance. Elaborated data lead to the activation energy of glass surface crystallization of the value of 182.8 kJ mol ^-1.     

Potential of thermo-optical methods for the study of molecular layers bonded to a flat glass surface

Thermal-lens spectrometry and photothermal deflection spectroscopy (mirage-effect spectroscopy) are used to study active bright red 5SKh bonded to a glass surface. The theoretically calculated dependence of the thermal-lens signal on the absorbance of the surface-absorbing layers on a nonabsorbing substrate is used to evaluate the concentration of the reagent in the bonded layer. The concentration of the reagent varies in the range of (2–8) × 10^?11 mol/cm², depending on the method of surface modification. Using photothermal deflection spectroscopy, it is shown that the thickness of the layer on the glass surface does not exceed 700 nm. Data obtained by the thermo-optical methods are supplemented with the results of the analysis of the surface relief obtained using electron microscopy in the secondary electron emission mode, and the uniformity of the modification of the sample surface is characterized. Based on the obtained data, a comparative analysis is performed for the potential of the considered methods for the investigation of the molecular layers bonded to the flat glass surface.     

Surface characterization of polyimide films

 Various kinds of poly(amide acid)s were prepared by the reaction of four kinds of acid dianhydride and three kinds of diamine in N,N-dimethylacetamide (DMAc). Polyimide films were prepared by casting the poly(amide acid) solution on soda glass substrates, followed by thermal imidization at various temperatures. Contact angles of polyimide films for the sides in contact both with air and with glass substrate (air side and glass side, respectively) were measured to evaluate the dispersive component (γ^D _S) and the nondispersive component (γ^P _S) of surface free energy (γ_S) of polyimide films. It was shown that, for the air side, γ^P _S value decreased greatly and γ^D _S values increased slightly with the development of imidization. Values of γ^P _S for the glass side were much higher than those for the air side. Poly(amide acid) solution was also cast on quartz glass and silicone rubber, and was thermally imidized. The γ^P _S for the quartz glass side was almost the same as that for the soda glass side. But the γ^P _S for the silicone rubber side was as low as the γ^P _S for the air side. Attenuated total reflection infrared spectroscopy of polyimide films showed that the degree of imidization for the glass side surface was not as high as that for the air side surface, and that the amount of polar groups for the glass side surface was higher than that for the air side surface. Among the various kinds of polyimides, there is a slight but clear difference in the values of γ_S and its components, which can be rationalized from the difference in the chemical structure. It was also found that thermal degradation and oxidation can be easily detected from the change of surface free energy and its components for the polyimide films after being treated at high temperatures in both air and vacuo. Received: 22 January 1998 Accepted: 10 March 1998     

Comparison of mechanical and molecular measures of mobility during constant strain rate deformation of a PMMA glass

We performed constant strain rate deformation and stress relaxation on a poly(methyl methacrylate) glass at T_g – 19 K, utilizing three strain rates and initiating the stress relaxation over a large range of strain values. Following previous workers, we interpret the initial rate of decay of the stress during the relaxation experiment as a purely mechanical measure of mobility for the system. In our experiments, the mechanical mobility obtained in this manner changes by less than a factor of 3 prior to yield. During these mechanical experiments, we also performed an optical measurement of segmental mobility based on the reorientation of a molecular probe; we observe that the probe mobility increases up to a factor of 100 prior to yield. In the post‐yield regime, in contrast, the mobilities determined mechanically and by probe reorientation are quite similar and show a similar dependence on the strain rate. Dynamic heterogeneity is found to initially decrease during constant strain rate deformation and then remain constant in the post‐yield regime. These combined observations of mechanical mobility, probe mobility, and dynamic heterogeneity present a challenge for theoretical modeling of polymer glass deformation. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1957–1967     

Strong effect of a solid surface on a liquid-phase radical chain chemiluminescence reaction. Oxidation of UIV by dioxygen

The reaction of uranium(IV) with O₂ in aqueous solutions of HClO₄ is accompanied by chemiluminescence (CL). The CL intensity and the reaction kinetics are appreciably affected by introduction of the glass beads with a radius of 1 mm. In the presence of 1600 beads (the surface area of the glass is 200 cm²), the yield of CL dereases 32-fold. The effect of the solid surface on the kinetic and chemiluminescence parameters of the liquid-phase radical chain reaction is due to the increased role of chain termination upon heterogeneous decay of the HO ₂ ^. and^.OH radicals.     

Viscoelastic properties of Nafion at elevated temperature and humidity

Tensile stress–strain and stress relaxation properties of 1100 equivalent weight Nafion have been measured from 23 to 120 °C at 0–100% relative humidity. At room temperature, the elastic modulus of Nafion decreases with water activity. At 90 °C, the elastic modulus goes through a maximum at a water activity of ～ 0.3. At temperatures ≥90 °C, hydrated membranes are stiffer than dry membranes. Stress‐relaxation was found to have two very different rates depending on strain, temperature, and water content. At high temperature, low water activity, and small strain, the stress relaxation displays a maximum relaxation time with stress approaching zero after 10³–10⁴ s. Water absorption slows down stress‐relaxation rates. At high water activity, the maximum stress relaxation time was >10⁵ s at all temperatures. No maximum relaxation time was seen at T ≤ 50 °C. Increasing the applied strain also resulted in no observed upper limit to the stress relaxation time. The results suggest that temperature, absorbed water, and imposed strain alter the microstructure of Nafion inducing ordering transitions; ordered microstructure increases the elastic modulus and results in a stress relaxation time of >10⁵ s. Loss of microphase order reduces the elastic modulus and results in a maximum stress relaxation time of 10³–10⁴ s. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 11–24, 2009.     

A novel method of preparing of glass capillary columns: Low-temperature plasma etching and polymerization

A novel method is described for the preparation of stable glass capillary columns (glass open tubular columns), including the etching and formation of a polymer film on the inner glass capillary surfaces. The approach used here is based on low-temperature plasma etching and polymerization. Under the influence of a field of radio frequency discharge, low pressure gases of fluoric compounds, introduced into the glass capillary tube, generate excited fluorine radicals which etch the inner surface. The plasma of organosilicone monomer in the glass capillary yields a uniform polymerized film on the inner surface. The resultant material functions as a good stationary phase for glass capillary gas chromatography (GC²). The inner surfaces treated with such a plasma, can be studied by means of a scanning electron microscope (SEM). The flexibility of this method permits the use of various stationary phases and surface modification.     

The effect of strain rate on craze yielding,shear yielding,and brittle fracture of polymers at 77°K

The tensile strength of poly(methyl methacrylate) (PMMA), polycarbonate (PC), polychlorotrifluoroethylene, and polysulfone was measured in liquid nitrogen over the strain rate range of 2 × 10^?4 to 660 min^?1. These polymers deformed by crazing which was induced by the liquid nitrogen. The stress versus log strain rate curve was sigmoidal in that its slope increased and then decreased with strain rate. Above a critical strain rate of about 200 min^?1, which varied somewhat with the polymer, crazing was not observed with the optical microscope; the behavior became brittle, and the tensile strength became constant. The nonlinear behavior of stress versus log strain rate at low strain rates was associated with a decrease in activation volume with increasing strain rate whereas the nonlinear behavior at high strain rates was associated with an increase in density and decrease in length of the crazes with strain rate. The strain rate effect was the basis for calculating the diffusion coefficient of nitrogen into the polymers at 77°K. The shear deformation mode of PC was measured under compression and under tension. The compressive strength versus log strain rate was linear throughout the entire range giving a compression shear activation volume of 360 ų. The shear tensile strength of PC varied only slightly with strain rate when compared to the compressive strength. The brittle fracture stress of PMMA, in the absence of crazing, in compression and in tension, did not vary with strain rate.     

Spectrophotometric study of coverage and acid-base equilibrium of a chemically bonded base

An aniline azo dye is immobilized on glass slides by reacting an organosilane reagent with the glass surface. The immobilized dye absorption spectral characteristics closely resemble those of the analogous solution form of the dye. The coverage of the immobilized dye, 1.1·10^?10 mol cm^?2, is consistent with simple monolayer coverage. The immobilized dye is a weaker base than the soluble form by an equilibrium constant factor of about 10, consistent with an electrostatic effect.     

Effect of Glass Substrates on the Formation of Gold-Silver Colloids in Nanocomposite Thin Films

A sol-gel route to synthesize nanocomposite thin films containing phase separated metal colloids of gold (Au) and silver (Ag) was developed. Ag—Au colloids were prepared in silica films using dip coating technique. The annealing of the samples in air results in the formation of phase separated Ag and Au colloids in SiO₂ thin films, showing the surface plasmon peaks at 410 nm and 528 nm. For the synthesis of phase separated Ag and Au colloids on float glass substrates, formation of the silver colloids was found strongly dependent on the surface of the float glass. On the tin rich surface formation of both gold and silver colloids took place, whereas, on the tin poor surface the formation of only gold colloids was observed. The surface dependence of the formation of silver colloids was attributed to the presence of tin as Sn²⁺ state on the glass surface, which oxidizes into Sn⁴⁺ during heat treatment, reducing Ag⁺ into silver colloids.