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Zollinger-Ellison syndrome is a very rare disease characterized by increased production of gastric acid that leads to peptic ulcer disease and diarrhea. It is caused by disproportionate ectopic secretion of gastrin from the cells of the endocrine tumor of digestive system – gastrinoma. It occurs sporadically or as a part of multiple endocrine neoplasia type 1. The tumor is localized mostly in duodenum and pancreas. The treatment combines control of gastric acid secretion, nowadays mainly by proton pump inhibitors and the treatment of gastrinoma itself – surgical or oncological (chemotherapy, biological treatment). In our paper, we present a case of a middle-aged woman examined for abdominal pain and diarrhea. Her initial gastroscopy showed erosive duodenitis; we excluded the infectious etiology of the diarrhea. During the examination process, there were severe complications of peptic ulcer disease for which several surgeries were done. From all the laboratory results and imaging methods, the final diagnosis was made – Zollinger-Ellison syndrome due to gastrinoma of the tail of pancreas and the patient underwent surgery. Unfortunately, there was a progression of the disease in a short time that requested another oncological and surgical treatment.

In this paper we model the volumetric and enthalpic response of a-PMMA during heating at constant rate through the glass transition region following an aging period of varying length. The modeling is based on a recently developed thermodynamically consistent non-linear viscoelastic theory of thermal and mechanical behaviors of glassy polymers in the glass transition range [Caruthers JM, Adolf DB, Chambers RS, Shrikhande P. Polymer 2004;45:4577–97. Adolf DB, Chambers RS, Caruthers JM. Polymer 2004;45:4599–621]. The original model is slightly modified by replacing the stretched exponential by a relaxation function based on a simplified cooperative model emulating the commonly observed linear variation of the relaxing quantity with logarithmic time. Good agreement is found with the observed temperature dependence of specific volume and enthalpy. Also the peaks in thermal expansivity and heat capacity are well described, both with regard to their intensity and position along the temperature axis.

This paper presents the results of measurements of physical aging on polystyrene with a narrow molecular weight distribution (Mw/Mn = 1.03). The evolution of the aging process was followed by recording the relaxed enthalpy and the accompanying decrease in volume, using differential scanning calorimetry and mercury-in-glass dilatometry, respectively. The measurements were carried out after cooling the sample at constant rate to the aging temperature. The cooling rate varied between 0.0037 and 1 °C/min. The aging data were fitted with the KWW and the TNM functions. The differences in the time scales of enthalpy, h, and volume, v, relaxation were relatively small. From the h(v) graphs the derivative dh/dv was found to amount to about 1.8 GPa, a value significantly exceeding the predictions of the thermodynamic model containing an additional internal variable.

This paper reports physical aging results for PMMA, PMMA/PEO blends, PS, PC, PVC and PET (semicrystalline). Also included in this study is amorphous selenium. Temperature down-jumps from equilibrium above T g to a temperature below T g were carried out. Relaxed enthalpy, Δ h and volume contraction, Δ v , were measured. From the aging records, the constant ratio Δ h /Δ v = K a was evaluated. For the polymeric samples K a values of about 2 GPa were observed, thus similar to the inverse value of the isothermal compressibility close to T g . Similarly for Se the K a value obtained from Δ h and Δ v was in fair agreement with its isothermal compressibility.

Enthalpy and volume relaxation data measured on atactic poly(methyl methacrylate) and its blends with poly(ethylene oxide) are used to calculate an aging bulk modulus, K a , tentatively defined as the derivative (∂ h∂ v) T . The K a values are mainly lower than the corresponding bulk modulus data determined directly with a piston-type PVT-device. On the whole, however, the K a values can be considered as lying close to the moduli measured in the liquid state. This notion is supported by K a data measured on amorphous selenium, which has a significantly lower compressibility in the liquid state than common amorphous polymers. The results are discussed in terms of thermodynamics of irreversible processes using a single internal variable, yet without arriving at suitable expression for the enthalpy/volume derivative.

Enthalpy, h , and volume, v , relaxation of poly(methyl methacrylate), PMMA, and polycarbonate, PC, were measured using differential scanning calorimetry and mercury-in-glass dilatometry. For PMMA both temperature down-jumps and up-jumps were studied. Attention was paid to the influence of cooling/heating rates on aging process with the result that it is related to the corresponding shift of glass transition temperature, T g . From the time dependence of enthalpy, h , and volume, v , the aging modulus was defined as K a = (∂ h /∂ v ) T , and was found to be ca. 2 GPa. This corresponds roughly to the inverse value of the compressibility, κ l , in the vicinity of T g . The results of the measurements on amorphous selenium supported the notion that K a assumes values close to 1/ κ l . The measured K a values were about 5 GPa, in fair agreement with compressibility data extracted from pvT measurements.

The present paper discusses the clustering of elementary events in a kinetic relaxation model based on a time-domain analog of Bose-Einstein statistics. The clustering results in a spectrum of relaxation times which are integer-valued fractions of the relaxation time of single events, thus providing a stretching of the overall process towards shorter times. The predictions of the model are combined with an expression describing the 'universal' behaviour of solids of various structure and composition in a stress relaxation experiment. This yields the remarkable result that the average cluster size relating to the entire process is six. Correspondingly, the average relaxation time is found to be 1/6 of the relaxation time of the single events.

Relaxation and consolidation processes in solids (stress relaxation, physical aging) exhibit a number of similarities, especially with regard to the kinetics involved. In this paper, we derive a kinetic formula representing the time-domain equivalent of a Bose-Einstein (BE) distribution. The approach is based on the master equation supplemented with a BE enhancement factor. The plausibility of the underlying physical mechanism is discussed, especially with regard to the clustering of the elementary transitions involved in such a process.

The polymer liquid crystal PLC is the PET/0.6PHB copolymer; PET=poly(ethylene terephthalate), PHB=ρ-hydroxybenzoic acid (LC): 0.6=the mole fraction of PHB. This is a multiphase system with PHB-rich islands in a PET-rich matrix. Tensile creep compliance was measured isothermally from 20 °C to 160 °C in 10 °C intervals. Master curves were determined using the time–temperature superposition for 20 °C and for the glass transition temperature of the PET-rich phase TgPET=62 °C. Experimental values of the temperature shift factor aT as a function of temperature T agree in the entire T range with those from Eq. (7) relating aT to the reduced volume ṽ and the Hartmann equation of state Eq. (10). Values of aT(T) calculated from the Williams–Landel–Ferry (WLF) formula give very large errors below Tg. A control 14 months creep experiment agrees with the theoretical predictions from Eq. (7). Stress relaxation experiments were performed under the constant strain of 0.5% from 20 °C to 120 °C, again master curves were de...

Service performance seems to be the ultimate test of successful synthesis, processing, characterization and testing of polymeric materials. Viscoelasticity and the time dependence of mechanical properties can be considered as a nuisance, but in fact they provide us with the capability of prediction of long-term performance in service on the basis of short-term tests. The material characteristics and their influence on long-term life prediction are evaluated using two concepts, the cooperative response of the material and the chain relaxation capability (CRC). We show quantitative predictions using the time-temperature correspondence, time-strain correspondence, frequency-temperature correspondence, stress-time correspondence and the slow crack propagation on the basis of these concepts. The predictions include the use of a generalized temperature shift factor aT formula. Our formula includes the W-L-F equation of 1955 as a special case and has a much larger range of applications. Our stress shift...

During physical aging of an amorphous polymer below its glass transition temperature, T g , thermodynamic properties such as enthalpy, H, and volume, V, will decrease with time towards their equilibrium value. This results, among others, in an overall increase of stiffness.

The ordering of electrically conducting polyaniline particles dispersed in a non-conducting liquid, 1,2,4-trichlorobenzene, in a weak electric field (of the order V mm −1 ) has been studied. The time needed for the formation of conducting chains, depending on the applied voltage, the polyaniline concentration and the viscosity of the suspension medium, was used to calculate the effective relative electric permittivity for polyaniline, ɛ p = 4.8, responsible for the dipole-dipole interaction leading to the structure formation. Nonlinear limit current-voltage characteristics typical of electrorheological suspensions were observed. The formation of the conducting chains could be followed in an optical microscope.

The relation between the electrical noise generated during flow of aqueous poly(ethylene oxide) PEO solutions in three capillaries of different lengths and entry shape but the same diameter and the flow behaviour was investigated. Flow into the square cut off capillary entry was characterised by a low pressure loss regime followed at higher shear rates by a high pressure loss regime while flow into the trumpet shaped capillary end exhibited only the low pressure loss regime. The high pressure loss flow was associated with strong electrical noise generation. The results suggest that the noise generation mechanism includes a current source supplied by the flow through the capillary volume with an associated streaming potential and a modulation of the current by mixing and turbulence at the capillary entrance. The noise measuring technique can potentionally be used to study disturbances in the entry flow to capillaries.

The stress-strain behavior in tension and the effect of temperature on the creep of poly(ether ether ketone) (PEEK) have been studied. At room temperature, ∼130° below the glass-transition temperature, the material does not become brittle, and the specimens show necking in tension over a wide range of elongation rates. The stress and strain at yield and the strain at break are almost linear functions of the logarithmic elongation rate. The values of stress and strain at yield increase slightly with increasing elongation rate, while the strain at break decreases markedly. The short-term creep tests were conducted at temperatures extending from 20 to 200°C. The glass-transition temperature was found to be about 155°C. The creep of PEEK is greatest at temperatures above 130°C. In the glass region the time dependence of the deformation is much weaker. It has been found that the time-temperature relation for PEEK corresponds well with its thermorheological simplicity in the temperature range investigated. The data on the temperature shift factor below and above the glass-transition temperature may be fitted separately to the Arrhenius and Williams-Landel-Ferry (WLF) equations, respectively. The long-term creep tests show that PEEK has excellent creep resistance at room temperature. After 14-month tests at a stress level of 30 MPa the total strain exceeds the instantaneous elastic strain only by a factor of 1.15.

Free volume plays an important role in the analysis of physical aging,both experimentally and theoretically. In this work, the Doolittle freevolume equation and the KAHR model are used to predict isothermal responsesfollowing temperature down- and up-jumps for poly(vinyl acetate), PVAc, inthe Tg-region. The constant B of the Doolittleequation is less than unity when the equation is combined with the Kovacsdifferential equation for free volume collapse for isothermal contractionpredictions. Using B values in the range 0.3 to 0.5 allows goodprediction of the isothermal contraction. It is shown that the equationyields aging rates approaching realistic values only for temperatures muchbelow Tg. Isothermal expansion could not be predicted. Volumerelaxation kinetics is analysed based on the Doolittle equation and comparedwith stress relaxation kinetics near Tg, where the slope ofthe inflexion region is calculated. The transient response of PVAc in thesame aging experiments is obtained using the KAHR model.

The physical aging behavior of poly(vinyl acetate), PVAc, in the T g -region has been investigated using dynamic mechanical analysis (DMA). Tests in shear and flexure modes were performed for samples quenched from a stabilization temperature of 40°C to aging temperatures 35.0°C and 32.5°C (down-jumps), and from equilibrium at a stabilization temperature of 32.5°C to aging temperature 36.2°C (up-jump). Volume recovery was investigated for the same thermal histories using mercury-in-glass dilatometry. The time scales for the evolution of the storage and loss moduli during physical aging have been compared with the volumetric time scales. Aging rates, μ, have been calculated in the frequency and time domains. The evolution of dynamic mechanical properties during aging is found to be different from the volumetric behavior. In the case of the temperature down-jump, the evolution of the volume ceases earlier than that for the storage and loss moduli. In the up-jump experiment, the opposite is found. Aging rates for the storage and loss moduli are analyzed. A suggestion is offered that the evolution of dynamic mechanical properties during physical aging is retarded by the cyclic mechanical stimulus, and that this effect (rejuvenation) is more noticeable the higher is the damping; consequently rejuvenation in the T g -region can be caused by small strains in dynamic mechanical analysis.

Polymers containing electrically conductive fillers show interesting electrical properties like semiconductors and metals without losing the processability of polymers. Typical applications are as antistatic (electrostatic dissipation) materials, electro-magnetic interference shielding materials, heaters and sensors. The selection of filler and polymer governs the properties obtained in such composites.

Experimental evidence on stress relaxation is analyzed first for a wide variety of classes of materials: metals and their alloys, synthetic and natural polymers, glasses and frozen non-polymeric organic liquids. Common features of curves σ(t) of relaxation of stress a as a function of time t are discussed, and the importance of the internal stress σi(∞) noted. Theoretical approaches are then reviewed, with particular attention to the cooperative model and its modifications; that model corresponds well to the experimental results. Some simulation results obtained with the method of molecular dynamics are reported for ideal metal lattices, metal lattices with defects, and for polymeric systems. In agreement with both experiments and the cooperative theory, the simulated σ(log t) curves exhibit three regions: initial, nearly horizontal, starting atσ 0; central, descending approximately linearly; and final, corresponding toσ i. In agreement with the theory, the slope of the simulated central part is proportional to the initial effective stressσ 0*=σ 0−σ i. The time range taken by the central part is strongly dependent on the defect concentration: the lower the defect concentration, the shorter the range. Imposition in the beginning of a high strain ɛ destroys largely the resistance of a material to deformation, resulting in low values of the internal stressσ i. On the joint basis of experimental, theoretical, and numerical results, we explain the mechanism of stress relaxation in terms of deformations occuring in the immediate environment of the defects. Simulations show several common features in the behavior of metals and polymers. Apart from the defect concentration, the amount of free volumev f is also important.

Electrical current noise and electrical resistivity as well as viscosity were measured in aqueous solutions of gelatin containing KCl during the sol—gel transition. While the viscosity rises toward infinity at the transition, the current noise power and the resistivity are continuous functions of the time and are therefore potentially useful probes for characterizing polymers undergoing gelation.

This paper presents the results of an experimental study of the stress relaxation behaviour of PE where the focus was on determination of the volume changes taking place during the relaxation process. The dimensions of the samples were followed using a specially designed non-disturbing extensometer. The extensometer data were confirmed in experiments where the volume was measured with a specially designed liquid stress dilatometer. The bulk of the results was obtained with LDPE and LLDPE. High density polyethylene was shown to behave similarly. The decrease in volume, corresponding to an increasing Poisson's ratio, during relaxation was approximately linear with log time. Volume vs. stress diagrams were linear; the values of the apparent bulk modulus calculated from them were only slightly higher than those obtained from the stress-strain curves. Gruneisen parameter was measured and compared with reported values. Possible similarities between the volume change during stress relaxation and that occuring during the process of physical ageing are discussed.

New types of composite systems based on polymeric matrices (polystyrene, polyethylene) containing conductive (Pd, Ag, Ag alloys) and dielectric (ferroelectric) components such as as BaTiO 3 or piezosonic powder were developed and investigated in bulk and as thick films. The latter were prepared using a screen-printing technique applying the composite paste on alumina, thermoset, enamelled steel and ceramic-coated Cu substrates. In samples with a spatially homogeneous distribution of the metal particles, as was the case with Pd, incorporation of the dielectric component caused a marked decrease of the percolation threshold. The opposite was true when the particles were inhomogeneously distributed (Ag), namely, in thick films. Dielectric measurements showed that the real part of the complex permittivity increased substantially with the content of both the dielectric additive and the metal component, especially at high resistivity levels. Thick films are interesting because the constant positive temperature coefficient of resistance can be used to make temperature sensors screen-printed on various substrates, for instance, on insulated metal.

Due to dipole interaction, the molecular polarization brought about by an external field is significantly lower in condensed matter (liquids) than in a gas. In addition to this, the response of interacting dipoles to stepwise changes of the external field does not follow a simple time exponential. Instead, a spectrum of relaxation times is required to describe such a response. Debye and Ramm [Ann. Phys. 28, 28 (1937)] have attempted to describe the effects associated with rotational hindrances due to dipole‐dipole interaction by the following differential equation: ∂f/∂t =(kT/ρ)Δf+(1/ρ)div(f grad u), where f denotes the distribution function specifying the number of dipoles pointing in a certain solid angle, t the time, ρ a friction coefficient, and u the potential of the forces acting on the dipoles. The latter quantity depends both on the external field and on the contribution from the dipole‐dipole interaction (internal field). Although unable to solve the above equation explicitly, Debye and Ramm (DR)...

Instrumentation for time-dependent stress dilatometry of polymers was constructed. Volumetric measurements in creep for polycarbonate and in stress relaxation for polyethylene were carried out. The advantage of the transverse and axial strain rate ratio for volumetric analysis, in creep, is emphasized. An experimental finding of volume-stress linearity in stress relaxation measurements is presented and discussed.

Molecular-dynamics simulations of stress relaxation have been performed for models of metals and polymers. A method that employs coupling between the simulation cell and an applied stress as well as an external thermal bath has been used. Two-dimensional models of the materials are defined with interactions described by the Lennard-Jones (Mie 6-12) and harmonic potentials. A special method is employed to generate chains in dense polymeric systems. In agreement with experiments, simulated stress-relaxation curves are similar for metals and polymers. At the same time, there exists an essential difference in the stress-strain behavior of the two kinds of simulated materials. During the relaxation, trajectories of the particles in different materials display a common feature: There exist domains in which movement of the particles is highly correlated. Thus, the simulation results support the cooperative theory of stress relaxation.

The time-domain dielectric response (after-effect) of poly(methyl methacrylate) (PMMA) and nitrile butadiene rubber (NBR) to a voltage step (100 V) was measured at varying temperatures. From the variation of the sample capacitanceC with time, we determined the ratioFd/ΔC, withFd=(dC/dlnt)max and ΔC denoting the difference between the initial and the extrapolated equilibrium capacitance values. For PMMA around room temperature (RT) this ratio assumed values similar to those reported for mechanical stress relaxation. With NBR, such values were observed only at temperatures significantly below RT. A modified Kohlrausch-Williams-Watts (stretched exponential) function provided a good fit to the measuredC(t) data.

Electrical noise measurements have been used to study electrical conduction in carbon black-filled linear low density polyethylene. Measurements of the resistivity, real part of the relative permittivity, thermal noise, and current noise have been carried out for carbon black contents and temperatures below, near, and above the percolation threshold for electrical conduction. The current noise shows a 1 /fα frequency dependence, with α near unity close to the percolation threshold, but increasing to 1.5 at higher resistivities or temperatures.

Some new polymeric composites exemplified by polyethylene or polystyrene filled with precious metal particles, especially with spherical palladium, silver and gold powders, of various diameters are discussed. In addition to bulk samples, thick films are prepared and studied. The determined critical parameters describing resistivity-content dependence are strongly influenced both by the polymer and by the shape and conductive particle dimensions. The percolation threshold varies from about 10 to 31 vol.%. All the composites exhibit a large positive temperature coefficient of resistance in a wide temperature range. Dielectric measurements show also a strong temperature dependence of capacitance up to the melting point. The results are compared with those obtained on polymers filled with carbon black of various types.

New types of heterogeneous systems based on polyethylene or polystyrene or polystyrene matrices containing precious metal powder or ruthenium dioxide were developed. Characterization of these materials included the determination of the dependence of the resistance on the powder content and the dependence of the impedance and phase angle or the capacitance on the powder content at various frequencies and temperatures. The critical parameters of the blending curves were evaluated for various composites. Frequency measurements made it possible to determine the complex permittivity and to show that its real part increased with increasing conductive ingredient content up to a value of about 1600 or more for a frequency of 1 kHz.

Polycarbonate, PC, and poly(methyl methacrylate), PMMA, were subjected to short-term unaxial creep tests in tension. Measurements were made of the axial and lateral strains. The creep curves were fitted to polynomials $$t = \sum\limits_{i{\text{ }} = {\text{ }}0}^n {a_i \varepsilon ^i } $$ (usually of 4th order), both for longitudinal and transverse strains, for the purpose of calculating axial and transverse strain rates. Viscous Poisson ratio, μ $$\mu = - \frac{{\dot \varepsilon _{33} }}{{\dot \varepsilon _{11} }}$$ was calculated and plotted vs. time. Simultaneously, deformational Poisson ratio ν = -33/911 and volume strain ɛν = ɛ11 + 2ɛ33 were measured and plotted. The volume strain increased with time during the initial stage of the creep process. After reaching a maximum value, it started to fall. The values of both μ and ν increased monotonically with time. By comparing the three plots it was seen that, while there was no relationship between the volume reversal and the deformational ratio ν, values of the viscous ratio μ 0.5, the opposite was true. It was concluded that for the purpose of characterizing the volumetric response of materials, such as PC and PMMA, in creep the viscous Poisson ratio represents a suitable and highly indicative parameter. Theoretical calculations of the volume strain behavior of the standard linear solid model and the generalized model of Voigt-Kelvin type were in agreement with the experimental data, predicting an increase or decrease in ɛν with time for μ 0.5, respectively.

Molecular dynamics simulations of stress relaxation have been performed for models of metals and polymers. The method which employs coupling between the simulation cell and an external thermal bath as well as an applied stress has been used. Two-dimensional models of the materials are defined with interactions described by the Lennard-Jones (Mie 6–12) and harmonic potentials. A special method is employed to generate chains in dense polymeric systems. Simulated stress relaxation curves are similar for metals and polymers, while there exist essential differences in the stress-strain behavior of the models. During the relaxation, trajectories of the particles in different materials display a common feature: there exist domains in which movement of the particles is highly correlated. This observation supports the cooperative theory of stress relaxation developed by one of us. The results of the simulations do not significantly depend on the number of particles in the system nor on other simulation details.

The possibility of describing transient phenomena associated with flow and consolidation of solids, such as stress relaxation or physical aging, in terms of a kinetic mechanism comprising spontaneous and induced events is discussed. The starting point is the differential equation dn/dt=−an[1−(b/a)n], with n denoting the number of relaxed entities and n=dn/dt (a,b are constants, t is time), yielding an n(t) function reminiscent of a Bose–Einstein distribution. The corresponding n(t) relation describes the linear variation of n with log t, and the exponential dependence of n on n, as often found experimentally. Replacing n in the starting equation by the relative rate n/n yields a power‐law‐type n(n) dependence. A further modification, where the induction term n/n is not linear but raised to a power ≳1, finally produces a generalized version of the stretched exponential. When interpreted formally in terms of a spectrum of relaxation times τ, all three equations produce response functions with disc...

An investigation of streaming potential and electrical noise generated during capillary flow of aqueous solutions of poly(ethylene oxide) showed that these effects do not depend on each other. While the former effect arises in the electrical double layer at the capillary wall, the latter is a bulk effect produced by turbulent movement of the flowing liquid.

This paper presents an investigation of the properties of composites of bacteria-produced polyesters reinforced with wood cellulose. Although cellulose fibers improved the strength and stiffness of the polyhydroxybutyrate (PHB), the samples were very brittle. The impact strength and elongation at break of these composites were significantly improved when PHB copolymers with increased amounts of hydroxyvalerate (HV) were used as the matrix. Dynamic mechanical properties of PHB copolymers of varying compositions and of cellulose-filled composites were investigated. The introduction of cellulose resulted in a decreased loss factor owing to restrictions of the chain mobility in the amorphous phase. An improvement was observed in the dynamic modulus, which was seen to be greatest at elevated temperatures. An excellent dispersibility of cellulose fibers was achieved in the PHB matrix as compared with such synthetic matrices as polystyrene or polypropylene. The degree of dispersibility was strongly affected by processing conditions. The defibrillation observed on extracted fibers suggests a possible hydrolysis of cellulose by crotonic acid formed in situ as a result of the thermal decomposition of the PHB matrix.

This paper is concerned with the creep behavior of short fiber-reinforced thermoplastics, especially with regard to the role of fiber orientation. Rectangular samples of low density polyethylene (LDPE) and polycarbonate (PC) containing varying amounts of carbon fibers were prepared by compression and injection molding. The materials were compounded using a technique producing a concentration independent fiber length distribution. The orientation distribution, on the other hand, was found to be strongly influenced by fiber concentration. The creep parameters were measured for both LDPE and PC. The contraction ratio was determined for the PC samples. In the case of LDPE reliable data could not be obtained due to the low modulus of the matrix. The creep properties of the PC and LDPE samples varied significantly with the orientation of the fibers. The creep strain was measured as a function of time for both polymers for different fiber concentrations and orientations. The predictions of the Halpin-Tsai equation underestimated the experimental strain figures somewhat when the stress direction coincided with that of the fiber orientation. When the stress acted across the fibers the theoretical and experimental results showed satisfactory agreement. A plausible explanation is that incorporation of carbon fibers changes the morphology of the matrix material. We conclude that the contraction ratio is an important measure of the volume and its changes during deformation, especially with regard to its relation to the free volume and similar quantities. We have shown that the necessary data can be obtained, in spite of experimental difficulties even for anisotropic samples.

The electrical noise generated during capillary flow of poly(ethylene oxide) solutions has been studied in detail in mixtures of water and isopropyl alcohol of various solvent powers. The noise level, which increased with flow rate, was found to be an increasing function of the linear dimension of the polymer coils as obtained from intrinsic viscosity data, if the system consists of entangled macromolecules, i.e. if the polymer concentration is higher than the coil overlap concentration.

This paper explores the properties of a relaxation function derived from a differential equation mimicking the distribution mechanism of Bose-Einstein statistics in the time domain. Within a significant portion of the process, the relaxation quantity n decreases linearly with log time. The relation between dn/dt and n is an exponential one. In this respect, the present approach produces results largely equivalent to those obtained using the hypothesis of stress-dependent thermal activation or a box-like spectrum of relaxation times, τ. The τ spectrum of the model proposed here is discrete, with integer valued fractions of a characteristic -centering the equations.

This paper demonstrates the usefulness of frequency analysis in monitoring melt flow instabilities. The flow behavior of a film-blowing grade of high density polyethylene has been studied using a capillary rheometer at constant piston velocities. The pressure in the reservoir was measured by a transducer fitted to the driving end of the piston. The signal was amplified and processed in a Fast Fourier Transform (FFT) frequency analyzer. The pressure fluctuations (noise) have a very low intensity at low flow rates in the sharkskin region. The noise signal increases dramatically at a critical shear stress of about 1.5 × 105 Pa, where the flow assumes a pulsating character (spurt defect). Above the pressure oscillation region the melt goes through a pseudostable region, followed by a secondary pressure oscillation region, eventually reaching a chaotic stage. The noise spectrum intensity seems to be related to the severity of the surface disturbances of the extrudate in this latter branch of the flow curve.

This paper demonstrates the usefulness of frequency analysis in monitoring flow behavior in a single-screw extruder fitted with a capillary die with a length to diameter aspect ratio of 20. The melt pressure in the barrel and die was measured by a transducer, and the pressure fluctuations processed in a Fast Fourier Transform (FFT) frequency analyzer. The pressure fluctuations were studied under various processing conditions for high density polyethylene, linear low density polyethylene and polypropylene. In general, stable extrudate flow generates a noise spectrum with low intensity, the intensity increasing with the level of instability. The machine vibrations appeared to have a direct effect on flow stability. Also, unstable melting of the resin along the screw resulted in an increase in noise level. It seems probable that the frequency of the surface distortions can be related to the shape of the noise spectrum. In general, although more complex in nature, the noise results related to melt flow in the extruder were in agreement with data reported for capillary flow using a piston rheometer.

It has been demonstrated that strong electrical noise is generated during capillary flow of an alkalized latex dispersion of copolymer carrying carboxylic groups where the dispersion particles are swollen. On the other hand, if the content of dissociated carboxylic groups in the alkalized dispersion is higher and swelling turns into decomposition of the particles to macromolecules or their aggregates, the thermal (Nyquist) noise at rest remains unaffected by flow, even if the flow curves are similar to corresponding ones when strong noise is generated. Viscosities and first normal stress differences measured at lower shear stresses suggest that the occurrence of noise generation during flow is affected, not only by pseudoplasticity and elasticity of the liquid, but also by its microstructure.

Thermal ageing of PC/PBT blends and alloys has been studied with dynamic mechanical, calorimetric, and tensile dilatometry tests during creep. The substantial embrittlement occuring in PC/PBT during ageing is tentatively explained in terms of an unusually high densification of the amorphous phase. The phase boundaries have been found to increase in strength during ageing, improving the load bearing properties of the material.

The dynamic-mechanical properties of high density polyethylene filled with 20% by volume of untreated glass spheres or glass spheres treated with a silane-based coupling agent were studied as a function of temperature and imposed tensile deformation. The coupling agent used is capable of providing covalent bonding between the polymeric matrix and the glass spheres. It is assumed that an interphase region is formed in the matrix around each filler particle with properties depending on the surface treatment of the filler, but different to that of the bulk matrix. It is shown how the mechanical loss factor can be used to characterize the properties of the interphase region and the degree of adhesion between the two phases, as affected by the surface treatment. We suggest that these kinds of measurements can be valuable when determining the effectiveness of various surface treatments of filler particles from a mechanical point of view.

High pressure injection molding of HDPE with relatively high molecular weight is known to give products with significantly increased modulus and strength. The extent of this improvement is reduced when the thickness of the molded sample increases. The present paper is an account of experiments with varying mold filling rates and gate geometries. After choosing proper gating and then optimizing the filling rate, thick samples with tensile moduli previously characteristic of thinner samples have been obtained. The modulus data are supplemented by measurements of the melting point of the normal and high-strength HDPE structures at different locations in the molded test bars. Corresponding crystallinity data are also included. The structure of the samples appears to provide some guidance in interpreting the observed modulus variations.

The results of an investigation of the flow behaviour of hard-metal carbide powder compounds in a capillary rheometer operating at constant piston speed are presented. The suspensions, consisting of a polymeric binder (PE + paraffin + copolymer EBA, EVA or EAA) and carbide powder, were prepared by melt mixing using a Brabender Plasticorder. The relations of Maron-Pierce, Quemada and Mooney were applied to calculate maximum loading values for the compounds used. Close to the maximum loading the character of the flow changed from pseudoplastic to plug flow. This was accompanied by the appearance of flow instabilities similar to pressure oscillations observed with unfilled polymers. The calculations of the average thickness of the polymer layer bound to the surface of the carbide particles were in fair agreement with microscopic observations.

Rheological behaviour plays a key role in powder processing by powder injection moulding and extrusion. The flow behaviour of concentrated suspensions consisting of polymeric binder and metal powder is to a large extent determined by the characteristics of the binder system. This has to provide an acceptable flow during the forming process, proper interaction with the powder, and an easy extraction after moulding. Low molecular weight polymers and waxes are often the basis for polymeric binders, mainly because of easier mixing and removal of short polymer chains. Due to their complexity, considerable difficulty is encountered in interpreting the flow behaviour of highly filled polymer systems. The literature provides only a limited amount of data relating to such systems, especially when the polymeric vehicle consists of several components.

New types of two- and multi-component polymeric composites based on polyethylene or polystyrene were studied. Conductive parts were formed from metal powders (Pd, Ag, Ag-compounds). Additional dielectric components (glass particles, BaTiO3 or piezosonic powder U5H-32) were used to modify mechanisms of electric charge transport. Investigation of the mixtures included blending curve measurements and evaluations, measurements of temperature dependence of DC resistivity and the complex impedance |Z| at various frequencies and the study of the composite structure by means of SEM. Some relations between the electrical characteristics and the structure are mentioned.

Simulations were performed using the method of molecular dynamics for ideal lattices and for lattices with defects generated by three different procedures. Curves of relaxation of stress \ensuremath{\sigma} vs logarithmic time t were obtained. In agreement with experimental results, the simulated curves exhibit three regions: initial, nearly horizontal, starting at ${\mathrm{\ensuremath{\sigma}}}_{0}$; central, descending approximately linearly; and final, corresponding to the internal stress ${\mathrm{\ensuremath{\sigma}}}_{\mathit{i}}$ as defined by Li. The existence of the central linear part has been predicted by a cooperative theory. In agreement with the theory, the slope of the simulated central part is proportional to the initial effective stress ${\mathrm{\ensuremath{\sigma}}}_{0}^{\mathrm{*}}$=${\mathrm{\ensuremath{\sigma}}}_{0}$-${\mathrm{\ensuremath{\sigma}}}_{\mathit{i}}$. The central part extends over approximately one decade of ${\mathrm{log}}_{10}$t for ideal lattices but over several decades for lattices with defects. High values of the imposed strain \ensuremath{\varepsilon} correspond to low internal stresses ${\mathrm{\ensuremath{\sigma}}}_{\mathit{i}}$, and vice versa. Stress relaxation is mainly due to deformations that occur in the vicinity of the defects, hence the process is related to the defect concentration and the amount of free volume ${\mathit{v}}^{\mathit{f}}$. Collective response of atoms in groups is observed. The origin of the defects does not seem to influence the relaxation.

There exist large amounts of experimental evidence on stress relaxation for metals and their alloys, synthetic and natural polymers, glasses and frozen non-polymeric organic liquids. The results, typically presented as curves σ (log t) of relaxation of stress σ as a function of logarithmic time t, exhibit common features, apparently independent of the type of material. All curves consist of three regions: initial, nearly horizontal, starting at σ0; central, descending approximately linearly; and final, corresponding to the internal stress σi = σ(∞). We discuss briefly the experimental evidence as well as the main features of the cooperative theory which does not involve specific features of different classes of materials. The bulk of the paper deals with computer simulations. Simulation results obtained with the method of molecular dynamics are reported for ideal metal lattices, metal lattices with defects, and for polymeric systems. In agreement with both experiments and the cooperative theory, the simulated σ (log t) curves exhibit the same three regions. In agreement with the theory, the slope of the simulated central part is proportional to the initial effective stress σ0* = σ0 − σi,. The time range taken by the central part Is strongly dependent on the defect concentration: the lower the defect concentration, the shorter the range. Imposition in the beginning of a high strain ɛ destroys largely the resistance of a material to deformation, resulting in low values of the internal stress σi. Since the cooperative theory assumes for particles (atoms, polymer chain segments) the existence of two states, unrelaxed and relaxed, and has a formal connection to the Bose-Einstein (B-E) distribution, we first simulate B-E systems, recording the formation of relaxed clusters of particles of different sizes. Differences in cluster sizes predicted from a B-E model and those obtained from the simulations are recorded and analyzed. On the joint basis of experimental, theoretical and simulation results, we explain the mechanism of stress relaxation in terms of deformations occurring in the immediate environment of the defects. These deformations, visible in simulations of both metals and polymers, correspond to cluster relaxations in the cooperative theory, and thus confirm a posteriori the assumptions made in developing the theory.