Investigation of transitions and relaxation processes in polystyrene by using positron annihilation

The glass transition and relaxation processes in polystyrene resins with the number average molecular weight ranging from 7.0·10² to 9.8·10⁴ were studied with the positron annihilation technique. The pick-off annihilation lifetime of ortho-positronium (₃) and its intensity (I ₃) were measured in the temperature range from 20 to 430 K. The glass transition temperature (T _g) was determined as an onset temperature coefficient of ₃.T _g shows the molecular weight dependence in these samples. BelowT _g, local motions were detected by measurements ofI ₃. The local motions could be observed above 100 K in this experiment.I ₃ show the minimum at around 250 K and it does not show molecular weight dependence.     

Study of relaxation processes in polyethylene and polystyrene by positron annihilation

Relaxation processes in polyethylene (PE) and polystyrene (PS) were studied by positron annihilation technique. For PE, above the glass transition temperature, T_g, the size of free volumes and its concentration were increased by the micro-Brownian motion of molecules. For PS, local motions of molecules in backbone chains were found to start above 260 K. However, these local motions were suppressed by an interphenyl correlation. For both PE and PS, below 250–260 K, the formation probability of positronium atoms increased with decreasing temperature. This fact was assigned to the freezing in of the local motions of molecules. For PS, an onset of the local motions of molecules was observed above 100 K. These motions were expected to be associated with liberation of phenyl groups. © 1996 John Wiley & Sons, Inc.     

Free volumes in polystyrene probed by positron annihilation

Free volume characteristics in three samples of monodisperse polystyrene were investigated by positron annihilation technique over a temperature range from 300 to 380 K. The number-average molecular weight of the samples ranged from 5730 to 1,524,000. The observed lifetime spectra were resolved into three components, where the longest lifetime, τ₃ was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by free volumes. The change of the temperature coefficient of τ₃ was observed at around 350 K, at which the value of τ₃ was a constant value of 2,3 ns for all specimens with different molecular weights. There was no discrete change of τ₃ in intensity, which is corresponding to the number of free volumes. The size of free volume at glass transition was evaluated to be 0.l nm³. © 1996 John Wiley & Sons, Inc.     

溶胶—凝胶法制备钕玻璃纤维

钕玻璃是一种重要的光学材料。它作为激光工作物质已在激光核聚变、中小能量器件、波导激光器和光纤通讯上获得重要的应用。制备钕玻璃及其纤维通常需要在高温熔融状态下进行,操作条件苛刻。溶胶-凝胶法是在较低温度下制备无机材料的新方法,目前正受到人们的重视。     

A finite element analysis of strain localization for soft rock using a constitutive equation with strain softening

Summary It is well known that deformation and/or strain of geological materials localize when they come close to the failure state. In the present: study, we try to analyze the deformation problem using a constitutive relation with strain hardening and strain softening. The constitutive model of a soft rock and overconsolidated clay using an elasto-plastic constitutive theory with memory was originally developed by Adachi and Oka. This type of formulation is shown to be easily applied to analyze the material behavior of strain softening because there is a similarity to that in viscoplasticity. Using the proposed model, the loss of uniqueness of the solution to the initial value problem can be avoided and a special or complicated numerical technique, e.g., an arc length method, does not need to be used. When we use constitutive equations with strain softening in a finite element analysis, there is a problem of strong mesh size dependency of numerical results. To remedy the mesh size dependency, we generalize the Adachi-Oka model based on the concept of non-localization by Bazant. We apply the proposed constitutive model to the behavior of a sedimentary soft rock in the drained triaxial compression test. It is found that mesh size dependency becomes smaller using the non-localization of the constitutive model.

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Eine Finite-Element-Analyse der lokalisierten Verformung von weichem Gestein bei einem Stoffgesetz mit Entfestigung

Übersicht Bekanntlich findet bei geologischen Stoffen in der Nähe des Versagenszustandes eine Lokalisierung der Verformung statt. Die Analyse dieses Problems wird in diesem Beitrag auf der Grundlage eines Stoffansatzes mit Ver- und Entfestigung unternommen. Entwickelt wurde das zugrunde gelegte elastischplastische Stoffmodell mit Gedächtnis von Adachi und Oka für weiches Gestein und übermäßig verdichteten Ton. Wegen der Ähnlichkeit zur Viskoplastizität läßt sich diese Formulierung des Stoffgesetzes leicht auf die Analyse des Verhaltens von entfestigendem Material anwenden, da der Eindeutigkeitsverlust der Lösung des Anfangswertproblems vermieden wird und besondere Rechenverfahren wie etwa die Bogenlängenmethode nicht benötigt werden. Bei der Benutzung von Stoffgesetzen in einer Finite-Element-Rechnung hängen die Ergebnisse stark von der Netzeinteilung ab. Um dies abzustellen, wird das Modell von Adachi und Oka auf der Grundlage von Bazant's Konzept der Nichtlokalisierung verallgemeinert. Anwendungsbeispiel ist das Verhalten von weichem Gestein im drainierten Triaxial-Test. Es zeigt sich, daß mit dem Konzept der Nichtlokalisierung im Stoffmodell der Einfluß der Netzeinteilung geringer wird.

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Presented at the workshop on Numerical Methods for Localization and Bifurcation of Granular bodies, held at the Technical University of Gdask (Poland), September 25–30, 1989.     

Open spaces and relaxation processes in the subsurface region of polypropylene probed by monoenergetic positron beams

Open spaces and relaxation processes in the subsurface region of isotactic polypropylene were investigated by monoenergetic positron beams. From measurements of the lifetime spectra of positrons, the size of the open spaces in the subsurface region (≤ 0.2 μm) was found to be larger than that in the bulk; their differences were estimated as 20% at 295 K and 10% at 395 K. From conventional positron annihilation experiments, the glass‐transition temperatures, T_{g (upper)}and T_g(lower) were determined as 306 K and 278 K, respectively. These transition temperatures were associated with the onset temperatures of the molecular motions under the constraint imposed by crystalline regions and those free from the constraint, respectively. In the subsurface region, although the onset temperatures of the molecular motions were close to those in the bulk, the molecular motions above T_{g (lower)} were found to be suppressed. The annihilation characteristics of positrons with different implantation energies were also discussed. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 101–107, 2000     

Mass spectrometric study on chemical transport of VO2

The analysis of gas in the chemical transport reaction of VO₂ using TeCl₄ as a transport agent was carried out with a quadrupole mass spectrometer. In the transport reaction from 600 to 500°C, it was found that the oxygen and vanadium of VO₂ were transported in the form of VOCl₃ and TeOCl₂ gases; the transport reaction was $\text{VO}{}_2\text{+}\text{3}\text{2}\text{TeCl}{}_4\text{= VOCl}{}_3\text{+ TeOCl}{}_2\text{+}\text{1}\text{2}\text{TeCl}{}_2$. The transport reaction from 900°C to 800°C was assumed to be $\text{VO}{}_2\text{+}\text{3}\text{2}\text{TeCl}{}_2\text{= VOCl}{}_3\text{+}\text{1}\text{2}\text{O}{}_2\text{+}\text{3}\text{4}\text{TeCl}{}_2$. In the transport at high temperature, the oxygen partial pressure estimated from the mass spectrum was considerably higher than that in equilibrium with VO₂ phase. In this paper a study of the chemical transport of the system VO₂-TeCl₄ is presented.     

Photophysical and Photochemical Properties of Ru(bpy)3 2+ in Si-Ti Mixed Oxides Xerogels Prepared by the Sol-Gel Process

The luminescence spectra and lifetime of tris(2,2-bipyridine)ruthenium(II), Ru(bpy)₃ ²⁺, were studied in sol-gel reaction systems of tetramethoxysilane (TMOS) and titanium(IV) isopropoxide (TTIP) with HCl. Luminescence lifetime in the TMOS system increased as the sol-gel reaction proceeded, because diffusion-controlled luminescence quenching such as oxygen and collisional quenching with solvent molecules were suppressed in the rigid matrices. On the other hand, luminescence lifetime in the TTIP system decreased during the sol-gel reaction. The decrease in lifetime was ascribed to electron transfer from photoexcited Ru(bpy)₃ ²⁺ to the conduction band of the TiO₂ xerogels. Extended X-ray absorption fine structure (EXAFS) measurements were done to associate lifetime in the Si-Ti xerogels with the structures of Ti⁴⁺ sites in the xerogels.     

Transition and relaxation processes of polyethylene,polypropylene, and polystyrene studied by positron annihilation

Transition and relaxation processes of polyethylene (PE), polypropylene (PP), and polystyrene (PS) were studied by the positron annihilation technique. From measurements of lifetime spectra of positrons as a function of temperature, the lifetime of ortho-positronium, τ₃, and its intensity, I₃, were found to increase above 260 K for PP. This fact was attributed to a cooperative motion of large segments of molecules above the glass transition temperature, T_g. For PE, above T_g (140 K), the value of τ₃ increased, but the temperature coefficient of I₃ was negative below 230 K. From this fact, for PE, the molecular motions that cause the glass transition were associated with a rearrangement of molecules by local motions such as kink motions. The discrepancy between the results for PE and PP was attributed to the presence of methyl groups in PP and the resultant suppression of the local motions. For PS (T_g = 340 K), the molecular motions were found to start above 260 K, but those were suppressed by an interphenyl correlation. Detailed annihilation characteristics of positrons in polymers were also discussed. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1601–1609, 1997     

Open spaces in the subsurface region of polyethylene probed by monoenergetic positron beams

Open spaces in the subsurface region (<10 μm) of very low density polyethylene were probed by a monoenergetic positron beam. From measurements of Doppler broadening spectra measurements of the annihilation radiation and the lifetime spectra of positrons as a function of incident positron energy, the size of the open spaces in the region of 0–3 μm was found to be larger than that in the deeper region. This was attributed to the cooperative motion of large segments of molecules which is activated near the surface. After the freezing in of such motions (below 230 K), although the lineshape parameter S in the bulk was almost constant, S in the region of 0–3 μm decreased with decreasing temperature. This discrepancy was associated with the presence of the open spaces with an excess content and the resultant contraction of amorphous structure near the surface. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2597–2605, 1998