Influence of Tacticity on the Retardation Mode Associated with T g in PMMA

The anelastic properties of PMMA in the temperature range of the main retardation mode are strongly dependent upon tacticity. The thermally stimulated creep (TSCr) technique was used to analyse the distribution function of retardation times. A series of PMMA, with various tacticities, synthesized by living polymerization, were investigated. One of the most important findings was the observation of a sub-mode, at 150°C, with a magnitude increasing with syndiotactic triads. The existence of physical interactions might explain this behaviour. On the low-temperature tail of the a retardation mode, a sub-mode was observed at 60°C: it was associated with remaining isotactic triads.     

Secondary retardation modes in diglycidyl ether of bisphenol A–diamino diphenyl methane networks

Thermally stimulated creep (TSCr) has been used to follow the viscoelastic behavior of some amine-cured epoxy networks below the glass transition. The investigation of the -180/+40°C temperature range has revealed two essential retardation modes characterizing localized motion of chain segments: the γ mode centered at ?155°C in all samples, and the well-known β mode observed around ?40°C in the stoichiometric network. The magnitude of the β mode was seen to decrease unexpectedly with the cross-link density, whereas its peak temperature and glass transition temperature both decreased. This evolution was confirmed by thermally stimulated currents (TSC) measurements and discussed on the basis of the antiplasticization concept. Water desorption under vacuum yielded additional information on the nature of the β mode and TSCr fractional loading experiments brought evidence that two types of relaxing units participate in β motions and furnished activation enthalpy data. © 1994 John Wiley & Sons, Inc.     

Retardation and relaxation behaviour of poly (ether-block-amide)s (PEBA) copolymers by thermally stimulated creep (TSCr) and current (TSCu)

The retardation and relaxation behaviour of poly(ether-block-amide)s PEBA copolymers was investigated as a function of hard polyamide sequence length by using thermally stimulated creep (TSCr) and Current (TSCu). Two primary TSCr and TSCu peaks, called ß_PE and ß_PA were observed in the temperature range 150–300 K. The “low temperature” peak ß_PE was characterized by a peak position T ß_PE which depends poorly upon hard segment content as the corresponding DSC glass transition temperature Tg_PE. The peak position Tß_PA of the “high temperature” relaxation showed a progressive shift towards higher temperatures as the hard segment average length was increased as it is observed in series of pure polyamide oligomers. So, complex spectra suggest that an amorphous phase separation occurs in PEBA copolymers. The TSCr ß_PE and ß_PA modes were analysed by thermal sampling method (TS). The elementary TSCr processes isolated in the ß_PE and ß_PA distributed retardation modes were characterized by retardation times following a compensation law. The comparison of activation and compensation parameters confirmed the existence of a biphasic amorphous phase in PEBA. In TSCr spectra, an additional peak, noted α, is observed at a temperature close to the DSC melting point T_mPE of the poly(tetramethylene glycol) segments.     

Molecular mobility in polymers studied with thermally stimulated recovery

Thermally stimulated recovery, TSR, like as thermally stimulated depolarisation currents, is a suitable technique that allows for the study of conformational mobility in polymeric systems. Due to its relatively low equivalent frequency and transient nature, the viscoelastic data obtained from this technique are complementary to conventional dynamic mechanical analysis (DMA). In this work TSR-like experiments, including TSR, thermally stimulated creep and thermal sampling (TS) experiments were carried out in the same commercial DMA equipment, allowing for the direct comparison of the data. Some advises for running TSR experiments are presented, such as the need of performing blank experiments and temperature calibrations. The analysis of the data to obtain the thermokinetic parameters of TS experiments is revised. In particular, from the direct fitting of the data, it is reported a tendency for a linear relationship between the pairs of values of (E _a, log τ₀) that best adjust any TS single experiment. It is concluded that the usual equation for describing TS experiments possesses an intrinsic compensation between these two thermokinetic parameters. This revised version was published online in August 2006 with corrections to the Cover Date.     

Research on viscoelastic behavior of semi-crystalline polymers using instrumented indentation

In this study, the viscoelastic behavior of a polyamide 12 (PA12) polymer was evaluated using instrumented indentation technology based on a rheological model. The creep compliance and retardation spectra were obtained to analyze the viscoelastic response during the holding stage according to the rheological model under different preloading conditions. The results showed that the viscoelastic responses were independent of the indentation depth or load under step loading conditions. However, the creep compliance increases, and the peak intensity of the retardation spectrum decreases with a decrease in the preloading rate owing to the structural relaxation observed during the preloading stage under ramp loading conditions. Furthermore, softening dispersion can be completed during the loading stage under continuous stiffness measurement (CSM) conditions. As the preloading strain rate changes, the peak of the retardation spectrum gradually decreases until it disappears completely. Moreover, studies on indentation creep using the CSM method are challenging because of the complicated viscoelastic response observed during the preloading stage.     

Broadening of relaxation and retardation spectra due to uniaxial orientational drawing of polyamide films

The initial and quasi-equilibrium values of relaxation modulus, the mean statistic retardation and relaxation times, and the parameters of retardation and relaxation time distributions have been estimated from the corresponding creep, elastic relaxation, and stress relaxation curves. Changes in the above characteristics with an increase in the draw ratio are analyzed.     

Creep and yield behaviour of polyethylene

Extensive studies of the creep and recovery behaviour of oriented polyethylene have been undertaken because of the potential engineering applications of high modulus polyethylene fibres and large section oriented materials made by the Leeds die-drawing process. Two primary requirements are a valid protocol for the practical assessment of the creep behaviour and an adequate formal representation which can provide the basis for a physical interpretation of the creep processes in terms of the structure of the oriented materials. The practical requirements of improved creep performance lead to examination of the effects of molecular weight and copolymerisation on creep and recovery, together with studies of the effects of radiation cross-linking either before or after drawing. A particular new feature has been the development of a process for cross-linking in acetylene, where it has been shown that diene bridge crosslinks are produced with very little chain scission. The protocol adopted for assessing creep behaviour is to construct creep strain rate/strain (Sherby-Dorn) plots which, for highly oriented fibres, show constant strain rates after an initial period. These equilibrium strain rates can be modelled by two thermally activated processes acting in parallel. In recent research these studies have been extended to fibres of lower draw ratio and to isotropic polyethylene. It has been possible to establish links between creep and yield behaviour, and to show that slow crack propagation can be understood in terms of a combination of yield and creep to failure.     

Specific features of the electret effect in two-layer polymeric films

Two-layer film corona-electrets based on low-density polyethylene and copolymers of ethylene with vinyl acetate of various brands were studied. The dependence of the electret characteristics of two-layer corona-electrets on the percentage content of polar groups in the copolymer constituting the bottom layer was analyzed. Data on thermally stimulated depolarization and thermally stimulated relaxation of the surface potential served as a basis for suggesting a mechanism of depolarization of two-layer corona-electrets.     

The influence of short-chain branching on the creep behavior of oriented polyethylene,and its effect on the efficiency of crosslinking by electron irradiation

Studies have been made of the creep behavior of oriented (15:1) polyethylenes containing 0.4 and 1.3 butyl branches per 1000 C atoms. Increasing the branch concentration reduces significantly the creep strain and the equilibrium strain rate. The data have been fitted to an established model comprising two thermally activated processes in parallel, relating to the amorphous network at low stress, and the crystal phase at high stress. Analysis based on this model indicates the similarity between branching, entanglements, and crosslinks on the creep response. The creep behavior of electron-beam-irradiated materials shows that increasing the branch concentration makes the polyethylene more susceptible to mainchain scission, indicated by increased creep flow rates at higher stress, consistent with previous rubber elasticity studies. Irradiation in an acetylene atmosphere with low (< 1 Mrad) doses is shown to reduce the creep rates at all accessible stresses, and this attributed to an increase in crosslinking compared with scission. © 1994 John Wiley & Sons, Inc.     

Chemical origin of thermally stimulated discharge currents in polyacrylonitrile

Thermally stimulated discharge has been used in the past to diagnose and estimate the magnitude of electrical polarization in polymers. Nevertheless, molecular characterization of operative phenomena by this technique alone is often a difficult task. In the present work, infrared attenuated total reflection spectroscopy has been used to investigate the origin of thermally stimulated discharge currents near 200°C in externally unpolarized films of polyacrylonitrile (PAN). Spectroscopic analysis of thermally degraded films reveals some unsaturation of the PAN backbone and possibly the generation of cyanide ions. Opposite surfaces in a solvent-cast film give different spectra, indicating a gradient in chemical degradation products across the film thickness. Data suggest that nonuniform generation of charged species and unsaturated bonds gives rise to internal potentials in PAN. The origin of thermally stimulated currents in PAN near 200°C is thus believed to be associated with the onset of chemical degradation.     

聚苯乙烯及其合金的热释放电流研究

聚苯乙烯及其合金的热释放电流研究盛京，李凤奎，胡静（天津大学材料科学与工程系，天津，３０００７２）关键词聚苯乙烯，聚苯乙烯松弛，热释电谱，聚苯乙烯合金，乙丙橡胶合金用热释放电流（ＴＳＣ）法研究聚苯乙烯（ＰＳ）的松弛过程只见Ｓｈｉｎｉｃｈｉ的报道［１］...     

Photo- and thermally stimulated relaxation processes in pre-irradiated atomic solids

Atomic and electronic relaxation processes in pre-irradiated atomic solids are considered using preliminary irradiated by an electron beam solid Ar as an example. The results of real-time-correlated study are presented for the first time with an accent on recently found anomalous low-temperature sputtering of Ar atoms from the surface. The experiments were performed using a set of activation spectroscopy methods—thermally stimulated exoelectron emission (TSEE) thermally stimulated luminescence (TSL) in combination with measurements of the sputtering yield. Solid evidence of thermally induced charge recombination mechanism is obtained. Mechanism of energy transfer based on the crowdion model is discussed. Photo-stimulated sputtering from pre-irradiated rare gas solids (RGS) is predicted.     

聚合物中激化的冷冻和热刺激

用等速升温、阶路升温、剩余电荷等方法测量了聚丙烯的热刺激电流。对于α和β峰，冷冻电荷方法有更高的分辨率和对称的线型。热刺激电流包含了快极化和慢极化机构的贡献，但线型主要决定于后者。慢极化涉及聚合物长分子链局部链段的运动与束缚空间电荷的相互作用。     

The evolution of the viscoelastic retardation spectrum during the development of an epoxy resin network

The evolution of the viscoelastic behavior of an epoxy resin at various stages of curing has been followed with the changes in the retardation spectrum. The creep J(t) and recoverable creep compliance J_r(t) curves of the neat epoxy resin Epon l00lF (Shell) were determined at temperatures between 30 and 77°C. The viscosity decreased over 8 orders of magnitude as the temperature was increased. Specimens with eight stages of network development were prepared by reacting all of the epoxy resin's oxirane rings with amine hydrogens from varying ratios of a monofunctional amine (methyl aniline) and a tetrafunctional amine 4,4′-diamino diphenyl sulfone (DDS). Preparations in which 25, 35, and 40% DDS were used did not result in a molecular network, so they were viscoelastic liquids. With 45% DDS, the product had a nascent network and was judged to be just beyond the point of incipient gelation. The remaining preparations from 0.50, 0.60, 0.70, and 1.0 DDS yielded tighter less compliant molecular networks. The creep and recoverable compliance curves were measured over a range of temperatures above the glass transition temperature, T_g. They were reduced to T_g, and retardation spectra L(ln τ) were calculated.     

A model for non-linear creep in polypropylene

Measurements of the creep behaviour of a polypropylene polymer under uniaxial tension have been modelled using a stretched exponential function with four parameters. Non-linear behaviour arises because one of the parameters, related to a mean retardation time for the relaxation process responsible for creep, is dependent on stress. Creep curves measured under a uniaxial tensile stress and a uniaxial compressive stress of the same magnitude are different. The differences can be described by relating the retardation time parameter to an effective stress that is determined by the magnitude of both the shear component of the stress and the hydrostatic component. This analysis has then been generalised to enable expressions to be formulated for creep behaviour under an arbitrary multiaxial stress state. This requires an assumption that either the Poisson's ratio or the bulk modulus is independent of time. The validity of this assumption has been evaluated through comparisons of predictions of creep under a pure shear stress with measurements, which show that a time-independent Poisson's ratio is the better approximation. Although not the main theme of the paper, examples are given illustrating the dependence of model parameters on the structure of the crystalline and amorphous regions of the polymer. This is particularly relevant to the application of the model to the analysis of the creep behaviour of welded polypropylene where properties will, in general, be influenced by the heat treatment.     

BaFCI:Tb晶体的热释发光和光激励发光

BaFCl:Tb晶体的光激励发光和热释发光过程为:在X射线辐照下,晶体中产生F色心,同时掺杂的Tb³⁺离子俘获空穴形成Tb⁴⁺离子。X线辐照后的晶体在可见光或热激励下,F色心中释放出的电子与Tb⁴⁺复合成Tb³⁺并处于激发态,随后发出Tb³⁺)子的跃迁辐射。     

A thermoreversible conformational change in two poly(zwitterions) evidenced by thermally stimulated depolarization currents

A thermally induced conformational change of zwitterionic groups of two polymers has been observed by thermally stimulated depolarization currents. Different dipole moments of the two conformations is a prerequisite. The appearance of peaks of opposite sign confirms the process. Giant dielectric constants can be obtained at room temperature using high-temperature poling and rapid cooling.     

A thermally stimulated discharge currents study of the molecular motions in two polysiloxane side-chain liquid crystalline polymers

The technique of thermally stimulated depolarization currents was used to study the dipolar relaxation mechanisms in two side-chain liquid-crystalline polysiloxanes. The studied polymers have the same mesogenic side-group, but they differ in the number (or density) of side groups attached to the main chain. In both cases three different relaxations were observed: one in the vitreous state, one in the liquid crystalline phase, and the glass transition relaxation. The features of those thermally stimulated discharges are discussed and some suggestions are made concerning the attribution of those discharges at the molecular level. © 1995 John Wiley & Sons, Inc.     

Recombination luminescence of poly(arylene phthalide) films: 1. Electronic and optical features of poly(arylene phthalide)s

Features of the luminescence and absorption spectra of poly(biphenylene phthalide), poly(fluorenylene phthalide), and poly(terphenylene phthalide) films were analyzed. Experimental results obtained for these films by optical methods and thermally stimulated current spectroscopy were compared. Long-lasting afterglow emission from the films after photoexcitation was observed. Possible explanations for the observed optical and thermally stimulated processes in the poly(arylene phthalide)s were suggested in terms of reversible electronic phase transitions induced by external factors in unconjugated polymers.     

Photo-oxidative degradation of polyvinyl alcohol and measurement of thermally stimulated current

An investigation was made of the correlation between changes in the chemical structure of polyvinyl alcohol during photo-oxidation and a thermally stimulated current. Photo-oxidation was initiated by dehydration and greatly accelerated as a result of chain scission, which resulted from the formation of carbonyl groups of ester, aldehyde, ketone and carboxylic acid types. Two main peaks of thermally stimulated current were observed at 220 and 250 K in photo-oxidized polyvinyl alcohol. It became clear that these two peaks, resulting from depolarization of dipoles, correspond to an increase in the content of carbonyl groups and a decrease in the content of hydroxyl group, respectively.