Thermoluminescence from extended-chain crystals of polyethylene

Thermoluminescence (TL) has been observed in γ-irradiated extended-chain crystals of polyethylene above room temperature. The TL curve, which exhibits four peaks at 50, 90, 120, and 140°C, is different from that given by folded-chain crystals, in both shape and intensity. In particular, a shape, strong glow peak is observed at 140°C, corresponding to the melting temperature of the extended chain crystals. These results are discussed in relation to independent measurements by differential scanning calorimetry and electron spin resonance.     

Radiothermoluminescence of linear polyethylene with extended-chain crystals

It has been shown that a radiothermoluminescence curve of polyethylene with extended-chain crystals can be separated into components corresponding to intracrystalline and disordered regions of the polymer by introducing an electron scavenger.     

Atomic force microscopy of extended-chain crystals of polyethylene

Extended-chain crystals of polyethylene grown at elevated pressure and temperature were analyzed for the first time by atomic force microscopy. It was possible to compare the typical fracture surface striation features with those obtained earlier by electron microscopy. High resolution atomic force microscopy on flat surfaces enabled the recording of an atomic scale regularity that could not be fully indentified. © 1993 John Wiley & Sons, Inc.     

Preparation and study of separated single crystals of extended-chain polyethylene

Etching of extended-chain crystals of polyethylene is shown to permit separation of the polycrystalline aggregate into single crystals. The single crystals are analyzed by density, molecular weight (lamellar crystal thickness), melting temperature, birefringence, superheating, and linear melting rate determination. The linear melting rate depends linearly on superheating and can be described by simple rate theory.     

Thermally stimulated depolarization current spectra of cross-linked polyethylene and the influence of cross-linking byproducts

Cross-linked polyethylene (XLPE) is notable for its use as power cable insulation. Its longevity is limited by space charge buildup linked to impurities such as the byproducts left behind by the cross-linking agent dicumyl peroxide (DCP). The goal of this work is to determine the impacts of these byproducts on charge trapping and detrapping in XLPE using the thermally stimulated depolarization current technique. XLPE with byproducts has one source of trapped charge, which originates from the byproducts. XLPE that was thermally treated via degassing exhibits two other sources of trapped charge, which are charge injection and dipolar relaxations. Oxidation from degassing was shown to control the trapping from these sources, which is useful knowledge for processing this material prior to its use. Reintroducing acetophenone, one of the major byproducts of DCP, suppresses those two peaks once more, showing that it controls the overall space charge buildup characteristics in XLPE.     

On the interfacial energy of extended-chain crystals from polyethylene melt by revised Flory equations of fusion

To analyze extended-chain crystalline systems composed of linear polyethylene, Flory's conventional theory of fusion is reconsidered by introducing a new concept of crystallinity. When this new treatment is applied to a melting case of a low molecular weight polyethylene fraction (M_n = 5600) isothermally bulk crystallized, a certain result that very large lamellar thickness was caused by a very small increase in crystallization temperature can satisfactorily be explained by a significant change in interfacial free energy of the crystallite end. Further, it shows 14–17 kJ/mol as a nonequilibrium value range of interfacial free energy for highly crystalline polyethylene fractions of low molecular weight M_n ≦ 5600 by using the previous data presented by other workers. A similar result is also obtained on the M_n = 5600 fraction by analyzing from a standpoint of equilibrium crystallinity. In either case, the estimated range of interfacial free energy is consistent with the conventional range. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1293–1303, 1998     

DTA and x-ray studies on extended-chain crystals of polyethylene under high pressure

The relation between the thermal behavior of extended-chain crystals (ECCs) of polyethylene and the phase transitions, i.e., orthorhombic ? hexagonal ? melt, of polyethylene at high pressures above about 400 MPa has been studied by high-pressure differential thermal analysis (DTA), and with a high-pressure and high-temperature x-ray diffraction apparatus equipped with a position-sensitive proportional counter measuring system. The original sample used in this study consists mainly of two kinds of ECC, which we designate as “ordinary extended-chain” crystals (OECCs) and “highly-extended-chain” crystals (HECCs). Experimental results at pressures below 300 MPa substantiate the results previously reported: i.e., the phase diagram indicating the relation between the melting temperatures and pressure for the OECCs and HECCs can be determined for pressures up to 500 MPa. In heating at pressures above about 500 MPa, the peak intensity of the (100) reflection of the hexagonal structure decreases in two stages with increasing temperature. The phenomenon corresponds to the thermal behavior determined by high-pressure DTA in which two small endothermic peaks can be observed at temperatures above that of the crystal transition evidenced by the strong peak. This phenomenon suggests melting in two stages of hexagonal structures with different thermal stabilities, and that the change at higher temperature may be due to fusion of the hexagonal phase annealed either below or above the transition temperature.     

Thermally stimulated depolarization current of novolac resin

A novolac phenol-formaldehyde resin was investigated via the thermally stimulated depolarization current, using integral and partial measurements in the temperature range from 137 to 270 K. Tow broadened peaks, assigned as Β₁, and Β₂, appeared at about 160 and 190 K. The influence of water and ¯M_n was investigated. The activation energy E_a vs. T relationship was analysed, and a search was made for compensation phenomena. The distribution of the relaxing dipolesN vs. E_a was approximated. Three different relaxation ranges were distinguished. All the motions were attributed to the rotation of the phenyl rings. The differences found are due to the heterogeneity in the resin. Contamination with water increases the polarizability, decreases the structure differences and relieves the motions in the resin. A resin with a smaller ¯M_n exhibits a higher polarizability and a decreases in structure variety.     

Thermally stimulated and isothermal depolarization currents in low-density polyethylene

Thermally stimulated and isothermal depolarization currents of three low-density polyethylenes are investigated in the temperature range 80–320°K. The thermally stimulated current spectra can be analyzed in terms of a continuous relaxation time spectrum. This spectrum can be resolved into three Gaussian distributions of activation energies centered near 140°K (γ-relaxation), 205°K, and 245°K (β-relaxation) in agreement with electrical and mechanical loss factor measurements. The experimental observation are explained in terms of reorienting dipoles, coupling the molecular motion to the electrical field.     

Thermally stimulated current study of the microstructure of peek

Thermally Stimulated Current (TSC) spectroscopy and Differential Scanning calorimetry (DSC) have been applied to the characterization of the microstructure of Poly (Ether Ether Ketone)/PEEK. the dielectric relaxation spectra show two modes, dependent upon crystallinity:

* for the mode stuated in the vicinity of the glass transition temperature, two components have been distinguished and attributed to the molecular mobility in the ‘true amorphous phase’ and in the ‘rigid amorphous region’.

* below 0°C, two sub-modes appear, situated around-110°C and-75°C, due to the two different crystal entities, beads and laths.

     

Thermally stimulated current investigation of copper octaethylporphyrin dimer Langmuir-Blodgett films

Langmuir-Blodgett films containing meso,meso'-buta-1,3-diyne-bridge Cu(II) octaethylporphyrin dimer have been deposited with the aim of carrying out a thermally stimulated current (TSC) investigation. This characterization was obtained upon cooling samples irradiated by light and others without irradiation: in this way TSC curves have been registered in the temperature range of 100-300 K. Analysis of experimental data has been performed through the application of three different approaches: the initial rate, the heating rate, and Chen's methods. In particular, a trap center at about 0.38 eV has been evidenced and the three different methods have given results in close accordance.     

Deformation and microstructure of extended-chain polydiacetylene crystals

The mechanical properties (Young's modulus, ultimate tensile strength, deformation processes) of extended-chain polydiacetylene crystals are investigated. The properties observed are similar to those of metal and ceramic whiskers. The elastic modulus is strain-dependent and the ultimate tensile strength increases with decreasing crystal size. The maximum tensile strength observed was 1700 Nmm^?2. The ultimate tensile strength seems to be controlled by the presence of a small number of defects near the surface at which fracture nucleates. Irreversible deformation of the crystals was observed to occur by crack propagation normal and parallel to the direction of the macromolecules. The observed mechanical behavior corresponds to exceptionally high per-chain properties. The per-chain modulus obtained for these crystals is nearly as high as that of diamond. A chain-aligned polyethylene fiber with the same per-chain mechanical properties would have an ultimate strength as high as 0.9 × 10⁴ Nmm^?2.     

Thermally stimulated current study of the molecular movements in bone

Thermally Stimulated Current (TSC) has been used for investigating the interface and interphase in a natural composite, bone. Young adult male femoral diaphysis has been studied at different stages of demineralization by EDTA. Analysis of the fine structure of complex TSC spectra has shown that in the early stages of demineralization, dielectric relaxations are due to the hydroxyl reorientations in the mineral apatite structure. After a certain stage of demineralization, the mineral response disappears and organic matrix mobility is observed. A dissociative buffer has been used to identify collagen relaxations. The contribution of the mineral-organic interface/interphase has been isolated.     

Triplet exciton stimulated current pulses in anthracene crystals

Current pulses of nanoseconds duration accompanied by delayed luminescence are induced by triplet excitons in the presence of a steady state hole current. The current pulses set in at 15.kV/cm and are brought about by the cooperation of two triplet excitons. The current pulse probability depends on the nature of the cathode. Current and luminescence pulses are attributed to triplet exciton induced free electrons in the vicinity of a blocking cathode.     

Thermally stimulated exoelectron emission from solid neon

In spite of the negative electron affinity of Ne atoms, appreciable concentrations of electrons can be trapped in solid neon layers formed by depositing the gas on a cold substrate with concurrent electron irradiation. These are trapped at defect sites, and can be promoted into the conduction band in an annealing experiment. They can then recombine with positive charges producing vacuum ultraviolet "thermoluminescence," but can also be extracted from the solid, and detected as an "exoelectron" current. The thermally stimulated exoelectron emission profiles of the electron current versus temperature reveal two broad features near 7.5 and 10 K. These are shown to correspond to two distributions of electron trapping sites with slightly differing activation energies. For the narrower, higher temperature maximum, an average activation energy of about 23 meV is deduced, in good agreement with predictions based on the theory of electronic defect formation.     

Crystallization and melting of polyethylene under high pressure. III. Mixed crystallization of two kinds of extended-chain crystals

Thermal behaviors of extended-chain crystals of polyethylene formed during various crystallization processes and conditions under about 5000kg/cm² were studied by high-pressure dilatometry and differential scanning calorimetry. The experiments indicate that by isothermal crystallization at small undercoolings for prolonged periods, the products show two endothermic peaks in the melting region of the usual extended-chain crystals. This means of the presence of a bimodal lamellar thickness distribution in the extended-chain crystals. A phase diagram has been made for pressures up to 5000 kg/cm². The experimental results confirm the existence of two kinds of extended-chain crystals, i.e., ordinary extended-chain and highly extended-chain crystals, as suggested previously by the authors.     

Thermally stimulated depolarization currents of crosslinked polyethylene relaxations in the fusion range of temperatures

With thermally stimulated depolarization currents, we researched the relaxations of crosslinked polyethylene as it is used in medium‐voltage cable insulation. Through conventional polarization two heteropolar peaks stand up in the spectra, at 80 and 105 °C. As the sample is annealed, a homopolar peak is developed at about 99 °C. With window polarization, our results indicated that the 80 °C peak is a structured peak related to polar crosslinking subproducts and impurities. The 105 and 99 °C peaks are fitted to the general kinetic‐order model because the 105 °C peak is related to free‐charge detrapping at the crystalline phase, in the bulk and maybe at the amorphous‐crystal interphases, and the peak that is observed at 99 °C is due to injected charge. Annealing at high temperatures promotes the creation of traps in the material. Charge trapping at T < 70 °C seems to be related to the increased insulator resistivity with annealing time. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1412–1421, 2003     

Current reversal in the thermally stimulated current spectra of polyethylene

Polyethylene thermoelectrets were prepared under static electric fields ranging from 70 to 750 kV/cm at various polarization temperatures. Thermally stimulated current (TSC) spectra were observed for those samples in the high-temperature region above room temperature. The TSC spectra of polyethylene were complex and were composed of two current peaks, of opposite polarity and dependent on the applied voltage and temperature. This result suggests that there are two trapping mechanisms which result in different charge injection modes. In addition, TSC spectra were obtained on polyethylenes of various morphologies to examine the relationship between the mechanical relaxations and the trapping mechanisms.