Dielectric relaxation studies of dipolar aromatics in polyethylene I. Spherulitic low-density polyethylene

Dielectric relaxation studies have been used as a means of investigating the behavior of aromatic molecules dissolved in a semicrystalline polymer. All molecules show a relaxation in the vicinity of the γ loss peak of polyethylene, while several show an additional peak at lower temperatures. The latter is interpreted as an indicator of molecular motions below the glass transition and the former is to be associated with the onset of molecular motions on a local scale as the glass transition is approached. The α loss process of polyethylene is seemingly unaffected by the presence of the molecules. Integration of loss curves shows that, at most, 50% of the molecules are able to relax but that the fraction relaxing increases with increasing temperature, suggesting the presence of a wide distribution of sites within the polymer.     

Dielectric relaxation study of gamma irradiated oriented low-density polyethylene

The influence of drawing, gamma irradiation and accelerated aging on the dielectric relaxation of low-density polyethylene has been studied using dielectric loss tangent measurements in the temperature range from 25 to 325 K and in the frequency range from 10³ to 10⁶ Hz. The intensity, position and activation energy of the γ- and β-dielectric relaxations were found to be strongly dependent upon the changes in the microstructure of the amorphous phase induced by uniaxial orientation, oxidation and crosslinking.     

Characterization and properties of polyethylene blends II. Linear low-density with conventional low-density polyethylene

Melting and crystallization phenomena in blends of a linear low-density polyethylene (LLDPE) (ethylene butene-1 copolymer) with a conventional low-density (branched) polyethylene (LDPE) are explored with emphasis on composition by differential scanning calorimetry (DSC) and light scattering (LS). Two endotherms are evident in the DSC studies of the blends, which suggests the formation of separate crystals. Light-scattering studies indicate that the blend system is predominantly volume filled by the LLDPE component whereby the LDPE component crystallizes as a secondary process within the domain of the LLDPE spherulites. In contrast to those of the LLDPE/HDPE blends, the mechanical and optical relaxation behavior of the LLDPE/LDPE blends are dominated by the LLDPE component in the vicinities of γ and β regions, whereas the trend reverses at high temperature α regions. This observation is accounted for on the basis of the relative restrictions imposed by the deformation of spherulites (which are primarily made up of the LLDPE component) at different time scales.     

Chain diffusion in semicrystalline polyethylene: Dielectric relaxation

The experimental data on high-temperature dielectric relaxation αcin linear PE samples with different thermal prehistories (slow cooling, quenching, and annealing) are described. The measurements are conducted at temperatures ranging from 0 to 80°C. Dielectric losses are measured with a high accuracy at frequencies varying from 10^?2 to 10–10⁶ Hz. These dielectric measurements allow one to reveal changes in the frequency dependence of losses with temperature. This effect of the thermal prehistory of the sample and applied stress is explained within the framework of the molecular model of chain diffusion between crystalline and amorphous phases in PE.     

Oriented crystallization of biphenyl in low-density polyethylene with a single texture

Doubly oriented specimens with a single texture can be obtained by unidirectional rolling of a sheet of low-density polyethylene. Swelling of the oriented samples with liquid biphenyl and in situ crystallization of the biphenyl give indirect information about the morphology of the polymer. In such samples, annealed a few degrees below the melting temperature, the orientation of the biphenyl crystals is a consequence of the interaction of the two crystalline lattices. The (001) biphenyl planes are parallel to the (h01) limiting planes of the lamellar polymer crystals. Theoretical considerations show that the epitaxial conditions are best fulfilled when the limiting planes of the lamellas are parallel to (201) planes. The experimental value of h is 3.     

Dielectric relaxation of polyethylene below 4° K

Summary Measurements of tan for polyethylene for frequencies around one kHz down to temperatures of 4.2° K show an increase in the dielectric loss (Fallou andBobo) (1). We have repeated these measurements with very sensitive equipment over a range of frequencies up to several kHz and down to temperatures of 1.2° K. We find maxima in tan as a function of temperature and frequency. It thus seems possible for there to be an activated mechanism at these very low temperatures. The values of the activation energy are of the order of only some cal/mol instead of the more usual kcal/mol. The loss peaks are observed in as received material, i. e. fabricated films and pressed plates. Additives and contaminations do not seem to play any role. The loss peaks are observed in stretched, particularly stretched crystalline, material. The dielectric losses vanish for melted and then slowly cooled or quenched material. Molecular rearrangements in the interface between crystalline and non-crystalline regions of the material may play a role in causing these losses.

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Zusammenfassung Messungen des tg von Polyäthylen mit Frequenzen bis zu einigen kHz und für Temperaturen bis herab zu 4,2° K zeigen einen Anstieg des dielektrischen Verlustes (Fallou undBobo) (1). Um zu sehen, ob dieses Verhalten real ist, wiederholten wir diese Experimente mit einer sehr präzisen Anordnung für Temperaturen bis herab zu 1,2° K. Wir finden Maxima für tg als Funktion der Temperatur, in der Lage abhängig von der Frequenz. So existieren also auch innerhalb des tiefsten Temperaturbereiches noch Möglichkeiten für aktivierte Mechanismen. Aus den experimentellen Daten ergeben sich Aktivierungsenergien der Größenordnungen von nur einigen cal/Mol, statt der üblicherweise bekannten kcal/Mol.Die aktivierten Prozesse treten für Material im Lieferzustand wie empfangen auf. Das bedeutet, für Material verarbeitet zu Folien und zu gepreßten Platten. Zuschläge und Verunreinigungen dürften keine Rolle spielen, aber zum Beispiel ein Verstrecken. Höhere Kristallinität vergrößert den Verlust, nach Aufschmelzen und Abschrecken fallen die Verluste unter die Meßgrenze. Es gibt einige Gründe, daß die Molekülanordnung in den Bereichen zwischen Kristall und amorph eine wesentliche Rolle spielen.

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One of us(O. Heybey) thanks theAlexander-von-Humboldt-Stiftung for a fellowship which enabled him to carry out this work at the Institut für Polymere in Marburg.

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We thankG. Knipsel for carrying out some of the measurements.We thank the members of the shop, particularlyG. Strohriegl andH. Sprenger, for their help in constructing the apparatus.Thanks, too, to theRöhm-Stiftung, for supporting our investigations in the polymer fields.     

Dielectric relaxation studies of aromatic polyamides

Dielectric relaxation spectroscopy (DRS) is presented for a family of four aromatic polyamides trying to relate the structure of the lateral groups to the molecular mobility. A prominent sub-T_g absorption is always seen followed in some cases by remanent dielectric activity at room temperature and a subsequent increase of the loss permittivity. The low temperature relaxation is analyzed in terms of a Fuoss–Kirkwood equation to obtain the broadness and the strength of these relaxations as well as the activation energy (ranging from 10 to 11 Kcal/mol). The low frequency conductive peak shows in each case a half-width higher (1.30) than those corresponding to a single relaxation time peak (1.144). These values of the half-width are an indication of the complex character of these phenomena. A final discussion of the rotational barriers of the lateral chains rules out that such motions are the only molecular origin for the gamma relaxation. Instead, some kind of motion involving the main chain and where the interchain interactions play a significant role should be considered as responsible for that relaxation. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 919–927, 1997     

Crystal orientation relaxation studies of polyethylene

Wide-angle x-ray relaxation studies were performed on films of low- and high-density polyethylene (PE). The samples were rapidly stretched by a pneumatic stretcher and held at a fixed length. The rate of crystal reorientation was monitored by measuring diffracted intensity changes using a count-rate meter with output displayed on a storage oscilloscope as well as a digital counter-timer. Experiments were completed at several temperatures and activation energies comparable to those reported for the α₁ relaxation in PE were obtained.     

Dielectric friction and solvation dynamics: Novel results on relaxation in dipolar liquids

In this article we present a new, general but simple, microscopic expression for time-dependent solvation energy of an ion. This expression is surprisingly similar to the expression for the time-dependent dielectric friction on a moving ion. We show that both the Chandra-Bagchi and the Fried-Mukamel formulations of solvation dynamics can be easily derived from this expression. This expression leads to an almost perfect agreement of the theory with all the available computer simulation results. Second, we show here for the first time that the mobility of a light solute ion can significantly accelerate its own solvation, specially in the underdamped limit. The latter result is also in excellent agreement with the computer simulations.     

Thermooxidative degradation of polyethylene. I and II. Structural changes occurring in low-density polyethylene,high-density polyethylene,and tetratetracontane heated in air

Samples of low-density polyethylene (LDPE), high-density polyethylene (HDPE), and tetratetracontane (n-C₄₄H₉₀) free from additives were heated in air at temperatures between 120 and 180°C. As a comparison, “as received” HDPE containing unspecified additives has also been included. The structural changes have been studied with gel chromatography, viscometry, infrared spectroscopy, differential scanning calorimetry, and gravimetric measurements. LDPE, HDPE, and n-C₄₄H₉₀ follow the same course of thermooxidative degradation when they are free from additives and present in the molten state. Both molecular-diminishing and enlargement reactions occur. At temperatures below 150°C molecular enlargement is not observed until after rather long exposure times, whereas at higher temperatures enlargement occurs immediately. The difference is because “peroxide curing” becomes increasingly important above 150°C, whereas ester formation is operating at all temperature levels. Degradation below T_m is restricted to the amorphous phase that results in a different degradation pattern. In accelerated testing work extrapolations of the Arrhenius type in the prediction of structural change are thus not justified, even within the actual narrow temperature range. Neither are changes in commonly used standards like carbonyl content justified as a measure of the changes; for example, in mechanical properties. The stabilizer in the unpurified HDPE not only influences the induction period but also the course of the thermooxidative degradation.     

Dielectric relaxation processes of some nearly spherical dipolar organic molecules in a few glassy media

Summary Dielectric absorption studies have been carried out on some dipolar, almost spherical organic molecules (camphane derivatives) in a polystyrene matrix in the frequency range 10–10⁵ Hz between 80 and 300 K. Some molecules have also been studied in other glass forming media such as carbon tetrachloride, glassy-o-terphenyl and polyphenyl ether (Santovac). All of these molecules show dielectric relaxation due to molecular reorientation in each dispersion medium with characteristic activation parameters. Alcohol molecules do not show dispersion due to hydrogen-bonded species below five percent concentration, a range usually used in dielectric studies. Relaxation due to the hydrogen-bonded species is observed at higher concentrations. High activation parameters for molecular reorientation of camphane derivatives are accounted for by the strong intermolecular interactions at low temperatures.

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Dielektrische Relaxationsprozesse einiger nahezu sphärischer dipolarer organischer Moleküle in glasartigen Medien

Zusammenfassung Es wurden die dielektrischen Absorptionen einiger dipolarer, nahezu sphärischer organischer Moleküle (Camphan-Derivate) in einer Polystyrolmatrix im Frequenzbereich 10–10⁵ Hz zwischen 80 und 300 K untersucht. Für einige Moleküle wurden auch andere glasbildende Medien herangezogen, z.B. Tetrachlorkohlenstoff, glasigeso-Terphenyl und Polyphenylether (Santovac). Alle untersuchten Moleküle zeigten eine dielektrische Relaxation mit charakteristischen Aktivierungsparametern, die auf molekulare Reorientierung in den jeweiligen Dispersionsmedien zurückzuführen sind. Alkoholmoleküle zeigen wegen wasserstoffbrückengebundener Spezies bei einer Konzentration unter 5% (ein bei dielektrischen Untersuchungen üblicher Bereich) keine Dispersion. Bei höheren Konzentrationen wird wegen wasserstoffbrückengebundener Spezies Relaxation beobachtet. Die hohen Aktivierungsparameter für die molekulare Reorientierung von Camphan-Derivaten wird auf starke intermolekulare Wechselwirkungen bei tiefen Temperaturen zurückgeführt.

     

Molecular association in polyethylene glycol II. Light scattering studies

Summary The present light scattering measurements reaffirm the fact that the polyethylene glycol molecules are associated in aqueous solution through the formation of both intra- and inter-molecular hydrogen bonds. The rupture of such association by the addition of either urea or thiourea is also noticed.

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Zusammenfassung Die hier vorliegenden Lichtstreuungsmessungen bekräftigen die Tatsache, daß Polyäthylenglykol-Moleküle in wäßrigen Lösungen durch Bildung von intraund inter-molekularen Wasserstoffbrücken assoziiert sind. Durch Hinzufügen von Harnstoff und Thioharnstoff brechen diese Brücken auf.

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With 1 figure and 1 table     

Dielectric relaxation studies of monoalcohol solutions with 1,2-diols

Dielectric relaxation processes in solutions of monoalcohols with ethanediol, 1,2-propanediol, and 1,2-butanediol have been studied using the dielectric time domain spectroscopy technique. The dielectric spectrum has been analyzed in terms of two dispersions. The influence of varying content of diol, and of chainlength of the alcohol, on the static dielectric constant _s and on the main dispersion is reported. For the long-chain alcohols _s is lowered and the main relaxation time successively shortened with the addition of diol to the solution. The results are compared to these found for aqueous alcohol solutions and discussed in terms of a structural model with rupturing of predominantly linear complexes on addition of water, diols, or on increasing the temperature.     

Dielectric relaxation studies in transition metal ions doped tellurite glasses

Vanadotellurite and vanado-cobalt tellurite glasses were investigated for dielectric properties in the frequency range 50 Hz to 5 MHz and temperature range 300–500 K. Hunt's model has been invoked to analyze the data and the results indicated that the relaxation process has a local character and can be described by hops between each pair of sites. Activation energies for dielectric loss process were determined and they were in close agreement with dc activation energies. Barton, Nakijama and Namikawa's (BNN) linear relation between ac and dc conductivities has been verified in both the glass systems.     

Electron trapping in low-density polyethylene

Thermoluminescence (TL) emission from low-density polyethylene has been investigated. The glow curves of gas-free samples x-irradiated at ?190°C and heated to room temperature were found to contain three peaks numbered I, II, and III in order of increasing temperature, in agreement with earlier results. The sites of all traps are accessible to absorbed gases; in the presence of air, O₂, N₂ or Ar, “gas” traps are formed, resulting in the appearance of an additional peak IV in the glow curve at a temperature between peaks I and II, large reductions in the intensities of peaks II and III, and various changes in peak I. The peak I, II, and III traps are formed from particular chain configurations occurring in the chain-fold regions of the samples, these configurations being broken up by different forms of molecular motion within the chains. It is unlikely that the peak IV traps are just the gas molecules themselves; they are probably formed from new chain configurations occurring in the amorphous regions of the samples in the presence of the gas, the properties of the gas influencing the associated TL intensity and emission temperature. These traps are also broken up by molecular motion. The samples can be divided into two main types, differing mainly in the height of peak I relative to peak II, which is of nearly constant intensity in all samples. We suggest that two types of trap which are not interconvertible are associated with peak I, and that the dominant type in a given sample depends on the fine details of the sample fabrication process.     

Long-chain branching in low-density polyethylene

A method is given for the analysis of long-chain branching in polymers by using combined GPC and intrinsic viscosity measurements. A computer program was written to evaluate branching indices by a tabular, iterative method. The method was applied to the evaluation of long-chain branching in low-density polyethylene.     

Oriented polymers from solution. II. Fibers and films of polyethylene/polypropylene blends

A version of the surface growth method by Zwijnenburg and Pennings is being used to produce blended fibers of polyethylene and polypropylene under conditions where polypropylene does not yield to this fiber growth technique but would grow films. The blended fiber contains intimately mixed crystalline regions of each pure component and combines thermal and mechanical properties of both. The growth method itself underlines the part played by adsorbed surface layers and entanglements within such layers which can involve the entanglement of chemically different species. This feature suggests a potentially new method for blend production.     

Characterization and properties of polyethylene blends I. Linear low-density polyethylene with high-density polyethylene

A blend system of linear low-density polyethylene (LLDPE) (ethylene butene-1 copolymer) with high-density (linear) polyethylene (HDPE) is investigated by differential scanning calorimetry (DSC), wide-angle x-ray diffraction (WAXD), small-angle x-ray scattering (SAXS), Raman longitudinal-acoustic-mode spectroscopy (LAM), and light scattering (LS). For slowly cooled or quenched samples, one single endotherm is evident in the DSC curve which depends on the composition. No separate peaks are observed in the WAXD, SAXS, Raman-LAM, and LS studies on the LLDPE/HDPE blends. This observation along with the fact that no peak broadening is observed suggests that these peaks are associated with the presence of a single component. In no case did we see double peaks or a broadened peak that might be associated with two closely spaced unresolved peaks. This suggests that segregation has not taken place at the structural levels of crystalline, lamellar, and spherulitic textures. A single-step drop in the scattered intensity (I_Hv) as a function of temperature is seen in the LS studies. It is therefore concluded that cocrystallization between the LLDPE and HDPE components occurs. The mechanical and optical α, β, and γ relaxations of these blends are explored by dynamic birefringence. The 50/50 blend displays the intermediate relaxation behavior between those of the components in all α, β, and γ regions. This observation is reminiscent of the characteristic of the typical miscible blends.