Differential scanning calorimetry of chlorinated polyethylenes

The heats of fusion of low-density polyethylene and chlorinated polyethylenes of varying chlorine content have been investigated by differential scanning colorimetry (DSC). Linear correlations are found between the chlorine content and the apparent heat of fusion, crystallinity, and the maximum specific heat measured in the temperature range 90–120°C. The effect of heat treatment on crystallinity is shown to be strongly dependent on the chlorine content of the polymers.     

Thermal behavior and characterization of solid-state chlorinated polyethylenes

The melting behavior of solid-state chlorinated polyethylene samples of various average molar mass has been studied by employing the differential scanning calorimetry method. Both the thermal and thermodynamic characteristics of the as-prepared chlorinated powdery polymer samples were found to be essentially different, compared to those of the melt-crystallized ones. The changes in the corresponding values depended mainly on the content of the comonomer units such as chloromethylene and dichloromethylene groups and their distribution along the polymer chains. Therefore, it can be assumed that the order of the polymer crystals arrangement is disturbed in the course of both the chlorination process and the subsequent melt crystallization.     

A study of the character of molecular rotation of nitroxyl radicals in liquids by ESR and dielectric relaxation methods

The possibility of combined application of ESR and dielectric relaxation methods in determining the character of molecular rotation of organic nitroxyl radicals in nonpolar liquids is studied for solutions of 2,2,6,6-tetramethyl-piperidine-N-oxide in decaline. Comparison of the two correlation times for rapid radical rotation obtained by both methods provides evidence for an essentially jump-like rotation of this radical.     

Vitamin E and related ether-phenols: A low-temperature dielectric relaxation study

Within a classs of phenols comprising durenols and methylated chromenols, chromanols and coumaranols, hydrogen-abstraction rate constants, and therefore radical-scavenging and antioxidant effectiveness are closely paralleled by low-temperature relaxation rates, and are apparently influenced in similar proportions by the inductive and resonance effects of para-alkoxy groups. Relaxation rates of 2,3,5,6-tetrachlorophenol and 2,6-di-tertiary-butyl phenol with and without 4-methoxy substituents are also reported and discussed.     

Hydration of monosaccharides: A study by dielectric and nuclear magnetic relaxation

NMR studies indicate that the relaxation rate of¹⁷O-enriched water is enhanced in monosaccharide solutions, and it is greater in hexose solutions than with a pentose, ribose. Three dielectric relaxations have been isolated and assigned to bulk water相似文献   

Microwave dielectric spectra and molecular relaxation in formamide-N,N-dimethylformamide binary mixtures

The dielectric dispersion and absorption spectra of formamide (FA), N,N-dimethylformamide (DMF) and their binary mixtures are investigated in the frequency range of 500 MHz to 20 GHz at 30 °C in view of the organic synthesis by microwaves heating using amides solvents. The concentration dependent values of molecular reorientation relaxation times lower than that of the ideal mixing behaviour have been attributed to the cooperative dynamics of H-bonded FA–DMF structures. The molar ratio of stable adduct is 2:1 of FA to the DMF, which is determined from the concentration dependent excess static dielectric constant and the relaxation time plots of these binary mixtures. Electrode polarization effect and ionic conduction in FA and DMF were investigated from their dielectric dispersion spectra in the low frequency region of 20 Hz to 1 MHz.     

A Raman-spectroscopic study of solution-crystallized polyethylenes

An analysis of the Raman internal modes of dilute-solution-crystallized homopolymers and co-polymers of ethylene has been made, similar to the work previously reported for the bulk-crystallized polymers. The crystallite structure can be described in terms of the relative amounts of the crystalline orthorhombic phase, the liquidlike amorphous phase, and the interfacial region. These quantities change with the molecular constitution of the chains and the crystallization conditions. The level of crystallinity decreases significantly with increasing counit content as would be expected. In addition, an appreciable interfacial structure develops in copolymers as compared with the homopolymers. A possible relationship between the interfacial content and the relaxation transitions in polyethylene is discussed.     

Studies of molecular relaxation of poly(propylene oxide) solutions by dielectric relaxation and brillouin scattering

Dielectric relaxation and Brillouin scattering are jointly used in studying molecular relaxation in poly(propylene oxide) (PPO) and its solutions in methylcyclohexane. The dielectric method was applied to the more concentrated (100%, 80%, 60%, by volume) solutions over a wide temperature and frequency range (30 Hz to 8 GHz) in order that the variation in activation energy characteristic of a glass-forming substance could be delineated. The present work extends previous work on the undiluted polymer to higher frequencies so that range of 12 decades in the dielectric loss maximum f_max as a function of temperature is now available. The “Antoine” equation is found to represent the behavior of log f_max, of the bulk concentrated solutions very well. The more dilute (40%, 20%) solutions were studied only in the high-frequency (GHz) region since phase separation occurred at low temperatures. Both the temperature and dilution effects were interpreted in terms of free-volume theory. Brillouin scattering spectra were obtained at several scattering angles and a wide range of temperatures. A maximum in the curve of hypersonic attenuation versus temperature was observed in each polymer solution. The attenuation maximum shifts toward lower temperature upon dilution, in agreement with the dielectric relaxation result. The Brillouin scattering follows different activation parameters and evidences a more rapid process than does the dielectric relaxation. It is speculated that it monitors a secondary or subglass relaxation, due perhaps, to damped torsional oscillations.     

Internal rotation of para-substituted phenols. A low temperature dielectric relaxation study

The dielectric relaxation rates of 18 phenols and 3 thiophenols have been determined in paraffin solutions at 4.2 or 77 K and used to obtain relative values of the tunnel splitting of the ground torsional state. Where comparison is possible the results are compatible with vapour phase microwave spectroscopic data. The barriers to hydroxyl rotation are calculated and are fairly consistent with barriers derived from far infrared data. Para-substituents F, CH₃, Cl, C(CH₃)₃, Br and I in order of decreasing effectiveness, lower the barrier. COOH and CHO raise it by 350–400 cm^?1.     

A dielectric study of molecular relaxations in irradiated high density polyethylene

The molecular relaxation behaviour of high density polyethylene (HDPE), exposed to gamma radiation in air to various absorbed doses (up to 700 kGy), has been investigated by dielectric loss (tanδ) analysis. All relaxation zones (γ, β and α, in order of increasing temperature), between 25 K and the melt temperature, were studied in the frequency range from 1 kHz to 1 MHz. The changes observed in the dielectric relaxation spectra were related to the modifications in the structural and morphological parameters attributed to exposure of the samples to radiation. Radiation-induced changes in the crystal fraction, oxidative degradation and degree of network formation, observed by wide angle X-ray scattering (WAXS), differential scanning calorimetry (DSC), infrared spectroscopy (IR) and gel measurements, were well connected with the changes in intensity, position and activation energy of α and γ relaxations. Complete disappearance of the already weak β relaxation with irradiation is attributed to the more restricted chain segment mobility in the net structure, but the contribution due to radiation-induced increase in crystallinity should also be taken into account.     

A molecular dynamics study of structural relaxation in tetrahedrally coordinated nanocrystals

The reorganisation of nanocrystals in order to reduce their surface energies has been examined in computer simulations. The relaxation takes a qualitatively different path for sphalerite- and wurtzite-structured particles. The surfaces of the sphalerite particles reconstruct into hexagonal nets, but the interior remains identifiable as sphalerite-like, whereas wurtzite particles form facetted, hexagonal nanorods by virtue of a reorganisation of the whole particle which involves the creation of a low energy internal interface between oppositely oriented domains. Despite the reorganisation, the diffraction patterns remain compatible with a wurtzite structure with some internal strain. The dipole moments of thermalized wurtzite particles are compared with experimental results for CdSe.     

Temperature dependent dielectric relaxation study of ethylene glycol-water mixtures

Using the picosecond time domain reflectometry method, dielectric relaxation measurements for 13 ethylene glycol (EG)-water mixtures have been studied from 0 to 40°C. The dielectric relaxations in the mixtures show a Debye-type behavior, whereas the relaxation in pure EG can be described by the Cole-Cole model. The static dielectric constant ₀, the relaxation time and the dielectric constant at high frequency have also been determined at various temperatures. The dielectric relaxation data suggests that there is no tendency to form hydrogen bonds with the addition of water to EG unlike other alcoholic systems but this tendency becomes increasingly important with decreasing temperature. The activation energy decreases with increased water content in the mixture as expected.     

On the molecular interpretation of the dielectric relaxation of nematic liquid crystals

This paper presents the results of studies of the dielectric relaxation of nematic 6CHBT obtained for different values of the angle between the directions of the macroscopic orientation of the sample (director n) and the probing electric field E. Analysis of the evolution of the relaxation spectrum from epsilon*(omega) (E n) to epsilon*(omega) (E n) allows one to explain the hitherto existing inconsistency in the molecular interpretation of the spectra. A model of the molecular dynamics in the oriented nematics is proposed.     

1H spin-lattice relaxation and observation of interfacial material in solid polyethylenes

The ¹H NMR laboratory- and rotating-frame (LF,RF) spin-lattice relaxation (SLR) behaviour in solid polyethylenes is reviewed in light of the two region spin diffusion coupled model. Highly crystalline high-density materials show single T¹ values indicative of the fast diffusion limit whilst RFSLR is close to the slow diffusion limit, allowing direct determination of the dimensions of the crystalline and disordered regions. The LFSLR and RFSLR of the HDPE's suggest the existence of an interfacial material which the SLR behaviour includes as part of the crystalline region in contrast to XRD and DSC. ¹³C CPMAS measurements of ¹H RFSLR in a LLDPE provide an additional insight into this problem. The ¹H RFSLR is best represented by a superposition of two exponentials, measured by either ¹H or ¹³C NMR. The ¹H RFSLR, measured at different chemical shifts in the ¹³C NMR spectrum, provides direct evidence of the interfacial material. The possible use of spin-locking at angles other than 90° in the rotating frame to scale the spin diffusion rates is reviewed and its possible utility for LLDPE materials explored.     

Surface molecular relaxation in smectic C* phase as studied by dielectric spectroscopy

The molecular relaxation process of a ferroelectric liquid crystal with a high tilt angle and a high spontaneous polarization in a homeotropically aligned cell has been studied by the dielectric relaxation method in the frequency range 10 Hz to 10 MHz. The measurements have been done using thin (3.5μm) cells with gold coated electrodes and samples aligned by a magnetic field. It has been observed that the molecular relaxation around the short axis of the molecule is detected in the chiral nematic and smectic C* phases. The surface molecular process is observed in the S*_c phase down to nearly 6 to 7 K below the transition temperature of the N* to the S*_c phase. The experimental results of the surface molecular process are analysed by theoretical calculations. The experimental results agree with the theoretical predictions.     

Non-markovian effects in dielectric relaxation

A generalized Nee—Zwanzig equation is proposed to describe both low and high frequency dielectric relaxation. It is shown that the inertial effect is closely related to the non-markovian character of the differential equation governing the time evolution of the dipole—dipole autocorrelation function. Comparison with experimental results is given.     

Dipole-phonon coupling and dielectric relaxation

Equations of motion for the average polarisation are derived from a microscopic approach by use of the local density matrix. A specific form of the dipole-phonon coupling in the memory kernel is investigated and the necessary conditions for the generation of a non-Debye form of response examined. It is shown that these conditions can only be fulfilled by allowing for dipole-induced distortion with the lifetime of the “phonons” determined by the lifetime of the dipole correlation.     

Pressure dependence of dielectric properties of chlorinated polyethylene vulcanizate. III. β relaxation

The effects of temperature and pressure on the shift factor and the dielectric increment of the β relaxation process were measured for vulcanized chlorinated polyethylene. The isobaric and isochoric activation enthalpies, H^*_P and H^*_V, the activation volume V^*, the pressure dependence of the glass–glass transition temperature, T_gβ/dP, and the apparent extinction temperature T_0β were obtained. The pressure dependences of both V^* and the dielectric increment would reach very small values near the liquid–glass transition temperature T_g, and the β process seems to be affected by the transition near T_g. The value of H^*v/H^*p for the β process is larger than that for the α process, and it is suggested that the molecular motions pertaining to the β process are more strongly restricted than those pertaining to the α process. The ratio T_0β/T₀, where T₀ is the characteristic temperature in the Vogel–Fulcher–Tammann–Hesse equation for the α process, follows the empirical relation of Matsuoka and Ishida, T_gβ/T_g ～0.75. The value of dT_gβ/dP estimated from T_g and T_0β/T₀ is consistent with the experimental value.     

Coexistence of magnetization relaxation and dielectric relaxation in a single-chain magnet

A novel single-chain magnet, [MnIII3O(Meppz)3(EtOH)4(OAc)] (1), has been successfully synthesized from a secondary building block [MnIII3O(Meppz)3(EtOH)5Cl] (2) with an S = 1 ground state. SCM 1 exhibits both magnetization relaxation and dielectric relaxation properties.