Molecular motion in melt samples of poly(propylene glycol) studied using dielectric and Kerr effect relaxation techniques

Dielectric relaxation times and electro-optical Kerr-effect relaxation times have been measured for melt samples of pure poly(propylene glycol) of nominal molecular weight 1025, 2025 and 4000, over the temperature range 209K–255K. Kerr-effect relaxation curves were analysed in terms of two main components: a ‘fast’ primary process (A), associated with a negative optical birefringence; and a ‘slow’ secondary process (B), associated with a positive optical birefringence. Measurement of dielectric loss at different frequencies for poly(propylene glycol) of molecular weight 2025 and 4000 indicated two main relaxation processes: a ‘fast’ process (the normal α-process), which carried with it most of the dielectric loss; and a ‘slow’ secondary process of much smaller amplitude. The close correspondence between dielectric and Kerr-effect relaxation times, for the primary and secondary relaxation processes, indicated that these techniques are probing effectively different aspects of the same molecular motion. Dielectric and Kerr-effect relaxation times for the secondary process depended greatly on the molecular weight of the polymer and were compared with the predictions of the models for the reptational motions of chains in the bulk polymer, as proposed by De Gennes and by Doi and Edwards.     

Molecular dynamics in semifluorinated side-chain polyesters as studied by broadband dielectric spectroscopy

The molecular dynamics in a variety of poly(ethylene isophthalate)s (PEIs) is studied by broadband dielectric spectroscopy (BDS). The materials comprise non-substituted main chain polyesters and polyesters with semifluorinated (oxydecylperfluorodecyl) side chains. Combining temperature-dependent small angle X-ray scattering (T-SAXS), differential scanning calorimetry (DSC) and BDS it can be shown that the microphase separated semifluorinated polymers exhibit independent dynamic glass transition relaxations taking place in the separate microphases. Additionally in the glassy state, the non-substituted polymers show an Arrhenius-type relaxation whose activation energy decreases gradually from 52 to 40 kJ/mol with increasing main chain flexibility. The semifluorinated polymers exhibit a relaxation assigned to fluctuations of the perpendicular component of the fluoroalkyl end group with activation energies between 38 and 40 kJ/mol. With increasing flexibility of the main chain, the dynamics of the backbone becomes faster for the non-substituted polymers while an opposite trend is observed in the oxydecylperfluorodecyl substituted side-chain materials. A detailed explanation of the molecular origin of the relaxations is provided.     

Role of epoxidation on segmental motion of polyisoprene as studied by broadband dielectric spectroscopy

In this article, the broadband dielectric spectroscopy (BDS) is used to investigate the relaxation behavior of polyisoprene (PI) epoxidized to various levels and the special consideration is devoted to the effect of epoxidation degree on segmental relaxation. To associate segmental relaxation with molecular structure, several empirical equations are used to fit experimental data. Furthermore, we want to discuss the origin of the change of segmental relaxation. Thermal measurement reveals that glass transition temperature of epoxidized PI (EPI) increase with the increase of epoxidation degrees. The epoxidation modification increases the breadth of the dielectric dispersion. Oxirane groups as steric interference enhance the cooperativity of segmental relaxation process and retard the relaxation process, which results in the increase of coupling parameter. Specifically, Vogel–Fulcher–Tammann equation shows the relationship between correlation length and temperature. These results further illustrate the effect of epoxidation group on segmental motion of PI. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43879.     

Thermal dynamics of side-chain copolymethacrylates as studied by the dielectric spectroscopy and relaxation of second-harmonic generation

A series of copolymethacrylates with different contents of tolane-based mesogenic groups have been synthesized. The mesogenic group content was characterized with ¹H-n.m.r.. The phase behaviours were determined using a differential scanning calorimeter and optical polarizing microscopy. A smectic A phase was obtained when the mesogenic group content was increased up to 80 mol.%. Dielectric relaxation results indicated that the amplitude of the -relaxation was suppressed significantly due to the presence of the liquid crystalline phase. The reduction of the molecular motion is beneficial to the enhancement of the temporal stability of the effective second-harmonic coefficient for the polymer with a higher mesogenic group content. Moreover, the second harmonic coefficient is enhanced as the mesogenic group content increases. The self-alignment nature of the liquid crystal phase is favourable for alignment of the NLO-active mesogenic group under an applied electric field and preserving such alignment after removal of the electric field. The relationship between thermal dynamic behaviour and second-order nonlinear optical properties is also discussed.     

Molecular motion in polyisobutylene and poly(propylene oxide) studied by 13C nuclear magnetic relaxation

The ¹³C spin-lattice relaxation times of low molecular weight (up to 2500) samples of polyisobutylene and poly(propylene oxide) have been measured as a function of molecular weight, temperature and concentration in chloroform solution. For both polymers there is little dependence on molecular weight indicating a flexible conformation in the liquid state, but the relaxation time increases with increasing dilution in CHCl₃. The motion of the polymer backbone therefore depends on the microviscosity of the solution, rather than the bulk viscosity. In polyisobutylene the methyl re-orientation rate increases in parallel with the backbone re-orientation rate, showing that the two motions are interlinked. In poly(propylene oxide) the methyl re-orientation rate is independent of the backbone motion.     

Molecular motion of small molecules in cellulose gels studied by NMR

Measurements of relaxation times, T₁ and T₂, and self-diffusion coefficients, D, for small molecules, viz., H₂O, dioxane and t-butanol, in the gel system cellulose/H₂O are reported and compared with those for H₂O in a polyacrylamide gel of the same polymer content. The temperature dependence of T₁ and D can for all the penetrants be represented by Arrhenius type relations which merely are parallel shifts by the same amount, towards smaller values, of those obtained without polymer. The T₂ values for H₂O in both gels pass through a shallow minimum over the considered temperature interval (14°?44°C). Furthermore, relaxation times T₁ of D₂O in the gel system cellulose/D₂O are reported; in a plot of ln T₁ vs. 1/T, a plateau region is observed at higher temperatures.     

Structural development in ion‐irradiated poly(ether ether ketone) as studied by dielectric relaxation spectroscopy

Structural alterations to amorphous poly(ether ether ketone) (PEEK) produced by ion irradiation (11.2 MeV H⁺ and 25.6 MeV He²⁺ ions) were investigated by dielectric relaxation spectroscopy. The analysis in terms of the Havriliak–Negami (HN) equation and the scaling model showed an increase in the intermolecular correlation with increasing irradiation dose. The dynamic fragility index (m) was estimated from Vogel–Fulcher–Tammann analysis. Ion irradiation not only elevated the glass‐transition temperature (T_g) but interestingly decreased m of the PEEK chains around T_g. This was due to increasing polar interaction and better packing efficiency of the irradiated samples compared with those of amorphous PEEK. The average size of the cooperative rearranging region decreased in line with decreasing m and indicated an increase in the rigid amorphous phase fraction after irradiation. The analysis of the direct‐current conductivity confirmed that there was a strong coupling between the macroscopic ion transport and concerted segmental motion. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2014 , 131, 39929.     

Electronic motion of HCIO4-doped polytoluidine studied by varying-frequency proton NMR relaxation

Summary Proton spin-lattice relaxation times (T₁) in poly-o-toluidine doped with HC10₄ were measured over a frequency range from 20 to 90 MHz at room temperature. It was found that there are at least two kinds of phase structures with definitely distinguishable T₁ values in moderate doped samples. The spin-lattice relaxation rate (T₁ ^–1) exhibits a maximum at the doping ratio of about 20%. The frequency dependence of (T₁ ^–1) reveals that the electrons predominantly perform an intrachain diffusive motion above 50 MHz and below this frequency an interchain hopping of electrons takes place.     

Molecular motion of syndiotactic poly(α-methylstyrene) in solution studied by carbon-13 n.m.r. relaxation measurements

The carbon-13 n.m.r. spin-lattice relaxation times, nuclear Overhauser enhancement factor (NOE), and line widths have been measured for a syndiotactic poly(α-methylstyrene) in solutions in toluene-d₈ and o-dichlorobenzene-d₄ as a function of temperature from 40° to 163δC. The single correlation time model of relaxation is inadequate to explain the data of spin-lattice relaxation time and NOE. But, within experimental error, these relaxation data in two solvents over a full temperature range can be interpreted in terms of either the Cole-Cole or the log-χ² distributions of correlation times, or a conformational jump model proposed by Monnerie et al. The internal rotation of the methyl group is relatively rapid, while that of the phenyl ring is slow and practically overshadowed by the backbone segmental reorientation over the temperature range examined. The solvent dependence of relaxation data was discussed.     

Critical slowing down in betaine phosphate as studied by nuclear magnetic relaxation

Proton and phosphorus spin-lattice relaxation time (T₁) measurements were applied to study the antiferroelectric phase transition in betaine phosphate (CH₃)₃NCH₂COO·H₃PO₄ at 86 K. In the whole temperature range studied, the T₁ or ¹H was governed by a dipolar fluctuation caused by the methyl- and the whole butyl-group reorientations. In the vicinity of the phase transition the T₁ of ³¹P shows a sharp dip which is a characteristic of critical slowing down. The analysis of the T₁ based on the order-disorder mechanism with respect to the position of the acidic protons leads to the critical exponent for the correlation length v=0.5.     

1H N.m.r. relaxation and molecular motion of polyethylene in solution as studied by the isotopic dilution technique

The molecular motion of polyethylene has been studied in decalin-d₁₈ solution by ¹H n.m.r. relaxation with the isotopic dilution technique which can separate the proton spin-lattice relaxation times into the intra- and intermolecular contributions, intra T₁ and inter T₁. The temperature dependences of intra and inter T₁s show that the intra- and intermolecular motions are in the extreme narrow region. These results, also, were compared with the results of molten polyethylene reported in our previous work.     

Molecular motions in low molecular weight melts of polystyrene studied by n.m.r. relaxation

The proton spin-lattice relaxation times of polystyrene were determined selectively for the phenyl and chain protons at 88 MHz in the temperature range of the α-process using a usual FT-inversion-recovery experimental procedure. The temperature and molecular weight dependence of the relaxation times are discussed qualitatively from a phenomenological point of view. The phenyl ring and main chain are coupled and have cooperative character.     

Molecular orientation in novolac cured epoxy resins as studied by rheo-optical FTIR spectroscopy

The molecular orientation and relaxation behavior was studied by rheooptical FTIR spectroscopy during the uniaxial deformation of epoxy resins prepared from the diglycidyl ether of butanediol and novolacs on the basis of bisphenol A. The investigation of orientation phenomena was performed in both the rubbery and the glassy state of the epoxies. Results are discussed with regard to the respective mechanism of deformation. Moreover, the effect of temperature, strain rate, and the molecular weight of the novolacs used on the orientation behavior and the mechanical properties was studied. A significant influence of these parameters on the molecular deformation behavior was observed. The reversibility of the orientation at temperatures above and below the glass transition temperature was examined. Epoxy films were subjected to successive loading–unloading cycles including elongation, relaxation, and annealing. The investigations show that the orientation is completely reversible in the rubbery state, but it is only partly reversible below the glass temperature. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 247–259, 1998     

Charge transport in diatomaceous earth studied by broadband dielectric spectroscopy

In the present work, broadband dielectric spectroscopy measurements in the frequency range of 10 mHz to 1 MHz and at elevated temperatures (room temperature up to 950 °C) were carried out in industrial diatomaceous earth samples, in order to study charge transport phenomena which play a key role to the transport of both organic and inorganic contaminants in this kind of minerals. Cole–Cole plots of complex impedance measurements may serve to distinguish the contributions of grain interior, grain boundaries and electrode polarization to the overall electrical behavior of diatomite. Complex electrical conductivity and electric modulus representations have been combined under the fulfillment of the BNN relationship over a wide temperature range, in order to investigate the temperature dependence of diffusion coefficients and concentration of charge carriers in thermally modified diatomite. Both diffusivity and mobility follow the same Arrhenius-type behavior at two distinct temperature regions as DC-conductivity does. The related activation energies were estimated and ascribed to proton conduction (Ea ≈ 1.00 eV) at low temperatures (T < 650 °C) and hopping conduction of small polarons (Ea ≈ 1.67 eV) at higher temperatures, due to the presence of transition metals in diatomite sample. The thermal treatment of diatomite at 950 °C enhances the available charge carriers and increases the diffusivity which might improve its adsorption capacity.     

Grain size dependence of dielectric relaxation in cerium oxide as high-k layer

Cerium oxide (CeO₂) thin films used liquid injection atomic layer deposition (ALD) for deposition and ALD procedures were run at substrate temperatures of 150°C, 200°C, 250°C, 300°C, and 350°C, respectively. CeO₂ were grown on n-Si(100) wafers. Variations in the grain sizes of the samples are governed by the deposition temperature and have been estimated using Scherrer analysis of the X-ray diffraction patterns. The changing grain size correlates with the changes seen in the Raman spectrum. Strong frequency dispersion is found in the capacitance-voltage measurement. Normalized dielectric constant measurement is quantitatively utilized to characterize the dielectric constant variation. The relationship extracted between grain size and dielectric relaxation for CeO₂ suggests that tuning properties for improved frequency dispersion can be achieved by controlling the grain size, hence the strain at the nanoscale dimensions.     

Oil splitting in industrial cleaning systems as studied by conductivity and interfacial tension

Characteristics of emulsion formation and splitting into aqueous and oily phases in simple and formulated surfactant systems were studied via conductivity. In systems composed of mixed nonionic ethoxylated alcohol surfactants, K₂CO₃, and emulsified n-hexadecane, conductivity decreased linearly with increasing oil volume fraction at HLB (hydrophile-lipophile balance) values of 12.9 and 13.9. The slope of the plot was ca. −3/2, in agreement with the Maxwell expression. At values less than or equal to an HLB of 11.3, conductivity first increased with a small addition of oil and then decreased nearly linearly with subsequent amounts. This was probably due to low HLB surfactants partitioning into the oily phase. When the type of oil was varied, the reduced conductivity also decreased linearly with volume fraction of emulsified oil. The slope was ca. −3/2 for oil weights ranging from very light (n-hexadecane) to very heavy (80W–90 gear oil), also in agreement with the Maxwell expression. Oil separation rates were measured by monitoring the change in conductivity in the lower region of the emulsion (where the aqueous layer formed) during splitting of the oily phase. Heavier oils were found to separate faster than light oils. Oils containing lubricity agents split at the slowest rate. Systems with lower HLB surfactants also displayed slower splitting rates. Splitting rates for a variety of systems, from simple oil and saline systems to more complex formulated systems, over temperatures from 23 to 75°C, were related to oil-aqueous interfacial tension values through a power law expression composed of the maximum splitting rate and the interfacial tension between saline and oil at 23°C.     

Complete relaxation map of polypropylene: radiation-induced modification as dielectric probe

The molecular relaxation behaviour of isotactic polypropylene (iPP) exposed to gamma radiation in air to various absorbed doses (up to 700 kGy) has been investigated by dielectric loss (tan  d { \tan }\;\delta ) analysis; the polar (mainly carbonyl and hydroperoxide) groups that were introduced by radiation-induced oxidation were considered as tracer groups. All relaxation zones (α, β, γ and δ in the order of decreasing temperature), between 25 K and melting 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 iPP samples to radiation. Wide-angle X-ray diffraction was used to investigate radiation-induced changes in the crystalline structure and degree of crystallinity, since the α relaxation is connected with this phase. Infrared spectroscopy and gel measurements were used to determine the changes in the oxidative degradation and the degree of network formation, respectively. Conclusions derived using different methods were compared. This study reveals high dielectric and/or relaxation sensitivity of iPP to gamma radiation. Disappearance of the low-temperature dielectric (γ and δ) relaxations together with large changes in intensity, position and activation energy of the dielectric α relaxation are observed with gamma irradiation and are mainly connected with oxidative degradation in iPP structure.     

Tunnelling mechanism of dielectric relaxation

The kinetic equation describing electric polarization in solid dielectrics is presented with complete consideration of the thermoactivated transition, tunnelling and reflecting of relaxing particles in the framework of the model of a one-dimensional potential barrier with two minima. Solving the equation under a series of simplifying conditions allows one to calculate the dielectric characteristics.

The analysis of the obtained results shows that evidence for the presence of this full quantum mechanism is displayed in solid dielectrics with light relaxing particles (of the type of H⁺, Li⁺, etc.).