Local forms of molecular mobility in comb-shaped chromophore-containing copoly(methacrylates)

The local forms of molecular mobility of comb-shaped random copoly(methacrylates) have been studied by the method of dielectric spectroscopy. Side branches of these copolymers are composed of alternating chromophore-containing chains and flexible fluoromethylene sequences. Two regions of relaxation of dipole polarization, γ₂ and β processes, related to the mobility of end polar groups and the reorientation of chromophore groups relative to their long axes, respectively, have been discovered in the glassy state. The times of relaxation of the γ₂ process depend on the ratio between the flexible and rigid parts of the copolymer. In the region of the β process, the molecular mobility appears to be slightly dependent on the chromophore structure and its intensity is determined by the degree of substitution of chromophore groups.     

Molecular mobility of a comb-shaped copolymethacrylate with chalcone chromophore-containing side groups

The molecular mobility of copolymethacrylate with chromophore-containing chalcone side chains is studied by broadband dielectric spectroscopy. Five regions of dipolar polarization relaxation are identified: namely, γ, β, β₁, α, and δ processes. It is shown that γ and β1 processes are related to the local mobility of methylene sequences and ester groups adjoining the backbone, while the α process is associated with the cooperative segmental mobility of the backbone. The β and δ processes are attributed to the mobility of chalcone groups: the local mobility in the glassy state (reorientation relative to the long axis of chromophores) and the cooperative mobility in the rubbery state (reorientation relative to the short axis of chromophores), respectively. The incorporation of 20% chalcone groups does not change the glass-transition temperature but enhances the cooperativity of the α process and intermolecular interactions.     

Dynamic relaxation behavior of copolymers of n-butyl methacrylate with n-butyl acrylate and of 2-hydroxyethyl methacrylate with ethyl acrylate,n-butyl acrylate,and dodecyl methacrylate

The effect of the α-methyl group on the mobility of the main and side chains of methacrylateacrylate copolymers has been investigated. Poly(ethyl acrylate) shows a small secondary loss maximum (attributed to the rotation of ? COOR side chains) at 145 K, while in the case of poly(n-butyl acrylate) this relaxation process is smeared out or possibly absent. On the contrary, poly(n-butyl methacrylate) and poly(2-hydroxyethyl methacrylate) exhibit secondary relaxations at about 278 and 301 K, respectively. From the dynamic mechanical response spectra of methacrylate-acrylate copolymers one can see that the removal of the α-methyl group causes a qualitative change in the molecular mechanism of the secondary relaxation, presumably as a consequence of the different participation of the main chains. The existing data, however, are insufficient to quantify these differences. The low-temperature relaxation attributed to internal motion within the side groups is not distinctly affected by the presence of α-methyl groups. If both components of the copolymer display the low-temperature relaxation (above 77 K), the loss maxima preserve their identity to a large extent. The effect of copolymer composition on the main (glass) transition temperature has been described by means of a one-parameter equation.     

Piezoelectric relaxation in poly(γ-methyl-L-glutamate) at high temperature

Piezoelectric relaxation has been studied on elongated poly(γ-methyl-L-glutamate) films with the α-helical molecular conformation. Relaxation processes are observed near 0 and 100°C. Each process has a dual character composed of relaxational and retardational frequency dependences. The low-temperature process is ascribed to thermal motion of side chains. The high-temperature process, discussed in relation to the dielectric relaxation, is attributed to the ionic dc conduction connected with the two-phase structure of crystalline regions and the electrode polarization.     

Probing methyl dynamics from 13C autocorrelated and cross-correlated relaxation

An understanding of side-chain motions in protein is of great interest since side chains often play an important role in protein folding and intermolecular interactions. A novel method for measuring dipole-dipole cross-correlated relaxation in methyl groups of uniformly 13C-labeled proteins without deuteration has been developed by our group. The excellent agreement between dynamic parameters of methyl groups in ubiquitin obtained from the cross-correlated relaxation and 13C spin-lattice relaxation and those derived previously from 2H relaxation data demonstrates the reliability of the method. This method was applied to the study of side-chain dynamics of human intestinal fatty acid binding protein (IFABP) with and without its ligand. Binding of oleic acid to the protein results in decreased mobility of most of the methyl groups in the binding region, whereas no significant change in mobility was observed for methyl groups in the nonbinding region.     

Charge transport properties for carbazolyl groups pendant poly(glutamate)

Carbazolyl groups pendant poly(glutamate) (PCLG) was prepared to analyze its charge‐transport properties by employing mobility measurements and thermally‐stimulated current (TSC) measurements. The mobility induced TSC (MITSC) model proposed by I.Chen was employed to evaluate the experimental TSC spectra with mobility results. Simulated MITSC spectra showed good agreement in its peak temperature with experimental TSC spectra for PCLG. This suggests that the carrier transport followed the trap‐limited mechanism estimated by the mobility results. Further, the peaks in experimental TSC spectra appeared over the same temperature range as that in thermally‐stimulated polarization current (TSPC) spectra. Since the TSPC spectra were found to be correlated with the dielectric tan δ spectra for PCLG, the peaks in TSPC spectra are attributed to the side‐chain relaxation for PCLG. Therefore, the similarity between TSPC and TSC spectra indicates that the charge‐transport mechanism for PCLG was considerably affected by side‐chain relaxation for PCLG, which would vary the energy state of trap sites and effectively reduces the energy for the release of the carriers trapped on the illuminated surface. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 61–69, 1999     

The universal electro-optic response of charged colloids in low electrolyte suspensions

The large dielectric dispersion of colloids in the low-frequency range, related to polarization of the particle surface electric layer (the alpha-relaxation), has been a subject of scientific interest for decades. In recent papers we advanced the idea that the process of particle surface polarization is partially detected by a second low-frequency relaxation displayed in the frequency domain of particle rotation. The aim of the present paper is to argument this view more consistently. The second low-frequency relaxation is as universal as alpha-relaxation and closely related to it. It is more sensitive to variations in particle electrophoretic mobility than the alpha-relaxation. The paper discusses several aspects concerning the phenomenon: the reasons for its difficult identification as a universal effect; the procedures helping its analysis; and the basic features and the origin of the phenomenon.     

Ionic transport in a sulfonated polystyrene–polyethylene copolymer

The conductivity of a grafted copolymer composed of 21% sulfonated polystyrene and 79% polyethylene has been investigated as a function of temperature and absorbed moisture for membranes containing the monovalent counterions H⁺, Li⁺, Na⁺, K⁺, Rb⁺, Cs, and Ag⁺. The mobility of Ag⁺ was measured directly in an experiment analogous to that used for the mobility of F-centers in alkali halides. The mobility and density of free counterions depend on the amount of absorbed water, but only the mobility and not the density of free counterions is a thermally activated process. Current is carried by both intrinsic counterions and, in samples containing absorbed water with greater than 1.5 V applied, by protonic carriers created when the absorbed water is electrolyzed. Thermal depolarization studies indicate that persistent polarization observed is the result of homogeneous volume polarization, due to the orientation of sulfonic acid side groups.     

An NMR study of mobility in a crystalline side‐chain comblike polymer

Carbon‐13 spin–lattice relaxation times are measured for poly(octadecyl acrylate) above and below the melting point of the crystalline side chains. The chain backbone has long spin–lattice relaxation times below the melting point that shorten by more than an order of magnitude as the melting point range is traversed. Below the melting point, the backbone is nearly immobilized with spin–lattice relaxation changing very slowly with temperature. Above the melting point, the shorter spin–lattice relaxation times are typical of a rubber above the glass transition and decrease with increasing temperature. The methylene groups in the side chain are quite mobile well below the melting point, indicating fairly rapid anisotropic motion within the crystal. The methyl group at the end of the chain and the adjacent methylene group have longer spin–lattice relaxation times, indicating the greatest side‐chain mobility at the end, which is in the middle of the crystal structure. The side‐chain carbon adjacent to the carbonyl group is as mobile as the majority of the side‐chain carbon, indicating side‐chain mobility extends to all of the side‐chain CH₂ groups. The abrupt transition in the mobility of the backbone is not typical of the amorphous phase in a semicrystalline polymer where the backbone units can crystallize. The close proximity of every backbone segment to the crystalline domain locks backbone segmental motion below the melting point. Melting and crystallization of the side chains switch segmental motion of the backbone on and off. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1548–1552, 2001     

Dielectric relaxation behavior of poly(methyl methacrylate) under high‐pressure carbon dioxide

An in situ dielectric measurement for atactic poly(methyl methacrylate) (at‐PMMA) was performed under high‐pressure CO₂ under various pressures and temperatures. The at‐PMMA has the acetate side group with a large dipole moment. In the glassy state, a local relaxation process (β‐process) can be observed using dielectric measurement. In the rubbery state, the micro‐Brownian motion of main chain (α‐process) occurs, and the β‐process changes into αβ‐process coordinated with the α‐process. The dielectric loss (ε″) spectrum of at‐PMMA in the glassy state is asymmetric because of the density fluctuation for the amorphous structure. The loss peak frequency shifted to higher frequencies, and the relaxation strength increased with increasing CO₂ pressure. In the glassy state, the shape of ε″ spectrum became more symmetric with increasing CO₂ pressure. These show that the molecular mobility enhanced by the plasticization effect of CO₂ allows the dipolar side groups in the high‐density region to contribute to the relaxation process. We also found that the apparent activation energy decreased under high‐pressure CO₂. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2951–2962, 2005     

Spin relaxation effects in photochemically induced dynamic nuclear polarization spectroscopy of nuclei with strongly anisotropic hyperfine couplings

We describe experimental results and theoretical models for nuclear and electron spin relaxation processes occurring during the evolution of 19F-labeled geminate radical pairs on a nanosecond time scale. In magnetic fields of over 10 T, electron-nucleus dipolar cross-relaxation and longitudinal DeltaHFC-Deltag (hyperfine coupling anisotropy--g-tensor anisotropy) cross-correlation are shown to be negligibly slow. The dominant relaxation process is transverse DeltaHFC-Deltag cross-correlation, which is shown to lead to an inversion in the geminate 19F chemically induced dynamic nuclear polarization (CIDNP) phase for sufficiently large rotational correlation times. This inversion has recently been observed experimentally and used as a probe of local mobility in partially denatured proteins (Khan, F.; et al. J. Am. Chem. Soc. 2006, 128, 10729-10737). The essential feature of the spin dynamics model employed here is the use of the complete spin state space and the complete relaxation superoperator. On the basis of the results reported, we recommend this approach for reliable treatment of magnetokinetic systems in which relaxation effects are important.     

PEG-POSS Star Molecules Blended in Polyurethane with Flexible Hard Segments: Morphology and Dynamics

A star polymer with a polyhedral oligomeric silsesquioxanne (POSS) core and poly(ethylene glycol) (PEG) vertex groups is incorporated in a polyurethane with flexible hard segments in-situ during the polymerization process. The blends are studied in terms of morphology, molecular dynamics, and charge mobility. The methods utilized for this purpose are scanning electron and atomic force microscopies (SEM, AFM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), and to a larger extent dielectric relaxation spectroscopy (DRS). It is found that POSS reduces the degree of crystallinity of the hard segments. Contrary to what was observed in a similar system with POSS pendent along the main chain, soft phase calorimetric glass transition temperature drops as a result of plasticization, and homogenization of the soft phase by the star molecules. The dynamic glass transition though, remains practically unaffected, and a hypothesis is formed to resolve the discrepancy, based on the assumption of different thermal and dielectric responses of slow and fast modes of the system. A relaxation α′, slower than the bulky segmental α and common in polyurethanes, appears here too. A detailed analysis of dielectric spectra provides some evidence that this relaxation has cooperative character. An additional relaxation g, which is not commonly observed, accompanies the Maxwell Wagner Sillars interfacial polarization process, and has dynamics similar to it. POSS is found to introduce conductivity and possibly alter its mechanism. The study points out that different architectures of incorporation of POSS in polyurethane affect its physical properties by different mechanisms.     

Dynamic Properties of Polyampholyte Hydrogel Elucidated by Proton NMR Spin-spin Relaxation Time

1H spin-spin relaxation time（T2） measurement of polyampholyte hydrogel poly（methylacrylic acidacryloyloxyethyl trimethylammonium chloride）[P（MA-DAC）] in different pH, ionic strength and temperature was carried out to reveal the molecular mobility. Spontaneous volume transition of the polyampholyte hydrogel was also investigated by spin-spin relaxation time measurement. Meanwhile T2 and the proton component fraction were acquired to study the swelling behaviour of the hydrogel. Moreover the changes of T2 characterized the molecular mo- bility of polyampholyte hydrogel in various swelling states. And the results suggest that the mobility of the main chains and a few free side chains（the long T2） of P（MA-DAC） was dominated by the mesh size in the hydrogel net- work, depending on the swelling ratio（Q） and the mobility of the side chains（the short T2） was influenced by electrostatic interaction between different charges in polymer side chains. Finally the T2 measurements of P（MA-DAC） hydrogel in the spontaneous swelling-deswelling process demonstrated the electrostatic interaction of the charged side chains caused deswelling behavior. At the same time, the mobility state transition temperature of the charged side chains was also studied by the lH spin-spin relaxation time measurements, and the transition activation energy of the side chains is 2.72 kJ.     

Local forms of molecular mobility in copolymethacrylates containing side‐chain nonlinear optical chromophores

Local forms of molecular mobility of comb‐like random copolymethacrylates, containing side‐chain nonlinear optical chromophores and amyl‐ or octafluoroamyl‐groups, are studied. In the glassy state, two relaxation processes are found, which are due to reorientation of terminal polar groups (γ₂‐process) and rotation of chromophore groups about their long axes (β‐process). For octafluoroamyl‐containing copolymers, the γ₂‐process is superposition of the two molecular motions due to reorientation of terminal chromophore groups and ? CF₂H groups. The intensity and relaxation times in the range of the γ₂‐process depend on the chemical structure and concentration of chromophores in a copolymer. For the β‐process, the molecular mobility weakly depends on the chromophore structure, being mainly determined by the relative lengths of the rigid chromophore and flexible side chains. On the global spectrum of depolarization current it was observed the γ₁‐process due to reorientation of (CH₂)_n spacers between chromophore groups and copolymer backbone. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1488–1496, 2008     

Dielectric relaxation in concentrated solutions of poly(γ-benzyl-L-glutamate)

Measurements of dielectric constant and loss, broad-line nuclear magnetic resonance, and differential thermal analysis of concentrated solutions of poly(γ-benzyl-L -glutamate) (PBLG) were carried out to determine the effects of intermolecular interactions on the mobility of the side group and the solvent (dichloromethane or dioxane). Only one dielectric loss peak due to the cooperative motion of the side group and the solvent was found. The activation energy of this relaxation process varied from 3 to 47 kcal/mole with increasing concentration of PBLG from 20 to 100% by weight accompanied with steep increases at about 40 and 80%. This result is explained as due to entanglement of neighboring side groups. In NMR, narrowing of the line was observed near the temperature where the dielectric loss was observed. The glass transition was also observed by differential thermal analysis. From these results it was concluded that the relaxation observed in PBLG solution and in pure PBLG have the nature of primary or α relaxations.     

Dynamic nuclear polarization assisted spin diffusion for the solid effect case

The dynamic nuclear polarization (DNP) process in solids depends on the magnitudes of hyperfine interactions between unpaired electrons and their neighboring (core) nuclei, and on the dipole-dipole interactions between all nuclei in the sample. The polarization enhancement of the bulk nuclei has been typically described in terms of a hyperfine-assisted polarization of a core nucleus by microwave irradiation followed by a dipolar-assisted spin diffusion process in the core-bulk nuclear system. This work presents a theoretical approach for the study of this combined process using a density matrix formalism. In particular, solid effect DNP on a single electron coupled to a nuclear spin system is considered, taking into account the interactions between the spins as well as the main relaxation mechanisms introduced via the electron, nuclear, and cross-relaxation rates. The basic principles of the DNP-assisted spin diffusion mechanism, polarizing the bulk nuclei, are presented, and it is shown that the polarization of the core nuclei and the spin diffusion process should not be treated separately. To emphasize this observation the coherent mechanism driving the pure spin diffusion process is also discussed. In order to demonstrate the effects of the interactions and relaxation mechanisms on the enhancement of the nuclear polarization, model systems of up to ten spins are considered and polarization buildup curves are simulated. A linear chain of spins consisting of a single electron coupled to a core nucleus, which in turn is dipolar coupled to a chain of bulk nuclei, is considered. The interaction and relaxation parameters of this model system were chosen in a way to enable a critical analysis of the polarization enhancement of all nuclei, and are not far from the values of (13)C nuclei in frozen (glassy) organic solutions containing radicals, typically used in DNP at high fields. Results from the simulations are shown, demonstrating the complex dependences of the DNP-assisted spin diffusion process on variations of the relevant parameters. In particular, the effect of the spin lattice relaxation times on the polarization buildup times and the resulting end polarization are discussed, and the quenching of the polarizations by the hyperfine interaction is demonstrated.     

Molecular mobility of substituted poly(p-phenylenes) characterized by a range of polymer relaxation techniques

The free volume and related mobility properties of substituted poly(p-phenylene) polymers are examined. The techniques used range from positron annihilation, dielectric relaxation, and dynamic mechanical spectroscopy to thermally stimulated currents. Fractional free volume is determined for the samples with different substituted side groups and related to the glass transition temperature. Bulkier groups lead to a greater fractional free volume and lower glass transition temperatures. Comparison of molecular relaxation times using the different characterization techniques demonstrates that there is strong coupling between motion of the main chain and the side groups, on which the dipoles reside. Intermolecular coupling between the main chains at the primary relaxation is shown in this work to be related to the nature of the side chains and resultant free volume, as are the temperature locations of local, secondary relaxations. A qualitative model describing the effect of regiochemistry on the motions and packing of these materials is also proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1465–1481, 1998     

Effects of the counterion on dielectric spectroscopy of a montmorillonite suspension over the frequency range 10(5)-10(10) Hz

Dielectric measurements were carried out on suspensions of montmorillonite clay exchanged with three different counterions: sodium, ammonium, and tetramethylammonium (TMA). Only two dielectric absorption peaks could be identified for the clay sample with the TMA counterion, whereas three peaks were found for the two inorganic counterions. The dielectric process observed at around 10 GHz is due to the orientation of bulk water molecules, judging from the relaxation time and relaxation strength. The relaxation strength of the process occurring at around 10 MHz was compared with the coefficient of adiabatic compressibility obtained from ultrasound velocity measurements. The increase in the relaxation strength with decreasing compressibility indicates that the process at around 10 MHz is caused by the orientation of bound water molecules on the clay samples. The relaxation strength of the process occurring at around 10 MHz for the TMA sample was remarkably small. Furthermore, the network structure of the bound water molecules can be characterized by a property peculiar to the TMA sample, taking into account the value of its Cole-Cole parameter. Results for the relaxation strength of the process occurring at around 100 kHz were compared with those for electrophoretic mobility. This comparison revealed that discrimination between bound ions and ions in the diffuse double layer is important, and both the relaxation and electrophoretic results could be satisfactorily explained by surface polarization of the clay.     

Probing the dielectric response of polyurethane/alumina nanocomposites

The dielectric properties of polyurethane (PUR) latex–boehmite alumina nanocomposites were investigated by means of broadband dielectric spectroscopy in the temperature range ?100 °C to 70 °C. The concentration of the filler (alumina) was kept constant at 10 phr for all specimens, whereas the mean particle diameter (namely 220, 90, and 25 nm) of the incorporated nanoparticles varied accordingly. For reasons of comparison, pure PUR was also examined. Four distinct relaxation modes were recorded in the spectra of all systems. They were attributed to interfacial polarization, glass transition (α‐relaxation), local motions of polar side groups, and chain segments (β‐relaxation and γ‐relaxation). All four relaxation processes exhibit a symmetric distribution of relaxation times, which in the case of interfacial polarization, becomes narrower. The intensity of interfacial polarization increases with the reduction of the mean particle diameter indicating enhanced interfacial area. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Polarization time effect on PMMA space-charge relaxation by TSDC

The effect of the polarization time in the formation of the PMMA space charge relaxation has been studied. The study has been carried out by thermally stimulated depolarization currents of electrets formed by windowing polarization. The results obtained have been fitted to the general kinetic order model, and they are consistent with the existence of a space-charge polarization mechanism with the kinetic order of 1.14. The intensity of the peak maxima results in being a good indicator of the trapped carrier number evolution. For high temperatures and high electrical fields the saturation of this mechanism is achieved faster, which is attributed to a carrier mobility increase with these parameters. Finally, a compensation law has been found for space-charge-relaxation in PMMA. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1971–1980, 1998