Segmental relaxation in semicrystalline polymers: A mean-field model for the distribution of relaxation times in confined regimes

The effect of confinement in the segmental relaxation of polymers is considered. On the basis of a thermodynamic model we discuss the emerging relevance of the fast degrees of freedom in stimulating the much slower segmental relaxation, as an effect of the constraints at the walls of the amorphous regions. In the case that confinement is due to the presence of crystalline domains, a quasi-Poissonian distribution of local constraining conditions is derived as a result of thermodynamic equilibrium. This implies that the average free-energy barrier F for conformational rearrangement is of the same order of the dispersion of the barrier heights, ( F) , around F . As an example, we apply the results to the analysis of the -relaxation as observed by dielectric broad-band spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized from either an isotropic or an oriented glass. It is found that in the latter case the regions of cooperative rearrangement are significantly larger than in the former.     

Hypersonic relaxation in amorphous polymers

In order to more fully understand the viscoelastic properties of amorphous polymers, it is desireable to have dynamic mechanical data over as wide a range in frequency as possible. One useful technique for studying the high-frequency behavior of polymers is to measure the velocity and attenuation of sound waves in the polymer fluid. When the frequency exceeds 109Hz. the sound waves are called hypersonic acoustic phonons. The present review examines the dynamic data obtained in the GHz frequency range for amorphous polymers.     

Transport of long neutral polymers in the semidilute regime through a protein nanopore

We investigate the entrance of single poly(ethylene glycol) chains into an α-hemolysin channel. We detect the frequency and duration of the current blockades induced by large neutral polymers, where chain radius is larger than pore diameter. In the semidilute regime, these chains pass only if the monomer concentration is larger than a well-defined threshold. Experiments are performed in a very large domain of concentration and molecular mass, up to 35% and 200 kDa, respectively, which was previously unexplored. The variation of the dwell time as a function of molecular mass shows that the chains are extracted from the semidilute solution in contact with the pore by a reptation mechanism.     

Calculation of spin-lattice relaxation times

A general expression is found for the probability for a direct (single-phonon) relaxation transition for arbitrary symmetry of the surroundings of a paramagnetic ion having an effective spin of S = 1/2. The expression is compared with experiment for the case of tetragonal symmetry. Since certain approximations frequently used in the calculations are not used here, a markedly improved agreement is found with the experimental relaxation times.Translated from Izvestiya VUZ. Fizika, No. 2, pp. 36–39, February, 1970.     

Spin relaxation times in disordered graphene

We consider two mechanisms of spin relaxation in disordered graphene. i) Spin relaxation due to curvature spin orbit coupling caused by ripples. ii) Spin relaxation due to the interaction of the electronic spin with localized magnetic moments at the edges. We obtain analytical expressions for the spin relaxation times τ_SO and τ_J due to both mechanisms and estimate their values for realistic parameters of graphene samples. We obtain that spin relaxation originating from these mechanisms is very weak and spin coherence is expected in disordered graphene up to samples of length .     

Temporal fluctuations in proton relaxation times

We studied mouse liver, heart and kidney for possible diurnal fluctuations of T₁ and T₂. In a subgroup of animals, we attempted to relate T₁ and T₂ of the organ samples to their water and lipid content (and in the liver, also to glycogen content). Diurnal periodic fluctuation was found only in liver T₂ and was of a very minor degree. Regression analysis of organ T₂ estabilished relationships with chemical composition which explained 25%–40% of the observed variation in T₂. No relationship with T₁ could be established.     

Commonality of elastic relaxation times in biofilms

Biofilms, sticky conglomerations of microorganisms and extracellular polymers, are among the Earth's most common life forms. One component for their survival is an ability to withstand external mechanical stress. Measurements indicate that biofilm elastic relaxation times are approximately the same (about 18 min) over a wide sample of biofilms though other material properties vary significantly. A possible survival significance of this time scale is that it is the shortest period over which a biofilm can mount a phenotypic response to transient mechanical stress.     

Propagation and relaxation of tension in stiff polymers

We present a unified theory for the longitudinal dynamic response of a stiff polymer in solution to various external perturbations (mechanical excitations, hydrodynamic flows, electrical fields, temperature quenches, etc.) that can be represented as sudden changes of ambient/boundary conditions. The theory relies on a comprehensive analysis of the nonequilibrium propagation and relaxation of backbone stresses in a wormlike chain. We recover and substantially extend previous results based on heuristic arguments. New experimental implications are pointed out.     

Calculations of anharmonic phonon relaxation times

Anharmonic phonon relaxation times in Ge are calculated using (i) an isotropic continuum model, and (ii) a dispersive model. A complete spectrum of calculated results is presented. Frequency-averaged values for normal- and umklapp-three-phonon relaxation times are also calculated. A comparison is made between our findings and some earlier works, and disagreements are discussed. The results are applied in calculating some results in the theory of lattice thermal conductivity. From February 1976: Department of Theoretical Physics, The University, Newcastle-Upon-Tyne, NE1 7RU U.K.     

Proton spin-lattice relaxation times in ytterbium hydrides

The measurements of the proton (NMR) spinlattice relaxation times have been made in a series of ytterbium hydrides, YbH _x . Results are reported forx=1.80, 1.95, 2.00 and 2.62 and temperatures 4.2T297K. In the orthorhombic phase (1.80x2.00), the spin-lattice relaxation times are dominated by the hyperfine interaction of protons with conduction electrons and the spin diffusion mechanism. In the cubic phase (x=2.62), the relaxation times are five orders of magnitude shorter than in the orthorhombic one. This is interpreted in terms of the proton coupling with the Yb³⁺ ion spin fluctuations.     

Spectral distribution of relaxation times in spin glasses

Recent neutron scattering measurements on spin glasses show that the dynamics of the spin systems can be best described in terms of wide spectral distribution of relaxation times evolving continuously with decreasing temperature but which is devoid of any critical behaviour, either speeding up or slowing down, at any finite temperature including the spin glass “freezing temperature T_sg”. It is argued that the latter temperature itself is dependent on the time constant of measurement for all spin glasses in general; the observed variation with frequency being less pronounced in some systems than others owing to some special characteristics of their spin dynamics such as, for example, the presence of parallel channels of rapid relaxation provided by the Korringa coupling in metallic spin glasses. The neutron scattering measurements presented here enable us to propose plausible forms for the density of relaxation times of the spin system and to show that the logarithmic frequency dependence of the freezing temperature observed in low frequency ac susceptibility measurements follows naturally from a uniform density of relaxation times at these frequencies.     

Measuring surface and bulk relaxation in glassy polymers

We present a comprehensive study of gold nanoparticle embedding into polystyrene (PS) surfaces at temperatures ranging from T _g + 8 K to T _g − 83 K and times as long as 10⁵ minutes. This range in times and temperatures allows the first concurrent observation of and differentiation between surface and bulk behavior in the 20nm region nearest the free surface of the polymer film. Of particular importance is the temperature region near the bulk glass transition temperature where both surface and bulk processes can be measured. The results indicate that for the case of PS, enhanced surface mobility only exists at temperatures near or below the bulk T _g value. The surface relaxation times are only weakly temperature dependent and near T _g , the enhanced mobility extends less than 10nm into the bulk of the film. The results suggest that both the concept of a “surface glass transition” and the use of glass transition temperatures to measure local mobility near interfaces may not universally apply to all polymers. The results can also be used to make a quantitative connection to molecular dynamics simulations of polymer films and surfaces.     

The concentration dependence of relaxation times of hydrogen proton in the aqueous solution of iron ferrite magnetic nanoparticles

By the direct coprecipitation of the aqueous solution of iron salt and tetramethylammonium hydroxide solution the stable iron ferrite nanoparticles were formulated. These nanoparticles were found to have uniform sizes of about 7 nm, and also showed no coalescence in the aqueous solution for a few months. The superparamagnetic behavior of these nanoparticles was checked by a vibrating sample magnetometer. Also, the temperature dependence of saturation magnetization of nanoparticles was observed using a superconducting quantum interface device magnetometer. The relaxation times of T₁ and T₂ of hydrogen proton in the colloidal aqueous solution of magnetic nanoparticles were measured using a nuclear magnetic resonance spectrometer for the wide range of concentration of nanoparticles in the aqueous solution. The inverse of relaxation times was observed to be directly dependent on the concentration of nanoparticles.     

Temperature dependence of proton relaxation times in vitro

Accurate measurement of tissue relaxation characteristics is dependent on many factors, including field strength and temperature. The purpose of this study was to evaluate the relationship between sample temperature, viscosity and proton spin-lattice relaxation time (T1) and spin-spin relaxation time (T2). A review of two basic models of relaxation the simple molecular motion model and the fast exchange two state model is given with reference to their thermal dependencies. The temperature dependence for both T1 and T2 was studied on a 0.15 Tesla whole body magnetic resonance imager. Thirteen samples comprising both simple and complex materials were investigated by using a standard spin-echo (SE) technique and a modified Carr-Purcell-Meiboom-Gill (CPMG) multi-echo sequence. A simple linear relationship between T1 and temperature was observed for all samples over the range of 20 degrees C to 50 degrees C. There is an inverse relationship between viscosity and T1 and T2. A quantity called the temperature dependence coefficient (TDC) is introduced and defined as the percent rate of change of the proton relaxation time referenced to a specific temperature. The large TDC found for T1 values, e.g. 2.37%/degrees C for CuSO4 solutions and 3.59%/degrees C for light vegetable oils at 22 degrees C, indicates that a temperature correction should be made when comparing in-vivo and in-vitro T1 times. The T2 temperature dependence is relatively small.     

Calculation of vibrational relaxation times in gaseous cyanogen

A calculation of vibrational relaxation times in gaseous cyanogen at 300°K is presented. It is suggested that two well separated dispersion regions should be found in sound velocity measurements at ultrasonic frequencies, the higher frequency region being associated with the relaxation of the combined vibrational specific heat contributions of the two lowest modes and the lower frequency region being associated with the relaxation of the corresponding contribution of the next highest mode.