Vibrational relaxation of DF at low temperatures

Temperature dependence of the probability of the deexcitation process DF(ν = 1) + DF(ν = 0) → DF(ν = 0) + DF(ν = 0) has been investigated near and below room temperature based on the model that the vibrational energy is transferred to the hindered rotational (oscillatory) motion as well as the translational motion. The deexcitation probability increases sharply with decreasing temperature; this inverse temperature dependence is attributed to the important contribution of hydrogen-bond attraction.     

Dielectric relaxation time of polymers

The temperature dependence of the dielectric relaxation time of amorphous polymers can be described quite satisfactorily by an expression derived from the theory of the relaxation time for local conformational transitions in a polymer chain. This theory was recently developed using the Kramers theory of the rate constant together with the free-volume theory.     

Photostimulated synchronous chlorination of polymers at low temperatures

High yields of free radicals (10¹⁸–10¹⁹ spin g^?1) were found upon the low-temperature (77 K) chlorination of solid synthetic polymers on irradiation with light with λ ≥ 360 nm. The stage of radical formation was considered in the framework of the mechanism of concerted reactions in the polymolecular complex [polymer—Cl₂].     

Dielectric modes in antiferroelectric liquid crystal observed at low temperatures

A broad temperature antiferroelectric binary mixture has been investigated by means of dielectric spectroscopy. The sample was cooled down to –70°C. It was found that the sample was still in antiferroelectric phase. This is the widest antiferroelectric mixture ever seen (～170°) in which three well-separated modes have been detected at room temperature. In addition, the bias field influence on existing modes has been observed. All modes change their strengths with bias field. Results show that the fastest mode, called X mode previously, gradually disappears around –30°C. When the temperature decreases below –40°C, one can indisputably observe additional mode, faster than X mode. This mode (named as Y mode) observed for extra low temperatures is bias independent. It can be the molecular mode, connected with rotation around long molecular axis. The rotation around short molecular axis seems to be blocked in antiferroelectric packing. To calculate parameters of observed modes, Cole–Cole model was used. The parameters of Y mode are discussed in this article.     

Dielectric relaxation and intramolecular mobility in LC polymers

Intramolecular motion and mechanisms of local mobility were considered in linear polyesters, Including thermotropic systems, on the base of experimental results of dielectric absorption research. In some cases the role of molecular motion in the formation of the mesomorphic structure is established.     

A new relaxation process in the high-elastic state at low temperatures

A new relaxation process, explaining the change of elasticity in rubberlike polymers at critical stress (0.1?0.5 kgf/cm²) has been discovered. This process is characterized by the low value of activation energy (weak temperature dependence of relaxation times) and large sizes of kinetic units (strong dependence of relaxation time on stress). Critical stress depend on temperature and for rubberlike polymers turns to zero at 40°?60°C. Mechanism of the phenomena can be explained by the existence of the ordered molecular microregions, creating additional crosslinking points of nonchemical nature with free chains of the network, breaking up at critical stress. Observed phenomena is analogous to the process of forced rubber elasticity of those polymers in glassy state. Critical stress is analogous to the limit of forced rubber elasticity below glass transition temperature.     

Compression stress relaxation in carbon black reinforced HNBR at low temperatures

Findings of a study of stress relaxation behaviour of hydrogenated nitrile butadiene rubber (HNBR) at nominal compressive strains up to 0.4 and temperatures above and below the glass transition temperature T_g are reported. Two formulations of a model HNBR with 36% acrylonitrile content and carbon black (CB) loading of 0 and 50 phr were investigated. The relaxation function of HNBR is found to be independent of strain at temperatures right above the T_g or at times longer than 10⁻³ s for the deformations employed. CB imparts higher long-term stiffness and also larger relaxation strength at times longer than 10⁻⁴ s to the HNBR, but it does not affect the relaxation behaviour of the rubber in the time span from 10⁻³ – 10⁴ s. In addition, the relationship between the strain energy function of HNBR and temperature is demonstrated to have a complex concave-downward shape which is affected by two competing contributions of entropy elasticity and the stress relaxation.     

Delayed formation and localisation of positronium in polymers at low temperatures

A theoretical model of the positron annihilation lifetime spectrum including the mechanisms of slow positronium (Ps) localisation and delayed Ps formation from a positron and a trapped electron was developed. The model was applied to two series of spectra for low-density polyethylene and high-density polyethylene (HDPE) collected at constant temperature (much below the glass temperature) as a function of measurement time. The Ps internal relaxation time and time of localisation of Ps in a free volume centre were determined. The results show that after long irradiation of the polymer a dominant fraction of positrons (unbound in Ps) annihilate from the trapped states. On the basis of parameters determined from the HDPE lifetime spectra, two S(t) curves (for sample in darkness and in light) were calculated. The predicted shapes of S(t) well agree with literature data obtained with the age–momentum correlation (AMOC) experiment. According to the new model the shapes of the para-Ps and the ortho-Ps (p-Ps) components are non-exponential. In spite of this, the multi-exponential decomposition of a polymer spectrum enables to determine correctly the value of the o-Ps lifetime, however the other parameters determined from the spectrum have no simple physical meaning.     

Vibrational relaxation of CF4 at low temperatures

An apparatus for low temperature acoustic measurements is described. The vibrational relaxation times of CF₄ have been determined between 183 K and 295 K, and the mechanism of vibrational energy transfer in this molecule is discussed.     

A theory for environmental craze yielding of polymers at low temperatures

It has been recently discovered that polymers craze at low temperatures in the presence of nitrogen or argon. A quantitative theory has been developed which explains (1) the critical temperature above which the phenomenon disappears, (2) the critical stress for nucleating a craze, (3) the effect of strain rate on the yield point and size of crazes, (4) the drop in the load during craze yielding, and (5) the increase in strength of the polymer in N₂ or Ar at high strain rates so that the ultimate strength may exceed that in He or vacuum. The crazing action of the gases is described qualitatively at the molecular level.     

Dielectric relaxation and effect of plasticisers on siloxane polymers

Studies of dielectric relaxation of polydimethyl siloxane have shown that there is anomalous behaviour near its melting point (≈ ?50°), probably due to the presence of “Rotator Phases”. A detailed study of polyphenyl methyl siloxane, with various polar and non-polar diluents, has been carried out. The most interesting aspect of this study is the systematic broadening of the loss curves with increasing concentration of diluents while retaining conformity with the empirical decay function π(t) = exp ? (t/τ_o)^β. The broadening of loss curves was explained in terms of a distribution of local concentration of the diluents. The range of concentration (ΔC) was calculated for various diluents. It was found that ΔC was larger for the polar diluents than for the non-polar diluents. This result shows that polar plasticiser molecules have changed considerably the environment of the segmental motion of the basic polymer. There is also a linear relationship between the loss factor maximum (?^″_m) with the number of carbon atoms in the plasticiser molecule.     

Host-assisted intramolecular vibrational relaxation at low temperatures: OH in an argon cage

The vibrational relaxation of hydroxyl radicals in the A (2)Sigma(+) (v=1) state has been studied using the semiclassical perturbation treatment at cryogenic temperatures. The radical is considered to be trapped in a closest packed cage composed of the 12 nearest argon atoms and undergoes local translation and hindered rotation around the cage center. The primary relaxation pathway is towards local translation, followed by energy transfer to rotation through hindered-to-free rotational transitions. Free-to-free rotational transitions are found to be unimportant. All pathways are accompanied by the propagation of energy to argon phonon modes. The deexcitation probability of OH(v=1) is 1.3 x 10(-7) and the rate constant is 4.7 x 10(5) s(-1) between 4 and 10 K. The negligible temperature dependence is attributed to the presence of intermolecular attraction (>kT) in the guest-host encounter, which counteracts the T(2) dependence resulting from local translation. Calculated relaxation time scales are much shorter than those of homonuclear molecules, suggesting the importance of the hindered and free motions of OH and strong guest-host interactions.     

Dielectric relaxation measurements on aerosol-OT/water/cyclohexane-solutions at low water content

The complex relative dieletric permittivity of aerosol-OT(AOT)/water/cyclohexane solutions has been measured within the frequency range 5 kHz-10 GHz. The investigated solutions were of oil-rich type with varying AOT- and water content. A marked dielectric relaxation has been found. The dielectric increment as well as the conductivity steeply increase with the water content, while the relaxation time decreases. Theoretical models of heterogeneous dielectrics consisting of polar, highly conductive inclusions in a nonpolar solvent with low conductivity have been applied to the results, allowing conclusions with regard to the inclusions, shape and conductivity.     

Spin-lattice relaxation of some polymer radicals of the peroxide type at low temperatures

At a frequency of 9300 Mc, the relaxation phenomena have been studied for radicals of the type R-OO in the lattices of the polymers-(CF₂)_n-,-(CF₂CFCl)_n-and-(CF₂CFH)_n-for polycrystalline specimens at liquid helium and liquid hydrogen temperatures. At liquid helium temperatures, strongly marked spin-spin cross-relaxation effects are observed. A model for the relaxation mechanism is presented which satisfactorily explains the data obtained.     

Dielectric relaxation studies of some linear crosslinked and branched polymers

Dielectric loss measurements are reported for polystyrene, crosslinked polystyrene, polyacrylamide, branched polyacrylamide, and poly(methyl methacrylate) at 1 and 10 kHz f over the temperature range ?85 to +100°C. Crosslinking and branching have a pronounced effect on the dielectric relaxation spectra of polymers. The methods of preparation of these polymers and their viscosity molecular weight data are also reported.