Relaxation at temperatures below

In addition to Tg (amorphous phases) and the melting temperature Tm (crystalline phases), polymers also manifest secondary relaxations at temperatures below those of major relaxations (Tg or Tm, which will collectively be referred to as T. The main secondary relaxation temperature wil be designated generically as Tp, although it may be labeled differently in the literature on specific polymers. For example, it is commonly labeled as Ty for bisphenol-A polycarbonate where Ty is for a relaxation of higher intensity than Tp, and occurring at a lower temperature, which is the main secondary relaxation of this particular polymer. 

Various methods, including dilferential scanning calorimetry (DSC), dielectric relaxation spectrometry, and NMR, are known to be useful to determine molecular mobility of freeze-dried formulations [16,17]. The glass transition temperature (Tg) has been used as a measure of molecular mobility of lyophilized formulations, since it indicates the critical temperature of a-relaxation for amorphous polymer materials. Freeze-dried formulations containing polymer excipients can be considered to exhibit low molecular mobility without a-relaxation at temperatures below Tg. 

The glass transition temperature, Tg, of polymer glasses could be identified as that of the first order hole phase transition by satisfying the criterion consisted of Eqs. (1) and (2), in which fx has been added under the operational definition concluding that the stable glasses could not be formed easily without the generation of "ordered p>art / hole" pairs during the enthalpy relaxation at temperatures below Tg ) ... 

The existence of secondary relaxations at temperatures below is clearly obvious... 

New results will be presented based on stress relaxation at temperatures below Tu Gat indicate Gat as the test temperature is increased Ge longer relaxation times disappear as Ge transition region is approached, Gen wiG fiirGer mcrease in test temperamre a new set of much longer relaxation times appear and Gen disappear at Ge upper end of Ge transition temperamre range. 

Fe(ptz)6] (BF4)2 and [Zni Fej.(ptz)6] (BF4,)2. The solid iron(II) complex of the unidentate ligand ptz = 1-propyltetrazole shows a reasonably sharp spin-state transition at about 130 K [112]. Optical excitation into the spin-allowed -> absorption band produces via LIESST the HS T2 state which remains trapped at temperatures below 50 K. Relaxation of the metastable J2 state to the LS state has been studied by following the changes of... 

Fe(6-Mepy)2(py)tren] (004)2 Doped in PSS. Magnetic susceptibilities measured for a microcrystalline sample of the complex produce a magnetic moment value = 0.36 pg at 10 K and 0.61 pg at 150 K, followed by a gradual increase to Peff = 2.80 pe at 311 K [138]. Thus 26% of the complexes are in the HS state at 300 K if a magnetic moment of 5.1 Pe is assumed for the pure HS compound. On the other hand, the complex doped into a polystyrene sulfonate (PSS) film does not provide any evidence for a thermal population of the HS state up to 340 K as demonstrated by variable-temperature UV-VIS and Mossbauer spectra. In fact, all the complexes doped into the PSS film are in the LS state at temperatures below 340 K. However, if irradiated by a single pulse of a Q-switched Nd/YAG laser (532 mp), the complex is excited from the LS ground state to the HS J2 states via an intermediate MLCT state and the metal states. The subsequent back relaxation from the excited T2 state to the... 

Homopolymers such as poly[(V)-3,7-dimethyloctyl-2-methylpropylsilylene], 117, were initially studied, and the helix-helix transition was discussed in terms of an entropically driven phenomenon in which at temperatures below Tc the side chains of the helical polymer are in a very ordered state and enforce a particular screw sense, whereas above Tc, the side chains become disordered such that the main chain can relax into the opposite screw sense.314 This concept is expressed in Figure 47. 

To this purpose, Fossheim et al. reported temperature sensitive liposomal Gd(III)-based probes (137-138). The composition of the liposomes was chosen in order to tune the temperature of transition between the gel-crystalline, where the liposome is water-impermeable, to the liquid-crystalline state, where water has free access to the interior of the liposome. This means that at temperatures below the transition, the relaxivity of the system is very low (paramagnetic contribution close to zero), whereas at higher temperature the Gd(III) complex ([GdDTPABMA(H20)] is the reference) is no longer silent... 

Indirect evidence of nonequilibrium flucmations due to CRRs in structural glasses has been obtained in Nyquist noise experiments by Ciliberto and co-workers. In these experiments a polycarbonate glass is placed inside the plates of a condenser and quenched at temperatures below the glass transition temperature. Voltage fluctuations are then recorded as a function of time during the relaxation process and the effective temperature is measured ... 

The original studies (McDonald et cd., 1965) that reported the formation of these species were limited by the available technology to temperatures above 77 K. At those temperatures, the minority S = species are the only ones detectable by EPR, but at temperatures below 25 K, attained by the use of liquid helium as a refrigerant, other species can be observed (Salerno and Siedow, 1979 Rich et al., 1978). These species have much faster relaxation rates because they... 

Similar expressions can be generated for holes simply by letting coc - — relaxation time xB needs justification, which will not be attempted here. Suffice it to say that this assumption is not bad for elastic scattering processes, which include most of the important mechanisms. A well-known exception is polar optical-phonon scattering, at temperatures below the Debye temperature (Putley, 1968, p. 138). We have further assumed here that t is independent of energy, although this condition will be relaxed later. 

The observed T1 can have two contributions 1/T = 1/T e + l/Tij. T e results from relaxation effects caused by conduction electrons, and Tm results from dipolar relaxation effects caused by the motion of nuclear spins. At temperatures below 110°C, the conduction electron effect appears to dominate T1 since a plot of T vs. 1/T can be fit with a straight line passing through the origin. At higher temperatures, the relaxation caused by the proton motion becomes... 

The behaviour of the polarized reflectivity and optical conductivity spectra of new quasi-two-dimensional organic conductor p -(BEDO-TTF)5[CsHg(SCN)4]2 versus temperature for E L and E1. L are quite different. For E . L, the temperature changes of R(ro) and ct(co) are due to the decrease of the optical relaxation constant of the free carriers as expected for a metal. For E L at temperatures below 200 K, the energy gaps in the ct(co) spectra at about 4000 cm 1 and at frequencies below 700 cm 1 appear simultaneously with the two new bands of ag vibrations of the BEDO-TTF molecule activated by EMV coupling. This suggests a dimerization of the BEDO-TTF molecules in the stacks, which leads to a metal-semiconductor transition.. In the direction perpendicular to L, the studied salt shows metallic properties due to a very favourable overlap of the BEDO-TTF molecular orbitals. 

The activation energy obtained by dielectric measurements for 8 -process is 28 kJ mol-1. This value is close to that obtained for PCHMA [30,38], Therefore the molecular origin of both relaxation processes should be related. This relaxation is observed in cyclohexyl compounds at temperatures below the y relaxation and, several authors [125] have related this relaxation to the motions of cyclohexyl group as a whole. In order to interpret the molecular origin of the 8 -transition, calculations... 

Higher-energy optical phonons may explain the exciton relaxation well above the exciton-band bottom (to > co0 + 50cm-1) for instance, the 140-cm 1 Bg lattice mode dominates the a-polarized exciton relaxation. However, acoustical and lower-energy (around 50 cm 1) optical phonons, according to our model, account by themselves for the observed broadening at temperatures below 77 K. 

The 1-2 cm 1 blue shift in the near-ambient spectra can be readily explained. Both B and B2 were found to blue shift nonexponentially with time after photolysis, with B experiencing a larger shift. This time-dependent blue shift is a consequence of the conformational relaxation that can occur under ambient conditions (39,46-48) but is inhibited at temperatures below the 185 K glass transition of the protein (24). Due to the similarity between the cryogenic and near-ambient temperature B-state center frequencies, the cryogenic B states appear to be the same as those measured near ambient temperature, but trapped within a conformationally unrelaxed protein. 

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