A-relaxation peak

The NMRD profile of the protein adduct shows a largely increased relaxivity, with the dispersion moved at about 1 MHz and a relaxivity peak in the high field region. This shape is clearly related to the fact that the field dependent electron relaxation time is now the correlation time for proton relaxation even at low fields. The difference in relaxivities before and after the dispersion is in this case very small, and therefore the profile cannot be well fit with the SBM theory, and the presence of a small static ZFS must be taken into account 103). The best fit parameters obtained with the Florence NMRD program are D = 0.01 cm , A = 0.017 cm , t = 18x10 s, and xji =0.56 X 10 s. Such values are clearly in agreement with those obtained with fast-motion theory 101). 

FIGURE 18.11 Thermally stimulated depolarization currents of PVP K30 demonstrating two different global relaxation peaksPi is the (5-relaxation peak (representing molecular motion belfry, and P2 is the a-relaxation peak (representing mobility ). 

NC samples maintain high elastic moduli to much higher temperatures. The E" curves for the blends show the PCL methylene relaxation at lower temperatures and prominent peaks above 0°C. The NC sample seems to lack a relaxation peak which would correspond to a Tg but does show a broad peak centered at about — 68°C. This peak also appears to be present in the 50 and 75% NC samples. 

The susceptibility minimum observed in the DS and LS relaxation spectra is a consequence of the interplay of the high-frequency tail of the a-relaxation peak and the contribution from certain fast dynamics dominating at frequencies close to but above the minimum. As was demonstrated by several experimental studies as well as by molecular dynamics simulations (cf. Fig. 13a), in addition to the vibrational contribution a fast relaxation process has to be taken into account for describing correctly the susceptibility minimum [5,9,19,55,64, 133,134,136,147]. This spectral contribution may usually be described by a power law with a positive exponent less than unity. In fact, the search for this fast relaxation process was mostly inspired by MCT, where it naturally appears in the solutions and is interpreted as rattling in the cage type of dynamics. Some authors discussed in addition a constant loss contribution [10,138,222,... 

In Fig. 12, G and 6 are plotted against cure time at various frequencies. For w = 10 rad/sec, there is no relaxation peak of 6" or cross-over point of G and G". This is due to the fact that at 140 C, the relaxation times of natural rubber at various states of cure are all longer than 0.1 sec (i.e., 1/w). However, there is a relaxation peak and cross-over point of G ... 

The a relaxation peak is strongly affected by the sustituents. In poly(para-halogenated styrenes), the peak temperature increases from 379 to 429 K more or less in proportion to the van der Waals radius of the halogen group, H, F, Cl, Br to I [30]. Gao and Harmon [6] showed that for para sustituents Ta and the width of the peak at half-height increases with increase in size of the pendant group. 

Similarly to the behavior of isotropic poly(ether ester)s the amplitude and position of the relaxation peaks in the loss curve of the extnidates were influenced by the composition of the amorphous phase. This is determined by the concentration and the degree of polymerization of the ester segments. For the extnidates the observed effect was pronounced only in the case of material C. Here, the glass transition temperature, as determined from the maximum of the so-called a-relaxation peak, increased linearly with decreeing extrusion temperature from - 4 C to 17 For the materials A and B the glass transition temperatures were found to be — 59 and - 50 °C, respectively, independently of the extrusion conditions. 

It was further shown that the structural a- relaxation peak shifts to the lower frequencies as the reaction proceeds, while the frequeney of maximum secondary relaxation loss remains constant, and only the signifieant inerease in amplitude of this relaxation process was observed. Such a behavior of y-relaxation confirms that in fact the secondary process takes its souree from intramolecular motions occurring within the monosaceharide unit. 

If the first scenario were real, the slower secondary relaxation should express its presence as an excess wing on high frequency side of the a- relaxation peak. To check this we superimposed dielectric loss spectra of octa-O-acetyl-lactose measured above and below Tg to that obtained at T=353 K. Next we fitted a master curve constructed in this way to the Kohlrausch-Williams-Watts function... 

However, it is difficult to interpret this in such a way. One can see that the coupling parameter n estimated for the a- relaxation peak of octa-O-acetyl-lactose is significant (it means that the distribution of a-relaxation times is quite broad). It implies that the separation between maxima of dielectric loss of the expected secondary relaxation and main structural relaxation peaks should be significant. Consequently, the p- mode should be clearly visible in the dielectric loss spectra. However, in the case of octa-O-acetyl-lactose the y- relaxation... 

The behaviour showed in Fig. 2 can be related to the occurrence of a post irradiation thermal cure, during the dynamic-mechanical thermal analysis test, at temperatures higher than the first relaxation. Samples irradiated at higher frequencies show only a relaxation peak at high temperature, thus indicating the occurrence of thermal cure during the irradiation itself. 

The dynamic mechanical analysis of virgin PP, untreated sisal/PP composites and MAPP treated sisal PP composites revealed an increase in the storage modulus (E ) in the PP matrix with the addition of fibers and MAPP (Figure 9.5a) [71]. The loss modulus displayed three relaxation peaks at -80°C (y), 8°C (P), and 100°C (a), respectively. The temperature of P relaxation maximum corresponds to the Tg of the matrix, while the a relaxation peak is related to the slip mechanism in the crystallites. The y relaxation peak is due to the... 

The shape of the segmental a-relaxation peak of the faster component in polymer blends depends only on the segmental a-relaxation time, Taf,... 

DDM = Diamino diphenyl methane For the mixtures of epoxy monomers, 1 1 mol ratio was used. Stoichiometric amounts were used in all cases. From differential scanning calorimetry method measurements (10 °C/min). Maximum of the loss modulus from dynamic mechanical thermal analysis measurements. a relaxation peak of the loss factor. Reproduced with permission from M. Sponton, L.A. Mercado, J.C. Ronda, M. Galia and V. Cadiz, Polymer Degradation and Stability, 2008, 93, 2025. 2008, Elsevier [22] ... 

FIGURE 26 Isothermal dielectric loss data of 1,4 poly butadiene that show resolved Johari-Goldstein relaxation in the supercooled liquid state above Tg. Representative KWW fit to the a-relaxation peak are shown as line. The value of n so determined is given in the figures. Each vertical arrow pointing toward certain data taken at some temperature indicates the location of the independent relaxation frequency, Vo = l/2 rToa, where To is calculated using Eq. (38). A, O, V, , are data taken at -97.5, -95, -92.5, and -91.2 C, respectively. 

Partial vitrification is also observed in isothermally cured epoxy systems. However, the effect is less pronounced since the glass transition domain at goo is narrower for these networks [80]. An example is given in Figure 2.14 for the system DGEBA-MDA T oo = 102°C). At 80°C, a stepwise decrease in Cp and a relaxation peak are observed. At 100°C, the system is partially vitrifjdng and the phase angle remains in the relaxation regime at the end of cure. At 120°C, no vitrification effect is noticed anymore, neither in Cp, nor in heat flow phase. 

The a relaxation peak in dynamic mechanical spectroscopy and dielectric relaxation spectroscopy of non-crystalUne polymers also reflect the glass transition phenomenon ... 

SAN-28 blends behaved similarly for compatible blends containing up to 50 wt % (or more) PCL, although differences were noted between the relaxation behaviours of as cast and quenched samples (the latter samples were quenched to low temperature after heating above the PCL Tj ). For incompatible blends containing 70% or more PCL the relaxation behaviour was a combination of a-relaxation peaks of the two component polymers and the P-relaxation of PCL [ 19]. Blends of SAN-30 (70 wt % PCL) simply exhibited a combination of a- and P-relaxations of the two polymers [20]. 

The viscoelastic properties of the blends were analysed using dynamic mechanical thermal analysis. The plot of tan 5 versus temperature showed a single relaxation peak around 130 °C, corresponding to the Tg of the epoxy-rich phase for the unmodified epoxy network and for the blends. On further examination, a relaxation peak of very low amplitude at around —65 °C (called p-relaxation), for both modified and neat epoxy matrices, is found, which is attributed to the motions of glycidyl units in the network. The Tg of the rubbery phase seems to be overlapped with p-relaxation and the Tg of the epoxy-rich phase slightly shifts toward the low-temperature side with the addition of the rubber. The decrease in Tg of the epoxy phase can be attributed to a dilution effect by the addition of the rubber phase, and may also result... 

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