Segmental a-relaxation process

On decreasing the content of the fast PI component in blends with PtBS, both the segmental relaxation and the NM have an increase in nonexponentiality (more stretched) and relaxation time. The effects increase with decrease of temperature. Observed at constant NM relaxation time equal to tr 3.2 x 10 s is an increase in separation between the NM time tr and segmental a-relaxation time of PI on decreasing the concentration of PI in the blends. This nontrivial relation between the two processes deserves an explanation. 

For polymers, dielectric spectroscopy is sensitive to fluctuations of dipoles, which are related to the molecular mobility of groups, segments, or the polymer chain as well [38]. The molecular mobility is taken as a probe for structure. The basic quantity is the complex dielectric function e f) = t (f) - it"(f) as a function of the frequency/and the temperature T. s (/) is the real whereas e"(/) is the loss part i = >f ). A relaxation process is indicated by a step-like decrease of s (/) with increasing frequency and a peak in e"(/). From the maximum position of the peak a mean relaxation rate can be deduced, which corresponds to the relaxation time of the fluctuation of the dipole moment of a given structural imit. For details see reference [49]. All shown measurements were carried out isothermally in the frequency range from 10 to 10 Hz by an ALPHA analyzer (NovocontroF). The temperature of the sample is controlled by a Quatro Novocontrol system with stability better than 0.1 K. 

For the most part, the timescales for the aforementioned kinetic processes are well beyond the accessible timescale for fuUy atomistic MD simulations. Local dynamics such as rotation of a methyl group or a polymer side chain can certainly be explored. For example, in a polymer melt at a temperature of lOOK above the T, the timescale for methyl-group rotations is about Ips and approximately 1-lOns for segmental a-relaxation in a polymer [4b]. Diffusion for even a small molecule such as water in... 

To discuss this in detail, we consider a segment of the polymer chain that has to overcome an energy barrier Q of a relaxation process (see section 8.1.1) to slide past a neighbouring segment and enable the deformation. The probability P to overcome this barrier by thermal activation is, according to appendix C.l, P oc exp —Q/kT), with the temperature T and Boltzmann s constant k. 

In 95-160°C region (above Tg), molecular dipoles are able to orient under the external field. An additional effect of polarization was observed which increases the permittivity of the P4FST (i.e., e = 5.35 at 140°C and 1.09 kHz). Through tan8 (Figure 20.19b), it is possible to note a relaxation process known under a-relaxation which is related to the cooperative reorientation motion of large segments above T. The dielectric loss is lower around 10 (i.e., 0.06 at 1.09 kHz). Similar behavior was observed in the atactic polystyrene [116]. 

Polymers can have dipoles in the monomeric unit that can be decomposed in two different components parallel or perpendicular to the chain backbone. The dipole moment parallel to the chain backbone giving rise to an "end-to-end" net polarization vector will induce the so-called dielectric normal mode dielectric relaxation that can be studied using theoretical models. The dipole moment perpendicular to the chain backbone will lead to the segmental a-relaxation that can only be described using empirical models, since no definitive theoritical framework exists for this universal process. 

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