Local chain reorientation

One example of mapping by chain diffusion involved the case of lOmers of polyisoprene at 413 K. A dynamic mapping between a fully atomistic and a very simple coarse-grained model was demonstrated. Only chain stiffness was used to perform the mapping in that study. The local chain reorientation in both simulations was the same after the time scales had been determined by the diffusion coefficient. The decay times of the Rouse modes, however, were not equal, indicating that mapping by stiffness alone is too simplistic. 

In the highest temperature range, reorientation of the glutarimide cycle around the local chain axis occurring within a MGI sequence, a reorientation that is associated with some MGI-MGI cooperativity, as in MGI-rich copolymers... 

For MGI-rich copolymers, MGI-MGI cooperativity accompanies reorientation of the glutarimide cycle around the local chain axis... 

Several attempts have been made to interpret the relaxation data of polysaccharides by employing a variety of dynamic models suitable for local chain motions of synthetic polymers. One motional model that has been applied to polysaccharides133135136 is the log 2) model72 (Eq. A-6 in the Appendix), which assumes that isotropic reorientation is characterized by a skewed distribution of correlation times, with tails toward longer correlation times (Fig. 13). The distribution func-... 

The lineshape changes observed for the amorphous fraction of the polymer between -128° and -60° (Figure 8) indicate some type of relaxation whose origin is similar to that just described. It appears that this process involves reorientation about the local chain axis and the rate of this process is 0.1 kHz at -80°C. The relaxation data of Figure 9 are consistent with this explanation and indicate that the rate of this motion is 30 kHz at -50°C. However, this process is coupled with a higher temperature process (see below) and does not give definitive lineshapes which are amenable to lineshape analysis. We can estimate that this relaxation has an activation energy of 16 5 kcal/mole. Therefore, we conclude that the process responsible for these results is the Y relaxation. 

These considerations lead us to conclude that the process responsible for the observed lineshape changes may be idealized as one in which the rate of the motion is fast ( 100 MHz) at all temperatures (-40°C to 260°C) and the amplitude of the motion grows as a function of temperature. Since the chemical shift lineshape is nearly axially symmetric at v -68°C (due to rapid reorientation about the local chain axis), we can describe this motion whose amplitude grows with temperature as reorientation of the local chain axis. 

The reorientation of the local chain axis gives rise to the partial narrowing of the amorphous REV-8 spectra and is of a random nature. In a short period of time, a particular segment of the macromolecular chain assumes a distribution of directions which deviate from its orientation at rest. In analogy to the description of molecular ordering in liquid crystals—, we use the concept of a local order parameter for the quantitative characterization of the extent of these fluctuations. 

Of course, LCPs are not static Iwt dynamical systems. The most important molecular motions, expected for these systems, are shown in Fig. 4. One can distinguish at least three different motional modes. The intramolecular motions consist of local internal reorientations such as tram-gmu isomerization or ring flips [6,10]. The intermolecular motion is the motion of several chain segments or the molecule as a whole. Two basic motional modes are distinguished, namely rotation about the long molecular axis and reorioitation of this axis, respectively [10]. 

Regarding local molecular dynamics, it is well established that all surfactant self-assemblies are characterized by very rapid short-range dynamics. The surfactant chains, as well as oil and water molecules, are in a liquidlike state. Local molecular reorientation occurs on time scales of 10 s, similar to pure water or hydrocarbon. On the other hand, due to the presence of the surfactant films, the reorientation is more or less anisotropic. Local translational motion is also quite fast, but long-range translation is strongly dependent on the presence of the surfactant films. 

Block copolymers with dielectrically active segments can provide insight into more local chain motions. A block polymer with a central active segment and dielectrically-silent wings shows relaxations in two frequency domains the relaxations are plausibly interpreted as local segmental motion and whole-body reorientation. 

The description of the chain dynamics in terms of the Rouse model is not only limited by local stiffness effects but also by local dissipative relaxation processes like jumps over the barrier in the rotational potential. Thus, in order to extend the range of description, a combination of the modified Rouse model with a simple description of the rotational jump processes is asked for. Allegra et al. [213,214] introduced an internal viscosity as a force which arises due to a transient departure from configurational equilibrium, that relaxes by reorientational jumps. Thereby, the rotational relaxation processes are described by one single relaxation rate Tj. From an expression for the difference in free energy due to small excursions from equilibrium an explicit expression for the internal viscosity force in terms of a memory function is derived. The internal viscosity force acting on the k-th backbone atom becomes ... 

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