Motion of chain segments

In the glassy amorphous state polymers possess insufficient free volume to permit the cooperative motion of chain segments. Thermal motion is limited to classical modes of vibration involving an atom and its nearest neighbors. In this state, the polymer behaves in a glass-like fashion. When we flex or stretch glassy amorphous polymers beyond a few percent strain they crack or break in a britde fashion. 

Spin-lattice relaxation times of carbon-13 in different polypropylene stereosequences differ slightly while nuclear Overhauser enhancements are almost identical (1.8-2.0) [533] isotactic sequences display larger Tx values than the syndiotactic stereoisomers. Other vinyl polymers behave correspondingly [534]. Carbon-13 spin-lattice relaxation times further indicate that dynamic properties in solution depend on configurational sequences longer than pentads. The ratio 7J(CH) 7J(CH2) varies between 1.6 to 1.9 thus, relaxation can be influenced by anisotropic motions of chain segments or by unusual distributions of correlation times [181],... 

The presence of cooperative motion of chain segments present in intercalated polymer chains can be examined using various analytical techniques such as Differential Scanning Calorimetry (DSC), thermally stimulated current (TSC) and dielectric spectroscopy. DSC measurements on an intercalated PEO, (Mw= 100,000)/montmorillonite hybrid (20 wt. % polymer), indicated the absence of... 

The rotation of the oscillator in a luminescent marker inserted into the polymer chain is always accompanied by a complex translational and rotational motion of chain segments adjoining it. The main task of the theory is to take into account the polymer effects proper in PL. This problem has been considered by several authors including the authors of this review. 

In spin-diffusion studies it is possible to detect not only two but three domain sizes. The third domain can be considered the interface (i) between the other two domains, which can be different chemical species in a polymer blend or rigid crystalline (r) and mobile amorphous (m) material in a semicrystalline polymer. To illustrate this point, a mobility timescale is depicted in Fig. 7.2.25(a) and the simplified ID domain structure of PE underneath in (b). Rigid crystalline and mobile amorphous materials exhibit motion of chain segments with different correlation times Tc. The chains at the interface between both domains exhibit intermediate mobility. The exact ranges of correlation times in the individual domains depend on the particular choice of filters. Therefore, the values of domain sizes derived through spin-diffusion NMR also depend on the type of filters used. In particular, the interface is defined solely by the NMR experiment and can only be detected if the filters are properly chosen. 

Larger-scale cooperative motions of chain segments become possible at Tg, and the value of... 

There have been several attempts to relate Kg to the critical molecular weight (Mcr, which is discussed in Chapter 13) based on considerations of the effects of polymer chain entanglements on the large-scale cooperative motions of chain segments involved in the glass transition for example, see references [97-99]. It was also suggested [100,101], from considerations of chain stiffness and the statistics of chain conformations, that Kg should be proportional to a power of... 

Secondary relaxations are usually measured either by mechanical methods such as dynamic mechanical spectroscopy or (somewhat less often) by electrical methods such as dielectric relaxation spectroscopy [159], The existence of Tp is generally ascribed to the onset of a significant amount of some kind of motion of the polymer chains and/or the side groups attached to them, on a much smaller and more localized scale than the large-scale cooperative motions of chain segments associated with Ta. These motions are usually inferred from the results of measurements using methods such as nuclear magnetic resonance spectroscopy. See... 

The activation energy for the a-relaxation decreases from 46 kcal/mole in the dry polymer to 18 kcal/ mole at 8.7% water with most of the decrease coming below 0.88% water. (12) The increase in the dielectric constant associated with the a-relaxation is greater when water is present. This Indicates that water molecules are bonded to the amide groups and participate in the motion of chain segments. 

Figure 3.1. Brownian motion of chain segment illustrating effect of extension on average segment dimension.

More complex models can be formed by connecting a number of such elements in series. However, even these are only linear visco-elastic models in which the rate of straining is directly proportional to the stress. For polymers at typical structural levels of load, the stress-strain rate is often highly non-linear. A molecular interpretation of this can be found in a thermally activated rate process model involving motion of chain segments... 

In the glassy state the major phase of a polymer is the frozen phase composed of frozen bonds, where conformational motions of chain segments are locked. In contrast, the active phase consists of active bonds and the free conformational motion can potentially occur and the polymer exists in the full rubbery state. Figure 20 shows a schematic picture of a simpUfled 3-D SMP model with frozen phase (dark shaded region) and active phase (light shaded region), and a 1-D simplification to describe uniaxial stretches. 

Hookean elasticity, where the motion of chain segments is drastically restricted and probably involves only bond stretching and bond angle deformation the material behaves like a glass. 

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