Movement of chain

For both investigations it will be necessary to utilize experimental techniques which have submolecular resolution since it is necessary to determine the movement of chain segments across the interface. 

In networks chain entanglements and local movements of chain segments with respect to each other also occur, but flow is not possible. 

In polyethylene the ac-relaxation process (see Section 3.4) enables the movement of chains into and out of the crystalline lamellae. Theoretical treatments have demonstrated that it most probably proceeds by propagation of a localized twist (180° rotation) about the chain axis extending over 12 CH2 units (Fig. 6.14). As the twist defect travels along the chain, it rotates and translates the chain by half a unit cell (i.e, by one CH2 unit) - this is termed the c-shear process (Mansfield and Boyd, 1978). The activation energy for this process is about HOkJmoF1, corresponding to the extra energy required to introduce the twist defect into the crystal. Once formed, the twist can freely... 

Figure 2.22 Movements of chain terminals, loops, and segments in the glass transition temperature range. (From Ref. 24.)...

These materials exhibit complicated behaviour, which depends on the degree of crystallinity and the detailed morphology. In general, relaxations can occur within the amorphous phase, within the crystalline phase, within both phases or be associated with specific details of the morphology, e.g. with the movement of chain folds. For these reasons the detailed interpre-... 

If impurities are added to the polymer at its melting point, fewer molecules return to the crystal phase because the impurity molecules displace polymer crystal molecules and fewer polymer molecules are able to deposit on the crystal phase. The rate of movement of chain molecules into the melt is not affected by the presence of impurities. So, the melting point of the polymer/impurity composition is lower than that of the pure polymer. 

Rubberlike elasticity, where the local freedom of motion associated with small-scale movement of chain segments is retained but large-scale movement (flow) is prevented by the restraint of a diffuse network structure. 

Safety system conveyors carrying inflammable materials need to be provided with sprinklers and easily accessible switches for stopping the drive motor (movement of chain) to prevent spread of fire. 

The Doi-Edwards theory assumes that reptation is the dominant mechanism for conformational relaxation of highly entangled linear chains. Each molecule has the dynamics of a Rouse chain, but its motions are now restricted spatially by a tube of uncrossable constraints, illustrated by the sketch in Fig. 3.38. The tube has a diameter corresponding to the mesh size, and each chain diffuses along its own tube at a rate that is governed by the Rouse diffusion coefficient (Eq. (3.37)). If the liquid is deformed, the tubes are distorted as in Fig. 3.39, and the resulting distortion of chain conformations produces a stress. The subsequent relaxation of stress with time corresponds precisely to the progressive movement of chains out of the distorted tubes and into random conformations by reptation. The theory contains two experimental parameters, the unattached mer diffusion coefficient T>o... 

The motion of a limited number of subchains is enough to impart a rubbery behavior to a polymer sample as observed in networks above their glass transition temperature. However, for very dense networks, the movement of chain segments is limited and the phenomenon is difficult to detect. Conversely, for polymers of short size, the motion of the whole chain can result in irreversible phenomena such as their creep and relaxation (see Chapter 12). 

An effective way to obtain amorphous polyethers is by grafting low molecular weight PEO to the polymer skeleton and form comb- or star-like structures. Since ionic conduction is closely related to the movement of chain segments, it can be expected that a polymer with an amorphous structure together with low Tg will meet the need for fast ionic conduction at room temperature. There are two common ways to synthesize comb polyethers [1] ... 

Each of the behaviors unique to coalesced PET can be attributed to the g tg + kink conformations assumed by its ethylene glycol fragments when included in the narrow channels of its y-CD-IC, which are compared schematically in Fig. 11.3 (bottom) to the dll-trans crystalline conformation (top) of PET. The kink conformations are nearly as extended as the crystalline, ll-trans conformation, but they have a smaller cross-section, which explains their preferential inclusion in the narrow channels of its y-CD-lC (see Fig. 11.1). When coalesced from its y-CD-IC the PET chains can readily and rapidly convert to the all-trans conformation through simple counter rotations about the —CH2—O— bonds and therefore crystallize. Normal melts of PET consist of chains with predominantly g —CH2—CH2— bonds, which must be rotated to the trans conformation during crystallization. This conformational transition is difficult to accomplish without long-range movements of chain segments, consistent with the normally slow crystallization rate of PET. Furthermore, both solid-state FTIR and C-NMR observations... 

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