Relaxation processes in semicrystalline polymers

Relaxation processes in semicrystalline polymers are complicated. Some of them occur in only one of the components. Others may involve both components. Some of them show a pronounced morphological dependence whereas others show many characteristics with no or only weak dependence on crystallinity and morphology. In isochronal (constant time or constant frequency) experiments between the temperature of liquid nitrogen (— 196°C) and the melting point of the polymer, at least two and often three relaxation processes are observed (Table 7.4). 

What are the molecular criteria for the presence of an a process Polymers with small or no pendant groups, with short repeating units and with weak intermolecular forces are the polymers most likely to have an a process. Examples of polymers belonging to this category are polyethylene, isotactic polypropylene, polyoxymethylene and polyethylene oxide. The pendant phenyl groups of isotactic polystyrene are too large and this polymer does not 

A central postulate of polymer crystallography is that the conformation of the polymer chains within the crystals is that of the lowest possible energy. Different crystal structures (so-called polymorphs) of a given compound may arise due to different types of packing of the low-energy chains. The chain 

The thickness of the lamellar crystals (LJ is small, typically about 10 nm, which leads to a considerable melting-point depression below the equilibrium (infinite crystal thickness) value (7°). The melting point (T ) is conveniently described by the Thompson—Gibbs equation  

Two or three relaxation processes occur in semicrystalline polymers. The low-temperature (y or P) process is a subglass process occurring in the amorphous phase. The medium or high temperature process (p or a,) is associated with the glass—rubber transition of the amorphous component. The glass transition is very weak, and in many cases difficult to find, in highly crystalline polymers like linear polyethylene. A certain class of polymers shows a high-temperature relaxation process denoted a, which is a combined crystalline and amorphous process. Reorientation of the chain by a 180° twist of the molecule in the crystals and a certain axial 

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The study of relaxation processes in Semicrystalline Polymers (qv) is a subject of continuing technological interest because of its practical importance. This is based on the observation that the stiffness of typical crystalline engineering thermoplastics at room temperature may be only one-third to one-fifth that of the same material at a low temperature. The drop in stiffness or modulus takes place in regions of temperature associated with a relaxation process. 

The study of relaxation processes in semicrystalline polymers is a subject of technological interest because of its practical importance. This is based on... 

The relaxation behavior of selected semicrystalline ESI is depicted in Figure 26.3. It can be seen that the loss peak evident in the temperature range —50 to +50 °C shows increasing breadth of the relaxation process as the styrene content in ESI decreases. The relaxation processes associated with this loss peak are complex in nature. The relaxation behavior of semicrystalline polymers is fundamentally different from that of amorphous polymers. The long-range segmental motions associated with the Tg process become hindered owing to the restrictions imposed by the crystallites. 

Figure 10 Typical relaxation processes in a semicrystalline polymer, is the relaxation process. The a and c subscripts refer to amorphous and crystalline phases.

Men Y, Rieger J, Strobl G (2003b) Role of the entangled amorphous network in tensile deformation of semicrystalline polymers. Phys Rev Lett 91 955021-955024 Men Y, Strobl G (2002) Evidence for a mechanically active high temperature relaxation process in syndiotactic polypropylene. Polymer 43 2761-2768 Plazek DJ, Chay I, Ngai KL, Roland CM (1995) Visoelastic properties of polymers. 4. Thermo-rheological complexity of the softening dispersion in polyisobutylene. Macromolecules 28 6432-6436... 

DMTA has been used to explore relaxation processes in amorphous and semicrystalline polymers (see Section 8.1.6). 

The glass transition is by far the most important one among the many transitions and relaxations observed in amorphous polymers. It has a drastic effect on the properties and processing characteristics of such polymers. It is important in semicrystalline polymers as well, but its role diminishes in importance with increasing crystalline fraction. 

Infrared (IR) spectroscopy plays a very important role in the physical characterization of polymers. IR absorption bands are well known for their marked specificity to individual chemical functionalities. Furthermore, the unique sensitivity toward the configuration, conformation, and other local sub- and supramolecular environments (e.g., different phases of semicrystalline polymers, moieties participating in specific interactions of miscible blends, and block polymer segments undergoing different stages of relaxation processes) makes IR spectroscopy a very powerful probing tool for numerous scientific investigations in polymer physics. 

For isochronal (constant frequency) experiments on Semicrystalline Polymers (qv) in the temperature range between the crystalline melting point and liquid nitrogen temperature (—196°C or 77 K), at least three relaxation processes are often foimd. The high temperature a process is often related to the crystalline... 

It must be emphasized that the site model is applicable only to relaxation processes showing a constant activation energy, examples being those assoeiated with localized motions in the crystalline regions of semicrystalline polymers. The... 

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