Relaxation processes semicrystalline polymers

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... 

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... 

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. 

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... 

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

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

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). 

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... 

---