Amorphous polymers orientation mechanism

The effects of orientation via mechanical deformation on Tg have been reviewed [65]. Tg increases in those amorphous regions of a semicrystalline polymer that are either attached to crystallites or so close to them that their chain segment mobilities are hindered because of the interference of the crystallites. On the other hand, orientation has little effect on Tg in amorphous regions far away from crystallites as well as in completely amorphous polymers. 

Linear viscoelasticity is valid only imder conditions where structural changes in the material do not induce strain-dependent modulus. This condition is fulfilled by amorphous polymers. On the other hand, the structural changes associated with the orientation of crystalline polymers and elastomers produce anisotropic mechanical properties. Such polymers, therefore, exhibit nonlinear viscoelastic behavior. 

Rheo-optical FTIR spectroscopy, therefore, not only provides a means to monitor strain-induced crystallization on-line to the mechanical treatment but also yields detailed information in terms of the orientation of the polynKr chains in the amorphous polymer relative to those in the strain-crystallizing domains. 

Structural Requirements for Piezoelectric Polymers. The piezoelectric mechanisms for semicrystalline and amorphous polymers differ. Although the differences are distinct, particularly with respect to polarization stability, in the simplest terms, four critical elements exist for all piezoelectric polymers, regardless of morphology. These essential elements are (.1) the presence of permanent molecular dipoles, (2) the ability to orient or align the molecular dipoles, (5) the ability to sustain this dipole alignment once it is achieved, and (4) the ability of the material to undergo large strains when mechanically stressed (3). 

The SSE process based on changes in polymer-billet shape is, first of all, characterized by the EDR value. The limited EDR value is determined by polymer type, molecular weight and morphology. For example, for HDPE it may reach more than 40 (1). For polyoxymethylene (POM), polypropylene (PP), PTFE the maximum values of draw ratio equal 10,6, and 4 (20-22), respectively. For polymethylmethacrylate (PMMA), polystyrene (PS), polycarbonate (PC), and other amorphous polymers they are even lower (21,23). The extruded specimens are highly oriented and possess improved mechanical properties. In the case of semicrystalline polymers, the extrusion results in a considerable increase of tensile modulus and strength (1,2). With amorphous polymers, a considerable increase of plasticity is observed alongside with increase of tensile moduli (21,23). 

Fig. 1-8. Schematic representation of an amorphous polymer [a], a crystalline polymer [b], and an oriented crystalline polymer [c]. (From Mechanical Properties of High Polymers, Turner Alfrey, Jr. Interscience, 1948)...

Orientation represents the alignment of the polymer molecules. Although usually associated with crystalline structures, orientation can and does occur in amorphous materials. If mechanical stress is applied, either the amorphous polymer molecule or the crystallite will be oriented. It is possible for orientation to bring... 

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