Linear crystalline polymers properties

Biphasic systems linear crystalline polymers and their properties... 

Linear crystalline polymers always contain a fraction of amorphous material. For this reason they are usually considered biphasic systems. They show the typical transitions of amorphous polymers (glass and secondary) but also the common transitions of crystalline polymers (polymorphic, order-disorder, melting). Mechanical and physical properties of this category of polymers depend on morphology and amorphous/crystalline ratio, but also on the molecular mobility of the amorphous phase. 

The most relevant property of stereoregular polymers is their ability to crystallize. This fact became evident through the work of Natta and his school, as the result of the simultaneous development of new synthetic methods and of extensive stractural investigations. Previously, the presence of crystalline order had been ascertained only in a few natural polymers (cellulose, natural rubber, bal-ata, etc.) and in synthetic polymers devoid of stereogenic centers (polyethylene, polytetrafluoroethylene, polyamids, polyesters, etc.). After the pioneering work of Meyer and Mark (70), important theoretical and experimental contributions to the study of crystalline polymers were made by Bunn (159-161), who predicted the most probable chain conformation of linear polymers and determined the crystalline structure of several macromolecular compounds. 

With these properties a wide field of application is revealed As the l.c. side chain polymers can be orientated in the l.c. state by an electric or magnetic field, it is possible to store any information obtained in the l.c. state by cooling the liquid crystalline polymer down to the glassy state. Obvious applications are e.g. optical filters or reflectors, prepared for linearly or circularly polarized light by cholesteric polymers. Furthermore the glassy polymers can serve as anisotropic matrices for dissolved molecules. 

The mechanical and thermal properties of a range of poly(ethylene)/ poly(ethylene-propylene) (PE/PEP) copolymers have been examined by Mohajer et al. (1982). They studied the effect of variation of composition and copolymer architecture on the polymer properties by synthesizing a range of PE-PEP-PE and PEP-PE-PEP triblocks and PE-PF.P diblocks with high molecular weights (M > 200 kg mol (.The crystallinity, density and melting enthalpy for all copolymers were found to be linearly dependent on the PE content, indicating microphase separation of PE and rubbery PEP in the solid state. The... 

By definition, thermoplastics have limitations at elevated temperatures. It is in this particular property that fibrous glass can lead to remarkable improvements. However, a sharp division exists for reinforced thermoplastics. The various reinforced thermoplastics can be put in two groups relative to DTUL. These consist of amorphous and crystalline or semicrystalline polymers. The amorphous polymers such as styrene-acrylonitrile, polystyrene, polycarbonate, poly (vinyl chloride), and acrylo-nitrile-butadiene-styrene are generally limited to modest DTUL improvements, usually on the order of 20°F with 20% glass. However, crystalline polymers such as the nylons, linear polyethylene, polypropyl-... 

The mathematical relationship between the stress and the strain depends on material properties, temperature, and the rate of deformation. Many materials such as metals, ceramics, crystalline polymers, and wood behave elastically at small stresses. For tensile elastic deformation, the linear relation between the stress, a, and strain, e, is described by Hooke s law as... 

A similar kind of association may be responsible for the mechanical properties of the polyimide discussed at the beginning of this article. Thus, the 10-second modulus shows only a gradual decrease over the temperature interval 30° to 450°C. rather than an abrupt decrease over a small temperature span, as is usually observed for linear amorphous polymers at temperatures near the glass transition (10). It was suggested that a small amount of crystallinity was responsible for this behavior, which is similar to the explanation advanced for the properties of BBB. Despite this indication of limited molecular mobility, the polyimide is reported to be ductile in elongation and thus again similar to BBB. 

It is well known that crystallinity affects the density and the moduli of polymers. Hence sound speed will also depend on those properties, but it is beforehand not clear how it depends on these properties. In Fig. 14.3 (Text is plotted vs. both density and crystallinity. In both cases the sound speed varies linearly with these properties, at least in the region shown, whereas according to Eq. (14.3) Hext is equal to (E/p)1/2. 

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