Hysteresis semicrystalline polymer

Another, presently unsolved problem in hysteresis of glassy materials is found in semicrystalline polymers. In these samples one finds that the remaining amorphous fraction is showing reduced or no hysteresis when compared to the fully amorphous polymer. The data on poly(ethyIene terephthalate) showed, for example, that a 10% crystallinity is enough to make the hysteresis peak disappear. 

The hysteresis behavior of the diblock copolymer HBI-50 is not shown but is very similar to that of HIBI-49. In summary then, the difference in hysteresis behavior of the HBIB series to that of HIBI and HBI is related to the ability of the members of the first series to form permanent entanglements, by entrapment of the end blocks in the semicrystalline domains, whereas no such arrangment is possible for neither HIBI nor HBI series. The permanent entanglement serves as a physical crosslink which promotes recovery of the polymer after the deforming stress has been removed. At the same time, much less energy is lost as heat. 

Menczel, J. and Wunderlich, B. Heat capacity hysteresis of semicrystalline macromolecular glasses. J. Polymer Sci., Polymer Letters Ed. 19, 261 (1981)... 

Ferroelectric materials are a subclass of pyro- and piezoelectric materials (Fig. 1) (see Piezoelectric Polymers). They are very rarely foimd in crystalline organic or polymeric materials because ferroelectric hysteresis requires enough molecular mobility to reorient molecular dipoles in space. So semicrystalline poly(vinylidene fluoride) (PVDF) is nearly the only known compoimd (1). On the contrary, ferroelectric behavior is very often observed in chiral liquid crystalline materials, both low molar mass and poljuneric. For an overview of ferroelectric liquid crystals, see Reference 2. Tilted smectic liquid crystals that are made from chiral molecules lack the symmetry plane perpendicular to the smectic layer structure (Fig. 2). Therefore, they develop a spontaneous electric polarization, which is oriented perpendicular to the layer normal and perpendicular to the tilt direction. Because of the liquid-like structure inside the smectic layers, the direction of the tilt and thns the polar axis can be easily switched in external electric fields (see Figs. 2 and 3). 

Thermal conductivity data as a function of temperature are presented for live commercial polymers, three amorphous and two semicrystalline. Most measurements have been carried out over a temperature range from the processing temperature down to room temperature. The polymers selected for these measurements are known to be reasonably stable. In many cases, it has been possible to carry out an entire heating scan to verify the cooling curve or examine hysteresis effects. 

The polymer in this composite is high density polyethylene, which is semicrystalline with a crystallinity of about 75%. The crystalline melting point is about 130 C. This corresponds well to the abrupt increase in resistance observed in Figure 10a. The width of the polymer melting transition also corresponds well with the width of the abrupt increase in resistivity. The polymer melting is a second order phase transition, and shows hysteresis. When this composite is cooled from 180 to 20 C the polymer does not recrystallize at the same temperature at which it melted (130 C) it recrystallizes at a lower temperature. The resistance vs. temperature... 

An important aspect of the tensile deformation of fibers of semicrystalline and crystalline polymers is the recovery after unloading. When the first extension of an oriented fiber reaches well into the second stage of the tensile curve, i.e. beyond the yield point, then the recovery is not complete. The permanent extension or set is approximately equal to the extension at which yielding occurs. The recoverable extension shows a spontaneous and latent recovery corresponding to elastic and viscoelastic or delayed elastic contributions. Further repeated extension of the fiber up to the same maximum extension hardly increases the permanent deformation but is still accompanied by a little hysteresis, as shown in Fig. 6.19. 

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