Semicrystalline polymers widths

In man-made fibres, any stretching will irreversibly alter the crystallinity and there is no control of the lateral size of polymer crystals. Semicrystalline polymer networks typically consist of platelet type crystals whose width exceeds their thickness by several order of magnitudes because only the thickness is controlled by the chain folding [61]. In contrast to synthetic fibres, spider silk does not need any mechanical treatment by external forces the constituents self-assemble directly during the spinning-process. These examples clearly demonstrate the need for more detailed control of the mesoscopic structures for further development of man-made materials. 

The diffraction peaks obtained with a perfect crystal are in theory expected to be infinitely sharp. The finite widths of the observed diffraction peaks as seen in Figure 3.2 reflect the fact that crystallites in semicrystalline polymers are not perfect, and the analysis of the line widths can tell us about the nature and degree of imperfection in the polymer crystal lattices and the size of the polymer crystallites if they are small. 

It is well known that the morphological and molecular structures of polymers play an important role in their wear behavior. It seems that the degree of crystallinity is also a structural factor of semicrystalline polymers important to their wear. Lontz et al. ( ) reported that the wear of poly(tetrafluoroethylene),(PTFE) decreased with the increase in crystallinity. Tanaka et al. (2 ) studied the wear of heat-treated PTFE specimens and concluded that the wear rate was affected by the width of the band in the fine structure rather than crystallinity. Recently, Hu et al. ( 3) have studied the effect of crystallinity on wear of PTFE using various heat-treated specimens. They have shown that the wear decreases with the increase in crystallinity, when molecular weight is constant. Eiss et al. ( ) reported that poly(chlorotrifluoroethylene) of a crystallinity of 65% exhib-ted higher wear than that of 45%. The results obtained by the authors mentioned above indicate that the effect of crystallinity on the wear of polymers is somewhat complicated and further investigation is needed to clarify the effect of crystallinity on polymer wear. 

At temperatures above their Tg, the resonance spectrum of noncrystalline polybutadiene (PB) (Fig. 8a) is clearly different from that of the semicrystalline polyethylene (Fig. 8c). Amorphous PB exhibits a narrow Lorentzian line shape with a width of 0.2 G. In contrast, the PE spectrum comprises two components, ie, narrow and broad line shapes. When the spectra of semicrystalline polymers are recorded in the glassy state (Figs. 8b and 8d), only abroad component is observed. This indicates that the line shape corresponds to molecular mobility and the line width reflects a correlation (or relaxation) time. Therefore, the broad and narrow components of semicrystalbne PE (Fig. 8c) are related to protons of methylene groups in rigid and mobile (amorphous) environments, respectively. On the basis of this, it was proposed that the degree of crystallinity could be determined by resolving the area of the broad component (rigid phase) from the spectrum. 

Two new parametets for describing the SAS data from semictystalline polymers are introduced. These are the ellipticity of the trace of the lamellar peak-maxima, and the orientation parameter determined from the increase in the longitudinal width of the lamellar peak with the distance from the meridional axis. These two parameters along with Ae lamellar spacing, tilt-angle of the lamellar plane, the diameter and the coherence length of the lamellar stack, and the lamellar intensity completely describe the SAS data from oriented semiciystalline polymers. These parameters can be obtained by fitting the 2-D SAS from uniaxially oriented semicrystalline polymers in elliptical coordinates. 

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... 

Figure 1.2 Schemes of a few one-dimensional nanostructures that can be reahzed by polymeric materials, (a) Flexible nanofiber (typically made of amorphous polymers), (b) nanowire (frequently made of semicrystalline organics) and (c) nanoribbon. The insets in (b) and (c) show schematics of the corresponding cross-sections of the nanostructures, r. nanowire radius w nanoribbon width h nanoribbon thickness.

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