WAXS Peaks and Peak Positions

Detection of Crystallization and Melting. A problem frequently tackled by monitoring WAXS peaks is the detection of crystallization and melting as a function of temperature, time, pressure, or other processing parameters As long as some of the characteristic peaks of a polymer material are observable, a fraction of the material is in crystalline state. 

On the other hand, if no peaks are observed, it cannot be concluded that there is no crystallinity. Because the peaks of polymer samples frequently are broad, the 

Peak positions may vary ° as a function of temperature due to thermal expansion or due to conformational changes. Peaks may rest on an inclined background. In this case the background must be subtracted before the peak position is determined, which is simple if the peak is symmetrical. 

Asymmetrical Peaks are rarely found in WAXS from polymers, but they are ubiquitous in the MAXS of liquid crystalline polymers. For asymmetrical peaks in isotropic patterns it is best to determine the peak position from the maximum of the peak, if peak asymmetry is a result of linear or planar disorder. Linear disorder means that the crystals are more or less one-dimensional (a tower of unit cells). Planar disorder means that the crystallites are made from only very few layers of unit cells (cf. Guinier [6] Chap. 7). 

Microfibrillar structure in isotropic materials makes asymmetrical peaks, because microfibrils are materials with linear disorder. Steep is the increase from small scattering angle. The peak shape can be quantitatively analyzed (S tribeck [106]) yielding extra information on the lateral extension of the microflbrils. 

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