The crystallinity of a material influences many of its characteristics, including mechanical strength, opacity, and thermal properties. In practice, crystallinity measurements are made both for research and development and for quality control. X-ray scattering occurs from both the crystalline and non-crystalline material illuminated with X-rays. The difference between the two types of scattering is in the ordering of the material. Materials, especially polymers, have some amorphous contributions. The ability to deconvolute the amorphous from the crystalline scattering is the key to obtaining a reliable value that is consistent with other techniques such as NMR and calorimetry.
As shown in Figure 1, X-ray scattering from amorphous material produces a "halo" of intensity, which when integrated obtains a broad, low-intensity "hump." X-ray scattering from a crystalline material produces well-defined spots or rings which integrate to sharp, higher-intensity peaks. Percent crystallinity as obtained by X-ray measurements is defined as the ratio of intensity from the crystalline peaks to the sum of the crystalline and amorphous intensities:
percent crystallinity = Icrystalline / (Icrystalline + Iamorphous)
With conventional X-ray diffractometers with line sources and point detectors, percent crystallinity measurements of polymer fibers and sheets are both time-consuming and error-prone. Long data collection times result from polymers being weak diffractors. Errors result from intensity of oriented reflections not being observed. The RAPID II imaging plate area detector overcomes these problems. This detector is able to collect the complete Debye rings of polymer samples measured in transmission, something conventional diffraction systems cannot do in practical amounts of time. From the Debye rings, sample orientation information is readily obtained, so one can be sure that all of the intensity from oriented reflections is accounted for and that accurate percent crystallinities are obtained.
Fibers are the most challenging samples for data collection. Usually, the fiber axis is close to the chain orientation direction in a fiber. This is described as the meridional direction. The direction normal to the fiber axis is defined as the equatorial direction. Fibers are usually rotationally symmetric. In other words, if a fiber were mounted vertically, the same diffraction pattern would be observed regardless of the φ setting. For any given 2θ range, a single sample position is required to obtain orientation information in an equatorial plane. The meridional reflections usually have a maximum intensity at the Bragg angle. This means that for an arbitrary sample position with respect to the incident beam different percent crystallinities would be determined based on the amount of the meridional reflection in the scan. To determine the percent crystallinity, all reflections that are not on the equator must be scanned.
Figure 2 is an example from a polyethylene fiber. The one-dimensional "Total scattering" spectrum is obtained by integrating two-dimensional detector data like that shown in Figure 1. This spectrum is then deconvoluted into amorphous and crystalline scattering components. In this case, the percent crystallinity is determined to be 46%.
The RAPID II has high spatial resolution by virtue of its point source and optics. This enables sample properties to be characterized as a function of depth. For example, skin-core effects in polymer sheets can be studied in transmission through thin sections. Depth-dependent crystallinity measurements on polypropylene-based material showed marked crystallinity differences 200 μm apart which caused premature failure in the material. This is one example of how X-ray diffraction can be used to probe of the effect of processing on material properties.