In order to transform polymers from raw materials into a completed product, there is a need for a processing stage involving a molten state, often called melt. After cooling, the final product will typically be in a semi-crystalline state.
The nanostructure of this semi-crystalline state, which develops at the point of solidification, has a large degree of control over the physical and mechanical properties of the polymer product. Studying and defining the crystallinity is therefore very important, both for fundamental studies, and for industrial R&D and process optimization.
In the following experiment, wide-angle X-ray scattering (WAXS) measurements were performed on isotactic polypropylene (i-PP), using a temperature control stage from Linkam.
Measurements and Results
An i-PP pellet (from Innovia Films, U.K.) was enclosed in thin aluminium foil in order to ensure good thermal contact and prevent direct air exposure. The polymer sample was then mounted and fixed into the temperature control stage (Linkam HFSX350). The WAXS configuration of the Xeuss 2.0 SAXS/WAXS system measured the crystallization of the i-PP in real-time.
A quenching experiment at 148°C from the fully molten state was performed, with an exposure time of 200s. Figure 1 shows the single-dimensional scattering curves, and corresponding analysis of the crystalline and amorphous fraction of the sample 16 minutes after the quench. The ratio of the integrals of the crystallized and amorphous curves is used calculate the crystallinity, as described by Panine et al1.
As the Xeuss 2.0 SAXS/WAXS system has a very low signal-to-noise ratio, studying low levels of crystallinity is possible even during real time crystallization. Hence, it is possible to determine crystallinity levels well below 5%2, as shown in figure 1. In this case, the crystallinity is estimated at 1.1%.
Figure 1. 1D scattering curves and estimation of crystalline fraction.
Simultaneous SAXS and WAXS Measurements
The physical and mechanical properties of the solid polymer are caused by its crystallization kinetics, which impact both the level of crystallinity and the nanoscale structure.
Characterization at long length scales (SAXS investigation) provides insights into the morphology of the crystalline phase, whereas the crystallographic order can be described by short length scale measurements (WAXS).
It is necessary to study the evolution as a function of temperature at both length scales concurrently, in order to gain insights into the underlying processes. The Xeuss 2.0 SAXS/WAXS system provides such simultaneous SAXS/WAXS measurement capability.
- Panine et al, Polymer, 2008, 49, 676-680
- Wang et al, Macromolecules, 2000, 33, 978-989
This information has been sourced, reviewed and adapted from materials provided by Xenocs.
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