Using an X-Ray Diffractometer to Determine the Crystallinity in Polymer Samples

Polymer materials are utilized for a diverse number of modern societal processes, ranging from packaging to aerospace engineering. Naturally, different types of processes necessitate different material characteristics, which are conditioned by the structural properties of the polymer product. Important observables for polymer materials range from the category of polymer (e.g., Polyethylene PE or Polypropylene PP) to the degree of crystallinity (D).

In a vastly industrialized context, the efficient screening of products for the latter properties is of equal importance to users and producers alike. A user-friendly and rapid means to distinguish the crystallinity and category of the compound is the execution of X-ray diffraction (XRD), in collaboration with whole pattern Rietveld refinements.

Instrument

The Thermo Scientific™ ARL™ EQUINOX 100 X-ray diffractometer engages a bespoke Cu (50 W) or Co (15 W) micro-focus tube fitted with mirror optics. These low wattage requirements negate the need for an external water chiller, as well as other peripheral infrastructure, and therefore permit the instrument to be effectively transported from the laboratory to the field, or between laboratories.

The ARL EQUINOX 100 (c.f. Figure 1) offers very rapid data acquisition rates in comparison to competing diffractometers, due to its distinctive curved position sensitive detector (CPS). The instrument calculates all diffraction peaks simultaneously, and in real time, and is consequently ideally suited for both reflection and transmission calculations.

ARL EQUINOX 100 X-ray diffractometer.

Figure 1: ARL EQUINOX 100 X-ray diffractometer.

Experimental

For XRD observations, sheet samples of PE (Polyethylene) and PP (Polypropylene) were computed in transmission geometry for five minutes under CuKα radiation. The quality of the data generated by the ARLEQUINOX 100 benchtop tool is similar to data obtained from a regular high-power floor standing tool. Qualitative and quantitative examination was undertaken with the use of MDI JADE 2010, in collaboration with the ICDD PDF4+ Organic database.

Results

Qualitative phase analyses explicitly demonstrate Polyethylene (PE) and Polypropylene (PP) category materials (c.f. Figure 2). MDI JADE 2010 yields the opportunity to directly ascertain the degree of crystallinity through the execution of a standard-less refinement technique, in which deconvolution of the diffraction pattern directly determines the intensity of the amorphous contribution, through the use of Rietveld’s method (c.f. Table 1).

Table 1: Results of crystallinity determination using Rietveld’s method in PE and PP materials

Sample Reference D
(in %)
D Rietveld
(in %)
Amorphous Wt
(in %)
PE 1 59.4 54.1 31.0
PE 2 29.3 23.1 61.9
PP 1 59.9 57.5 47.2
PP 2 45.3 39.7 73.6

For the measurement of the amorphous content (in wt%), a density of 0.85 g/cm3 was utilized. This technique offers univocal results for PE and PP samples with high and low values for degree of crystallinity (c.f. Figure 2).

Diffraction patterns of PE and PP with 5 min measurement time.

Figure 2: Diffraction patterns of PE and PP with 5 min measurement time.

Conclusion

In collaboration with the MDI JADE 2010 software package, and the ICDD pdf4+ Organic database, the ARL EQUINOX 100 benchtop XRD instrument represents an effective solution for the convenient determination of both the category and the crystallinity of polymer materials.

This is even the case for users lacking experience. The values are in the range of high (PE 1) and low (PE 2) density PE and isotactic (α-) PP, whereby the differences in crystallinity are impacting on the mechanical characteristics of the polymers. With this method, it is possible to ascertain the quality of polymer products in less than ten minutes, in both research and production contexts. Doing so will minimize the cost and maximize the quality.

Authors: Dr. Simon Welzmiller, Ju Weicai, XRD Application Specialists

This information has been sourced, reviewed and adapted from materials provided by Thermo Fisher Scientific - Elemental Analyzers.

For more information on this source, please visit Thermo Fisher Scientific - Elemental Analyzers.

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