Introduction

The application of polymer plastics is very common in daily life and industrial production. According to their structural properties, they can be divided into crystalline polymers and amorphous polymers. The crystallinity of crystalline polymers is affected by many factors, such as crystallization temperature, cooling rate, crystal nucleus density, etc. Changes in crystallinity will affect various properties of polymer materials, such as mechanical properties, thermal properties, density and optical properties. Changes in properties will affect the use and quality of polymer materials. Therefore, in industrial production, crystallinity is an important parameter for evaluating the production process of polymer materials. A more convenient and faster method is needed to monitor crystallinity. This article will use X-ray diffraction method to calculate the crystallinity of four typical crystalline polymers.

Crystalline form classification of plastics

Polymer materials are mainly classified according to the type of polymer (such as polyethylene PE or polypropylene PP) or its crystallinity, and according to the crystalline form of the plastic, which is generally divided into crystalline plastics and amorphous plastics.

Crystalline polymer means that under appropriate conditions, the polymer macromolecular chains can form a three-dimensional orderly arrangement according to certain rules (such as: PE, PP, PA, POM, PET, PBT, etc.), and most of them are partially crystalline. .

Amorphous refers to polymers whose macromolecular chain shape and arrangement do not show a crystal structure but are in a disordered state (such as ABS, PC, PVC, PS, PMMA, EVA, AS, etc.). Amorphous plastics exhibit the same mechanical properties in all directions, that is, they are isotropic.

Importance of Polymer Crystallinity to Polymer Materials

The crystallinity of polymers has an important impact on their performance and characteristics, and different forms show different process characteristics and physical and mechanical properties. Therefore, controlling the crystallinity of polymers is a key factor in plastics engineering, which can be tailored to specific application needs to obtain the desired material properties. 

Application of XRD in polymer crystallinity analysis

There are multiple methods to determine the crystallinity of polymer materials: X-ray diffraction, calorimetry, infrared spectroscopy, and density methods. Among the above methods for measuring crystallinity, X-ray diffraction is a recognized method with clear instruments and wide application. Commonly used measurement and calculation methods for measuring the crystallinity of polymers using X-ray diffraction technology include graphing method, regression line method, Ruland method, and X-ray diffraction curve fitting peak splitting method.

Test cases

This experiment uses the benchtop X-ray diffractometer to analyze polymer plastic samples provided by a company, collects the diffraction patterns of the samples, and obtains their crystallinity information. The four polymer plastic samples are labeled UPE, PP, PTFE, and POM.

（1） Calculating formula

This experiment is based on the standard "SH/T1827-2019 Determination of Plastic Crystallinity X-ray Diffraction Method" and uses the X-ray diffraction curve fitting peak splitting method as the calculation method. The technical formula is as follows:

X_e----crystallinity                               I_ci----diffraction peak area

F_ci----crystal plane diffraction peak correction factor              I_ai----amorphous scattering peak area

F_ai----amorphous scattering peak correction factor

k----The ratio of the used diffraction peak area count value to the sum of the diffraction peak and scattering peak area count values

（2）Sample display : omitted

（3）Test parameter settings

（4）Sample testing and calculation results

Sample—UPE

By fitting the spectrum, it is found that the amorphous scattering peak position of UPE is located at 19.843°, and the crystal plane diffraction peaks selected for calculation are 21.446°, 23.906°, and 29.961°. The corresponding crystal faces are (1 1 0), (2 0 0) and (2 1 0) respectively. According to the formula, the crystallinity Xe=86.55% can be calculated.

UPE (high-density polyethylene) sample characteristic peak display chart

Sample—PP

By fitting the spectrum, it is found that the amorphous scattering peak position of PP is located at 16.935°, and the crystal plane diffraction peaks selected for calculation are 13.886°, 16.713°, 18.393°, 21.134°, and 21.777°. The corresponding crystal faces are (1 1 0), (0 4 0), (1 3 0), (1 1 1), and (-1 3 0). According to the formula, the crystallinity Xe=75.84% can be calculated.

PP (polypropylene) characteristic peak position display chart

Sample—PTFE

By fitting the spectrum, it is found that the amorphous scattering peak position of PTFE is located at 16.29°. The crystal plane diffraction peak selected to be added to the calculation is 18.046°, and the corresponding crystal plane is (1 0 0). According to the formula, the crystallinity Xe=76.54% can be calculated.

PTFE (polytetrafluoroethylene) characteristic peak position display chart

Sample—POM

By fitting the spectrum, it is found that the amorphous scattering peak position of POM is located at 20.745 °. The crystal plane diffraction peak selected to be added to the calculation is 22.812 °, and the corresponding crystal plane is (1 0 0). According to the formula, the crystallinity Xe=76.357% can be calculated.

POM (polyoxymethylene) characteristic peak position display chart

（5）Conclusion

The spectrum obtained by FRINGE series X-ray diffractometer from LANScientific can meet the requirements for calculating the crystallinity of plastics, and the testing and calculation process is simple and fast. Under correct sample preparation conditions, the calculated plastic crystallinity is close to the actual value. It can be used for plastic manufacturing process control and plastic product quality control.