Qualitative and Quantitative Identification of Polymer Samples Using ATR

Infrared spectroscopy or IR spectroscopy is used for identifying a group of chemical species in a wide range of sample materials, especially for classifying organic chemical materials based on the presence of carbon atoms in the molecular structure. To that end, many types of polymeric and plastic samples, which can be incorporated in the category of organic molecular materials, can be divided into specific family groupings. Using the Attenuated Total Reflectance (ATR) method as an IR measurement, the types of sample families can be identified quantitatively and qualitatively.

Application

The Specac Quest ATR accessory with the single reflection AR coated diamond crystal puck was utilized to determine different types of polymer and plastic samples. The combination of the ATR technique and the ATR accessory for measurement prevents the necessity to prepare samples for testing, which may often be the case for a transmission spectral analysis.

During an ATR measurement, the polymer sample form is presented as a rod, pellet, bead, shard, fragment, and shard can be simply placed over the ATR crystal and secured tightly into position to make appropriate contact with the ATR crystal by means of an anvil and force arm assembly. This helps in achieving a reliable and reproducible ATR spectrum for the sample type.

Materials and Method

To perform the ATR measurement of different types of polymer samples and also to acquire a spectral range for the sample between 4000cm^-1 to 400cm^-1, the Quest ATR Accessory (p/n GS10800-B) (Figure 1) was installed with the AR coated diamond crystal puck on its optical unit. Then the stainless steel flat anvil option was integrated into the anvil and force arm assembly for use against each sample type for precise and consistent contact with the diamond ATR crystal.

Figure 1. Specac’s Quest ATR Accessory (p/n GS10800-B)

The ATR spectra were then obtained on a Thermo Nicolet ¡S5 system by means of the traditional room temperature detector system configured at a resolution of 4cm^-1 for 32 scans. The Quest ATR Accessory was used to study 13 plastic and polymer material types. Table 1 shows the sample types utilized for the studies.

Table 1. Samples used for the studies

From table 1, it can be seen that there are 7 different polymer family types, which have been color-coded as identical family types for their sample number, for instance, the 2 polycarbonate type samples are coded in red color. Figures 2-14 shows the classified type materials.

Figure 2. Spectrum Quest1 - Sample 1 - PVC Classified Type Material

Figure 3. Spectrum Quest2 - Sample 2 - PVC classified type material

Figure 4. Spectrum Quest3 - Sample 3 - polycarbonate classified type material

Figure 5. Spectrum Quest4 - Sample 4 - polypropylene classified type material

Figure 6. Spectrum Quest5 - Sample 5 - polyester classified type material

Figure 7. Spectrum Quest5 - Sample 6 – polypropylene classified type material

Figure 8. Spectrum Quest7 - Sample 7 - polyethylene classified type material

Figure 9. Spectrum Quest7 - sample 8 - polyethylene classified type material

Figure 10. Spectrum Quest9 - sample 9 - polycarbonate classified type material

Figure 11. Spectrum Quest10 - sample 10 – cellophane classified type material

Figure 12. Spectrum Quest11 - sample 11 – polystyrene classified type material

Figure 13. Spectrum Quest12 - sample 12 – PVC classified type material

Figure 14. Spectrum Quest13 - sample 13 – polypropylene classified Type Material

Discussion

The 13 separate ATR spectra thus obtained for the polymer and plastic samples represent the family type of the polymer material. The spectra denoting the same family type of polymer material depict them to be analogous however, there are unique features in each spectrum that make it possible to distinguish the specific sample from another sample type.

Among the seven different types of polymer family spectra obtained, four types can be compared from an overlay of their separate spectra. Here, just one sample type for polystyrene, polyester and cellophane family type were determined.

The sample spectra that were compared for overlay are as follows:

• Samples 1, 2 and 12 for a PVC family type polymer material
• Samples 3 and 9 for a polycarbonate family type polymer material
• Samples 4, 6 and 13 for a polypropylene family type polymer material
• Samples 7 and 8 for a polyethylene family type polymer material

The superimposed spectra have been presented between 3800cm^-1 to 500cm^-1 and have been completely scaled for the strongest absorbance peak in the spectrum.

Conclusion

The Specac Quest ATR Accessory with a single reflection AR coated diamond crystal puck can be utilized to distinguish different polymer and plastic types from a surface reflectance measurement. The ATR method requires only minimal sample preparation and in all the instances for the sample types tested in this article, they were introduced “as is” to make excellent contact with the ATR crystal from the particular sample form as presented.

Comparisons made between the polymer family types reveal both differences and similarities from the ATR spectra alone. The variations observed in the ATR IR spectrum can be utilized to discriminate from one specific sample type to another and hence an ATR IR measurement technique may be sufficient to detect and determine a specific polymer type material. From an ATR measurement of a polymer and plastic type sample, while the the polymer family type can be determined, the measurement sensitivity for the effective pathlength allowed may not be adequate to measure any particular sample constituents such as fillers, additives, and so on in the make-up of the polymer material that exists in low concentration. Hence, it may be important to produce a thin film of the polymer and plastic sample for transmission spectral analysis from an appropriate film-making accessory.

This information has been sourced, reviewed and adapted from materials provided by Specac.

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