18 Sep 2019

Polymer quality analysis | Spectroscopy Solution

Polymers are extremely important in modern manufacturing. Pretty much any multi-component product will contain polymer as part of its structure. The examples extend from cars to airplanes and kitchen equipment to televisions and personal electronics such as mobile phones. Each of these examples is comprised of multiple polymers with specific properties, functions, and purposes.

polymers

Polymers need to be consistent, reproducible and predictable having definite analysis parameters including tensile strength, glass transition point and melting point, and chemical stability. As industry moves towards the greater demand for nanoscale and smart materials such as wearable electronics, flexible digital screens, and solar panels then quality and consistency become more and more critical. This means that reliable polymer analysis is vital for quality control processes and developing the properties required for products both old and new.  

Mechanical Properties of Polymers

At synthesis, polymer chains are amorphous and the chains are tangled and randomly ordered. The process of heating, tempering, drawing and stretching allows the polymer molecule chains to mature, disentangle and become ordered and oriented with predictable properties. In general, polymer film mechanical properties, e.g. tensile strength and tear resistance hinge very strongly on the direction of orientation of the polymer chains. Tensile strength and tear resistance are measuredly stronger in a direction parallel to the molecule chains than in a 90-degree normal direction.

These ‘synthetic molecule chains’ are semi-crystalline in nature and contain various regions with ordered polymer chain arrangement in some regions and disordered amorphous areas as well. Experiment shows that stretching polymer films in a single direction will usually align the molecules in that direction. Polymer ‘morphology’ describes how polymer chains are arranged and directly relates to macroscopic properties and in due course its quality and physical properties. 

Assessing polymer morphology indicates the handling of the polymer during production such as crystallization, annealing, and deformation. The importance of this is that a polymer may exhibit amorphous regions where the chain packing/density may vary, and this could be the focus of a fault in the structure. 

FTIR Spectroscopy in Polymers

Fourier transform infrared (FT-IR) spectroscopy is the analytical method best suited to measure the degree of orientation in the plane of the polymer film and at the normal angle to it. When using FT-IR it is usual to examine the film in a 3-D fashion to gain a full perspective. This requires the film to be tilted so that dipole changes normal to the plane can be evaluated. There are two methods to set up polymer films for analysis. Attenuated total reflectance (ATR) is one, but measurements can be sensitive to the orientation of the polymer molecules both in the plane and normal to it. And ATR only measures the surface regions. Although using a variable angle ATR accessory allows probing at various depths.

Polymer orientation can vary as a function of sample depth in some polymers due to stresses from the manufacturing processes. The orientation of polymer chains is important because there are implications for macroscopic mechanical properties, e.g. Young’s modulus and tensile strength. Polymer orientation can be explored using ATR-FTIR in conjunction with a polariser, which is especially good for the classification of polymer groups. Thin-film transmission provides another viable method. However, the resulting spectra often have interference fringes that need to be filtered out and this can be accomplished by using polarising filters. 

IR Polarizers are an Essential Tool

Molecular orientation studies of polymers provide good information on the mechanisms involved in polymer deformation. The orientation of polymer chains in thin-films is commonly anisotropic as a consequence of the manufacturing method. FT-IR absorption is based upon the interaction of the electric-field vector of the incident light with the electric dipole-transition moment associated with specific vibrational modes. When polymer film is manufactured, the macromolecular polymer chains tend to line-up in a very specific direction. The IR absorption of any oriented film is known to be sensitive to the state of polarization of incident radiation in a reference direction defined normally as the ‘stretching’ direction.

Polarised IR spectroscopy may be used to determine the crystalline and amorphous orientation of the polymer molecules. A study of polyamide fibers in 2005 used FT-IR and compared it as a technique to more established methods such as x-ray diffraction. For PA66 the crystalline orientation was estimated from a single IR band at 936 cm-1and for PA6 crystalline orientation used the band at 930 cm-1and amorphous orientation used the band at 1124cm-1. A good correlation was found between the crystalline orientation values obtained via the IR measurements and the other methods. 

Specac Accessories

Specac provides a range of filters to analyze polymer structure and chain orientation. Polarizing filters from Specac are all custom designed to work in the 5000 cm-1to 285 cm-1range. They have a grid periodicity of 4000 lines per mm and offer a high extinction ratio and high transmission with a 25mm clear aperture for the sample. Polarised FTIR is generally used to scan polymer surfaces and samples to find morphological issues that relate to polymer quality.

ftir polarizers

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Holographic Wire Grid IR Polarizers from Specac are perfect for supporting FTIR analysis of polymers. They are designed for integration with Specac accessories, such as the Golden GateTMATR. 

Conclusion

Polarised ATR-FTIR spectroscopy has become the favored method for analysis of all varieties of polymers. The method is outstanding to examine properties such as polymer chain orientation and relaxation and can relate polymer structure morphology to quality. Polarizing FTIR, as a method, is rapid, easy to interpret and can provide a range of informative data on polymer microstructure and properties. Polarised FTIR analysis can be used in real-time and is a good method for checking the quality of polymer products during manufacturing by direct feedback.

References

  1. T. Buffeteau et al. Molecular orientation studies in polymer films by polarization modulation FTi.r. spectroscopy. Polymer, Volume 35, Number 12, 1994
  2. Measuring Orientation in Polymer Films. Spectroscopy; Aug 01, 2014, Volume 29, Issue 8
  3. R.A. Spragg, Spectroscopy supplement FT-IR Technology for Today's Spectroscopists 28(s8), 14–21 (2013).
  4. Vasanthan N. Determination of Molecular Orientation of Uniaxially Stretched Polyamide Fibers by Polarized Infrared Spectroscopy: Comparison of X-ray Diffraction and Birefringence Methods. Applied Spectroscopy 59(7):897-903, August 2005.
  5. Momose, M. and Ando, S. (2010), Quantitative analysis of near surfaces three- dimensional orientation of polymer chains in PET and PEN films using polarized ATR FTIR spectroscopy. J. Polym. Sci. B Polym. Phys., 48: 870–879.
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