Determining the Quality of Polymers Using Polarized FTIR Spectroscopy

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Polymers have become ubiquitous with the modern age to such a point that nearly any multi-component product will contain some polymer content. From cars to kitchen appliances and airplanes to windows, each one will contain numerous polymers each with its own specific functions and properties. This makes their analysis crucial for quality control processes, and IR polarizers, provided by Specac, are a vital tool for this.

Polymers are macromolecules, made up of chains of many repeated subunits. Polymers can differ from the familiar synthetic plastics such as polyethylene, polystyrene, and polypropylene to natural biopolymers such as DNA, which is central to biology. Kevlar, a synthetic polymer, is used in the manufacture of bulletproof vests for law enforcement officers. Polypropylene is used for pipes, fishing nets and structural materials and nylon (Dupont) has numerous uses as fabric, bearings or line.

These applications are only the tip of the iceberg, but for every application, the quality of the polymer must be reliable to allow the manufactured products a consistent service life and good mechanical strength. If a Kevlar vest fails then an officer’s life will be put at risk, if a polypropylene or nylon rope snaps then a load may be dropped and damaged.

Polymers must be consistent, predictable and reliable with clear parameters such as glass transition point, tensile strength, and melting point as well as reliable and monitorable chemical properties. The trend today is towards the production of nanoscale and smart materials such as stimulus-response polymers or photovoltaic solar panels then quality/consistency becomes even more crucial.

How Polymer Morphology and Quality are Linked

Polymers display several degrees of organization from chain length to interatomic spacing, branching and how the chains are composed together. Polymer morphology is the study of the organization of polymer chains and how this connects to the polymer’s macroscopic properties and ultimately its physical properties and quality.

Evaluating polymer morphology can also provide an indication of the treatment the polymer has received during fabrication such as annealing, crystallization and deformation. The significance of this is that a polymer may display amorphous regions where the chain packing/density may differ.

The regular state of a polymer chain is amorphous where the chain is in a tangled, unordered state. Using a process of controlled cooling and heating, drawing and stretching, the chains within a polymer can disentangle and become more systematic as well as oriented with more predictable properties. The more organized a polymer becomes the more crystalline it is. However, crystalline polymers still have amorphous regions, with crystalline and amorphous regions in variable degrees.

Amorphous regions in crystalline polymers can be the focus of stress relief and mechanical failure such as creep or cracking, the development of cracks that happen over time because of exposure to constant stress. Analytical techniques such as polarized FT-IR can be used to scan polymer surfaces and samples to locate morphological problems that relate to product quality.

Using Polarized FTIR to Determine Polymer Morphology

IR spectroscopy is typically used in the field of polymer analysis to establish the molecular orientation of polymers and to examine the chemical bonding in materials such as polymers, fibers, and biopolymers.

For some materials, the degree of polymer orientation differs as a function of sample depth because of stresses added to the manufacturing processes. Understanding the orientation of polymer chains is vital as it has repercussions on macroscopic mechanical properties such as the tensile strength and Young’s modulus (stiffness). Polymer orientation can be studied using ATR-FTIR in conjunction with a polarizer.

Using a variable angle ATR accessory allows for exploring at numerous depths. Polarizing FT-IR is especially good for the grouping of polymer groups.

Polarization and Filters in Polymer Analysis

A polarizer can offer a beneficial sampling enhancement to FTIR, allowing researchers to establish chemical information not otherwise found when using non-polarized light. The dichroic ratio and the dichroic difference can be acquired from spectra recorded sequentially using infrared radiation polarized parallel and then perpendicular to a reference direction.

To optimize the sensitivity of this method and to be able to accurately pursue the dynamics of orientation, FTIR spectroscopy can be combined with a polarization modulation (PM) method. The principle of polarization modulation is the use of a linearly polarized infrared beam dividing a p-polarized beam, perpendicular to the sample surface, and the linearly polarized light into an s-polarized beam, parallel to the surface of the sample.

The s-polarized absorptions are then subtracted from the p-polarized absorptions and normalized using the overall intensity of both the p- and s-polarized IR beams to offer a normalized surface-specific IR absorption signal, which can be considered independent of the environmental conditions.

Polarized FTIR can be used to provide near real-time feedback during polymer production processes. Image Credit: muph/Shutterstock.com

Using FTIR for Feedback During Production

Polarized FTIR analysis can be set up in real time and is a robust technique for checking the quality of polymer products during manufacturing, based on the morphology parameters selected.

By feeding back FTIR data changes can be made to the production process or the formulation of the polymer to optimize the product quality.

Specac’s Infrared Polarizers

Specac offers a variety of infra-red wire grid polarizing filters and holders for examining the orientation effects of polymer chains. These superior-quality polarizing filters are custom designed to function in the wavelength range 5000 cm^-1 to 285 cm^-1 (wave numbers) and with a grid periodicity of 4000 lines per mm they all offer a high extinction ratio and high transmission with a 25 mm clear aperture.

The thin profile of these polarizers also makes them ideal for FTIR applications in which there are space constraints. They are engineered to fit directly into the aperture ports of all Specac Benchmark™ baseplate compatible accessories, such as Silver Gate™ Evolution ATR, Gateway™ horizontal ATR systems, the Golden Gate™ ATR and the Cyclone™ and Tornado™ long pathlength gas cells. Substrates include BaF₂, CaF₂, KRS-5, ZnSe and Ge.

Infrared Polarizers from Specac.

Conclusion

Polarized ATR-FTIR spectroscopy has become a popular analysis technique for all types of polymers. The technique is excellent for analyzing properties such as polymer chain orientation and relaxation offering appropriate data that can relate polymer structure morphology to quality.

Polarizing FTIR is fast and easy to interpret and can deliver a range of informative data regarding the relationship of polymer micro-structure and properties.

References and Further Reading

1. Michel Pézolet, Christian Pellerin, Robert E.Prud'homme, Thierry Buffeteau, Study of polymer orientation and relaxation by polarization modulation and 2D-FTIR spectroscopy, Vibrational Spectroscopy, Volume 18, Issue 2, December 1998, Pages 103-110.
2. http://www.scienceclarified.com/everyday/Real-Life-Chemistry-Vol-3-Physics-Vol-1/Polymers-Real-life-applications.html#ixzz4xHAlUBqx
3. Su Cheol Park,Yongri Liang, and, and Han Sup Lee, Quantitative Analysis Method for Three-Dimensional Orientation of PTT by Polarized FTIR-ATR Spectroscopy, Macromolecules, 2004, 37 (15), 5607-5614.
4. 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.
5. Hoffman J.D., Davis G.T., Lauritzen J.I. (1976) The Rate of Crystallization of Linear Polymers with Chain Folding. In: Hannay N.B. (eds) Treatise on Solid State Chemistry. Springer, Boston, MA
6. Lewis, Peter Rhys (2010). Forensic polymer engineering: why polymer products fail in service. Cambridge, Woodhead Publishing. ISBN 1-84569-185-7.

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

For more information on this source, please visit Specac.