Polymer measurement using FTIR Polarizers | Spectroscopy Solutions
Polymers have become ubiquitous with the modern age to such a point that almost any multi-component product will have some polymer content. From cars to kitchen appliances and windows to airplanes, each one will contain multiple polymers each with its own specific properties and functions. This makes their analysis an important for quality control processes, and IR polarisers are an essential tool for this.
Polymer quality can be optimized in polymer production using FTIR and the thin profile of Specac FTIR Wire Grid Polarizers makes them ideal for optimizing this small-scale spectroscopic measurement, operating well in tight space constraints.
ASTM and other polymer standards
Polymers are macromolecules, composed of chains of many repeated subunits. Polymers can vary from the familiar synthetic plastics such as polystyrene, polyethylene and polypropylene to natural biopolymers such as DNA, which is fundamental to biology. A synthetic polymer – Kevlar - is used in the construction of bulletproof vests for law enforcement officers. Polypropylene is used for fishing nets, pipes and structural materials and nylon (Dupont) has a multitude of uses as fabric, line or bearings.
These applications are just the tip of the iceberg, but for every application the quality of the polymer must be dependable to allow the manufactured products a consistent service life and good mechanical strength. If a Kevlar vest fails then the police may lose one of their officers, if a nylon or polypropylene rope snaps then a load may be spilled.
Polymers must be consistent, reliable and predictable with defined parameters such as tensile strength, glass transition point and melting point as well as dependable and monitorable chemical properties. As we continue to move towards the production of nanoscale and smart materials such as photovoltaic solar panels or stimulus response polymers then quality/consistency becomes even more important.
Standards are comonly applied to the production of polymers, such as those set by American Standard for Testing and Materials (commonly known by the ASTM acronym). Following the correct procedure in polymer production, such as ASTM Plastic Standards, can be an industry essential. Appropriate FTIR measurement, potentially facilitated by the use of FTIR Wire Grid Polarizers, is often a key part of this.
How polymer morphology and quality are linked
Polymers show several degrees of organisation from chain length to interatomic spacing, branching and how the chains pack together. Polymer morphology is the study of the arrangement of polymer chains and how this relates to the polymer’s macroscopic properties and ultimately its physical properties and quality.
Assessing polymer morphology can also provide an indication of the treatment the polymer has received during production such as crystallisation, annealing and deformation. The importance of this is that a polymer may exhibit amorphous regions where the chain packing/density may vary.
The normal state of a polymer chain is amorphous where the chain is in a tangled, unordered state. Using a process of controlled heating and cooling, drawing and stretching, the chains within a polymer can disentangle and become more ordered as well as oriented with more predictable properties. The more ordered a polymer becomes the more crystalline it is. However, crystalline polymers still have amorphous regions, with amorphous and crystalline regions in varying degrees.
Amorphous regions in crystalline polymers can be the focus of stress relief and mechanical failure such as cracking or creep, the formation of cracks that occur over time due to exposure to constant stress. Analytical methods such as polarised FTIR can be used to scan polymer surfaces and samples to find morphological issues that relate to product quality.
Using polarized FTIR to determine polymer morphology
IR spectroscopy is typically used in the field of polymer analysis to determine the molecular orientation of polymers and to investigate the chemical bonding in materials such as fibres, polymers, and biopolymers.
An FTIR spectrometer accessory, such as the Golden Gate ATR, is suitable for this analysis as samples require little or no sample preparation before spectroscopic measurement. Request a free demo
For certain materials, the degree of polymer orientation varies as a function of sample depth due to stresses introduced in the manufacturing processes. Understanding the orientation of polymer chains is important because it has implications on macroscopic mechanical properties such as the Young’s modulus (stiffness) and tensile strength. Polymer orientation can be explored using ATR-FTIR in conjunction with a polarizer.
Using a variable angle ATR accessory allows for probing at various depths. Polarizing FTIR is especially good for the classification of polymer groups.
Polarization and filters in polymer analysis
A polarizer can provide a valuable sampling enhancement to FTIR, allowing researchers to determine chemical information not otherwise found when using non-polarized light. The dichroic ratio and the dichroic difference can be obtained from spectra recorded sequentially using infrared radiation polarised parallel and then perpendicular to a reference direction.
To improve the sensitivity of this technique and to be able to accurately follow the dynamics of orientation, FTIR spectroscopy can be coupled with a polarization modulation (PM) technique. The principle of polarization modulation is the use of a linearly polarized infrared beam dividing the linearly polarised light into an s-polarized beam, parallel to the surface of the sample, and a p-polarized beam, perpendicular to the sample surface.
The s-polarized absorptions are then subtracted from the p-polarised absorptions and normalised using the total intensity of both the p- and s-polarized IR beams to give a normalised surface specific IR absorption signal, which can be considered independent of the environmental conditions.
Using FTIR for feedback during production
Polarized FTIR analysis can be executed in real time and is a powerful method for checking the quality of polymer products during manufacturing, according to the morphology parameters chosen.
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 Infrared Polarizers
Specac provide a range of infra-red wire grid polarizing filters and holders for studying the orientation effects of polymer chains. These high-quality polarizing filters are custom designed to operate 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 provide a high extinction ratio and high transmission with a 25mm clear aperture.
The thin profile of these polarizers also makes them suitable for FTIR applications in which there are space constraints. They are designed to fit directly into the aperture ports of all Specac BenchmarkTM baseplate compatible accessories, such as the Golden GateTM ATR, Silver GateTM Evolution ATR, GatewayTM horizontal ATR systems, and the CycloneTM and TornadoTM long pathlength gas cells. Substrates include CaF2, BaF2, ZnSe, KRS-5, and Ge.
Polarized ATR-FTIR spectroscopy has become a go-to analysis method for all varieties of polymers. The method is excellent for examining properties such as polymer chain orientation and relaxation providing good data that can relate polymer structure morphology to quality.
Polarizing FTIR is rapid and easy to interpret and can provide a range of informative data about the relationship of polymer micro-structure and properties.