Polymers can be characterized by numerous methods. There is an alphabet soup of analytical equipment available: FTIR, DSC, DMA, TGA, GC-MS, NMR, just to name a few. Each can be used to provide information on a polymer property. But which one to use? What is the “best”?
The questions that need to be answered before diving in are:
If you are running an injection molding machine using a glass filled polyamide to make automotive parts, you probably do not need to know the solution viscosity of the polyamide. What you are interested in is batch-to-batch consistency – the material performing as it should. It is when the material does not perform as expected that the specification data sheets are found, questions about polymer property characterization are asked and analytical equipment investigated.
To figure out which piece of equipment to use, first you need to ask yourself the following questions:
What do you need to know?
- Chemical identity: Which polymer is this? What additives are present? Is there contamination?
- Morphology: Amount of crystallinity, optical properties.
- Molecular weight: Molecular weight distribution.
- Solution behavior: Viscosity, flow, gel.
- Thermal behavior: Glass transition, melt and decomposition temperature, melt viscosity.
- Mechanical properties: Tensile strength, modulus, impact, creep, dimensional stability.
In the table below, are some suggestions for which piece of analytical equipment can give which types of information. There is no magic piece of equipment that can give you all the answers.
| What do you need to know? |
Analytical equipment (suggestions only) |
| Chemical identity |
FTIR, Raman, NMR, GC-MS |
| Molecular weight |
Viscometer, chromatography (various kinds) |
| Solution behavior |
Viscometer, rheometer |
| Thermal behavior |
DSC, DMA, TMA, melt flow indexer |
| Mechanical properties |
Universal testing machine, DMA |
| Morphology |
Optical microscopy, DSC |
FTIR (Fourier Transform Infrared Spectrometry) and DSC (Differential Scanning Calorimetry) are perhaps the most popular analytical instruments used to characterize polymeric materials.
Let's explore these analytical instruments in detail.
Fourier Transform Infrared Spectrometry (FTIR)
FTIR can be used to answer the “What is this?” question. FTIR is relatively easy to use and a
wide range of materials can be quickly analyzed. There are four main sampling methods used in FTIR:
- Transmission
- Attenuated total reflectance (ATR)
- Diffuse reflectance, and
- Specular reflectance
In all cases, the sample is exposed to a beam of IR energy. The sample absorbs and transmits this IR energy in a way that is characteristic to its chemical structure. The result is a spectrum that is specific for each compound.
FTIR Sampling Methods (Source: ResearchGate)
FTIR using ATR is quite popular now as there is
little or no sample preparation required, however, it may not the best choice for all materials. In ATR, the sample is held in intimate contact with an optically dense crystal. The IR beam is directed into this crystal, it bounces and interacts with the sample. Say you want to characterize the outer layer of a multi-layer film used in packaging. You know that this layer is quite thin, perhaps only a few microns. Depending on the crystal used in ATR, the depth of penetration or how far the IR beam penetrates the sample, may be too deep. The beam could penetrate past the layer you want to analyze. The resulting spectrum could be of the second layer and not the outer layer you are interested in. You need to be aware of the limitations of each sampling method and pick the one that matches your material.
FTIR with ATR (Souce: ResearchGate)Differential Scanning Calorimetry (DSC)
DSC is used along with
TGA (Thermal Gravimetric Analysis) and
DMA (Dynamic Mechanical Analysis) to evaluate the thermal properties of polymeric materials. DSC measures the heat flow produced in a sample when it is heated, cooled or annealed.
Glass transition temperature,
crystallization behavior and melting point can be measured by DSC.
When looking at any type of thermal analysis, know that the thermal history of polymer sample has a huge impact on the measured glass transition temperature and measured crystallinity. Annealing a semi-crystalline polymer at a temperature above its glass transition temperature but below its melting, allows the chains to move and crystals to form. This annealing will greatly increase the percent crystallinity.
The amount of crystallinity of a polymer will impact its physical properties. Injection molders take advantage of this relationship and will use the mold temperature to control the percent crystallinity of
semi-crystalline thermoplastics.
Molten polymer fills the mold and cools in a somewhat controlled manner before the part is ejected. This is done to get the level crystallinity needed to meet the physical requirements for the molded part.
DSC curve showing heat flow, heat capacity depending on the temperature; Tg; Exothermic & Endothermic Peaks
(Click on Image to Enlarge)
Now say something changed resulting in “bad” parts and you want to analyze the polymer used in the last few batches. DSC is a great choice. To get the best idea of what is happening in the injection molding line, the polymer sample should have the same thermal history as what it sees in “real life”. In this example, it should be melted and annealed at the same temperature and rate in the DSC as it is on the production line.
There are advantages and limitations for each technique. Make sure that the technique chosen gives consistent results that are meaningful to you. An overarching theme is to have a correlation between a measurable polymer property and the performance of the material.
» Learn How to Best Combine DMA, DSC, FTIR Tools to Achieve Proficiency in Optimal Material Analysis
Where will you get this information from?
Material Supplier
The material supplier can provide you with
certificate of analysis and data sheets. But they may not measure and/or report the property you are interested in. Depending on your relationship, you might be able to work quite closely with your supplier. However, this is not always possible, and you may want to have your own in-house instrumentation.
In-house Analytical Instrumentation
You have the most control when you own the analytical equipment. Analytical instrument suppliers have made great strides in making their equipment more user friendly – intuitive software interface, simplified sample preparation, preloaded libraries or even making the maintenance of the instrument easier. If you are looking to purchase a piece of equipment,
make sure you have a place to put it and someone who will take care of it.
For optimal performance, any piece of analytical equipment will need a clean, dry, dust free, temperature controlled, vibration free space. Some will need additional cooling (chilled water, liquid nitrogen, liquid helium), others need additional gasses (nitrogen, helium, dry air), all will need calibration.
You need someone who will handle the “care & feeding” of the instrument and interpret the results. The equipment is worthless if it is not set up and maintained correctly. The results are worthless if you do not know what they mean.
Outside Testing Laboratory
Sending material to an outside laboratory for analysis is also an option. It can be expensive, but they are doing all the equipment maintenance, sample preparation and will help with
interpretation of the results.
No matter where you get the information from, you need to understand how the analysis was done and what the results mean.
How will knowing this information help you?
You spent time and money to obtain this information, what you going to do with it? Are you characterizing the polymer as part QC? Are you collecting property information as part of a failure analysis project? How does this information help you?
Remember, the bottom line is to
correlate a measured physical property of the polymer with its performance. Always ask yourself how knowing a certain property will help you understand the behavior of the material.
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