Exploring the Techniques Used in Polymer Analysis

Synthetic polymers are a crucial, yet often overlooked, aspect of daily life. From the hard plastics used in packaging and consumer products to biocompatible coatings for medical materials, to advanced lightweight materials for automotive and aerospace engineering – these polymer molecules underpin a huge variety of modern advancements.¹

As such polymers have become ubiquitous in everyday life, polymer analysis techniques have become increasingly important tools for scientists, allowing them to properly assess the performance of these materials in different environments.

There is a wide variety of analytical techniques that can be applied to synthetic polymers, each offering a slightly different look at the chemical or physical properties of these compounds. Combining these methods, academics and R&D scientists can better understand and predict how these polymers will behave in different scenarios, paving the way for more advanced materials and breakthroughs in sustainability and plastics recycling.

The importance of polymer analysis

Polymer materials are made up of hundreds or thousands of repeating smaller “chain links,” known as monomers. The average molecular weight of these long polymer chains, as well as their density, crystallinity, molecular mass distribution, degree of cross-linking and the addition of any stabilizers or plasticizers can all have a significant impact on the eventual physical, thermal and mechanical properties of the polymer material.²

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Modern polymer chemical analysis technologies offer R&D scientists a way to look at this fundamental structural information, which can be used to better understand why a polymer is behaving in a certain way.

Industrial producers of polymers and plastic products will also want to make use of mechanical testing methods in their quality control processes to ensure that their products meet all relevant performance and safety standards.

Together, the broad range of existing polymer analysis techniques can be used to support and drive the creation of novel polymer materials with even more favorable properties for specialist applications.

Chromatography and sustainable plastics research

Chromatography is one of the first tools that a research lab might apply when creating a novel polymer material.

“We usually start with chemical composition and molecular weight (length of the polymer chains) to assess new plastic materials,” said Erin E. Stache, an assistant professor of chemistry at Princeton University. “We can gather this information by using nuclear magnetic resonance (NMR) and size exclusion chromatography (SEC), sometimes also called gel permeation chromatography (GPC).”

Despite recent advances, polymerization reactions do not produce a completely uniform set of polymers – there will be some natural variation in the chain length and molecular weight of the final molecules.³ This molecular weight distribution can affect the processability, mechanical strength and morphological phase behavior of the polymer.⁴ As a result, chromatographic methods play an important role in allowing scientists to separate these molecules into different fractions according to their size or weight and analyze their relative distribution.

SEC uses porous gel beads packed inside a chromatographic column to separate a polymer dissolved in a solvent mobile phase according to their size.⁵ Smaller polymer molecules and ones with a lower molecular weight are more likely to get trapped in the beads’ pores, and as a result will take more time to pass through the chromatography column.

This molecular weight data can also be useful to scientists wanting to study how polymers degrade in certain environments. This is a central research theme for the Stache Lab, in addition to its work investigating new polymerization techniques that yield easily degradable polymers and the recycling of waste plastics back into their monomer building blocks or other commodity chemicals.

“Degradation or depolymerization requires characterization of the products,” Stache said. “These techniques help determine changes in molecular weight or modification of the polymer backbone. Then, we use this information to assess the effectiveness of our method and establish a mechanism for degradation or depolymerization. This information helps us optimize the process to be more efficient, always working towards the goal of a single product with high recovery.”

Studying degradation with spectroscopic techniques

Spectroscopy techniques can be a useful complement to chromatographic techniques in polymer structural characterization.

The advent of high-resolution mass spectroscopy methods has seen this technique become increasingly more utilized for polymer analysis. Used together with chromatic separations, it is possible to generate extremely accurate molecular weight distribution measurements, as well as information relating to the polymer’s end groups and topology.⁶

Nuclear magnetic resonance (NMR) spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy and Raman spectroscopy are also common techniques applied in polymer analysis. These techniques are normally utilized for determining which functional groups are present in a polymer.⁷

“During degradation, we focus on the impact on molar mass, so GPC is really important for that where we can [use it],” said Andrew Dove, a professor of chemistry at the University of Birmingham. “NMR is also a very useful technique for us and we are starting to look at more advanced methods there, like diffusion NMR and extending out at the moment to MRI [magnetic resonance imaging] techniques for imaging.”

The Dove Research Group is heavily involved in exploring different aspects of polymer degradation. This includes the creation of biodegradable polymers and new polymers that can be more easily recycled, as well as polymers that can resist degradation in certain environments, such as polymer materials being used in the body for medical implants.

Just like the Dove group, the Stache lab also makes use of spectroscopic techniques in their research to create a fully-formed chemical and structural characterization of the polymers they are working with.

“For small molecule (<500 g/mol) identification, we use nuclear magnetic resonance (NMR), gas chromatography (GC), mass spectrometry (usually GC-MS), and infrared spectroscopy (FT-IR),” Stache said. “Combining all these techniques allows us to unequivocally determine the identity and quantity of products being formed.”

Thermal techniques for polymer materials characterization

Chromatographic and spectroscopic techniques allow scientists to study what is going on at the molecular level inside a polymer or plastic product. Thermal analysis techniques provide feedback on a more macro scale, giving an insight into the materials’ mechanical properties and thermal behavior.

The most commonly applied thermal polymer characterization techniques are: differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), thermomechanical analysis (TMA) and dynamic mechanical thermal analysis (DMTA).⁸ Using one or a combination of these techniques, it is possible to build up an idea of how a new polymer will react during manufacturing or when exposed to various external forces.

Each of these methods operates on the principle of exposing a polymer material to heat, cold or a changing temperature and studying how the material and its properties vary under these conditions.

TGA is concerned with studying changes in mass as a polymer is heated, which can be used to investigate the material’s thermal stability as well as the determination of any volatile species or fillers in the material. DSC is a calorimetry technique commonly used to study a material’s phase transitions. TMA and DMTA are both used to study a material’s response to external forces at different temperatures, with TMA using a constant force and DMTA applying periodically oscillating forces to assess deformation and kinetic properties such as thermal expansion.⁹

“TGA tells us the degradation temperature of the polymer, that is, the temperature at which the polymer will start to break down,” said Stache. “DSC tells us more about the physical properties of the polymer, such as glass transition temperature (T_g) and melting temperature (T_c),”

“T_g will tell us if the polymer is hard and glassy or soft and rubbery at a given operating temperature,” she explained. “T_c tells us the temperature at which the polymer will start to melt and flow.”

Beyond assessing the performance of newly synthesized polymer materials, these thermal analysis methods are also routinely applied for studying polymer degradation.

“We are interested in how degradation affects the materials’ properties, so we do a lot of the characterization by thermo-mechanical methods, like DSC, TGA, DMTA, tensile testing etc.,” said Dove.

In addition to chromatographic, spectroscopic and thermal analysis methods, there are a raft of other analytical techniques that can be applied to polymer analysis. X-ray diffraction and X-ray scattering data can be used to provide additional insights on the crystallinity of semi-crystalline polymers.¹⁰ Microscopy techniques can also provide extra information on a polymer’s microstructure and micromechanical properties.⁷

Collectively, these techniques all contribute to the fabric of the polymer science sector, allowing researchers to produce more advanced, high-performance polymer materials with more favorable recycling and degradation profiles tailored to suit the modern world.

About the interviewees:

Erin Stache is an assistant professor of chemistry at Princeton University. Research in her lab integrates organic chemistry, photo chemistry, inorganic materials, and polymer chemistry to pioneer fresh advancements in materials science and synthesis. A major aim of the Stache Lab is finding solutions for a more sustainable plastics economy through the application of innovative catalytic methods.

Andrew Dove is a professor of chemistry at the University of Birmingham. He leads the Dove Research Group, a multinational collection of vibrant and dynamic researchers that are focused on challenges in polymer and materials science. He is part of the Birmingham Plastics Network, an interdisciplinary team of more than 40 academics working together to shape the fate and sustainable future of plastics.  This unique team brings together chemists, environmental scientists, philosophers, linguists, economists, and experts in many other fields, to holistically address the global plastics problem.