Polymer characterization techniques and applications

Accurate characterization of new materials is vital to verify their chemical identity and to understand their properties. However, with polymers, this can be far from trivial. Polymeric organic materials can be amorphous, crystalline, or semi-crystalline, and their physical states can range from solids to oils and gels. The presence of a range of chain lengths can further add to the complexity. In addition, solvents can have a significant impact causing swelling or contraction, affecting the surface area, surface energy, and overall polymer activity.

Guidelines for method selection

To address the challenges inherent to the characterization of polymeric materials, a combination of techniques is usually required. For good scientific practice, at least one analytical technique should give an indication of purity, and it is advisable to use at least one technique that addresses each of the following three areas:

• Chemical characteristics; Including identification of functional groups, chemical bonds, intentionally and non-intentionally added chemicals and components, and intermolecular interactions.  

• Molecular characteristics; Including behavior in different solvents, molecular weight (MW), scattering behaviors, and crosslink density. 

• Bulk mechanical and dynamic properties; Including thermal properties, viscoelasticity, mechanical strength, compressibility, relaxation, and creep.  

There are several analytical techniques for studying each of these areas, with the technique chosen depending on the nature of the polymer. This is useful, as in some instances a particular polymer may not give clear data under a particular testing regimen. Table 1 outlines some of the common testing techniques used to probe different aspects of a polymeric material.

In the next section, we’ll go into more detail on the types of information five commonly used techniques can reveal about polymer properties.

Polymer characterization with NMR

Nuclear magnetic resonance (NMR) spectroscopy can give detailed information about bonds present in the primary structure, as well as network structure and behavior. Importantly, NMR can give an indication of product purity. Solution-state 1H and 13C experiments are commonly used, but 19F, 31P, and 15N can also give useful information.

Two-dimensional (2D) NMR techniques for bonded moieties (e.g. COSY, HMQC, and HMBC) can provide molecular detail for polymers with complex structures, and other advanced techniques can probe the overall structure and interactions, as well as polymer size distribution. Solid-state NMR can yield useful information about the structures of solid polymers beyond their chemical composition, but it has a reduced spectral resolution compared with liquid NMR.

However, there are some drawbacks to NMR spectroscopy. Spectra for polymeric materials may be complex and resonances may be broad. At present, determination of the molecular weight is only possible with short polymers with a degree of polymerization N < 100, therefore mass spectrometry, or a chromatographic technique such as size-exclusion chromatography, may be better options.

Polymer characterization with SEC

Size exclusion chromatography (SEC), also known as gel permeation chromatography (GPC), is commonly used to measure the molecular weight distributions of a polymer. Data collected can include number-average molecular weight (Mn), weight-averaged molecular weight (Mw), and dispersity, (Đ).

Calibration with standards is necessary when measuring the distribution using only an RI detector, and this approach will not determine a numerically accurate molecular weight. Another technique utilizing a multi-angle light scattering detector (SEC-MALS) allows for the determination of an exact molecular weight average.

Polymer characterization with FTIR and Raman spectroscopy

Fourier transform infrared (FTIR) and Raman spectroscopy measure the interaction of light with particular bonds within a molecule, revealing the presence or absence of a functional group. Both techniques can be used to collect fingerprint spectra of the material, which can be used to identify the general structure of the polymer when compared to a spectral library.

FTIR spectroscopy is a quick and easy technique for identifying the presence or absence of groups with strong dipoles (e.g. esters, azides, and alcohols), whereas Raman is best for identifying groups with weak dipoles (e.g. alkynes, disulfides, and thiols). However, neither FTIR nor Raman will give quantitative information or the exact molecular structure of a polymer, so they should be used in conjunction with another technique, such as NMR.

Polymer characterization with DSC

Differential scanning calorimetry (DSC) can effectively determine the physical and chemical properties of polymers undergoing glass transitions and thermodynamic phase transformations. The amount of heat released or absorbed by the polymer sample as it undergoes heating or cooling is monitored in relation to a reference. The rate of change of temperature in each sample will be affected by compositional and physical phase changes, leading to differences in heat flow as a function of temperature that correlate to the specific heat (Cp) of the material.

DSC can be used to determine the melting point (Tm), crystallization point (Tc), glass transition (Tg), thermal stability, and heat capacity of polymers. DSC may also be used to monitor the polymer’s purity and quality, as impurities and structural differences often lead to changes in phase transition temperatures. However, DSC is a destructive technique and does not provide direct elemental information.

Measurlabs offers organic polymer characterization services using all of the methods described above, and more. Do not hesitate to contact our experts through the form below to discuss your analysis needs and to request a quote for your material.