A brief guide to polymer characterization: structure (IUPAC Technical Report)

1 Introduction

To bolster the series of brief guides released by International Union of Pure and Applied Chemistry (IUPAC) [1], [2], [3], [4], [5], here we introduce the first Brief Guide to Polymer Characterization. This article provides a concise overview of characterization methods for teachers, students, non-specialists, and newcomers to polymer science as well as being a useful manual for researchers and technicians. This guide focuses on the structural characterization (i.e., molecular and multi-molecular aspects) of polymers (see Fig. 1).

Fig. 1:

Overall structure for the characterization of polymers based on the type of information sought, highlighting that this Brief Guide deals with structural characterization. This serves as a guide for this document.

Through the use of a succinct table, the intention is to enable the reader to navigate from the polymer property that one wishes to measure to the technique(s) required to measure it, and vice versa. This is a starting point for the user and not a comprehensive operating manual of all of the characterization techniques that are available. Figure 1 provides an overview of polymer characterization and can be used to navigate Table 1 more easily. A glossary of acronyms and abbreviations used in the manuscript is provided at the end of the paper.

Unlike pure low molar mass chemical substances, polymers are not composed of identical molecules. The macromolecules which comprise a single polymer sample vary from one another, primarily in terms of size and shape, but often also in the arrangement or positioning of atoms within macromolecules (e.g., chain branching, isomerism). Polymer properties are drastically different from those of other substances and their characterization relies on specialist equipment and/or common equipment used in a specialized way (e.g., particular sample preparation or data analysis). This Brief Guide aims to provide a starting point to assist the reader to navigate some of the most useful techniques to determine the various structural characteristics of polymers. It is also worth noting that the guide focuses on the characterization of single (one component) polymers, rather than multicomponent polymer blends. Notably, most commercial polymers contain additives, which are also not discussed herein.

2 Structural characterization

The structural analysis of polymers can be further categorized into (A) the characterization of the macromolecules that make up the polymer substance, such as molar mass (averages) or chemical functionality, and (B) the analysis of multi-molecular assemblies or the effects thereof (such as density or degree of crystallinity). For the latter category, there are clearly examples where the entity to be analyzed lies within the overlap between structural and behavioral characterization. In these cases, we have attempted to place the measured characteristic and associated technique in the most commonly used and logical category. For example, if the property and/or technique are more suitable in a discussion of the structural analysis of polymers, then they are included herein. In contrast, if the property and associated technique are more appropriate for the discussion of polymer performance, then they should be dealt with separately in a more detailed discussion of the behavioral characteristics of polymers. Examples of the latter include, but are not limited to, glass transition temperature (T _g), melting temperature (T _m), and intrinsic viscosity ([ƞ]), which are a direct result of polymer structure, but bear a strong influence on the behavior or performance of the polymer via the so-called structure–property relationships and are therefore more appropriately categorized as behavioral characteristics. Table 1 lists the structural features of polymers alongside techniques commonly used to characterize them.

Notably, there are several different molar mass averages that can be used to describe a polymer (e.g., z-average (zentrifuge-average) molar mass, M _z, z+1-average molar mass, M _z+1, viscosity-average molar mass, M _v). Here we focus on the two most commonly reported parameters (i.e., number-average molar mass, M _n, and mass-average molar mass, M _m or M _w) and the ratio between them (molar-mass dispersity, Ɖ _M); Ɖ _M is routinely used as a descriptor of the breadth of polymer molar mass distribution. Additionally, it should be noted that a number of the techniques used to characterize the surface of polymers can also be used to probe deeper into the sample to, for example, gain information about compositional changes going from the surface into the bulk.

It is important to appreciate that when measuring a certain molecular structural parameter (e.g., molar mass, radius of gyration), some techniques may only give the average of this parameter for all of the molecules in the sample, while other techniques may give an indication of the distribution with regards to the specific parameter. While, in essence, these characteristics are brought about by the analysis of all the macromolecules that comprise the polymer and hence could be thought of as multi-molecular, they are included in Section A of Table 1 because the specific properties being measured are at the molecular or submolecular scale. Moreover, as a direct consequence of polymers being comprised of a mixture of molecules with many different structural parameters, many characterization methods rely on a pre-analysis sorting step (fractionation) that is used to divide up a polymer into molecularly more uniform batches (fractions) prior to analysis. The most common pre-analysis separation steps involve chromatography or time-of-flight procedures (included in Table 1). Other processes, such as various types of fractionation (e.g., CEF, CRYSTAF, FFF, TREF, step crystallization), not included in Table 1, should not be overlooked as powerful tools in a polymer analyst’s armory. These techniques can separate the polymer according to some inherent characteristic (e.g., molecular size, propensity to crystallize) into fractions, which, when analyzed, can provide valuable information about the distribution of that characteristic in the given polymer.

Finally, one should also note that computational modelling is a powerful theoretical tool to complement experimental characterization techniques. There are several different approaches that can be applied, such as molecular mechanics (MM), molecular dynamics (MD), and self-consistent field theory (SCFT), to name but a few. For example, density functional theory (DFT) can be applied to assign vibrational, UV–Vis, and NMR spectra, helping the interpretation of both intramolecular (conformation, tacticity, etc.) and intermolecular features (e.g., hydrogen bonding or other weak interactions). Computational modelling is an important pillar of analytical polymer science and, as such, is not covered in this Brief Guide and should be dealt with by a dedicated piece of work.

3 Summary

The complex nature of the structural variables possible in macromolecular materials often presents a challenge with regard to the detailed structural characterization of polymers. This Brief Guide provides a useful starting point to direct the reader to the most commonly used and useful techniques to characterize these structural variables. Common methods used to determine behavioral characteristics of polymers (such as viscoelasticity, conductivity, etc.) often rely on international standards and form a much larger body of work that is beyond the scope of this Brief Guide.

4 Glossary of acronyms and abbreviations

5 Membership of sponsoring bodies

Membership of the IUPAC Polymer Division Committee for the period 2022–2023 is as follows:

President : C. K. Luscombe (USA); Vice President : I. Lacik (Slovakia); Secretary : P. D. Topham (UK); Past President : G. T. Russell (New Zealand); Titular Members : M. C. H. Chan (Malaysia); Tanja Junkers (Australia); P. Mallon (South Africa); J. B. Matson (USA); Y. Men (China); M. Peeters (UK); P. Théato (Germany); Associate Members: A. Aguiar-Ricardo (Portugal); C. M. Fellows (Australia); D. Haase (USA); R. Hutchinson (Canada); J. Merna (Czech Republic); M.-H. Yoon (Korea); National Representatives : R. Adhikari (Nepal); J.-T. Chen (China/Taipei); S. Guillaume (France); J. E. Imanah (Nigeria); A. Kishimura (Japan); G. Mechrez (Israel); S. Ramakrishnan (India); G. Raos (Italy); M. A. A. Tasdelen (Turkey); J. van Hest (Netherlands).

Membership of the Subcommittee on Polymer Terminology during the preparation of these Recommendations (2015–2022) was as follows:

Chair : R. C. Hiorns (France) 2014–2020; P. Théato (Germany) 2021-present; Secretary : C. K. Luscombe (USA) 2014–2015; P. D. Topham (UK) 2016–2019; J. B. Matson (USA) and P. Théato (Germany) 2020; J. B. Matson (USA) 2021-present; Members : V. Abetz (Germany); R. Adhikari (Nepal); G. Allegra (Italy); R. Boucher (UK); B. Brettmann (USA); P. Carbone (UK); M. C. H. Chan‡ (Malaysia); T. Chang (Korea); J. Chen (USA); C. dos Santos (Brazil); W. Farrell (USA); C. M. Fellows (Australia); A. Fradet (France); M. Gosecka (Poland); C. F. O. Graeff (Brazil); F. Giuntini (UK); D. Haase (USA); J. He (China); K. H. Hellwich (Germany); M. Hess (Germany); P. Hodge (UK); W. Hu (China); A. D. Jenkins‡ (UK); J.-I. Jin (Korea); J. Kahovec (Czech Republic); D. J. Keddie (UK); T. Kitayama (Japan); P. Kratochvíl‡ (Czech Republic); R. G. Jones‡ (UK); P. Kubisa (Poland); M. Malinconico‡ (Italy); P. Mallon (South Africa); S. V. Meille (Italy); J. Merna (Czech Republic); G. Moad (Australia); W. Mormann (Germany); T. Nakano (Japan), C. K. Ober (USA); M. Peeters (UK); S. Penczek (Poland); O. Philippova (Russia); M. D. Purbrick (UK); G. Raos (Italy); G. Russell (USA); C. Scholz (USA); F. Schué‡ (France); S. Słomkowski (Poland); L. Sosa Vargas (France); R. F. T. Stepto‡ (UK); N. Stingelin (USA); A. Šturcová (Czech Republic); J. P. Vairon (France); L. S. Vargas (France); M. Vert (France); J. Vohlídal (Czech Republic); M. G. Walter (USA); E. S. Wilks (USA); A. Yerin (Russia); M.-H. Yoon (Korea).

‡ Deceased.