Practical Polymer Analysis

Synthetic resins, in which plastics are also included, vary widely in their chemical composition and in their physical properties. The number of synthetic resins which can be made is vast; relatively few, however, have achieved commercial importance. Some of the polymers that have achieved commercial importance and their uses are tabulated in Table 1 and some of their important physical properties are listed in Appendix 2.

It is emphasized at the start that the examination of a polymer can fall into one of several broad categories;

Most polymers contain quite complex additive systems which are incorporated during manufacture to impart beneficial properties during manufacturing operations, e.g. protective antioxidants, slip agents etc, and during end-use, e.g. antioxidants, u.v. stabilizers, plasticisers and anti-static agents, flame retardants, antioxidants and thermal stabilizers. The first stage in the examination of a polymer, either from the point of view of identifying the polymer or identifying and determining additives present must be to separate gross polymer from gross additives. It may then be necessary as discussed in Chapter 5.1.16 to separate the gross additive fraction into individual additives by a chromatographic procedure in order to facilitate the identification of individual additives.

The polymer analyst working in industry is frequently asked to identify a polymer or at least to give an opinion as to the type of a polymer. This might arise as a result of an interest in the materials being used by competitors or manufacturers in components or packaging materials. Obviously, if a detailed examination of a polymer or copolymer and its additive system is required then other chapters of this book should be referred to. However, if a quick opinion is all that is required, then some simple physical and chemical tests and the fingerprinting approach discussed in this chapter, might suffice. If on the other hand, the polymer does not yield to such tests, or, if a more detailed structural elucidation is required, then the full range of elemental and functional group analysis (Chapters 4.3 and 9.2 and microstructural and spectroscopic studies (Chapters 9 and 10) will be required.

In order to appreciate fully the techniques which have been developed for the analysis of additives in polymers, it is necessary to be familiar with the difficulties involved in such an undertaking, and also with the chemical and physical properties of the additives themselves.

Functional groups can occur in polymers over a wide range of concentration, ranging from a few parts per million, as occurs for example in the case of end-groups, or micro-unsaturation functional groups to the percentage range. The occurrence of two or three double bonds per thousand carbon atoms in, for example, polyethylene can effect intrinsic polymer properties and can certainly help to distinguish between polyethylene manufactured by different manufacturers using different processes. As such, this type of determination falls within the province of micro-structure, which is discussed in Chapter 10. At the other end of the concentration scale a copolymer of, for example, ethylene and vinyl acetate will contain between 1 an 90% of either monomer. Analysis of functional groups in polymers in this concentration range is considered below.

Polymers normally do not consist of a particular molecule with a unique molecular weight, but rather are a mixture of molecules with a molecular weight range which follows a distribution. With some types of polymers the picture is further complicated by the appearance of what are known as crosslinks. These are chemical bonds which link one polymer chain to another. Crosslinking will, therefore increase the molecular weight of a polymer and, incidentally, decrease its solubility in organic solvents. These are some of the features which make it possible to produce for a given polymer, say polypropylene, a range of grades of the polymer, each with different physical properties and end-uses and each characterized by a different molecular weight distribution curve and degree of crosslinking. The factors which control these parameters in a polymer are complex, and are linked with the details of the manufacturing process used. They will not be discussed further here. The measurement of the molecular weight is a task undertaken in its own right by polymer chemists, and is concerned with the development of new polymers and process control in the case of existing polymers. Additionally, however, it is necessary to separate a polymer not into unique molecules each with a particular molecular weight, but into a series of narrower molecular weight distribution fractions.

Molecular weight obtained by size exclusion chromatography and other chromatographic methods are not true determined volumes but are obtained via a calibration process which involves the preparation of a universal calibration curve relating log known molecular weight versus elution volume obtained for standard polymer fractions of precisely known molecular weight. Frequently standard polystyrenes are used for this purpose as these are easily available. If the molecular weight (MA) i of a species of polymer A is related to the molecular weight (M p/s)i of a polystyrene standard eluting at the same retention volume i.

The elucidation of the composition and structure of a copolymer or terpolymer is a challenging task for any polymer analyst. Firstly, it is necessary to be absolutely sure of the elemental composition of the sample, (Chapter 4.3), and preferably that the copolymer has been completely separated from any additives present, (Chapter 3). Accurate data on the type and concentration of elements present will often simplify the task ahead.

Microstructural features are often of tremendous importance in deciding the physical, and to some extent the chemical, properties of polymers. The term microstructure does not necessarily infer that the feature of concern occurs at low concentrations. It is more concerned with the particular detail of the structure of the polymer. Thus polybutadiene unsaturation exists in various cis and trans forms, all present as constituents of the polymer. Some forms of unsaturation might exist at high concentrations, and some at low. In the case of polyethylene, however, butyl hexyl and octyl side-groups, which are of greater interest from the microstructural point of view, all exist at low concentrations usually expressed as the number of such groups present per 1000 carbon atoms in the polymer chain.

Thermal methods of analysis predominate in the measurement of this characteristic of polymers. Thermal methods of analysis of polymers are important in that these techniques can provide information about the thermal and oxidative stability of polymers, their lifetime or shelf-life under particular conditions, phases and phase changes occurring in polymers, and information on the effect of incorporating additives in polymers. As will be seen in Chapters 12 and 13, these methods have also been applied to the measurement amongst others of melting points, percent crystallinity, crystallization temperature, glass transition temperature, melting temperature, cure rate, degree of cure, composition, thermal stability, expansion coefficient, and softening temperature.

The glass transition temperature (Tg) is defined as the temperature at which a material loses its glasslike, more rigid properties and becomes rubbery and more flexible in nature. Practical definitions of Tg differ considerably among different measurement methods; therefore, specification of Tg requires indication of the method used.

Crystallinity is a state of molecular structure referring to a long range periodic geometric pattern of atomic spacings. In semicrystalline polymers, such as polyethylene, the degree of crystallinity (% crystallinity) influences the degree of stiffness, hardness and heat resistance.