Polymers on the Crime Scene

The words of Paul Kirk, one of the most eminent criminalists of the twentieth century, summarise the reason why forensic science elicits so much fascination. Criminalistics (or in its broadest sense forensic science) combines all those analytical and characterization techniques that support all the stakeholders in the judicial system in the investigation or explanation of crimes. As such, a vast array of scientific disciplines can be incorporated within it and, indeed, this comprehensiveness is its greatest strength.

What are polymers, one of the most pervasive class of materials in our everyday life? The most immediate answer defines polymers as complex and giant molecules which, due to their large size, are starkly different from low molecular weight compounds. To put such difference in a numerical context, the molecular mass of table salt is 58 g/mol, that of aspirin is about 180 g/mol, that of nitroglycerin is 227 g/mol, whereas the typical molecular weight of a polymer ranges from the tens of thousands up to millions of grams per mole. A high molecular weight, though, is not enough to have a polymer. Another important prerequisite is that the giant chemical species is composed of smaller moieties, called repeat units, which recur along the molecule. Such nature is reflected in the name: the word polymer comes in fact from the Greek πολύς, which means ‘many’ and μέρος which means ‘part’. The small molecules, which combine to form a big molecule, can share the same chemical nature or be different. To illustrate, imagine a train, which is a big vehicle composed of a sequence of basic units, railroad cars (Fig. 2.1). In some trains all the cars are of the same type (for example they are all similar passenger cars), in some others there are mixed types of cars (for example passenger and sleeping cars). Analogously, polymers can be composed of repeating units which are all equal, or they can contain two or more different building blocks. The former species are called homopolymers, the latter are defined as copolymers.

Polymers are ubiquitous in our everyday life, so it is highly likely that they may be somewhat involved in the commission of crimes. Fibres will be shed by the garments of a burglar, paint smears and plastic fragments can be left in car accidents, adhesive tape can be used for the packaging of illegal drugs, wires insulated by a polymeric coating are part of many bombs, etc.

Some polymers are more common than others, and so they were more deeply studied, researched and reported by forensic scientists. For these kinds of items, protocols and analytical guidelines are available, and in some cases abundant literature was published. The aim of this chapter is presenting a summary of the types of polymeric items most commonly encountered in casework, in order to show the reader the state-of-the-art in this branch of forensic analysis. An examination of the existing literature on these materials is also informative to understand the issues and the features of well-established analytical strategies, and to catch how the many concerns and requirements discussed in Sect. 3.2 can be satisfied. For each kind of evidence, the chemical composition, the manufacturing process, the issues regarding recovery and sample preparation and the techniques useful for its characterisation will be presented.

Polymers are extremely rarely used as pure materials. Many applications have a large number of functional and structural requirements. Probably the most graphic example of the complex requisites which materials must meet is found in tissue engineering. In this branch of science, the aim is that of devising materials which can act as scaffolds for the substitution, regeneration or healing of damaged organic tissues. If a polymer is intended for such biomedical applications, it must be compatible with certain tissues or cells, but on the other hand it must be unreactive with other molecules present in the organism, for instance it must not be considered an extraneous substance by the immune system. Such materials must have mechanical properties compatible with the intended use, not too stiff, but not too soft either. They must be degradable in the body, yielding harmless residues which can be easily excreted, in a time span compatible with the concurrent growth of the tissue, which is regenerated by the organism. This is definitely an extreme example of a material very demanding for properties and performance. A similar amount of complexity can however be found in much more mundane objects, such as car tyres, textile fibres or paper sheets. A single polymeric material usually does not have the right mix of properties to be ready to use for practical applications.

As outlined in Chap. 1, any polymeric object is actually a composite, containing a number of ingredients with different functions. Of course, the matrix is the fundamental component because it constitutes the largest portion of the material. The scheme in Fig. 1.1 shows that the characterisation of the polymer matrix can be organised along two main directions. One is focused on the molecular features directly related to the polymerisation process, the other is centred upon the description of the structure and morphology attained by the material. In the former approach, the constitution and the configuration (Sect. 2.7) of the polymer are investigated. Constitution defines which repeat units are present in the macromolecules and how they are connected together. Section 5.7 introduced the issue of the identification of the polymer matrix, mostly with the aim of broadly classifying the type of material within the families described in Sect. 2.8. However, much more information can be obtained, in order to discriminate between materials pertaining to the same class. Molecular weight, the presence of comonomers or the regularity of the sequence of repeat units are constitutional features which can be exploited for this purpose. The tacticity of the polymeric chains, i.e. the evenness of the succession of the configuration of the repeat units, is another important attribute which is directly dependent on the ability of the polymerisation process to control the stereoregularity of the synthesis. All these characteristics cannot be modified without breaking chemical bonds, and are therefore the result of the reactions involved in the synthesis of the material. On the other hand, processing has a very negligible role in influencing them.

Processing is the mix of operations which are applied to materials when objects are made. A whole branch of engineering deals with the design of viable solutions to all the problems which can arise in the transformation of polymers. With a maybe rough approximation, it can be said that processing consists in the transfer of mechanical or thermal energy to the polymer, with the aim of mixing it with other substances and of shaping it in the desired final form. Processing is rarely mild, because the quest for maximisation of industrial production imposes the conditions which minimise the manufacturing time and which often correspond to high temperatures and shear forces. The polymer thus suffers a relevant mechanical and thermal stress, which is the main driving force determining the structure of the material in the solid state. The reason why this should be of interest to the forensic scientist is that processing-dependent parameters contain information extremely indicative of the manufacturer.