Wide-Angle X-ray Diffraction and Small-Angle X-ray Scattering Studies of Rubber Nanocomposites

With their predominantly amorphous nature, elastomers would not seem very prone to X-ray diffraction or scattering studies. As will be shown in the following pages, this is not so true. It must be borne in mind that many elastomers, when stretched, can indeed crystallize. Moreover the preparation of rubber-based nanocomposites consists in the introduction in an elastomeric matrix of fillers, which are crystalline or at least produce a material with fluctuations in electron density within its bulk. The nanocomposite approach has its roots in the vivid academic and industrial research activity spurred by the increasing demand for high performance materials. Rather than designing and synthesizing novel polymeric materials employing innovative monomers, it is more convenient under an applicative and industrial point of view to prepare composite materials. The double objective of filler addition is enhancing properties while keeping the cost of the material at an affordable level. Carbon black is nowadays by far the most important filler employed in the rubber industry. It is produced by pyrolysing oil or natural gas under controlled conditions, and is thus associated to pollution. Moreover, it confers a black color to the materials limiting their application in medical, sports and domestic product segments. The quest for other fillers to substitute carbon black in rubber compounds included testing of fillers such as sepiolite, kaolin and silica, but their performance was not adequate and remained below that obtained with carbon black. Due to their inorganic nature, these fillers are not compatible with the polymer matrix and it is therefore very difficult to homogeneously disperse them. More recently, a novel approach has been introduced for the preparation of effective compounds. Clay-based nanocomposites were firstly discovered by the Toyota group [1,2]. The dispersion of the reinforcement agents on a nanometer scale and the high aspect ratio of fillers confer to nanocomposites innovative physical and chemical properties with respect to their bulk counterparts [3-5]. A wide variety of nanofillers has thus been tested for reinforcing rubbers, generating the need of techniques that can accurately describe their degree of dispersion in the matrix and the morphology of the obtained composite. This is the aspect that X-ray scattering studies can contribute to shed light on. A quantitative approach to wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS), although somewhat labor intensive, allows to obtain a very thorough description of the structure and morphology of nanocomposites. An especially interesting advantage of X-ray diffraction methods is that, differently from microscopy techniques, they sample the whole bulk of the specimen, thus giving a more generalized picture of its morphology. Being able to exploit this peculiarity at its full extent offers an invaluable tool for a complete characterization of polymer-based nanocomposite materials. This chapter, after a brief introduction on WAXD and SAXS, will present a number of examples of literature works where these techniques were determinant for the investigation. It should not thus be intended as a “textbook” on these techniques (many excellent books of this kind already exist [6-13]) but as a collection of stimulating approaches, among which the reader could find the most suitable solution for his or her particular characterization problem.