Polymer Characterization: Electron Paramagnetic Resonance

Abstract

Polymers and their nanocomposites are widely used in our daily life. The field of their use is permanently expanding. They are successfully used in pharmaceutics,medical therapy, aircraft and spaceship construction, and so on (1). One of the main scientific goals is to reinforce human brain with computer ability (2). To create such a symbiotic intelligence, appropriate neurochips were planned to be developed and created. However, a convenient modern computer technology is based on three-dimensional silicon crystals, whereas human organism consists of lower dimensional biological object. So, the combination of a future computer based on organic insulating and conjugated polymers with biopolymers is expected to considerably increase the power of human apprehension. The information in such elements can be transferred through conjugated polymers, for example, transpolyacetylene (trans-PA), poly(p-phenylene) (PPP), polypyrrole (PP), polyaniline (PANI), polythiophene (PT), and their derivatives (3). Their electrical conductivity of either por n-type can be changed by more than 12 orders of magnitude by chemical or electrochemical introduction into their volume of various anions (BF4 , ClO4 , AsF − 5 , J − 3 , FeCl − 4 , MnO − 4 , and so on) or cations (Li +, K+, Na+, and so on) or cations (Li+, K+, Na+, and so on), respectively (4). The handling of charge transfer in such polymers becomes possible due to the existence in their backbone of alternating single and double bonds. This originates the appearance of midgap in their band structure as a result of the overlapping of π orbitals of monomer rings that depends on polymer structure, morphology, chain packing, and doping level y (the number of the dopant molecules per each polymer unit). Both the molecular and band structures of PT are shown in Figure 1 as an example. For this and analogous polymers, a resonance form can be derived, which corresponds to a quinoid structure. So, energetically lower and higher, benzenoid and quinoid forms can be stabilized in conjugated polymers under their chemical or electrochemical doping up to intermediate level y or irradiation (Fig. 1). As a result, the nonlinear excitations, polarons with spin S = 1⁄2 and elemental charge e, are formed on polymer chains. Their energy level lies in the midgap above the valence band (VB) and below the conducting band (CB) (Fig. 1). At the polymer doping up to intermediate level, polaron pairs may collapse and form spinless bipolarons (Fig. 1). The width of the polaron and bipolaron is 3–5 and 5–5.5 polymer units, respectively (5).With the further increase in a doping level y, bipolaron states overlap forming bipolaron