Conductivity in polymer systems

When considering the electrical conductivity of polymers, the phenomenon of interest is the movement of electrical charge and the way that this depends on the chemical structure and motion of the polymer molecules. The charge movement may be electronic or ionic. 

---

The experimental basis for and the application of the percolation theory to conductivity in polymer systems derive mainly from studies of metal powders in polymers. From these a number of (inadmissible) conclusions have been deduced, among them,... 

Reversible oxidation and reduction of polymers is commonly used to increase conductivity in these systems. Ions from the electrolyte are usually incorporated into the polymer as part of this process (see Electrically conducting polymers). 

Dyes for WORM-Disks. Regarding their memory layer, dye-in-polymer systems show advantages over metal layers in their higher stabiHty, lower toxicity, lower heat conductivity, lower melting and sublimation temperature, and simpler manufacturing technique (substrate coating by sublimation or spincoating). 

Table 2 shows the present state-of-the-art for the electrical conductivity of doped conjugated polymers. The magnitude of the electrical conductivity in polymers is a complex property determined by many stmctural aspects of the system. These include main-chain stmcture and TT-ovedap, molecular... 

The mobility of the ions in polymer electrolytes is linked to the local segmental mobility of the polymer chains. Significant ionic conductivity in these systems will occur only above the glass transition temperature of the amorphous phase, Tg. Therefore, one of the reqnirements for the polymeric solvent is a low glass-transition temperature for example, Tg = —67°C for PEO. 

In view of the above, conductivity measurements were conducted in asymmetric systems Au-polymer-Si for polystyrene and polysilazane, and Au-polymer-In for polysiloxane. The difference in barrier height between Au-polymer and Si-polymer estimated on the basis of measurements of the Au-Si barrier is ca. 0.5 eV (M) which, in the case of the conductivity limited by the electrodes, should produce a difference in the intensity of the currents of opposite polarizations equal to about 8 orders of magnitude. The difference in work function of Au and In, on the other hand, is ca. 1 eV so, on the assumption of the Schottky mechanism of conductivity, the difference in the intensity of opposite polarizations should amount to 17 orders of magnitude ( ). As can be seen in Fig. 4 in the case of an asymmetric polysilazane sample there is a difference in the intensity of the currents although this difference does take the expected course, it is several times smaller than expected, and is thus virtually negligible. A similar result was obtained for the polystyrene sample, while in the case of the asymmetric system based on polysiloxane there was no difference in the intensity of the opposite-biassed fields over the entire range of fields used - up to 3 x 10 V/m. It can thus be assumed that the conductivity in the films under study is dominated by the Poole-Frenkel volume generation independent of the contact effects. Such were also the conclusions of the workers who studied the conductivity in polystyrene (29) and polysiloxane (21). 

Excimers, which are simply excited state conplexes formed from equivalent chemical species, one of which is excited prior to complexation, were first reported in small molecule systans. However, daring the past 20 years one of the more vigorous research areas in photophysics has been the investigation of excimers formed in polymers (1). In most of the cases reported to date, the excimer studies in polymer systems have been conducted on polymers bearing pendant aromatic chromophores. Only a few papers have been published on excimers formed from polymers with the two species participating in excimer formation spaced at relatively large... 

A high ambient temperature conductivity of 10 Scm was measured with LiTFSI dissolved in the P2S5-PEO matrix, i.e, in salt-in-polymer materials. The decoupling of Li+ motions from polymer segmental motions was demonstrated, suggesting that, in certain cases, the transport number for Li+ ions in the phosphorus sulfide-PEO matrix will be much higher than in a typical PEO-based salt-in-polymer system. 

In acidic conditions, glycosidic bonds are cleaved in the cellulose structure. The rate of this reaction is controlled by the crystallinity of the cellulose and the temperature of the system (3, 5, 6). The resultant hydrolyzed cellulose has a shorter chain length than the original polymer, which greatly decreases the strength of the wood (7). Hackney (8) noted embrittlement and discoloration of sulfur dioxide exposed cellulose fibers. Such acid hydrolysis experiments have been conducted in aqueous systems, with the degradation rate dependent on the difiiision characteristics of the acidic solution into the wood structure. Because museum artifacts will not normally... 

Solvated proton condncting polymers and composite membranes should govern the transport of protons and water by micro structural control. The objective of this attempt is to take advantage of the high proton conductance in watery systems and to control the proton condnctivity by a sophisticated water management. Nevertheless, the condnctivity is strongly dependent on the water content, which also inflnences the mechanical properties by e.g. swelling. 

Ionic conductivity in polymer-based, water-containing, solid systems has been with us for a long time. A review of that history, which is intimately associated with the history of analytical electrochemistry and the physical chemistry of electrolyte solutions, will help to put the present work into perspective. 

Dirty" conductors with electrical conductivities of the order of 10 S/cm or less Such systemsscan be achieved with relative ease all that is required is a moderately high density of carriers. Carrier delocalization is neither required nor implied by such values. In such cases, the heavily doped polymers are highly disordered systems which can be considered as examples of Ae "Fermi glass" concept. Indeed, rc-conjugation is not even needed to achieve this low level of conductivity in polymers. ... 

The solid polymer eleetrolytes are mixtures of an inorganic salt and an organic polymer, preferably polyether like poly(ethylene oxide) (PEO). Relaxation and diffusion phenomena in context of ionic conductivity in those systems are our concern in this paragraph. 

The temperature dependence of the conductivity in polymer electrolytes has often been taken as indicative of a particular type of conduction mechanism In particular, a distinction is generally made between systems which show an Arrhenius type of behaviour and those which present a curvature in log a or log aT inverse temperature plots. In the latter case, empirical equations derived from the free-volume theory have... 

Quantitative DTAs have been modified by experienced researchers in order to measure the thermal conductivity of polymer systems [6-8], The procedure outlined below is reservedfor experienced DSC users. The sample is placed in direct contact with the sample holder, so that the instrument can record the temperature and the heat flow on one side of the sample. A heat sink of known temperature is placed in contact with the other side of the sample. Knowing the sample dimensions, the temperature difference between the sample holder and the heat sink and the recorded heat flow, the thermal conductivity of the sample can be calculated as follows ... 

Conductivity in organic systems is broadly divided into conductivity in crystals or films and conductivity in polymers. Single crystals without impurities are well suited for investigating the intrinsic charge transport properties of organic semiconductors. 

---