Conductively filled polymers

2 TRADITIONAL ROUTES TO CONDUCTIVE POLYMERS 1.2.1 Conductively filled polymers 

Polymers have found increasing use in all walks of life, both industrial and consumer, and in many cases they have replaced metals. In some cases, however, although the polymer may offer many advantages over its metal analogue, it may still be desirable to have the electrical conductivity of the replaced metal. Alternatively, polymers may be used in applications that are totally new but that still require good electrical 

An example of a relatively new application for conductive polymers, which has evolved with the semiconductor industry, is in the so-called die-attach adhesives. These typically epoxide- or polyimide-based materials are used to attach silicon integrated circuits to their substrates and leadframe components, and are often required to be electrically conducting so that the back of the chip is effectively earthed. 

There are a number of conductive additives and fillers that are used in producing this type of conductive polymer, including the carbon blacks, carbon fibre, aluminium flake, stainless steel fibres, metal-coated fillers, and other metals such as silver and gold. 

Carbon-black-filled polymers probably constitute the largest proportion of all conductive polymers produced, and there are many varieties of carbon black available. A big advantage of the carbon blacks is their low cost, although, as they are black, it is not possible to produce coloured products. Also, carbon-black-filled polymers are not suitable for EMI/RFI shielding applications, because the required conductivities cannot be achieved without very high loadings that severely impair the product s physical properties. 

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Another mode of operation derived from contact AFM that is relevant to polymer studies is the scanning thermal microscope (SThM) [118, 146], In SThM the tip is a special device that has a resistive element. If a current is passed through this resistive element, its temperature depends on the heat transferred to the specimen. It thus acts as a thermal probe, as seen for example in Fig. 5.36. During scanning, thermal control of the probe can be used to generate images based on variation of either sample temperature or thermal conductivity. Filled polymers and polymer blends are candidates for this kind of study, but the resolution is relatively poor. Microfabricated thermal probes can give a resolution of 100 nm. 

This article addresses the synthesis, properties, and appHcations of redox dopable electronically conducting polymers and presents an overview of the field, drawing on specific examples to illustrate general concepts. There have been a number of excellent review articles (1—13). Metal particle-filled polymers, where electrical conductivity is the result of percolation of conducting filler particles in an insulating matrix (14) and ionically conducting polymers, where charge-transport is the result of the motion of ions and is thus a problem of mass transport (15), are not discussed. 

The results of the above section show that the significant nonuniformity of the distribution of the filler particles in the thickness of sample is observed during injection moulding of the filled polymers. This nonuniformity must affect the electrical properties of CCM owing to the strong dependence of the CCM conductivity on the filler concentration. Although there are no direct comparisons of the concentration profiles and conductivity in the publications, there is data on the distribution of conductivity over the cross-section of the moulded samples. 

The main reasons for this lie in feasibility. Conducting fillers are rather expensive and their use increases the cost of an article. Besides, filled polymers have worse physical-mechanical properties, especially impact strength and flexural modulus. The use of fillers is also detrimental to the articles appearance and calls for additional treatment. The continuous development of electronics has also contributed to a loss of interest to conducting composites as screening materials the improvement of components and circuits of devices made it possible to reduce currents consumed and, thereby, noise level a so called can method is practised on a wide scale in order to cover the most sensitive or noisy sections of a circuit with metal housings [14]. 

Composites. See also Composite materials Composites. See also Laminates aluminum-filled, 10 15-28 carbon fiber, 26 745 ceramic-filled polymer, 10 15-16 ceramic-matrix, 5 551-581 conducting, 7 524 from cotton, 8 31 ferroelectric ceramic-polymer,... 

Dalmas F, Dendievel R, Chazeau L, Cavaille JY, Gauthier C (2006) Carbon nanotube-filled polymer of electrical conductivity in composites. Numerical simulation three-dimensional entangled fibrous networks. Acta Materialia 54 2923-2931. 

Polymers may also be converted from nonconductors to conductors of electricity by the addition of conductive fillers. Graphite-filled polymers are semiconductors, and polymers filled with aluminum flakes or aluminum filaments are relatively good conductors of electricity. 

Carbon-filled polymers, especially those made from acetylene black, are fair conductors of heat and electricity. Polymers with fair conductivity have... 

Electronic devices can also generate electromagnetic and radio frequency interference waves that can interfere with other electronic devices. These waves must be modulated and leakage to the environment prevented. Plastics, silicones, acrylics, and polyesters (qv) that are filled with conductive fillers, such as silver, nickel, and copper, are used for this application (1). Although nickel-filled polymers are low cost and efficient, these are not preferred because of the carcinogenic nature of nickel powder. 

The higher thermal conductivity of inorganic fillers increases the thermal conductivity of filled polymers. Nevertheless, a sharp decrease in thermal conductivity around the melting temperature of crystalline polymers can still be seen with filled materials. The effect of filler on thermal conductivity for PE-LD is shown in Fig. 2.5 [22], This figure shows the effect of fiber orientation as well as the effect of quartz powder on the thermal conductivity of low density polyethylene. 

Conducting carbon polymer ink, which filled a UV-ablated microchannel, was used to construct the integrated microelectrode on a plastic chip. Both chronoamperometry and CV were employed to detect a model compound (fer-rocenecarboxylic acid) down to 3 iM, corresponding to 0.4 fmol within a volume 120 pL [758], In another report, a carbon-paste electrode was constructed by filling a laser-ablated (PET or PC) channel with C ink. The whole structure was then cured at 70°C for 2 h [189]. 

Electrodes in a capacitively coupled conductivity detector were made by injection molding carbon-filled polymer into a preformed PS chip. The polymer consisted of three conducting formulations 8% carbon black filled PS, 40% C fiber filled nylon-6,6, and 40% C fiber filled high-impact PS [774]. In another report, a movable contactless conductivity detector was also developed to allow the distance of the electrode to be adjustable [775],... 

We should also develop the processes of producing filled polymers during their synthesis, which is economically feasible and justifiable. Researchers have already developed a process to obtain filled conductive silicone rubbers by the technique stated above. 

FIG. 3.22. J—V characteristics of the conducting organic polymer diode ITO/PEDOT PSS/MEH-PPV/Au at 98 K. The squares are the experimental data while the dotted line shows the trap-filling region and the solid fine represents trap-filled V2 region [53]. 

Nonmetallic conductors and corrosion products. Carbon brick in vessels is strongly cathodic to the common structural alloys. Impervious graphite, especially in heat-exchangers, is cathodic to structural steel. Carbon-filled polymers can act as active cathodes. Some oxides or sulfates are conductors, such as mill scale (magnetite Fe304), iron sulfides on steel, lead sulfate on lead can act as effective cathodes with an important area to that of the anodes. Very frequently, the pores of the conductive film are the preferable anodic sites that leads to localized corrosion (pitting).5... 

This property is usually not measured for the carbon black itself, but in the system containing the carbon black, i.e. in a polymer or binding agent Conductivity of a filled polymer increases with the specific surface area and the structure of the carbon black being incorporated into the system. It is also a function of carbon black concentration and dispersion and of the type of polymer or binding agent used. 

Trans-polyacetylene, tra 5-(CH) was the first highly conducting organic polymer [1,2]. The simple chemical structure, -CH- units repeated (see Fig. IVB-la), would imply that each carbon contributes a single p electron to the tr-band. As a result, the rr-band would be half-filled. Thus, based upon this stmcture, an individual chain of neutral polyacetylene would be a metal since the electrons in this idealized metal could move only along the chain, polyacetylene would be a one-dimensional (Id) metal. However, experimental studies show clearly that neutral polyacetylene is a semiconductor with an energy gap greater than 1.5 eV. Rudolf Peierls [86] showed many years ago that Id metals are... 

A much more general mechanism for this kind of application is schematically represented in Fig. 3(b). A third phase (3) is inserted as an active material between the contact plane (1), made of metal or any inert metal-free material such as glassy carbon or carbon-black-filled polymers with a good electronic conductivity, and the... 

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