Oxidized polyacrylonitrile

Process. Any standard precursor material can be used, but the preferred material is wet spun Courtaulds special acrylic fiber (SAF), oxidized by RK Carbon Fibers Co. to form 6K Panox B oxidized polyacrylonitrile (PAN) fiber (OPF). This OPF is treated ia a nitrogen atmosphere at 450—750°C, preferably 525—595°C, to give fibers having between 69—70% C, 19% N density less than 2.5 g/mL and a specific resistivity under 10 ° ohm-cm. If crimp is desired, the fibers are first knit iato a sock before heat treating and then de-knit. Controlled carbonization of precursor filaments results ia a linear Dow fiber (LDF), whereas controlled carbonization of knit precursor fibers results ia a curly carbonaceous fiber (EDF). At higher carbonizing temperatures of 1000—1400°C the fibers become electrically conductive (22). 

Oxide-water interfaces, in silica polymer-metal ion solutions, 22 460—461 Oxidimetric method, 25 145 Oxidization devices, 10 77-96 catalytic oxidization, 10 78—96 thermal oxidation, 20 77-78 Oxidized mercury, 23 181 Oxidized polyacrylonitrile fiber (OPF), 23 384... 

Semicarbon or oxidized polyacrylonitrile fibers, produced by thermo-oxidative stabilization of either viscose or acrylic fibers, have excellent heat resistance, do not melt or burn, and have excellent resistance to molten metal splashes. Panox (RK Textiles), Panotex (Universal Carbon Fibers), and Pyron (Zoltek Corp) are some examples, produced from acrylic fibers. 

Carbon molecular sieve membranes. Molecular sieve carbons can be produced by controlled pyrolysis of selected polymers as mentioned in 3.2.7 Pyrolysis. Carbon molecular sieves with a mean pore diameter from 025 to 1 nm are known to have high separation selectivities for molecules differing by as little as 0.02 nm in critical dimensions. Besides the separation properties, these amorphous materials with more or less regular pore structures may also provide catalytic properties. Carbon molecular sieve membranes in sheet and hollow fiber (with a fiber outer diameter of 5 pm to 1 mm) forms can be derived from cellulose and its derivatives, certain acrylics, peach-tar mesophase or certain thermosetting polymers such as phenolic resins and oxidized polyacrylonitrile by pyrolysis in an inert atmosphere [Koresh and Soffer, 1983 Soffer et al., 1987 Murphy, 1988]. 

Tsai JS, Tension effects on the properties of oxidized polyacrylonitrile and carbon-fibers during... 

Trade names for such commercially available impervious isotropic carbon products are "glassy carbon" (9) and "vitreous carbon" (10). This conservation of the polymer shape is also the basis for the carbon fibre formation from crosslinked polymer fibres (fig.4 b) i.e. rayon or oxidized polyacrylonitrile. 

Figure 2 Microscopic images of oxidized polyacrylonitrile particles dispersed in silicone oil under different electric fields (a) E l.O kV/mm, (b) E=1.5 kV/mm, (c) E=2.5 kV/mm. the left and right side stripes are electrodes and the gap between them are 1 mm. Reproduced with permission from T. llao, and Y. Xu, J. Colloid Interface Sei., 181(1996)581.

Figure 15 Carbonating process from the polyacrylonitrile to oxidized polyacrylonitrile...

Particle conductivity has a strong influence on ER performance. Block [24J studied how ER effect is influenced by the particle conductivity using the acene-quinone radical polymer/silicone oil, and found that the static yield stress peaks at a particle conductivity around 10 S/m (sec Figure 17). The molecular structure of the acene-quinone radical polymers is shown in Figure 18 and Figure 3 in Chapter 3. A similar tendency was found in oxidized polyacrylonitrile/silicone oil ER system [61J. However, the yield Stress peaks at the particle conductivity of about 10 S/m, rather than 10 ... 

S/m (see Figure 19). The ER effect of the polyaniline particle of different conductivity dispersed into silicone oil was studied and the largest ER effect was found to occur in the suspension of polyanilinc particle of conductivity 10 S/m [621. Besides the influence on the ER effect, the particle conductivity also determines the current density of the whole suspension and the response time of the ER fluid. The current density of the oxidized polyacrylonitrile(OP)/silicone oil suspensions obtained at 2.5 kV/mm as a function of particle conductivity is shown in Figure 20 [61]. The current density almost linearly increases with the conductivity of particle. The response time was found to be inversely proportional to the particle conductivity both experimentally [63] and theoretically [64]. The response time can be determined from the relationship between the shear stress and the frequency of applied electric field. Such an example is shown in Figure 21, in which the shear stress of two aluminosilicate/silicone oil suspensions is plotted vs. frequency, fhe suspension with particle of conductivity 6.0 x 10 S/m displays a response time 0.6 ms, much shorter than that of the suspension of the particle conductivity 8.4 xlO S/m, 0.22s (42]. 

Figure 20 The current density of the oxidized polyacrylonitrile(OP)/silicone oil suspensions obtained at 2.5 kV/mm as a function of particle conductivity. The particle volume fraction of all suspensions is 35 vol%. Reproduced with permission from T. Hao, Z. Xu, Y, Xu, J. Colloid Interface Sci. 190 (1997)...

Particle surface modification could be characterized with the surface energy measurement, ilow the surface energy of dispersed particle affects the ER efiect was systematically addressed by Hao [95]. A set of oxidized polyacrylonitrile (OP) materials of different surface properties were employed for such a purpose. The five kinds of water-free ER fluids composed of oxidized polyacrylonitriles (OP) particle dispersed in a low viscosity silicone oil were used for correlating the ER effect with the particle surface energy. The powdered OP materials with average particle size 0.1-10 pm were treated at 150 C for 8 h, and then dispersed in silicone oil immediately at the particle volume fraction of 35 vol /o. The surface energy was measured by means of the dynamic wicking method [96]. Water and... 

Figure 46 Complex viscosity of oxidized polyacrylonitrile/silicone oil suspension vs. the particle volume fraction at different electric fields. The mechanical strain is 1 and frequency is 5 s. Reproduced with permission from Y. Xu, R. Liang, J. Rheol. 35 (1991) 1355.

Figure 12 The conductivity (subtracted from the one of silicone oil) vs. the particle volume fraction for oxidized polyacrylonitrile/silicone oil suspensions. Redrawn with permission from T, Hao, Y. Chen, Z. Xu, Y. Xu and Y. Huang, Chin, J. Polym. Sci 12(1994)97...

Figure 55 The dc current against time obtained at 0.5 kV/mm for oxidized polyacrylonitrile/silicone oil suspension. Particle volume fraction is 27 vol%. The mechanical angular frequency =1, and strain amplitude 50 %. The stress t and the strain y against time in tlie same time frame are also presented. Reproduced with permission from T. Ilao, and Y.Xu, J. Colloid Interf. Sci., 181(1996)581.

In summary, the dc current absorption is observed in the oxidized polyacrylonitrile based-ER fluids. The conductive behaviors of ER suspensions with or without an oscillatory mechanical field are confined by the microstructure—the fibrillated chain structure induced by an external electric field. The de current oscillates with the mechanical frequency and strain amplitude, implying that ER suspensions could be used as a mechanical sensor transferring a mechanical signal to an electric one. The conductive mechanism of an oxidized polyacrylonitrile-based ER suspension can be well described by a Quasi-Id-VRH model, where the localized charges hop from one localized state to another along the chains. This conduction model can be used to quantitatively describe the dc current oscillation phenomena. 

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