Cathodic polymerization

Electronic and Electrical Applications. Sulfolane has been tested quite extensively as the solvent in batteries (qv), particularly for lithium batteries. This is because of its high dielectric constant, low volatUity, exceUent solubilizing characteristics, and aprotic nature. These batteries usuaUy consist of anode, cathode polymeric material, aprotic solvent (sulfolane), and ionizable salt (145—156). Sulfolane has also been patented for use in a wide variety of other electronic and electrical appHcations, eg, as a coil-insulating component, solvent in electronic display devices, as capacitor impregnants, and as a solvent in electroplating baths (157—161). 

Electrochemical synthesis utilizes the ability of a monomer to be self-coupled upon irreversible oxidation (anodic polymerization) or reduction (cathodic polymerization). While this method does not always produce materials with well-defined structures (as do the three other polymerization methods to be discussed), electropolymerization, nonetheless, is a rather convenient alternative, avoiding the need for polymer isolation and purification. Of these two routes, anodic polymerization is the most widely explored as monomers such as pyrrole and thiophene are relatively electron-rich and prone to oxidation. For this reason the anodic route will be the focus of the remainder of this presentation. 

Electrolytically initiated polymerization may either depend on a direct electron transfer between electrode and monomer, or on the formation of an intermediate which interacts with a monomer molecule in a fast chemical step, thus creating a chain initiator. As an example of the former type of process, the formation of a living polymer from the cathodic polymerization of a -methylstyrene by electrolysis in sodium tetraethylaluminate - tetrahydrofuran may be cited 639 whereas a typical case of the latter type is the anodic polymerization of vinyl monomers by electrolyzing them together with sodium acetate in aqueous solution 63 7,640) Here it is assumed that acetate ion is discharged to form an acetoxy or methyl radical which attacks the monomer molecule in a fast chemical step. 

A nanofilm of plasma polymer (up to about 100 nm) has sufficient electrical conductance as evidenced by the fact that an LCVD-coated metal plate can be coated by the electrolytic deposition of paint (E coating), i.e., plasma polymer-coated metals can be used as the cathode of the electrolytic deposition of paint (see Chapter 31). Thus, the plasma polymer layer remains in the same electrical potential of the cathode (within a limited thickness) and the work function for the secondary electron emission does not increase significantly. When the thickness of plasma polymer deposition increases beyond a certain value, the coated metal becomes eventually insulated, and DC discharge cannot be sustained. DC cathodic polymerization is primarily aimed to lay down a nanofilm (10-100 nm) on the metal surface that is used as the cathode (see Chapter 13). 

The cathodic polymerization was found to be pressure dependent (with a fixed flow rate) and independent of the flow rate, whereas the negative glow polymerization is pressure independent (at a fixed flow rate) but flow rate dependent (at a fixed pressure). The cathodic polymerization yields a much tighter network, manifested by high refractive indices, than the product of the negative glow polymerization. 

Figure 4.3 Change of the luminous gas phase of trimethylsilane (TMS) with reaction time for cathodic polymerization with two magnetron-anodes, 5 W left column closed system from the initial pressure of SOmtorr, right column flow system at SOmtorr, top row 5 s, middle row 60s, bottom row 180s.

The ionization of an organic molecule is the basic principle of mass spectroscopy however, it should be recognized that mass spectroscopy of a simple molecule such as ethane generally shows multiple ions, covering the fragmented species to partially polymerized species, which indicates that the dissociation of the molecule and some extent of polymerization of fragmented species occurred in the mass spectrometer. The presence of the DG (cathode glow) in DC cathodic polymerization implies that the formation of chemically reactive species via ionization, as depicted by Eq. (4.1), is a very unlikely primary event under the conditions of LCVD. 

XPS Cls/Si2p ratio steadily increases with the reaction time (film thickness) by a closed-system plasma polymerization, while the ratio more or less stays at a constant level by a flow system cathodic polymerization, as shown in Figure 4.10. Such a graded ultrathin film was found to provide an excellent corrosion protection of aluminum alloy when an organic coating was applied on top of the ultrathin film [8]. 

In DC discharge, the situation is quite different. First, the cathodic polymerization is aimed at the short-term polymerization to coat the metal substrate... 

When the equation for plasma polymerization [Eq. (8.2)] is applied to express the thickness growth rate of the material that deposits on the cathode cathodic polymerization), it becomes quite clear that the deposition kinetics for the cathodic polymerization is quite different. There is a clear dependence of the deposition rate on WjFM, but no universal curve could be obtained. In other words, the relationship given by Eq. (8.2) does not apply to cathodic polymerization. The best universal dependency for cathodic polymerization was found between D.R./M (not D.R./F M) and the current density IjS), where / is the discharge current and S is the area of cathode surface [5]. Figure 8.7 depicts this relationship for all cathodic polymerization data, which were obtained in the same study, covering experimental parameters such as flow rate, size of cathode, and mass of hydrocarbon monomers but at a fixed system pressure. The details of DC discharge polymerization are described in Chapter 13. 

Figure 8.7 A master eurve for the relationship between GRjM and the eurrent density for DC cathodic polymerization data obtained under various conditions for methane and n-butane, at a fixed system pressure of 50mtorr.

The equation indicates that cathodic polymerization is controlled by the conditions of the local environment near the cathode. The normalized deposition rate in DC (deposition E) is D.R./[M], not D.R./[FM], and the normalized power input parameter is Wc/S, not WjFM. In DC discharge, the dissociation glow virtually adheres to the cathode surface. Therefore, the equation proves that the dissociation glow controls the deposition rate on the cathode surface. 

An electrode in an AC discharge is the cathode for half of the deposition time and the anode for the other half of the time. Comparing Eq. (8.2) and Eq. (8.7), the contribution of the cathodic polymerization can be estimated by examining the system pressure dependence of the deposition rate (at a fixed flow rate). If plasma polymerization (deposition G) is the dominant factor, it is anticipated that the deposition rate would be independent of the system pressure. If cathodic polymerization (deposition E) is the dominant factor, the deposition rate onto an electrode is dependent on the system pressure, and the value of deposition rate is expected to be one-half of that for DC cathodic polymerization. 

These findings indicate that cathodic polymerization takes place on the electrode in a 40-kHz discharge. This assessment agrees with the observation that... 

It is important to note that plasma deposition occurs predominantly on the cathode surface in the cathodic polymerization, and a new cathode (substrate) was used in every plasma coating operation. In other words, the contamination of the reactor is considered to be minimal. 

CATHODIC POLYMERIZATION VS. PLASMA POLYMERIZATION IN NEGATIVE GLOW... 

The DC cathodic polymerization, 40-kHz (HF) and 13.5-MHz (RF) plasma polymerization of trimethylsilane (TMS) were compared in a bell jar type of reactor [1-3]. The bell jar has the dimensions of 635 mm height and 378 mm diameter. A pair of stainless steel plates (17.8 x 17.8 x 0.16cm) was placed inside the bell jar with spacing of 100 mm and used as parallel electrodes. The substrate used in the plasma deposition process was an aluminum alloy panel positioned in the midway between the two parallel electrodes. 

If one considers that the overall DC plasma polymerization is a mixture of cathodic polymerization (material formation in the dissociation glow that is adhering to the cathode surface) and plasma polymerization (material formation in the diffused luminous gas phase), the deposition on the cathode is primarily cathodic polymerization. With an insulating layer between the substrate and the cathode surface, there is no cathode glow, and hence no cathodic polymerization that deposits polymer on the substrate. The substrate on the cathode surface without electrical contact or any noncathode surface receives the products of plasma polymerization in negative glow. 

In a 40-kHz discharge (Fig. 8.10), the deposition onto the surface of the electrode, regardless of electrical conductivity or contact, is significantly different from deposition onto a floating substrate. The cathodic aspect of the electrode is less (one-half of DC discharge), but because of this the overall cathodic aspects of polymerization extend beyond the surface of the electrode yielding cathodic plasma polymer on an electrically insulated substrate placed on the electrode. Thus, the features of cathodic polymerization dominate in the vicinity of the electrode regardless of electrical contact. 

The major difference between the cathodic polymerization and the glow discharge polymerization is the influence (or absence) of ion bombardment during the process of material deposition. With an organic compound as monomer, ions that bombard the cathode are mainly those of hydrogen and other non-polymer-forming elements. 

In DC cathodic polymerization conducted in a bell jar reactor, the cathode (substrate) is positioned in the middle between the two anodes. In such electrode arrangement, the distance between the cathode and the anode is expected to have some effects on the deposition rate and deposition profile with respect to those without anode assembly. Figures 13.4 and 13.5 show the influence of the distance between two anodes (one-half of which is the cathode-anode distance) on TMS deposition rate on cathode (i.e. substrate) and anode, respectively. 

Figure 13.4 The influence of electrode distance on the deposition rate on Cathode in DC cathodic polymerization 1 seem TMS, SOmtorr, DC 5 W, d the distance between two anodes, d/2 is the distance between the cathode and an anode.

From Figure 13.4 it can be seen that, with the increase of anode spacing from 60 mm to 160 mm, the deposition rate on cathode (substrate) showed an increasing trend. The deposition on the cathode (substrate) surface seemed to reach the maximum when the anodes were removed from the plasma system, i.e., no anode assembly was present and the grounded reactor wall functioned as anode. In contrast, it is noted that, from Figure 13.5, the deposition on the anode surface decreased with the increase of anode spacing. These results clearly indicated that the too-close anode spacing not only reduced the preferred plasma polymer deposition on substrate (cathode) but also induced more undesired deposition on the anode surface. In other words, DC cathodic polymerization without anode assembly seems to be a more efficient and realistic approach in its practical applications. 

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