Buffer coat using polyimides

In electronics, polyimides are now extensively used in the form of self-standing films for flexible circuitry, deposited films for interlayer dielectrics, passivation and buffer coatings, moulding thermoplastic powders for PCBs, and adhesive pastes or tapes. The basic polyimide chemistry has been adapted to fulfill the specific requirements of these applications. A series of books provides complete information not only on the chemistry of polyimides but also on their utilization in electronics [4,23,24]. The following figures summarize the chemical formulae of the most important categories of polyimide precursors or precyclized polymers that are commonly used in electronics. 

Most instruments use a cassette design to position the separation capillary in the thermostat and to connect the column with the buffer reservoirs and detector. Columns can be purchased with a detection window located at the correct position for insertion into the cassette. Alternatively, a number of different types of ring heaters, optical splicers and microbumers are available for those who wish to prepare their own columns at a lower cost [294,315,317]. A detector window is prepared by vaporizing a small section of the polyimide coating to leave a transparent section of fused silica about 1-2 mm long at a fixed distance from the column outlet. Once the window is formed, the column is fragile, and must be handled carefully. It is also important that the column ends are cut square and free of debris since misshapen column ends can results in distorted peak shapes and poor quantitative precision. 

We first consider the case of an open, but buffer-filled capillary column (Type A, shown schematically in Fig. 1), which is similar to that used in capillary electrophoresis. This is typically made of polyimide-coated fused silica. Let its length be L (cm) with an interior cross-sectional area as A (cm ). If a voltage, V (Volts), is applied across the ends of the column and the current I (Amps) is measured, the resistance, R (Ohms), of the buffer can be calculated from Ohm s law, as given in Eq. 1 ... 

Capillary electrophoresis was carried out with a Capel 105 system (LUMEX, Russia) equipped with a UV monochromator (190-380 nm). A polyimide-coated fused silica capillary with an inner diameter of 75 jj.m, an outer diameter of 375 m and a total length of 61.5 cm (52 cm to the detector) was used. Data control was performed with personal computer software for Windows 95. The voltage used in all experiments was 20 kV (cathodic detection side). Between runs, the capillary was rinsed at high pressure for 3 min with 0.5 M NaOH and then with distilled water and flushed with the carrier buffer. 

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