Electrical breakdown and electrostatic phenomena in ultra-thin polyimide Langmuir-Blodgett films - Polyimides and Other High Temperature Polymers - page 241

on Al, Ag and Au electrodes, which were charged under a needle-plane electrode
system by applying a dc voltage to the needle electrode. Here the needle-plane
electrode system was very similar to that used in the experiments on the electrical
breakdown of insulating polymers [23]. Electrical breakdown voltage was also
examined. We then studied the interfacial electrostatic phenomena in PI LB films,
and electrical breakdown process in ultrathin PI LB films.
2. EXPERIMENTAL
Polyimide LB films (denoted as PI), whose chemical structure is shown in Fig. 1,
were used in this study. PI has a large electron affinity and thus it has a strong
tendency to accept electrons from metal electrodes such as Al and Au. PI LB
films were prepared onto Al and Au evaporated glass slides (10 mm x 50 mm) by
means of the LB technique coupled with a precursor method developed by Kaki-
moto et al . [22], in a manner as described in our previous study [24, 25]. Briefly,
amphiphile monolayers of poly (amic acid) that have a chain of long alkyl-amine
salt, a precursor of PI, were transferred onto the glass slides by the vertical dip-
ping method at a surface pressure of 35 mN/m. After the chemical imidization
with acetic anhydride and pyridine, PI LB films with a chemical structure shown
in Fig. 1 were obtained [22]. The number of deposited layers was between 10 and
70, and the monolayer thickness was 0.4 nm.
The electrode configuration for the sample is shown in Fig. 2. All samples were
placed in a vacuum vessel with a pressure of 1x10 -3 Torr, and they were then an-
nealed at a temperature of 100°C for 1 hour to remove extrinsic charges and ad-
sorbed water molecules because these charges and molecules affect the electro-
static properties of PI LB films. The surface potentials of deposited PI LB films
biased under a needle tungsten-electrode were measured by means of the Kelvin
method (TRek Model 320B), where the detector D of the surface potential meter
and the needle electrode moved parallel to the sample (see Fig. 2). The surface
potential of PI LB films (position Q) was measured with reference to the potential
of the clean metal electrodes (position P).
Figure 1. Chemical structure of polyimide used.