Polyimides relative humidity effect

Figure 3.26 Effect of relative humidity on the dielectric constant of polyimide.

Effect of Humidity Because the water content of Kapton polyimide film can affect its electrical properties, electrical measurements were made on 0.025 mm film after exposure to environments of varying relative humidities at 23°C.f l... 

A similar study of the effects of relative humidity on the CC>2 permeability of Kapton polyimide showed only a 5% reduction compared to the pure component value at 250 psia. This finding is consistent with the previous discussion, since the pure component CO2 permeability shown in Fig. 20.4-14 is approaching its limiting value of kpOp at a CO2 pressure of 250 psia. This rapid saturation arises as a result... 

FIGURE 20.4-14 Demonstration of the flux-depiessing effect of low relative humidities on the CO2 peimeability of Kapton polyimide. Paulson et al. has shown that at high relative humidities, the observed flux of CO2 in a number of polymers is higher than the pure component level. At much higher relative humidities for Kapton, it may be that this effect also will be observed, consistent with the right-hand side of Fig. 20.4-4. 

This finding is consistent with the earlier cited quote by Pye et al. It can be understood from Eq. (20.4-1 ), in which the second term effectively is suppressed, causing the permeability to approach the limiting value of koDp. If the polymer is water sensitive, of course, plasticization may occur as the relative humidity is increased further, thereby causing increases in the value of Dp with a resulting upswing in the observed permeability as shown in Fig. 20.4-4. Data for the series of superselective high-flux aromatic polyimid studied by Pye and coworkers are shown in Table 20.4-6. The table compares dry (0% RH) behavior to wet behavior (exposed to only 15 mm Hg of water vapor at 30 C (50% RHl). 

In addition, the development of a resistive-type humidity sensor based on multiwall carbon nanotube (MWCNT)/polyimide composite films attracted more attention for development of sensing technologies (Lee et al. 2013). The relative humidity sensors showed a good linearity (1.10) over a wide humidity range, with a sensitivity of 60 RH and a response time of 10 min. In addition, an increase of resistance to water absorption was observed. This effect was explained by the charge transfer between the adsorbed water molecules and the MWCNTs. 

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