6FDA-pMDA polyimide

Xu and Coleman [76] modified the 6FDA-pMDA (polyimide) films by irradiating ion beam and studied the structure and morphology by AFM. The AFM images data indicated that free-standing polyimide films had deep surface valleys which could extend to a depth of several micrometers. 

XL. Xu, M.R. Coleman, Atomic force microscopy images of ion-implanted 6FDA-pMDA polyimide films, / Appl Polym Sci, 66 (3), 459-469,1997. 

Table 15.2 shows the characteristics of the high-fluorine-content 6FDA/TFDB polyimide and KAPTON (nonfluorinated PMDA/ODA type polyimide, DuPont). The Tg of 6FDA/TFDB (335°C) is a little lower than that of PMDA/ODA because of its flexible -C(CF3)2- groups. However, 6FDA/TFDB has a high decomposition temperature of 569°C. 

Figure 6. Excitation spectra of synthesized PMDA and 6FDA based polyimides with same cure history indicating effect of flexibility of diamine on relative intensity of PMDA-ODA vs PMDA-IPDA and effect of flexible dianhydride on spectral structure and relative intensity of PMDA-IPDA vs 6FDA-IPDA.

Figure 11. Normalized intensity as a function of normalized reciprocal distance for substituted PMDA polyimides and thermally treated 6FDA-IPDA.

We would like to thank Dr. K. O Brien, Dow Chemical USA, Walnut Creek Research Center, CA Dr. G. R. Husk, U.S. Army Research Office, Research Triangle Park, NC and Prof. W.J. Koros, University of Texas, Austin for synthesizing and preparing the PMDA and 6FDA based polyimide films used in the Structural Effects sections of this paper. This work was supported in part by the Stanford Institute for Manufacturing and Automation. P. S. M. would like to thank the Natural Sciences and Engineering Council of Canada for a postdoctoral fellowship. 

Table 12 lists the properties of this PI with those of PMDA-TFDB for comparison. In spite of the presence of electron-withdrawing -CF3 substituents, the maintained reactivity of TFDB is most likely based on the m a-substitution onto benzidine. If the (7r /i(7-substituted diamine counterpart was used, it must be difficult to obtain high molecular weight PAA in the conventional way because of its expected much lower reactivity. The transmission spectra of a series of TFDB-based Pis in Fig. 58 indicate how the 6FDA-TFDB polyimide film is optically transparent. A secondary positive effect of the -CF3 substituents in TFDB on the film transparency is the weakened intermolecular cohesive force due to lower polarizability of the C-F linkage. This functions negatively to interchain CTC formation. 

The diamines, DAFLI and DAFL2 were reacted with PMDA, s-BPDA, 3,3, 4,4 -ODPA and 4,4 -(hexafluoroisopropylidene)diph-thalic anhydrtde (6FDA), and polyimides with low e values were also prepared without decreasing their thermal stability (Table 7.13). 

This equation differs from the conventional model (6) for positronium-for-ming media in having l/2.5r instead of l/2r as the left hand side term. This form has been dictated by the following considerations (a) The positron annihilation in polyimides reportedly (7) differs considerably from that observed in most polymers. It proceeds from the free or trapped positron states without the formation of positronium atoms (b) Positron lifetime spectra in all of the polyimides (PMDA, BFDA, BTDA and 6FDA-based polyimides etc.) investigated in this laboratory exhibit only two lifetime components. The shorter lifetime (r,) ranges from 100 to 300 picoseconds and arises firom free positron... 

In the overview of structure-property relationships that follows, it should be borne in mind tliat these comparisons are (wherever possible) for those materials where hydrogen has been replaced with fluorine. Cases in the literature where dramatically different monomer structures are used for the fluorinated and unfluorinated polyimides cannot easily be interpreted for effects of fluorine, although such comparisons are often casually made in the literature anyway. For instance, to ascribe die property differences in PMDA-ODA and 6FDA-0DA to fluorine substitution is of little value toward understanding the effect of fluorine, though this is often die extent to which data are available. 

Table 14.4. Mechanical Properties of Perfluorinated (10FEDA/4FMPD), Partially Fluorinated (6FDA/TFDB), and Unfluorinated (PMDA/ODA) Polyimide Films ...

The fluorinated copolyimides (including homopolyimides) are all more transparent than the nonfluorinated PMDA/ODA. The color of 10-pm-thick films changes gradually from bright yellow to colorless as the 6FDA/TFDB content increases. All these copolyimide films are also homogeneous compared with polyimide blends of 6FDA/TFDB and PMDA/TFDB. 

The fluorinated polyimides PMDA/TFDB and 6FDA/TFDB and the copolyimides have the high optical transparency, controllable refractive index. 

The effect of an additional 310 °C cure on the emission intensity and intermolecular spacing of a 6FDA-IPDA film, as compared to a 250 °C cured 6FDA-IPDA film, is also shown in Table III. The intensity of the 310 °C cured sample is significantly greater than that of the 250 °C cured sample. This behavior is identical to that observed in the cure study of the commercial polyimide films. Furthermore, this increase in intensity corresponds to a decrease in intermolecular distance, as shown by the data in Table III, analogous to the trend observed with the PMDA films. Finally, this extra cure appeared to result in a more ordered film as is evident by the addition of two minor diffraction peaks corresponding to 3.4 and 2.1... 

Most of the polyimides were amorphous in nature. This was because of the various structural modifications onto the polymer backbone. In general, polyimides derived from dianhydrides such as PMDA, BPDA and BTDA exhibited higher crystallinity than the other dianhydrides such as OPDA, DSDA and 6FDA. The higher crystallinity reflected on their poor solubility. 

Xu and Coleman studied 6FDA (2,2 -bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride)-pMDA (pyromellitic dianhydride) polyimide films irradiated by an ion beam [84]. A beam of 140 keV N ions with a low-current density was used, and three irradiation fluences (2 x 10 cm , 1 x 10 cm , and 5 x 10 cm ) were chosen. It was reported that even a small dose altered the microstructure of the surface layer. The AFM analysis of those films showed that low-fluence irradiation induced microvoids in the surface layer of the polymer, and high-fluence irradiation resulted... 

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