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Figure 1. Imidization reactions of two poly(amic acid)s: PMDA-ODA and BPDA-PDA.
2. EXPERIMENTAL
2.1. Materials and processing
The poly(amic acid)s of pyromellitic dianhydride-co-4,4'-oxydianiline (PMDA-
ODA, 14±1 wt%, in NMP/aromatic hydrocarbon, viscosity: 10-12 poises) and
3,3',4,4'-biphenyltetracarboxylic dianhydride-co-1,4-phenylenediamine (BPDA-
PDA, in NMP, 10.5±1 wt%, viscosity: 25-30 poises) were purchased from Al-
drich Co. and used without further purification. After standard RCA cleaning pro-
cedures on 4-in. silicon wafers, the wafers were spin-coated with poly(amic acid)s
to form uniform thin films. The spin-coating was carried out at 500 rpm for 30 sec
followed by 4000 rpm for another 60 sec. These films were then heat treated for
imidization kinetics study.
2.2. Degree of imidization (x)
The FTIR spectra of cured polyimides were obtained using a Bio-rad QS300
FTIR spectrophotometer. Conversion of the poly(amic acid)s to polyimides was
monitored by a comparison of the peak intensity at 1380 cm
-1
between the curing
temperature and 400°C. Here, it is assumed that the poly(amic acid)s are fully
converted to polyimides after curing at 400°C for two hours. To eliminate the dif-
ference in absorption due to variation of film thickness after curing, the peak at
1500 cm
-1
was used as an internal standard for comparison. This peak represents
vibration of the benzene ring of the diamine part in the polyimide and this part
would not participate in the imidization reaction. The degree of imidization of
poly(amic acid)s to polyimides was determined using eq. (2):
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