Chemistry Reference
In-Depth Information
Table 5.
Activation energy for thermal degradation of BTDA and BPDA based copolyimides
o-TDA
(mole %)
SiDA
(mole %)
E
a
( kJ/mole)
BTDA based copolyimide
BPDA based cop
olyimide
Coats-Redfern
method
Chang
method
Coats-Redfern
method
Chang method
90
10
106.7
117.8
93.2
92.2
85
15
101.6
112.4
73.2
76.4
75
25
98.2
106.4
65.6
74.95
)
n
] against 1 / T yields a straight line if the decom-
position order n is selected correctly. A straight line was obtained when n is equal
to one. The slope and intercept of this line provide the (-E
a
/R) and ln (A) values,
respectively. The results are shown in Table 5.
The thermograms of both series of materials showed a two-step weight loss in
air atmosphere. The first step was obviously due to imidization to polyimide and
loss of water produced as a by-product and the second step was due to degrada-
tion of polyimide. The activation energies determined by Coats-Redfern as well
as Chang methods decreased as the silicon content in the polymer increased, indi-
cating that less energy was required to degrade the polymer with silicon diamine
(SiDA). The extent of decrease in activation energy was comparable.
A plot of ln [(d
α
/ dt) / (1-
α
5. CONCLUSIONS
For both series of PAA, Co-PAA and Co-PDPAA, the bulk viscosity decreased
upon storage at room temperature for a period of one month, irrespective of the
chemical composition or the chemical structure of the Co-PAA or Co-PDPAA.
Also molecular weights, inherent viscosity and acid number decreased as a func-
tion of time at room temperature, which indicated that the most likely reaction
mechanism was anhydride formation. The weak linkage was the amide bond be-
tween the dianhydride moiety and the siloxane diamine (SiDA) moiety. Ionic salt
formulation (Co-PDPAA) was less stable than PAA. BPDA based PAA showed
less viscosity drift than BTDA based PAA. This behaviour can be explained on
the basis of E
a
values of BPDA and BTDA. The viscosity drift of Co-PDPAA
formulation with DEEM was lower than with DMEM.
Shear thinning was observed in both PAA and Co-PDPAA. The shear tempera-
ture dependence of the bulk viscosity of PAA and Co-PDPAA followed the expo-
nential Arrhenius relationship.
Thermal stability behaviour of both DEEM and DMEM systems showed simi-
lar trends and distinctly showed two-step weight losses. Imidization occurred
from 50 to 350°C (removal of water, and DEEM or DMEM) followed by thermal
degradation in the second step.
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