Polyimides Containing 1,3,4-Oxadiazole Rings
1639
All th e polyim ides as well as th eir polyam ic acid precursors gave tran s-
paren t free-stan din g film s by castin g 8–10% NMP solution s of th e polym ers
on glass plates. Such film s h ad th e th ickn ess of th e order of ten s of m icrom -
eters. All th e film s prepared from polyim ides con tain in g h exafluoro-
propan e-2,2-diyl groups were flexible, tough an d with stood repeated
ben din gs, wh ile in case of polyim ides based on ben zoph en on etetra-
carboxylic dian h ydride on ly th ose con tain in g 1,3-ph en ylen e un its (2c)
were flexible. Th e film s of polyim ides based on 6F dian h ydride sh owed a
very stron g adh esion to th e glass substrate an d th ey could on ly be rem oved
by boilin g in water for 1–2 h . Th ese facts sh ow a good in fluen ce of h exa-
fluoropropan e-2,2-diyl groups on m ech an ical properties of such polyim ide
film s. Dyn am ic m ech an ical an alysis (DMA) of free-stan din g th in film s
m ade from polyam ic acids an d polyim ides was perform ed to get a deeper
in sigh t in to th e im idization reaction an d oth er ph en om en a takin g place
durin g h eatin g. It is gen erally recogn ized th at dyn am ic m ech an ical an alysis
offers a valuable h elp in un derstan din g such ph ysical processes, bein g m ore
sen sitive to m olecular m otion th an oth er th erm al m eth ods for determ in a-
tion of tran sition s an d tran sform ation s in polym ers18,19. As an exam ple,
two typical DMA curves are sh own in Fig. 6 an d discussed.
Figure 6 presen ts th e storage m odulus (E′), th e loss m odulus (E″), an d th e
loss factor tan gen t (tan δ) depen den ces on tem perature for film s of poly-
am ic acid 2′c an d polyim ide 2c. Th e drops in E′ curves an d th e peaks on E″
an d tan δ curves in dicate th e ph ysical tran sition s in polym ers. Usually, th e
tran sition tem peratures are recorded at th e m axim um rate of turn down of
th e storage m odulus E′ or at th e m axim um loss m odulus E″ or of tan δ
peaks. At low tem perature both polym ers sh ow a plateau in th e E″ depen d-
en ce above 109 Pa, wh ich is typical of glassy polym ers. Th is is th e glassy re-
gion wh ere th e m acrom olecules are in th e “frozen ” state. As to th e E′ curve,
th e E′ value in th e glassy region for polyim ide 2c is h igh er th an th at for
polyam ic acid 2′c, wh ich sh ows th at polym er ch ain s lose flexibility
th rough im idization . Th e curve (red dots, ●) sh ows a m axim um peak of
elastic m odulus (E″) at about 185 °C, sh owin g th at th e glass tran sition (Tg)
of polyam ic acid 2′c starts. Th is value of glass tran sition tem perature is also
foun d on th e tan δ curve (blue dots, ●). At about 210 °C th e in clin ation of
th e m odulus E′ curve ch an ges an d becom es lower due to th e begin n in g of
im idization of polyam ic acid. Th e gradual decrease in storage m odulus E′
con tin ues up to 225 °C as can be observed on th e tan δ an d E″ curves. Start-
in g from 225 °C, a pron oun ced decrease in E′ occurs alon g with a drop in
th e m odulus curve at 237 °C as a result of glass tran sition of th e resultin g
polyim ide 2c. Th e value 237 °C can be also foun d on th e tan δ curve of th e
Collect. Czech. Chem. Commun. 2008, Vol. 73, No. 12, pp. 1631–1644