Imidization process dissociation reaction is not primarily imidized amic acid dissociation containing anhydride end groups and amino end groups of the molecular chains, anhydride occurs in the still present in the system due to the effects of moisture and imidization reaction generated water hydrolysis, ortho acid produced by hydrolysis and dehydration can anhydride under the action of heat.
IR 45500px-1 by the wave number measured, anhydride initially appeared in 50 ~ 100 ℃ (Oil amido acid dissociation generated). At 175 ~ 225 ℃ maximum, disappeared in 250 ~ 300 ℃. Macromolecular chains more flexible, the less produce anhydride [1%~2%( Mole fraction)]. For rigid molecules, anhydride to produce up to 10%
Above 200 ℃ complex reaction and imidization by the anhydride end groups and amino end groups gradually dominant. Peak at 45500px-1 that became weaker and finally disappeared. For high molecular weight polyamide acid, even when under the action of heat degradation of the mechanical properties of the films is not large, throughout its toughness during heat treatment are at a high level. Low molecular weight polyamide acid due to the relatively low molecular weight, are inherently brittle.
Reducing the relative molecular mass of 150 ~ 200 ℃ and no major effect on the toughness. Further increasing the temperature to make anhydride end groups reacted with an amine is not enough to make the molecular weight of the polymer is increased to a sufficient degree show toughness. Polyamide acid medium relative molecular mass of the most frequently encountered, which already has a film at low temperature high toughness. Since the polyamic acid under the action of heat dissociation, reducing the relative molecular mass between 150 ~ 200 ℃, the toughness of the polymer film appears a trough, as the temperature continues to rise, a gradual increase in toughness, to above 350 ℃, its performance and the high molecular weight polyamide acid obtained polyimide is not much differences.
Thus 150 ~ 200 ℃ should be avoided final imidization temperature. In this temperature region because the resulting polymer has the lowest molecular weight, so it has the worst mechanical properties.