Synthesis of Various Polyimides Using Tetrahydrofuran/Methanol as Solvent

Abstract

KEY WORDSMethanol I Poly(amic acid) I Polyimide I Solvent I TetrahydrofuranThe presence of residual solvent in the polyamic acid precursor films affects processing conditions in the fabrication of polyimide films due to the need to remove it from the system. The aprotic solvents typically used in the manufacture of polyamic acids, dimethylacetamide (DMAc) and N-methyl pyrrolidone (NMP), form molecular complexes with polyamic acid making the solvent particularly difficult to remove under ambient conditions. 1 -4 Echigo et al., have created a novel solvent system consisting of tetrahydrofuran (THF) and methanol for the fabrication of ODA-PMDA polyimides. 5 -? They found that this solvent system can be used in the synthesis of high molecular weight polyamic acids that can later be thermally cured into polyimides. They developed this system to assist the incorporation of water into the solution mixture which upon curing introduces porosity into the films. This solvent system results in the easy removal of nearly all solvent from the polyamic acid prior to thermal cure. The synthesis of other polyimides using this easy to remove solvent system has heretofore not been attempted. Here is reported the synthesis of a variety of polyimides using the THF /MeOH solvent system. GENERAL EXPERIMENTAL PROCEDUREPolyamic acids, as 3-12 wt% solutions were prepared by reacting stoichiometric amounts of diamines and pyromellitic dianhydrides for 2-12 hours. In a three necked flask under a nitrogen purge, the diamine was first dissolved in 40-lOOmL solvent, a 80/20wt% mixture of THF and methanol, by mechanical stirring. Upon complete dissolution of the diamine, addition of the dianhydride as one batch resulted in a curdlike mixture. Continued stirring resulted in complete dianhydride dissolution and homogeneous solutions. Films were then cast from these solutions and after evaporation of the solvent under ambient conditions, subjected to 125oc for one hour and two hours at 250°C to imidize the films. All diamines and dianhydrides were purified by either sublimation or recrystallization from toluene prior to use. THF and methanol were used as received. The generalized reaction scheme is shown in eq 1. Various diamines and dianhydrides are drawn in t To whom correspondence should be addressed. . Viscosity measurements were conducted in concentrated sulfuric acid (H 2 S0 4 ) on polyamic acids precipitated from the reaction solutions by pouring the solutions into water. Ubbelohde capillary viscometry using a concentration of 0.5 g/lOOmL H 2 S0 4 at 30°C was conducted for all polyamic acids. RESULTS AND DISCUSSIONDuring the synthesis of the poly(amic acid)s very unusual solution behavior was noted. Upon the addition of the dianhydride, all solutions took on a cottage cheese-like texture. However, with continued stirring for various times, 2-12 hours, this texture disappeared and clear solutions of various colors resulted. This was accompanied by increase in the molecular weight as seen by the increase in the viscosity of the solution. The...

Preparation of Polyimides by Using Mixtures of Tetrahydrofuran and Methanol and Their Properties

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