SEMICRYSTALLINE POLYIMIDES BASED ON CONTROLLED MOLECULAR-WEIGHT PHTHALIMIDE END-CAPPED 1,3-BIS(4-AMINOPHENOXY)BENZENE AND 3,3',4,4'-BIPHENYLTETRACARBOXYLIC DIANHYDRIDE - SYNTHESIS, CRYSTALLIZATION, MELTING, AND THERMAL-STABILITY

The synthesis of controlled molecular weight semicrystalline polyimide s based on 1,3-bis-(4-aminophenoxy)benzene (TPER diamine) and 3,3',4,4 '-biphenyltetracarboxylic dianhydride (BPDA), end capped with phthalic anhydride, is reported herein. The above polyimide henceforth referre d to as TPER polyimide (M(n) = 20k, 30k) displayed excellent thermal s tability, as evidenced by dynamic thermogravimetric analysis in both a ir and nitrogen atmospheres. This polyimide displayed a glass transiti on temperature of ca. 210 degrees C based on DSC measurements, and a m elting temperature of 395 degrees C. A unique feature of this polyimid e was the fact that quenching the polymer from the melt, even at very high cooling rates (ca. 200 degrees C/min), did not result in an amorp hous polymer, implying very high crystallization rates from the melt. The subsequent melting endotherm was also shown to be extremely narrow , as evidenced by a sharp endotherm in the DSC trace, which was attrib uted to a narrow distribution of crystal thicknesses. On the basis of the results of the melting behavior of nonisothermally and isothermall y crystallized samples, a process of melting/recrystallization has bee n shown to occur in the system during the DSC heating scan. This melti ng/recrystallization phenomenon has been shown to give rise to the obs erved multiple melting endotherms in the DSC scans of isothermally cry stallized samples. The equilibrium melting temperature of this polymer estimated using a Hoffman-Weeks plot was shown to be 408 degrees C. T he thermal stability of the TPER-based system has been investigated by monitoring the crystallization and melting response after residence i n the melt at various times and temperatures. Melt time and temperatur e studies showed the exceptional thermal stability of the TPER polyimi de versus corresponding results for the commercial polyimide ''New TPI '' and for a polyimide based on 1,4-bis(4-aminophenoxy)benzene and 4,4 '-oxydiphthalic anhydride (TPEQ polyimide). Polyimide samples with ami ne end groups, as well as samples partially end capped with phthalic a nhydride were shown to display distinctly lower thermal stability comp ared to phthalimide end-grouped samples. The improved behavior was dem onstrated by melt rheological and crystallization experiments.