The synthesis of controlled molecular weight semicrystalline polyimides 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 referred to as TPER polyimide (M n = 20k, 30k) displayed excellent thermal stability, as evidenced by dynamic thermogravimetric analysis in both air and nitrogen atmospheres. This polyimide displayed a glass transition temperature of ca. 210 °C based on DSC measurements, and a melting temperature of 395 °C. A unique feature of this polyimide was the fact that quenching the polymer from the melt, even at very high cooling rates (ca. 200 °C/min), did not result in an amorphous 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 attributed to a narrow distribution of crystal thicknesses. On the basis of the results of the melting behavior of nonisothermally and isothermally crystallized samples, a process of melting/recrystallization has been shown to occur in the system during the DSC heating scan. This melting/recrystallization phenomenon has been shown to give rise to the observed multiple melting endotherms in the DSC scans of isothermally crystallized samples. The equilibrium melting temperature of this polymer estimated using a Hoffman−Weeks plot was shown to be 408 °C. The thermal stability of the TPER-based system has been investigated by monitoring the crystallization and melting response after residence in the melt at various times and temperatures. Melt time and temperature studies showed the exceptional thermal stability of the TPER polyimide 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 amine end groups, as well as samples partially end capped with phthalic anhydride were shown to display distinctly lower thermal stability compared to phthalimide end-grouped samples. The improved behavior was demonstrated by melt rheological and crystallization experiments.

Controlled molecular weight, thermoplastic polyimides have been prepared via poly(amic acid) salt precursors. 2,2-Bis[4-(3,4-dicarboxyphenoxy)phenyl]propane dianhydride and m-phenylenediamine (the Ultem monomers), together with calculated amounts of phthalic anhydride were reacted in N-methylpyrrolidinone as well as in tetrahydrofuran to form poly(amic acid)s with controlled molecular weights. Poly(amic acid) salts were prepared in heterogeneous reactions of the poly(amic acid)s with quaternary ammonium bases or triethylamine dissolved in methanol or water, to yield soluble salts. The poly(amic acid) salts were then melt imidized in air at 250 or 300 °C for 30 min. Results suggest that the poly(amic acid) salt counterion controls the mechanism by which the salt imidizes, which in turn controls the properties of the final polyimide. The triethylammonium poly(amic acid) salts yielded linear, thermoplastic, molecular weight controlled polyimides upon melt imidization. The polyimides prepared from the poly(amic acid) salts containing the triethylammonium, tetraethylammonium, and tetrapropylammonium counterions showed dynamic weight loss profiles comparable to the polyimide produced directly from the control poly(amic acid).

Novel high performance semicrystalline polyimides, based on controlled molecular weight phthalic anhydride (PA) endcapped 1,4‐bis(4‐aminophenoxy)benzene (TPEQ diamine) and oxydiphthalic dianhydride (ODPA), were synthesized. They exhibited excellent thermal stability in nitrogen and air atmospheres as determined by thermogravimetric analysis (TGA). The glass transition temperatures (Tg) for these polymers ranged from 225°C for the 10,000 Mn (10K) polymer, to 238°C for the 30,000 (30K) Mn material. The observed melting temperatures for all the polymers were ∼420°C. The crystallization behavior of these polymers showed a strong molecular weight dependence, as illustrated by the observation that the 10K and 12.5K polymers crystallized with relative ease, whereas the 15K, 20K, and 30K polymers showed little or no ability to undergo thermal recrystallization. The thermal stability of these polymers above Tm was investigated by studying the effect of time and temperature in the melt on the cold crystallization and melting of these polymers. Increased time and temperature in the melt resulted in lower crystallinity because of melt state degradation, such as crosslinking and branching, as evidenced by an increase in melt viscosity, which was more prominent for the higher molecular weight polymers.

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