Cross-Linked Structures of Nadic-End-Capped Polyimides at 371 °C

Abstract

The thermally induced cross-linking of nadic-end-capped polyimides has been studied by a variety of spectroscopic techniques. Two nadic-end-capped polyimides were subjected to 371 °C heat treatment for 100 h in order to study cross-linking resulting from elevated temperature. The glass transition temperature of the treated samples increased to greater than 400 °C, indicating that both polyimides underwent extensive chemical changes during heat treatment. 13C spin counts were measured before and after heat treatment by 13C cross-polarization magic angle spinning nuclear magnetic resonance. Electron spin resonance measurements showed an increase in the concentration of stable free radicals in the treated samples. Dynamic nuclear polarization and electron spin resonance results show that two species of free radicals are present in the treated samples; the first is located on the polymer backbone and the second on the cross-linking network that arises from the reaction of the nadic end-capping groups. On the basis of these results, new cross-linking structures of nadic-end-capped polyimides before and after heat treatment at 371 °C are proposed.

Anionische, mikroporöse Polymernetzwerke durch Polymerisation eines schwach koordinierenden Anions

“…Das Spektrum (Abbildung S10) wird durch ein intensives, unstrukturiertes Signal mit einem LandØ-Faktor g von 2.00 dominiert, das stabilen Radikalen im Polymernetzwerk zugeordnet werden kann, was für große konjugierte, organische Materialien nicht überraschend ist. [17] Das Spektrum zeigt des Weiteren die typischen sechs Linien der Hyperfeinstruktur, die für Mn 2+ ESR-spektroskopisch untersucht (Abbildung S11). Das Muster ist sehr ähnlich, jedoch treten neben den erlaubten Übergängen die verbotenen Übergänge mehr in Erscheinung, mçglicherweise hervorgerufen durch eine chemische Veränderung in der Manganumgebung.…”

Quantum Mechanical Computations and Microkinetic Modeling to Obtain Mechanism and Kinetics of Oxidative Degradation of a Polyimide

“…These studies identified methylenedianiline (MDA) component and cross‐links as the most reactive moieties toward oxidation and benzophenone tetracarboxylic dianhydride (BTDE) component as least susceptible to oxidation . The oxidation of nadic end‐cap of PMR‐15 was found to be slow till 1 200 h and experienced a rapid weight loss thereafter .…”

“…ESR studies by Ahn et al showed the involvement of free radicals in the oxidative degradation of PMR‐15 in the presence of O 2 . ESR, SQUID, and ESRI experiments done by different groups identified free radicals on the cured and heat‐treated PMR‐15 samples . Stability of the observed free radicals present in the heat‐treated PMR‐15 samples at 644 K can be attributed to the delocalization of electrons due to the aromatic structures formed from the nadic end‐cap on heat treatment .…”

“…ESR, SQUID, and ESRI experiments done by different groups identified free radicals on the cured and heat‐treated PMR‐15 samples . Stability of the observed free radicals present in the heat‐treated PMR‐15 samples at 644 K can be attributed to the delocalization of electrons due to the aromatic structures formed from the nadic end‐cap on heat treatment . Even though the experiments could identify the degradation products, the mechanism of formation of radicals and other short‐lived intermediates is still a matter of debate.…”

“…The pathways used to model the oxidation of these fragments are based on well‐documented radical mechanisms that are initiated by O 2 or or . Formation of oxidation layer and radicals on the polymer on exposure to flowing air and high temperature also supports the initiation of oxidation by O 2 and radicals. The mechanistic routes investigated here incorporate several previously reported intermediates and end products.…”