On the Oxidative Degradation of Nadic End-Capped Polyimides. 3. Synthesis and Characterization of Model Compounds for End-Cap Degradation Products

Abstract

The oxidative degradation of PMR (for polymerization of monomeric reactants) polyimides at elevated temperatures was followed by cross-polarized magic angle spinning (CP-MAS) NMR. Labeling of selected sites in the polymers with 13C allowed for direct observation of the transformations arising from oxidation processes. The formation of several degradation products has been proposed to be occurring in the cross-links derived from the nadic end caps on oxidation. Model compounds have been synthesized and characterized by CPMAS NMR with both normal and delayed decoupling to distinguish between protonated and unprotonated carbons. Comparison of these spectra to predicted chemical shifts of proposed products for the aged polymer provides further insight to degradation occurring in the cross-linked moieties.

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

“…Oxidation layer formation and oxidative degradation of PMR‐15 were explored by several thermomechanical, spectroscopic, and modeling studies . These studies concluded that oxidation begins at the surface and then proceeds toward the inner core of the polymer.…”

“…The oxidation was also found to be influenced by the aging parameters like temperature, exposure to O 2 , moisture, etc. Possible mechanistic pathways and details of chemical changes occurring during the oxidative degradation of PMR‐15 were monitored by various spectroscopic studies on the oxidatively aged PMR‐15 or on model compounds …”

“…Breakage of polymer chains can occur with the formation of 9 (which is the major product of oxidation), and thus affects the mechanical properties of the polymer. Among the oxidation products from the cross‐links, 50 can result in chain cleavage and is thought to be responsible for the shift observed for the C‐13 NMR peak of carbon α to the imide CO group upon aging . The oxidation products 70b , which is formed by the oxidation of rDA products based cross‐links can also result in the polymer chain breakage.…”

“…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 . C‐13 NMR studies on oxidatively aged PMR‐15 and model compounds identified the carbon α to the imide group and the methylene carbon as the major reactive centers in this fragment …”

BPDA-PDA Polyimide: Synthesis, Characterizations, Aging and Semiconductor Device Passivation

“…The effect of long time aging of polyimide at high temperature (>200 °C) and in oxidative environment on the mechanical properties [39], weight loss [40,41], and chemical properties [42,43], was widely investigated for thick polyimide matrix composites (1 mm thick) used in high temperature aerospace applications. It was found that while thermal degradation occurred throughout the material, the oxidative degradation occurs mainly within a thin surface layer where oxygen diffuses into the material.…”