Optical and Flame-Retardant Properties of a Series of Polyimides Containing Side Chained Bulky Phosphaphenanthrene Units

Abstract

Among the multitude of polymers with carbon-based macromolecular architectures that easily ignite in certain applications where short circuits may occur, polyimide has evolved as a class of polymers with high thermal stability while exhibiting intrinsic flame retardancy at elevated temperatures via a char-forming mechanism. However, high amounts of aromatic rings in the macromolecular backbone are required for these results, which may affect other properties such as film-forming capacity or mechanical properties; thus, much work has been done to structurally derivatize or make hybrid polyimide systems. In this respect, flexible polyimide films (PI(1–4)) containing bulky 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) units have been developed starting from commercial dianhydrides and an aromatic diamine containing two side chain bulky DOPO groups. The chemical structure of PI(1–4)) was characterized by 1H NMR, 13C NMR and 31P NMR spectroscopy. The optical properties, including absorption and luminescence spectra of these polymers, were analyzed. All polyimides containing DOPO derivatives emitted blue light with an emission maxima in the range of 340–445 nm, in solvents such as N,N-dimethylformamide, N-methyl-2-pyrrolidone, chloroform, and N,N-dimethylacetamide, while green light emission (λem = 487 nm for PI-4) was evidenced in a thin-film state. The thermal decomposition mechanism and flame-retardant behavior of the resulting materials were investigated by pyrolysis-gas-chromatography spectrometry (Py-GC), scanning electron microscopy (SEM), EDX maps and FTIR spectroscopy. The residues resulting from the TGA experiments were examined by SEM microscopy images and FTIR spectra to understand the pyrolysis mechanism.

The Synergistic Effect of Triazine and Phosphaphenanthrene Units on the Physico-Chemical Behavior of Polyimides

“…In a condensed form, the fluorescence of PI1–PI3 was almost quenched ( Figure 8 b), mostly due to the molecular aggregation that led to non-radiative decay by intermolecular energy transfer. This behavior is opposite to that found for related DOPO-containing polyimides [ 46 ] which displayed a strong fluorescence emission in film state, with maximum emission wavelength at about 510 nm. Thus, it can be assumed that 1,3,5-triazine brought the most significant contribution to florescence suppression of present polyimides in solid state.…”

“…This emission was associated with 1,3,5-triazine since this heterocycle is a well-known electron-deficient moiety used in green and blue TADF emitters [ 54 ]. Previous studies on related polyimides containing DOPO units have highlighted that their emission in polar solvents occurs in the UV-blue spectral range [ 46 ], which definitely provides support for our statement. Additional evidence for the origin of the fluorescence (FL) bands of our polymers associated with triazine emission comes from the position of their maxima.…”

“…From our knowledge, the number of studies in the literature reporting on DOPO-containing polyimides is relatively low [ 31 , 45 ], and they rely mainly on describing the influence of DOPO derivatives on solubility, crystallinity, thermal, and flame retardant properties. Lately, the studies on DOPO-containing pristine polyimides have focused on analyzing the gas-transport properties, optical or flame-retardant properties [ 43 , 46 ].…”

Enhancing flame retardancy and optical functionality in multifunctional devices through advanced design of phosphorus‐containing copoly(imide‐oxadiazole)s

“…Moreover, these compounds offer environmentally friendly alternatives, minimizing the production of toxic byproducts during combustion. The incorporation of flame‐retardant additives, such as 9,10‐dihydro‐9‐oxa‐10‐phosphaphenanthrene oxide (DOPO), into polyimide matrices is a common strategy for enhancing their fire resistance 21–26 . These phosphorus compounds can interfere with the combustion process, reducing the formation of flammable gases and slowing the propagation of flames.…”

“…These phosphorus compounds can interfere with the combustion process, reducing the formation of flammable gases and slowing the propagation of flames. As a result, polyimides modified with DOPO exhibit improved fire resistance, making them suitable for applications where fire safety is a concern 21 . In this context, the exploration of novel phosphorus‐containing poly(1,3,4‐oxadiazole‐ether‐imide)s not only contributes to the development of safer and more environmentally friendly materials but also fosters innovation.…”