The effect of thermal treatment on the decomposition of phthalonitrile polymer and phthalonitrile-polyhedral oligomeric silsesquioxane (POSS) copolymer

Abstract

Phthalonitrile polymer and phthalonitrile-polyhedral oligomeric silsesquioxane (POSS) copolymer were sintered at 500 °C, 600 °C, or 800 °C. The samples before and after sintering were evaluated by using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) coupled to FT-IR (TG-FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results suggest that phthalonitrile-polyhedral oligomeric silsesquioxane copolymers have better thermal stability than phthalonitrile polymers. After sintering at 800 °C, the phthalonitrile polymers decomposed completely while the phthalonitrile-polyhedral oligomeric silsesquioxane copolymers retain their original structure. The presence of the rigid POSS structure in the copolymer may limit the thermal movement of polymer chains at higher temperatures, thereby increasing the intermolecular forces between polymer chains. Moreover, POSS generates spatial steric hindrance, further restraining movement of chains and hindering degradation of labile groups; the overall effect is to improve the thermal stability of the composite under higher temperatures.

Introduction

The demand for resin matrix composite materials with high temperature resistance, ablation resistance, and flame retardant properties has grown enormously in aerospace and automobile manufacturing. To meet these demanding requirements, the development of new high-performance resin matrix composite materials has become a very important focus area. Phthalonitrile polymers [[1], [2], [3], [4], [5]] are a new class of thermal materials combining low flammability and high strength. They have great potential in the aerospace sector as components for maintaining airframe loads for the next generation of aeronautical and space vehicle systems.

Phthalonitrile-based composites [[6], [7], [8]] have been developed in an attempt to improve the mechanical, thermal, and oxidative stability properties which are superior to many state-of-the-art thermal composites such as polyimides and phenolic triazine. In terms of fire resistance, phthalonitrile-based composites are one of the few to meet the United States Navy's stringent requirements for the usage of polymer composites aboard Navy submarines (MIL-STD-2031). One of the most appealing properties of phthalonitrile polymers are their thermal stability at high temperature as shown by Keller and Dominguez, etc. [[9], [10], [11], [12], [13], [14], [15], [16]] at the Naval Research Laboratory, who first synthesized phthalonitrile monomer, [4,4′-bis(3,4-dicyanophenoxy)biphenyl (BPh).

There has been renewed interest recently in these polymers and their composites, with most work focusing on lowering the melting temperature of the monomer and thus widening the processing window. Additionally, many different phthalonitrile monomers [[17], [18], [19], [20], [21]] have been synthesized. With regards the formation of phthalonitrile-based composites, the two common approaches in their preparation involve the formation of block copolymers or physical blends with structures in the nano-size range. Following this line of research, in our previous work [22] we developed phthalonitrile-POSS (polyhedral oligomeric silsesquioxane) copolymers. In our work, we found that addition of POSS in the polymer chain led to a higher thermal stability. In addition, there was a higher weight retention (48%) when the polymer was heated at 900 °C compared to the neat polymer (31%). However, the thermal degradation mechanism that leads to this high char yield and the nature of the thus formed products in the POSS-phthalonitrile copolymers system is unclear. An understanding of the mechanism and/or the nature of the products formed is of great theoretical and practical importance for determining the processes which hinder or promote thermal decomposition.

The thermal degradation of polymers has been studied intensively for decades. The Hamciuc group has studied thermal degradation of various heterocyclic polyethers and polyimides [23,24]. Later [25], they synthesized polyimide-polydimethylsiloxanes (PI-PDMS) and characterized the thermal decomposition products with TG/MS/FTIR analysis, proposing a mechanism for their formation. Peterson et al. [26] simplified the pyrolysis model and developed thermal decomposition kinetic models for 11 polymers. Grochowicz and Kierys [27] studied the oxidative decomposition of polymer-silica composites via TG/DSC/FTIR. Pagacz et al. [28] has studied the thermal degradation of POSS-containing polymers in POSS/PU composites by FTIR-QMS. Analysis of the degradation products suggested that POSS has an important role in the degradation mechanism, leading to the formation of silica-containing char products under pyrolytic conditions. Lewichi et al. [29] found that POSS/PU composite elastomers were indeed more thermally stable than an unmodified PU matrix, with significantly reduced levels of volatile degradation products and a ∼30 °C increase in onset degradation temperature. However, to the best of our knowledge there are no reports on the degradation of phthalonitrile and phthalonitrile-POSS copolymers. The aim of this paper is to develop a clearer understanding of the processes involved in the thermal degradation of phthalonitrile and phthalonitrile-POSS composites by examining the nature of its degradation products with DSC/TGA/XPS/XRD/TG-FTIR analyses. Based on this information, we aim to gain a better understanding of the increased stability of POSS-containing polymers, and propose possible routes for the thermal decomposition of the neat polymer and its POSS-based composite.