Aromatic heterocyclic resin : precursor for carbon materials and high temperature foams

Abstract

Phthalonitrile based polymers have been intensively and persistently studied over the past few decades due to their potential as high temperature (HT) resistant thermoset. All studies so far emphasised on the resin formulation as HT matrices. This study focused on understanding the detailed structural formation responsible for the superior thermal properties of resorcinol-based phthalonitrile (RPh) polymer and extending its applications as organic precursors for carbon materials and resin for ultra-low density HT foams.

High crosslinking density was essential in order to acquire the exceptional thermal performances, although Fourier Transform Infrared Spectroscopy indicated the formation of less thermally stable linear structure formation at high curing additive content. It was proposed that the trapping of the linear chains inside the cavities of the aromatic heterocyclic structures formed a highly crosslinked and thermally resistant system. Systematic studies on kinetic of curing and degradation further supported the hypothesis. The 86% char yield at 800 C makes RPh an excellent carbon precursor. The results showed that the carbon films obtained through pyrolysis of RPh precursor were smooth and crack free, exhibiting excellent mechanical robustness and integrity which were comparable or even surpassed the carbon films obtained from epoxy or pitch. The obtained bulk electrical conductivity of 87 S/cm made RPh carbon film a potential candidate for electronic applications.

RPh foams were prepared for the first time via a synchronized single step gelation-foaming process based on the established resin viscosity profile and chemical blowing agent gas liberation. The density could be precisely controlled with the lowest obtainable density of 0.04 g/cm3. Well-distributed closed cells were obtained. The RPh foams showed excellent short and long term thermal stability and strength retention (>90%) after thermal aging at 280 °C for 100 hours in air which surpassed any reported polymer foams.

Three types of nanofillers with distinct aspect ratios, 0D fumed silica (FS), 1D multiwall carbon nanotubes (MWNT) and 2D expanded graphite nanosheets (GH) were selected for nanocomposite foam preparation. Results showed the nanofillers functioned as nucleation agents, rheological modifiers and reinforcement fillers. The cell density was increased by 3 orders of magnitude and the cell size distribution was narrowed. Unique ‘cage-like’ structures consisted of closed cells interconnected by microvoids were obtained. The formation of physical gel upon MWNT made foaming possible when synchronization of gelation and gas liberation cannot be established. Three fillers were found to provide varying levels of enhancement of the nanocomposite foam properties with GH being the most effective filler. Reinforcement mechanisms were proposed to explain the findings.