The overall research objectives for this project were to determine if an experimental polyhedral oligomeric silsesquioxane (POSS) diamine was a suitable drop-in replacement for methylene dianiline (MDA) in Polymerizable Monomeric Reactants-type (PMR) resin systems, and if the diamine could be used to develop alternative high performance resin chemistries to PMR-15. The effectiveness of an experimental, non-commercial diamine monomer as a replacement for MDA in the commercial polyimide PMR-15, was assessed in this project. The monomer, designed and synthesized at Air Force Research Laboratory (AFRL), featured a polyhedral oligomeric silsesquioxane (POSS) core with an aromatic periphery to maximize its thermal stability.
MDA is a known irritant, causative agent for liver damage, and possible carcinogen to humans as it has been found to cause liver and thyroid cancer in mice. In response, the Occupational Safety and Health Administration (OSHA) has set a low permissible exposure limit and replacement efforts for materials containing MDA are ongoing. Unfortunately, many efforts spanning approximately 35 years seeking an alternative to MDA in PMR-15 have not produced a solution that affords an equivalent balance of properties. The POSS monomer was chosen based on speculation of non-toxicity based on its chemical structure and high molecular weight. Furthermore, premise for this work was in part because of the novelty of POSS and its chemical framework for this particular application. Due to the limited scope of this SEED project, the POSS diamine was not examined for any biological effects but could be the focus of future studies. The subject POSS monomer was used as a drop-in replacement for MDA in heritage PMR-15 chemistry and resultant oligoimide, cured polyimide, and carbon fiber reinforced composite properties were examined.
Two strategies were explored to synthesize different families of oligomers containing the POSS diamine. In a more conventional approach, the POSS diamine was reacted with three ratios of the dianhydride monomers utilized in PMR-15, namely 3,3´,4,4´-benzophenonetetracarboxylic dianhydride (BTDA) and nadic anhydride (NA), to render oligomers of variant average molecular weight and POSS content. In the second approach, pursued as a parallel path for the sole purpose of risk reduction, the POSS dianiline was reacted with phthalic anhydride (PA), p-phenylene dianiline (p-PDA), and NA endcap. Oligomerization of PA and p-PDA is known to render an intractable material. However, impetus for this approach was the expectation that PA and p-PDA may result in a material with very high thermal stability, and POSS typically imparts improved processing characteristics manifested in lower viscosity.
Oligomers were characterized in screening studies for their uncured and cured properties, and compared with PMR-15. The properties that were selected for comparison were chosen based on their criticality to processing and performance, including flow viscosity, post-cure glass transition temperature, moisture diffusion and saturated weight content, and short and long-term thermal stability. In general, properties quantified to be within 5% of those of PMR-15 based on literature values, when available, or from direct comparison of measured values, met a threshold for viability. The candidate featuring the highest glass transition temperature was selected for a synthesis reaction at a suitable scale to facilitate composite fabrication. Resultant composite laminates were sectioned and characterized mechanically for interlaminar and flexural strength properties, chosen for examination because of risk identification as these are resin dominate properties. Interlaminar shear strength was an indicator of the adhesion between fabric and resin, while the flexural strength was affected by resin toughness. Composites were also tested for their long-term thermo-oxidative stability (TOS) and micro-cracking behavior.
PMR-15 oligomers were commercially synthesized in methanol via anhydride esterification, and the resultant solution was utilized to impregnate (pre-preg) textile tapes and fabrics. It was found that the subject POSS monomer was insoluble in methanol, unlike MDA. All synthesis reactions involving POSS were therefore conducted in tetrahydrofuran (THF), and conversations with the pre-pregging industry revealed that this solvent could be used in manufacturing as it was considered relatively nontoxic.
Pure substitution of MDA with POSS in heritage PMR-15 chemistry (BTDA2-POSS3-NA2) produced mixed results. Fortuitously, integrity of the POSS cage was confirmed by nuclear magnetic resonance (NMR) spectroscopy after synthesis and amic acid imidization. POSS was found to improve the processing characteristics by reducing flow viscosity above the glass transition temperature, but also reduced the cured glass transition temperature by 77 °C. The longterm thermo-oxidative stability at 316 °C was not affected, behaving equivalently to PMR-15, up to almost 700 hours. Saturated moisture uptake in boiling water was lessened by 80%.
The negative impact of POSS oligomerization on cured glass transition temperature was due to its large molecular dimensions, and probably to a lesser extent, the ability for the POSS cage to relax under mechanical or thermal stimuli (the cage was not rigid but somewhat flexible). Consequently, POSS segments yielded oligomers of greater length and volumetric footprint than PMR-15, and in the cured state, the distance between crosslink junctions must be greater. In attempts to offset this, two shortened oligomers in a series were synthesized, one less a repeat unit (BTDA1-POSS2-NA2), and another devoid of BTDA (POSS1-NA2). Although the cured glass transition temperature increased with decreasing average oligomer length, the thermo-oxidative stability suffered, and this was presented to be due to the higher concentrations of the nadic group, and its cross-linked products. Nonetheless, the POSS1-NA2 oligomer was selected for composite fabrication and testing because it demonstrated the highest cured glass transition temperature of the series. The alternative strategy to oligomerize POSS with PA, p-PDA, and NA resulted in intractable and insoluble materials for the stoichiometries examined.
A 200 gram batch of POSS1-NA2 was synthesized and pre-pregged using Hexcel IM7 four harness satin weave fabric. A composite laminate panel was fabricated, sectioned, and tested for interlaminar shear strength, flexural properties, and long-term thermo-oxidative stability. All properties were measured to be significantly less than those of PMR-15 composite: short beam shear, -71%, flexural strength, -78%, and TOS, 3.1% weight loss vs. 0.7% for PMR-15 composite after 625 hours at 316 °C. One significantly positive finding was that the POSS1-NA2 exhibited no micro-crack initiation and growth during long-term TOS, as opposed to PMR-15.
The POSS dianiline examined in this effort improved resin processability and moisture resistance, and prevented aging induced micro-cracking at elevated temperature, was sufficiently thermally stable to be utilized in high performance polyimide chemistries, but reduced cured glass transition temperature. This reduction was offset by shortening average oligomer chain length, but resin dominated mechanical properties such as those quantified herein suffered as a result. Although weight loss during elevated temperature aging in air was accelerated for some of the experimental oligomers, this result was traced to nadic end group concentration rather than the POSS monomer itself. Implications for structure manufacturing are that POSS could facilitate the fabrication of composite parts using high temperature capable oligomers by resin transfer molding (RTM), where the textile preform was infiltrated with resin via a simple flow process, only possible with resin systems of sufficiently low viscosity. Currently, RTM is viable with lower performing resin systems such as epoxies and BMIs, but not many polyimides. The RTM method is gaining momentum in industry as it does not require solvent and is simpler in practice, therefore there is a growing need for high temperature capable, low flow viscosity resin systems. In service, POSS containing composite materials are expected to have enhanced survivability, especially in hot-wet conditions and due to their observed resistance to micro-cracking.
The observation of lower cured glass transition temperatures with POSS utilization required additional oligomer design to maximize the effectiveness of its implementation, and should be the subject of future studies to enhance technological viability. Use of an alternative crosslinking group was superficially studied as part of this effort, namely phenylethynyl phthalic anhydride (PEPA), well known to afford higher cured glass transition temperature and thermal stability than nadic anhydride. Initial results were promising, thus deserving of further focus. The results produced from this effort have been presented to the Department of Defense (DoD), National Aeronautics and Space Administration (NASA), relevant industry firms, and academia at several conferences including the American Chemical Society (ACS), High Temple, and the Society for the Advancement of Material and Process Engineering (SAMPE). The general response is that the observations of moisture uptake resistance, but most noteworthy the elimination of micro-cracking induced by thermo-oxidative aging, at least within the conditions implemented in this study, provide hope that the issues that have long plagued composite materials might one day be overcome.
During the design phase of aerospace components, the use of organic matrix composites is often precluded due to their current service life, but POSS could provide a solution to extending this attribute thereby expanding their utility in reducing the weight of systems in a sustainable manner. The manufacturing shift towards RTM due to its simplicity, speed, and reliability is expected to increase in intensity, but the implementation of high temperature resin systems in this process is thus far impossible due to their intrinsically high viscosities. The incorporation of POSS into the backbone of thermosetting oligomides reduces viscosity to an extent that it could be possible to use resin systems conventionally disregarded for RTM processing without compromising their thermal stability. To promote adoption of this technology by industry, future work should therefore focus on detailing the amount of POSS monomer needed in a high temperature matrix resin to eliminate micro-cracking in concert with developing an oligomeric architecture that is amenable to RTM processing. The coupling of these two attributes with high temperature capability and non-toxic character would provide a revolutionary yet sustainable organic composite material technology available for the manufacture of next-generation aerospace systems.