Environmental effects and cure monitoring of multifunctional composites of carbon nanotubes

Date
2012
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University of Delaware
Abstract
In this work, composites of dispersed carbon nanotubes (CNTs) in a thermosetting polymer matrix were investigated. It was the goal of the study to develop in situ sensing technologies based on the measurement of electrical resistance of a percolating network of CNTs in the polymer matrix. Three characterization methods were developed and changes in state of the composites were correlated to changes in electrical resistance. Composites of dispersed CNTs and vinyl ester (VE) resin were tested for thermoresistive behavior using a simple in situ electrical resistance measurement during thermomechanical analysis. Specimens ranging from 0.1 to 1 wt% CNT were tested. The results showed that the thermoresistive behavior was strongly dependent on the CNT content and was highly tailorable in terms of sensitivity to temperature and polymer segmental motion. Specimens with CNT concentrations far above the electrical percolation threshold showed positive and negative temperature coefficient of resistance (TCR) over the temperature range from 25 to 165 ̊C. Specimens with CNT concentration closer to the percolation threshold showed a near-zero and negative TCR. It was found that thermoresistive analysis of the CNT/VE composites was capable of detecting the glass transition and likely other polymer segmental and side chain processes previously observable only through more complex electrothermic analysis techniques. Cross-ply composites of glass fiber and dispersed CNTs in VE resin were tested in an environmental chamber while constrained in a constant displacement weathering fixture. Temperature was repeatedly cycled at a high rate from -73 to 150 ̊C for approximately 56 hours. Electrical resistance was monitored in real-time during the duration of the testing regime, including the initial pre-tensioning step, and the evolution of damage observed in the electrical resistance data was assessed by edge replication. It was found that the electrical resistance response was sensitive to the combined thermal and mechanical loading condition and that significant cracking occurred in the 90 ̊ plies during environmental testing. A novel method of in situ polymer matrix composite materials process monitoring was developed. The method of torsional braid analysis (TBA) was used in conjunction with a real-time data acquisition system to determine the viscoelastic behavior of VE/CNT/glass fiber composite test specimens. The data acquisition system was used to collect electrical resistance data from electrodes placed at either end of the specimen during TBA tests. A rheometer with torsion accessory served as an analog for the torsional pendulum. The sensitivity of nanotube-based sensing to viscoelastic changes during cure was investigated. It was observed that electrical resistance was strongly correlated to the progression of cure. The proposed sensing method has the potential to give real-time sensing capability during critical phases of manufacturing as well as in-service monitoring for enhanced safety and reliability.
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