Kenneth A. Pestka II –
Jacob W. Hull – ‪
Jonathan D. Buckley – ‪

Department of Chemistry and Physics
Longwood University
Farmville, Virginia, 23909, USA

Stephen J. Kalista Jr. –
Department of Biomedical Engineering,
Rensselaer Polytechnic Institute
Troy, New York, 12180, USA

Popular version of paper 5aPA3
Presented Friday morning, May 11, 2018
175th ASA Meeting, Minneapolis, MN

In our lab at Longwood University we have recently used Resonant Acoustic and Ultrasonic Spectroscopy to improve our understanding of a self-healing thermal plastic ionomer composed of polyethylene co-methacrylic acid (EMAA-0.6Na) both before and after damage [1]. Resonant Ultrasound Spectroscopy (RUS) is a prodigious technique ideally suited for the characterization and determination of the elastic properties of novel materials, especially those that are often only accessible in small sample sizes or with exotic attributes, and EMAA-0.6Na is among one of the more exotic materials [1,2]. EMAA-0.6Na is a thermal plastic material that is capable of autonomously self-healing after energetic impact and even after penetration by a bullet [3].

Material samples, including those composed of EMAA-0.6Na, exhibit normal modes of vibration and resonant frequencies that are governed by their sample geometry, mass and elastic properties, as illustrated in Fig. 1. The standard RUS approach uses an initial set of approximate elastic constants as input parameters in a computer program to calculate a set of theoretical resonant frequencies. The resulting theoretically calculated resonant frequencies are then iteratively adjusted and compared to the experimentally measured resonant frequencies in order to determine the actual elastic properties of a material.

Figure 1. 3D-model of a self-healing EMAA-0.6Na sample illustrating the first six vibrational modes.

However, EMAA-60Na is a relatively soft material, leading to sample resonances that are often difficult to isolate and identify. A partial spectrum from an EMAA-0.6Na sample is shown in Fig. 2. In order to extract individual resonant frequencies a multiple peak-fitting algorithm was used as shown in Fig. 2 (b).

Thermal Plastic

Figure 2. Undamaged Sample Behavior: Time dependence of the partial resonant spectrum of an approximately 7✕7.5✕1.4 mm3 EMAA sample over 48 hours (a). Lorentzian multi-peak fit to the signal used to extract individual resonances (b). Time evolution of the resonant frequencies at approximately 8.7 kHz (c) and 9.8 kHz (d) for the undamaged EMAA sample, adapted from[1].

Interestingly, the resonant frequencies of undamaged EMAA-0.6Na samples changed over time as shown in Fig. 2(c) and 2(d), but the observed rate of elastic evolution was quite gradual. However, once the samples were damaged, in this case by a 3mm pinch punch hammered directly into approximately 1mm thick samples, dramatic changes occurred in the resonant spectrum, as shown in Fig. 3. Using this approach we were able to determine the approximate healing timescale of several EMAA-0.6Na samples after exposure to damage.

Thermal Plastic

Figure 3. Partial time dependent spectrum of an approximately 7✕7.5✕1.4 mm3 EMAA sample before damage (a) and after damage (b). The lorentzian multi-peak fits are shown just after damage (c) and over an hour after damage (d), adapted from [1].

Building on this approach we have been able to identify a sufficient number resonant frequencies of undamaged EMAA-0.6Na samples to determine the complete material elastic constants. In addition, it should be possible to assess the evolution of EMAA-0.6Na elastic constants for both undamaged and damaged samples, with the ultimate goal of quantifying the material parameters and environmental conditions that most significantly affect the elastic and self-healing behavior of this unusual material.

[1] Pestka II, K. A., Buckley, J. D., Kalista Jr., S. J., Bowers, N. R., Elastic evolution of a self-healing ionomer observed via acoustic and ultrasonic resonant spectroscopy Rep. vol. 7, Article number: 14417 (2017). doi:10.1038/s41598-017-14321-z

[2] Migliori, A. and Maynard, J. D. Implementation of a modern resonant ultrasound spectroscopy system for the measurement of the elastic moduli of small solid specimens.  Rev. Sci. Instrum. 76, 121301 (2005).

[3] S. J. Kalista, T.C. Ward, Self-Healing of Poly(ethylene-co-methacrylic acid) Copolymers Following Ballistic Puncture, Proceedings of the First International Conference on Self Healing Materials, Noorwijk aan Zee, The Netherlands: Springer (2007).

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