Self-healing silicone elastomers

Seonghyeon Jeong

Research output: Book/ReportPh.D. thesis

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Abstract

As the field of soft robotics has matured, the development of sustainable dielectric elastomer actuators (DEAs) has become all the more relevant. This can be accomplished by both extending the lifetime of DEAs and establishing end-of-life perspectives for the actuators. The aim of this thesis was thus to develop a new soft thermoplastic elastomer (TPE) system, in order to design self-healing DEAs. The self-healing behaviour of DEAs offers the possibility of extending lifetime without the need for any external intervention. In addition, the sustainability of the TPE actuators was further increased by the possibility of thermally recycling the TPE from the DEAs. In a similar way to DEAs, polymeric capacitors also exhibit the self-healing behaviour of devices at elevated temperatures through the thermoplastic behaviour of the polymer dielectric. However, the fundamental thermal stability of these devices is currently insufficient to meet industry requirements, and so the final part of the project was therefore dedicated to the development of polymer capacitors with improved thermal stability, while still retaining their self-healing behaviour.

First, a supramolecular TPE was developed by combining polydimethylsiloxane (PDMS), urea and urea-adjacent ureidopyrimidone (urea-UPy), whereby the strongly hydrogen-bonded urea-UPy allowed the TPE to maintain its mechanical properties as a result of cyclic deformation being faster than elastomer relaxation.

The thermally reversible properties of TPE enabled a simple method to fabricate a surface-corrugated DEA by hot-pressing. In addition, the fabricated DEA exhibited autonomous self-healing behaviour by exploiting the generated heat, in combination with self-cleared electrodes, to recover after multiple breakdowns. This ongoing self-healing behaviour was achieved by the local liquid-like behaviour of the TPE during the breakdown process.
In addition to improving the lifetime of the DEA, the TPE DEA also helps in the recovery of dielectric material and recycling into new devices after use. This was demonstrated witha new cleaning process, enabling reclamation of the TPE with minimal impurities (i.e.traces of silver). The recovered material had linear viscoelastic and dielectric propertiessimilar to those of pristine TPE, thereby demonstrating that the recovered TPE couldsupport five recycling cycles for the fabrication of new devices.

Finally, a number of materials were tested as alternatives for thin film polymer capacitors, targeting higher thermal stability and self-healing properties. The initial screening revealed a number of possible candidates, but eventually a semi-interpenetratin gpolymer network (IPN) based on a cross-linked PE (polyethylene) network with free PP (polypropylene) was identified as an optimal candidate. The precursor mixtures allowed spin-coating to obtain films with a low surface roughness, whereby a simple in-situ radical grafting and cross-linking reaction could be carried out. Free PP embedded in the system was confirmed as being mobile at higher temperatures, and an annealing process demonstrated thermally induced self-healing behaviour, smoothening small defects in the thin films’ surface. Effectively, smoothening behaviour by embedded free PP demonstrated the concept of self-healing. Further analysis will have to be conducted, in order to evaluate the ultimate thermal stability of fabricated capacitors. 
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages118
Publication statusPublished - 2022

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