Electroactive self-healing elastomers and nanocomposites for actuation and energy harvesting

Dielectric elastomers are of great interest for actuation and energy harvesting devices due to their ability to transduce mechanical energy input to electrical energy and vice versa. The dielectric elastomer generators can be designed for harvesting waste mechanical and vibration energy from automotive vehicles and converting to electrical energy to power the wireless sensors. This is to reduce the use of copper cabling and batteries, therefore to achieve smart and sustainable operation systems. The electroactive properties of dielectric elastomers are related to the relative permittivity, electrical breakdown strength and Young’s modulus, although secondary factors such as dielectric loss (tan δ), strain at break and cycle lifetime can influence the performance of the materials and devices. Whilst dielectric elastomers have intrinsically high breakdown strength and good deformability as compared to dielectric ceramics, typically their relative permittivity is low (2~3), impacting the charge that can be stored within the elastomer during an energy harvesting cycle. To enhance the relative permittivity, both intrinsic and extrinsic modification approaches are investigated in a number of commodity elastomers in this work. Firstly, an unsaturated styrenic block copolymer of styrene-butadiene-styrene (SBS) was selected, which is a type of thermoplastic elastomer that are widely applied in industry. A number of polar groups were chemically grafted to the polymer backbone via a one-step thiol-ene ‘click’ reaction under UV light at ambient conditions. The grafting of polar groups enhanced the electromechanical properties of the elastomers, as well as enabled autonomous self-healing behaviour at room temperature. The mechanisms were investigated from the aspects of polar group types, grafting ratios, intra- and inter-molecular interactions. Three types of polar groups: methyl thioglycolate (MG), methyl-3- mercaptopropionate (M3M) and thioglycolic acid (TG) were selected for grafting modification of SBS. The M3M grafted SBS elastomers demonstrated higher selfhealing, strain at break, deformability and breakdown strength as compared to MGSBS, due to the longer carbon chain of M3M. The grafting of TG introduced hydrogen bonding through carboxyl functionality into the modified SBS elastomers, which also introduced a temperature-dependent self-healing behaviour, giving rise to the potential of using the elastomer for actuation or energy harvesting in higher temperature environments. MG-SBS showed enhanced relative permittivity and low tan δ loss, whereby the relative permittivity increased to 11.4, and was able to autonomously self-heal under room temperature conditions. Both M3M-SBS and MG-SBS exhibited excellent energy harvesting performances, whilst showing that they could be readily actuated and recover their actuation ability up to 85% after failure. Finite Element Modelling was used to demonstrate that defects in the elastomers acted as sites for initiation of breakdown, preventing the elastomers from recovering their electrical properties fully. Furthermore, styrene-butadiene rubber and polybutadiene were investigated and modified with both methyl thioglycolate and thioglycolic acid to enhance both the electrical properties and mechanical strength through hydrogen bonding. In addition, the grafting of a conducting polar group, TANI, to the backbone of SBS enhanced the relative permittivity of the elastomers while maintaining a flexible and stretchable nature. This was achieved due to the compatibility between the styrene block of SBS and tetraaniline via π-π stacking. Extrinsic modification of SBS and MG-SBS was investigated via melt compounding with polyvinylidene fluoride (PVDF) and multi-walled carbon nanotubes (MWCNTs). A third polymer, ethylene vinyl acetate (EVA), was introduced to improve the compatibility between SBS and PVDF. Significant enhancements in the relative permittivity of the PVDF/SBS/MWCNT composites were observed, whilst tan δ loss was suppressed after the first percolation threshold due to a dual-percolation behaviour at 1 wt% and 3 wt%. This was in addition to a rheological dual percolation behaviour at the same MWCNT weight fractions. Overall, this thesis presents a series of intrinsic and extrinsic modification pathways for dielectric elastomers and nanocomposites. The demonstrated facile and scalable approaches and self-healing dielectric elastomer actuators and energy generators will make contribution to smart and high performance soft devices applications