Stretchable Conductive Elastomers

Flexible electronics have gained considerable attention in both the academic and industrial fields in recentyears. However, classic conductors such as metals are limited in flexible electronics, due to their essential stiffness and brittleness. Different strategies have been proposed in order to fabricate conductive materials that are able to undergo significant levels of deformation. Elastomers are considered a promising substrate for stretchable conductive materials, owing to their elasticity, processibility, stability and low cost. The use of elastomeric polymers in stretchable conductive materials is usually combined with highly conductive particles, due to their poor electrical performance, whilst stretchability as well as electrical properties under deformation are also important factors that need to be considered in this regard. In the present thesis, these concerns are addressed by developing new methods to locate conductive particles selectively in heterogeneous bimodal networks, in order to achieve high conductivity and stretchability simultaneously.

In the first part, a two-step process was employed to prepare heterogeneous bimodal networks, in order to selectively distribute multi-walled carbon nanotubes (MWCNTs) in polydimethylsiloxane (PDMS). This approach was used to explore the influence of MWCNTs concentration in different phases on the conductivity and stretchability of a PDMS-MWCNTs composite. MWCNTs concentration was varied by changing the amounts of MWCNTs added throughout different stages in heterogeneous bimodal networks, which was found to have a significant effect on conductivity and stretchability. Therefore, this method is considered an effective means of preparing stretchable conductive material.

In the second part, the proposed method was applied to a commercial soft silicone adhesive, in order tobroaden the applicability of the proposed method to other systems and to prepare a stretchable conductive PDMS-MWCNTs composite with proper adhesive properties that can be used for a health-caredevice. This material was indeed prepared, but its relatively low conductivity failed to meet the requirements of the health-care device; nevertheless, the feasibility of the proposed method for application in other material systems was proven.

In the third part, the PDMS-MWCNTs composite prepared in the first part was used to fabricate astretchable conductive fiber via a modified wet spinning method. A strategy named “self-orientation” was utilized during this fiber fabrication process with the purpose of enhancing conductivity. The main objective was to produce a highly stretchable and conductive PDMS-MWCNTs composite fiber with enhanced conductivity by orientating MWCNTs along the fiber direction. The resulting material exhibited significantly enhanced conductivity, improved stretchability and self-enhanced conductivity under elongation, thus demonstrating the effectiveness of the applied methods.In the final part, efforts were devoted to developing various applications based on the previously prepared PDMS-MWCNTs composite fibers, in order to explore possible uses in electronic fields. Compression sensors were manufactured by simply stacking coated PDMS-MWCNTs composite fibers, which showed high sensitivities and are able to accurately react to a low forces. Coating two parallel PDMS-MWCNTs composite fibers resulted in a strain sensor that in a similar fashion showed sensitivity to low tensile strain.The successful preparation of the compression sensors and the strain sensor demonstrates the application prospects of prepared PDMS-MWCNTs composite fibers in wearable electronics. Furthermore, a preliminary exploration of prepared PDMS-MWCNTs composite fibers in energy harvesting was conducted, albeit time limitations curtailed the opportunity to conduct an in-depth study. However, the current work points in this direction for further study.