Next Generation Rubbers Based on Self-Reinforcing Materials

Objective

The objective of the research project is to create silicone elastomers for e.g. artificial muscles, adhesives and medical devices that are strong without the use of reinforcing fillers.


Background

Silicone based rubbers (elastomers) find countless applications as adhesives, seals, food storage products, electronics and cable insulation. In medical applications silicone elastomers are used as tubing for dialysis and transfusion equipment, catheters and implants. Since the fundamental physics and chemistry of silicone elastomers are still not fully understood, many potential applications of elastomers remain unexplored. Elastomers are basically polymer networks formed by end-linking polymer chains in cross-linking reactions with multi-functional cross-linker molecules. Such silicone networks suffer from low tear strength and low overall mechanical properties. As a result, almost all applications require filler reinforced silicone elastomers. The most commonly used reinforcing filler is silica particles which are usually added in concentrations of 20-60 wt%. The use of such fillers does, however, have several drawbacks. As particles are often mechanically blended into the silicone matrix it is difficult to obtain a high level of dispersion. Agglomeration is therefore commonly observed. Furthermore, reinforcing fillers increase the stiffness and thereby decrease the extensibility of the elastomer (at high filler loadings). Furthermore, the particles hinder recyclability, which means that the current materials have a poor life-cycle. Silicone elastomers can, however, be reinforced with no optical distortion or increase in density or stiffness by creating heterogeneous polymer networks. 

The project

The project aims to obtain an understanding of such heterogeneous networks and to use this understanding to develop novel strong and ultra-soft elastomer materials with superior properties compared to traditional particle filled elastomer systems. The new self-reinforcing silicone dielectric elastomers will be strong enough to be used without reinforcing fillers, which would make the elastomers more lightweight, stretchable and soft. When used as for example artificial muscles, the materials will therefore obtain a larger electromechanical response (higher output). The silicone materials will be recyclable giving the technology another push towards implementation as a green energy technology. The heterogeneous silicone networks are prepared by combining heavily cross-linked short-polymer chain domains within a long-polymer chain network. In contrast to traditional polymer networks, the heterogeneous networks are prepared in a two-step procedure where short polymer chains are reacted below the gelation point (the point where an infinite network is formed) into hyperbranched structures. These hyperbranched structures are subsequently mixed and cross-linked together with long polymer chains and consequently short-chain domains are created within a long-chain network. The preparation procedure of heterogeneous networks is, at present, not simple and there are many (time-dependent) parameters which make reproducibility difficult. Furthermore not much is yet understood about the structure-property relationship of such networks and therefore the technology is still very far from being applicable in industry for commercial products. Therefore, a full investigation into all time-dependent parameters of such networks are needed before further development of the new materials can take place.