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Self-Monitoring/Healing SMPC Materials

Shape memory polymeric materials with self-monitoring and self-healing properties

Introduction

Shape memory polymers (SMP) and polymer composites (SMPC) are lightweight and are able to be deformed and fixed into a temporary shape and, in response to a stimulus, subsequently revert to the original, permanent shape (see Figure 1), giving them wide applications in industry. SMPs are reported to have large recoverable strains of 400%, low cost and high manufacturability and processability, while also being of low density. These properties make them of great interest for many industries.

Challenges

SMPs and SMPCs exhibit inferior mechanical properties and therefore improvements to their poor mechanical properties become essential, especially in demanding applications such as in aerospace and the marine industry, which are prone to damage by fatigue, impact, and large loads. Initial damage in polymer composites can be difficult to detect, as it can occur deep within the component and be on a micrometer scale. Traditionally, embedded sensors have been used to monitor the structural conditions, but these also can lead to degradation of mechanical performance of the material, poor durability, and difficulty of repair. Therefore, it is essential to incorporate self-monitoring and self-healing abilities into SMPs or SMPCs to increase the lifetime of the components.

       Self-monitoring indicates the capability of a material to intrinsically sense some properties without any external sensor. It can be observed by electrical or optical measurement. Self-healing refers to the material that can self-repair any deterioration incurred while in service without any external intervention.

Figure 1. Schematic diagram illustrating the basic shape memory effect in a polymer with a switching transition temperature of Ttrans. State A shows the temporary (programmed) shape. State B shows the permanent (recovered) shape. The polymer is heated to a temperature of T1 and cooled to a temperature of T2.
Figure 1. Schematic diagram illustrating the basic shape memory effect in a polymer with a switching transition temperature of Ttrans. State A shows the temporary (programmed) shape. State B shows the permanent (recovered) shape. The polymer is heated to a temperature of T1 and cooled to a temperature of T2.

Approach

Based on the possible strategies of incorporating self-healing and self-monitoring capabilities into shape memory polymeric materials, Joule heating was identified as a strategy which could enable and include all three of these effects in shape memory polymers reinforced with conductive additives (such as carbon nanotubes, carbon fibers, graphene or carbon black). Incorporation of carbon additives into polymers found to lead to an increase the mechanical performance of a SMP and also provide multi-functionality through self-monitoring and self-healing.

        With the use of carbon additives, an interconnected electrically-conductive network can be formed in an SMP. When damage occurs, the electrical resistance of these composites will change due to disruption of the conductive pathway. Therefore, even small amounts of damage (micro-cracks) can be detected by a significant change in resistance. As a consequence of the conductive properties of the additives, applying a voltage to these composites can induce Joule heating. Joule heating at these areas could then induce shape memory behavior to close the micro-cracks and the matrix polymer is re-melted/compressed together (self-healing process) at an elevated temperature. This is called shape memory assisted healing process: close-then-heal (CTH). The process for the self-monitoring and self-healing by Joule heating is demonstrated in Figure 2.

 

Future Developments

This concept can be explored for those needed for deployable structures (e.g. sunshields, or reflectors/antennas) or large structures (e.g. aircraft wings). For future investigations into Joule heating of shape memory polymer composites, it would be beneficial to test and assess the viability/stability of Joule heating in large shape memory structures. This can be achieved by imaging with an infrared camera to ensure an even distribution of heat, as the volume affects the voltage required to heat to a given temperature. It may be possible that the voltages needed to heat to temperatures required for the temperature effect would render this method unsuitable for large structures. Contact resistance affecting the uniformity of heating also remains an issue. Moreover, it may be that, for some conductive networks that are formed in large materials, the conductive paths travelled by electrons is such that the resultant heating is not uniform or predictable between components.

Relevant factors to consider when assessing the viability of these materials to a given application include: the rate of healing; the repeatability of healing at a given site of damage; the degree to which the original mechanical performance is recovered on healing; and the stability of the system and the sensitivity of monitoring, the balance between the self-monitoring and Joule heating ability.

Figure 2. Self-monitoring and self-healing process for shape memory polymer composites
Figure 2. Self-monitoring and self-healing process for shape memory polymer composites
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