Shape memory polymers and their composites are promising materials showing great shape memory behavior. Stimuli sensitive SMPs enhanced with reinforcing materials such as particles and fibres have various application areas including aerospace, aviation, biomedicine, MEMS and NEMS, automotive, etc.

Shape memory polymers (SMPs) are categorized under shape-memory materials (SMMs) that have attracted considerable attention in the last decades. The first investigated SMMs were shape memory alloys (SMAs). The discovery of SMPs was later in the 1980s. Since then, international research on SMPs has shown a rapid increase. These stimuli-responsive materials show shape memory effect (SME) which is defined as the ability to recover their original form after a large deformation upon various external stimuli. Depending on the active

Introduction

Shape memory behavior of SMPs stems from their two-phase structure which consists of a fixed and reversible phase. The fixed phase network ensures the original shape's recovery is achieved through chemical cross-linking, crystallization, and interpenetrating networks. On the other hand, the reversible phase can fix the temporary shape through the glass transition, crystallization, and transition between different liquid crystalline phases. Upon exposure to a stimulus, the switching/transition is triggered and strain energy stored in the temporary shape is released, which consequently results in the shape recovery. Thermo-responsive SMPs are among the most common SMP materials whose shape recovery is triggered by thermal changes. Transition temperature (Ttrans), which corresponds to the glass transition temperature (Tg) of chemically cross-linked thermoset materials and physically cross-linked thermoplastic polymers and melting temperature (Tm) of semicrystalline polymer networks and chemically cross-linked rubbers, is the most important property of SMPs. Ttrans is different for each SMP, allows the formation of temporary and recovery of the original shape. Thermo-responsive SMPs can be categorized based on their structure as thermoplastic (physically crosslinked) and thermosetting (chemically crosslinked) shape memory polymers. SMPs show advantages over other SMMs such as good shape recoverability (up to 400% recoverable strain), low density, ease in processing, and in the tailoring of properties (e.g., transition temperature, stiffness, biodegradability, and ease of functionally grading), programmability and controllability of recovery behavior, and low cost. It is important to note that SMPs show only a one-way shape-memory effect. The SMP at the ‘‘soft’’ stage can only deform with the help of external force rather than automatically deform by cooling. The main drawbacks of SMPs are lower recovery stress, smaller energy output, longer recovery time, and shorter cycle life compared to SMAs. Out of these disadvantages, lower recovery stress is the most challenging problem while other disadvantages could be utilized in different applications. To overcome these drawbacks, the development of shape memory polymer composites (SMPCs) has been suggested.

Composites of Shape Memory Polymers

SMPCs are reinforced polymer materials with enhanced properties compared to SMPs. Reinforcing materials introduced to the SMPC structure improve the mechanical and thermomechanical behavior as well as the response to external stimuli of the SMP. SMPCs can show better electrical conductivity, magnetism, optical properties, and bio functionality thanks to the unique combination of polymer matrix and reinforcing materials such as ceramics, metals, organic, and inorganic materials. The effect of these materials depends on their size, shape, distribution, volume fraction, and alignment. These reinforcing materials can be fibres, tubes, or particles in nano- and micro-scales. The dispersion of reinforcing materials in the polymer matrix and the interfacial adhesion between the polymer and reinforcing materials are the most important factors affecting the SMPC properties. This is why nano-sized reinforcing materials are advantageous in SMPCs. Their high surface area provides better dispersibility and interfacial interaction enhancing the mechanical and chemical properties of composites. In SMPCs, polymer matrix constitutes to the shape memory behavior while reinforcing materials enhance the structural and responsive properties.

Particle reinforced SMPCs are investigated due to their promising potential. Carbon black, carbon nanotube, carbon nanofiber, SiC, Ni, Fe3O4, and clay particles are commonly utilized in SMPCs to achieve enhanced properties. These materials can improve the electrical conductivity, magnetic behavior, stiffness, and optical properties of the composite. Studies show that carbon powders and SiC enhance the recovery stress and elastic modulus of SMPs. Electrically conductive particles such as carbon black, carbon nanotubes, nickel powders, and chopped carbon fibres are incorporated into SMPs to develop electro-active SMPCs. Furthermore, Fe3O4 articles embedded in SMPs can lead to magnetism-induced SMPCs. While particle reinforced SMPCs are used as promising functional materials, they are not suitable for use as structural materials.

Fibre reinforced SMPCs can offer great advantages as structural materials due to their enhanced strength, stiffness, and resistance against relaxation and creep. While particle and short fibre reinforced SMPCs are not suitable to be used as structural materials, fibre reinforced SMPCs can provide the required strength for this purpose. Fibre additives are especially utilized in thermoplastic SMP resins which show great shape memory behavior but poor thermal and mechanical properties. Thus including fibre reinforcing materials can enhance these SMPs greatly. Carbon-, glass- and Kevlar-fibre are the most common SMP reinforcing materials in this category. These additives are proven to provide better mechanical and thermal properties to the SMP material. For example, a study shows that glass-fibre reinforced thermoplastic SMPCs offer a 140% increase in failure stress and a 62% decrease in recovery rate.

What are the Application Areas of Shape Memory Polymers?

Smart SMP and SMPC materials have attracted a lot of attention in various applications due to their shape memory behavior, response to external stimuli, and adjustable mechanical properties. Currently, the aerospace industry is heavily interested in SMP/SMPC materials for deployable components such as hinges, trusses, booms, antennas, optical reflectors, and morphing skins. The traditional deployable devices used in aerospace applications require the use of a complex assembling processes, massive mechanisms, large volumes, and undesired effects during deployment. Thus utilizing multi-functional SMPs and SMPCs can offer huge benefits to these applications reducing the complexity and weight of the system.

SMP-based materials have also inspired novel applications in the aviation industry. Shape-changing structures such as folding wings and variable camber wings can be utilized to adjust the functionality of the aircraft parts. These structures can allow efficient cruising and high maneuverability at the same time while providing high speed to the aircraft and reducing energy consumption.

The biocompatibility of SMPs and SMPCs is considered to be a great asset in biomedical applications. Light-, thermo-, and moisture-responsive behavior of SMPs are very useful in biomedical applications because stimuli response can be manipulated to achieve desired results in specific parts of the body. SMPS can be used for the deployment of different clinical devices, for effective drug delivery, removal of blood clots, and use of micro/nanodevices for surgery. Furthermore, biodegradable SMPs can be used for wound closure.

Other application areas of SMPs/SMPCs include automobile parts and accessories, smart textiles, MEMS and NEMS applications, toys, food equipment, deployable structures such as shelters, and packaging materials.

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