Shape Memory Polymers (Şekil Hafızalı Polimerler, İng)

Shape memory materials, which can be called adapting to environmental conditions, are mainly obtained in alloys, ceramics and polymers. This adaptation is the state of changing some properties of the material against external factors such as heat, light, electrical field and pH, and regaining its former properties when these factors are removed. In other words, the material has a permanent shape at room temperature. It deforms at a high transition temperature and retains its shape after cooling. It returns to its original shape upon reheating.

The low density, easy fabrication and easily adaptable glass transition temperature of shape memory polymers (SMPs) compared to shape memory alloys and shape memory ceramics are outstanding factors. Mostly amorphous polymers, semicrystalline polymers and liquid crystalline elastomers have this shape memory effect.

Figure 1 Scheme of shape memory cycle [1].

Shape memory cycle (Fig. 1) indicates the three steps that polymer transforming pathway. First is shape deforming: Increasing the entropy of polymer to make easy move of its molecular chain. Second is shape fixing: Maintaining the structure by decreasing temperature and external stress (physical and chemical). Lastly, evacuation of the external stress. This pathway can be applied several times, however, at the end we have temporary shapes but one defined shape. Nevertheless, a condition that should be noted is that polymers that do not change under this pathway are not considered shape-memory. An example of this is the swelling of hydrogels in water.

Thermoviscoelastic theory modeling examines the behavior of SMPs from a thermodynamic point of view. As is known, the chain structure in polymers is small in diameter but large in length, and the chains are entanglements. In SMPs, entropy is large due to the complexity of the chains; When the temperature is increased, the chains stretched and become oriented. This ordered state lowers entropy. When the temperature is lowered, the polymer lowers its thermoviscoelasticity and the molecular motion slows down. At this stage, the stored stress in the molecular chain is elastic potential energy. When the temperature increases again, thermoviscoelasticity is regained. Additionally, from a thermomechanical point of view, the phase transition theory is more suited to explaining the shape transition behavior of SMPs.

Figure 2 Two phases of SMPs [2].

Frozen and active phases are compositions of SMPs. In frozen phase, internal structure remains and in active phase, deformation can occur. Parts of the frozen phase shift into the active phase when the polymer transitions from the glass to the rubbery state, and the ratio of the frozen phase to the active phase varies.

The transformations of these phases express the glass transition behavior in the thermodynamic cycle and explain the storage and release process of stress in the shape memory process.

One-Way SMPs’ Mechanism

In this structure, the cold stated material can be bent and stretched and protects this structure until it is heated. After heating, the structure returns to its original shape. Additionally, SMPs in this structure consist of switchable segments and netpoints. Switchable segments are responsible for the temporary shape change of the structure; netpoints are responsible for the permanent shape change. When the polymer is heated and exposed to an external stimulus, changes in structure occur with switchable segments. The connection points of these segments are netpoints and are usually chemically crosslinked.

Figure 3 Light-induced cycloaddition reaction (left) and shape memory effect (right) of photoresponsive multiblock polyesterurethane [3]

Wu et al. examined the one-way shape memory properties of cinnamamide-doped polyester urethane. In this work, a two-step polyaddition reaction of N,N-bis(2-hydroxyethyl) cinnamamide (BHECA), biodegradable poly(l-lactide) (PLLA), and poly(ε-caprolactone) (PCL) diols was studied. As Fig. 3 indicates, (A) Original shape; (B) Temporary shape obtained by an external force (UV light, >260nm) to form temporary chemical crosslinks and then releasing the external force; (C) Final shape after irradiation. According to the results obtained, a photosensitive shape memory effect was observed.

Two-Way SMPs’ Mechanism

In the two-way effect, the material remembers two different shapes. One is the low-temperature shape and the other is the high-temperature shape.

Gao et al. reported that the study of polyolefin elastomers’ shape memory effect. This study consists of heating a flat shaped material to 85°C and cooling to 50°C. After this step C type temporary shape was observed. Further cooling, helix type temporary shape is obtained at 0°C. Lastly, reheating to 50°C, C type is obtained (Fig. 4).

Figure 4 Reversible shape memory effect of polyolefin thermoplastic elastomer [4].

 

Multiple SMPs’ Mechanism

In this type of effect, more than one temporary shape is observed.

Tao Xie studied the shape memory properties of perfluorosulphonic acid ionomer (PFSA). As indicated in the Fig. 5, S0 represents the permanent shape and can memorize the three different temporary shapes. Also, further heating led to the recovered shapes.

Figure 5 Quadruple-shape memory properties of PFSA. S0: permanent shape; S1: first temporary shape; S2: second temporary shape; S3: third temporary shape; S2rec: recovered second temporary shape; S1rec: recovered first temporary shape; S0rec: recovered permanent shape [5].

Recent Advances in Applications of SMPs

SMPs have promising applications in both macro and nanoscale. Examples include packaging, electronics, textile, biomedical, and aerospace applications. Wings that can change shape under different conditions, for example, can be employed in airplanes to save energy while also taking appropriate shapes during flight and, depending on the scenario, during take-off and landing. Smart fabrics made with SMP, which changes shape with temperature, can have varied air and moisture permeability at low and high temperatures, allowing for the production of garments that can adapt to the climate.

Aerospace Applications

Usually, SMPs used for weight reduction purposes are also used as shape-changing wings. Thus, smart wings will save energy by changing shape during takeoff or landing [6].

Figure 6 The configuration of Mission SMS-I, I: packed configuration, II: deployed configuration, III: oblique view of the first observation, IV: oblique view of the third observation [7]

As Figure 6 indicates, the sunlight-stimulated shape memory substrate was put into stable orbit with an experimental satellite in 2016 to conduct deployable and long-term anti-cosmos irradiation tests. Under sunlight, the substrate was able to return from a bent shape to a flat shape, registering a recovery rate of over 100% 13 days after its launch [1].

Biomedical Devices

Implants and hydrogels, especially smart suture threads used in surgeries, are the application areas of SMPs. Threads are preferred because of their self-knotting and compression feature. The implants required for surgery are placed inside the body through a small section and can change their shape when they reach body temperature. The biodegradable nature of some prevents the secondary surgical procedure necessary to re-extract the material from the body. Hydrogels are also at the forefront of controlled drug release systems.

Figure 7 Schematic illustration of the atrial septal defect prototype after interventional therapy with an occlude [8].

Electronics, Robotics & 4D Printings

Other works are wearable electronics for harvesting energy and mechanic sense, parts of a robotic system that lifts objects, and 4D technology based on digital control of stress on a 2D membranes with SMP.

Figure 8 a) Demonstration of the setup for strain testing and shape memory polymer (SMP)-based triboelectric nanogenerators [9]. b) Demonstration of high load capacity and good shape adaptivity by a versatile gripper equipped with three fast-response and stiffness-tunable actuators [10]. c) Process illustration from a planar sheet with patterned concentric circles swollen into a cap-shape 3D structure and cap geometries controlled by crosslinking density distribution [11].

External stimuli like as light, heat, magnetism, and electricity can cause SMPs to revert to their basic shape from a programmed temporary shape.

Shape memory polymer composites (SMPCs) with extensive recoverable deformation, increased mechanical characteristics, and programmable remote actuation are the consequence of the incorporation of functional components and nanostructures.

Aerospace engineering, biomedical devices, flexible electronics, soft robotics and 4D printing are all potential applications for SMPCs.

All in all, the usual one-way, two-way, and multiple SMPs were demonstrated in this review. A thorough examination of the shape recovery methods, multifunctionality, applications, and current breakthroughs in SMPs and SMPCs was also given.

REFERENCES

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[2]	R. Huang, Z. Shoujing, Z. Liu and T. Y. Ng, "Recent Advances of the Constitutive Models of Smart Materials-Hydrogels and Shape Memory Polymers," International Journal of Applied Mechanics, vol. 12, no. 2, 2020.
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[5]	T. Xie, "Tunable polymer multi-shape memory effect," Nature, vol. 11, no. 464, pp. 267-270, 2010.
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[8]	C. Lin, J. Lv, Y. Li, F. Zhang, J. Li, Y. Liu, L. Liu and J. Leng, "4D-Printed Biodegradable and Remotely Controllable Shape Memory Occlusion Devices," Advanced Functional Materials, vol. 29, no. 51, 2019.
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[10]	Y.-F. Zhang, N. Zhang, H. Hingorani, N. Ding, D. Wang, C. Yuan, B. Zhang, G. Gu and Q. Ge, "Fast-Response, Stiffness-Tunable Soft Actuator by Hybrid Multimaterial 3D Printing," Advanced Functional Materials, vol. 29, no. 15, 2019.
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