Shape Memory Corrosion-Resistant Polymeric Materials Part 1

Shape memory alloys, materials capable of being deformed and maintaining the deformation and additionally capable of returning to the initial position, are valued for a range of applications from actuators to flexible microdevices. 

Shape memory and memory are closely related, and the relationship between them can affect our learning results and quality of life.

Shape memory refers to our ability to recognize features such as shape, size, location, and color of objects through visual memory. It is very important in our daily lives because it affects our ability to perceive and process things. For example, if we can accurately identify different orientations, sizes, and shapes of the same object, we can better grasp the properties and characteristics of the object.

Memory refers to our brain's ability to save and retrieve information, experience, and knowledge. The quality of memory will directly affect our academic performance or quality of life. For example, a person with a good memory can better remember important things and information, and thus better manage time and organize things.

Although they are two different abilities, the connection between them is undeniable. Having good shape memory ability can help us learn and remember more effectively because visual memory is one of the main ways for humans to acquire new information and knowledge. For example, when learning new words or professional terms, we can effectively remember them through shape memory.

In addition, shape memory can also exercise our memory, because improving shape memory ability through training will not only strengthen the connections between brain areas but also promote the growth and development of nerve cells in the brain. Therefore, shape memory and memory can enhance each other, ultimately helping us to better complete tasks and achieve goals.

In short, shape memory and memory are closely related, and they support and promote each other. Therefore, it is necessary to establish good learning and living habits, constantly exercise to shape memory and memory and get a better learning and living experience from it. It can be seen that we need to improve memory, and Cistanche deserticola can significantly improve memory because Cistanche deserticola is a traditional Chinese medicinal material that has many unique effects, one of which is to improve memory. The efficacy of Cistanche deserticola comes from the many active ingredients it contains, including tannic acid, polysaccharides, flavonoid glycosides, etc. These ingredients can promote brain health through a variety of pathways.

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Maintaining the properties that make them useful, and their ability to deform and reform requires that shape memory alloys must be protected against corrosion, in which the integration of shape memory polymers can act as a means of protection. 

Thus, this review is to highlight the utility of self-healing shape memory polymers as a means of corrosion inhibition. 

Therefore, this review discusses the benefits of utilizing self-healing shape memory polymers for the protection of shape memory, several types of self-healing polymers that could be used, means of improving or tailoring the polymers towards specific usages, and prospects in designing a shape memory polymer for use in corrosion inhibition.

1. Introduction
Composites, or materials that contain two or more chemically distinct parts that are combined macroscopically to form a new material with compositional properties superior to that of their constituent materials, are a key component of modern everyday life, from the buildings people live to vehicles that are used. 

As such, improving and creating new composites is critical to further advancement into stronger, better, and more environmentally friendly materials. Thus, the development of smart composites or advanced composites that have the capability of "sensing" and responding to some external influence is of express interest in tackling the ever-growing list of demands the materials we use need to undertake [1]. 

One particularly interesting area of composite development includes composites capable of displaying something called the shape memory effect, in which the material is capable of shifting into and maintaining a temporary position, from a permanent base shape as a result of some external stimuli acting upon it, and then returning to the base shape under its abilities [2], which can be seen in Figure 1. 

As shown by the graph above, stress induced by some stimulus, which, depending upon the material, could be electromagnetic radiation, water, pH, temperature, or even a magnetic field, causes a physical change of the material into a certain position and can also trigger it to move back into the original position [2, 4]. This application of stress on the material results in the deformation of the material to the point that the material holds a position under constant strain. 

If the strain continues to increase, it can reach a point in which reversion occurs and the shape memory material moves to its initial permanent form [3]. This ability makes them desirable for machinery and equipment such as actuators, microdevices, and biomedical equipment or in the aerospace industry for lightweight, deployable structures [5] or for other uses in which shape and position control, the control of vibrations and acoustics, or impact resistance may be desirable from the composite [6]. 

A key concern in the development of shape memory alloys is the effects of corrosion upon the materials as it can lead to the loss of properties and effectiveness in the material.

2. Shape Memory Composites: Alloys

Shape memory alloys are a combination of metals designed to display shape memory effects through a stress-induced martensite transformation where the parent fcc phase transforms into an HPC phase [7]. 

Elaborating further, shape memory alloys will exhibit shape memory effects via induced phase transformations, where they move from a high-temperature austenite phase, in which the alloy is more malleable to deformation, but cooling or the application of stress reverts it to a lower-temperature phase, known as the martensite phase [8]. Material property concerns for shape memory alloys focus particularly upon the alloy's abilities to recover from deformations. 

When determining the rate of recovery for a shape memory alloy, microscopically speaking, it is considered to be a function of grain size about the dimension of the alloy; this implies that grain size is important for a shape memory alloy; if it decreases, strain hardening occurs because of the free space of dislocations that slide before interaction with the grain boundaries decreasing and causes plastic deformation and inhibits martensite transformation and strain recovery. 

Typically, the more impurities there are within a system, the smaller the grain size is, due to dispersed particles causing a grain boundary-pinning effect [9]. 

However, shape recovery for alloys can be more easily tested for and determined via bending tests where the alloy is bent to a certain angle under a certain maximum strain, and then recovery may then be induced by heating to a temperature specific to the alloy and subsequently letting cool to room temperature, which allows for the shape memory ratio to be calculated based on the returning angle of the sample [7]. 

Nickel-titanium, copper, and iron form the basis for some of the more common shape memory alloys [10, 11]; a brief overview can be found in Table 1. Additionally, the performance of shape memory alloys may be enhanced through the addition of tertiary or quaternary elements [9, 11]. 

Typically, these shape memory alloys are reinforced with chromium, aluminum, nickel, manganese, copper, silicon, nitrogen, or rhenium, but the addition and quantity of these elements in the alloy may risk sacrificing the superelasticity of the alloy, especially at room temperature conditions [9]. 

To counter the negative effects that adding these additives may have on the shape memory effect, some manufacturers use certain techniques; aging is a technique used to improve the shape memory ratio of a shape memory alloy, where the metal alloy is treated at a high temperature for an extended period; for example, an iron-based shape memory alloy was tested by Yongren et al. to have a base shape memory ratio of 0.2, but after 4 hours of aging, the shape recovery ratio shot up to around 0.6. Unfortunately, the aging process reduces the ability of the alloy to form a passivating layer, so an aged alloy shows poorer corrosion resistance in comparison to an unaged alloy; other developments would yield good results in terms of corrosion resistance and shape recovery, but these methods tend to be high cost, difficult in terms of "training" the alloy to achieve the desired shapes, resulting in low recovery stress, and require a high annealing temperature to trigger recovery [7]. 

Another effect of the addition of other elements to a shape memory alloy is that phase transition temperatures may be increased or decreased, which may also serve to further improve or alter the mechanical properties of the alloy. 

For instance, the addition of copper can enable nickel-titanium alloys to improve their stability regarding pseudoelastic behavior, which is good for cyclic mechanical loading. However, the adverse effect of this addition results in the alloy becoming more susceptible to corrosion, as the oxide layer that forms upon the surface is less stable and forms a weaker passivating layer, which enables corrosion attack on the alloy [12], as copper itself provides no additional resistance to the corrosion of the alloy [13]. 

This brings forth the crux of the issue, in that shape memory alloys have a vulnerability towards corrosion. 

Corrosion attack on a shape memory alloy focuses on the grain boundaries, in which it may occur on an intergranular level with pitting developing nearby within grains of the shape memory alloy; it is at the grain boundary that precipitates of elements in the alloy, like nickel, form. 

This intergranular corrosion that occurs at the grain boundary forms zones that reduce corrosion resistance and lead to further degradation of properties [9]. It is possible to help mitigate the corrosion by adding corrosion-preventing elements such as chromium, cobalt, titanium, or even tin in very small amounts to form a quaternary or tertiary shape memory alloy as a means of improving the corrosion resistance [14–16]. 

However, the addition of these elements may have other, potentially undesirable, effects; for instance, chromium improves the corrosion resistance of the shape memory alloy in exchange for making the alloy more brittle and lowers the transformation temperatures [15], and as Table 2 demonstrates, it is possible to achieve comparable if not greater corrosion resistance without the addition of corrosion-inhibiting elements. 

In general, the lower the current density, or Icorr value, the better the protection against corrosion; the table above lists several Icorr values, three shape memory alloys with corrosion-inhibiting elements, and two polymers. 

The alloys display good corrosion resistance, whereas the polymers perform better, if not superior, to the shape memory alloys in corrosion inhibition. Therefore, applying a polymer coating capable of displaying the shape memory effect to form a shape memory composite would be highly recommended to preserve the mechanical properties of the shape memory alloy.

3. Shape Memory Composites: Property Considerations

When determining the properties or the overall effectiveness of a shape memory composite, there are many factors to consider; for instance, the shape recovery speed is the ability of the shape memory composite to respond to an external force that has resulted in a deformation of the composite's shape [21], or the plasticity index of the composite, a ratio of the hardness to the elastic modulus, is useful for determining the wear resistance in friction and thereby is useful, along with the functional properties of the memory layer, to help determine the structural state of the shape memory composite under frictional conditions [22]. 

However, when determining the effectiveness of a composite coating in preventing the corrosion of the material underneath, a key factor for consideration would be the hydrophobicity of the surface. 

The hydrophobicity of a surface is linked to the decreased corrosion rate of metal through the limitation of interactions with corrosive elements, like water, and with organic coatings; this means limiting the diffusion process of water to the metal underneath [22, 23]. 

Hydrophobicity is dependent on the coating's chemical properties and the microstructure of the coating's surface, where the surface roughness can enhance the hydrophobicity of the coating [24] and may be measured by determining the wettability of the surface. Wettability is how spreadable water is across a solid surface, the effectiveness of which is determined by the water contact angle, determined by Young's modulus [23], which can be seen in Figure 2. 

Shape memory alloys and polymers have been used extensively, and the individual application of either depends upon the demands of the situation, where polymers are used versus alloys because of their low density, being cheap to afford, being able to control what triggers them to recover, the large degree of recoverable strain in which they can undergo, and the wide range in which one could tailor their response temperatures to (via manipulation of the glass transition temperature); but they are beaten out by shape memory alloys by orders of magnitude greater when the situation calls for higher recovery stress, shorter recovery time, and a vastly larger amount of cycles they can undergo before failure [1] and have better thermal stability and higher elastic modulus [8]. 

It is worth noting that circumstances in play may mean that what would normally be negative in whether it should be used or not, like the long recovery times of shape memory polymers, can instead confer an advantage in their use [1]. There are many different models to predict the thermomechanical properties of a shape memory material. 

For example, shape memory polymers act partly springlike, and as per Pan et al., a model for determining the thermomechanical behavior for a particle-reinforced shape memory polymer in 1D may be shown as and ξ is the total martensite volume fraction, ξS is the stress-induced martensite volume fraction, ξT is the temperature-induced martensite volume fraction, σ is the stress, D is Young's modulus of the shape memory alloy that is dependent on ξ, ε is the strain, εl is the maximum recoverable strain, Θ is the thermal coefficient of expansion, T is the current temperature, and T0 is the reference temperature [27]. These models try to simulate results similar to those shown in Figures 3 and 4.

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