Shape memory polymers for composites

https://doi.org/10.1016/j.compscitech.2018.03.018Get rights and content

Abstract

Shape memory polymers (SMPs) are a class of active, deformable materials that can switch between a temporary shape, which can be freely designed, and their original shape. With their large deformation, low density, various stimulation methods, good biocompatibility and other advantages, SMPs have become widely accepted as smart materials. However, SMPs have many limitations and weaknesses that are exposed in engineering applications. For this reason, the significance of SMP composites (SMPCs) has been analyzed in terms of four aspects: reinforcement, innovation and improvement of driving methods, the creation of specific deformations and the creation of multifunctional materials. We then introduce the constitutive theory of SMPs and the post-buckling analysis of SMPCs. Afterward, we introduce the extensive applications of SMPCs in the fields of aerospace, biomedical equipment, self-finishing, deformable mandrels and the 4D printing of active origami structures, demonstrating their ability to undergo active driving and deformation, their adaptiveness, their ease of transport and their rapid production capacity, which fully demonstrate the unique advantages of SMPs in solving application problems. Finally, the advantages and disadvantages of SMPCs in applications are summarized, and the prospects for new SMPCs and new SMPC structures are described.

Introduction

The shape memory effect (SME) is a special mechanical phenomenon usually described by the shape memory cycle (SMC). Fig. 1.1 shows a highly common shape memory cycle. SMC represents the mechanical process that embodies the shape memory effect of that kind of material, and a more exact definition will be explained later. The shape memory effect is common in polymers, but most examples are inferior. One class of polymers can provide excellent shape memory effects and they are predominantly used in shape memory applications. They are called shape memory polymers (SMPs). Shape memory polymers are active deformable materials that undergo large deformations, first mentioned by Vernon et al. in a dental patent in 1941 [1]. In the sixties, heat-shrinkable tubes entered the market [2,3]. Their applications have drawn substantial research attraction. As the seventies began, a number of commercial companies developed their own shape memory polymers [4]. Since the end of the last century, researchers have begun to systematically study shape memory polymers. The principle of shape memory has been increasingly elucidated, and diverse shape memory effects have been observed [5].

Taking the initiative to change shape is vitally necessary for animals and even other organisms. Compared to inorganic ceramics and metal materials, polymer materials show natural advantages such as lower density, better biological and organic compatibility, and easier modification and processing. Accordingly, shape memory polymers are blossoming in radiant splendor in the field of active polymers. Actively moving materials can be effectively deformed in shape by external stimulation [[6], [7], [8], [9]]. Examples such as shape memory polymers, electroactive polymers [10,11], photo-induced polymers [12], and hydrogels [13,14] have been the subject of substantial research. Actively moving materials are often categorized by response behavior, and the differences in properties between different actively moving materials are significant.

Shape memory polymers are materials driven by external stimuli that actively switch between multiple shapes. Compared with other materials, shape memory polymers have the advantages of high stress tolerance [15], the ability to undergo large deformations [2], a rich selection of driving methods (including heat 3456, light [7,8], electricity [[9], [10], [11]], magnetism [12,13] wetting [14], and pH [16]), excellent radiation resistance and good biocompatibility [17], which make them a research hotspot in the field of actively moving materials.

At present, shape memory polymers have many applications in aerospace [18], medicine [[19], [20], [21]], self-finishing smart textiles [22,23] and electronic devices [24], and self-assembling structure [[25], [26], [27]]. Specific applications, including low-impact release mechanisms in the aerospace field, large spatial deployable structures, shape memory polymer sutures, minimally invasive surgical instruments with good biocompatibility, the active deformation or self-finishing of textiles, electronic devices, and variable mandrels, which effectively solve the thorny problems of the corresponding fields, show the great power of shape memory polymers. The above applications involving composites will be described in detail in Section 5.

Section snippets

Basis of shape memory effect in polymers

In this section, the basic characteristics of shape memory polymers are reviewed, and the topic of shape memory polymer composites is not addressed. The majority of this section is thus a classic topic, which can be found in a standard shape memory polymer review. Therefore, this section is brief. More detailed description can refer to the supplementary documents.

This section does not discuss an important property of the shape memory polymer, the stimulation method. Because taking into account

Why use SMPCs instead of SMPs?

In general, the significance of composite materials is that several components complement each other, result in synergies, or improve or enrich the function of the matrix material. For shape memory polymers, composites are made with two basic objectives, i.e., reinforcement and finding new and effective stimulation methods. (For shape memory polymer composites, the stimulation can be more precise and highly selective, so more sophisticated shape memory behaviors can be achieved, such as the

The mechanics of SMPs and SMPCs

As mentioned earlier, the shape memory effect is a polymer mechanical behavior. With the principle of shape memory effect explained, increasing numbers of people are aware of the universality of this effect. Once it is recognized that the shape memory effect is to a considerable extent a process property, the description of the mechanical behavior becomes very urgent. There are significant differences in the shape memory behavior of polymers with respect to different programming and recovery

Applications of SMPCs

Applications have always been the focus of studies on shape memory polymers and their composites. During the 1960s, shape memory polymers gained their first large-scale applications, namely, the use of PE thermal contraction tubes [263,264], which now are made from nylon [265] or polystyrene [266] materials. Thermal contraction tubes have good flame retardance, insulation and temperature tolerance properties and have been widely used in insulated joint protection, harnesses and erosion

Conclusion and outlook

It is not appropriate to discuss shape memory polymers without discussing shape memory effect, so I hope that increasing numbers of people will be able to learn about the shape memory effect as a general phenomenon of polymer mechanics that occurs through the material learning of shape memory polymers, i.e., it is determined by the properties of the material and the external factors together. This concept can guide their design, especially in cases of active deformation behavior. We distinguish

Acknowledgement

This work is supported by the National Natural Science Foundation of China: Grant Nos. 11672086, 11772109 and 11632005.

Thanks to Xisu Wang for translating and collating some of the manuscripts in English. Thanks to Hongrui Suit finishing the article and the reference document format.

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