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Abstract: This review paper summarizes the recent research progress in the underlying
mechanisms behind the shape memory effect (SME) and some newly discovered shape
memory phenomena in polymeric materials. It is revealed that most polymeric materials,
if not all, intrinsically have the thermo/chemo-responsive SME. It is demonstrated that a
good understanding of the fundamentals behind various types of shape memory phenomena
in polymeric materials is not only useful in design/synthesis of new polymeric shape
memory materials (SMMs) with tailored performance, but also helpful in optimization of
the existing ones, and thus remarkably widens the application field of polymeric SMMs.
Introduction
Traditionally, the shape memory effect (SME) refers to the following interesting phenomenon, i.e.,
after being severely and quasi-plastically distorted, a material is able to recover its original shape at the
presence of the right stimulus [1–3]. Figure 1 reveals the SME in three commonly used engineering
polymers, namely, polytetrafluoroethylene (PFTE), polylactide (PLA) and ethylene-vinyl acetate (EVA).
The SME is fundamentally different from another commonly observed phenomenon, namely, the
shape change effect (SCE), in which a material returns its original shape either instantly or gradually
when the applied stimulus is removed [4,5]. A typical example of the SCE is reversible elastic distortion
when a piece of material is elastically distorted. Note that upon unloading, shape recovery might be
either instantly or in the case of a viscous-elastic material, gradually in the SCE.
From an energy point of view, the difference between the SME and SCE is due to the magnitude of
energy barrier between two states (marked as A and B in Figure 2), in which one is with, while the
other is without, the presence of the right stimulus [3]. As illustrated in Figure 2, in the case of high
energy barrier (H), additional driving force is required for shape recovery (i.e., the SME); while in the case of low energy barrier, the SCE can be realized either instantly or gradually (in the case of low
energy barrier H′).
As pointed out in [3], the SCE and SME may coexist in one material, i.e., a material may behave
and thus be classified either as a shape change material (SCM) or as a shape memory material (SMM),
dependent on the exact working condition/environment. For instance, loading within the elastic range
of a piece polymer at low temperatures, the material is SCM; while loading to beyond its elastic range,
the quasi-plastic deformation may be recovered upon heating or immersing into a particular chemical
(without any apparent temperature variation), so that the material is SMM.
Although the term SME originally referred to the shape memory phenomenon observed in an AuCd
alloy in the 1930s, heat shrinkable polymer and water shrinkable polymer actually have a much longer
history that began even well before the coining of the term SME [5–7].
Currently, the SME in polymeric materials can essentially be triggered by four basic types of
stimuli, namely temperature variation (thermo-responsive, including both cooling and heating either
directly or indirectly); chemicals (chemo-responsive, including water, ethanol and pH change etc.); light
(photo-responsive, without apparent temperature fluctuation); and, mechanical force (mechano-responsive,
including impact and pressure) [2–5,8–18].
In the past decade, extensive and continuous research efforts have been devoted to developing new
polymers with the SME and/or improving the existing ones for higher performance [14,19–30]. On the
other hand, a few new shape memory phenomena have been discovered [31–47], which not only enhance
the flexibility of the current shape memory technology, but also add in new dimensions for extended
versatility and adoptability. Consequently, a number of novel concepts have been proposed for a range
of engineering applications [45,48–54], in particular in the field of biomedical engineering as of the
last couple of years [24,43,55–70].
The purpose of this paper is to present a brief review about the underlying working mechanisms for
various commonly observed shape memory phenomena in polymers. A good understanding of the
fundamentals behind various types of shape memory phenomena in polymers is not only useful in design/synthesis of new polymeric SMMs with tailored performance and optimization of the existing
ones, but also very helpful for modeling and simulation of their stimulus-responsive behaviors.
The outline of this paper is as follows. Section 2 discusses the basic working mechanisms behind
the classic SME. Section 3 extends the discussion from the basic shape memory phenomenon to other
shape memory phenomena recently discovered. Based on the background knowledge presented in
Sections 2 and 3, in Section 4, the design of new polymeric SMMs with required features and optimization
of existing ones for tailored performance are presented. Some major conclusions are summarized
in Section 5.
2. Working Mechanisms
There are many ways to achieve the SME in materials based on various working mechanisms.
In Figure 3, a line-shaped indent is made on the top of an EVA-based melting glue droplet.
After heating to above the melting temperature of the melting glue, the indent disappears and the
droplet recovers its original shape. In this example, shape recovery is driven by surface tension when
melting glue is heated to fully melt, which pulls the droplet surface back to the original spherical shape
for energy minimization. As we can see, like many others, such a working mechanism is limited to
some very special situations only.
Instead of exhausting efforts to have a complete list of every possible working mechanism in all
real engineering practice, in this review paper, we will focus on the generic ones, which are applicable
for a wide range of polymeric materials and their composites.
In this section, we take heating-induced SME as an example to reveal possible working mechanisms
which can be easily implemented in most of polymers, if not all, and their composites.
Different from their metallic counterpart, namely shape memory alloys (SMAs), in which the reversible
martensitic transition either induced thermo-mechanically or thermo-magneto-mechanically, is the driven
mechanism [1,6,71–73]. In a general sense, the SME in polymeric materials is fundamentally based on
a dual-component system, in which one component (segment or domain) is always elastic within the
temperature range of our interest (for the thermo-responsive SME), while the other component
(segment or domain) is able to reversibly change its stiffness (and also easily deform in a plastic manner in the low stiffness state) depending on if the right stimulus (heat in the case of thermo-responsive SME) is
presented [2,4,10,49]. The glass transition and melting are the two most commonly utilized transitions
in polymers for the thermo-responsive SME.
A typical SME cycle for a heating-responsive SMM includes two parts, namely programming and
shape recovery [1,4,45,74,75]. As illustrated in Figure 4, in the first step “a”, a piece of SMM is
strained to a maximum strain of εm at high or low temperatures. Subsequently, with or without the step
of cooling back to room temperature, the SMM is unloaded (“b”), and a residual strain of εu is resulted
at room temperature. Shape fixity ratio (Rf), which is normally defined as,
f
u
m
R ε
ε = (1)
is one of the key parameters in characterizing the shape memory phenomenon of a SMM. This ends
the programming process.