Shape-Memory Polymers.pdf

SHAPE-MEMORY POLYMERS

Introduction

Besides ﬁnding high performance materials, material scientists concentrate on

designing “intelligent” and “self-repairing materials.” In this context, materials

showing a thermally induced shape-memory effect, such as metallic alloys or gels,

have been studied intensively, and a class of polymers showing shape-memory

behavior has found growing interest (1–5). The relatively easy manufacturing

and programming of shape-memory polymers makes these materials a cheap and

efﬁcient alternative to well-established metallic alloys. These polymers usually

surpass by far the recovery properties of shape-memory alloys, which can recover

deformations of 8% at maximum. Taking into consideration the importance of

polymeric materials in daily life, a very broad spectrum of possible applications for

intelligent polymers opens up, covering an area from minimally invasive surgery

to high performance textiles, and to self-repairing plastic components in all kinds

of technical devices.

Stimuli-sensitive implant materials have a high potential for applications

in minimally invasive surgery. Degradable implants could be inserted into the

human body in a compressed (temporary) shape through a small incision where

they obtain their shape relevant for the speciﬁc application after warming up to

body temperature. After a deﬁned time period the implant is degraded. In this

case subsequent surgery to remove the implant is not necessary. For applica-

tions in biomedicine, it is necessary to have tailor-made shape-memory polymers

whose thermal, mechanical, or degradation properties can be varied over a wide

range. A substantially new development in this context is polymer systems. These

are families of polymers in which macroscopic properties can be controlled by a

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speciﬁc variation of molecular parameters. In this way shape-memory polymers

having a speciﬁc combination of properties required for speciﬁc applications

can be obtained by slight variation of the molecular structure and chemical

composition.

Deﬁnitions of Shape-Memory Polymer and Related Technical Terms

Shape-memory polymers are stimuli-responsive materials. Upon application of an

external stimulus they have the ability to change their shape. A change in shape

initiated by a change in temperature is called thermally induced shape-memory

effect . The shape-memory effect results from the polymer’s structure, that is, its

morphology in combination with a certain processing and programming technol-

ogy. Therefore, shape-memory behavior can be observed for several polymers that

may differ signiﬁcantly in their chemical composition.

The process of programming and recovering the shape of a polymer is shown

schematically in Figure 1. The polymer is processed into its permanent shape

by conventional methods. Afterwards, it is deformed and the desired temporary

shape is ﬁxed. The latter process is called programming. The programming pro-

cess consists of heating the sample, deforming and cooling the sample, or drawing

the sample at a low temperature (“cold drawing”). The permanent shape is now

stored while the sample is in the temporary shape. Heating the programmed

polymer above a temperature higher than the transition temperature T trans re-

sults in activating the shape-memory effect. As a consequence, the recovery of the

memorized, permanent shape can be observed. Cooling of the polymer below the

transition temperature leads to solidiﬁcation of the material. However, the poly-

mer sample does not return to its temporary shape. The described effect is called

a “one-way” shape-memory effect . The programming process including a mechan-

ical deformation can be repeated. The new temporary shape is not necessarily

Fig. 1. Schematic demonstration of the thermally induced one-way shape-memory effect.

By the programming process the permanent shape is transferred to the temporary shape.

Heating up of the sample to a temperature above the switching transition T trans initiates

the recovery of the permanent shape. From Ref. 2.

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Fig. 2. The series of photos demonstrates (top–down) the transition from the temporary

shape (spiral) to the permanent shape (rod) for a shape-memory network that has been

synthesized of poly( ε -caprolactone)dimethacrylate and butylacrylate (comonomer content:

50 wt%). The switching temperature of this polymer is 46 ◦ C. The recovery process takes

35 s when heated up to 70 ◦ C.

supposed to match the temporary shape resulting from the former programming

process.

In Figure 2, a photo sequence demonstrates impressively the perfor-

mance of shape-memory polymers. The permanent shape of the polymer

sample is that of a rod. This rod has been deformed to a spiral (tempo-

rary shape) during the programming process. Under the inﬂuence of hot air

having a temperature of 70 ◦ C the permanent shape is recovered as soon as

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the switching temperature T trans is reached. With optimized programming con-

ditions the permanent shape can be recovered with an accuracy of more than

99% (1).

Molecular Mechanism of the Shape-Memory Effect

All shape-memory polymers discussed henceforth are elastomers . On the molecu-

lar level they represent polymer networks consisting of segment chains that are

connected by netpoints. The netpoints can be formed by entanglements of the

polymer chains or intermolecular interaction of certain polymer blocks. These

cross-links are called physical netpoints . Cross-links in the form of covalent bonds

form chemical netpoints . An elastomer exhibits a shape-memory functionality if

the material can be stabilized in the deformed state in a temperature range that is

relevant for the particular application. This can be achieved by using the network

chains as a kind of molecular switch. For this purpose, it should be possible to

limit the ﬂexibility of the segments as a function of the temperature. This process

is supposed to be reversible. The ability to incorporate a control function into the

material provides a thermal transition T trans of the network chains in the tem-

perature range of interest for the particular application. At temperatures above

T trans the chain segments are ﬂexible, whereas the ﬂexibility of the chains below

this thermal transition is at least partly limited. In the case of a transition from

the rubber-elastic, ie viscous, to the glassy state the ﬂexibility of the entire seg-

ment is limited. The molecular mechanism of programming the temporary and

recovering the permanent shape is demonstrated schematically in Figure 3 for

a linear multiblock copolymer as an example of a thermoplastic shape-memory

polymer with a crystalline hard segment.

Fig. 3. Schematic demonstration of the molecular mechanism of the thermally induced

shape-memory effect for a multiblock copolymer, T trans = T m . If the rise in temperature

is higher than T trans of the switching segments, these segments are ﬂexible (marked red,

here) and the polymer can be deformed elastically. The temporary shape is ﬁxed by cooling

down below T trans (marked blue, here). If the polymer is heated up again the permanent

shape is recovered.

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SHAPE-MEMORY POLYMERS 129

Table 1. Possible Combinations of Hard-Segment- and

Switching-Segment-Determining Blocks in Linear, Thermoplastic Block

Copolymers with Thermally Induced Shape-Memory Effect a

Hard-segment-

Switching-segment-

Phase-separated

determining

block copolymers

Thermal transition

block (Block A )

block (Block B)

Possible

determining the

Highest

Second

Highest

Second

switching

permanent shape

thermal

transitions

at T perm

transition transition transition transition T perm T trans

Melting point

T m , A

T g , A

T m , B

T g , B T m , A T m , B , T g , A , T g , B

T perm = T m , A

T m , A

T g , A

T m , B

T g , B T m , A T m , B , T g , mix

T m , A

T g , A

T g , B

T m , A T g , A , T g , B

T m , A

T g , A

T g , B

T m , A T g , mix

Glass transition

T g , A

T m , B

T g , B T g , A T m , B , T g , B

( T perm = T g , A )

T g , A

T m , B

T g , B T g , A T m , B , T g , mix

T g , A

T g , B

T g , A

T g , B

a Ref. 2.

T perm , and on the

other hand the ﬁxation determining the permanent shape. The materials are di-

vided into two categories according to the thermal transition of the particular

switching segment the shape-memory effect is based on. Either the transition

temperature T trans is a melting temperature T m or a glass-transition temperature

T g . In case of a melting temperature one observes a relatively sharp transition

in most cases, whereas glass transitions always extend over a broad tempera-

ture interval. In case there is no sufﬁcient phase separation between the “hard-

segment-determining” block (Block A) and the “switching-segment-determining”

block (Block B), mixed glass-transition temperatures T g , mix between the glass

transiton of the “hard-segment- and the switching-segment-determining” blocks

may occur. Mixed glass-transition temperatures are also qualiﬁed to act as

switching transitions for the thermally induced shape-memory effect. Table 1

gives an overview about possible combinations of “hard-segment- and switching-

segment-determining” blocks in linear, thermoplastic shape-memory polymers

(2).

In chemically cross-linked shape-memory networks the permanent shape

is stabilized by the covalent netpoints. In analogy to linear block copolymers

with shape-memory, the temporary shape of covalently cross-linked shape-

memory networks can be ﬁxed either by crystallizable segment chains or by

a glass transition of the segment chains that is in the temperature range of

interest.

The mechanism of the thermally induced shape-memory effect of linear block

copolymers is based on the formation of a phase-separated morphology with one

phase acting as molecular switch. The phase showing the highest thermal tran-

sition T perm is by the formation of physical netpoints, on the one hand provid-

ing the mechanical strength of the material, especially at T

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