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Shape memory polymer composite structures with two-way shape

memory effects

Zhengdao Wang n, Weibin Song, Liaoliang Ke, Yuesheng Wang

Department of Mechanics, School of Civil Engineering, Beijing Jiao-Tong University, Beijing 100044, China

a r t i c l e i n f o

Article history:

Received 22 June 2012

Accepted 24 August 2012Available online 4 September 2012

Keywords:

Shape memory materials

Thermal analysis

Two-way

Buckling

a b s t r a c t

Shape memory polymers (SMPs) usually exhibit one-way shape memory effect. A simple and effective

way to obtain two-way shape memory effect by SMP composite structures is proposed. The system

consists of a SMP film with higher glass transition temperature Tg surrounded on another SMPcylindrical core with lower Tg. A special thermomechanical process is performed. During this process

the mismatched deformation between the SMP film and cylindrical core intrigues the structure

developing from smooth to buckled configuration, and then recovering its original smooth shape. In

this study, we confirmed such two-way transition by the theoretical prediction and experimental

observation. The means to produce two-way graded shape memory effect is also proposed.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Smart materials and intelligent structure systems are receiving

increasing attention due to their great scientific and technological

significance. Shape memory polymers (SMPs) are a new classof smart materials with the capability of keeping a temporary

shape and subsequently recovering its original shape. The potential

applications of SMPs cover biomedical engineering [1], space

deployable structures [2], micro-electromechanical systems [3],

etc. Compared with traditional shape memory alloys and ceramics,

SMPs have many advantages such as low density, high fixture

strain, easy processing, wide shape transition temperature and even

biocompatibility. Great work has been performed on the fabrication

and mechanism researches of SMPs in the past two decades[48].

It is known that shape memory alloys (SMAs) and liquid

crystalline elastomers (LCEs) exhibit two-way shape memory

effect that reversibly switches shapes without the need of

external mechanical manipulation. Most SMPs, however, are

one-way. They can only switch from a temporary shape to theoriginal shape, and the transition is not reversible. Recently, SMPs

with two-way and multi-shape memory effects are receiving

more interests [911]. This paper aims to present a simple

method to realize two-way shape memory effect by using SMPs.

The structure changes from a smooth composite cylinder to a

gear-like shape and reversibly recovers its smooth shape via a

special thermomechanical process. Moreover, how to produce

two-way graded shape memory effect is also proposed.

2. Analysis and verification

As shown in Fig. 1a, a composite cylinder consists of a SMP

film and another SMP cylindrical core with different Tg. The glass

transition temperature of the SMP film (Tg1) is higher than that ofthe SMP substrate (Tg2). The thermomechanical process includes

the following steps: (1) both the film and substrate are in the

rubbery state at the temperature above Tg1, in which the compo-

site cylinder can have a large deformation under the applied load;

(2) keeping that deformation and decreasing the temperature to

be lower than Tg2, both the SMP film and core are fixed, and

thus the compressed deformation is kept even if removing the

applied load; (3) reheating and increasing the temperature to be

higher thanTg2but lower thanTg1, the SMP core has a tendency to

recover the original state (radial contraction), while the SMP film

still keeps the frozen state. The mismatched deformation leads to

the micro-buckling of the structure; (4) when further increasing

the temperature to above Tg1, the SMP film is also unfrozen,

the structure reversely recovers its original state. Similar processis inFig. 1b.

The physical mechanism is by using mismatched deformation

between the thin film and underlying compliant substrate to

intrigue micro-buckling. The critical buckling and unbuckling

expressions are derived inAppendix A. As an example, two epoxy

SMPs prepared by epoxy resin E-51 with varying contents of

curing agents of 4,4-methylenedianiline (DDM) are employed as

the film and cylindrical core in this study. The mass ratio of E-51

to DDM in the film and substrate are 100:19 and 100:15,

respectively. Fig. 2 shows experimental results of the shape

recovery rate and storage modulus of the SMP film and cylindrical

core as functions of temperatures, where we reasonably use the

Contents lists available at SciVerse ScienceDirect

journal homepage: www.elsevier.com/locate/matlet

Materials Letters

0167-577X/$ - see front matter& 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.matlet.2012.08.112

n Corresponding author. Tel.: 86 10 5168 7257; fax: 86 10 5168 2094.

E-mail address: zhdwang@bjtu.edu.cn (Z. Wang).

Materials Letters 89 (2012) 216218

http://www.elsevier.com/locate/matlethttp://www.elsevier.com/locate/matlethttp://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.matlet.2012.08.112mailto:zhdwang@bjtu.edu.cnhttp://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.matlet.2012.08.112http://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.matlet.2012.08.112mailto:zhdwang@bjtu.edu.cnhttp://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.matlet.2012.08.112http://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.matlet.2012.08.112http://localhost/var/www/apps/conversion/tmp/scratch_1/dx.doi.org/10.1016/j.matlet.2012.08.112http://www.elsevier.com/locate/matlethttp://www.elsevier.com/locate/matlet

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storage modulus achieved by Dynamic Mechanical Analysis

(DMA) instead of Youngs modulus at different temperatures.

More information about the material fabrications and behaviors

can be referred in [8].

Fig. 3 shows the theoretical results about the dependence of

the buckling amplitude on temperatures during heating, where

t0.5 mm, R10 mm, epre20%. It is clear that three stages canbe obtained. The buckling amplitude is zero in stage I and III. That

means the structure keeps the smooth-surface state. The buckling

amplitude, however, is not zero in stage II. That means gear-like

buckling shape is developed. Moreover, the buckling amplitude

increases at first, and then keeps decreasing in stage II.

Moreover, thermomechanical experiments were preformed.

Fig. 4 shows the result. The SMP composite cylinder is com-

pressed with an axial pre-strain of 20% at 393 K, and then keeping

that deformation and decreasing to room temperature. After that

the load is removed and the structure is fixed ( Fig. 4a). When we

heat the specimen to 378 K at the rate of 10 min/K, the local

buckling is developed (Fig. 4b), and the buckling amplitudeincreases with further heating (Fig. 4c). When the temperature

is higher than 393 K, the localized buckling is decreased and

finally disappeared at 408 K (Fig. 4f). The structure recovers its

initial smooth cylinder. It is noted that the theoretical buckling

temperatures in Fig. 3 are in the range of 381409 K and the

measured values in Fig. 4 are 378408 K. They are in well

agreement.

Here we theoretically and experimentally confirmed a simple

way to obtain SMP composite structures with two-way shape

memory effect. More complicated two-way shape memory pro-

files can be spontaneously formed by this technique. For example,

Fig. 1b in the above schematically illustrates how to realize two-

way graded shape memory effect by employing a SMP conical

substrate/film system.

3. Concluding remarks

In summary, we proposed an effective and easy-operating way

to realize two-way shape memory effect by a SMP composite

structure consisting of a SMP film surrounded on another SMP

cylindrical core. The key point is that the shape transition

temperature of the SMP film must be much higher than that of

the SMP cylindrical core. The ideal is confirmed by the theoretical

prediction and experimental observation. Moreover, a concept

design of two-way graded SMP composite structure is proposed.

Fig. 1. Schematic illustration of the process of a two-way shape memory composite structure.

320 340 360 380 400 420 440

0

20

40

60

80

100

Recov

eryrate(%)

Temperature (K)

Substrate

Film

320 340 360 380 400 420 440 460 480

1E7

1E8

1E9

1E10

substrate

Storagemodulus(Pa)

Temperature (K)

Film

Fig. 2. Experimental curves of the frozen recovery rate and storage modulus vs. temperatures [8].

340 360 380 400 420 440

0.0

0.2

0.4

0.6

0.8

Amplitude(mm)

Temperature (K)

Fig. 3. Buckling amplitude as the function of temperatures: no buckling appears in

stage I and III, and buckling happens in stage II.

Z. Wang et al. / Materials Letters 89 (2012) 216 218 217

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Acknowledgement

This work was funded by Natural Science Foundations of China

(No. 11272044).

Appendix A

Considering the elastic strain of SMPs in the rubbery state can

be up to 100%, linear elastic buckling theory is used in thefollowing analysis. For an isotropic and linear elastic film/sub-

strate composite cylinder, the critical stress scr, critical bucklingwavelengthlcr, and the critical wave number ncrare given by[12]

scr t

R

1=2 Ef ~Es3

!1=2,lcr 2pt

R

t

1=4 Ef12 ~Es

!1=4,

ncr R

t

3=4 12 ~EsEf

!1=41

whereEfEf=1u2f,

~EsEs=12us1 us; Efand ufare Youngs

modulus and Poissons ratio of the film respectively; Es and usYoungs modulus and Poissons ratio of the cylindrical core; R and

tthe radius of cylindrical core and thickness of the film. It is noted

that Ef and Es are strongly temperature-dependent in SMPs. Forthe sake of simplicity, we assume that Poissons ratios of both the

film and core are same during the thermomechanical process, i.e.

ufus0.45.

When a SMP composite cylinder with radial pre-frozen strain

epre is heated at room temperature, the SMP cylindrical coreshrinks more quickly than the film at first due to its lower Tg-

value. The mismatched deformation causes the increase of com-

pressive stress in the film, namely

sT EfEs2R

2 2Rtt2eprefsff

2Es1u2fR

2 Es1 uf Ef1 us12us2Rtt

22

where fs and ff are shape recovery rates of the substrate and

film. Both of them are temperature functions. Naturally, buckling

happens on condition ofs(T)4scr. Therefore, substituting Eq.(1)into Eq.(2) the critical buckling condition is obtained as

EfEs2R2

2Rtt2eprefsff

2Es1u2f

R2 Es1 uf Ef1 us12us2Rtt2

Zt

R

1=2 Ef ~Es

3 !

1=4

3

Similarly, when we further increase the temperature above Tg1,

the shape recovery rate of the SMP film becomes quicker, and the

mismatched stress is decreasing. The composite cylinder will

elastically recover its original unbuckling state on condition of

EfEs2R2 2Rtt2eprefsff

2Es1u2fR2 Es1 uf Ef1 us12us2Rtt

2

rt

R

1=2 Ef ~Es3

!1=44

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Fig. 4. Two-way shape memory effect of a SMP composite cylinder during heating. Localized buckling structures are developed from (a) to (c), and then slowly

disappearing from (d) to (f). (a) 298 K, (b) 378 K, (c) 388 K, (d) 393 K, (e) 398 K, (f) 408 K.

Z. Wang et al. / Materials Letters 89 (2012) 216 218218

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