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Synthesis and Characterization ofPolyurethane-Based Shape-Memory Polymersfor Tailored Tg around Body Temperature forMedical Applications

Manzoor Ahmad, Jikui Luo,* Bin Xu, Hendra Purnawali, Peter James King,Paul Raymond Chalker, Yongqing Fu, Weimin Huang, Mohsen Miraftab

Various polyurethane-based SMPUs were synthesized using five types of polyols as softsegments and two different diisocyanates as hard segments. The effects of diisocyanateconcentration on material properties such as crystallinity, transition temperature, shape-memory effect and tensile strength were investigated. SMPUs with a maximum strain near1 000%, recovery rate up to�98%, fixity up to�90% andTgs of 35–45 8C were obtained. A high MDI contentresults in SMPUs with better shape-memory effect,whereas increasing IPDI content leads to a weakershape-memory effect. High IPDI concentration seemsto prevent or restrict chemical reactions and crosslinksbetween the polyols and the hard segments, leading tolarge phase separation and coexistence of soft and hardsegments in the macrophases.

Introduction

Shape-memory materials are one type of smart materials,

and have found tremendous applications in space explora-

M. Ahmad, J. K. Luo, M. MiraftabInstitute of Materials Research & Innovation, University ofBolton, Bolton BL3 5AB, UKE-mail: [email protected]. Xu, Y. Q. FuDepartment of Mechanical Engineering, School of Engineeringand Physical Sciences, Heriot-Watt University, Edinburgh EH144AS, UKH. Purnawali, W. M. HuangSchool of Mechanical and Aerospace Engineering, NanyangTechnological University, 50 Nanyang Avenue, 639798 SingaporeP. J. King, P. R. ChalkerDepartment of Materials, University of Liverpool, Liverpool L693BX, UK

Macromol. Chem. Phys. 2011, 212, 592–602

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonline

tion, engineering and recently in medical devices.[1–3]

Shape-memory polymers (SMPs) are better for certain

applications than shape-memory alloys (SMAs) owing to

their capability of large deformation up to 1 000%, light

weight, low density, low cost and easy processing etc.[4]

SMPs have been proposed for use in several medical devices

such as sutures for sewing wounds, and cork-screw-type

wires to pull blood clots out of a vein of a stroke patient

using light sensitive polymers.[2,5] We have recently

proposed to use SMPs to fabricate medical bandages which

can produce a gradient pressures acting on an ulcer leg to

speed the blood circulation and healing process for an ulcer

leg patient.[6]

Although SMPs are useful, their applications in medical

devices are limited due to limited choices of SMP materials

and requirement of specific properties. Most commercially

available shape-memory polyurethanes (SMPUs) have a

high elastic strength and high transition temperatures, Tg,

library.com DOI: 10.1002/macp.201000540

Synthesis and Characterization of Polyurethane-Based Shape-Memory . . .

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typically in the range of 50–90 8C[7] that are much higher

than the human body temperature, therefore are not

particularly suitable for medical device applications as they

would need high-temperatures for activation, though low

temperature SMPs have been synthesized by a number of

groups. SMPs with a relatively low elastic strength and a

near-body transitional temperature are desirable for

medical devices such as the laser-activated cork screw

surgical tool[8] and the SMP-based bandages.[6] Therefore it

would be useful to develop low Tg and low elastic strength

pure SMPs for medical applications, and further modifica-

tion can be made to change these SMPs into ones which can

be activated by other mechanisms such as light and

electrical voltage by adding different nanoparticles, etc.

Thermo-responsive SMPUs are one type of SMP, having

distinct properties such as high resistance to organic

solvents and aqueous solutions,[9] long-term stability

against exposure to sunlight,[10] consistent elastic proper-

ties[11] and biocompatibility.[12] SMPUs typically consist of

two major molecular segments: soft segment such as

different polyols and hard segment such as various

diisocyanates as shown in Figure 1. The hard segment is

formed when bifunctional diisocyanate reacts with mono-

mer polyols, and is responsible for the stiffness and recovery

stress of the SMPUs. The soft segment is formed from long-

chains of polyols and determines the flexibility and

transitional temperature, Tg, of the polymers. High micro-

phase separation (block polymers), large thermodynamic

phase incompatibility and crystallinity are prerequisites for

better shape-memory effect.

It is known that the properties of SMPUs can be modified

and tailored using different chemistries at different

contents.[13] Various diisocyanates as a hard segment

have been developed and studied, including 4,40-diphenyl-

methanediisocyanate (MDI), toluenediisocyanate (TDI),

hexamethylenediisocyanate (HDI), 1,6-diphenyldiisocyanate

(PDI)[14] and isophoronediisocyanate (IPDI). Yang et al.

synthesized SMPUs using PDI, and demonstrated that

SMPUs have better shape-memory properties over those

with MDI molecules owing to more rigid planar PDI

structure than the MDI bent structure.[15] Lee et al. reported

the loss of the shape-memory effect with an excess or

Figure 1. Representation of the basic soft and hard segments in ashape-memory polymer. Microphase separation and crosslinksbetween hard and soft segments are the necessary condition toform a shape-memory polymer.

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Macromol. Chem. Phys. 2

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shortage of hard segments in SMPUs.[16] Polyurethanes

with 20–25 wt.-% hard segment content did not show shape

recovery behavior as they do not have sufficient crosslinks

with hard segment due to their low concentrations. SMPUs

with an excellent shape recovery rate were obtained when a

hard segment content of 30–45 wt.-% was used.[17] Poly-

(ethylene glycol) (PEG-200) is able to produce high cross-

links owing to its high functionality ( f¼ 550), and act as a

plasticizer to increase the distance between the soft

segment chains. Thus PEG-200 is one of the chemicals

commonly used together with diisocyanates to enhance the

shape-memory effect and flexibility of the SMPUs. Lee

et al.[18] found that high crosslinks with hard segment

produce a high shape recovery rate, but low shape fixity.

Combination of diisocyanates was also used to improve the

quality of SMPUs as different diisocyanates have different

properties. High crosslink is necessary to form SMPUs with

better thermomechanical properties and good shape-

memory effect. Ding et al. used two different diisocyanates

(HDI and MDI) together with PEG-200 and chain extender

1,4-butanediol (1,4-BDO) to synthesize SMPUs and obtained

SMPUs with Tg¼ 10–40 8C, and found that the degree of

crystallinity is dependent of the chemical structure of the

polyols and hard segment concentration.[19,20] Not only the

chemistries and the content of the segments, but also

synthesis process,[21] additional molecules and nanoparti-

cles can change the properties of SMPs. By adding

reinforcements like nanoclays, fibers and nanotubes, SMPs

were found to have much enhanced hardness and elastic

modulus.[1,22,23] However they also have a significant effect

on the thermal properties of the SMPs such as on Tg values.

By adding photosensitive chemicals or nanoparticles, it is

also possible to synthesize SMPs which can be activated by

light or electromagnetic waves.[24,25]

Although various SMPUs have been developed and the

effects of some chemistry and the concentration on the

properties of SMPUs have been investigated to some degree,

there are still numerous choices and combinations of

chemicals which can be used to synthesize SMPs. For

example, the role of HDI, MDI and PEG200 in SMPUs has

been investigated,[16] while IPDI which is one of the major

hard segments in polymers remains largely unexplored.

IPDI has high resistance to light-related degradation and

possesses high hydrophobicity. Owing to its high phase

compatibility with soft segments, IPDI is believed to restrict

the segmental and short-range conformational movements

of soft segment after deformation above transition

temperature. Inclusion of IPDI in SMPUs therefore is

expected to improve the long-term stability and resistance

to moisture of the SMPUs significantly, allowing the

development of SMPU-based high quality medical devices,

and to enhance the shape fixity of SMPUs. Also specific

design requests for applications are rarely known. There-

fore it is important to have SMPU systems that will allow for

011, 212, 592–602

H & Co. KGaA, Weinheim593

Table 1. Fomulations with different MDI and IPDI molar concen-tration (M) and molar ratio.

Samples MDI (M) IPDI (M) g

PCL-PU-1 0.080 – 1PCL-PU-2 0.067 0.013 5.15

PCL-PU-3 0.055 0.025 2.20

PCL-PU-4 0.040 0.040 1.00

PCL-PU-5 0.025 0.055 0.45

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M. Ahmad et al.

changes in the material’s properties to account for the

design requests and optimized device performances. In this

research, different polyols and hard segments (MDI and IPDI

at different molar ratios) were used to conduct a systematic

investigation on their effects on the SMPUs properties, and

to obtain a process which can be used to produce SMPUs

with tailored and controllable properties for suitable

medical device applications. Also to investigate how the

IPDI affect the properties of SMPUs at different concentra-

tions.

PCL-PU-6 0.013 0.067 0.19

PCL-PU-7 – 0.080 0
Experimental Part
In order to produce SMPUs, five different polyols and two

diisocyanates were selected. The polyols used as soft segments

are polycaprolactone (PCL), poly(ethylene adipate) 2 200 (PEA),

polyether-type polyols of poly(tetramethylene glycol) (PTMG) and

two polyester-type polyols of poly(butylene adipate) (PBA) and

(PBA-U). The latter (PBA-U) has a shorter reaction time and

narrower molecular weight distribution than the former (PBA).

These polyols have good resistance to wear, abrasion, solvents and

oil, are very stable in long-term heating, and possess strong

hydrophobic characteristics.[26] The molecular weights and func-

tionality are Wm¼ 2 000 g �m�1 and f¼55–58, respectively, for all

the polyols used which is believed to be able to produce high

crosslinks in soft segments.

HDI and MDI hard segments in SMPs have been investigated

intensively, while IPDI hard segment in SMPs has received little

attention though it is one of the main cycloaliphatic diisocyanates

used for the preparation of light-stable and stiff polyurethanes. IPDI

and MDI were used as a part of the hard segment. Besides, chain

extender 1,4-BDO and PEG-200 were also chosen as the hard

segments to increase the crosslink of hard segment and increase the

flexibility. The molar ratio of MDI and IPDI was varied to optimize

the SMPUs with high quality.

A series of SMPUs were synthesized by keeping a constant molar

ratio of the hard segment to the soft segment (hard segment was

chosen at 42% as it was confirmed to be the optimal content.[17])

while varying the chemistry of the hard segment and soft segment.

The concentrations of polyols, PEG200 and BDO were fixed at 58,

11.5 and 2.5%, respectively, for all the samples, while the ratio of

MDI to IPDI (defined as g ¼MDI/IPDI hereafter) was varied from

MDI¼ 0 to 100%. Initially a series of SMPUs using PCL as a soft

segment were synthesized at different molar ratios of MDI to IPDI,

and their thermomechanical properties were investigated. From

these experiments, the reasonable MDI to IPDI molar ratios

(formulation 3, 4 and 5 as shown in Table 1 with g ¼2.2, 1 and

0.45, respectively) were selected to synthesize SMPUs with other

polyol soft segments.

The synthesis process established is as follows: a water bath was

used and purged with a continuous flow of dry nitrogen and

equipped with a mechanical stirrer and a thermometer to monitor

the temperature. A desired quantity of the polyol was added into

the flask containing N,N-dimethylformamide (DMF) solvent,

followed by adding a required amount of IPDI. The chemicals were

allowed to react for 2 h at 90 8C, then PEG-200 was added into the

reaction flask, followed by adding MDI in the flask. The mixture was



stirred for 1 h at 90 8C. Two drops of dibutyltin dilaurate were then

added as a catalyst, and finally the required amount of the chain

extender was added and mixed by stirring for an hour at 60 8C. To

cast a SMP film, the synthesized SMPU solution was poured onto a

PTFE-coated glass mold, and baked at 60 8C for 12 h, followed by

baking at 80 8C for 24 h, and a further baking at 100 8C for 8 h, all in a

vacuum oven. The typical film thickness for all the samples is

between 0.5 and 1.0 mm after drying.

Wide-angle X-ray diffraction (XRD) was used to investigate the

crystal structures of the SMPUs with a scanning angle 2ubetween 5

and 408 (D8 Discover X-ray diffractometer equipped with a Goebel

mirror and Cu Ka radiation with a wavelength of 1.542 A) at 40 keV

and 40 mA.

The thermal properties of the polyols and SMPUs were measured

by differential scanning calorimetry (DSC, Perkin-Elmer DSC7)

purged with nitrogen gas. The specimens were scanned from 20 to

150 8C at a heating rate of 10 8C �min�1. The weight of the

specimens used for analysis was typically 10.5–15.0 mg. Repeated

X-ray and DSC measurements were conducted. It showed good

repeatability for all the samples. The results shown in the paper are

typical behavior measured.

Tensile testing properties including stress/strain, tensile

strength, shape recovery and shape fixity were tested using Instron

Universal Tester. The specimens were made into dumb-bell shapes

with a length of 25 and a width of 7 mm. The crosshead speed was

set at 50 mm �min�1 for all the experiments. The tests were

performed at ambient temperature. The typical stress-stain curves

were used to extract the maximum strain and tensile strength at

break.

The typical thermomechanical cyclic test has been used to

characterize the SMPU synthesized. A test process of the thermo-

mechanical cycles has been previously described by Lendlein and

Kelch[13] and Tobushi et al.[28] For convenience, the test procedure is

schematically shown in Figure 2.

In the first path (A-B), the SMP sample is deformed to a fixed

extension, 50% for all the samples tested here, by applying a stress

sm, at a temperatureTh> Tg, resulting in a strain maximum em (note

this is different from the maximum strain which is defined as

elongation at break). In the second path (B–C), the strain is

maintained and the SMP is allowed to cool to a temperature TI<Tg.

The cooling process results in an increased stress referred to as

‘‘stress thermal’’. In the third path (C–D), the generated stress in the

011, 212, 592–602

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Figure 2. Thermomechanical cycle test routines used to test theshape-memory polymer.[27]


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SMP sample is removed completely by unloading the sample. The

strain reduces slightly at a value of eu. If the unconstrained sample

is heated to a temperature aboveTm, most of the strain generated at

the first stage of the cycle will disappear (D–F), corresponding to the

shape recovery. The remaining stress is defined as residual ep. If the

constrained sample is heated above Tm (D-E), it leads to

development of stress called ‘‘recovery stress.’’ The relationships

between the strain em, eu and ep are used to calculate the rates of

Shape fixity, Rf and Shape recovery, Rr, of the SMP as expressed by

Equation (1 and 2):

www.M

Rf ¼"u

"m� 100% (1)

Rr ¼"m�"p nð Þ

"m�"p n�1ð Þ � 100% (2)

where n and n�1 refer to the nth and nth�1 cycle for cyclic testing

using the above defined stress, strain, temperature cycle. Three

cyclic tests were used for all the samples. Also all values such as Tg,

strain to break etc extracted from DSC, tensile and shape-memory

effect tests were average values of three tests. There are no

standard deviation values due to limited test number, but the

deviations from the average values were all less than 10%.

A physical shape recovery test was performed on a hot plate to

examine the shape-memory effect and actuation behavior of the

synthesized SMPUs. The temperature was measured by infrared

digital thermometer.

Figure 3. X-ray curves of the PCL (a), PTMG (b) and PBA-U based(c) SMPU samples.

Results and Discussions

XRD

The typical XRD curves in Figure 3(a–c) show the number of

prominent peaks and the intensity of the peaks. The

distance between the crystal planes of the SMPU can be

calculated from these X-ray curves and are summarized in

Table 2, together with the corresponding results of the pure

polyols for comparison. It is worth mentioning that the

peaks in different sample series with similar peak positions

in XRD curves were classified as the same X-ray peak

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number only for simplicity. Most of them originate from

different crystalline structures as different polyols were

used for these samples, but they will not be investigated in

detail as it is beyond the scope of the paper.

As shown in Figure 3(a), the PCL sample series are

dominated by three peaks around 2u¼ 21, 22 and 248,similar to those observed in the pure PCL polyol with some

shifts to high angle due to stress once the hard segments

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Table 2. Listed calculated distances between parallel crystal planes.

Sample Distance between parallel lattice planes (nm)

Peak 1 Peak 2 Peak 3 Peak 4 Peak 5

PCL -2000 (P) 0415 0405 0.367

PCL-PU-1 0.415 0.404 0.377

PCL-PU-2 0.416 0.405 0.377

PCL-PU-3 0.418 0.405 0.379

PCL-PU-4 0.417 0.406 0.379

PCL-PU-5 0.418 0.407 0.379

PCL-PU-6 0.405 0.371

PCL-PU-7 0.408 0.371

PTMG-2000(P) 0445 0.365

PTMG-PU-3 0.448 0.368

PTMG-PU-4 0.447 0.367

PTMG-PU-5 0.442 0.364

PBA-U-2000(P) 0 417 0405 0397 0.370

PBA-U-PU-3 0.417 0.405 0.399 0.369

PBA-U-PU-4 0.418 0.406 0.399 0.370

PBA-U-PU-5 0.415 0.401 0.394 0.367

PEA-2000(P) 0.438 0406 0.367

PEA-PU-3 0.435 0.406 0.364

PEA-PU-4 0.433 0.405 0.363

PEA-PU-5 0.425 0.401 0.357

PBA-2000(P) 0 415 0402 0398 0.367

PBA-PU-3 0.417 0.405 0.399 0.369

PBA-PU-4 0.417 0.405 0.400 0.369

PBA-PU-5 0.410 0.397 0.396 0.365

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M. Ahmad et al.

were added. The intensities of the peaks for the pure polyol

are the highest, and decrease dramatically once high

concentration MDI diisocyanate was added. However

increase in IPDI content gradually increases the XRD

intensities for all peaks, indicating that the crystallinity

of the polymers increases with IPDI content, in agreement

with the observation by Yang et al., i.e., linear shape PDI

seems to have some advantage over bent-shaped MDI in

getting more crystalline structure when they stacked in

polymeric chains.[15] Although the peak intensities

increase, the crystal structures remain almost unchanged

from those of the pure polyols when the IPDI content

increases, leading to an important conclusion useful in

explaining the shape-memory results later: high concen-

tration IPDI in polymers may prevent or restrict the reaction

and crosslink between polyol and diisocyanate, and a large

amount of soft polyol and diisocyanate hard segments may

co-exist as separated ‘‘macrophases’’. Figure 3(b) and (c) are



for PTMG and PBA-U sample series, showing similar

behaviors as those of PCL sample series. A slight shift

towards higher diffraction angles was observed for all

SMPUs series with increased IPDI content, and it is believed

to be due to increased phase separation between soft and

hard segment and increased stress as the concentration of

hard segment IPDI increases.

Thermal Properties

Before investigating the thermomechanical properties of

the SMPUs synthesized, the thermal properties of the five

pure polyols were investigated using DSC. The scan curves

of the pure polyols are shown in Figure 4. The results

showed these polyols are thermoplastic materials with

melting temperature, (the transition temperature Tg for

these polyols is dominated by melting temperature) Tm, in

the range between 40 and 60 8C, much lower than those of

011, 212, 592–602


Figure 4. DSC curves of five pure polyols used as the soft segmentto synthesize the SMPUs.

Figure 5. DSC results for PCL and PBA-U-based SMPU samples.


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the hard segments used in SMPUs. The PTMG polyol has the

lowest melting temperature range of 35–45 8C, while

the PBA-U sample has the highest melting temperature,

in the range of 55–62 8C.

Figure 5(a) and (b) show the DSC curves of the SMPUs

with PCL and PBA-U polyols as the soft segment. In forming

SMPs, the melting temperatures of the polyol soft segments

determine the transitional temperatures of the correspond-

ing SMPUs. The SMPUs exhibited a reduced transition

temperature compared to the melting temperature of the

pure polyols, from a peak temperature of 52 to 39–44 8C for

the PCL sample series with different MDI/IPDI molar ratios,

and from a peak temperature of 59 to 45–48 8C for the PBA-U

SMPU sample series. The ranges of the starting melting

temperature to stopping one for the SMPUs remained

largely unchanged as compared to those of the pure polyols.

Although the reaction of polyols with hard segments leads

to a reduction in the peak transition temperature by more

than 10 8C, the large change of the molar ratio seems to have

a limited impact on the transition temperature range of the

SMPUs. On the other hand, the SMPUs with different polyols

have a large variation in the transition temperature as

summarized in Table 3, largely determined by the soft

segments used. These results clearly show that the

chemistry of polyols plays a major role in thermal

properties of the SMPUs.

The results of the enthalpy or the heat of fusion, DHf, of

the transition are summarized in Table 3 together with

other parameters for all the samples. The pure PCL polyol

has the highest DHf value of 76.6 J � g�1, implying a high

crystalline nature. The DHf values decrease drastically once

high MDI ratio hard segments were introduced to form the

shape-memory polymer, possibly due to disruption and

reduction of crystallinity.




As it can be seen from Table 3, PCL-PU-1 with 100% MDI

(no IPDI was added) has the lowest DHf value of 8.6 J � g�1.

The DHf increases rapidly as the IPDI content increases and

reaches a value of 32.64 J � g�1 for the sample with 100%

IPDI. The same trend can be seen from the SMPUs with other

polyols. A high value of DHf is an indication of high

crystallinity. Compared with the pure polyols, the drastic

decrease of the enthalpy value at high MDI concentration

implies a better reaction and crosslink between soft and

hard segments to form microphases as shown in Figure 1.

However, the DHf value increases with the IPDI content,

indicating increased crystallinity, consistent with the

observation from the XRD measurements. On the other

hand, as shown in Figure 3, there is no additional peak

appeared except those from the pure polyols when the hard

segments are introduced in all the polymers. The results

further indicate that a high concentration of IPDI may

prevent or restrict the reaction and crosslink between the

polyols and hard segments. It is generally believed that high

crystallinity and microphase separation are two of the

necessary conditions for shape-memory effect. Our results

011, 212, 592–602


Table 3. Heats of fusion (DH), peak temperatures (Tpm), andtensile strengths of the samples.

No. Sample Tpm DH Stress

at break

Strain

at break

-C J � g�1 MPa %

1 PCL(P) 51.6 76.6

2 PCL-PU-1 40.3 8.6 6.99 930

3 PCL-PU-2 41.3 18.8 6.24 869

4 PCL-PU-3 44.3 23.4 5.64 790

5 PCL-PU-4 42.6 20.5 4.39 334

6 PCL-PU-5 43.3 27.2 3.06 192

7 PCL-PU-6 39.0 30.1 2.81 104

8 PCL-PU-7 39.3 32.6 2.21 37

9 PTMG(P) 41.3 106.8

10 PTMG-PU-3 27.4 30.6 2.47 985

11 PTMG-PU-4 24.4 27.8 2.09 625

12 PTMG-PU-5 26.0 33.8 0.94 17

13 PBA-U(P) 58.3 73.6

14 PBA-U-PU-3 44.6 23.6 8.55 796

15 PBA-U-PU-4 46.3 33.1 4.61 411

16 PBA-U-PU-5 47.3 40.0 3.44 63

17 PEA(P) 55.3 63.2

IS PEA-PU-3 42.3 19.4 5.53 784

19 PEA-PU-4 41.3 26.7 4.43 328

20 PEA-PU-5 42.3 27.3 1.53 119

21 PBA(P) 55.0 55.2

22 PBA-PU-3 4 8.6 35.8 7.32 631

23 PBA-PU-4 43.6 34.4 4.53 293

24 PBA-PU-5 46.6 38.0 3.23 61

Figure 6. Stress/strain tests for PTMG SMPUs at ambienttemperature, showing decreased elongation at break as the IPDIconcentration increases.

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M. Ahmad et al.

showed that SMPUs gradually lose the shape-memory

effect, as shown later, as the IPDI concentration increases

even though the crystallinity of the polymer increases.

Therefore it is believed the crosslink between the IPDI and

polyols is very poor, and polyols and hard segments coexist

as ‘‘macrophases’’ which are different from the micro-

phases or block polymers required for a shape-memory

effect. In other words, inclusion of high content MDI leads to

better and uniform reaction and crosslink of polyol and

hard segment to form separated microphases required for

shape-memory effect, while increase of IPDI content

restricts the chemical reaction and cross link.

It should also be noted that the physical appearance of

SMPUs with low and high IPDI concentrations is very

different. At low IPDI content, the SMPUs are semitran-

sparent, and they soften when heated to above Tg, showing

a typical thermoplastic characteristic. Whereas the SMPUs



with high IPDI content are whitish, rigid like wax with poor

elongation as shown later, and they melt at T> Tg.

Tensile Properties

Elongation at Break

The maximum strain, defined as the elongation strain at

break, of the SMPUs synthesized varies significantly with

the change of the molar ratio of MDI to IPDI. Figure 6 shows

an example of the stain in percentage produced from the

PTMG-based SMPUs series at room temperature (�20 8C).

The stress increases rapidly with the strain initially, reaches

a maximum and then drops suddenly. It is followed by a

slow increase with strain. The drop of the stress is believed

to be associated with the breakdown of some molecular

chains/bonds which are destroyed and are unrecoverable.

The maximum strain for sample PTMG-PU-3 with a high g

value of 2.2 reaches � 985%, and is followed by � 625% for

PTMG-PU-4 (g ¼ 1.0). The maximum strain for the PTMG-

PU-5 (g ¼ 0.45) is about 17%, as the shape-memory polymer

is a wax-like polymer with less flexibility.

Similar trends were observed for the other series of

SMPUs synthesized and the results are summarized in Table

3. All the SMPUs with a molar ratio of g ¼ 2.2 showed an

elongation at break between 600 and 1 000%, about 300–

600% for those with a molar ratio of g ¼ 1.0, and 15–200%

for those with a molar ratio of 0.45. It is clear that a high

molar ratio, i.e., high MDI and low IPDI contents can

significantly enhance the elongation at break. The samples

with molar ratio less than 0.45 are ‘‘wax-like’’ polymers, and

are unable to be stretched at an extension beyond 30%. As

shown in the XRD and DSC measurements, as the IPDI

content increases, the crystallinity of the SMPUs increases

with large phase incompatibility between polyols and hard

segments. This is believed to be responsible for the

deteriorated elongation at break for SMPUs with high IPDI

011, 212, 592–602


Figure 7. Stress vs. strain up to 50% extension at 40 8C (a), stressthermal generated during cooling down (b) and recover stressgenerated as the temperature rises for PTMG-PU-3 with the cyclictest number as a variable.


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concentrations. It therefore can be concluded that increase

in MDI molar ratio can result in an enhanced elongation.

It is worthy to mention that a maximum strain up to

�1 000% is one of the best values reported for SMPs,

showing their excellent properties and great potential

for applications.[28]

The maximum tensile strength, defined as the stress at

which the sample breaks, varies with the types of the

polyols used. Regardless of the type of the polyols used, the

maximum tensile strength decreases as the molar ratio

decreases or IPDI content is increased, as summarized in the

Table 3. The results confirm that a high MDI content leads to

high tensile strength and high elongation at break for these

SMPUs.

Comparison of the results also revealed that the PTMG-

based SMPUs have the lowest maximum tensile strength of

�2.5 MPa though they have the highest maximum strain of

�1 000%, the PCL and PBA samples series have similar

tensile strengths, in the range of 7–8.6 MPa, 2–3 times

higher than that of the PTMG-U samples series, while the

PEA sample series have the values between the PCL and

PTMG-U sample series. The results clearly show the nature

of the polyols and bonds of polyol with diisocyanate, as well

as the contents of the hard segment determine the tensile

properties of the SMPUs.

Recovery Stress

Figure 7(a) is a typical behavior of the stress vs. strain of the

synthesized SMPUs extended up to 50% at 40 8C. The stress

obtained at 50% extension for the first cycle is around

0.32 MPa, and it decreases rapidly to 0.24, and 0.21 MPa for

the second and third tests, respectively. The stress drop from

the 1st test to the 2nd test is large, and slows down from the

second test to the third test, which is mainly due to broken

chains in the polymer which are unrecoverable. This is also

reflected in the increased residual stress as the cyclic test

was increased.

The stress thermal is defined as the stress generated

when deformed SMPU sample cools down to room

temperature by holding a constraint in a thermomechani-

cal cyclic test as shown in Figure 2. The curves shown in

Figure 7(b) were obtained after the samples were stabilized

for �5 min at 50% extension for the stress/strain tests

[Figure 7(a)]. Initially there is a quick decrease in stress as

the temperature is reduced from 40 to 25 8C as shown in

Figure 7(b), then it gradually slows down. A slight increase

in stress value was observed as the temperature was

decreased further from 18 to 10 8C. This is probably due to

freezing out of the molecular chains at �10 8C. For the 1st

cycle test, the stress thermal generated is �0.18 MPa, and

become �0.15 and �0.13 MPa at the second and third cycle

tests, respectively. The broken molecular chains/bonds and

relaxation are responsible for the reduction of the stress

thermal for each cyclic test.




For our proposed SMP bandage, the recovery force (stress)

generated by a pre-extended SMP sample when heated is the

most important parameter. The recovery stress for

these samples was measured by reheating the pre-extended

samples which were reloaded at the lowest temperature

with no initial stress, and the results are shown in Figure 7(c).

The recovery stress starts to build up even at �20 8C, much

lower than the transitional temperature, and gradually

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Figure 8. Cyclic recovery stress for all the SMPU samples with a pre-extension of 50%. The values were extracted at 40 8C as shown inFigure 7(c).

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M. Ahmad et al.

increases with temperature. The full recovery stress develops

at a temperature where the sample initially was deformed.

The recovery stress generated in the 1st cycle at the highest

testing temperature is 0.12 MPa, and becomes 0.08 and

0.07 MPa for the second and third cycle tests, respectively. A

dramatic reduction in recovery stress after one cyclic test is

due to the broken molecules chains/bonds and relaxation of

the molecular structures in the polymer.

The same tests up to an extension of 50% were applied to

all the other SMPUs synthesized, and the recovery stresses

are summarized in Figure 8. The reduction in recovery stress

is significant from the first to the second cycle test, and

decreases as the cyclic test progressed for all the test

samples. On the other hand, the values of the strain, stress

thermal and recovery stress for each sample series increases

as the molar ratio g value decreases. It is believed that

increased crystallinity at high IPDI content increases the

stiffness, hence the recovery stress of the SMPUs. For

samples with a molar ratio g < 1.0, no recovery cyclic tests

were performed as all of them break before being extended

to 50%, as discussed above.

Figure 9. SMPs shape recovery rate for all the SMPUs studied.



Shape Recovery and Shape Fixity

Shape recovery is one of the most important parameters

used to assess the quality of a SMP. It can be calculated from

the results of thermomechanical cycles by Equation (2). The

results of the shape recovery for all the samples with

different polyols are summarized in Figure 9.

The shape recovery of all the SMPUs with a maximum

strain >100% ranges from 70 to 98%, depending on the

molar ratio and polyols used. Sample PTMG-PU-3 has the

highest shape recovery rate of 98% among all the tested

SMPUs, while the PBA sample series have the lowest

recovery rate of less than 80%. The shape recovery rate for a

series of SMPU using the same polyol decreases with

increase of IPDI for all the tested samples, typically at the

rate of �10% as the molar ratio was decreased from 2.2 to

1.0. For the PCL-based SMPUs, it deceases from 87 to 79% as

the molar ratio g was decreased from 1 to 1.0. As all the

SMPUs with a molar ratio g < 1.0 melt at >40 8C, no

shape recovery experiment can be performed for these

samples.

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Figure 10. Shape fixity rate for all the SMPUs investigated.


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The shape fixity is another key parameter to assess the

ability of SMPs for maintaining a temporary shape after the

applied load is removed. The shape fixity can be calculated

from Equation (1) from the thermomechanical cycles.

Usually the temporary shape is different from that achieved

by straightforward deformation. Figure 10 is the summary

of the shape fixity for all the SMPUs synthesized. No shape

fixity test was performed for these with a molar ratio lower

than g < 1 as they melt at T> 40 8C. The shape fixity for all

the SMPUs synthesized is in the range of 65–90%,

depending on the polyols used in the cyclic tests conducted.

SMPUs made of the PCL, PBA-U and PEA with a molar ratio

g ¼ 1 have the highest fixity of �90%. The fixity of the

SMPUs was found to increase from 78 to �90% for the PCL

sample series as the IPDI content increases from 0 to �70%.

The same trend was found for all other SMPUs. The shape

recovery and fixity characteristics observed can be

Figure 11. Shape recovery of a PCL-PU-3 based actuator on a hot plate at 60 8C. It returnsto its original shape completely at� 50 s. The temperatures were measured by infrareddigital thermometer.

explained by the crystallinity and macro-

phase separation in the samples. Gener-

ally it is believed that more crosslinks

between the soft and hard segments

improve the shape recovery for SMPUs.

The above X-ray and DSC results showed

that a high MDI in SMPU leads to more

crosslinks and uniform reaction. As the

IPDI concentration increases, it restricts

crosslink and chemical reaction between

soft and hard segments to form good

polymers, the polyols and hard segments

co-exist as macrophases. The polymers

become wax-like materials with poor

flexibility as IPDI restricts the segmental

and short-range conformational move-

ments of soft segment chains. Its recovery

rate therefore drops while the fixity rate

increases. It is worthwhile to mention

that the fixity higher than 90% is among

the best results reported so far. Unlike the




recovery stress, both shape recovery rate and fixity do

not show significant reduction during the multi-cyclic

tests.

Shape Recovery Test

Physical shape recovery tests were performed using

samples with a thickness of 0.5 mm, width of 25 mm and

length of �70 mm, one example (sample of PCL-PU-3) is

shown in Figure 11. The polymer sample was rolled after

being heated to 60 8C, and the deformed shape was fixed by

quick cooling to ambient temperature (�20 8C) as shown in

Figure 11(a).

The sample was then put on a hot plate at 60 8C and its

shape changes were recorded. The sample started to unroll

gradually at 10 s, and fully recovered to its original shape at

�50 s, showing an excellent actuation capability. The tests

showed all SMPUs have the ability to return to their original

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M. Ahmad et al.

shapes, and the actuation time did not show visible change

from sample to sample with different polyols.

Conclusion

Polyurethane SMPs have been synthesized using five

different polyol soft segments and two different diisocyan-

ates as hard segments. The effects of the molar ratio of the

two diisocyanate hard segments and the chemistry of the

polyols on SMP properties have been investigated using

XRD, DSC, thermomechanical and shape-memory effect

tests. The transition temperature of the SMPUs can be

tailored between 30 and 45 8C by varying the MDI to IPDI

molar ratio, close enough to the body temperature for

medical device applications. SMPUs with a maximum

strain in the range of 600–1 000%, recovery rate of 80–98%,

fixity of 70–90% have been obtained. SMPUs with a high

content of MDI hard segment have a high maximum strain

and stress and high recovery rate. Increase in the IPDI

content in the SMPUs deteriorates the shape-memory

effect. The results indicate the existence of high content IPDI

may restrict chemical reaction and crosslink between

polyols and hard segments. SMPU-based prototype actua-

tors showed excellent shape recovery at temperatures

higher than the transitional temperature.

Acknowledgements: The authors would like to acknowledgepartial financial support from the Engineering and PhysicalSciences Research Council under grant no. of EP/F06294, theLeverhulme Trust under the grant no. of F/01431 and the RoyalSociety for the UK/China International Joint Project. The Knowl-edge Centre for Materials Chemistry under the grant no. ofX00680PR, and Royal Academy of Engineering-Research Exchangeswith China and India Awards, Royal Society-Research Grant(RG090609), Royal Society of Edinburgh and Carnegie TrustFunding. We are particularly grateful to Willy Rau (Vast SpringEnterprize Co., Taiwan, China), Michael Austin (Perstorp, UK) andJames Hoyland (Huntsman Polyurethanes, UK) for their technicalsupport and free of charge samples for this project.

Received: September 7, 2010; Revised: November 26, 2010;Published online: January 28, 2011; DOI: 10.1002/macp.201000540



Keywords: diisocyanates and polyols; maximum strain;nanoparticles; polyurethanes; thermomechanical properties

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