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