Shape memory polyurethanes containing azo exhibiting photoisomerization function

Article abstract of DOI:10.1039/c0jm01944e

A series of azobenzene-containing polyurethanes (azoPU) was synthesized. The structure of the azoPU and the synthesis process were detected by FTIR and nuclear magnetic resonance (NMR), and the transition temperature was determined by differential scanning calorimetry (DSC). Tensile and cyclic thermomechanical experiment results revealed that excellent mechanical properties, shape fixity (R_f), and shape recovery (R_r) were obtained by the addition of azo to the chain of PU. R_r and R_f of azoPU increased with the increase of hard segment (HS) content. The higher HS content enhanced interaction among polymer chains as the chances of induced dipole-dipole interaction between aromatic rings increased in the presence of azo in the main chain. The materials presented trans-cis isomerization under UV irradiation in addition to the shape memory effect. The UV-vis spectrum indicated that photoisomerization occurred both in solution and solid state. It is expected that the work may be helpful in expanding the application of shape memory PU in areas of drug release and optical data storage.

Full text of DOI:10.1039/c0jm01944e

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Shape memory polyurethanes containing azo exhibiting photoisomerization
function†
ab
ab
a
a
a
Yaoming Zhang, Chao Wang, Xianqiang Pei, Qihua Wang* and Tingmei Wang
Received 19th June 2010, Accepted 30th July 2010
DOI: 10.1039/c0jm01944e
A series of azobenzene-containing polyurethanes (azoPU) was synthesized. The structure of the azoPU
and the synthesis process were detected by FTIR and nuclear magnetic resonance (NMR), and the
transition temperature was determined by differential scanning calorimetry (DSC). Tensile and cyclic
thermomechanical experiment results revealed that excellent mechanical properties, shape fixity (R
f
),
and shape recovery (R ) were obtained by the addition of azo to the chain of PU. R and R of azoPU
r
r
f
increased with the increase of hard segment (HS) content. The higher HS content enhanced interaction
among polymer chains as the chances of induced dipole–dipole interaction between aromatic rings
increased in the presence of azo in the main chain. The materials presented trans-cis isomerization
under UV irradiation in addition to the shape memory effect. The UV-vis spectrum indicated that
photoisomerization occurred both in solution and solid state. It is expected that the work may be
helpful in expanding the application of shape memory PU in areas of drug release and optical data
storage.
2,5,23
1
. Introduction
could control the transition temperature (T ) and SME.
s
Recently, more work concentrates on the design of variable
groups in the chain. Hu et al. tried to introduce the ionomers into
SMPU, which has a pyridinium-containing HS, suggesting the
new polymer possessed an antibacterial function as well as good
Shape memory polyurethane (SMPU) composed of both
reversible phase and fixed phase are attractive candidates for
potential smart applications, such as in biomedical fields,
because the shape memory polymers can be tailored to exhibit
biocompatibilities and body temperature capabilities by intro-
24
SME. Jeng and co-workers have demonstrated that dendritic
chains as extender in PU could improve the mechanical and
shape memory properties due to the formation of more hydrogen
25
ducing special functionalized soft segment (SS) and hard segment
1
(
HS) into the polymers. SMPU possesses shape memory effect
SME) due to microphase separation between soft domain and
bonds in the PU molecular chains. So far, the other operations
(
that could induce shape recovery of the SMPU have been
drawing research interest. For example, Huang et al. found that
moisture could also trigger the thermal SMPU. The reason for
hard domain; the soft domain changes its shape under external
stress and then the external stress was released with the subse-
quent cooling. When reheated to the Ttrans, SMPU recovers its
g
this phenomenon is that the T of SMPU samples decreases as
2–7
original shape which is controlled by the HS.
Nowadays,
the hydrogen bonding among PU chains is weakened due to the
26
bound water.
increasing focus has been paid to shape memory polymer, and
a variety of new polymers with SME have been studied, such as
It is well known that the azo-containing polymers (azopoly-
8
–11
12,13
PCL block segment polymers,
polyester,
crosslinked poly-
18–20
mers) have the capacity of reversible trans-cis isomerization when
27–30
14
15–17
(
lactic acid), blend polymers,
biomaterials
and so on.
exposed to UV-vis irradiation.
The azopolymers are more
Additionally, SMPU is widely studied in the bionic field due to its
thermally stable and have better mechanical properties than their
respective monomers, which have been used to produce optical
sensors. In the present paper, azo was introduced into the PU
main chains, and its photoisomerization function was studied. It
is expected that this new type of SMPU with azo-groups may find
potential applications in the field of medicine, such as drug
21,22
biocompatibility, biodegradation,
temperature.
and tunable transition
In the past few years, a large amount of papers dealing with the
alteration of the SMPU architecture have been published, and
these contributions mainly focus on the synthesis and charac-
terization of the new function of the SMP. At the beginning,
Jeong and Kim studied the SMPU by adjusting the number
9,31,32
release and cerebral aneurysm reparation.
average molecular weight M and the ratio of HS and SS, which
n
2
. Experimental section
a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, Lanzhou, 730000, P. R. China.
E-mail: wangqh@lzb.ac.cn; Fax: +86 931 4968180; Tel: +86 931 4968180
2.1 Materials
b
0
Graduate School of Chinese Academy of Sciences, Beijing, 100039, P. R.
China
Epsilon-caprolactone (CL) and 4,4 -methylenediphenyl diiso-
cyanate (MDI) were received from Acros. 1,4-Butanediol
1
Electronic supplementary information (ESI) available: H NMR
†
spectra of the PCL and 4,4 -dihydroxyazobenzene; the synthetic
procedure for 4,4 -dihydroxyazobenzene in DMSO-d
samples A–F. See DOI: 10.1039/c0jm01944e
(
BDO), 4-nitroaniline, ethylene glycol, dibutyltin dilaurate
0
0
(DBTDL) and N,N-dimethylformamide (DMF, 99%) were
bought from Tianjin Chemical Reagents Company. CL and
6
; DSC curves for
9
976 | J. Mater. Chem., 2010, 20, 9976–9981
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DMF were dried with CaH2 for 24 h and freshly distillated
before use.
was added in a dry three-necked tube. The mixture was stirred
ꢀ
under nitrogen at 65 C for 2 h to prepare the pre-polymer. The
azo solution in DMF was subsequently added, followed by
addition of DBTDL, and then the temperature was adjusted to
ꢀ
2
.2 Preparation of polycaprolactone diols prepolymer PCL-
1
10 C for another 11 h to complete the polymerization.
diols (4500)
A certain amount of CL and ethylene glycol with a stoichio-
metric ratio were mixed in a flask, and then 0.05% stannous
octoate was added as catalyst. The reaction was carried out
2
.5 Film preparation
To prepare the film samples, PU solution in DMF was cast on
ꢀ
ꢀ
under pure argon at 130 C for 24 h. The polymer was precipi-
a Teflon plate and dried at 80 C for 24 h to evaporate most of the
tated in cold diethyl ether and then deterged by methanol. Then
ꢀ
DMF, after which the film samples were further solidified in
ꢀ
the product was dried in vacuum at 30 C to a constant mass. The
a vacuum oven at 80 C for another 24 h. The thickness of the
molecular weight of the prepolymer was determined by gel
permeation chromatography (GPC).
film was controlled to be about 0.2 mm.
The proton resonance data are in agreement with the expected
2
.6 Nuclear magnetic resonance (NMR)
values: (chloroform-d
1
): d (ppm) 4.24 (s, –O–CH
–CH –); 2.28 (m, –O–CO–
2
–CH –); 1.37 (m, –CH –CH –
2 2
–CH –O–));
1
H NMR (400 MHz) spectra were recorded in DMSO-d and
6
4
.03 (t, –O–CH –); 3.62 (t, HO–CH
2
2
x
CDCl as solvent.
3
CH
CH
2
–CH
–); T
x
); 1.58 (m, –CH
x
–CH
2
x
x
ꢀ
¼ 107.16 J gꢁ1.
x
m
¼ 59.94 C, DH
m
2
.7 Differential scanning calorimetry (DSC)
0
2
.3 Preparation of 4,4 -dihydroxyazobenzene
DSC was carried out with a Mettler-Toledo instrument
e
DSC 822 ) with a heating or cooling rate of 10 C min . The PU
ꢀ
ꢁ1
(
The diazotization-coupling reaction was conducted according to
ꢀ
33
sample was heated to 250 C and kept at this temperature for
ꢀ
the methods outlined in the literature. Briefly, 4-hydroxy-
aminobenzene was firstly dissolved in hydrochloric acid solution,
and then the sodium nitrate solution was added dropwise with
5
min and then cooled to ꢁ100 C in the first run in order to
remove the thermal histories. In the second scan, the sample was
ꢀ
ꢀ
ꢀ
heated again to 250 C and cooled to ꢁ50 C, from which the
glass transition temperature (T ), melting temperature (T ) and
stirring at 0–5 C. After the diazotization, the sodium hydroxide
g
m
solution containing phenol was added to the diazonium salt
solution. Meanwhile, the pH value of the mixture was adjusted to
crystal temperature (T ) were determined.
c
6
–7 by adding HCl. Then the orange red precipitate was collected
by filtration. The crude product was purified by recrystallization
1
from a mixture of ethanol and ethyl acetate (1 : 1, v/v). H NMR
Table 1 Composition and characterization of polyurethanes
Polymer
code
wt% azo
diols PCL : MDI : diol of HS (%)
X
c
(
DMSO-d
6
): d (ppm) 10.116 (s, 2H); 7.705 (d, 4H); 6.898 (d, 4H);
ꢀ ꢀ
m g
T / C T / C (%)
3
.334 (s, H O); 2.490 (s, DMSO).
2
A
B
C
D
E
F
azo 1 : 6 : 5
azo 1 : 5 : 4
azo 1 : 4 : 3
azo 1 : 3 : 2
azo 1 : 2 : 1
BDO 1 : 5 : 4
36.36 15.13 48.91 ꢁ24.72 24.6
31.89 12.96 49.32 ꢁ32.22 29.8
26.75 10.45 50.27 ꢁ28.42 32.5
20.73 7.47 51.01 ꢁ36.68 34.1
13.70 4.10 50.03 ꢁ37.24 37.6
2
.4 Preparation of azo-based PCL-PU [see Scheme 1]
The compositions of PU and its main characteristics are given in
Table 1. The following procedure was typical: MDI was dis-
26.37
0
52.13 ꢁ48.41 26.8
ꢁ1
solved in DMF and PCL with a molecular weight of 4500 g mol
Scheme 1 Synthetic procedure for SMPU.
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2
.8 UV-vis and FTIR
3. Results and discussion
UV-vis spectra were recorded on a Hitachi U-3010 spectropho-
tometer. IR spectra were measured using a Bruker IFS 66v/s IR
3
.1 Structural analysis of azoPU
ꢁ1
Recently, a series of azo-containing PU were synthesized. The
formulations of these PUs with different compositions are
shown in Table 1. A series of SMPU extended by diols were
previously synthesized by a two-step method. However, due to
the relatively low reactivity of phenol in comparison to
spectrophotometer within a range of 4000–400 cm .
2
.9 Atomic force microscope (AFM)
AFM was performed with a Nanoscope IIIa Multimode atomic
force microscope (AFM, Digital Instruments) at the tapping mode
using FESP probe (curvature radius < 10 nm, 87 kHz). The azoPU
was spin-coated onto silicon substrates. The films were allowed to
1
,4-butanediol (BDO), it was difficult to obtain PU extended by
phenolic hydroxyl. In the present study, the reaction time was
prolonged to 11 h at elevated temperature and in the presence
of a catalyst. The FT-IR spectra of (A) prepolymer, (B) sample
ꢀ
dry in an oven at 80 C for 24 h, and then annealed in a vacuum
ꢀ
ꢁ1
A are shown in Fig. 1. In Fig. 1A, the peak at 2273 cm ,
corresponding to the isocyanate group stretching vibration,
disappeared in the spectra of sample B, indicating the success
of the reaction between phenol extender and the residual
oven at 80 C for another 24 h to obtain the phase equilibrium.
2
.10 Mechanical and cyclic thermomechanical experiment
34,35
1
isocyanate group.
sample A diluted in DMSO. The peaks at 6.88–6.90 ppm and
.68–7.71 ppm, which could be ascribed to the phenolic
Fig. 2 shows the H NMR spectrum of
Tensile tests were performed on a Shimadzu AG-X. The strain
ꢁ1
rate was 10 mm min . To check the SME of the PU, the test were
carried out on a SANS tester (Shenzhen SANS Material Test
Instrument Co. Ltd., China) equipped with a controlled thermal
chamber. The shape memory test of films was carried out
7
hydroxyl (c, f) of azo, confirm the existence of azo groups.
Compared to the normal PU, there are two peaks at 10.11 and
9
.49 ppm corresponding to the amide group (a, b), which
h l
between T and T , during which the stress and strain was
indicates that there are two kinds of group attached to the
1
recorded. Samples were cut to standard dimensions according to
ISO 527-2/1BB. The cyclic thermomechanical tensile test was
2
4
amidocyanogen. The H NMR and FTIR spectra confirm the
expected structure of the azoPU.
carried out in the following order: (1) heating the sample to T
h
and stretching the sample to 100% extension with a constant
ꢁ1
crosshead speed of 10 mm min and holding for 5 min; (2)
cooling the sample to T with holding the shape, (3) reducing the
l
3.2 Thermal properties of azo-based PCL-PU
stress to zero at T and maintain at T ; (4) raising the temperature
l
l
The thermal transitions of azo-based PU were determined by
DSC. The curves from the second heating run are shown in
from T
l
to T
h
and keeping at T
h
for 5 min and measuring the
length of the sample. Under the above conditions, shape reten-
tion (R ) and shape recovery (R ) are defined as follows:
m c
Fig. S3, and the T and crystallinity (X ) are summarized in
r
f
Table 1. X was calculated according to the follow equation
c
3
u
R
f
¼
¼
ꢂ 100%
3
m
À
Á
X
c
¼ (DH
m
/DH100%) ꢂ 100%
3
m
ꢁ 3
p
R
r
ꢂ 100%
3
m
where DH
100%
is the theoretical heat of fusion of PCL
ꢁ
1 36,37
ꢀ
ꢀ
(135 J g ).
m
As can be seen, the T values of the PU were in
where T
h
¼ T
m
+ 25 C, T
l
¼ T
m
ꢁ 25 C, 3
m
¼ strain at 100%
ꢀ
ꢀ
the range between 48 and 51 C, which was lower than that of
ꢀ
elongation, 3
u
¼ retention strain at T
m
ꢁ 25 C, and 3
p
¼
ꢀ
pure PCL (59 C) as previously studied. It could be attributed the
recovery strain at T
m
+ 25 C.
fact that the introduction of azo and MDI to the molecular chain
decreased the crystallinity of the PCL segment and inhibited
crystal formation. The speculation was proven by the X values
c
in Table 1. Besides, it can be seen from Table 1 that the transition
temperature as well as the X modestly decreases with increasing
c
azo concentration. This is in agreement with the previous PCL
systems, which predicted that the incorporation of HS and PCL
inhibited the crystal formation and decreased the crystallinity of
SS. It is also noticeable that an inconspicuous T
ꢀ
g
was observed
g
of the SS of
between ꢁ37 and ꢁ24 C, corresponding to the T
PCL. In contrast, the azo-PU has higher crystallinity and lower
than those of PU extended with BDO. The difference between
T
m
these two materials indicated that the chain structure was
changed by introduction of azo. With the addition of azo to the
main chain, the rigidity increased and the induced dipole–dipole
interaction between aromatic rings increased. The crystallinity
was improved due to the better combination among the poly-
2
meric chains. The T
g
of PU was increased since the local chain
Fig. 1 FTIR spectrum of prepolymer azoPU (A) and sample A (B).
978 | J. Mater. Chem., 2010, 20, 9976–9981
mobility was constrained by enhanced interaction among HS.
9
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1
Fig. 2 H NMR spectrum of azoPU (sample A) in DMSO-d
1
(signal * existed also in the H NMR spectrum of pure MDI).
34
6
2
SS. In this paper, we choose PCL (4500) as the SS, which
3
.3 Mechanical and shape-memory properties of azo-based
PCL-PU
exhibits an obvious Tm and acts as switch segment. The shape
fixity was negligibly influenced by the varying amounts of HS
As described in the experimental section, the mechanical prop-
erty of PU was examined by tensile tests at room temperature,
and the results are summarized in Table 2.
content since the R is determined by the SS, except for sample E,
f
all the samples have a high R
f
above 96% with changing HS
was
content from 13.7% to 36.36% it indicates that the R
f
As can be seen from Table 2, Young’s modulus (E) increased
negligibly influenced by varying the amount of HS. In Table 2, it
is found that the shape recovery decreases gradually with the
decrease of HS content. When HS content increases from 13.70%
and elongation at break (3
R
) decreased with increasing HS, while
) remained almost constant
the maximum tensile strength (s
m
with values around 20–30 MPa. E was found to be strongly
influenced by the content of HS. With the azo increased from
r
to 36.36%, the R increased from 80% to 99.9%. As the HS
content increased, the ability to maintain a permanent shape is
increased by physical crosslinking. Sample A exhibited a perfect
ability to fix the temporary deformed shape and recover the
permanent shape. This excellent SME is attributed to the
enhancement of phase separation because of the strong interac-
tion among HS caused by azo. Lee et al. suggests that the domain
4
1
.10% to 15.13%, E was increased over a large range from 58 to
81 MPa, which can be ascribed to the enhancement of the
rigidity. However, it was not the case for 3
from 791% to 460%. Comparing azo-PU B with BDO-PU,
relatively higher E and s were observed for B in the presence of
R
, which decreased
m
azo, due to the enhanced rigidity and interaction between the
molecules as mentioned above after the introduction of azo
groups in the main chains.
2
formation is helpful to the SME. For example, the shape
deformation and recovery process for azoPU sample A is shown
in Fig. 3.
f r
The R and R of samples A–F are shown in Table 2. In the
previous studies, the shape memory properties of SMPU with
different kinds and variable content of SS and HS have been
studied, and the results indicated that the key factors influencing
SME were the molecular weight of SS and relevant content of
Table 2 Mechanical properties and shape memory properties of poly-
urethanes
Polymer
code
R
f
R
r
E/MPa
s
m
/MPa
R
3 (%)
A
B
C
D
E
F
100 ꢃ 0.1
99.90 181.07 ꢃ 5.10 21.74 ꢃ 0.86 468 ꢃ 24
98.52 ꢃ 3.5 95.17 164.11 ꢃ 3.05 31.68 ꢃ 1.20 780 ꢃ 12
97.03 ꢃ 0.1 87.50 100.73 ꢃ 1.02 20.01 ꢃ 0.20 677 ꢃ 53
96.31 ꢃ 2.2 85.00 66.48 ꢃ 2.30 24.06 ꢃ 1.56 709 ꢃ 67
83.79 ꢃ 0.4 80.00 58.25 ꢃ 2.86 21.51 ꢃ 2.22 791 ꢃ 90
95.02 ꢃ 0.3 90.00 80.30 ꢃ 4.52 21.41 ꢃ 1.57 611 ꢃ 32
ꢀ
ꢀ
Fig. 3 azoPU (sample A) showing shape memory effect at 75 C:
ꢀ
(
(
a) initial state; (b) deformed state (stretched at 75 C and fixed at 25 C);
ꢀ
c) recovered state after heating in oven at 75 C.
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3
.4 UV absorption and photochromic properties of azoPU
linked to the azo group, the isomerization would induce the
macroscopic change of the polymers. This result indicates that
the azo in azoPU can undergo trans-cis reversible photo-
isomerization both in solution and in the solid state.
Polymers containing azo units exhibited special strong absorp-
tion bands in the UV-vis spectra, which would be changed when
38–40
the isomerization occurred.
The UV-vis spectra are often
Fig. 5 shows the phase image of azoPU films before and after
UV irradiation. Before irradiation, the surface displayed phase
separation at the nanometre scale, while a more homogeneous
used to characterize the trans-cis isomerization of azo. The
azoPU in chloroform solution was irradiated with UV light (l ¼
2
54 nm) in 10 min intervals, shown in Fig. 4(a). The absorption
ꢀ
ꢀ
phase image was seen after the UV irradiation at 80 C (30 C
above the T ). The results revealed that UV irradiation led to
intensity of the p / p* transition band at 360 nm decreased and
the n / p* intensity of the transition band at 500 nm increased
with increasing irradiation time, which indicated the trans-
formation from trans form to cis form. To investigate the UV-vis
spectra and photochromic properties of the polymers in the solid
state, thin films of azoPU were prepared by spin-coating of 5% w/
w azoPU DMF solution onto clean glass plates (2 ꢂ 2 cm). For
comparison, films of sample A were subjected to UV irradiation
m
reformation of phase structure of the film. The bright regions
could be related to the HS due to its high modulus, which was
41,42
consistent with previous studies,
and it became more homo-
geneous under the UV irradiation. The change of morphology
indicated a high sensitivity of the film to UV light. The trans-cis
isomerization should change the length of the chain or dipole
43
moment, induced by the UV light, heat or other stimuli. Many
azopolymer films can form surface relief gratings induced by
a laser, which can be erased by laser or heating. This attracted
considerable attention for potential applications such as holo-
ꢀ
ꢀ
m
for 5 min under a temperature of 80 C (30 C above the T ), and
the UV-vis absorption spectra of film A is reported in Fig. 4 (b).
A significant shift of absorption peak was observed compared to
the sample in solution. It was inferred that the blue shift in
chloroform was probably due to the solvent effect. Under UV
irradiation at 254 nm for 5 min, the spectra change corre-
sponding to isomerization was exhibited. The cis form of the azo
which is thermodynamically instable was induced by UV light
due to the absorbed energy. When the macromolecules were
44
graphic gratings, optical information storage, and so on. The
reversible change between hydrophilicity and hydrophobicity
arising from trans-cis isomerization could induce dissociation
and aggregation, which has potential applications in drug
45
formulation.
Fig. 5 AFM phase image for azoPU film spin-coated on a silicon wafer:
films prior to UV irradiation (a) and the same film after UV irradiation
(254 nm) (b).
Fig. 4 UV-vis spectra of azoPU (sample A) with UV irradiation at
254nm, in chloroform solution (a) and film sample (b).
9
980 | J. Mater. Chem., 2010, 20, 9976–9981
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1
5 S. C. Li, L. N. Lu and W. Zeng, J. Appl. Polym. Sci., 2009, 112,
4
. Conclusions
3
341–3346.
6 H. Zhang, H. T. Wang, W. Zhong and Q. G. Du, Polymer, 2009, 50,
596–1601.
1
Shape memory PU with azo as hard segments were synthesized
and characterized in the present study. Due to the increased
interaction, especially the induced dipole–dipole between the
chains, high strength and R and R of almost 99.9% were
1
1
1
7 W. Zhang, L. Chen and Y. Zhang, Polymer, 2009, 50, 1311–1315.
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r
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azo as chain extender can achieve excellent mechanical properties
and shape memory effect. Photoisomerization was observed both
in chloroform solution and the solid state. The trans-cis photo-
isomerization of the shape memory polyurethane is expected to
have very promising applications in specific fields. The light-
triggered shape memory polymers will be studied in the future.
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This journal is ª The Royal Society of Chemistry 2010
J. Mater. Chem., 2010, 20, 9976–9981 | 9981