UV-controlled shape memory hydrogels triggered by photoacid generator

Article abstract of DOI:10.1039/c5ra14421c

Light-induced shape memory polymers represent a class of stimuli-responsive materials that can recover their permanent shapes from temporarily trapped ones upon exposure to light illumination. Although much effort has been devoted to developing various light-responsive shape memory polymers, fabrication of such a light-responsive shape memory hydrogel still remains a challenge compared to neat polymers in their dry state. Herein, we developed a facile and general strategy to endow conventional hydrogel systems with ultraviolet (UV)-controlled shape memory performance simply using a photoacid generator (PAG) as a trigger. The process involves shape fixity through coordination interaction between imidazole groups and metal ions, and shape recovery by switching off the complexation via PAG photolysis reaction which leads to the protonation of imidazole groups. Furthermore, this convenient strategy is proved to be applicable to other pre-existing hydrogels such as a boronate ester cross-linked melamine-poly(vinyl alcohol) (PVA) hydrogel. We believe this method could provide a new opportunity with regard to the design and practical application of light-controlled shape memory hydrogels.

Full text of DOI:10.1039/c5ra14421c

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UV-controlled Shape Memory Hydrogels Triggered By Photoacid
Generator
Wei Fenga, Wanfu Zhoub, Shidong Zhangb, Yujiao Fan a, Akram Yasina and Haiyang Yang a*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Light-induced shape memory polymers represent
a class of employed to achieve anisotropic bending of polymer films in
stimuli-responsive materials that can recover their permanent response to light.6 Photothermal shape memory property was
shapes from temporarily trapped ones upon exposure to light obtained when photothermal effect fillers(e.g. gold nanoparticle,8,
10, 12
14
illumination. Although many efforts have been devoted to
gold nanorod,13 carbon nanotube,7 dye/ligand,11,
and
developing various light-responsive shape memory polymers, graphene15-17) were introduced into a polymer matrix bearing
fabrication of such a light-responsive shape memory hydrogel still thermo-sensitive microdomain structures.
remains a challenge compared to neat polymers in their dry state.
However, when it comes to fabrication of a light-controlled shape
Herein, we developed a facile and general strategy to endow memory hydrogel, the strategies above-mentioned for neat
conventional hydrogel systems with ultraviolet (UV)-controlled polymer in their dry state are not suitable. The main confinement is
shape memory performance simply using photoacid generator mainly attributed to the low solubility of the photochromic moieties
(PAG) as a trigger. The process involves shape fixity through and similar organic functional groups in water. Analogously, the
coordination interaction between imidazole groups and metal ions, hydrophobic nature of photothermal gradients makes them prone
and shape recovery by switching off the complexation via PAG to aggregation in water without functionalization strategies.11, 12, 16
photolysis reaction which leads to the protonation of imidazole Thus, photo-responsive shape memory hydrogels have been rarely
groups. Furthermore, this convenient strategy is proved to be reported.
applicable to other pre-existing hydrogels such as a boronate ester
Previously, we reported chemically or thermally activated shape
19
cross-linked melamine-poly(vinyl alcohol) (PVA) hydrogel. We memory hydrogels bearing ligand-Fe3+ complexation18,
or
believe this method could provide a new opportunity with regard crystalline domains20, respectively. Herein, we develop a facile and
to the design and practical application of light-controlled shape general Photoacid Generator strategy to endow different hydrogel
memory hydrogels.
systems with light-controlled shape memory performance simply
using photoacid generator (PAG) as a trigger. For instance, as shown
Light-responsive shape memory polymers (SMPs) have attracted in Fig. 1, after treated with a mixture solution of metal ions (Cu2+,
increasing attention mainly for the distinct features of light such as Zn2+, or Ni2+) and PAG (diphenyliodonium nitrate), an organic cross-
remotely, spatially, and easily controlled “on-off” activation.1-3 linked poly(acrylamide-co-N-vinylimidazole) hydrogel is trapped in a
Light-responsive shape memory bulk polymer composites are temporary state because of a drastic enhancement of mechanical
commonly based on photo-cross-linking/isomerization reaction,4, 5,6 strength resulted from coordination interaction between metal ions
or photothermal effect.1, 7-11 For example, photochromic moieties and imidazole groups. Upon UV irradiation, PAG is photolyzed to
such as coumaric acid or cinnamylidene acetic acid, undergo generate protons to lower the pH below the pKa (~5.8) of imidazole
desired reversible photo-cross-linking reaction upon absorption of groups, leading to disassociation of the complexation effect.
photons and afford additional network to fix a temporary shape. Consequently, the temporary shape recovers its permanent form
Trans-cis photoisomerization of azobenzene groups could be which is determined by organic cross-linkers. This simple method,
where PAG plays
a
critical role, is also applicable to other
boronate ester cross-linked
conventional systems such as
a
melamine-poly(vinyl alcohol) (PVA) physical network. Thus, we
hope that this facile strategy may provide new opportunities for
both designing and application of light-controlled shape memory
hydrogels.
P(AM-co-VI) hydrogels were prepared through free radical
polymerization.(Detailed synthesis procedure can be found in
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Fig. 1 Illustration of UV light-responsive shape memory hydrogel mechanism.
Electronic Supporting Information) Briefly, after preparing mixture
solutions containing different amounts of acrylamide, N-
vinylimidazole and cross-linker polyethylene glycol diacrylate
(a)
I
OH
UV
I
HNO3
+
+
(PEGDA),
ammonium
persulfate
(initiator)
and
H2O
-
NO3
tetramethylethylenediamine (accelerator) were added to initiate
the polymerization to obtain hydrogels with varied compositions.
(b)
co
co
CH2 CH
O
CH2
CH
CH2 CH
C
The chemical structures of prepared hydrogels (Scheme 1) are
confirmed by FTIR spectra. The FTIR spectra of samples with
x
z
y
N
O
NH2
O
N
different VI contents are listed in Fig. 2. The band at 1668 cm−1 is
CH2
attributed to C=O in acrylamide and PEGDA.21,
Bands from
22
CH2
imidazole groups are verified at 1500 (C=C and C=N stretching
vibrations), 1416 cm-1 (imidazole ring stretching vibrations), 1228
(CH and C-N in-plane bending) and 1083 cm−1 (CH in-plane bending
and ring stretching). The strong band from –NH2 group is verified,
with stretching vibrations of N-H at 3433 cm−1. These characteristic
peaks suggest successful synthesis of P(AM-co-VI) hydrogels.
O
O
C
H2C CH
z
Scheme 1. (a) Photolysis reaction of photoacid generator PAG
(diphenyliodonium nitrate); (b) Chemical structure of P(AM-co-VI)
hydrogel.
Fig. 3a shows that the as-prepared hydrogels swelled in deionized
water with water content (detailed swelling data can be found in
Table S1, ESI). Meanwhile, the swelling ratio increased with
increasing acid concentration. The EWC of PAM-VI-50 hydrogel
increased from 60% to 87.6% in deionized water and increased
from 87.6% at pH 7.01 to 99.1% at pH 3.21, and then decreased to
96.2% at pH 1.04. In the range 3.21<pH<7.01, electrostatic
repulsion between protonated imidazole groups causes increase in
the swelling of hydrogels. And in the range 1.04<pH<3.21, the
decrease in the swelling ratio of hydrogels is attributed to the
increasing ionic strength of the medium.23, 24 Meanwhile, for the
same pH, hydrogel with higher VI content exhibits higher EWC,
which is attributed to the increasing repulsive electrostatic
interaction between neighbouring protonated imidazole groups.25
Fig. 2 FTIR spectra of samples with different VI/AM ratios.
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Fig. 4 (a) Storage moduli of different VA/AM rartios as a function of
frequency. (b) Frequency sweep of PAM-VI-50 hydrogel before and
after immersion in a mixture solution of ZnSO4 (10mM) and PAG
(20mM).
Fig. 3 (a) Swelling behaviour of hydrogels in water with different pH.
(b) Adsorption kinetics of Zn2+ by PAM-VI-50 hydrogel from 10mM
ZnSO4 solution.
affinity of imidazole groups to Cu2+, Ni2+, Zn2+ could provide
sufficient strength to lock a temporary shape, whereas Co2+ is
poorly efficient for shape memory. Taking into consideration the
To study the adsorption kinetics of Zn2+, atomic absorption
spectroscopy assay was performed. Fig. 3b exhibits the adsorption shape recovery, we choose Zn2+ as an example, with Cu2+ and Ni2+
kinetics of Zn2+ by PAM-VI-50 hydrogel in 10mM ZnSO4 solution.
as comparisons.
The adsorption of Zn2+ rises over time is revealed by corresponding
Fig. 4b shows the effect of ZnSO4-PAG on the mechanical
decrease of Zn2+ concentration in the external solution. An
properties of hydrogels. The G’ values for as-prepared samples all
absorption equilibrium is observed within 4 hours, implying that a
exhibit substantial elastic response and are considerably larger than
maximum amount of Zn2+ is immobilized at the final state.
Fig. 4a depicts the effect of VI concentration on the mechanical
predominantly elastic nature. After treated with ZnSO4-PAG, both
strength of hydrogels. Decreasing the VI/AM molar ratio leads to
obvious increase of the G’ values, revealing a relatively weaker
G’ suggests that a stronger network is formed. Undoubtedly,
network formed at a higher VI/AM ratio. It is reasonable that the
the G’’ values over the entire range of frequencies, indicative of
the G’ and G’’ values display drastic enhancement. The increase of
reinforcement of the network is mainly attributed to strong
coordination interaction between VI groups and Zn2+. As another
evidence of this, with the increase of VI/AM ratios, the gels show
prominent increment in G’ values. For instance, storage moduli of
PAM-VI-70, PAM-VI-50 and PAM-VI-30 hydrogels increase to 37-
fold, 12-fold and 1.38 fold (100 rad/s) of corresponding original
values, respectively. (Fig. S1, ESI) Meanwhile, as a reference, the
PAM hydrogel without VI exhibits a slightly higher G’, which could
be attributed to ion-dipole interaction between Zn2+ ions and amide
hydrogel containing higher acrylamide content tends to exhibit
stronger mechanical strength due to strong hydrogen bonding
between amide groups, which serves as physical cross-linkages and
contributes to denser polymeric network.26, 27 In contrast, fewer
hydrogen bonding points exist in the presence of higher VI/AM
ratios.
Previous studies have confirmed that various metal ions can
coordinate with imidazole groups and the stability constants of
typically adopted metal ions are in the order: Cu2+>Ni2+>Zn2+>Co2+.28,
29
In order to achieve desirable shape memory performance, the
31
groups of polacrylamide.30, It is worthwhile noting that slight
metal ions used here need to be selected delicately. Metal ion with
relatively higher stability constant to imidazole group is appropriate
for memorizing a temporary shape. Experiments show that binding
frequency dependence shows up for the samples in the presence of
Zn2+ ions compared to those as prepared ones, indicative of the
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existence of additional dynamic non-covalent interaction in
hydrogels.
It is well-known that photoacid generator can undergo photolysis
under UV irradiation.32-36 Here, we employ this facile method to
endow hydrogel with shape memory ability. A straight strip gel with
a length of 6 cm and radium of 4mm was bent into a “V”-form
shape and immersed in the solution containing ZnSO4 (10 mM) and
PAG (20 mM) for 4 hours to fix the temporary shape. Then it was
subjected to UV irradiation with an Oriel 100 W mercury arc lamp
to perform the shape recovery process. The hydrogels could be
recycled for shape memory performance. Between cycles, to
remove Zn2+ ions from the hydrogel, the used hydrogel was firstly
soaked in 20mM EDTA solution for three hours and then soaked in
deionized water for two hours. The angle was recorded at certain
time. According to previously published calculation methods,37-40
the shape fixity ratio Rf and shape recovery ratio Rr were defined by
the following equations:
ꢀ
θ
Rf = u(N ) ×100%
θm
ꢀ
θp(N ) −θu(N )
Rr =
×100%
θp(N −1) −θu(N )
Where ꢀθm is the maximum angle change applied to the sample,
ꢀθu is the fixed angle change reached after releasing external force.
As depicted in Fig.S2 (ESI), θu stands for the “V” angle value in the
end of fixity process, θp stands for the “V” angle value in the end of
recovery process, and N represents the cycle number. In this
definition, for an initial straight strip hydrogel, θ=180°; for a curled
“V”-shaped hydrogel, θ<180°. As shown in Fig. 1, the strip gel is
curled into a “V” shape under external stress and immersed in the
mixture solution of ZnSO4 and PAG (10mM, 20mM, respectively).
Complexation between imidazole groups and Zn2+ enables the
maintenance of the temporary shape. When subjected to UV
irradiation, it regresses to the initial straight shape in 15 minutes.
Organic cross-linker, polyethylene glycol diacrylate, is utilized to
determine the permanent shape and prevent chain slippage. The
recovery ratio accompanied by pH change over time is portrayed in
Fig. 5a. Upon exposure to UV illumination, the pH value decreases
rapidly within several minutes, suggesting the production of
protons from photolysis of diphenyliodonium nitrate. Meanwhile,
the “V” shape gel begins to return to its permanent shape quickly.
The complete recovery process finishes in 15 minutes with a final
pH value around 3.7. Compared to those conventional pH-
responsive or ion-extraction shape recovery systems, the present
one possesses a much faster recovery capability.41 The cyclicity of
shape memory process is evaluated by measuring shape fixity ratio
(Rf) and shape recovery ratio (Rr) of three samples in three cycles.
(Fig. 5b) The mechanical response also shows corresponding
reversibility. (Fig. S3, ESI) In every cycle, the ZnSO4-PAG solution is
freshly prepared and there are enough Zn2+ ions to form
complexation with imidazole groups to enhance hydrogel’s
mechanical strength and lock its temporary shape. So Rf is quite
stable during three cycles. The samples show compromised shape
recovery ability with increasing cycle number, which can be
attributed to the remaining Zn2+ ions in the hydrogel,19 slightly
enhancing the mechanical strength of hydrogel (as shown in Fig. 6a,
the hydrogel treated with UV light has a slightly higher G’ than
Fig. 5 (a) pH curve of 20 mM PAG solution and shape recovery
ratio (Rr) as a function of UV irradiation time. (b) Cyclicity of PAM-
VI-50 hydrogel shape memory process.
pristine hydrogel), preventing the hydrogel fully returning to its
original shape.
Frequency sweep (Fig. 6a) after UV irradiation demonstrates an
almost identical strength of the sample in comparison with its initial
state, indicating that the complexation effect is completely
screened because of protonation of imidazole groups. FTIR
spectroscopy has been employed in order to further probe
coordination interaction change associated with UV irradiation.(Fig.
6b) On the formation of Zn2+-imidazole group coordination, the
indicative ring mode (R6) peak at 915 cm-1 of imidazole decreases in
intensity, accompanied by the formation of a new band at 954 cm-
.
1 21 After UV illumination, the imidazole groups become protonated
and thus the peak at 954 cm-1 disappears.
In order to testify whether this PAG strategy is generally
applicable to other conventional pre-existing shape memory
hydrogels,
a physical network, boronate ester cross-linked
melamine-poly(vinyl alcohol) (PVA), is used. It is known that PVA
43
and boronic acid can form pH-sensitive boronate ester.42,
Therefore, it holds promise for designing a light-responsive shape
memory hydrogel with a combination of the simple and practical
PAG strategy.
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returns to its permanent form due to the deconstruction of the
boronate ester cross-linkages with hydroxyl groups from PVA. This
is further proved by the drastic decrease of G’ values after
irradiation. (Fig. S4, ESI) Thus, the facile PAG strategy is well suitable
for designing light-responsive shape memory hydrogel systems.
Note that although UV is critical to construct such a shape memory
hydrogel, the present strategy may be confined to those pH-
sensitive systems. Application to more comprehensive systems
needs to be investigated in further detail.
Conclusions
In conclusion, we propose a facile and general strategy to endow
conventional hydrogels with ultraviolet (UV)-controlled shape
memory performance. This convenient method is proved to be
suitable
for
organic
cross-linked
poly(acrylamide-co-N-
vinylimidazole) hydrogel and
a
melamine enhanced polyvinyl
alcohol hydrogel. Photolysis of PAG generates proton, which is
necessary to disassociate the additional networks that are essential
for fixity of a temporary shape. Considering the difficulty for
preparing light-responsive shape memory hydrogels and the
simplicity of the present strategy, we believe this method
undoubtedly provides a new opportunity with regard to the design
and practical application of light-controlled shape memory
hydrogels.
Acknowledgements
Fig. 6 (a) Storage moduli of 1) pristine PAM-VI-70 hydrogel, 2)
hydrogel after immersed in 10 mM Zn2+ and 20 mM PAG solution
for 4 hours, then hydrogel subjected to 3) UV light for 15 minutes, 4)
pH 3 solution for 15 minutes. (b) FTIR spectra of freeze-dried (i)
PAM-VI-50 hydrogel, (ii) the hydrogel treated with 10 mM ZnSO4 +
20 mM PAG solution and (iii) hydrogel then treated with UV
irradiation.
We gratefully acknowledge financial support from the National
Natural Science Foundation of China (Grant no. 51273189), National
Science and Technology Major Project of the Ministry of Science
and Technology of China (2011ZX05010-003), PetroChina
Innovation Foundation (2012D-5006-0202) and China Postdoctoral
Science Foundation funded project (no. 2013M531513).
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