Bending and stretching actuation of soft materials through surface-initiated polymerization

Article abstract of DOI:10.1002/anie.201008252

A redhead: Surface-grafted hydrophilic polymer brushes (see picture) with high molecular weight and graft density caused reversible bending and stretching of soft polymeric substrates on a macroscale. The shape change of the substrate was tuned to respond to different stimuli including humidity, temperature, and pH. Copyright

Full text of DOI:10.1002/anie.201008252

Communications
DOI: 10.1002/anie.201008252
Smart Soft Materials
Bending and Stretching Actuation of Soft Materials through Surface-
Initiated Polymerization**
Yuquan Zou, Adriel Lam, Donald E. Brooks, A. Srikantha Phani,* and
Jayachandran N. Kizhakkedathu*
Shape–memory materials (SMMs) and actuators possess the
ability to respond to external stimuli such as temperature,
[
1]
[2]
electricity, magnetic field, and light, and change their
shapes. During the process, energy is converted into mechan-
ical deformation which makes them attractive for various
applications in biomedical devices, deployable structures,
[
3]
artificial muscles, microdevices, sensors, etc. Traditional
SMMs and actuators rely on the properties of bulk material
irrespective of their nature (e.g. polymers, metallic alloys,
[
1–4]
composite materials).
Very recently, polymer-brush-based
[
5]
nanoscale bending actuators were reported. As a conse-
quence of the strong interchain repulsion, the overcrowded
polymer chains within the polymer brush can exert forces
onto the underlying substrate and deform the substrate.
However, there is no empirical evidence that bending
observed on the nanoscale can be adapted for actuator
applications on the macroscale. Furthermore, bending alone
may not be sufficient to provide the desired macroscopic
actuation; axial stretching may also be required.
Herein we demonstrate the bending and stretching of a
soft polymeric substrate, plasticized poly(vinyl chloride)
(
pPVC, thickness 400 mm, Youngꢀs Modulus 6.89 MPa), on
the macroscale by grafting a model hydrophilic polymer,
poly(N,N-dimethylacrylamide) (PDMA), at high graft density
on the pPVC surface. As shown in Figure 1A, when PDMA
Figure 1. A) Illustration of bending and stretching of a soft material
driven by the growth of polymer chains by SI-ATRP. B) Photographs
showing the effect of the polymerization time on the bending of the
plasticized PVC substrate. C) Effect of the polymerization time on
bending (C1) and stretching deformation (C2; s is the grafting density
of the polymer brushes).
[
*] Dr. Y. Zou, Prof. Dr. D. E. Brooks, Prof. Dr. J. N. Kizhakkedathu
Centre for Blood Research, Department of Pathology
and Laboratory of Medicine, Department of Chemistry
University of British Columbia
Vancouver, BC V6T 1Z3 (Canada)
Fax: (+1)604-822-7742
E-mail: jay@pathology.ubc.ca
chains were grafted on one side of the pPVC surface, bending
of the substrate was observed. In contrast, when PDMA
chains were grafted on both sides of the pPVC surface,
stretching of the substrate was observed. Proof-of-concept
bending and stretching actuators with relatively high actuator
strain and reasonable force per unit length were developed
based on this observation. The reversible bending and
stretching can also be tailored to respond to various external
stimuli by changing the chemical structures of the grafted
polymer chains.
We first investigated the bending of the pPVC film on the
macroscale. As a proof of concept, an initiator-modified
pPVC substrate obtained by single-side atom-transfer radical
polymerization (ATRP) without noticeable initial curvature
was used (see the Supporting Information for sample
preparation). The initiator-modified pPVC was precut into
samples of 1.5 cm ꢁ 0.25 cm dimensions and they were used
for surface-initiated ATRP (SI-ATRP) of N,N-dimethylacry-
A. Lam, Prof. Dr. A. Srikantha Phani
Department of Mechanical Engineering
University of British Columbia
Vancouver, BC V6T 1Z4 (Canada)
E-mail: srikanth@mech.ubc.ca
[
**] We thank Dr. Johan Janzen for helpful discussions. This work was
supported by the Canadian Institutes of Health Research (CIHR),
the Natural Science and Engineering Council of Canada (NSERC),
the Canadian Blood Services (CBS), Canada Foundation for
Innovation (CFI), and the Michael Smith Foundation for Health
Research (MSFHR). J.N.K. is a recipient of a CIHR/CBS new
investigator award in transfusion science. A.S.P. holds a Canada
Research Chair in dynamics of lattice materials and devices.
Supporting information (including detailed experimental protocols,
instruments, synthesis of the functional atom transfer radical
polymerization initiator, engineering model and calculation of the
force density and energy) for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201008252.
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lamide (DMA) for 2, 6, 12, and 24 h. As shown in Figure 1B,
the pPVC substrates were deformed with increased polymer-
ization time. All pPVC substrates curved in the direction of
the side onto which the PDMA chains were grafted. Longer
polymerization times resulted in a gradual decrease in the
radius of the curvature of the substrates (Figure 1C1). The
molecular weight (M : number-average molecular weight,
n
M : weight-average molecular weight) of the PDMA chains
w
formed in solution along with the surface-grafted chains
remained constant after 2 h (M and M /M values were 1.5 ꢁ
n
w
n
6
6
6
6
1
0 , 1.8 ꢁ 10 , 2.1 ꢁ 10 , 2.0 ꢁ 10 and 1.68, 1.75, 1.87, 2.00,
respectively for 2, 6, 12, 24 h of SI-ATRP), suggesting that the
increase in bending deformation can be attributed to the
increase in graft density of the polymer chains (i.e. increase in
chain–chain repulsion) on the surface. Although the direct
[6]
measurement of the molecular weight is desirable, the Mn
values of the grafted PDMA chains were estimated from the
solution polymers in the current study because the amide
linkage between the polymer and the surface was cleaved
incompletely. The presence of amide bonds in the PDMA also
complicated the cleavage process. The gradual increase in the
PDMA graft density and the ultrahigh molecular weight of
the chains are consistent with our previous observation of SI-
ATRP of DMA from unplasticized PVC in aqueous solu-
Figure 2. A) Effect of dehydration–rehydration on the bending of a
pPVC substrate with PDMA chains grafted on one side (12 h SI-ATRP).
B) Effect of different drying processes on the reversibility of the
bending. B1) Photograph of a wet PDMA-grafted pPVC substrate,
B2) photograph of the substrate dried under vacuum, B3) photograph
of the substrate dried by a hot gun, B4) ATR-FTIR spectra of a PDMA-
brush-grafted pPVC substrate dried in vacuum (dot-dashed line) and
with a hot gun (solid line), B5 and B6) relationship of the bending
angles to different wetting–drying cycles under different drying con-
ditions (B5: vacuum drying, B6: hot-gun drying). C) Cross-sectional
back-scattered electron SEM images of C1: unmodified pPVC, C2:
pPVC grafted with PDMA (24 h SI-ATRP), and C3: spin-coated PDMA
on pPVC. The scale bars in C1–C3 represent 5 mm.
[
7]
tion.
A high degree of reversibility is essential for an ideal
actuator design. Figure 2A shows the effect of dehydration–
rehydration under atmospheric conditions on the bending–
flattening of the PDMA-grafted pPVC substrate (12 h SI-
ATRP). Under these conditions (45% relative humidity,
2
28C), the PDMA-grafted pPVC gradually dehydrated,
flattened, and finally reached an equilibrium state after nine
minutes. The dry sample reverted to its original shape upon
rehydration within eight seconds (see video 1 in the Support-
ing Information). The flattening and bending of the PDMA-
brush-grafted pPVC is due to conformational changes of the
PDMA chains on the surface during the drying–wetting
process. The chain dimensions of the grafted PDMA
decreased during the drying process which resulted in reduced
chain–chain interactions. During the rehydration, the poly-
mer chains regained their original dimensions and the
substrate reverted to its original shape. A control nontreated
pPVC substrate did not show noticeable shape changes during
the wetting–drying process (see video 2 in the Supporting
Information).
hot-gun drying (Figure 2B4). The differences in the behavior
of the substrate subjected to the two drying methods reflect
the importance of the residual water for the reversibility of
the bending–flattening process. One possible reason for the
irreversible deformation in the case of hot-gun drying could
be entanglement of the polymer chain. The residual water in
the specimen after vacuum drying might have prevented such
entanglement, resulting in the high reversibility of the
process.
To verify that the covalently grafted PDMA chains are
responsible for the bending, we examined the bending–
flattening process of a pPVC substrate spin-coated with
PDMA on one side. The M value of the spin-coated PDMA
n
To obtain quantitative information on the reversibility of
the bending–flattening process, a wet PDMA-grafted pPVC
substrate was dried by two approaches: vacuum drying (228C.
was comparable to that of the grafted chains and the thickness
of the dry coating was approximately 31 mm. There was no
bending observed for the spin-coated sample in the hydrated
state and it bended slightly to the side of the PDMA coating
upon drying, which is presumably due to the contraction of
the PDMA layer. The structures of unmodified pPVC, the
PDMA-grafted pPVC substrate (24 h SI-ATRP), and the
pPVC spin-coated with PDMA were compared by scanning
electron microscopy (SEM). The topography of the pPVC
and the PDMA-grafted pPVC substrates is rougher than that
of the spin-coated PDMA (see Figure 6S in the Supporting
Information). Cross-sectional images of the samples are
shown in Figure 2C. A sparsely spaced interface filled with
vertically aligned fiberlike structures was observed for
0
.1 Pa, 15 minutes) and hot-gun drying (1808C, 10 seconds).
As shown in Figure 2B1, B2, and B5, the vacuum drying
afforded a highly reversible bending–flattening process indi-
cated by the minimal variation in the bending angles when the
process was repeated (Figure 2B5). In contrast, hot-gun
drying (complete drying) led to a gradual decrease in the
bending angle (Figure 2B1, B3, B6) with repeated wetting–
drying cycles, suggesting a more pronounced initial irrever-
sibility. In the attenuated total reflectance Fourier transform
À1
infrared (ATR-FTIR) spectrum the water peak (3400 cm )
observed for the vacuum-dried substrate disappeared after
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Communications
PDMA-grafted pPVC (Figure 2C2). In contrast, unmodified
PVC and spin-coated PDMA did not show any special
interfacial features (Figure 2C1 and 2C3). The thickness of
the PDMA-grafted layer on the pPVC was approximately
5
mm as determined by SEM (approximately 1% of the
thickness of the pPVC substrate), correlating well with the
ultrahigh molecular weight of the PDMA chains.
To understand the experimental observations better and
to illustrate the application of this shape–memory mechanism
for the generation of bending and stretching actuator devices
(
Figure 3), we applied engineering principles. An engineering
analysis of the experimentally observed bending deforma-
[
8a]
tions has been performed by using beam theory.
The
polymer-brush layer grafted onto the surface is assumed to
exert an in-plane membrane force, f per unit width, at the
polymer-brush–substrate interface, in the plane of the inter-
face as shown in Figure 3A (the details of the engineering
model and the analysis are described in the Supporting
Information). This allows two deformations. In the first case,
the substrate bends when either the upper or the lower
surface alone is coated. This bending is similar to the
observation by Stoney and co-workers of the bending of
[8b,c,9]
nickel-plated metal strips as a result of residual stresses.
In the second case, the substrate stretches when the same
polymer-brush structure is present on both sides (Figure 3B).
Consequently, both stretching and bending actuation of the
substrate is possible with the polymer-brush coatings. Based
on the engineering analysis of the deformations we estimate a
À1
membrane force of 24 Nm for a polymerization time of 4 h
6
and a Mn value of 1.6 ꢁ 10 for bending actuators (see
Figure S8 in the Supporting Information). For the stretching
case, the estimated force per unit length (FPL) for the upper
À1
and lower surfaces was 18.3 and 12.9 Nm , respectively. A
detailed calculation of the force density is provided in the
Supporting Information. When the polymerization time was
increased from 4 to 24 h, much higher values of 74.7 and
À1
6
7.3 Nm were deduced for the upper and the lower surfaces,
respectively (see Figure S10 in the Supporting Information).
This is attributed to the increase in graft density with
increasing polymerization time. The elastic modulus of the
pPVC did not change significantly after PDMA chains had
been grafted on the surface.
Figure 3. A) Engineering model to describe the bending behavior of
pPVC substrates with PDMA grafted on one side. The length of the
coated and uncoated segment was denoted as L1 and L2, respectively.
B) Engineering model predicting the stretching of pPVC coated with
PDMA on both sides. C) Photographs of the pPVC substrate with
PDMA-grafted on one side under dry (C1) and wet (C2) conditions.
D) Photograph of the stretching actuator based on a PDMA-brush-
grafted pPVC sample. Comparison of the length of the pPVC surfaces
grafted with PDMA on both sides under dry (top) and wet (bottom)
conditions. E) Comparison of the polymer-brush actuator to other
available actuators with regard to two performance indices: force per
unit length and actuator strain.
[7]
It is instructive to compare the polymer-brush actuator
with other existing actuators based on two mechanical indices
for actuator performance: force per unit length and actuator
strain. The actuator force per unit length is deduced from the
engineering analysis. Actuator strain is the maximum strain
the actuator undergoes. The FPL versus actuator strain
diagram is shown in Figure 3E, wherein actuators drawn
[8d]
from an actuator data base are compared with polymer-
brush-based actuators. It can be observed that there is a trade-
off between FPL and actuator strain in general for all
actuators. Actuators with high FPL, such as piezoelectric
materials, cannot provide large strains. Shape–memory metals
such as Ni–Ti are attractive due to their larger strains. The
polymer-brush-based actuators developed herein are shown
to provide even higher actuator strain (typically 10%) and
moderate force per unit length: slightly better than some
electroactive polymers. This comparison diagram underlines
the potential for polymer-brush actuators in soft material
systems. We note that graft density and molecular weight
govern the FPL and actuator strain, and the data reported
here is for the experiments we conducted.
To explore the application of this shape-change phenom-
enon further, we used two other external stimuli, namely
temperature and pH. To generate SMMs and actuators
sensitive to temperature, a classical temperature-sensitive
polymer, poly(N-isopropylacrylamide) (PNIPAm), and its
copolymer with PDMA were used. As shown in Figure 4A,
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modification of soft polymeric substrates. We anticipate that
this will give a new direction in the generation of soft shape–
memory materials and devices. We have demonstrated that
conformational changes at the nanoscale associated with the
grafted polymer chains (polymer brushes) can be transformed
to the macroscale. The deformation of the grafted soft
polymeric materials can be achieved by external stimuli
such as humidity, temperature, and pH, and possibly other
stimuli. We have also demonstrated the application of this
mechanism for the generation of proof-of-concept bending
and stretching actuators. The achieved force per unit length
and the actuation strains make these soft material systems
potential candidates for artificial muscles and components in
hitherto unexplored applications in biomedical sciences and
nanotechnology.
Received: December 30, 2010
Revised: February 15, 2011
Published online: April 21, 2011
Keywords: bending–stretching actuators · polymers ·
.
shape–memory materials · soft materials · polymerization
Figure 4. Effect of different stimuli on the bending of polymer-brush-
grafted pPVC substrates. A) Photographs of the PNIPAm-grafted pPVC
substrate at different temperatures (A1, A2); A3: Relationship of
bending of angles and heating–cooling cycles. B) Photographs of
PDMA-co-PNIPAm-brush-grafted pPVC substrate at different temper-
atures (B1, B2); B3: Relationship of bending angles and heating–
cooling cycles. C) Photographs of PDMA-co-PAPMA-brush-grafted
pPVC substrate at different pH values (C1, C2); C3: Relationship of
bending angles and pH-change cycles. D) Chemical structures of three
polymers.
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(
PNIPAm-co-PDMA; 75:25), resulting in a nearly reversible
bending–flattening process during the heating–cooling cycles
Figure 4B3). The value of the bending angle was lowered by
(
about 25% during the first cycle and remained constant
afterwards. The reversibility of the process may be because
the PDMA component remains hydrated during the collapse
of the copolymer chains induced by a temperature change.
Similarly, the grafting of a pH-sensitive copolymer, poly(N,N-
dimethylacrylamide-co-aminopropyl-methacrylamide)
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(
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Figure 4C). Again, the introduction of a neutral PDMA
component in the copolymer significantly improved the
reversibility of the process.
In summary, we have illustrated a novel and universal
mechanism of generating soft shape–memory materials based
on bending and stretching actuators through the surface
[9] While Stoneyꢀs analysis is concerned with the initial curvature
resulting from residual stresses, we are interested in the reversible
curvature owing to forces induced by polymer chains on the
substrate.
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