Room temperature rapid photoresponsive azobenzene side chain liquid crystal polymer

Article abstract of DOI:10.1021/ma2013173

A new room temperature photoresponsive side chain liquid crystalline polymer has been synthesized and tested. It combines a side-on azobenzene LC with a low glass transition temperature polysiloxane backbone to obtain a nematic LCP at room temperature. Th

Full text of DOI:10.1021/ma2013173

ARTICLE
pubs.acs.org/Macromolecules
Room Temperature Rapid Photoresponsive Azobenzene Side
Chain Liquid Crystal Polymer
Michael Petr and Paula T. Hammond*
Department of Chemical Engineering, Rm 66-352, Massachusetts Institute of Technology, 77 Massachusetts Avenue,
Cambridge, Massachusetts 02139, United States
ABSTRACT: A new room temperature photoresponsive side
chain liquid crystalline polymer has been synthesized and tested.
It combines a side-on azobenzene LC with a low glass transition
temperature polysiloxane backbone to obtain a nematic LCP at
room temperature. The LCP also exhibits a crystalline phase that is
kinetically trapped by the nematic phase at room temperature.
Thermal transitions were observed at ꢀ7, 42, and 73 °C for the T ,
g
T , and T , respectively, and polarizing optical microscopy with
m
iso
in situ UV irradiation was used to characterize the room tempera-
ture photoresponsive behavior of its metastable nematic phase. Upon irradiation, the nematic phase almost completely disappeared in
ꢀ
11
about 35 s. The isotropization was linear with a rate of 1.9 ꢁ 10
mol/s at short times, but the final clearing process was
exponential with a time constant of 4.6 s. These relatively short response times can be achieved at room temperature and are fast
enough to be relevant for actuator and responsive elastomer applications.
’
INTRODUCTION
absorption of a UV photon promotes an electron from the HOMO
π-orbital to the LUMO π*-orbital, which switches the azobenzene
to its cis conformation. In its trans conformation, azobenzene is
straight and extended and able to pack effectively with its neighbors
to form an LC phase; however, in its cis conformation, the
azobenzene takes on a bent configuration and is no longer able to
pack sufficiently to form an LC phase leading to the formation of an
isotropic phase. This process is reversible and can be catalyzed by
higher wavelengths of light as well as thermal energy because the
Liquid crystal polymers have been studied for several decades
because they combine the properties of liquid crystals (LC’s) with
the advantages of polymer systems. When designing a side chain
liquid crystalline polymer (SCLCP), there are three main structural
components to consider: the LC side group, the polymeric back-
bone, and the linker group or functional group by which the two are
attached to one another. The polymer can be from any number
of backbone categories, including vinyl,
strained cycloalkene product of ring-opening metathesis polymer-
ization (ROMP),
transition temperature (T ) polysiloxane backbones are important
because they are flexible and do not constrain the ordering of the
attached LC’s; when functionalized with LC’s, they are generally
not glassy at room temperature, enabling the exhibition of LC
phases at ambient conditions.
to exhibit numerous phases, with the most common being
smectic
or low
can be attached “end-on”
1
ꢀ3
1,4ꢀ8
methacrylate,
a
73
trans conformation is thermodynamically favored. Historically,
this nematic formation has been the process used to study the phase
9ꢀ11
12
or even a dendrimer. In particular, low glass
change kinetics, and the nematic domains grow linearly with time
g
74ꢀ77
when subject to a large enough temperature quench.
On the
other hand, there have been few kinetic studies on the nematic-to-
isotropic transition, though one simulation did predict a linear
78
1
3ꢀ34
decrease in the order parameter with respect to time.
The LC can also be selected
By the process described above, quick, reversible, and large
responses have been achieved in photoresponsive SCLCP’s. This
observation is notable in that rapid responses in the bulk have,
thus far, only been achieved at elevated temperatures. For
example, the nematic side-on azobenzene polymethacrylate
2
,4ꢀ9,11,35ꢀ37
,38,43
11,38ꢀ41
4,5,9ꢀ11,42
or nematic
with a high
3
isotropization temperature (T ). Finally, the LC
iso
1
,2,4,5,7ꢀ10,36,37
3,39ꢀ41,44,45
or “side-on”
with respect to the unit director of the LC. More recently,
photoresponsive SCLCP’s, which contain azobenzene moieties,
have gained interest for their ability to change properties by
merely irradiating them with the correct wavelength of light in
the appropriate temperature range.
particular interest for applications in which a quick response or
2
made by Li required 25 min of irradiation at 2.3 mW/cm and
38
room temperature for an 80.5 nm film or a couple of minutes of
2
40
irradiation at 100 mW/cm and 70 °C for a 20 μm film for a
1
6,29,33,38ꢀ40,46ꢀ72
complete response because the T of the polymer backbone
g
They are of
was 60 °C. A photoresponsive smectic end-on azobenzene
polysiloxane system was made by Verploegen; however, in that
case, the transitions were only observed at or above 100 °C.
1
6,33,47,49,52,56ꢀ59,61,62,71,72
actuation
muscles,
is necessary, such as artificial
3
9,40
51,62,66,69
photomechanical cantilevers,
photome-
70
68
chanical oscillators, and even robotics.
Azobenzene SCLCP’s are responsive to UV light by means of a
trans-to-cis isomerization of the azobenzene moiety. The azoben-
zene starts in its thermodynamically stable trans conformation; then,
Received: June 14, 2011
Revised:
October 6, 2011
Published: October 25, 2011
r 2011 American Chemical Society
8880
dx.doi.org/10.1021/ma2013173
|Macromolecules 2011, 44, 8880–8885

Macromolecules
ARTICLE
This is because, although the T of the polymer was very low, the
g
68
Tiso of the smectic phase was 126 °C. Below 100 °C, the
smectic phase was too viscous for a fast response. In fact, these
two examples illustrate the importance of the T of the polymer
g
backbone and the Tiso of the LC because they define the
temperature range in which the bulk photoresponsive SCLCP
can be used. The T defines the absolute lower limit, and the T
g
iso
defines the upper limit as well as a potential lower limit whereby,
if it is too far below the T , the LC becomes too viscous for a
iso
quick response. Consequently, a photoresponsive SCLCP with a
low T and a low T is necessary for a quick response at room
g
iso
temperature; this work is an example of such a material.
’
EXPERIMENTAL SECTION
Materials. 1,3,5-Trivinyl-1,3,5-trimethylcyclotrisiloxane (V3) and
1,4-bis(hydroxydimethylsilyl)benzene were purchased from Gelest, Inc.,
and all other reagents were purchased from Sigma-Aldrich. No purifica-
tion was performed on any of the reagents or solvents, except for the
tetrahydrofuran (THF) polymerization solvent, which was taken from
an Innovative Technologies, Inc., solvent purification system and dried
with sodium.
Instrumentation. Anionic polymerization in an Innovative Tech-
nologies, Inc., glovebox was used to make poly(vinylmethylsiloxane)
(
7
PVMS), and gel permeation chromatography (GPC) with a Waters
17plus autosampler running THF as the mobile phase was used to
determine the molecular weight (MW) and the polydispersity index
1
(PDI) of the PVMS. H NMR on a Bruker AVANCE-400 was used to
determine molecular structure and degree of conversion for the LC
synthesis and attachment. Differential scanning calorimetry (DSC) on a
Thermal Analysis (TA) Instruments DSC Q1000 was used to measure
the various transition temperatures of the material. Polarizing optical
microscopy (POM) with a Zeiss Axioskop 2 MAT and an AxioCam MRc
camera recorded with CamStudio screen-capture software was used to
characterize the LC phase, and during this characterization, UV light
centered at 360 nm from a Dymax Blue Wave 200 with a Thorlabs, Inc.
FGUV W53199 UV filter was shined on the material. Finally, anisotropic
spectroscopic ellipsometry on a J.A. Woollam Co., Inc., XLS-100 was
used to measure refractive indices and birefringence.
Figure 1. Synthesis of new photoresponsive SCLCP.
Pt(0)ꢀ1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution in
xylene (Pt ∼2%) was mixed. This solution was added dropwise to the
first solution to obtain a red solution and then stirred overnight at 60 °C
under N . The orange 4 was separated with 1:9 v/v ethyl acetate:hexane on
2
a silica gel column to obtain 1.922 g for a conversion of 30%. H
1
0
0
Synthesis of 4-Butoxy-2 ,4 -dihydroxyazobenzene (1). This
NMR: δ 6.8ꢀ8.2 (m, 11H, Ar), 4.73ꢀ4.62 (m, ꢀSi(CH ) ꢀ
3
2
38
synthesis has been reported previously. 7.821 g of crude 1 was obtained
OꢀSiH(CH
3
)
2
), 0.0ꢀ0.3 (m, ꢀSi(CH
3
)
2
ꢀOꢀSiH(CH
3 2
) ).
for a conversion of 90%.
Synthesis of 4-Butoxy-2 -hydroxy-4 -(4-butoxybenzyloxy)
Synthesis of Poly(vinylmethylsiloxane) (PVMS) (5). This
0
0
32
synthesis has been reported previously. Roughly 1 g was made, and
3
8
azobenzene (2). This synthesis has been reported previously. 6.320 g
of red 2 was obtained for a conversion of 57%.
Synthesis of 4-Butoxy-2 -allyloxy-4 -(4-butoxybenzyloxy)
a sample was run on the GPC, which measured the number-average MW
as 18 000 g/mol and the PDI as 1.18 against polystyrene standards.
LC Attachment. This attachment procedure has been reported
0
0
3
8
azobenzene (3). This synthesis is similar to that reported previously
32
previously. The resulting 6, made from 1.922 g of 4 and.134 g of 5, was
but with allyl alcohol in place of 4-hydroxybutyl methacrylate.
To 100 mL of dichloromethane was added 6.320 g of 2, and the solution
was cooled in an ice bath. Next, 1.001 g of allyl alcohol and 5.402 g of
triphenylphosphine were added, and then 3.541 g of 40 wt % DEAD in
toluene was added dropwise. The solution was stirred overnight in the
ice bath, and then the dichloromethane was removed. The orange 3 was
separated with 1:9 v/v ethyl acetate:hexane on a silica gel column and
1
a dark brown oil. H NMR confirmed removal of the excess 4 by
disappearance of the SiꢀH peak as well as the complete disappearance of
1
the vinyl peak of 5 for an attachment of 100%. H NMR: δ 8.12
(
d, ꢀO CꢀArH H ꢀOꢀbu), 7.90 (d, ꢀN ꢀArHH ꢀO C), 7.82
2
2
2
2
2
2
(
d, buꢀOꢀArH H ꢀN (Z to spacer)), 7.06ꢀ6.64 (m, buꢀOꢀ
2
2
2
ArH
2
H
2
ꢀN
2
ꢀ buꢀOꢀArH
2
H
2
ꢀN
2
(E to spacer) ꢀN
2
ꢀArHH
2
ꢀ
O
4
ꢀ
ꢀ
2
Cꢀ ꢀO
2
CꢀArH
H
2 2
ꢀOꢀbu), 4.04 (td, ꢀOꢀCH
2
CH
2
CH CH ),
2
3
recrystallization in ethanol from 60 °C to room temperature to obtain
.15ꢀ3.67 (br t, ꢀOꢀCH CH CH ꢀSiꢀ), 1.96ꢀ1.56 (br m, ꢀO
2
2
2
1
5
6
5
4
.060 g of 3 for a conversion of 74%. H NMR: δ 6.8ꢀ8.2 (m, 11H, Ar),
CH
2
CH
2
CH
2
ꢀSiꢀ), 1.80 (p, ꢀOꢀCH
CH CH
), 1.23, (t, ꢀOꢀCH
CH CH CH
), 0.94ꢀ0.32 (br m, ꢀOꢀSiꢀ(CH
2
CH
2
CH
2
CH
3
), 1.50 (sex.,
.10 (m, ꢀOꢀCH
2
ꢀCHdCH
2
), 5.47 (t, ꢀOꢀCH
2
ꢀCHdCH
2
(Z),
OꢀCH
2
CH
2
2
3
2
CH
2
CH
2
ꢀSiꢀ), 0.98
.34 (q, ꢀOꢀCH ꢀCHdCH (E)), 4.74 (d, OꢀCH ꢀCHdCH ),
2
2
2
2
(
t, ꢀOꢀCH
2
2
2
3
3
)
2
ꢀ
.65 (br s, OꢀCH
2
ꢀCHdCH
2
).
CH CH ꢀSi(CH )ꢀOꢀ), 0.17ꢀ0.07 (br m, ꢀSi(CH )).
2
2
3
3
0
Synthesis of 4-Butoxy-2 -(3-(1,1,3,3-tetramethyldisiloxanyl)
propoxy)-4 -(4-butoxybenzyloxy)azobenzene (4). This synthesis
0
’
RESULTS AND DISCUSSION
3
2
is similar to that reported previously. 13.468 g of 1,1,3,3-tetramethy-
ldisiloxane and 25 mL of toluene were mixed at room temperature.
A second solution with 5.060 g of 3, 25 mL of toluene, and 10 drops of
Synthetic Scheme. The procedure used to synthesize the new
SCLCP, shown in Figure 1, uses the mesogen synthesis described
8
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Macromolecules
ARTICLE
Figure 2. Differential scanning calorimetry (DSC) at 10 °C/min with
two cycles on SCLCP 1 week after annealing.
3
8
by Li and the PVMS backbone and side chain attachment
Figure 3. Polarizing optical microscopy (POM) image of SCLCP at a
dm i sa cg l ni ni fia tc iao t ni os n. of ꢁ50 showing a Schlieren texture with 2- and 3-point
32
approach of hydrosilylation described previously by Verploegen.
The PVMS backbone is a siloxane with a glass transition tempera-
ture approaching ꢀ100 °C in its nonfunctionalized form. It was
chosen to enable a low glass transition even after the attachment of
polymer side chains. The LC has its point of attachment at the
center of the mesogen core so that its LC unit director falls parallel,
rather than perpendicular, to the polymer main chain, yielding a so-
called “side-on” SCLCP. This side-on arrangement creates polymer
systems whose chain conformations are highly responsive to the
3,39ꢀ41,44,45
arrangement of the LC mesogens
and was specifically
chosen for this work to induce a stronger rheological response to
mesogen isotropization as compared to the end-on side chain LC
68
functionalization studied in the earlier polysiloxane-based system.
SCLCP Properties and Characterization. The freshly synthe-
sized, solvent-cast, or annealed SCLCP is a dark brown gel-like
viscous liquid. After being stretched, spread, or strained in any
way or after sitting for a few days to a week at room temperature,
the SCLCP turns into a light brown crystalline solid. This
behavior can be explained by the DSC in Figure 2, which shows
two scan cycles for a sample of SCLCP a week after annealing at
Figure 4. UVꢀvis absorbance spectrum.
1
00 °C. Both cycles show an LC isotropization (T ) peak at
iso
about 73 °C, but the first cycle shows an extra crystalline melting
peak (T ) at 42 °C. The T does not show up on the second
m
m
cycle because, after the crystalline phase has melted from the first
heating cycle, it takes at least several days to reform again. The
glass transition temperature (T ) of the LC-functionalized poly-
g
siloxane backbone is approximately ꢀ7 °C; although higher than
79
that of most polysiloxanes, this value is quite low compared to
that of most photoresponsive SCLCP’s and is also well below
4
,5,9ꢀ11,42
room temperature.
Figure 5. Trans-to-cis conformation change resulting in the nematic-to-
isotropic phase change.
As for the LC phase itself, the POM image in Figure 3 is pink and
green, which is due to the birefringence of an LC phase. The
ordinary and extraordinary refractive indices and the birefringence
of a spin-cast sample were measured as 1.59, 1.66, and 0.07,
respectively, using anisotropic ellipsometry. Figure 3 also shows a
Schlieren texture with 2- and 3-point disclinations, which is positive
evidence for a nematic LC phase. Furthermore, this nematic phase
from the trans isomer, which is thermodynamically stable, to the cis
73
isomer. Figure 4 shows a UVꢀvis absorption spectrum for the
SCLCP, which has an absorbance peak in the near-UV and visible
region from about 306ꢀ417 nm corresponding to this π to π*
transition.
persists down to the T because the crystalline phase forms so
g
The SCLCP forms a nematic phase because the straight
azobenzene and benzoate moieties in the trans conformation
tend to line up to reduce free volume, as in the first schematic in
Figure 5. Upon irradiation with UV light in this π to π* region,
the azobenzenes' change from trans to cis disrupts the nematic
slowly, thus making it a long-lived metastable phase.
Photoresponsive Characterization. The SCLCP is photo-
responsive because the azobenzene moiety can absorb a photon to
promote an electron from the HOMO π-orbital to the LUMO π*-
orbital, resulting in a conformational change of the azobenzene
8
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ARTICLE
Figure 7. Integrated color intensity from polarizing optical microscopy
7
(
POM) images. The first portion has rate a of 1.0 ꢁ 10 intensity/s and a
2
linear r of 0.9995, and the second portion has an exponential time
2
constant of 4.6 s and an r of 0.998.
ꢀ
11
1
.9 ꢁ 10 mol/s by applying eq 1, which uses the initial volume
of the film divided by the initial color intensity as a scale factor:
A h F
i
i
r ¼ rI
ð1Þ
Ii
M
where r is the rate in mol/s, r is the rate in intensity/s, A is the
I
i
ꢀ4
2
initial area of the film calculated as 2.4 ꢁ 10 cm from the
manufacturer’s technical data, h is the initial height of the film
i
estimated at 13 μm using a MichelꢀLevy plot, I is the initial
i
8
Figure 6. Polarizing optical microscopy (POM) at a magnification
of ꢁ50 of SCLCP with UV irradiation. The first picture was taken before
the UV light was turned on, and it shows a nematic Schlieren texture with
intensity of the film found to be 2.1 ꢁ 10 , F is the density of the
3
film estimated to be 0.95 g/cm , and M is the molar mass of the
SCLCP repeat unit calculated to be 753.16 g/mol. The film
height was estimated with a MichelꢀLevy plot because, even
though the film height was uniform in the small viewing window
of the microscope, it was nonuniform across the entire sample, so
direct measurement in the specific area of the POM images
would have been difficult. Basically, a MichelꢀLevy plot is a
graphical link between film thickness, birefringence, and viewed
POM color. Given two of these, the third can be estimated.
It was a somewhat counterintuitive result to have two regimes
for the SCLCP’s isotropization. Because the azobenzene’s iso-
merization is unimolecular, it is expected to follow first-order
2-, 3-, and 4-point disclinations. The next pictures are from 5 to 30 s and
show the nematic phase disappearing, and the last picture is at 35 s and
shows the nematic phase almost completely gone.
phase of the SCLCP because, when they change to cis, they
become bent and, therefore, cannot line up with each other, as in
the second schematic in Figure 5.
This nematic disruption was monitored using POM with in situ
2
UV irradiation at 60 mW/cm .The first imagein Figure 6 was taken
with no UV light. At 0 s, the UV light was turned on, and the
Schlieren texture was monitored with screen-capture software that
recorded an image every couple of seconds. The last seven images
in Figure 6 were taken every 5 s and show the disappearance of the
birefringent Schlieren texture in the exposed area over a total of
about 35 s. For these images, as well as the intermediate ones, the
red, green, and blue 24-bit color intensities were evaluated for each
pixel and summed over the entire picture for an overall color
intensity. This color intensity is plotted as a function of time in
Figure 7. The decrease in birefringence first exhibits a linear regime,
producing data similar to that of previous photoresponsive nematic
95
kinetics with rates on the order of minutes to hours, and indeed,
this was, most likely, the case given the previous photochemical
38
characterization of the LC. However, the SCLCP’s isotropiza-
tion is not necessarily linearly correlated with the azobenzene’s
isomerization. The bent cis LC moieties act as bulky impurities in
50
the nematic phase to lower its Tiso and to effectively destroy it
at a given temperature below the original T . Once the UV light
iso
was introduced, the nematic phase was destabilized, and the
linear regime followed by the exponential regime is consistent
with the kinetics of a nematic phase with a distribution of domain
6
7,80ꢀ94
7
77,78
systems,
with a rate of 1.0 ꢁ 10 intensity/s and finishes
sizes that is supersaturated with cis-azobenzene impurities.
with an exponential regime with a time constant of 4.6 s. The
rate in the linear regime can be converted to a traditional rate of
Figure 6 actually shows isotropization starting from the domain
boundaries and moving inward, leading to linear kinetics, followed
8
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Macromolecules
ARTICLE
by convergence of the growing isotropic domains—i.e., disappear-
ance of the nematic domains—leading to exponential kinetics.
Because there is a distribution of nematic domain sizes in the
beginning, the individual domains disappear at different times,
smaller ones followed by larger ones. Coincidentally, the camera is
centered on a cluster of smaller domains, which disappear faster and
produce the artifact of isotropization seeming to start in the center
of the screen. The fourth image in Figure 6 then shows some of the
smaller nematic domains in the center as they disappear, signifying
the transition to the exponential regime whereby the growing
isotopic domains begin to coalesce and stop growing. A decreasing
number of growing isotropic domains means a decreasing isotro-
pization rate, which corresponds to the exponential kinetics.
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1
on azobenzene LC from Li with the polysiloxane backbone and
32
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and this process started with a linear regime, with isotropic domains
emanating from the nematic domain boundaries, and finished with
an exponential regime, corresponding to convergence and coales-
cence of the growing isotropic domains. In the future, reversibility
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The authors of this paper thank the United States Army’s
(
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