The effect of light penetration depth on the LCST phase behavior of a thermo- and photoresponsive statistical copolymer in an ionic liquid

Article abstract of DOI:10.1002/pola.29075

A reflection cloud point technique allows for rapid screening of light-dependent phase separation temperatures of thermo- and photoresponsive polymer/ionic liquid solutions as a function of sample thickness, molecular weight, and copolymer composition. We systematically investigate the lower critical solution temperature (LCST) phase behavior of poly(benzyl methacrylate-stat-(4-phenylazophenyl methacrylate)). Under UV light, the photoresponsive azobenzene-based repeat unit becomes more polar as the cis form dominates, increasing its solubility in the ionic liquids 1-ethyl-3-methyl imidazolium and 1-butyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)imide. This light-dependent polarity change leads to two phase separation temperatures, depending on the illumination wavelength. Under visible light, which drives the azobenzene moiety into the trans ground state, the LCST shows no sample thickness dependence. Under UV light, however, sample thickness plays a significant role. Samples of around 1 mm thickness show no apparent difference under UV and visible light, whereas thinner samples show an increasing difference between the phase separation temperatures with decreasing sample thickness. Neither phase separation temperature exhibits a significant dependence on molecular weight. Increasing the photoresponsive monomer content did not lead to an increase in the difference between the phase separation temperatures at fixed thickness, due to a concomitant increase in UV light absorbed at the sample surface.

Full text of DOI:10.1002/pola.29075

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The Effect of Light Penetration Depth on the LCST Phase Behavior of a
Thermo- and Photoresponsive Statistical Copolymer in an Ionic Liquid
1,2
Cecilia C. Hall ,1 Cecelia A. Rivera,2 Timothy P. Lodge
1Department of Chemistry, University of Minnesota, Minneapolis, Minnesota, 55455
2Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, 55455
Correspondence to: T.P. Lodge (E-mail: lodge@umn.edu)
Received 28 March 2018; accepted 16 May 2018; published online 00 Month 2018
DOI: 10.1002/pola.29075
ABSTRACT: A reflection cloud point technique allows for rapid
screening of light-dependent phase separation temperatures of
thermo- and photoresponsive polymer/ionic liquid solutions as
a function of sample thickness, molecular weight, and copoly-
mer composition. We systematically investigate the lower criti-
cal solution temperature (LCST) phase behavior of poly(benzyl
methacrylate-stat-(4-phenylazophenyl methacrylate)). Under UV
light, the photoresponsive azobenzene-based repeat unit
becomes more polar as the cis form dominates, increasing its
solubility in the ionic liquids 1-ethyl-3-methyl imidazolium and
1-butyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)i-
mide. This light-dependent polarity change leads to two phase
separation temperatures, depending on the illumination wave-
length. Under visible light, which drives the azobenzene moiety
into the trans ground state, the LCST shows no sample thickness
dependence. Under UV light, however, sample thickness plays a
significant role. Samples of around 1 mm thickness show no
apparent difference under UV and visible light, whereas thinner
samples show an increasing difference between the phase sepa-
ration temperatures with decreasing sample thickness. Neither
phase separation temperature exhibits a significant dependence
on molecular weight. Increasing the photoresponsive monomer
content did not lead to an increase in the difference between the
phase separation temperatures at fixed thickness, due to a con-
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comitant increase in UV light absorbed at the sample surface.
2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem.
2018, 00, 000–000
KEYWORDS: azobenzene; ionic liquid; photoresponsive; poly(-
benzyl methacrylate); stimuli-responsive
temperature phase behavior (LCST). Fundamental studies
elucidating the phase separation behavior of these polymers
enables their extension to applications such as thermorever-
sible physically associated gels, where the stimuli-responsive
polymer is the endblock of an ABA triblock copolymer, with
the B block being soluble in the ionic liquid.9,16
INTRODUCTION Appropriate combinations of polymers and
ionic liquids yield materials with the desirable properties of
the ionic liquid and the mechanical robustness of poly-
mers.1,2 The high thermal stability, wide electrochemical win-
dows, high conductivity, and negligible vapor pressures make
ionic liquids an attractive choice as solvents for a variety of
applications.3 Immobilizing the ionic liquid with polymers
gives a wide range of possible applications including electro-
chemical mechanical actuators,4 gate dielectrics in field effect
transistors,5 gas separation membranes,6,7 and fuel cell
membranes.8 Using stimuli-responsive polymers adds
another dimension to the control over the structure of the
material, allowing for reversible gel formation or the con-
trolled motion of an actuator.10 Thermoresponsive polymers
have been well-studied in ionic liquids, such as poly(N-iso-
propyl acrylamide) (PNIPAm)11,12 and poly(benzyl methacry-
late) (PBzMA).13–15 PNIPAm in ionic liquids undergoes phase
separation on cooling, that is, upper critical solution temper-
ature (UCST) phase behavior. PBzMA in ionic liquids phase
separates on heating, following lower critical solution
Changing temperature to induce phase separation has been
well-studied, but this limits potential applications because
the system is being physically altered. Controlling the phase
separation in a manner that does not physically or chemi-
cally alter the system could open up a new range of applica-
tions for responsive ionic liquid/polymer systems, such as
self-healing materials.17–19 Using light as a stimulus could
enable such contactless transitions. Azobenzene is a very
common light-responsive molecule, where the wavelength of
light and temperature control the polarity of the azobenzene,
shown in Figure 1.20
The trans ground state under visible light is relatively non-
polar, but upon irradiation with UV light the azobenzene unit
Additional Supporting Information may be found in the online version of this article.
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Consequently, the AzoMA is not evenly distributed through the
chain, which was implicated in the observed broadening of the
phase transition for higher AzoMA loadings.
There have also been some studies on BzMA/AzoMA copoly-
mers, focusing on a small range of copolymer compositions,
at or below 4 mol% AzoMA incorporation, and low concen-
trations,24 or as a gel.25 Adding even a small amount of
AzoMA depresses the phase separation temperature signifi-
cantly—PAzoMA homopolymers are insoluble in common
ionic liquids. For example, a 28 kDa PBzMA at 3 wt% in 1-
ethyl-3-methyl imidazolium bis(trifluoromethanesulfonyl)i-
mide (EMI TFSI) undergoes phase separation at 105 8C.14
For a 3 wt% solution of a BzMA/AzoMA copolymer with 4
mol% AzoMA incorporation reduced the phase separation
temperature under visible light to 80 8C.24 The more polar
cis isomer increases the solubility of the polymer in the
polar ionic liquid, bringing the phase separation temperature
up to 102 8C.
FIGURE 1 cis/trans isomerization of azobenzene as a function
of light and temperature.
undergoes trans to cis isomerization, inducing a significant
dipole moment.20 It returns to the trans ground state via
irradiation with visible light, or through thermal relaxation.
This change in polarity can be used to change the solubility
of a polymer chain in solvent when the azobenzene moiety
is incorporated in a monomer.19,21–25 4-Phenylazophenyl
methacrylate can be copolymerized with thermoresponsive
monomers, where the change in polarity of the azobenzene
pendant changes the phase separation temperature. Ideally,
the photoresponsive monomer should be a minority compo-
nent, randomly distributed among the majority thermores-
ponsive monomer. Photoresponsive phase separation has
been well-studied in NIPAm and 4-phenylazophenyl methac-
rylate (AzoMA) copolymers in ionic liquids, over a range of
both copolymer compositions and concentrations. Figure 2
shows the two thermo- and photoresponsive copolymers
that will be discussed.
For this work, BzMA/AzoMA copolymers are examined in
more detail, exploring the phase separation temperature as a
function of molecular weight, copolymer composition, and
especially sample thickness. These monomers are chemically
similar, leading to reactivity ratios that are close to 1 (Sup-
porting Information, Fig. S1). Thus, the monomers are dis-
tributed more evenly, making it less likely that the phase
transition will broaden with increasing AzoMA content. This
allows for investigation of a large range of copolymer com-
positions, to examine whether the phase transition tempera-
ture window increases with increasing AzoMA content in
this system. The absolute temperature of the phase transi-
tion may change with molecular weight, as is the case with
PNIPAm in ionic liquid.11 Finally, sample thickness is a con-
sideration for photoresponsive compounds. The strong
absorbance of light drives the isomerization, but it can lead
to inhomogeneity in light exposure.26 If the sample is too
thick, the light is only able to penetrate a short depth into
the sample before IT IS entirely absorbed. Previous studies of
both PNIPAm/AzoMA and BzMA/AzoMA copolymers focused
on testing a single sample thickness of 200 mm.22,24 Under-
standing the sample thickness dependence on the phase sep-
aration behavior is crucial for extending these polymers to
any potential applications. For each polymer examined here,
a range of sample thicknesses was measured to investigate
how significantly the absorption of the azobenzene chromo-
phore impacts the measured phase transition temperature.
For PNIPAm in ionic liquid, the phase separation temperature
is affected by the molecular weight of the polymer12; this
effect is carried over to PNIPAm/AzoMA copolymers. By incor-
porating 29 mol% AzoMA in the copolymer, differences in
phase separation temperatures under UV and visible light of
up to 43 8C were observed.22 In comparison, using 8 mol%
AzoMA produced a window of only 8 8C. This difference in
phase separation temperatures gives a bistable temperature
window, where phase separation can be triggered by light at
any intermediate temperature. This approach has been
extended to AB diblock copolymers, showing light-induced
micellization and de-micellization,23 as well as to light-
induced gelation and gel melting in ABA triblock copoly-
mers.19 While this system has been demonstrated to be effec-
tive, the reactivity ratios of the two monomers are fairly
dissimilar. This leads to long reaction times, as both mono-
mers prefer to add AzoMA, which is loaded in small quantities.
EXPERIMENTAL
Materials
4-Phenylazophenyl methacrylate (AzoMA) was synthesized
from methacryloyl chloride and 4-phenylazophenol, as
described previously.27 Benzyl methacrylate was purchased
from Sigma Aldrich and purified over neutral alumina.
1,1,4,7,10,10-Hexamethyltriethylenetetramine
(HMTETA),
ethyl a-bromoisobutyrate (EBriB), and anhydrous anisole
FIGURE 2 Statistical copolymers of (a) NIPAm and AzoMA and
(b) BzMA and AzoMA.
were used as purchased from Sigma Aldrich. 1-Ethyl-3-methyl
2
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TABLE 1 Molecular Characteristics of P(BzMA-stat-AzoMA) Statistical Copolymers
Polymera
mol% incorporation of AzoMAb
Mn (kDa)c
Ðc
Average AzoMA Monomers per Chaind
BsA (17-6)
BsA (35-6)
BsA (86-6)
BsA (28-17)
6
17
35
86
28
1.05
1.06
1.09
1.10
5
6
10
24
24
6
17
a
c
The numbers in parentheses represent the number average molecular
weight in kDa and the mol% incorporation of AzoMA, respectively.
The number average degrees of polymerization and dispersities were
determined by SEC.
b
d
The incorporation of AzoMA was determined using 1H NMR
The average AzoMA molecules per chain were determined using the
spectroscopy.
number average molecular weight from SEC and the mol% incorpora-
tion of AzoMA from NMR spectroscopy.
imidazolium bromide and 1-butyl-3-methyl imidazolium chlo-
ride were used as purchased from Iolitec. Lithium bis(trifluor-
omethanesulfonyl)imide was used as purchased from 3M.
Copper (I) bromide was used as purchased from Riedel-de-
S4) and size-exclusion chromatography (SEC); their proper-
ties are summarized in Table 1.
Measurements
Proton nuclear magnetic resonance (1H NMR) spectra of the
polymers were collected using a 500 MHz Bruker Avance II
spectrometer, with deuterated chloroform as a solvent. The
molecular weight distributions were analyzed using size
exclusion chromatography (SEC) paired with refractive index
and light scattering detectors, with tetrahydrofuran (THF) as
the eluent. The refractive index increment of PAzoMA was
€
Haen. Stainless-steel washers were purchased from McMas-
ter–Carr.
Synthesis of EMI TFSI and BMI TFSI
The ionic liquids were prepared by an ion exchange reaction.
The ethyl methyl imidazolium bromide salt (EMI Br, 0.5 mol)
or butyl methyl imidazolium bromide salt (BMI Br, 0.5 mol)
was mixed with lithium bis(trifluoromethanesulfonyl)imide
(Li TFSI, 0.5 mol) in 250 mL of water, then stirred at 70 8C
for 48 h. Dichloromethane was added to the reaction, and
the water was extracted. The organic layer was washed with
deionized H2O until there was no excess salt present in the
water, as determined by a silver nitrate test.
ꢀ
measured using an Abbe refractometer (Supporting Informa-
tion, Fig. S5). UV–vis spectra of the polymers were obtained
using a Varian Cary 100 Bio UV–vis spectrophotometer with
temperature control (Fig. 5 and Supporting Information, Fig.
S6). The UV lamp used was a handheld UVP light source fil-
tered to 365 nm, with a power of 10 mW/cm2. The visible
light source was a Thor Labs LED with a peak wavelength of
420 nm.
The dichloromethane/ionic liquid mixture was transferred to
a round bottom flask and stirred with activated charcoal for
24 h to remove any residual color. After 24 h, the activated
charcoal was removed by filtering over neutral alumina, and
the dichloromethane was removed via rotary evaporation.
The final ionic liquid was dried in a vacuum oven at 70 8C
for 72 h. The resulting ionic liquids were characterized by
1H and 19F NMR spectroscopy, as shown in Supporting
Information, Figures S2 and S3.
Ionic Liquid Solution Preparation
All polymer/ionic liquid solutions were prepared using the
cosolvent method. Polymer and ionic liquid were combined
at the desired concentrations to achieve an overall mass of
5 g. THF was added to achieve full dissolution of both poly-
mer and ionic liquid, and the resulting solution was stirred
overnight. The THF was removed by purging with nitrogen,
and residual solvent was removed by drying in a vacuum
oven at room temperature for 72 h.
Synthesis of P(BzMA-Stat-AzoMA)
Copolymers were synthesized using atom transfer radical
polymerization (ATRP). In a representative synthesis, benzyl
methacrylate (170 mmol), AzoMA (9.0 mmol), HMTETA
(0.78 mmol), and EBriB (0.78 mmol) were combined with
anisole (90 mL) in a Schlenk flask. The mixture was
degassed through three freeze–pump–thaw cycles, then
placed under positive pressure of argon. The septum was
removed from the reaction flask under a flow of argon, and
copper (I) bromide (0.39 mmol) was added. After resealing
the flask under argon, the reaction was allowed to proceed
at 75 8C for 4 h. The reaction mixture was quenched by plac-
ing it an ice water bath and purified by precipitation in hex-
anes three times. The resulting polymers were characterized
using 1H NMR spectroscopy (Supporting Information, Fig.
FIGURE 3 Schematic of the thin-film cloud point experimental
setup. Laser exposure to the sample is controlled via a shutter
between the laser and the sample. The laser is transmitted
through the sample using mirrors. The temperature is con-
trolled by placing the sample on a heated, polished aluminum
plate. [Color figure can be viewed at wileyonlinelibrary.com]
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sample. For visible light measurements, the samples were
exposed for 10 min at room temperature, and the experiments
were performed in the dark. For UV light measurements, the
samples were exposed to the light for 1 h at room tempera-
ture, and the UV light remained on for the experiment. The
samples reach the steady state of cis isomer, shown in Figure 5
and Supporting Information, Figure S6, after 6–7 min. An hour
ensures that the higher concentration samples are in the
steady state—longer irradiation times did not have an effect
on the cloud point. Similarly, a higher UV power of 20 mW/
cm2 did not affect the cloud point. The samples were heated
on the polished aluminum plate using heating cartridges at a
rate of 1 8C/min. The temperature was recorded from a ther-
mocouple secured to the top of the microscope slide, to ensure
that the temperature reading is as close to sample tempera-
ture as possible. A motorized shutter controlled with an Ardu-
ino microcontroller reduces the exposure time of the laser on
the sample, blocking the beam for 30 s then allowing for trans-
mittance for 10 s. The cloud point is chosen as the point where
transmittance is 80% of the maximum value achieved.
RESULTS AND DISCUSSION
To study the effect of molecular weight on the phase separa-
tion behavior, the chosen polymers should have narrow molec-
ular weight distributions, with a significant range of average
molecular weights. Three polymers of varying molecular
weight with a targeted AzoMA incorporation of 5 mol% were
synthesized. The actual incorporation of AzoMA obtained was
slightly higher, at 6 mol%, due to the reactivity ratios of the
two monomers (Supporting Information, Fig. S1). The SEC
traces of the three polymers containing 6 mol% AzoMA and
varying molecular weights are shown in Figure 4(a). Each
polymer shows a single, narrow peak, and the number average
molecular weights vary by a factor of five. As well as investi-
gating the impact of molecular weight, it is interesting to con-
sider how changing comonomer composition will affect the
phase separation behavior. Accordingly, a fourth polymer was
synthesized with a similar molecular weight to BsA (35-6)
with the AzoMA incorporation approximately tripled to 17%;
the SEC trace is shown in Figure 4(b).
FIGURE 4 SEC traces of P(BzMA-stat-AzoMA) with (a) varying
molecular weights and 6 mol% AzoMA incorporation, and (b)
similar molecular weights and varying AzoMA incorporation.
[Color figure can be viewed at wileyonlinelibrary.com]
The four polymers were prepared as 10 wt% solutions in
ionic liquids. Concentrations below the overlap concentration
of the polymer, ca. 5 wt% for BsA (35-6), were too dilute to
reliably detect a cloud point in the thin film geometry. The
cloud point of 28 kDa PBzMA as a 3 wt% solution in EMI
TFSI is ca. 105 8C,14 whereas PAzoMA is completely insoluble
in both EMI TFSI and BMI TFSI. All BsA polymers with 6
mol% AzoMA were soluble in EMI TFSI at room tempera-
ture, with phase separation temperatures in the range of the
experimental limits of the home-built cloud point setup. BMI
TFSI was used for BsA (28-17), as it was insoluble in EMI
TFSI at room temperature.
Cloud Points
The cloud point of each polymer solution was measured in
reflection on a home-built instrument, shown in Figure 3.
The samples were pipetted onto a microscope slide in the
center of a stainless steel washer of a given thickness. A
glass coverslip was placed over the sample, and the slides
were sealed with epoxy. A HeNe laser (k 5 633 nm) was
reflected down onto the sample using a mirror. When the
sample is transparent, the laser is transmitted through the
sample and reflected from a polished aluminum plate up to
another mirror, which directs the laser toward the detector.
For each of the four polymer solutions, the cloud points
were measured across a range of sample thicknesses. The
absorbance of UV light in the AzoMA monomer enables the
trans to cis isomerization that changes the solubility of the
When the sample undergoes phase separation, the transmit-
tance of the laser beam drops as the sample becomes turbid.
The desired illumination source was placed directly above the
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polymer in ionic liquid, but it also means that the UV light
has a finite penetration depth through the sample. Figure 5
shows the UV–vis spectrum for 0.02 wt% BsA (28-17) in
BMI TFSI. The trans isomer has a strong absorbance in the
UV region, even at a low concentration and a path length of
2 mm. With this strong absorbance, it was of interest to
determine how sample thickness affects the observed phase
separation temperature, which required the use of reflective
cloud point measurements.
The reflection geometry used for these cloud point measure-
ments was designed to have the laser reflect from a polished
aluminum plate when the sample is homogeneously mixed.
However, there are also reflections that can occur at the various
interfaces (glass–air, coverslip–sample) that can give rise to a
measured laser power even when the sample is turbid. As a
result, the samples typically never reach 0% transmittance after
phase separation occurred. A representative cloud point for BsA
(35-6) is shown in Figure 6, where the transmittance shows a
clear drop, indicating the cloud point. The cloud point tempera-
ture is taken as 80% transmittance scaled to the initial and final
transmittance values. As previously demonstrated,24 the cis iso-
mer increases the solubility of the polymer under UV light, giv-
ing a cloud point ꢀ15 8C higher than under visible light.
FIGURE 6 Representative cloud points of 10 wt% BsA (35-6) in
EMI TFSI under UV (closed symbols) and visible light (open
symbols) at a thickness of 100 mm. [Color figure can be viewed
at wileyonlinelibrary.com]
a substantial dependence on sample thickness, as expected. For
both 1500 and 500 mm, the UV cloud points occur at approxi-
mately the same temperature as under visible light. For these
samples, the surface layer that is isomerized by the UV light
before it is absorbed is small in comparison to the bulk. Once
the visible light phase transition temperature is reached, the
bulk will become turbid, giving the same cloud point as under
visible light. As the samples become thinner, the visible and UV
light phase separation temperatures diverge, as the sample
thickness approaches the penetration depth of the UV light.
Increasing the window between the two cloud points allows for
more reliable light-triggered phase separation—minor temper-
ature fluctuations, such as increased heat from turning on a UV
lamp, will not change the temperature enough to move outside
of the window. Notably, the difference between the UV and visi-
ble light phase separation temperatures do not reach a plateau.
This suggests that the penetration depth is still <25 mm for
these samples. Due to signal limitations, this reflection setup
was not able to measure below 25 mm. A penetration depth can
be estimated based on the extinction coefficient obtained from
UV–vis spectrophotometry. This can be done at 324 nm, the
peak of absorbance in the UV region, as well as at 365 nm, the
peak output of the UV lamp utilized. The output of the UV lamp
is centered at 365 nm; however, the filter does not fully cut out
shorter wavelength UV light, thus the true penetration depth
likely falls between these two. For both wavelengths, the extinc-
tion coefficient depends on the isomerization state of the
AzoMA, where the trans isomer has a higher absorptivity than
the cis. As the surface is exposed to UV light, the isomerization
of the surface to cis will increase the penetration depth. There-
fore, the estimated penetration depth is expressed as a range
for each polymer. For BsA (17-6), the penetration depth for a 10
wt% solution in EMI TFSI was estimated as 10–90 mm at
324 nm and 80–600 mm at 365 nm (calculation discussed in
Supporting Information). As the azobenzene content is constant
for all molecular weights, the penetration depths for all three
polymers with 6 mol% AzoMA incorporation should be the
Each polymer was measured across a range of sample thick-
nesses from 25 to 1500 mm. The data for the three polymers
with 6 mol% AzoMA incorporation are shown in Figure 7(a).
The values for all measured cloud points are listed in Support-
ing Information, Table S1. The cloud point under visible light,
shown in open symbols, is independent of sample thickness.
Under these conditions the sample is in the trans ground state;
the 420 nm LED serves to drive any cis isomers formed from
ambient UV light back to the ground state. There is no signifi-
cant dependence on molecular weight for the cloud points
under visible light. The UV light cloud points, in contrast, show
FIGURE 5 UV–vis spectrum for fully trans (green) and fully cis
(black) isomerized AzoMA in BsA (28-17) at 0.02 wt% in BMI
TFSI. [Color figure can be viewed at wileyonlinelibrary.com]
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behavior such as PNIPAm in water, which has an LCST of ca.
32 8C across a range of molecular weights.28
The effects of increasing the AzoMA content in the copoly-
mer did not follow the expected trend. In a range of compo-
sitions of P(NIPAm-stat-AzoMA) copolymers in ionic liquid, it
was previously found that increasing the AzoMA content
increased the window between the phase separation temper-
atures under UV and visible light.22 In contrast, Figure 7(b)
shows that the window is virtually the same. In increasing
the AzoMA content, it is expected that a larger polarity
change will be induced in the polymer sample. This larger
change in polarity should lead to a bigger difference between
the visible and UV light phase separation temperatures.
However, increasing the AzoMA content also increases the
absorbance of UV light, decreasing the penetration depth in
the sample. The penetration depth for a 10 wt% solution of
BsA (28-17) in BMI TFSI was estimated as 4–40 mm at
324 nm and 30–450 mm at 365 nm. This is significantly
reduced from the penetration depths for the 6 mol% AzoMA
incorporation polymers. As a result, the range of sample
thicknesses accessible for the reflective cloud point measure-
ments are further from reaching full penetration of UV light
for BsA (28-17). If the samples were able to be tested at
thicknesses equal to or less than the penetration depth of
the UV light, BsA (28-17) would likely show the largest cloud
point temperature window. The previous work on the
BzMA/AzoMA copolymers used only EMI TFSI, and as such
was unable to get copolymers with compositions higher than
4 mol% AzoMA to dissolve in the ionic liquid.24 Additionally,
heating rate may play an effect on the cloud point tempera-
ture window. Ref. 24 used a heating rate of 0.1 8C/min, an
order of magnitude less than what was used for these tests.
While the slower heating rate may allow for a more precise
FIGURE 7 Cloud point temperatures of (a) varying molecular
weights of 10 wt% P(BzMA-stat-(6 mol%)AzoMA) in EMI TFSI and
(b) varying AzoMA content in 10 wt% P(BzMA-stat-AzoMA) in
EMI/BMI TFSI under UV and visible light as a function of sample
thickness. [Color figure can be viewed at wileyonlinelibrary.com]
same. Given these ranges, it is not unreasonable to assume that
the samples are still thicker than the penetration depth of UV
light. There is possibly a minor molecular weight dependence
for the two thinnest samples, where the lowest molecular
weight has the widest temperature window between UV and
visible light phase separation. This may be a result of higher
chain mobility, allowing some of the isomerized AzoMA chains
under UV light near the surface to move further in the sample,
giving a higher phase separation temperature under UV light
only. There is minimal difference for the other two molecular
weights, or for samples thicker than 100 mm. In general, there-
fore, the data are consistent with the conclusion that the cloud
points are independent of molecular weight. This follows expec-
tation for polymer/solvent systems that have LCST phase
FIGURE 8 Temperature window between UV and visible light
cloud point temperatures for the four polymers tested, across
the range of sample thicknesses. [Color figure can be viewed
at wileyonlinelibrary.com]
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determination of the phase separation temperature, it also
drastically increases the experiment time per sample.
Laaser and Peter Schmidt for help putting together the home-
built cloud point setup.
The data for cloud point temperature windows are collected
in Figure 8. All four polymers show a similar trend with
respect to sample thickness, and all can achieve a bistable
window of 20–30 8C. In general, BsA (28-17) shows the nar-
rowest window, with BsA (35-6) and BsA (86-6) being very
similar to each other and having a slightly broader window.
These two polymers can both be compared with BsA (28-17)
in different ways—BsA (35-6) has a similar molecular weight
and BsA (86-6) has the same average number of AzoMA mol-
ecules per single polymer chain (Table 1). As the samples
have not reached the thickness equal to or less than the pen-
etration depth of UV light, we cannot state definitively which
of these variables matters more. Finally, the smallest molecu-
lar weight chain BsA (17-6) shows the broadest temperature
window for the two thinnest samples tested, potentially as a
result of diffusional mixing.
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cloud point measurements was developed. This setup
allowed for the screening of the phase separation behavior
of four polymers under a range of six sample thicknesses,
from 25 to 1500 mm. Studying the effect of sample thickness
gives an idea of how thin the samples need to be to
approach full penetration of UV light through the sample.
This was not achieved for any of the samples, even at 25
mm, as evidenced by a continuing increase in the window
between the phase separation temperatures under UV and
visible light. However, even without full penetration of the
UV light, windows as large as 20–30 8C were measured. This
temperature difference is large enough to envision using pol-
y(BzMA-stat-AzoMA) copolymers for further work in photo-
responsive systems, such as light-induced micellization and
gel network formation. The lack of molecular weight depen-
dence in the phase separation temperature is also favorable
for these applications. By knowing the phase separation tem-
perature of a poly(BzMA-stat-AzoMA) copolymer at the con-
centration used, the bistable window for triggering
micellization or gelation with light can be predicted. Target
copolymer compositions can maintain low AzoMA content,
and still have a broad temperature window for triggering as
well. The less AzoMA that is necessary, the less “specialty”
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This work was supported by the National Science Foundation
through award DMR-1707578. The authors thank Prof. Jennifer
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