Photochromism and photopolymerization induced mesophase transitions in mixtures of spiropyran and mesogenic diacrylate

Article abstract of DOI:10.1021/jp108113b

A phase diagram of a binary mixture of photochromic molecule (spiropyran) and mesogenic diacrylate monomer has been established by means of differential scanning calorimetry and polarized optical microscopy. Subsequently, a theoretical phase diagram has been calculated by self-consistently solving the combined Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for nematic ordering, and phase field free energy for crystal solidification. The phase diagram thus obtained consists of various coexistence regions involving single-phase crystals, pure nematic, crystal + liquid, crystal + nematic, and crystal + crystal coexistence gaps. Under UV irradiation, both SP and SP/RM257 mixtures showed the lowering trend of the melting points, which may be attributed to the plasticization effect by the merocyanine isomers. When UV light is illuminated on the 2/98 SP/RM257 mixture for an extended period, mesogenic diacrylate in the mixtures gets polymerized, showing the permanent fixation of isotropic and nematic structures due to the network formation of RM257 caused by the biradicals in the merocyanine intermediate.

Full text of DOI:10.1021/jp108113b

J. Phys. Chem. B 2010, 114, 16381–16387
16381
Photochromism and Photopolymerization Induced Mesophase Transitions in Mixtures of
Spiropyran and Mesogenic Diacrylate
†
‡
,†
Namil Kim, Harris Lam, and Thein Kyu*
Department of Polymer Engineering, UniVersity of Akron, Akron, Ohio 44325-0301, United States, Department
of Mathematics, UniVersity of New Orleans, New Orleans, Louisiana 70148, United States
ReceiVed: August 26, 2010; ReVised Manuscript ReceiVed: NoVember 3, 2010
A phase diagram of a binary mixture of photochromic molecule (spiropyran) and mesogenic diacrylate monomer
has been established by means of differential scanning calorimetry and polarized optical microscopy.
Subsequently, a theoretical phase diagram has been calculated by self-consistently solving the combined
Flory-Huggins free energy for isotropic mixing, Maier-Saupe free energy for nematic ordering, and phase
field free energy for crystal solidification. The phase diagram thus obtained consists of various coexistence
regions involving single-phase crystals, pure nematic, crystal + liquid, crystal + nematic, and crystal +
crystal coexistence gaps. Under UV irradiation, both SP and SP/RM257 mixtures showed the lowering trend
of the melting points, which may be attributed to the plasticization effect by the merocyanine isomers. When
UV light is illuminated on the 2/98 SP/RM257 mixture for an extended period, mesogenic diacrylate in the
mixtures gets polymerized, showing the permanent fixation of isotropic and nematic structures due to the
network formation of RM257 caused by the biradicals in the merocyanine intermediate.
Introduction
surface modification by taking advantage of their photoreversible
properties.1
7-21
Recent advances in optoelectronic devices have focused on
development of new materials for fast information processing.
Among many potential materials, organic chromophores have
received considerable attention because of their photorevers-
ibility and precision controls. Taking advantage of different
optical properties in the bistable isomeric states, such chro-
mophores may be incorporated in a variety of matrixes to
produce various functions: specifically, photomechanical re-
sponse, anisotropic photoalignment, molecular assembly or self-
On the other hand, reactive mesogenic diacrylate monomers
have been used to fabricate the macroscopically aligned polymer
films. Upon polymerization, the reactive mesogens containing
more than two polymerizable terminal groups yield the mac-
roscopically ordered polymeric networks while maintaining their
1
-3
orientations as well as their mesophase structures permanently
up to their degradation temperatures.2
2-24
By virtue of the
inherent anisotropy of these mesogenic LC units and the
concomitant mesophase ordering, the shrinkage due to polym-
erization is relatively low as compared with the conventional
nonmesogenic networks. The blends of spiropyran (i.e., pho-
toisomerizable chromophore) and mesogenic diacrylate (RM257,
photopolymerizable monomer) are therefore of considerable
interest because the alignment of polymeric LC network can
be controlled by the SP-to-MC transformation. However, phase
segregation and crystallization of these photochromic units have
often led to deterioration in photophysical properties of these
composite films. Therefore, understanding the phase diagram
and associated phase transitions of these SP/RM257 mixtures
is of paramount importance from the fundamental as well as
practical standpoints.
4
-8
organization, and so forth.
Especially, liquid crystalline
chromophores and their mixtures with liquid crystal (LC)
counterparts have been investigated extensively because of their
potential applications, including image display or recording, in
which the molecular ordering/disordering of liquid crystalline
system can be manipulated by light-induced preferential
alignment.9 The light-manipulation of molecular architecture
of these LC/chromophore systems has practical significance;
for example, thin LC films containing photochromic molecules
can be easily inscribed by means of photolithographic patterning.
Moreover, the coherent light induces surface relief of the
structure by mass migration at the surface as well as in the bulk
-13
1
4-16
for a variety of volume holographic gratings.
In the present paper, phase behavior of spiropyran and
mesogenic diacrylate (RM257) mixture has been investigated
by means of differential scanning calorimetry (DSC) and optical
microscopy. A theoretical phase diagram was calculated by self-
consistently solving the combined free energies of Flory-Huggins
Photochromic spiropyran (SP) and its derivatives are one of
the frequently used chromophores because of their photoinduced
transformation of molecular geometry (or shape) and bistability.
Upon exposure to UV light (320-400 nm), the C-O bond of
the SP molecules undergoes heterolytic cleavage by transforming
from the colorless closed SP structure to the colored open
merocyanine (MC). Merocyanine is capable of returning to the
initial SP form by irradiation with visible light or thermal
treatment. Various applications using SP chromophores have
been reported in the fields of optical switch, image storage, and
(
FH), Maier-Saupe (MS), and phase field (PF) models. From
the nematic and crystal phase order parameters of each constitu-
ent, the phase transition temperatures were determined, and the
coexistence points were evaluated in the context of a double
tangent method via balancing the chemical potentials of each
phase. Thermal quenching experiments were performed to
identify the single and coexistence regions in comparison with
the theoretical phase diagram. Finally, the photochromism and
photopolymerization-induced mesophase transition has been
*
Corresponding Author: tkyu@uakron.edu.
University of Akron.
University of New Orleans.
†
‡
1
0.1021/jp108113b  2010 American Chemical Society
Published on Web 11/17/2010

1
6382 J. Phys. Chem. B, Vol. 114, No. 49, 2010
Kim et al.
(2)
examined by irradiating UV light at the single-phase nematic
and isotropic phases, and the effect of UV induced SP-to-MC
chromism on the phase diagram of the SP/RM257 blend has
been explored.
φ
2
MS
1
2
2
f
)
-ln Z + νφ s
2 2
(
)
r2
where ν is the anisotropic interaction parameter, and Z is the
partition function. Equation 2 can be understood by dividing
the total free energy density of MS theory into the entropic and
Experiments
MS
2
2
2
Materials and Methods. The spiropyran chromophore (SP,
enthalpic terms; that is, f ) -(1/2)νφ
2
s
2
2 2 2
- φ (ln Z - νφ s ),
M
w
) 322.4 g/mol) and reactive mesogenic diacrylate monomer
where the entropy is dominant initially, contributing to an
increase in the total free energy. With increasing director
alignment, the enthalpic contribution due to the attractive
interaction (i.e., the first term) becomes more negative, offsetting
the entropic contribution (i.e., the second term). Consequently,
the total free energy density exhibits a complex trend, giving
rise to an unstable free energy hump at some intermediate value
of s. However, with further increase in the director alignment,
it decreases to a large negative value and exhibits a potential
well for a nematic phase to stabilize. The detailed explanation
of the variation of the free energy density with orientational
(RM257, M ) 588.6 g/mol) were purchased from Aldrich and
w
Merck Ltd. respectively, and used without further purification.
Spiropyran transforms to merocyanine upon exposure to UV
light and reverts back to its ground state by thermal treatment
or irradiation with visible light. The mesogenic diacrylate
monomer possesses acrylate double bonds at both ends of the
backbone, capable of undergoing photoinitiated radical reaction
(
or photo-cross-linking). The blend samples containing different
SP contents were prepared at room temperature by dissolving
in a common solvent, carbon tetrachloride (CCl ). These
4
2
9
order parameter pertaining to eq 2 can be found elsewhere.
The orientational order parameter (s ) is defined as follows:
2
mixtures were mechanically stirred and homogenized and then
cast onto slide glasses. The residual solvent was further removed
by drying in a vacuum oven at 40 °C for 24 h.
The closed SP/open MC molecular shape transformation was
monitored by means of a UV/vis spectrophotometer (model
1
2
2
s2 )
∫
f(cos θ)
(
3 cos θ - 1
)
dΩ
(3)
8453, Hewlett-Packard). The spiropyran/n-hexane solution was
piped into a quartz cell, and then absorption spectra were
recorded as a function of UV wavelength. Phase transition
temperatures of pure and mixed samples were determined using
differential scanning calorimetry (DSC, model Q-1000, TA
Instruments). The DSC scan rate was 5 °C/min unless indicated
otherwise. Only the first DSC scan was analyzed because neat
RM257 can undergo self-polymerization during annealing at
where θ is the angle between the director of LC molecules and
the reference axis and u(θ) represents the nematic potential of
the LC director orientation. The orientation distribution function,
f(cos θ), and the partition function, Z, are defined as
1
πZ
f(cos θ) )
exp[-u(cos θ)/kT]
(4)
(5)
4
1
50 °C if the annealing time exceeds 1 h. The cell was purged
with nitrogen gas at a flow rate of 40 mL/min. For optical
microscopy (BX60, Olympus) analysis, the pictures of various
coexistence phases were taken using a digital camera (EOS
u(cos θ)
Z )
∫
exp -
d(cos θ)
[
]
kT
3
00D, Cannon) following several thermal quenches. The UV
unit (ELC403, Electrolite Corporation) having an intensity of
2
The anisotropic interaction parameter is given as ν )
4
0 mW/cm at a wavelength of 350-380 nm was utilized to
4
.541TNI/T.
trigger photochromism.
The free energy density of crystal solidification may be
Thermodynamics Description. To describe the phenomena
of liquid-liquid demixing, nematic ordering, and crystal
solidification, we combined free energies of Flory-Huggins
described in the context of the phase field model in which the
free energy of pure crystal has the Landau-type asymmetric
double-well form; namely,3
0,31
FH
MS
PF
(
f ), Maier-Saupe (f ), and phase field (f ) model along with
coupling
their coupling interaction (f
) energy. The free energy
ψi
density of liquid-liquid demixing can be generally described
f
(
ψi
)
) Wi
i i i i i
( )( )
ψ ψ - ꢁ ψ - 1 dψ
∫
2
5,26
0
according to the Flory-Huggins (FH) theory,
(6)
ꢁ
i(T)
ꢁi(T) + 1
2
3
1
4
4
)
Wi
ψ -
ψ + ψi
i
[
i
]
2
3
φ
φ
2
FH
1
f
) r1 ln φ +
ln φ + ꢀ φ φ
aa 1 2
(1)
1
2
r2
The coefficient of the third-order term in eq 6 should be finite
to describe the first-order phase transition. ψ
crystal order parameters of each constituent. The coefficient W
is related to the energy to overcome the unstable energy hump
), which may be estimated experimentally from the melting
1
and ψ
2
are the
i
where r
by constituent molecules 1 and 2, which may be taken as unity
for a small molecular system. φ and φ are the volume fractions
1 2
and r represent the numbers of lattice sites occupied
(ꢁ
i
u
1
2
point (Ti,m) and heat of fusion (∆H
); that is,
i
of components 1 and 2, respectively. ꢀaa is the Flory-Huggins
interaction parameter representing the amorphous-amorphous
interaction given empirically by ꢀaa ) A + B/T, where A and B
are constants.
u
i
0
i,m
1/2 - ꢁi)-
)
(
1
W ) 6∆H /RT
(
1 - T/T
i
Maier-Saupe (MS) mean-field theory is employed to account
In addition, the coupling interaction term may be expressed in
terms of crystal-amorphous (noncrystal), amorphous-crystal,
and crystal-crystal interactions; that is,
for the nematic ordering of liquid crystal molecules through
2
7-29
2
the orientational order parameter, s , in what follows:

SP, SP/RM257 Mixtures Mesophase Transitions
J. Phys. Chem. B, Vol. 114, No. 49, 2010 16383
coupling
2
2
2
f
)
(
ꢀ ψ1 - 2ꢀ ψ ψ + ꢀ ψ + Rs ψ ψ2)φ φ
ca
cc
1
2
ac
2
2
1
1
2
(
7)
The last term in eq 7, indicating the nematic-crystal higher-
order coupling interaction, is very small relative to the lower
order coupling terms, and thus, it can be ignored. Hence, the
total free energy of the binary crystalline and single component
nematic system becomes
φ
φ
2
1
f(φ, ψ, s) ) φ1f
(
ψ1
)
+ φ2f
(
ψ2
)
+
ln φ +
ln φ +
2
1
r1
r2
2
2
(
ꢀaa + ꢀ ψ1 - 2ꢀ ψ ψ + ꢀ ψ
)
φ φ +
ca
cc
1
2
ac
2
1
2
2
φ
νφ s
Figure 1. UV/vis absorption spectra of spiropyran in n-hexane solution
2
2 2
-
ln Z +
(8)
(
)
as a function of UV light irradiation time. The intensity of the UV
r2
2
2
source was 40 mW/cm.
where ꢀca ≡ ꢀc a is the crystal-amorphous interaction and ꢀac
1
2
≡
ꢀa c represents the amorphous-crystal interaction. These
1
2
interaction parameters are taken to be proportional to the heat
u
of fusion of crystalline component; that is, ꢀca ∼ ∆H
1
/RT; ꢀac
u
∼
∆H
2
/RT, and the crystal-crystal interaction parameters can
1/
be expressed by their geometric mean; that is, ꢀcc ) c
w
(ꢀca ·ꢀac
)
2
, where c
w
signifies the departure from the ideality (i.e., c
w
)
1
for the ideal solid solution). The crystalline-amorphous
interaction parameter, ꢀca, of a crystalline-amorphous blend can
be determined experimentally from the liquidus front (i.e., the
melting point depression approach), as reported by Rathi et al.32
The phase transitions from nematic-isotropic and crystal-nematic
may be obtained by minimizing the total free energy with respect
to the respective order parameters as follows:
Figure 2. Comparison of DSC thermograms and optical micrographs
of neat RM257 before and after photopolymerization. (a) Optical
micrographs of monomeric RM257, corresponding to the crystal +
nematic, nematic + isotropic coexistence regions, and the isotropic
phase. (b) Upon photopolymerization of RM257/photocurative system,
the crystal, nematic, and isotropic morphologies got permanently fixed,
showing no signs of degradation up to the maximum temperature of
the experiment (i.e., 250 °C).
∂
f(φ, ψ, s)
∂f(φ, ψ, s)
∂ψ1
∂f(φ, ψ, s)
∂ψ2
)
0
) 0
) 0
∂
s2
(9)
These minimized values of s and ψ
i
thus obtained may be
subsequently substituted in the free energy expression (eq 8),
and then the coexistence points may be calculated by balancing
the pseudo chemical potentials for each phase along with the
double tangent method; namely, (∂f/∂φ
detailed procedures for seeking the self-consistent solution by
show a tendency of leveling off as the SP-to-MC conversion
reaches asymptotic equilibrium in hexane solution.
)|φ iR ) (∂f/∂φ )|φ iꢂ . The
i
i
The DSC thermogram of neat RM257 exhibits two major
endothermic peaks at ∼71 and 128 °C, along with a weak minor
peak at around 43 °C. The weakest broad peak at 43 °C may
be a consequence of the melting of nonequilibrium crystals
formed during the cold crystallization. As manifested by the
optical micrographs in Figure 2a, the crystalline, nematic, and
isotropic phases can be identified at 26, 100, and 130 °C
respectively. This observation suggests that the low-temperature
3
3
a double tangent algorithm can be found elsewhere.
Results and Discussion
Phase Diagram of SP/RM257 Mixtures. The initial SP/n-
hexane solution is transparent and colorless, showing no
discernible UV/vis absorption peak (Figure 1). Upon UV
irradiation, the transparent SP/n-hexane solution turned to a dark
blue merocyanine (MC), which may be attributed to the cleavage
of a C-O bond. When the UV light is turned off, the solution
gradually returns to the transparent state, implying a reversible
nature of the SP-MC transformation. With the UV illumination,
a broad absorption peak develops at around 590 nm and a minor
shoulder appears at 420 nm, which are ascribed to excited state
absorptions from merocyanine isomers. The pathway of SP-
to-MC transition is shown in Figure 1, where the C-O bond
cleavage has led to the generation of the biradicals in the
intermediate that eventually transform to a more stable Zwit-
terion form of the MC molecule. The intensity of both peaks
progressively increases with irradiation time, indicative of the
forward SP-to-MC conversion. After 3 min, the peak intensities
DSC peak at 71 °C corresponds to the crystal-nematic (TCr-N
)
transition, whereas the highest peak at 128 °C is due to
nematic-isotropic (TNI) transition. When photopolymerization
was carried out on the RM257/photocurative (Irgacure 651)
system at various reaction temperatures corresponding to the
isotropic, nematic, and crystalline regions of neat RM257, these
hierarchical morphologies got permanently fixed due to the
formation of the diacrylate networks. More importantly, these
structures persisted up to the highest experimental temperature of
250 °C without any sign of degradation (Figure 2b).
3
4
Figure 3 depicts the DSC thermograms of the SP/RM257
blends obtained at a heating rate of 5 °C/min. The crystal melting
point (Tm,1 ) 178 °C) of neat SP and crystal-nematic (TCr-N,2
) 71 °C) and nematic-isotropic (TNI,2 ) 128 °C) transition
3
5

1
6384 J. Phys. Chem. B, Vol. 114, No. 49, 2010
Kim et al.
Figure 3. DSC thermograms of SP/RM257 mixtures, showing the
systematic shift of phase transitions as a function of blend ratio. The
inset shows the enlarged DSC melting transitions of SP at intermediate
compositions. The heating rate was 5 °C/min.
temperature of neat RM257 decline in their blends, as indicated
by the arrows. The enlarged view of Tm,1 at 40-20 wt % SP is
shown in the inset. Compared with Tm,1, the depression of TCr-N,2
of RM257 in mixtures is not significant because of the limited
plasticizing effect by SP. On the other hand, the TNI of RM257
decreases drastically from 128 to 88 °C with addition of 10 wt
%
of SP, and thereafter, it levels off. To identify the single and
coexistence phases bound by the liquidus and solidus lines, the
theoretical calculation has been carried out and compared with
the experimental phase diagram as well as with the morphology
development in the thermal quench experiments.
In the theoretical calculation, we examined the variation of
crystal and nematic order parameters of each constituent in neat
constituents as well as their blends. As can be seen in Figure
Figure 4. Temperature dependence of crystal and nematic order
parameters of SP and RM257 in their neat and blend concentrations:
(
a) crystal order parameter of SP (ψ
1
) and (b) crystal and nematic order
parameters of RM257 (ψ , s ).
2
2
4
1
a, the order parameter of pure SP crystal (ψ ) drops discretely
at 178 °C. Upon adding RM257, a similar discontinuous
transition takes place at a lower temperature, and concurrently,
its value decreases with concentration. The lower value of crystal
order parameter suggests that the emerged SP crystal is solvated
and becomes less perfect. A similar trend is observed for RM257
crystal (ψ ) in Figure 4b, in which the crystal-nematic phase
2
transition of pure RM257 declines from 71 to 62 °C with
increasing SP (to 15 wt %) with its value around 0.99 and an
additional discontinuous change occurring at a higher temper-
ature arising from the nematic-isotropic phase transition (Figure
4
b). The isotropization temperature of RM257 drops from 128
to 68 °C as the SP content is increased from 0 to 15%. By
inserting these values into the respective PF and MS free
energies and combining together, the total free energy curve
can be constructed for each temperature, and the coexistence
points can be determined in accordance with the common
tangent. Subsequently, the liquidus and solidus lines were
obtained by connecting the loci.
The predictive capability of self-consistently calculated phase
diagram is further compared with the experimental observations.
In Figure 5, the calculated lines represented by the solid lines
are in good accord with the DSC melting transition points
denoted by the open circles. The model parameters utilized in
Figure 5. The calculated liquidus and solidus lines in comparison with
the experimental phase diagram of SP/RM257 mixture. The circular
symbol represents the phase transition points determined by DSC, and
the solid line denotes the theoretical curves.
coincided with the pure axes of the constituents. In a descending
order of temperature, the crystal + liquid (Cr
1
+ L
2
), crystal +
2
+ Cr )
nematic (Cr + N ), and crystal + crystal (Cr
1
2
1
coexistence regions are discernible over the broad compositions.
At extreme RM257 compositions (e.g., <10 wt % SP), the
single-phase nematic (N ) appears below the TNI of RM257.
2
the self-consistent calculation were r
) ) 346 K at T ) -100 °C, and c
experimentally accessible material parameters: ∆H
1
) r
w
2
) 1, A ) 0, B ()
With descending temperature, the isotropic phase transforms
to a single-phase nematic (N ) by passing through the narrow
liquid + nematic (L + N ) coexistence gap. The single-phase
crystal of respective constituent (Cr , Cr ) is hard to identify in
the phase diagram because the solidus lines of Cr + Cr
coexistence region are virtually overlapped with the pure axes,
ꢀ
c
T
c
c
) 0.01, along with the
u
1
) 20.3 kJ/
) 30.7 kJ/mol at TCr-N,2 ) 71 °C,
2
u
mol at Tm,1 ) 178 °C, ∆H
2
1
2
and TNI,2 ) 128 °C. Of particular interest is that the calculated
liquidus line captures the experimental trend of the depressed
melting points, whereas the crystal solidus line is virtually
1
2
1
2

SP, SP/RM257 Mixtures Mesophase Transitions
J. Phys. Chem. B, Vol. 114, No. 49, 2010 16385
Figure 6. Polarized optical micrographs of 5/95 and 60/40 SP/RM257
mixtures taken after 5 h following several temperature quenches into
various coexistence regions corresponding to (a) single nematic (N
2
),
Figure 7. DSC thermograms of pure SP and its blends with RM257
after UV irradiation at the isotropic region, displaying a slight depression
of crystal melting point of SP. No transition peak was observed above
(
b) crystal + liquid (Cr
1
+ L
2
), (c) crystal + nematic (Cr
1
+ N
2
), and
(
d) crystal + crystal (Cr
1
+ Cr
2
) coexistence regions.
8
0 wt % RM257. DSC scans were conducted at a heating rate of 5
°C/min.
implying that the SP and RM257 molecular crystals prefer to
grow within their own species, forming separate crystals.
The thermal quenching experiments are further undertaken
to test the validity of the calculated phase diagram. Figure 6a-d
shows the polarized optical micrographs of 5/95 and 60/40 SP/
RM257 mixtures taken after 5 h following various temperature
quenches to the single and coexistence regions (marked on the
phase diagram). At 5 wt % SP (Figure 6a), the Schlieren textures
with two and four brushes representing the strength of line
disclination of (1/2 and (1 are fully developed at 90 °C,
indicative of the single-phase nematic (N
presence of Cr + L , Cr + N , and Cr
regions has been confirmed by changing the quench depth of
0/40 SP/RM257 mixture. The sample was heated to the
2
) (Figure 6a). The
1
2
1
2
1
+ Cr coexistence
2
6
isotropic state (170 °C) and then quenched to the corresponding
coexistence regions. When the sample was quenched to 110
°
C, the numerous needle-like crystals were found to disperse
in the continuum of surrounding isotropic liquid (Figure 6b).
Since the morphology shows no structural change after 5 h, it
Figure 8. Comparison of DSC thermograms of the 2/98 SP/RM257
mixture before and after UV irradiation, showing the complete
disappearance of the crystal-nematic and nematic-isotropic transitions
of RM257 upon UV irradiation.
may be inferred that Cr
1
2
+ L coexistence region reaches
asymptotically a stable state. Upon thermal quenching from 170
to 75 °C (Figure 6c), the bright area with dark brushes develops
at the edge of the crystals, since the isotropic RM257 transforms
to the nematic phase (please see the enlarged view). However,
the dark region still persists, the origin of which is presently
unclear. One possible conjecture is that it may be associated
with the homeotropic alignment of RM257 mesogens. Figure
The same study was extended to various SP/RM257 blends,
in which the melting temperature (Tm,1) of SP was systematically
reduced with increasing RM257 content (Figure 7). There are
two possible scenarios for the reduction of Tm,1 in the SP-MC/
RM257 mixtures: the plasticization effect by RM257 (provided
that there is no cross-linking) or the formation of MC during
UV irradiation, or combination of both. At higher RM257
loading levels above 80%, the DSC thermograms show no
melting peaks, suggesting that the original eutectic melting
behavior of SP/RM257 blend is no longer discernible upon UV
irradiation.
To illustrate further, the DSC thermograms of the 2/98 SP/
RM257 mixture before and after UV irradiation at the isotropic
state (125 °C) were compared in Figure 8. Prior to irradiation,
the 2/98 SP/RM257 mixture shows two endothermic peaks
corresponding to the crystal-nematic (69 °C) and the nematic-
isotropic (121 °C) transitions, but are a few degrees lowered
than those of neat RM257 (see Figure 2), which is simply due
to the plasticization by SP. However, upon UV irradiation, these
peaks completely disappear, implying that the photoinduced SP-
MC transformation might have prevented the crystallization of
RM257. Even though Zwitterion is a major product of photoi-
somerized MC, the biradicals are found in the MC intermedi-
ates.35 It may be hypothesized that the biradicals resulting from
the cleavage of the C-O bonds of SP during the SP-to-MC
6
d shows the optical micrograph taken after a temperature
quench to 28 °C, in which the whole microscopic view area is
covered by both constituent crystals, confirming the Cr
coexistence region.
1
+ Cr
2
Effect of Photochromism on Phase Behavior of SP/
RM257 Mixtures
The effect of SP-MC photochromism on phase transition
behavior of pure SP was examined by irradiating with UV light
for 5 min in the isotropic state, that is, 4 °C above the
isotropization temperature. After cooling the sample to ambient
temperature, DSC scans were acquired at a rate of 5 °C/min.
Since the SP-to-MC chromism can hardly reach full conversion,
the residual SP may undergo crystallization during cooling. As
shown in Figure 7, the crystal melting temperature of the UV-
exposed neat SP was found to be depressed from 178 to 174
°
m
C. The observed lowering of T of neat SP may be due to the
plasticizing effect by the UV irradiated noncrystalline mero-
cyanine or its intermediate.

1
6386 J. Phys. Chem. B, Vol. 114, No. 49, 2010
Kim et al.
Conclusions
We have demonstrated the eutectic behavior of SP/RM257
blends that consisted of single component mesophases (N ) and
Cr + L , Cr + N , and Cr + Cr coexistence regions in the
2
1
2
1
2
1
2
temperature versus composition phase diagram. The correspond-
ing single and coexistence regions have been further confirmed
by the emerged morphologies following thermal quenching. The
photochromism of SP to MC was found to lower the melting
transition temperatures of neat SP as well as those in the SP/
RM257 mixtures. Moreover, the UV irradiation of the SP/
RM257 blend in the isotropic state has led to the formation of
infusible nematic phase over the broad temperature range
investigated, which may be attributed to the photopolymerization
of RM257 triggered by biradicals of the merocyanine intermedi-
ates. These observations imply that either the radical curing
reaction of RM257 must be slow, the conversion may be low,
or both. However, it is still sufficient to anchor the underlying
mesophase or crystalline structures permanently in the present
SP/RM257 system.
Figure 9. Morphology development of the 2/98 SP/RM257 mixture
subjected to photochromism at (a) isotropic (i.e., at 125 °C), showing
the appearance of infusible birefringence entities; and (b) nematic phase
Acknowledgment. Support of this work by National Science
Foundation through Grant nos. DMR-0514942 and NSF REU-
DMR-0648318, is gratefully acknowledged. H.L. is indebted
to Goodyear Tire and Rubber Company for their generous
support of the REU program. This research is supported in part
by the Collaborative Center for Polymer Photonics sponsored
by the Air Force Office of Scientific Research, Wright-Patterson
Air Force Base, and the University of Akron. Acknowledgment
is made to the donors of the American Chemical Society
Petroleum Research Fund (PRF #48735-ND7) for in part support
of this research.
(
i.e., at 100 °C), showing the fixation of the nematic Schlieren texture
upon UV irradiation in the nematic gap.
chromism have triggered the photo-cross-linking of RM257 in
the presence of MC intermediates.
To elucidate possible photopolymerization of RM257 in the
presence of SP, the morphological investigation of the 2/98 SP/
RM257 mixture during the course of photochromism was carried
out without adding any curatives. When UV light is illuminated
on the above mixture at the isotropic state (i.e., 125 °C),
numerous birefringence droplets appear and persist up to 200
References and Notes
(
(
1) Rau, H. Photochemistry and Photophysics; Boca Raton: FL, 1990.
2) Irie, M. PhotoreactiVe Materials for Ultrahigh Density Optical
°
C (Figure 9a), which is consistent with the DSC result showing
no phase change. Similarly, when photopolymerization was
Memory; Elsevier: Amsterdam, 1994.
(
(
3) Bouas-Laurent, H.; D u¨ rr, H. Pure Appl. Chem. 2001, 73, 639–665.
taken at the single-phase nematic temperature of 100 °C (Figure
4) Ichimura, K.; Oh, S.-K.; Nakagawa, M. Science 2000, 288, 1624–
9
b), the Schlieren texture was permanently fixed, that is,
1
626.
(5) Yu, Y.; Ikeda, T. Angew. Chem., Int. Ed. 2006, 45, 5416–5418.
6) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. J. Chem. Soc., Chem.
Commun. 1993, 18, 1439–1442.
7) Lendlein, A.; Jiang, H.; J u¨ nger, O.; Langer, R. Nature 2005, 434,
879–882.
8) O’Neill, M.; Kelly, S. M. J. Phys. D: Appl. Phys. 2000, 33, R67–
infusible up to the maximum experimental temperature of 200
C. The fixation of the nematic Schlieren texture is one of the
(
°
signatures of the RM257 reactive mesogen undergoing photo-
polymerization.22 As seen in Figure 2, the anisotropic polymer
network prepared from the neat RM257 sustained its crystalline
and nematic structures up to 250 °C. As pointed out earlier, the
product of photoisomerized MC intermediates is the biradicals,
which may be responsible for the structural vitrification
phenomenon of the present SP/RM257 system.
(
(
R84.
(
9) Ikeda, T. J. Mater. Chem. 2003, 13, 2037–2057.
(10) Gibbons, W. M.; Shannon, P. J.; Sun, S.-T.; Swetlin, B. J. Nature
991, 351, 49–50.
1
(
11) Hasegawa, M.; Yamamoto, T.; Kanazawa, A.; Shiono, T.; Ikeda,
T. AdV. Mater. 1999, 11, 675–677.
As pointed out above, there are two opposing effects of the
UV irradiation on phase transitions of the SP/RM257 blends:
(12) Legge, C. H.; Mitchell, G. R. J. Phys. D: Appl. Phys. 1992, 25,
92–499.
4
(13) Ruslim, C.; Komitov, L.; Matsuzawa, Y.; Ichimura, K. Jpn. J. Appl.
(1) the formation of MC that can lower the SP melting associated
Phys. 2000, 39, L104–L106.
with the plasticization effect, and (2) the biradicals thus
generated in the merocyanine intermediates trigger the curing
reaction of RM257 such that the resulting network will raise
the melting transition temperature of SP, that is, polymerization-
induced crystallization. Since the melting point depression was
found to be prevalent in the UV irradiated SP in its blends with
RM257 (Figure 7), it is reasonable to conclude that the
plasticization effect predominates over the polymerization-
induced crystallization of SP. The above finding implies that
the free radical curing reaction of RM257 in the presence of
biradicals of MC intermediates must be slow, and the conversion
may be low, but it is just enough to fix the underlying mesophase
or crystalline structures permanently in the SP/RM257 system.
(14) Yoneyama, S.; Yamamoto, T.; Tsutsumi, O.; Kanazawa, A.; Shiono,
T.; Ikeda, T. Macromolecules 2002, 35, 8751–8758.
(
15) Ubukata, T.; Seki, T.; Ichimura, K. AdV. Mater. 2000, 12, 1675–
678.
(16) Stracke, A.; Wendorff, J. H.; Goldmann, D.; Janietz, D.; Stiller, B.
AdV. Mater. 2000, 12, 282–285.
1
(
17) Dvornikov, A. S.; Malkin, J.; Rentzepis, P. M. J. Phys. Chem. 1994,
9
8, 6746–6752.
(18) Berkovic, G.; Krongauz, V.; Weiss, V. Chem. ReV. 2000, 100,
741–1753.
1
(19) Bunker, B. C.; Kim, B. I.; Houston, J. E.; Rosario, R.; Garcia, A. A.;
Hayes, M.; Gust, D.; Picraux, S. T. Nano Lett. 2003, 3, 1723–1727.
(20) Mele, E.; Pisignano, D.; Varda, M.; Farsari, M.; Filippidis, G.;
Fotakis, C.; Athanassiou, A. Appl. Phys. Lett. 2006, 88, 203124/1–203124/
3
.
(
21) Stitzel, S.; Byrne, R.; Diamond, D. J. Mater. Sci. 2006, 41, 5841–
5844.

SP, SP/RM257 Mixtures Mesophase Transitions
J. Phys. Chem. B, Vol. 114, No. 49, 2010 16387
(
22) Hikmet, R. A. M.; Lub, J.; Broer, D. J. AdV. Mater. 1991, 3, 392–
(29) Shen, C.; Kyu, T. J. Chem. Phys. 1995, 102, 556–562.
(30) Harrowell, P. R.; Oxtoby, D. W. J. Chem. Phys. 1987, 86, 2932–
2942.
3
94.
(
23) Broer, D. J.; Finkelmann, D. J.; Kondo, K. Makromol. Chem. 1988,
1
3
2
1
89, 185–194. Broer, D. J.; Mol, G. N. Makromol. Chem. 1989, 190, 19–
0. Broer, D. J.; Boven, J.; Mol, G. N. Makromol. Chem. 1989, 190, 2255–
268. Broer, D. J.; Hikmet, R. A. M.; Challa, G. Makromol. Chem. 1989,
90, 3201–3215.
(31) Matkar, R. A.; Kyu, T. J. Phys. Chem. B 2006, 110, 16059–
16065.
(32) Rathi, P.; Huang, T.-M.; Dayal, P.; Kyu, T. J. Phys. Chem. B 2008,
112, 6460–6466.
(33) Koningsveld, R.; Stockmayer, W. H.; Nies, E. Polymer Phase
(
24) Kihara, H.; Tamaoki, N. Macromol. Rapid Commun. 2006, 27, 829–834.
25) Flory, P. J. Principles of Polymer Chemistry; Cornell University
(
Diagrams: A Textbook; Oxford University Press: New York, 2001.
Press: Ithaca, NY, 1953.
26) Olabisi, O.; Robeson, L. M.; Shaw, M. T. Polymer-Polymer
Miscibility; Academic Press: New York, 1979.
27) Maier, W. Z. Naturforsch. Teil A 1959, 14, 882–889. Maier, W.;
Saupe, A. Z. Naturforsch. Teil A 1960, 15, 287–292.
28) Chandrasekhar, S. Liquid Crystals; Cambridge University: Cam-
bridge, 1992.
(
34) Shirinian, V. Z.; Besugliy, S. O.; Metelitsa, A. V.; Krayushkin,
(
M. M.; Nikalin, D. M.; Minkin, V. I. J. Photochem. Photobiol. A: Chem.
2
007, 189, 161–166.
35) Heller, C. A.; Fine, D. A.; Henry, R. A. J. Chem. Phys. 1961, 65,
908–1909.
(
(
1
(
JP108113B