Low Temperature Thermochromic Polydiacetylenes: Design, Colorimetric Properties, and Nanofiber Formation

Article abstract of DOI:10.1021/acs.macromol.5b02683

Owing to their stimulus responsive color changing properties, polydiacetylenes (PDAs) have been extensively investigated as colorimetric sensors. Thermochromic properties of PDAs have been the central focus of a number of investigations that were aimed not only at gaining a fundamental understanding of the physical basis of the color change but also at developing practical applications as temperature sensors. The thermochromic transition temperature of a PDA polymer is closely related to the melting point of the corresponding diacetylene (DA) monomer. In addition, the majority of PDAs described to date undergo a blue-to-red color change above room temperature because PDAs are generally derived from DA monomers that have melting points above room temperature. In the current study, we developed a series of low temperature colorimetric PDAs that were designed based on the reasoning that removal of the sources for strong headgroup interactions would lower the melting points of the corresponding DA monomers. This strategy was used to design and fabrication of PDA sensors that display color transitions in the range of 5-30°C. Moreover, the thermochromic transition temperatures of the PDAs were found to decrease by ca. 10°C when the alkyl chain length in the DA monomer is truncated by two methylene units. The results of FTIR and Raman spectroscopic analyses suggest that the PDA alkyl chain adopts an all-trans conformation in the blue-phase and some gauche forms exist in the alkyl chain in the red-phase PDA. Finally, the new PDAs are stable up to 300°C, and their processable nature enables them to be fabricated in nanofiber forms by employing an anodized aluminum oxide (AAO) membrane as a template.

Full text of DOI:10.1021/acs.macromol.5b02683

Article
pubs.acs.org/Macromolecules
Low Temperature Thermochromic Polydiacetylenes: Design,
Colorimetric Properties, and Nanofiber Formation
In Sung Park, Hye Jin Park, Woomin Jeong, Jihye Nam, Youngjong Kang, Kayeong Shin,‡
†
‡
†
†
‡
‡,§
,†,§
Hoeil Chung, and Jong-Man Kim*
†
‡
§
Department of Chemical Engineering, Department of Chemistry, and Institute of Nano Science and Technology, Hanyang
University, Seoul 133-791, Korea
*
S Supporting Information
ABSTRACT: Owing to their stimulus responsive color changing properties,
polydiacetylenes (PDAs) have been extensively investigated as colorimetric
sensors. Thermochromic properties of PDAs have been the central focus of a
number of investigations that were aimed not only at gaining a fundamental
understanding of the physical basis of the color change but also at developing
practical applications as temperature sensors. The thermochromic transition
temperature of a PDA polymer is closely related to the melting point of the
corresponding diacetylene (DA) monomer. In addition, the majority of PDAs
described to date undergo a blue-to-red color change above room temperature
because PDAs are generally derived from DA monomers that have melting
points above room temperature. In the current study, we developed a series of
low temperature colorimetric PDAs that were designed based on the reasoning
that removal of the sources for strong headgroup interactions would lower the
melting points of the corresponding DA monomers. This strategy was used to
design and fabrication of PDA sensors that display color transitions in the range of 5−30 °C. Moreover, the thermochromic
transition temperatures of the PDAs were found to decrease by ca. 10 °C when the alkyl chain length in the DA monomer is
truncated by two methylene units. The results of FTIR and Raman spectroscopic analyses suggest that the PDA alkyl chain
adopts an all-trans conformation in the blue-phase and some gauche forms exist in the alkyl chain in the red-phase PDA. Finally,
the new PDAs are stable up to 300 °C, and their processable nature enables them to be fabricated in nanofiber forms by
employing an anodized aluminum oxide (AAO) membrane as a template.
INTRODUCTION
of the corresponding diacetylene (DA) monomers, which for
the most part are solids at room temperature because they
contain moieties that enable intermolecular hydrogen bonding
as well as arene−arene and electrostatic interactions.
■
1
−11
Polydiacetylenes (PDAs)
are a family of supramolecular
conjugated polymers that have received great attention because
they display stimulus-responsive color (generally blue-to-red)
and fluorescence (non-to-red) changes. A wide range of
Low temperature thermochromic PDAs have potential utility
as temperature sensors for goods that require refrigeration or
freezing. In this type of application, a colorimetrically
irreversible sensor is more desirable because it would indicate
when goods have been exposed to an environment above room
1
2−20
21,22
chemical/biochemical (e.g., solvents,
ions,
surfac-
2
3−27
28−36
37
tants,
biomolecules,
explosives ) and physical (e.g.,
3
8−45
46−48
49
temperature,
electric current
mechanical strain,
magnetic field,
5
0−52
) stimuli promote colorimetric and
55
temperature any time during the storage period. We and
fluorometric transitions of properly designed PDAs. Colori-
56
others have already described low temperature thermochro-
mic PDAs derived from isocyanate containing and aliphatic DA
monomers. In the current study, we designed and prepared a
new series of PDAs that undergo a blue-to-red color transition
in the temperature range between 5 and 30 °C. The PDAs are
prepared by polymerization reactions of DA monomers that
contain methyl or methoxyethyl ester groups. These DAs were
found to be liquids below 30 °C, and they solidify in a freezer at
metrically responsive PDAs also respond to water when they
53
contain hygroscopic elements in headgroups or when they are
54
embedded in a hydrogel matrix.
Thermochromic properties of PDAs have been the central
focus of a number of investigations since the time of the
discovery of these conjugated polymers. This effort has not
only provided a fundamental understanding of the physical
basis for thermochromism but also led to practical applications
of PDAs as temperature sensors. The majority of thermochro-
mic PDAs reported to date have colorimetric transition
temperatures above room temperature (typically >40 °C).
This property is a consequence of the fact that the transition
temperatures of PDAs are closely related to the melting points
−
5 °C. UV irradiation of the solid DAs results in generation of
Received: December 11, 2015
Revised: January 25, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.5b02683
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
the corresponding blue PDAs, which undergo colorimetric
transition at temperatures close to the melting points of the
respective monomers. The results of FTIR and Raman
spectroscopic analyses suggest that alkyl chains in the blue-
phase of these PDAs adopt all-trans conformations, whereas
some gauche conformations exist in the red-phase. In addition,
the soluble and thermally stable (up to 300 °C) nature of PDAs
enables them to be used in a straightforward approach for the
preparation of PDA nanofibers, which employs an anodized
aluminum oxide (AAO) membrane as a template. The results
of these studies reveal important information about strategies
needed to design DA monomers that will generate PDAs that
have desired thermochromic properties and provide a
mechanistic basis for the color changing phenomenon and
fiber formation of low temperature thermochromic PDAs.
(calcd C28
H
48
O
3
, 432.3603). Other diacetylene monomers were
prepared by employing a similar strategy.
-Methoxyethyl Tricosa-10,12-diynoate (TCDA-EGME). 510 mg,
2
−1
8
8%; mp 15.45 °C. IR (NaCl) νmax (cm ): 2927, 2854, 1737, 1463,
1
1
376, 1240, 1178, 1130, 1099, 1035, 931, 863, 723. H NMR (300
MHz, CDCl ): δ = 4.22 (2H, m), 3.59 (2H, m), 3.39 (3H, s), 2.34
3
(
1
2H, t, J = 7 Hz), 2.24 (4H, t, J = 7 Hz), 1.62 (2H, q, J = 7 Hz), 1.53−
13
.46 (4H, m), 1.40−1.26 (22H, m), 0.88 (3H, t, J = 7 Hz). C NMR
(150 MHz, CDCl ): δ = 174.0, 77.7, 77.6, 70.7, 65.4, 65.3, 63.4, 59.1,
3
34.3, 32.0, 31.7, 29.7, 29.6, 29.4, 29.2, 29.2, 29.2, 29.0, 29.0, 28.9, 28.5,
28.4, 25.0, 22.8, 22.8, 19.3, 19.3, 14.3. HRMS m/z 404.3293 (calcd
C
H
44
O
3
, 404.3290).
26
2
-Methoxyethyl Heneicosa-8,10-diynoate (HCDA-EGME). 472 mg,
−1
8
0%; mp 4.86 °C. IR (NaCl) νmax (cm ): 2926, 2854, 1737, 1462,
1
1
377, 1348, 1252, 1200, 1175, 1130, 1092, 1038, 988, 864, 723. H
NMR (300 MHz, CDCl ): δ = 4.23 (2H, m), 3.59 (2H, m), 3.40 (3H,
3
s), 2.35 (2H, t, J = 7 Hz), 2.24 (4H, t, J = 7 Hz), 1.61 (2H, q, J = 7
Hz), 1.56−1.49 (4H, m), 1.41−1.26 (18H, m), 0.88 (3H, t, J = 7 Hz).
13
EXPERIMENTAL SECTION
C NMR (150 MHz, CDCl ): δ = 173.9, 77.8, 70.6, 65.5, 65.3, 63.5,
3
■
5
2
3
9.1, 34.2, 32.0, 29.7, 29.6, 29.4, 29.2, 29.0, 28.7, 28.6, 28.5, 28.2, 24.9,
2.8, 19.3, 19.3, 14.3. HRMS m/z 376.2976 (calcd C H O ,
76.2977).
Methyl Pentacosa-10,12-diynoate (PCDA-Me). 470 mg, 91%; mp
Materials and Instruments. 1-Ethyl-3-(3-(dimethylamino)-
propyl)carbodiimide (EDC) was purchased from TCI Chemicals
2
4
40
3
(
Japan). 2-Methoxyethanol and anhydrous solvents were obtained
from Sigma-Aldrich (USA). 10,12-Pentcosadiynoic acid (PCDA),
0,12-tricosadiynoic acid (TCDA), and 8,10-heneicosadiynoic acid
−1
2
9.00 °C. IR (NaCl) νmax (cm ): 2924, 2854, 1741, 1464, 1435, 1362,
1
(
1
1
1323, 1238, 1196, 1171, 1099, 1016, 881, 854, 723. H NMR (300
MHz, CDCl ): δ = 3.67 (3H, s), 2.30 (2H, t, J = 7 Hz), 2.24 (4H, t, J =
HCDA) were purchased from GFS-Chemicals (USA). H NMR
spectra were recorded on a Varian Unitylnova (300 MHz)
3
spectrometer, and 13C NMR spectra were recorded on a Varian
7 Hz), 1.61 (2H, q, J = 7 Hz), 1.55−1.46 (4H, m), 1.39−1.25 (26H,
m), 0.88 (3H, t, J = 7 Hz). 13C NMR (150 MHz, CDCl ): δ = 174.4,
Premium COMPACT NMR Magnet System (150 MHz) spectrom-
eter using CDCl3 as solvent. DSC spectra were obtained on a
DSC2010 (TA Instruments, Inc.). IR spectra were recorded on a
MAGNa-IR E.S.P (Thermo Fisher Scientific, Inc.). The absorption
spectra were examined on an USB2000 miniature fiber-optic
spectrometer (Ocean Optics). Raman spectra were collected by direct
illumination of laser radiation (785 nm, Invictus, Kaiser Optical Inc.).
Optical and fluorescence images of the samples were obtained using an
Olympus (BX51 W/DP70) microscope. Scanning electron microscope
3
7
2
1
7.7, 77.6, 65.4, 65.3, 51.6, 34.2, 32.1, 31.7, 29.8, 29.8, 29.7, 29.6, 29.5,
9.2, 29.2, 29.2, 29.0, 29.0, 28.9, 28.5, 28.4, 25.0, 22.8, 22.8, 19.3, 19.3,
4.3. HRMS m/z 388.3344 (calcd C H O , 388.3341).
26 44
2
Methyl Tricosa-10,12-diynoate (TCDA-Me). 1.56 g, 75%; mp 20.61
−1
°
1
C. IR (NaCl) νmax (cm ): 2924, 2854, 1741, 1466, 1434, 1360, 1323,
238, 1196, 1170, 1099, 1016, 879, 856, 723. H NMR (300 MHz,
1
CDCl ): δ = 3.67 (3H, s), 2.30 (2H, t, J = 7 Hz), 2.24 (4H, t, J = 7
3
Hz), 1.61 (2H, q, J = 7 Hz), 1.54−1.47 (4H, m), 1.39−1.26 (22H, m),
0
7
2
3
.88 (3H, t, J = 7 Hz). 13C NMR (150 MHz, CDCl ): δ = 174.4, 77.7,
7.6, 65.4, 65.3, 51.6, 34.2, 32.0, 31.7, 29.7, 29.6, 29.4, 29.2, 29.2, 29.0,
9.0, 28.9, 28.5, 28.4, 25.0, 22.8, 22.8, 19.3, 19.3, 14.3. HRMS m/z
60.3026 (calcd C H O , 360.3028).
(
SEM) images were obtained using a JEOL (JSM-6330F) FE-SEM at
3
an accelerating voltage of 15 kV. A homemade Peltier device was used
to investigate the thermochromism of the polydiacetylene at low
temperatures. Molecular weight of the polymer was estimated with a
Waters 717plus autosampler which is equipped with a refractive index
detector (Waters 2415) and four series columns (Styragel, HR 5,
24 40
2
Methyl Heneicosa-8,10-diynoate (HCDA-Me). 510 mg, 88%; mp
−1
9
.76 °C. IR (NaCl) νmax (cm ): 2925, 2854,1741, 1463, 1435, 1365,
1
1
323, 1252, 1201, 1167, 1093, 1076, 1016, 877, 842, 723. H NMR
5
0K−4M; Styragel, HR 4, 5K−600 K; Styragel, HR 2, 500−20K;
(300 MHz, CDCl ): δ = 3.67 (3H, s), 2.31 (2H, t, J = 7 Hz), 2.25 (4H,
Styragel, HR 0.5, 0−1K; columns are packed with 5 μm particles and
the size of the column is 7.8 mm × 300 mm). In GPC measurements,
chloroform was used as the eluent at a flow rate of 1 mL/min. GPC
samples were taken at 0.2 wt % concentration and filtered with a 0.2
μm hydrophobic PTFE filter (Advantec Syringe Filters) to remove
aggregates.
3
t, J = 7 Hz), 1.63 (2H, q, J = 7 Hz), 1.56−1.26 (22H, m), 0.88 (3H, t, J
=
7 Hz). 13C NMR (150 MHz, CDCl ): δ = 174.3, 77.8, 77.4, 65.5,
3
6
5.3, 51.6, 34.1, 32.0, 29.7, 29.6, 29.4, 29.2, 29.0, 28.7, 28.6, 28.5, 28.2,
24.9, 22.8, 19.3, 19.3, 14.3. HRMS m/z 332.2716.
Representative Procedure for Preparation of Polydiacety-
lenes. PCDA-EGME (640 mg) was placed on a Peltier device and the
surface temperature was lowered to −5 °C. After UV irradiation (254
nm, 1 mW/cm ) for 1 h at −5 °C, unreacted monomer (331 mg) was
recovered by using a cotton-filled syringe filter with hot ethyl acetate.
The residual red precipitate was triturated with hot chloroform, and
the organic solvent was concentrated in vacuo to afford poly(PCDA-
EGME) (267 mg, 42%). H NMR (300 MHz, CDCl
Representative Procedure for Preparation of Diacetylene
Monomers. A tetrahydrofuran (THF) solution containing 10,12-
pentacosadiynoic acid (PCDA) (500 mg, 1.33 mmol), 1-ethyl-3-(3-
2
(
dimethylamino)propyl)carbodiimide (510 mg, 2.67 mmol), 2-
methoxyethanol (1.05 mL, 13.35 mmol), and 4-(dimethylamino)-
pyridine (20 mg) was stirred for 18 h at room temperature. The
mixture was concentrated in vacuo, and the residue was dissolved in
hexane. The hexane solution was washed with saturated NaCl(aq), and
1
): δ = 4.22 (2H,
3
t, J = 4.4 Hz), 3.59 (2H, t, J = 4.6 Hz), 3.38 (3H, s), 2.47 (2H, br),
2.34 (3H, t, J = 7.3 Hz), 1.61 (7H, br), 1.42−1.18 (27H, m), 0.88 (3H,
the organic layer was dried over MgSO , concentrated to give an oil
4
t, J = 6.8 Hz). 13C NMR (150 MHz, CDCl
): δ = 173.7, 70.5, 63.2,
that was subjected to a silica gel column chromatography
3
(
hexane:ethyl acetate (20:1 v/v)) to afford 2-methoxyethyl pentaco-
58.9, 34.2, 31.9, 29.8, 29.7, 29.6, 29.4, 29.3, 29.2 28.7 24.9, 22.7, 14.1.
sa-10,12-diynoate (PCDA-EGME) (495 mg, 86%); mp 24.01 °C. IR
Other polydiacetylenes were prepared by employing a similar strategy.
−1
1
(
1
NaCl) νmax (cm ): 2925, 2854, 1737, 1464, 1377, 1348, 1240, 1176,
Poly(TCDA-EGME). H NMR (300 MHz, CDCl
3
): δ = 4.22 (2H, t, J
1
130, 1097, 1035, 931, 865, 723. H NMR (300 MHz, CDCl ): δ =
= 4.4 Hz), 3.59 (2H, t, J = 4.6 Hz), 3.38 (3H, s), 2.47 (2H, br), 2.34
(3H, t, J = 7.3 Hz), 1.56 (7H, br), 1.40−1.18 (23H, m), 0.88 (3H, t, J
3
4
.23 (2H, m), 3.56 (2H, m), 3.40 (3H, s), 2.34 (2H, t, J = 7 Hz), 2.24
= 6.8 Hz). 13C NMR (150 MHz, CDCl ): δ = 173.8, 70.5, 63.2, 58.9,
(
4H, t, J = 7 Hz), 1.62 (2H, q, J = 7 Hz), 1.56−1.46 (4H, m), 1.41−
.26 (26H, m), 0.88 (3H, t, J = 7 Hz), 0.88 (3H, t, J = 7 Hz). 13
NMR (150 MHz, CDCl ): δ = 174.0, 77.7, 77.6, 70.7, 65.4, 65.3, 63.4,
3
1
C
34.2, 31.9, 29.7,29.6, 29.4, 29.3, 29.2 28.7, 24.9, 22.7, 14.1.
1
Poly(HCDA-EGME). H NMR (300 MHz, CDCl ): δ = 4.22 (2H, t,
3
3
5
2
9.1, 34.3, 32.1, 29.8, 29.8, 29.7, 29.6, 29.5, 29.2, 29.2, 29.2, 29.0, 29.0,
8.9, 28.5, 28.4, 25.0, 22.8, 19.3, 19.3, 14.3. HRMS m/z 432.3607
J = 4.4 Hz), 3.59 (2H, t, J = 4.6 Hz), 3.38 (3H, s), 2.47 (2H, br), 2.34
(3H, t, J = 7.1 Hz), 1.61 (7H, br), 1.40−1.18 (20H, m), 0.88 (3H, t, J
B
DOI: 10.1021/acs.macromol.5b02683
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Macromolecules
Article
=
6.6 Hz). 13C NMR (150 MHz, CDCl ): δ = 173.6, 70.5, 63.2, 59.0,
3
Scheme 1. Preparation of Diacetylene Monomers Used for
Preparation of Low Temperature Thermochromic
Polydiacetylenes
3
4.1, 32.0, 29.8, 29.7, 29.4, 29.3, 29.2, 28.9, 28.7, 28.5, 24.9, 22.7, 14.1.
1
Poly(PCDA-Me). H NMR (300 MHz, CDCl ): δ = 3.66 (3H, s),
3
2
.47 (2H, br), 2.29 (3H, t, J = 7.6 Hz), 1.61 (6H, br), 1.40−1.26 (28H,
13
m), 0.88 (3H, t, J = 7.0 Hz). C NMR (150 MHz, CDCl ): δ = 174.2,
3
5
1.4, 34.1, 31.9, 29.8. 29.7, 29.6, 29.4, 29.3, 25.0, 22.7, 13.1.
1
Poly(TCDA-Me). H NMR (300 MHz, CDCl ): δ = 3.66 (3H, s),
3
2
.48 (1H, br), 2.30 (3H, t, J = 7.6 Hz), 1.61 (6H, br), 1.39−1.24 (24H,
13
m), 0.88 (3H, t, J = 7.0 Hz). C NMR (150 MHz, CDCl ): δ = 174.2,
3
5
1.4, 34.1, 31.9, 29.8, 29.7, 29.6, 29.4, 29.3, 24.9, 22.7, 14.1.
1
Poly(HCDA-Me). H NMR (300 MHz, CDCl ): δ = 3.66 (3H, s),
3
2
.47 (1H, br), 2.30 (3H, t, J = 7.1 Hz), 1.63 (6H, br), 1.39−1.24 (18H,
13
m), 0.88 (3H, t, J = 6.8 Hz). C NMR (150 MHz, CDCl ): δ = 174.1,
3
5
1.4, 34.1, 31.9, 29.8, 29.7, 29.4, 29.3, 29.2, 28.8, 24.9, 22.7, 14.1.
Preparation of Polydiacetylene Nanofiber. Poly(PCDA-
EGME) powder was coated on the surface of an anodized aluminum
oxide (AAO) (pore diameter: ca. 200 nm; thickness: 60 μm)
membrane, and the membrane was placed on a hot plate at 150 °C.
The red poly(PCDA-EGME) powder turned yellow as the polymer
melts, and yellow polymer penetrates into the porous membrane. The
PDA embedded AAO membrane was cooled to room temperature,
and the residual PDA on the membrane surface was removed by using
a razor blade. Incubation of the PDA embedded AAO membrane in
aqueous KOH solution (20 wt %) results in dissolution of the
membrane. The PDA nanofibers were collected by centrifugation and
washed with DI water.
RESULTS AND DISCUSSION
■
Preparation and Properties of Diacetylene Mono-
mers. In line with the general trend observed for long alkyl
chain diacetylenic acids, 10,12-pentcosadiynoic acid (PCDA,
mp: 62−63 °C), 10,12-tricosadiynoic acid (TCDA, mp: 54−56
°
C), and 8,10-heneicosadiynoic acid (HCDA, mp: 51−52 °C)
are solids at room temperature. Again following the general rule
of thumb that the temperature needed to promote
thermochromism of a PDA is close to the melting point of
the corresponding DA monomer, the color changes of PDAs
derived from PCDA, TCDA, and HCDA were observed to be
initiated at around 65, 55, and 52 °C, respectively.
Conversion of a carboxylic acid group to an ester moiety
removes the source for intermolecular hydrogen bonding, and
as a result, it causes the melting temperature of a substance to
decrease. Based on this trend, six ester group containing DA
monomers were prepared from the corresponding carboxylic
acids (Scheme 1). Three DA monomers PCDA-Me, TCDA-
Me, and HCDA-Me were obtained using standard esterification
reactions of diacetylenic acids with MeOH in the presence of 1-
ethyl-3-(3-(dimethylamino)propyl)carbodiimide (EDC) and
Figure 1. Nomalized DSC thermograms of DA monomers monitored
in endothermic regions: HCDA-EGME (a), HCDA-Me (b), TCDA-
EGME (c), TCDA-Me (d), PCDA-EGME (e), and PCDA-Me (f).
The heating rate is 10 °C/min.
also Figure S1, Supporting Information). DSC analyses provide
information about the thermal properties of the DA monomers.
For example, owing to the structural flexibility of methoxyethyl
moiety, members of the EGME family of PDAs have lower
melting temperatures (4.9−5.2 °C) than the methyl ester
family. Second, the melting points of DA esters was found to
decrease ca. 8.5 °C (Me family: 8.39 °C; EGME family: 8.56
°C) as the number of methylene groups between the
diacetylene and terminal methyl group (m value) decreases.
In addition, a decrease in the number of methylene groups
between the diacetylene and ester groups (n value) of the DA
monomers causes an approximate 10.7 °C decreases in melting
point (Me family: 10.85 °C; EGME family: 10.59 °C). The
observed relationship between melting point and chain length
suggests that the carbon chains of the DA monomers play an
important role in governing intermolecular interactions.
(
dimethylamino)pyridine (DMAP). Ester containing DA
monomers PCDA-EGME, TCDA-EGME, and HCDA-EGME
were prepared by employing a similar strategy.
One of the advantageous features of PDAs that is not shared
by other conjugated polymers is their ability to contain alkyl
chains that are readily modified in order to manipulate
colorimetric transition properties. In general, PDAs having
shorter alkyl chains are colorimetrically more sensitive to
environmental stimuli. Thus, PDAs arising by polymerization of
the monomers displayed in Scheme 1 are also expected to have
variable thermochromic transition temperatures.
In Figure 1 are shown normalized differential scanning
calorimetry (DSC) thermograms for six DA monomers.
Analysis of these thermograms shows that the maximum heat
flow is in the following order: PCDA-Me (29.0 °C), PCDA-
EGME (24.0 °C), TCDA-Me (20.6 °C), TCDA-EGME (15.5
Because they are liquid at room temperature, structural
analysis of the DA monomers by using X-ray diffraction (XRD)
and transmission electron microscopy (TEM) is difficult. Much
°C), HCDA-Me (9.8 °C), and HCDA-EGME (4.9 °C) (see
C
DOI: 10.1021/acs.macromol.5b02683
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Figure 2. (A, B) Infrared (A) and Raman (B) spectra of liquid (black line) and solid (blue line) state diacetylene monomer PCDA-Me. (C)
Schematic representation of PCDA-Me molecules in liquid (left) and solid (right) states.
information exists in the literature about how the conforma-
tions of hydrocarbons having long alkyl chains in the solid state
can be determined by FTIR and Raman spectroscopic
liquid PCDA-Me in the C−C stretching region (Figure 2B, see
also Figure S3 for other DA monomers) contains major broad
−1
bands in the 1075−1084 cm region. In contrast, three major
5
7−62
−1
methods.
As a result, the CH stretching regions (3050−
bands at around 1063, 1100, and 1127 cm are present in the
2
−1
2750 cm ) in FTIR spectra of the liquid and solid states
spectrum of solid PCDA-Me. This Raman spectroscopic data
along with those accumulated for the five other DA monomers
in both liquid and solid states suggest that these substances
exist in all-trans conformations in the solid state, and they have
random coil structures with some gauche conformations when
they are in the liquid state (Figure 2C).
PCDA-Me (Figure 2A, see also Figure S2 for other DA
monomers) were analyzed. The C−H stretching region in the
FTIR spectrum of PCDA-Me in the liquid state contains two
broad bands corresponding to asymmetric stretching at 2924−
1
−1
2
927 cm− and C−H symmetric stretching at 2854−2856 cm .
The FTIR spectrum of solid state PCDA-Me was obtained by
placing the liquid monomer on a NaCl plate followed by
cooling to below the melting temperature by using a Peltier
device. The C−H stretching region in the spectrum of solid
PCDA-Me contained bands in the same region as that of the
liquid state of DA monomer except that they split into groups
with 3−5 bands in the asymmetric stretching region and 2−3
sharp peaks in the symmetric stretching region. A similar peak
splitting phenomenon was observed in the C−H wagging and
rocking vibration regions of the FTIR spectra of PCDA-Me.
The broad bands in the spectrum of the liquid state DA
correspond to overlapping vibration bands associated with
various C−C and C−H bond conformations. However, at low
temperature the solid DA monomer will likely exist in a single
conformation, and as a result, the FTIR spectrum will contain
Preparation and Thermochromic Properties of Poly-
diacetylenes. In order to investigate the feasibility of
photopolymerization of the DA monomers and the thermo-
chromic properties of the resulting PDAs, each monomer was
immobilized on filter paper. The monomer-wetted paper was
placed on a homemade Peltier device, and the surface
temperature of the device was lowered to below the melting
point of the monomer. Solidification of the monomer was
readily observed by using the naked eye. Irradiation of the solid
sample with a hand-held laboratory UV lamp (254 nm, 1 mW/
2
cm ) leads to formation of a blue PDA. Polymers derived from
six DA monomers were soluble in chloroform, and the
molecular weight of the polymers was dependent on the
headgroup structures. For instance, the polymer derived from
PCDA-Me had a weight-average molecular weight (M ) of
w
59
well-defined bands (Figure 2A).
208 000 with a polydispersity (PD) of 2.93. Similarly,
poly(TCDA-Me) and poly(HCDA-Me) displayed Mw of
210 000 (PD: 1.98) and 160 000 (PD: 3.16), respectively.
Interestingly, polymers derived from EGME containing DAs
In order to gain more information about the molecular
structure, Raman spectra were recorded for both liquid and
solid states of the DA monomers. It is known that the Raman
−1
bands in the region of 1150−1000 cm provide useful
information about the conformational preferences of long alkyl
chains of hydrocarbons and, in particular, that the presence of a
showed lower M values compared to those obtained with Me
w
derivatives. Thus, poly(PCDA-EGME) and poly(TCDA-
EGME) had molecular weights of 50 000 (PD:2.25) and
89 000 (PD: 2.53), respectively. The molecular weight of the
polymer derived from HCDA-EGME was not obtained owing
to the precipitation of the polymer in the column during the
GPC analysis.
−1
broad band at 1089 cm indicates the existence of some gauche
57
conformations in liquid state. In addition, Raman spectra of
solid state hydrocarbons contain two bands at approximately
−1
1
065 and 1129 cm . Inspection of the Raman spectrum of
D
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Figure 3. Absorption spectra of PDAs derived from PCDA-Me (A), PCDA-EGME (B), TCDA-Me (C), TCDA-EGME (D), HCDA-Me (E), and
HCDA-EGME (F) upon heating.
Real-time spectroscopic monitoring of color change under-
gone by the PDAs as the temperature increases was carried out
using a fiber-optic absorption spectrometer over the following
temperature ranges: PCDA-Me (0−30 °C), PCDA-EGME (0−
26 °C), TCDA-Me, (−2−22 °C), TCDA-EGME (0−18 °C),
HCDA-Me (−2−11 °C), HCDA-EGME (−4−10 °C). As can
be seen in Figure 3, the absorption maximum (635−607 nm) of
the blue phase PDA shifts to lower wavelength (547−542 nm)
upon gradual heating.
Plots of colorimetric responses (CR) of thermochromic
PDAs as a function of temperature often provides useful
information about the thermochromic properties of the
3
polymer. CR, a relative evaluation of color changes, is defined
as CR = (PB − PB )/PB × 100%, where PB is the percentage
0
f
0
Figure 4. Plots of colorimetric response (CR) values of PDAs derived
from HCDA-EGME (a), HCDA-Me (b), TCDA-EGME (c), TCDA-
Me (d), PCDA-EGME (e), and PCDA-Me (f) as a function of
temperature.
of absorbance of blue before (PB ) and after (PB ) the
0
f
thermochromic transition as given by PB = Ablue/(Ablue + Ared)
×
100%. In Figure 4 are displayed plots of CR values of the
PDAs as a function of temperature in the range of −5−30 °C.
The plots show that each PDA has characteristic temperature
range over which the CR values sharply increase. The
temperature regions are as follows: PCDA-Me (28−29 °C),
PCDA-EGME (21−24 °C), TCDA-Me (15−18 °C), TCDA-
EGME (10−12 °C), HCDA-Me (7−10 °C), and HCDA-
EGME (2−5 °C). Interestingly, the temperature at which the
PDA undergoes a sharp colorimetric transition was found to be
related closely to the melting point of the corresponding
monomer. In general, PDAs derived from Me ester containing
E
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DA monomers display a gradual increase of CR before the
sharp transition takes place while the PDAs derived from
EGME containing DA monomers undergo abrupt colorimetric
transitions without an preliminary gradual increases.
chains containing all-trans conformations. However, upon
heating to 40 °C these bands disappear being replaced by
−1
one major band at lower frequency (1070 cm ). The results
suggest that the major all-trans C−C conformation of the alkyl
chain is altered during the color transition, becoming at least
partially gauche. In addition, when a red-PDA sample is
subjected to a cooling, the three characteristic bands associated
with of the all-trans alkyl chains of the blue phase are not
restored, and the red phase PDA band at 1070 cm− remains
unchanged (Figure S5). These spectroscopic features indicate
that the original conformation of the blue phase PDA is not
restored by cooling of the thermally stimulated PDA.
Changes occurring in the molecular structures of PDAs
during the blue-to-red color transition caused by a temperature
increase were indirectly demonstrated by using Raman
spectroscopy. In the Raman spectra of both the blue phase of
the PDAs (Figure S4), bands associated with ene−yne groups
1
in the conjugated backbone appear at ca. 1450 (CC) and ca.
2
080 cm− (CC), respectively. The Raman bands for the
1
ene−yne groups in the red phase PDAs created by heating are
−1
shifted to higher frequencies (ca. 1520 and ca. 2120 cm ,
respectively). This spectral change is promoted by the
thermally induced conformational distortion of PDA backbone.
In a similar manner, Raman bands in the C−C stretching
region give useful information about the molecular structure of
The mechanism for operation of the low temperature color
transition PDAs described above is interesting. The close
relationship that exists between the DA melting point and the
colorimetric transition temperature of the PDA indicates that
conformational changes in the macromolecule play a significant
role in thermochromism. Evidence gained from Raman
spectroscopic studies support the proposal that the con-
formations of alkyl chains in the PDAs change from all-trans in
the solid state (monomer) and in the blue-phase PDA
57
aliphatic alkyl chains in the PDA backbone. The blue-phase
PDA prepared by UV irradiation of solid PCDA-Me contains a
−1
strong band at 1081 cm and two weak bands at 1107 and
1
128 cm− (Figure 5). The three bands are associated with alkyl
1
(
polymer) to some gauche above the monomer melting and
PDA color transition temperatures. Thus, it appears that
thermally induced conformational changes of alkyl chains cause
distortion of the conjugated p-orbital array in the PDA
(
Scheme 2).
Fabrication of Polydiacetylene Nanofiber. Recently,
generation of PDA nanofibers using nanoporous inorganic
templates has been reported, and the method enables
fabrication of nanofibers with controlled diameters and
63
lengths. PDA nanofibers were prepared from polymerization
of monomeric diacetylenes that are encased within templates.
However, owing to difficulties associated with UV penetration,
only monomers on the template surface undergo polymer-
ization. In order to circumvent this limitation, we have
developed a technique for nanofiber construction that utilizes
a soluble PDA instead of the monomer as the starting material.
Inspection of the TGA curves displayed in Figure 6 shows that
poly(PCDA-Me) and poly(PCDA-EGME) have nearly identi-
cal thermal stabilities and that decomposition of both begins at
around 300 °C. In DSC curves, endothermic peaks of
poly(PCDA-Me) are observed at 133.7 and 149.9 °C, while
Figure 5. (A) C−C stretching region of Raman spectra used to
monitor the blue-to-red color transition of poly(PCDA-Me) upon
gradual heating from 10 to 40 °C. (B) Single bond conformational
change of a long alky chain attached to PDA backbone upon heating.
Scheme 2. Proposed Mechanism for the Blue-to-Red Color Transition of a Low Temperature Thermochromic Polydiacetylene
F
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poly(PCDA-EGME) displays endothermic peaks at 110.8 and
1
37.2 °C. The data obtained from TGA and DSC analyses
suggest that the two polymers should be able to be processed in
their liquid states at reasonably high temperatures.
Anodized aluminum oxide (AAO) membrane was chosen as
a template to prepare poly(PCDA-EGME) nanofibers using the
new procedure we have devised. At 150 °C, a film of
poly(PCDA-EGME) melts on the surface of the AAO template
accompanied by a red-to-yellow color transition. After cooling
to room temperature, a red PDA filled AAO membrane is
obtained. The residual PDA film on the AAO template surface
was removed by scratching with a knife, and the PDA nanofiber
is generated by etching the AAO template with aqueous NaOH
and collected by using centrifugation. In Figure 7 are shown
optical, fluorescence, and SEM images of the poly(PCDA-
EGME) nanofibers formed in this manner. Inspection of the
optical microscopic image (Figure 7A) demonstrates that
bundles of PDA fibers are generated. In addition, because the
red phase PDAs are fluorescent, red-emitting fibers are clearly
visible in the fluorescence microscopic image (Figure 7B).
Importantly, analysis of the SEM image (Figure 7E) indicates
that the diameter of the PDA fibers (ca. 300 nm) is similar to
the pore diameter of the template AAO membrane (Figure 7F).
Furthermore, although some short PDA fibers are produced,
the length of the majority of the fibers is similar to that of the
AAO membrane. Finally, the surface of the PDA nanofiber is
relatively smooth and absent of any porous structures (Figure
Figure 6. TGA (A) and DSC (B) of PDAs derived from PCDA-Me
7
E). The simple and straightforward method described above
(
black line) and PCDA-EGME (red line). The heating rate is 10 °C/
should be applicable to the preparation of PDA nanofibers
having controlled dimensions.
min.
Figure 7. Optical (A), fluorescence (B), and SEM images (C−E) of poly(PCDA-EGME) nanofibers obtained using the AAO templating method.
F) SEM image of top view of an AAO membrane.
(
G
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Several low temperature thermochromic PDAs, derived from
methyl and methoxyethyl ester containing DA monomers, were
designed and prepared. Removal of the source of intermo-
lecular interactions between head groups leads to a significant
decrease in the melting point of the DA monomer, an effect
that is directly reflected in the colorimetric transition
temperature of the corresponding PDA. As a result, we were
able to prepare PDAs that display thermochromic transition
temperature in the range of 5−30 °C. FT-IR and Raman
spectroscopic analyses provided useful information about the
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alkyl groups, which exit in the blue-phase, are converted to
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nanofibers using anodized aluminum oxide (AAO) as a
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b02683.
■
(
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AUTHOR INFORMATION
Corresponding Author
E-mail: jmk@hanyang.ac.kr (J.-M.K.).
1
(
■
*
(
Notes
The authors declare no competing financial interest.
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