Correlation between the secondary structure and hydrogen bonding in optically active polyurethane and its effect on infrared emissivity

Article abstract of DOI:10.1016/j.reactfunctpolym.2012.05.006

Optically active polyurethanes (PUs) with different specific rotations were synthesized using a hydrogen transfer addition polymerization procedure with isocyanate-phenols of varying enantiomeric excess. The optical activities of the PUs were enhanced with an increase in the enantiomeric excess of the monomers. A helical secondary structure in the PUs with a more compact arrangement of the macromolecular backbones facilitated interchain hydrogen bonding, and the infrared emissivity of the corresponding polymers decreased. The results indicated that the level of order and number of hydrogen bonds in the macromolecule played significant roles in controlling the infrared emissivity.

Full text of DOI:10.1016/j.reactfunctpolym.2012.05.006

Reactive & Functional Polymers 72 (2012) 574–579
Contents lists available at SciVerse ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Correlation between the secondary structure and hydrogen bonding
in optically active polyurethane and its effect on infrared emissivity
a
b
Yong Yang a, Yuming Zhou a, , Jianhua Ge , Xiaoming Yang
⇑
a School of Chemistry and Chemical Engineering, Southeast University, Jiangsu Optoelectronic Functional Materials and Engineering Laboratory, Nanjing 211189, PR China
b Suzhou Sidike New Material Technology Co., Ltd., Suzhou 215400, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Optically active polyurethanes (PUs) with different specific rotations were synthesized using a hydrogen
transfer addition polymerization procedure with isocyanate–phenols of varying enantiomeric excess. The
optical activities of the PUs were enhanced with an increase in the enantiomeric excess of the monomers.
A helical secondary structure in the PUs with a more compact arrangement of the macromolecular back-
bones facilitated interchain hydrogen bonding, and the infrared emissivity of the corresponding polymers
decreased. The results indicated that the level of order and number of hydrogen bonds in the macromol-
ecule played significant roles in controlling the infrared emissivity.
Received 15 February 2012
Received in revised form 15 May 2012
Accepted 16 May 2012
Available online 23 May 2012
Keywords:
Secondary structure
Hydrogen bonding
Optically active
Infrared emissivity
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
Special attention has been paid to the development of cloak-
ing technologies to prevent an object from detection. Infrared
Optically active macromolecules are of interest for their unique
characteristics such as an orderly secondary structure, adjustable
chiral parameters, and abundant interchain interactions [1–3].
The formation of highly ordered architectures enhances the prop-
erties of the materials and allows for applications in the fields of
chiral recognition, asymmetric catalysis, liquid crystal displays,
chemical sensors, life science and so forth [4–8]. Helicity in syn-
thetic polymers is usually induced by the presence of a chiral cen-
ter either in the polymer backbone or in a side chain. As the
building blocks of proteins, amino acids are the most common chi-
ral resources for use in the synthesis of optically active materials
[9–11]. Advances in organic and polymer syntheses increase their
utility in the production of well-ordered helical polymers. Such
studies are valuable not only because they mimic biomacromole-
cules but also because they provide insight into how the helical
sense can be controlled. Non-covalent chemical bonds and steric
hindrance are the two most significant driving forces causing the
polymers to adopt helical conformations [12–14]. The well-or-
dered hydrogen bonds constructed between the functional groups
are primarily responsible for facilitating the stabilization of the
helical secondary structure. As the hydrogen bonding is disturbed
by heat or polar solvents, a decrease in the intensity of the Cotton
effects and the absolute value of the specific rotations can be ob-
served indicating a breakup of the helical architectures [15–20].
stealth is the rapidly developing principal field for stealth
research. In addition to decreasing the skin temperature of the
object, control of its infrared emissivity is critical for successful
camouflage [21–23]. Extensive work toward the development of
coatings with low infrared emissivity has been reported. These
materials include semiconductive materials, metallic thin films,
conductive polymeric materials, and inorganic/organic composite
coatings [24–28]. Materials with controllable emissivity have
attracted considerable attention due to improvements in military
detection technologies and the mutable environmental condi-
tions around the object. Among the many materials examined
for infrared camouflage, organic polymers possess special poten-
tial given that their adjustable structures can provide the
required properties.
The present study focused on the relationship between the reg-
ular secondary structure and the hydrogen bonds in the macromol-
ecules. The infrared emissivity values were also investigated. We
prepared optically active polyurethanes (PUs) based on different
enantiomeric excesses of isocyanate–phenols derived from tyro-
sine. The degrees of hydrogen bonding in the PUs varied with
changes in the enantiomeric excess of the monomers and were
attributed to the differing quantities of one-handed helical confor-
mations. In addition, their infrared emissivity was affected. The
structure of polymers greatly influenced the infrared emissivity
of them, and adjustment of the secondary structure and intermo-
lecular interactions could be regarded as a promising method for
tuning the infrared emissivity.
⇑
Corresponding author. Tel./fax: +86 25 52090617.
E-mail address: ymzhou@seu.edu.cn (Y. Zhou).
1381-5148/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2012.05.006

Y. Yang et al. / Reactive & Functional Polymers 72 (2012) 574–579
575
(300 MHz, CDCl3): d 0.84–0.88 (t, J = 6.15 Hz, 3H, ACH3), 1.27 (m,
6H, ACH2A), 1.58–1.60 (m, 2H, ACH2A), 2.91–2.93 (m, 2H,
APhCH2A), 4.09–4.13 (t, J = 6.54 Hz, 2H, AOCH2A), 4.38 (m, 1H,
˜CHA), 6.67–6.96 (m, 4H, AC6H4A). 13C NMR (75 MHz, CDCl3): d
13.9, 22.5, 25.4, 28.4, 31.3, 39.1, 58.8, 66.7, 115.5, 127.5 (N@C@O),
2. Experimental part
2.1. Materials
L-tyrosine, DL-tyrosine, and triethylamine were purchased from
Shanghai Chemical Reagent Company and used as received
without further purification. Triphosgene was produced by
Sigma–Aldrich (Shanghai, China). Methyltrioctylammonium chlo-
ride (AliquatÒ336) was purchased from Acros Organics. Thionyl
chloride, 1-hexanol, and diethyl ether were all obtained from
Shanghai Chemical Reagent Company and distilled using standard
methods.
130.6, 154.9 (CAOH), 170.8 (C@O). FT-IR (m
/cmÀ1): 3419 (OAH),
2957, 2932, 2259 (AN@C@O), 1738 (C@O), 1516, 1004, 834.
2.4. Polymerizations
The polyaddition of isocyanate–phenols was carried out in situ
with magnetic stirring under nitrogen at room temperature for
24 h with the addition of triethylamine (3 mol%) as the catalyst.
The reaction mixture was poured into a large quantity of diethyl
ether, filtered and dried under reduced pressure to yield the off-
white polymer. The prepared polyurethanes were record as PU1,
PU2, PU3, PU4, PU5, and PU6 representing the various enantiomeric
excesses of the monomers.
2.2. Measurements
The FT-IR spectra were recorded using a Bruker TENSOR™ 27
FT-IR spectrometer at room temperature with KBr pellets. The
spectra were obtained at 4 cmÀ1 resolution and recorded in the re-
gion of 4000–400 cmÀ1 1H and 13C NMR spectra were recorded
.
The spectroscopic data of the PUs are as follows (using PU6 as an
example). 1H NMR (300 MHz, CDCl3): d 0.90 (3H, ACH3), 1.31 (6H,
ACH2A), 1.63 (2H, ACH2A), 3.16 (2H, APhCH2A), 4.14 (2H,
AOCH2A), 4.67 (1H, ˜CHA), 5.61 (1H, ANHA), 7.08–7.16 (4H,
using a Bruker AVANCE 300 MHz NMR spectrometer. Chemical
shifts were reported in ppm. The molecular weights and molecular
weight polydispersities were determined by gel permeation chro-
matography (Shodex KF-850 column) calibrated using polystyrene
with THF as the eluent. The UV-vis spectra were measured in solu-
tion on a Shimadzu UV 3600 spectrophotometer. The CD spectra
were determined with a Jasco J-810 spectropolarimeter using a
10-mm quartz cell at room temperature. The specific rotations of
all samples were measured in a WZZ-2S/2SS digital automatic
polarimeter at room temperature. The wavelength of the sodium
lamp was 589.44 nm. Thermal analysis experiments were per-
formed using a TGA apparatus operated in the conventional TGA
mode (TA Q-600, TA Instruments) at a heating rate of 10 K/min
in a nitrogen atmosphere with a sample size of approximately
50 mg. X-ray diffraction (XRD) measurements were recorded using
a Rigaku D/MAX-R with a copper target at 40 kV and 30 mA. The
powder samples were spread onto a sample holder, and the diffrac-
tograms were recorded in the range of 5–80° at a speed of 5°/min.
The infrared emissivity values of the samples were investigated
using an IRE-I infrared emissometer at the Shanghai Institute of
Technology and Physics, China.
AC6H4A). FT-IR
(m
/cmÀ1): 3330 (NAH), 2956, 2930, 1736
(ACO2C6H13), 1711 (AC(O)NHA), 1507, 1217, 1035, 898.
3. Results and discussion
3.1. Synthesis and characterization of the polymers
As shown in Scheme 1, the monomers were synthesized with
varying enantiomeric excess by altering the mixture of L and DL-
tyrosine hexyl ester hydrochloride. The hydroxyl and isocyanato
moieties were able to coexist stably in solution [32]. The polyure-
thanes of different optical activities were prepared in CH2Cl2 at
room temperature using triethylamine as the catalyst. The IR and
1H NMR characterizations both confirmed that the reaction had ta-
ken place as expected. Table 1 summarizes the results of the poly-
merizations; the monomers satisfactorily undergo polymerization
to produce adequate yields of the corresponding polymers with
similar molecular weights. All polymers except PU1 exhibit optical
activities, and their specific rotations gradually increase with the
increasing enantiomeric excess of the monomers. The solubility
of PUs is investigated in several solvents, and all are found to be
soluble in many common organic solvents such as chloroform, tet-
rahydrofuran, acetone and dioxane. The time required for the poly-
mers to dissolve increases with increasing optical rotatory power
most likely because of the increase in interchain interactions that
will be discussed below in more detail.
2.3. Synthesis of the monomers
L and DL-tyrosine hexyl ester hydrochlorides were prepared
using the thionyl chloride technique [29,30]. The procedure used
to synthesize monomers is described as follows: Phosgene was ob-
tained from the decomposition of triphosgene with AliquatÒ 336
according to the literature [31]. The phosgene developed during
the reaction was collected in CH2Cl2 and preserved below 0 °C. A
1.51 g tyrosine hexyl ester hydrochloride sample with varying
enantiomeric excess (containing different mole fractions of L and
DL-tyrosine hexyl ester hydrochloride) was added to the stirred
phosgene solution (5–10 g, 0.05–0.1 mol) in 50 ml CH2Cl2, which
was kept at À5 °C. A 50-ml saturated aqueous sodium bicarbonate
solution was added dropwise over 10 min. The reaction mixture
was stirred at 0–5 °C for an hour then bubbled with nitrogen to re-
move the residual phosgene. The biphasic mixture was poured into
a 250-ml separatory funnel. The organic layer was collected, and
the aqueous layer was extracted with three 5-ml portions of
CH2Cl2. The combined organic layers were dried (MgSO4), vacuum
filtered, and concentrated to 20 ml to obtain a colorless monomer
solution. The solution was used in the polymerization without fur-
ther isolation.
3.2. Hydrogen bond analysis of the PUs
In each polymer model, the hydrogen bonding has been exten-
sively studied using infrared spectroscopy, which is a particularly
useful tool for characterizing hydrogen bonding. Participation in
hydrogen bonding can be evidenced by a frequency shift in the
absorptions to values lower than those for free groups with a
simultaneous reinforcement of the intensity [33–36]. Fig. 1a and
b shows the FT-IR spectra of the PUs. The absorptions at approxi-
mately 3500–3300 cmÀ1 correspond to the amide NAH groups of
the polyurethanes, although differences appear in the NAH
stretching region. The bands at 3428 and 3330 cmÀ1 are assigned
to the free NAH groups and the hydrogen-bonded NAH groups,
respectively. These two bands exhibit different intensities with
the type of PU optical activity, indicating different degrees of
hydrogen bonding in the macromolecules. The free NAH groups
The spectroscopic data of the isocyanate–phenol are as follows
(for an enantiomeric excess of 100% as an example). 1H NMR

576
Y. Yang et al. / Reactive & Functional Polymers 72 (2012) 574–579
OH
OC6H13
OC6H13
SOCl 2
C6 H13OH
O
O
O
x
NH2
N
+H3Cl -
NCO
OH
OH
OH
DL-tyrosine hexyl ester hydrochloride
DL-tyrosine
isocyanate of D-tyrosine hexyl ester
COCl2 ,NaHCO3
CH2Cl2,H2O
+
+
OH
OC6H13
OC6H13
SOCl2
O
O
y
O
C6H13OH
NH2
N+H3Cl -
NCO
OH
OH
OH
L-tyrosine
isocyanate of L-tyrosine hexyl ester
L-tyrosine hexyl ester hydrochloride
PU1: x=0.5,y=0.5
PU2: x=0.6,y=0.4
PU3 : x=0.7,y=0.3
PU4 : x=0.8,y=0.2
PU5 : x=0.9,y=0.1
O
OC6H13
O
OC6H13
O
O
TEA (3%mol)
O
CH Cl
2,r.t.,24h
O
N
H
2
N
H
PU6 : x=1,y=0
x
y
PU1-6
Scheme 1. Synthetic route of monomers and corresponding polyurethanes.
Table 1
Polymerization of isocyanate–phenols.
c
c
d
Polymer
ee (%)a
Yieldb (%)
Mn  10À4
Mw/Mn
[a
]
D
(°)
PU1
PU2
PU3
PU4
PU5
PU6
0
20
40
60
80
86
90
93
92
95
93
2.1
2.2
1.9
2.4
1.8
2.1
1.68
1.61
1.84
1.54
1.56
1.58
0
26
48
78
104
126
100
a
b
c
The enantiomeric excess of the monomers.
Ether-insoluble part.
Determined by GPC eluted with THF based on polystyrene standards.
d
Measured by polarimetry at room temperature, c = 0.50 g/dL in CH2Cl2.
in PU1 are primarily derived from the racemic monomer, while
their absorption decreases and undergoes a red shift to lower wave
numbers with an increase in the enantiomeric excess of the mono-
mers. This result indicates a gradual intensification in hydrogen
bonding with the enhancement of the PU optical activity. The car-
bonyl stretching regions of the PU spectra are shown in Fig. 1b;
participation in hydrogen bonding decreases the vibration fre-
quency of the carbonyl groups and complicates their absorptions.
The carbonyl region in the PU spectra is composed of two different
carbonyl absorptions centered at 1741 and 1712 cmÀ1, corre-
sponding to the free carbonyl groups belonging to ester and carba-
mate, respectively. The introduction of optical activity in the PUs
results in lower vibrational frequencies and broader peaks in this
region compared with the absorptions of free carbonyl groups.
The degree of hydrogen bonding reaches a maximum when the
enantiomeric excess of the monomers is 100%.
3.3. Secondary structural analyses of the PUs
The helical conformation of the macromolecules can be tested
using polarimetry and circular dichroism (CD) spectroscopy when
the helix has an excess screw sense [37,38]. To characterize the
secondary structure of the polymers, the CD and UV-vis absorption
spectra of the PUs were measured and are presented in Fig. 2. All
PUs exhibit absorption at 235 and 265 nm corresponding to the
Fig. 1. IR spectra of PUs at (a) 3000–3800 cmÀ1 and (b) 1600–1800 cmÀ1. The circle
indicates the hydrogen bonded carbonyl bands around 1662 cmÀ1. This absorption
is very weak in the PU1 and intensifies gradually with the enhancement of optical
activity of the PUs.
p–
pà transition of the aromatic rings and the n–pà transition of
force field generated by the chiral center affects the secondary
structure of the polymers, and the persistence length of the helical
domain of PU6 is the largest among the PUs. Meanwhile, L-tyrosine
hexyl ester was prepared and its specific rotation and CD were
compared with the PU6 (see Supporting Information). Furthermore,
the carbonyl groups, respectively. As shown in the CD spectra,
the Cotton effect was observed in each of the polymers except
PU1, and its magnitude increased with increasing enantiomeric ex-
cess of the monomers. This result suggests that the asymmetric

Y. Yang et al. / Reactive & Functional Polymers 72 (2012) 574–579
577
exhibit a Cotton effect and solvent dependence in dioxane and THF
solutions. The specific rotation of PU6 decreases to 46° (in dioxane)
and 38° (in THF) because the helical structure of the PUs is stabi-
lized by the hydrogen bonding between the NAH and carbonyl
groups. The interactions are readily broken by polar solvents, which
force the main chain to adopt a randomly coiled conformation.
The WAXS study of the PUs is presented in Fig. 4. The diffraction
patterns for the PUs exhibit a primary crystalline peak A and an
amorphous region. Peak A is present regardless of the optical activ-
ity of the polymers; thus, it can be assigned to the presence of hard
aromatic ring segments in the polymers. A crystalline peak B
emerges with the generation of optical activity. All PUs have the
same constitutional unit, and no other ingredients are present that
could bring about crystallinity. Therefore, the asymmetric force
field generated by the chiral compound could increase order in
the new region of the resulting polymers, and thus give rise to in-
creased crystallinity. In addition, this diffraction peak strengthens
with an increase in optical activity, which could be attributed to
the enhancement of the helical content in the macromolecular
chains as indicated by the CD analysis.
The hydrogen bonding, which is strongly influenced by the sec-
ondary structure of main chain, can originate from either intermo-
lecular or intramolecular interactions. The longer helical pitch
resulting from the larger dihedral angles of the main chain most
likely induces the molecular layers to assume a more compact
arrangement, which facilitates the formation of interchain hydro-
gen bonds. The crystal layer spacing in the PUs is approximately
4.25 nm, which is 21° to the plane of the bifurcated hydrogen
bonded structures. The increase in crystallinity and the emergence
of an orderly B region further prove that the degree of hydrogen
bonding increases from PU1 to PU6. Therefore, the hydrogen bonds
could be constructing as proposed in Fig. 4.
Fig. 2. CD and UV-vis spectra of PUs measured in CH2Cl2 (c = 0.5 g/dL) at room
temperature.
3.4. Thermal stability analysis of the PUs
The thermal properties of the PUs were investigated using TGA
techniques at a heating rate of 10 °C/min under a nitrogen atmo-
sphere. Fig. 5 shows the TG curves for all samples, which exhibit
a smooth and stepwise manner suggesting a two-step thermal deg-
radation. PU1, created with racemic monomers, undergoes an ini-
tial weight loss at approximately 150 °C, whereas PU6, with an
enantiomeric excess of 100%, shows no decomposition until
265 °C. The temperatures at which 5% and 10% weight loss occur
(T5, T10) have been calculated using the thermograms. These values
can be used to evaluate the thermal stability of the polymers. The
thermogravimetric data for these polymers are summarized in Ta-
ble 2. PU6 clearly has the highest thermal stability among the poly-
mers. The PUs have the same repeating units and compositions;
therefore, their differences in thermal stability must be attributed
to differences in their macromolecular conformations. The pres-
ence of a helical structure has a notable influence on the thermal
stability of the polymers. A regular secondary structure is better
able to dampen the internal thermal energy. PUs with more highly
ordered backbones have higher degradation temperatures. The
presence of intermolecular hydrogen bonding increases the stabil-
ity at high temperatures. In contrast, a random coiled molecular
chain with structural flexibility in its backbone undergoes chain
separation, thus decreasing the interchain interactions. Therefore,
the degree of hydrogen bonding in the PUs varies, further support-
ing the previously discussed analysis of the macromolecular chain
conformations.
the red shift occurring from PU1 to PU6 may be due to the extended
main-chain conformation. The coplanarity of the polymer back-
bones is enhanced, and the helices become loose. Consequently,
PU6 may possess a longer helical pitch than the other PUs [39,40].
The variation in the helicity induced by the external stimuli is
unique to dynamic helical polymers. Achiral stimuli, such as tem-
perature, solvent and irradiation have been extensively studied
[41–44]. If the CD signals shown in Fig. 2 resulted from any confor-
mational chirality in the PUs, the intensity of the CD signals would
vary with a change in solvent. The conformation study of PU6 is car-
ried out in other solvents for comparison (Fig. 3). These CD spectra
3.5. Infrared emissivity analysis
The infrared emissivity values of the PUs derived from different
Fig. 3. CD spectra of PU6 in different solvents (c = 0.5 g/dL) at room temperature.
enantiomeric excess of monomers at wavelengths of 8–14 lm at

578
Y. Yang et al. / Reactive & Functional Polymers 72 (2012) 574–579
d=4.25nm
C
O
C
O
C
O
C
O
C
O
C
O
C
O
C
O
H
N
H
N
H
N
H
N
H
N
H
N
C
O
C
O
C
O
C
O
H
N
H
N
C
O
C
O
C
O
C
O
H
N
H
N
H
N
H
N
H
N
H
N
H
N
H
N
Fig. 4. WAXS pattern and proposed helical packing arrangement in PUs.
Fig. 6. Dependence of the infrared emissivity and CD signal intensity on the
enantiomeric excess of monomers.
Table 2
Fig. 5. TGA thermograms of PUs under N2 atmosphere at a heating rate of 10 °C/
Thermal properties of PUs.
min.
Polymer Decomposition temperature
Decomposition temperature
(°C) T5
(°C) T10
room temperature are presented in Fig. 6. The intensity of the CD
effects is plotted as a function of the enantiomeric excess of the
isocyanates to provide a clear comparison. The intensity of the
CD signals progressively increases from zero to 20.6, 37.5, 65.9,
85.9, 95.8 (Â103 deg cm2 dmolÀ1). The increase in the chiral unit
is favorable for polymer main chains to adopt the preferred helical
conformation, and their relationship is nonlinear. The majority rule
effect, however, is less pronounced in the PUs than in the optically
active polyisocyanates and polyacetylenes [45,46], which can most
likely be attributed to the existence of a chiral center in the main
chain of the macromolecules and in the smaller side chains with-
out a synergistic effect between the pendent groups. For PU1 based
on a racemic monomer, the infrared emissivity value is 0.866. The
infrared emissivity values of the PUs decrease with increasing
orderliness. The vibration of the massive covalent bonds, the con-
formational variation in the macromolecular backbones and the
unsaturated groups in the substances related to the thermal energy
are known to induce infrared radiation. Obviously, optically active
polymers based on chiral compounds possess regular stereostruc-
tures and a better ability to attenuate internal thermal energy. A
closely packed helical backbone is favored over the formation of
a large number of intermolecular interactions such as hydrogen
bonding, which reduces the index of hydrogen deficiency and the
PU1
PU2
PU3
PU4
PU5
PU6
251
258
269
278
283
285
267
274
286
291
293
294
unsaturated degree, according to the above analyses. Therefore,
the orderliness in the macromolecular structure significantly con-
tributes to the decrease in infrared emissivity.
4. Conclusion
A series of optically active PUs with different specific rotations
were prepared by varying the enantiomeric excess of the mono-
mers derived from tyrosine. Variation of the enantiomeric excess
of the monomers altered the optical activities of the corresponding
PUs, which was related to the conformation of the macromole-
cules. With increasing enantiomeric excess of isocyanate–phenols,
the quantity of one-handed helical structures increased with ex-
tended main chain conformations in the macromolecular chains,
thus facilitating interchain hydrogen bonding. The orderliness

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strategy could be used to obtain organic materials with specific
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Acknowledgments
The authors are grateful to the National Nature Science Founda-
tion of China (51077013, 50873026), Special funds for Jiangsu
Province Scientific and Technological Achievements Projects of
China (BA2011086), the Program for Training of 333 High-Level
Talent, Jiangsu Province of China (BRA2010033), and the Key Pro-
gram for the Scientific Research Guiding Found of Basic Scientific
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Operation
Expenditure,
Southeast
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(3207041102) for financial support.
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