Synthesis, characterization and infrared emissivity property of optically active polyurethane derived from tyrosine

Article abstract of DOI:10.1016/j.polymer.2011.06.027

Optically active polyurethanes (LPU and DPU) and racemic polyurethane (RPU) were synthesized by the self-polyaddition of the isocyanate-phenols which derived from the chiral and racemic tyrosine. All of the polymers were characterized by FT-IR, ¹H NMR, GPC, UV-Vis spectroscopy, circular dichroism (CD) spectroscopy, TGA and X-ray diffraction (XRD), and the infrared emissivity values were investigated in addition. LPU and DPU were two enantimorphs, they possessed helical configurations and higher degree of hydrogen bondings compared to the RPU which presented random coiled molecular chain. The crystallinity and thermal decomposition temperature of LPU and DPU were higher than that of the RPU due to the more regular secondary structure which facilitate the formation of a large number of inter-chain hydrogen bonds. Consequently, the LPU and DPU exhibited lower infrared emissivity values (8-14 μm), which came down to 0.611 and 0.625.

Full text of DOI:10.1016/j.polymer.2011.06.027

Polymer 52 (2011) 3745e3751
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis, characterization and infrared emissivity property of optically active
polyurethane derived from tyrosine
*
Yong Yang, Yuming Zhou , Jianhua Ge, Yongjuan Wang, Xinglan Chen
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received in revised form
15 June 2011
Accepted 19 June 2011
Available online 25 June 2011
Optically active polyurethanes (LPU and DPU) and racemic polyurethane (RPU) were synthesized by the
self-polyaddition of the isocyanate-phenols which derived from the chiral and racemic tyrosine. All of
the polymers were characterized by FT-IR, 1H NMR, GPC, UVeVis spectroscopy, circular dichroism (CD)
spectroscopy, TGA and X-ray diffraction (XRD), and the infrared emissivity values were investigated in
addition. LPU and DPU were two enantimorphs, they possessed helical configurations and higher degree
of hydrogen bondings compared to the RPU which presented random coiled molecular chain. The
crystallinity and thermal decomposition temperature of LPU and DPU were higher than that of the RPU
due to the more regular secondary structure which facilitate the formation of a large number of inter-
chain hydrogen bonds. Consequently, the LPU and DPU exhibited lower infrared emissivity values (8
Keywords:
Optically active
Polyurethanes
Tyrosine
e14 mm), which came down to 0.611 and 0.625.
Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction
stationary phase(CSP) in high-performance liquid chromatogra-
phy(HPLC), liquid crystals for display, catalysts for asymmetric
synthesis and so on [17e19]. Especially, conformational changes of
polymers induced by the external stimuli lead to the changes of
physical and chemical properties including viscosity, conductivity,
solubility, wettability, morphology, etc., many appealing researches
aimed at changing and switching the secondary structure of opti-
cally active polymers to develop smart and intelligent macromol-
ecules have been reported [20e22]. Amino acids are the most
common chiral resources that have been widely used in the
synthesis of optically active materials [23e26]. Moreover, amino
acid-based chiral polymers could have induced crystallinity with
the ability to form higher ordered structures that exhibit enhanced
properties. Poly(c-alkyl a-glutamic acid)s are used as surface
treatment reagents of artificial leathers, and examined as liquid
crystalline materials, piezoelectric materials, and LB films [27,28].
Polymethionine shows high biocompatibility and is expected as
medical materials [29e32] (e.g., contact lenses, artificial skins,
internal organs, and blood vessels). Polyleucine catalyzes the
asymmetric epoxidation of chalcone with approximately quanti-
tative optical purity [33]. However, to the best of our knowledge,
there is no work referred to the investigation of infrared emissivity
properties of optically active polymers which derived from amino
acids.
With the development of infrared technologies, emissivity
control has attracted more and more attention both in civil and
military field [1,2]. Various new materials with low infrared emis-
sivity have been reported so far, such as nanosized semiconduor
particles, metallic thin films, strongly magnetic materials, and
inorganic/organic composite coatings [3e8]. Moreover, the mate-
rials are supposed to meet a wide range of requirements for
application besides having low infrared emissivity [9,10]. Organic
polymers possess unique properties including light weight,
corrosion-resistant, tractability, and the most important, the
adjustable structures which can provide the possibility of tuning
the infrared emissivity according to our need [11].
Special interest in the properties of polymers possessing optical
activity arises from the realization that the helices are significant
for organisms in maintaining their usual functions [12]. Macro-
molecules with optical activity possess not only the advantages of
any other polymers but also some unique characteristics, such as
ordered secondary structure, adjustable chiral parameter, and
abundant inter-chain interaction as well [13e16]. Many naturally
and synthetic optically active polymers have been used as the chiral
Polyurethanes (PUs) [34,35] are types of high-performance
polymeric materials that have been widely used in adhesives and
coatings of various materials because of their outstanding thermal
* Corresponding author. Tel./fax: þ86 25 52090617.
E-mail address: ymzhou@seu.edu.cn (Y. Zhou).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2011.06.027

3746
Y. Yang et al. / Polymer 52 (2011) 3745e3751
and chemical stabilities, mechanical properties and electrical
properties. Because of the reactivity of hydroxyl and isocyanate
group, there were no linear polyurethanes with head-to-tail
structures until Versteegen [36] and Nagai [37e39] with their co-
2.3. Monomer synthesis
2.3.1. Synthesis of L, D and DL-Tyrosine hexyl ester hydrochloride
To 1-hexanol (75 mL) at ꢁ5 ꢀC, thionyl chloride (6.55 g,
workers synthesized
a
,
u
-isocyanato alcohols, isocyanate-phenols
0.055 mol) was dropped slowly in order to maintain the temper-
and the corresponding polyurethanes respectively. The key
reagent for the preparation of the monomers was di-tert-
butyltricarbonate which demanded a large volume of pentane for
crystallization and low temperature to preserve [40e42]. Further-
more, amine salts needed to be converted to the corresponding free
bases beforehand. In the present study, we used the solution of
phosgene in the synthesis of the monomer containing phenolic
hydroxyl and isocyanate group coexisting in the same molecule
under a mild condition. In consideration of the solubility of esters in
methylene dichloride, tyrosine hexyl ester hydrochloride was used
and the corresponding isocyanate was polymerized with triethyl-
amine as the catalyst. The optically active polyurethanes LPU and
DPU possessed lower infrared emissivity values than the racemic
one. The incorporation of the chirality in the resulting polymers
provided a great promise in the development of macromolecules
with low infrared emissivity and the adjustment between them is
in progress.
ature under 0 ꢀC. Then
L-tyrosine (9.05 g, 0.05 mol) was added. The
resulting mixture was stirred at 95 ꢀC for 12 h. As the mixture
cooled, the product was precipitated by the addition of diethyl
ether (200 mL). The precipitate was collected, washed with ether
(2 ꢂ 50 mL) and dried to give the white powdery
ester hydrochloride. and DL-tyrosine hexyl ester hydrochlorides
were prepared in the similar way using and DL-tyrosine instead of
-tyrosine.
L-tyrosine hexyl
D
D
L
L
-Tyrosine Hexyl Ester Hydrꢀochloride: yield 89%, a white solid,
mp 162e164 ꢀC; ½a D25
ꢃ
¼ þ7.6 (C ¼ 1 g/dL, methanol). 1H NMR
(300 MHz, DMSO-d6):
d
0.82e0.8 (t, J ¼ 6.48 Hz, 3H, eCH3),
1.20e1.25 (m, 6H, eCH2-), 1.43e1.47 (m, 2H, eCH2-), 2.89e3.10 (m,
2H, -PhCH2-), 4.00e4.05 (t, J ¼ 6.42 Hz, 2H, eOCH2-), 4.10e4.14 (t,
J ¼ 6.99 Hz, 1H, >CH-), 6.68e7.01 (m, 4H, eC6H4-), 8.55 (s,
3H, ꢁNþH3Clꢁ), 9.41 (s, 1H, eOH). FT-IR (
n
/cmꢁ1): 3297 (OeH),
2953, 2869, 1738 (C]O), 1516, 1230, 843.
D
-Tyrosine Hexyl Ester Hydrochloride: yield 90%, a white solid,
mp 157e159 ꢀC; ½a D25
IR data are similar to
omitted.
ꢃ
¼ ꢁ7.2ꢀ (C ¼ 1 g/dL, methanol). Its 1H NMR,
2. Experimental part
L-tyrosine hexyl ester hydrochloride and thus
2.1. Materials
DL-Tyrosine Hexyl Ester Hydrochloride: yield 95%, a white solid,
mp176e179 ꢀC; ½a D25
ꢃ
¼ 0 ꢀ (C¼ 1 g/dL, methanol). Its1HNMR, IR data
L
-tyrosine, DL-tyrosine and triethylamine were purchased from
Shanghai Chemical Reagent Company and used as received without
further purification. -tyrosine was the product of Aladdin. Tri-
are similar to L-tyrosine hexyl ester hydrochloride and thus omitted.
D
2.3.2. Isocyanate-phenols
phosgene was obtained from Aldrich. Methyltrioctylammonium
chloride (AliquatÒ336) was produced by Acros Organics. Thionyl
chloride, 1-hexanol and other solvents were all obtained from
Shanghai Chemical Reagent Company and distilled by the standard
methods.
Phosgene was obtained by decomposition of triphosgene with
Aliquat 336 according to the literature [43]. The phosgene that
developed during the reaction was collected in CH2Cl2 and
preserved below 0 ꢀC. 1.51 g
L-tyrosine hexyl ester hydrochloride
was added to the stirred solution of phosgene (5e10 g,
0.05e0.1 mol) in 50 mL CH2Cl2 which was kept at ꢁ5 ꢀC. Then
50 mL of saturated aqueous sodium bicarbonate was dropped
slowly within 10 min. The resultant mixture was stirred at 0e5 ꢀC
for an hour and then bubbled with nitrogen in order to remove the
residual phosgene. 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,
2.2. Measurements
Melting point (mp) was measured by an X-4 micro-melting
point apparatus. FT-IR spectra were carried out on a Bruker
Tensor 27 FT-IR spectrometer at room temperature using KBr
pellets and liquid cell with the chloroform as the solvent. The
spectra were obtained at a 4 cm-1 resolution and recorded in the
region of 4000e400 cm-1 1H and 13C NMR spectra measurements
were recorded on a Bruker AVANCE 300 NMR spectrometer.
Chemical shifts were reported in ppm. The molecular weights and
molecular weight polydispersities were determined by GPC
(Shodex KF-850 column) calibrated by using polystyrenes with
THF as the eluent. UVevis spectra in solution were measured on
a Shimadzu UV 3600 spectrometer. CD spectra were determined
with a Jasco J-810 spectropolarimeter using a 10 mm quartz cell at
room temperature. The optical rotation of all samples was
measured in a WZZ-2S (2SS) digital automatic polarimeter at room
temperature. The wavelength of sodium lamp was 589.44 nm.
and concentrated to 20 ml to afford the colorless isocyanate of
L-
tyrosine hexyl ester solution. The solution was used in the poly-
merization without further isolation. The isocyanate of
tyrosine hexyl ester was prepared in the similar way using
tyrosine instead of -tyrosine.
The spectroscopic data of isocyanate-phenol are as follows (take
isocyanate of
-tyrosine hexyl ester for example). 1H NMR
(300 MHz, CDCl3):
D
and DL
-
D
and DL
-
L
L
d
0.84e0.88 (t, J ¼ 6.15 Hz, 3H, eCH3), 1.27 (m,
6H, eCH2-), 1.58e1.60 (m, 2H, eCH2-), 2.91e2.93 (m, 2H, ꢁPhCH2-),
4.09e4.13 (t, J ¼ 6.54 Hz, 2H, eOCH2-), 4.38 (m, 1H, >CH-),
6.67e6.96 (m, 4H, eC6H4-). 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), 130.6, 154.9
Thermal analysis experiments were performed using
a
TGA
(CeOH), 170.8 (C]O). FT-IR (
n
/cmꢁ1): 3419 (OeH), 2957, 2932,
apparatus operated in the conventional TGA mode (TA Q-600, TA
Instruments) at the heating rate of 10 K/min in a nitrogen atmo-
sphere, and the sample size was about 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 on a sample holder, and the diffractograms were recorded
2259 (eN]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
by addition of triethylamine (3 mol%) as the catalyst. The reaction
mixture was poured into a large amount of diethyl ether, filtered
and dried under reduced pressure to give the off-white polymer.
The polyurethanes prepared were record as LPU, DPU, RPU which
ꢀ
in the range 5 ꢀe80 at the speed of 5 ꢀ/min. Infrared emissivity
values of the samples were investigated on the IRE-I Infrared
Emissometer of Shanghai Institute of Technology and Physics,
China.

Y. Yang et al. / Polymer 52 (2011) 3745e3751
3747
represented the polymerization of isocyanate of
L-tyrosine hexyl
ester, isocyanate of
tyrosine hexyl ester.
D-tyrosine hexyl ester and isocyanate of DL-
The spectroscopic data of PU are as follows (take LPU for
example). 1H NMR (300 MHz, CDCl3):
0.90 (3H, eCH3), 1.31 (6H,
eCH2-), 1.63 (2H, eCH2-), 3.16 (2H, -PhCH2-), 4.14 (2H, eOCH2-),
d
4.67 (1H, >CHe), 5.61 (1H, eNH), 7.08e7.16 (4H, eC6H4-). FT-IR (n/
cmꢁ1): 3330 (NeH), 2956, 2930, 1736 (eCO2C6H13), 1711 (eC(O)
NHe), 1507, 1217, 1035, 898.
3. Results and discussion
3.1. Monomer synthesis
Phosgene, diphosgene and triphosgene were widely used in the
synthesis of isocyanates. The reaction usually required refluxing at
high temperature in order to decompose the intermediate of car-
bamyl chloride into isocyanate. It is not suitable for preparing
unstable isocyanates. Nowick et al. [44,45] reported that amino acid
ester hydrochlorides and peptide hydrochlorides could readily be
converted to isocyanates by treatment with a solution of phosgene
in toluene and methylene chloride under the catalyzing of pyridine
or saturated aqueous sodium bicarbonate solution at 0 ꢀC. Based on
the research of them, we successfully synthesized the monomers of
isocyanate-phenol using the solution of phosgene in a facile way.
The length of ester chain was selected based on the solubility of the
esters of tyrosine in CH2Cl2 in which the preparations of monomer
and polyurethane were conducted. Tyrosine hexyl ester had good
solubility in CH2Cl2 while the esters became poorly soluble below
hexyl chain length [46]. The synthesis of monomers was carried out
according to Scheme 1. Tyrosine hexyl ester hydrochlorides were
prepared by reacting tyrosine with 1-hexanol in the presence of
thionyl chloride. The methylene chloride solutions of tyrosine hexyl
esters were also prepared by the same method without the use of
phosgene in order to compare and confirm the formation of
isocyanate-phenols. The structure of isocyanate-phenols was char-
acterized by IR,1H NMR and 13C NMR spectroscopy. Fig.1a illustrates
Fig. 1. (a) 1H NMR in CDCl3 and (b) IR spectra of (1) L-tyrosine hexyl ester and (2)
isocyanate of L-tyrosine hexyl ester in the reaction.
the 1H NMR spectra of
L-tyrosine hexyl ester and the corresponding
isocyanate in CH2Cl2. The signal assignable to the protons of primary
amine (eNH2) around 3.5 ppm was completely disappeared after
the reaction with phosgene, from which the formation of the
isocyanate group was supposed. Fig.1b shows strong IR absorptions
assignable to the isocyanate and hydroxyl groups at 2259 and
3419 cmꢁ1 of the monomer, thus confirmed the synthesis of
isocyanate-phenol. The 13C NMR spectrum in Fig. 2 also supports the
structure of the monomer. The isocyanate of L-tyrosine hexyl ester
was stable in solution and could not be isolated which probably led
to spontaneous polymerization. The same results were obtained in
3.2. Polymer synthesis
Zirconium acetylacetonate, dibutyltin dilaurate and tertiary
amines are commonly used as the catalysts in the synthesis of
polyurethane. In order to get the pure linear polyurethanes
excluding the nylons, the polymerization here were performed by
addition of triethylamine but not others [37]. Fig. 3a shows the FT-
IR spectrum of LPU, the strong absorptions at 1736 cm-1 and
1711 cm-1 are assigned to the C]O groups of the hexyl ester and
carbamate, respectively. The bands between 2980 cm-1 and
the preparation of isocyanates of
D and DL-tyrosine hexyl ester.
Scheme 1. Synthetic procedure of isocyanate-phenol and polyurethane.

3748
Y. Yang et al. / Polymer 52 (2011) 3745e3751
Table 1
Polymerization results and characterization of PUs.
Polymer
Yielda(%)
Mn
Mw/Mn
½ ꢃ
a 2D5
b
b
c
LPU
DPU
RPU
93
91
86
21,243
24,094
21,235
1.58
1.54
1.68
þ126 ꢄ 5
ꢁ120 ꢄ 5
0
a
Ether-insoluble part.
Determined by GPC eluted with THF based on polystyrene standards.
Measured by polarimetry at room temperature, c ¼ 0.50 g/dL in CH2Cl2.
b
c
CH Cl
2
2
CDCl
3
the NeH proton of amide group at 5.6 ppm is observed, con-
firming the formation of linear polyurethane. These results indi-
cate that the reaction has taken place as expected. Table 1
summarizes the results of the polymerization. Polyurethanes
with moderate molecular weights and narrow molecular weight
distributions were obtained in good yields. These polymeric
materials are soluble in common organic solvents including
chloroform, acetone, and THF, and show good film forming ability
that would be of great importance for some applications.
j
g,h
i
k
m
c,d,e
f
a
b
200
150
100
50
0
(ppm)
δ
Fig. 2. 13C NMR of isocyanate of L-tyrosine hexyl ester in CDCl3 in the reaction.
2900 cm-1, in the spectrum, are associated with asymmetric and
symmetric eCH2 groups. Absorption of amide NeH group appears
around 3300e3500 cmꢁ1. Moreover, the characteristic band at
2259 cmꢁ1 belonging to N]C]O group has disappeared. The 1H
NMR spectrum of LPU is shown in Fig. 3b. The signal assignable to
3.3. Hydrogen bond analysis
The IR spectrum is not only for the structure confirmation, it is
also useful in the study of hydrogen bond effect. Participation in
Fig. 3. (a) IR and (b) 1H NMR spectra of LPU in CDCl3.
Fig. 4. IR spectra of PUs at (a)3000-3700 cmꢁ1 and (b)1600e1800 cmꢁ1
.

Y. Yang et al. / Polymer 52 (2011) 3745e3751
3749
hydrogen bonding would decrease the vibration frequency of the
NeH and C]O groups [47e49]. As shown in Fig. 4a, the broad band
centered at 3428 cm-1 corresponds to mainly free NeH groups for
RPU, this absorption decreases and undergoes a red-shift to
3330 cm-1, which assigns to the hydrogen bonded NeH groups for
LPU and DPU. This phenomenon indicates that most of the free
NeH groups have changed to be hydrogen bonded. It is noteworthy
that compared with the enveloping of NeH stretching region by the
absorptions of hydrogen bonded NeH groups in DPU, the presence
of free and hydrogen bonded NeH groups can be observed in LPU.
This probably originated from the slightly different degrees of
hydrogen bonding in chiral polymers with the opposite confor-
mation [50]. The carbonyl stretching regions of the spectra of PUs
are showed in Fig. 4b, there are two overlapping bands, apparently
centered near 1741 and 1712 cmꢁ1, due to the free carbonyl groups
belonging to esters and carbamates, respectively. A hydrogen
bonded carbonyl band is observed at a lower frequency of
1662 cm-1 whereas it is weak in the RPU. Compare to the free ester
carbonyl peak, the free carbamate carbonyl absorption decreases as
the hydrogen bonded carbonyl band increases in the spectrum of
LPU and DPU, which indicates that the NeH groups are involved in
hydrogen bonding with carbonyl groups in the carbamate.
DPU
LPU
RPU
5.90
5.80
5.70
5.60
5.50
5.40
5.30
(ppm
)
δ
Fig. 6. 1H NMR spectra of PUs in CDCl3 at 5.20e6.00 ppm.
The solubility of PUs in many common organic solvents allowed
us to investigate their properties by “wet” spectroscopic methods.
The IR spectrum of LPU is also conducted in CHCl3 (Fig. 5) in order
to demonstrate which kind of hydrogen bond is formed in the
optically active polyurethanes [14,15]. The peak of hydrogen
bonded NeH groups in CHCl3 diminishes remarkably comparing to
that in the solid state. With the decrease of the concentrations from
5 to 1 mM (Fig. 5aed), the absorptions of hydrogen bonded NeH
groups reduce together with the increasing of free NeH groups.
Meanwhile, the bands of carbonyl groups become narrower. This
indicates that the hydrogen bondings are mostly intermolecular.
However, there are no significant differences when the concen-
trations are below 1 mM, and some degree of hydrogen bonding
exists even at a very low concentration, which implies the existence
of intra-molecular hydrogen bonding.
a reasonable conclusion that both inter- and intra-molecular
hydrogen bondings are constructed in optically active poly-
urethanes. From the IR and NMR analyses, the existence of plentiful
hydrogen bonds can be concluded in the optically active LPU and
DPU, and their enhanced properties will be further demonstrated.
3.4. Optically activity and secondary structure of polymers
The secondary structure of PUs was examined by polarimetry,
CD, and UVevis spectroscopies. Table 1 summarizes the specific
rotations of optically active and racemic polyurethanes measured in
CH2Cl2. In contrast to the corresponding HCl salts, the LPU and DPU
display large optical rotations with an opposite value to each other,
suggesting that they took a helical structure with predominantly
one-handed screw sense. However, the RPU still has no optical
activity.
The formation of hydrogen bond is also investigated by 1H NMR
spectra obtained from CDCl3 solutions in Fig. 6. Although the inter-
chain hydrogen bond has been disrupted by the solvent, the peak of
NeH proton becomes slightly broader and shifts to lower field upon
hydrogen bonding association with the carbonyl. These data lead to
Fig. 7 depicts the CD and UVevis spectra of LPU, DPU and RPU
measured in CH2Cl2 at room temperature. The three polymers
almost have the same UVevis absorption. The peak at 235 nm
corresponds to the
p-p
* transition of the aromatic rings, and the
broad absorption peak at about 265 nm is assigned to the n-
p
*
transition of carbonyl groups. The CD spectroscopic analysis is
important for the investigation of secondary structure [51,52].
Hence, we conduct it to confirm the helicity of the PUs. The LPU and
DPU exhibit similar strong Cotton effects with wonderful mirror
image symmetry at 235 and 265 nm. The result unambiguously
proves that the polyurethanes based on
a couple of enantiomorphs which have the right handed and left-
handed helical conformation with preferred screw sense,
L and D-tyrosine are
a
respectively. On the contrary, the RPU is CD inactive which resulted
from the random coiled molecular chain or 1:1 mixture of both left
and right handed helices (see below for detail discussion).
3.5. Crystallizability properties
Wide-angle X-ray diffractograms for the polymers are displayed
in Fig. 8. As it can be seen, the diffraction pattern for every PU shows
a main crystalline peak A at 21 ꢀ as well as amorphous region. Peak
A is present regardless of the optical activity of polymers, thus it can
be assigned to the presence of hard segments of aromatic ring in
the PUs. It is worth noting that a crystalline peak B appeared at
about 12 ꢀ for the optically active LPU and DPU. All of the PUs with
Fig. 5. IR spectra of LPU in CHCl3 at the concentrations of (a)5 mM, (b)3 mM, (c)2 mM,
(d)1 mM, (e)0.5 mM and (f)0.1 mM.

3750
Y. Yang et al. / Polymer 52 (2011) 3745e3751
Fig. 9. TGA curves of PUs.
regular helical secondary structure of the molecular chain in the
light of CD analysis above. The result indicates that the asymmetric
force field generated by the chiral compound could induce more
crystallinity as well as optical activity in resulting polymers,
consequently generated orderliness in new region. In addition, the
crystallinity of LPU and DPU are better than RPU which doesn’t
have this crystalline region, the random coiled macromolecular
chain but not the equivalent left and right handed helices in the
RPU can thus be proved.
3.6. Thermal stability analysis
The thermal properties of PUs were investigated by TGA tech-
niques at a heating rate of 10 ꢀC/min under a nitrogen atmosphere.
As shown in Fig. 9, the TGA curve for each sample exhibits
a
smooth, stepwise manner, suggesting a two-step thermal
degradation. The thermogravimetric data of these polymers are
summarized in Table 2. The temperature of 5% and 10% weight loss
(T5, T10) of the polymers have been calculated by means of ther-
mograms and used as criterion for evaluation of thermal stability of
the polymers. Although LPU, DPU and RPU have the same repeating
units and compositions, their thermal stabilities are different. The
RPU shows initial weight loss around 150 ꢀC, whereas, LPU and DPU
show no decomposition until 265 ꢀC. It is clear that optically active
polymers have higher thermal stability than the racemic one, and
this could be pertained to their different chain conformations. LPU
and DPU have better ability to damping the internal thermal energy
which benefit from the more regulated secondary structure.
Furthermore, the higher crystallinity of LPU and DPU which
confirmed by the XRD study would increase closer chain interac-
tions, and therefore enhances the thermal stability. On the other
hand, random coiled polymer chains can bring about chain sepa-
ration and decrease the inter-chain interactions. This is why the
degree of hydrogen bond of LPU and DPU is higher than that of RPU,
and further supports the macromolecular chain conformations
analyses above.
Fig. 7. CD and UVevis spectra of PUs measured in CH2Cl2 (c ¼ 0.5 g/dL) at room
temperature.
the same composition are obtained by the self-polyaddition of
monomers derived from tyrosine. There are no other ingredients
which could bring about crystallinity. It can be inferred that the
emergence of peak B is perhaps attributed to the formation of
Table 2
Thermal properties of PUs.
Polymer
Decomposition
temperature (ꢀC) T5
Decomposition
temperature (ꢀC) T10
LPU
DPU
RPU
285
283
251
294
295
267
Fig. 8. WAXS Pattern for PUs.

Y. Yang et al. / Polymer 52 (2011) 3745e3751
3751
Table 3
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RPU derived from racemic monomer, the infrared emissivity value
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favors the transmission loss of heat. Meanwhile, the single-handed
helical conformation easily leads to the formation of hydrogen
bond according to above analyses. This can alter the original
vibration mode of molecules and reduce the index of hydrogen
deficiency and the unsaturated degree, thus reduce the infrared
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Isocyanate-phenols derived from chiral and racemic tyrosine
were synthesized and then polymerized in the presence of trie-
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active and racemic polyurethanes (LPU, DPU and RPU) in high
yields. The LPU and DPU are a couple of enantimorphs, the optical
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The asymmetric force field in the chiral isocyanate-phenols induces
orderliness in new region. They possess a helical configuration and
higher degree of hydrogen bond compared to the RPU which pre-
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thermal stability of LPU and DPU provide further proofs of regular
secondary structure and more inter-chain interactions. Both of
them result in the reduction of infrared emissivity. The optically
active polyurethanes based on amino acid could be regarded as
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Acknowledgments
The authors are grateful to the National Nature Science Foun-
dation of China (50873026,51077013), the Key Program for the
Scientific Research Guiding Found of Basic Scientific Research
Operation Expenditure, Southeast University (3207041102),
Program for Training of 333 High-Level Talent, Jiangsu Province of
China (BRA2010033) for financial support.
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