A novel vegetable oil-lactate hybrid monomer for synthesis of high-T g polyurethanes

Article abstract of DOI:10.1002/pola.23759

A study was conducted to introduce a vegetable oil-lactate hybrid monomer for synthesis of high T_g polyurethanes (PU). The conjugation of epoxidized soybean oil (ESO) was investigated, which was prepared by epoxidation of soybean oil using lipase as catalyst, with lactic acid (LA) through ring-opening reaction. It was demonstrated that 1000 mg of ESO and 600 mg of LA were charged in a reactor that contained a stirrer bar. The reactor was purged with nitrogen for around 15 minutes to remove the air and it was sealed. The reaction was allowed to continue for 6 hours and the final reaction mixture was washed with water to remove unreacted chemicals till the pH value of the washing solution turned to 7.0 and the desired product, lactic acid-epoxidized soybean oil (LA-ESO) was recovered by evaporating the water off under reduced pressure.

Full text of DOI:10.1002/pola.23759

A Novel Vegetable Oil–Lactate Hybrid Monomer for Synthesis of
High-Tg Polyurethanes
SHIDA MIAO,1,2,3 SONGPING ZHANG,2 ZHIGUO SU,2 PING WANG1
1Department of Bioproducts and Biosystems Engineering and Biotechnology Institute, University of Minnesota,
St. Paul, Minnesota 55108
2National Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences,
Beijing 100190, China
3Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Received 17 July 2009; accepted 24 September 2009
DOI: 10.1002/pola.23759
Published online in Wiley InterScience (www.interscience.wiley.com).
KEYWORDS: biodegradable; biomaterials; biopolymers; polyurethanes
INTRODUCTION Plant oils are attractive renewable alterna-
tives to petroleum for production of both materials and
fuels.1–7 In recent years, a variety of polymeric materials
have been developed and tested with plant oils as feed-
stocks. Among other products, plant oil-based polyurethanes
(PUs) are the most extensively studied attributing to their
simple preparation yet greatly promising applications.8–10
However, the plant oil-based PUs reported hitherto possess
glass transition temperature (Tg) generally below 40 ꢀC.8,10
That will limit the stiffness of the materials under warm con-
ditions. Ideally, PUs with tunable Tg in a much broader tem-
perature range can significantly enhance the potential of
such materials for general-purpose applications.
bean oil-based polyols have been reported by treating epoxi-
dized soybean oil (ESO) with hydrochloric acid, hydrobromic
acid, methanol, or hydrogen.10,12,13 Epoxide rings in epoxi-
dized plant oils could also be partially opened by water in
the presence of glacial acetic acid by heating to ꢁ75 C.
Such polyols prepared by ring-opening reaction possess
hydroxyl groups mostly located in the middle of the fatty
acid chains, and partial fatty acid chains, the portion of fatty
acids between hydroxyl group and the chain end, form the
pendant groups.10
14,15
ꢀ
Plant oil-based PUs were always amorphous because of ran-
dom crosslinking structure and displayed low glass transi-
tion temperatures.8,10 The Tgs reported so far were generally
<40 ꢀC, attributing mainly to both the low functionality of
the polyols and the presence of pendant groups. Generally,
higher functionality would lead to higher Tg because of the
formation of more crosslinked structures. For example, poly-
ols with functionalities in the range of 3.0–5.2, prepared via
ring opening of several epoxidized plant oils, were used to
generate PUs by polymerization with 4,40-methylene-
bis(phenyl isocyanate) (MDI).10 It was shown that the Tg
increased with increasing of the functionality of the polyols,
with the linseed oil-based polyols possessing the highest
functionality 5.2 giving a Tg as high as 77 ꢀC.10 However,
Plant oil-based PUs have been generally prepared by first
converting the plant oils into polyols, followed by curing
with diisocyanate reagents.8–10 The preparation of polyols
from plant oils can be realized through several approaches.
It was reported that plant oils including trilinolein (or trio-
lein), low-saturation canola oil, and soybean oil could be ozo-
nolyzed in methylene chloride or methanol and were then
hydrogenated sequentially to form polyols.8 Similarly, canola
oil was ozonolyzed in water to produce ozonide that was
then reduced to polyol in tetrahydrofuran.9 The functionality,
in terms of the number of hydroxyl groups contained in each
polyol monomer molecule, was often less than 3.0 for poly-
ols prepared through this approach. Alternatively, plant oils
have been alcoholyzed with glycerol to produce monoglycer-
ides, with each of which contains two hydroxyl groups at the
glycerol end.2,11 Such polyols would generate PUs with the
long fatty acid moiety as the pendant group. Another widely
adopted approach is to oxidize the fatty acid chains to form
epoxide rings and produce polyols with various ring-opening
reagents. For example, halogenated and nonhalogenated soy-
general plant oil-based PUs only showed low Tgs around
10
ꢀ
ꢀ
30 C, such as soybean oil-based PU showing a Tg of 31 C.
It was suspected that the pendant groups in the PUs acted
as plasticizer, leading to the low Tg of the final material.
Ozonolysis method could reduce the content of pendant fatty
acid chains and produce polyols mainly with terminal pri-
mary hydroxyl groups.8 However, the functionality of the
polyols was not more than 3.0, and soybean oil-based PU
prepared by this method gave a low Tg of only 22 ꢀC.8 The
Correspondence to: P. Wang (E-mail: ping@umn.edu)
C
V
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ꢀ
low Tgs make plant oil-based PUs mostly in a leathery state
(transient from glassy to rubbery) at room temperature and
ideal for use as soft materials, resulting, however, in some-
what low strength and moduli,10 which limit their use as
general stiff plastic materials.
rotating speed of 300 rpm and maintained at 40 C. Epoxida-
tion reaction was initiated by adding 700 lL of H2O2 (35 wt
%, out of bottle) to the above mixture and was allowed to
continue for 48 h. Lipase was then removed from the reac-
tion solution by filtration, and the clear filtrate was rinsed
with water for at least three times to remove any residual
H2O2. The ESO product was recovered by drying, and gener-
ally 1050 mg was obtained. According to NMR results, there
were 4.3 double bonds per soybean oil molecule and 3.2 ep-
oxy groups per ESO molecule, respectively. Thus, the conver-
sion rate of double bonds was 74.4%.
We have developed a new class of oleic acid-based PUs
through an epoxy ring-opening reaction by carboxylic group,
generating PUs that have short pendant fatty acid groups.16
This work seeks to broaden the physical properties of plant
oil-based PUs by introducing a biorenewable hybrid mono-
mer, thus increasing the functionality of the monomer. In
particular, we investigated the conjugation of ESO, which
was prepared by epoxidation of soybean oil using lipase as
catalyst, with lactic acid (LA) via ring-opening reaction. We
expect that through such a synthetic strategy, a new class of
partially biorenewable PUs with controllable Tg in a broad
temperature range can be produced.
1H NMR [CDCl3/tetramethylsilane (TMS), d, ppm] of ESO:
5.25 (m, ACH(OH)A), 4.29 (q, ACH2A, glycerol), 4.16 (q,
ACH2A, glycerol), 2.80–3.20 (m, ACH2OCH2A), 2.30 (t,
ACH2COOA), 1.20–1.80 (m, ACH2A aliphatic backbone),
0.90 (t, ACH3).
Preparation of Polyol LA-ESO from ESO and LA
Typically, 1000 mg of ESO and 600 mg of LA were charged
in a reactor that contained a stirrer bar. The reactor was
purged with nitrogen for roughly 15 min to remove the air
and then was sealed. Put the reactor on a 90 ꢀC hot plate
with a magnetic stirring at 300 rpm. The reaction was
allowed to continue for 6 h. The final reaction mixture was
washed with water to remove unreacted chemicals till the
pH value of the washing solution turns to 7.0, and the
desired product lactic acid-epoxidized soybean oil (LA-ESO)
was recovered by evaporating the water off under reduced
pressure. Generally, 1190 mg of product was obtained.
Around 2.1 epoxy groups per ESO molecule were opened by
LA, and the conversion rate of epoxy group was ꢁ70%.
EXPERIMENTAL
Materials and Instrumental Analysis
Soybean oil, oleic acid, ESO, ethanol, Chloroform-d (contain-
ing 0.03% TMS), 4,40-methylenebis(phenyl isocyanate), and
lipase from Candida Antarctica immobilized on an acrylic
resin (Novozym 435) were purchased from Sigma–Aldrich
(MO). Hydrogen peroxide (H2O2, 35% w/w in water solu-
tion) was obtained from Fluka (MO). Lactic acid (LA) with
purity >98% was purchased from MP Biomedicals (OH).
Water was purified with a Mill-Q system from Milllipore
(Billerica). All other reagents were of analytical grade.
1H Nuclear Magnetic Resonance (1H NMR) analysis was per-
formed on a VAC 300 spectrometer, an upgraded IBM instru-
ment with a VXR-300 of Unity (Varian, CA, USA) with chloro-
form-d containing 0.03% tetramethylsilane (TMS) as the
solvent. Electrospray ionization mass spectrometry (ESI-MS)
was performed in positive-ion model on a LTQ XL Linear Ion
Trap Mass Spectrometer (Thermo Fisher Scientific, MA). A
Fourier transform infrared (FTIR) spectrometer (Nicolet Se-
ries II Magna-IR System 750, Nicolet Instrument, WI)
equipped with a horizontal germanium attenuated total re-
flectance accessory was used to monitor preparation of PU
materials starting from soybean oil and LA. Differential scan-
ning calorimetry (DSC) experiments were conducted on a TA
Instruments Q1000 Differential Scanning Calorimeter (DE,
USA) at a programmed ramp rate of 10 ꢀC/min and with a
1H NMR [CDCl3/tetramethylsilane (TMS), d, ppm] of LA-ESO:
5.25
(m,
ACH(OH)A,
in
glycerol),
4.90
(m,
ACOOACH(ACH2A)A), 4.30 (m, CH3ACH(OH)ACOOA), 4.29
(q, ACH2A, in glycerol), 4.16 (q, ACH2A, in glycerol), 3.60
(m, ACH(OH)A, in aliphatic backbone), 3.10–3.85 (m, poly-
ether backbone), 2.30 (t, ACH2COOA), 1.20–1.80 (m, ACH2A
aliphatic backbone), 0.90 (t, ACH3).
Preparation of Ethyl Epoxy Stearate
Ethyl epoxy stearate (EES) was synthesized from oleic acid
through sequential epoxidation and esterification reactions.
The epoxidation reaction of oleic acid was first conducted
with hydrogen peroxide with lipase as the catalyst following a
modified procedure as reported previously,20 and the esterifi-
cation reaction between the oleic acid epoxide and ethanol
was subsequently performed. Typically, 1500 mg of oleic acid
and 10 mL of toluene wꢀere mixed in a 20-mL capped glass
vial under stirring at 40 C for 10 min before the addition of
150 mg lipase (Novozym 435). The mixture was stirred for
additional 10 min, and the epoxidation reaction was initiated
by the addition of 600 lL of H2O2 (35 wt %, out of bottle) to
the reaction mixture. The reaction was allowed to continue
for 10 h. The epoxy stearic acid product was recovered from
the reaction solution by the same procedure as that applied
for ESO. Generally, 1550 mg of product was obtained. Nearly,
all the double bonds of oleic acid were converted into epoxy
groups according to NMR result.
ꢀ
temperature range from ꢂ100 to 200 C. Thermogravimetric
Analysis (TGA) spectra were recorded on a Perkin-Elmer
Pyris Diamond Thermal Analyzer (MA, USA). The change in
weight loss of the samples was measured by heating in
nitrogen atmosphere (20 mL/min) at a programmed ramp
ꢀ
ꢀ
rate 10 C/min from 50 to 580 C.
Preparation of ESO
Epoxidation of soybean oil was conducted following a
modified procedure as previously reported.17–19 Typically,
1000 mg of soybean oil, 100 lg of oleic acid, 100 mg of
Novozym 435, and 5 mL of toluene were mixed in a 20-mL
capped glass vial that was then placed in a shaker with a
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SCHEME 1 Reaction route for
preparation of polyurethane
materials from soybean oil and
lactic acid.
1H NMR [CDCl3/tetramethylsilane (TMS), d, ppm] of epoxy
stearic acid: 2.94 (m, ACH2OCH2A), 2.36 (t, ACH2COOA),
1.20–1.80 (m, ACH2A aliphatic backbone), 0.90 (t, ACH3).
ACH(OH)A), 3.10–3.85 (m, polyether backbone), 2.30 (t,
ACH2COOA), 1.20–1.80 (m, ACH2A aliphatic backbone),
0.90 (t, ACH3).
EES was then produced by esterification of epoxy stearic
acid with ethanol. One gram of epoxy stearic acid was mixed
with 10 mL of ethanol in a 20-mL capped glass vial that was
placed in a shaker with a rotating speed of 300 rpm at
Preparation of PU Materials
The PU materials were prepared by mixing LA-ESO with
4,40-methylenebis(phenyl isocyanate) (MDI) at 70 ꢀC. Typi-
cally, 100 mg of LA-ESO was mixed with MDI with an
amount ranged from 10 to 90 mg, and the mixture was then
ꢀ
40 C. Esterification was initiated by adding 100 mg of Novo-
zym 435 to the mixture, and the reaction was continued for
24 h. Clear EES was recovered by removing the enzyme
through filtration, followed by evaporation to remove
unreacted ethanol. Generally, 1070 mg of product was
obtained. Nearly, all the epoxy stearic acid was converted
into EES according to NMR result.
ꢀ
applied on clean glass plates that were maintained at 70 C.
The glass plates were then moved into an 80 ꢀC oven for a
20 h curing process. The synthesized PU materials were
peeled off from the glass plates for characterization tests.
1H NMR [CDCl3/tetramethylsilane (TMS), d, ppm] of ethyl ep-
oxy stearate: 4.12 (q, ACOOCH2A), 2.91 (m, ACH2OCH2A),
2.30 (t, ACH2COOA), 1.20–1.80 (m, ACH2A aliphatic back-
bone), 0.90 (t, ACH3).
RESULTS AND DISCUSSION
Preparation of LA-ESO Polyol
As shown in Scheme 1, we expect that the addition of lactate
to the fatty acid chains of lipids will not only increase the
amount of hydroxyl group of each lipid monomer (function-
ality) but also allow the hydroxyl groups being more flexible
and reactive by stemming out from the main chain of lipids
through the pendant lactate. Increasing the functionality and
reactivity will afford higher degree of crosslinking in the
final PU products and, thus, increase their Tg. Accordingly,
we first epoxidized lipids via a lipase-catalyzed oxidation
reaction, and the epoxidized lipids were then reacted with
Ring-Opening Reaction between EES and LA
Typically, 200 mg of EES and 68 mg of LA were applied, fol-
lowing the same procedure as described for the reaction
between ESO and LA, except that the reaction only continue
for 3 h.
1H NMR [CDCl3/tetramethylsilane (TMS), d, ppm] of the
product: 4.90 (m, ACOOACH(ACH2A)A), 4.30 (m,
CH3ACH(OH)ACOOA), 4.12 (q, ACOOCH2A), 3.60 (m,
LA to generate
a
polyol monomer, LA-ESO that was
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FIGURE 1 1H NMR spectra of soybean oil
(SOY), epoxidized soybean oil (ESO),
commercially available ESO (ESO-Stand-
ard), and LA-ESO polyol.
FIGURE 2 1H NMR spectra of ethyl ep-
oxy stearate, and its ester product after
reacting with lactic acid (LA-EES)
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RAPID COMMUNICATION
FIGURE 4 IR spectra of reaction intermediates and products
(SOY: soybean oil; ESO: epoxidized soybean oil; PU: polyur-
ethane networks)
FIGURE 3 ESI-MS analysis of soybean oil (SOY), epoxidized
soybean oil (ESO), and and LA-ESO polyol.
subsequently reacted with 4,40-methylenebis(phenyl isocya-
nate) (MDI) to form PUs (Scheme 1).
Figure 1 shows the 1H NMR spectra of the chemicals
involved in the preparation procedure. The formation of
epoxide rings on the fatty acid chains of soybean oil was evi-
dent from the peaks shown at 2.8–3.2 ppm in the 1H NMR
spectrum of the epoxidized soybean oil (ESO in Fig. 1) when
compared with that of its parent soybean oil (SOY). The
NMR spectrum of our ESO product is the same as that
observed for a commercially available ESO standard. For the
same consideration, the disappearance of those peaks in the
LA-ESO spectrum should be a confirmation of the ring-open-
ing reaction between ESO and LA. Furthermore, the occur-
rence of the peaks at 4.9 and 3.6 ppm in the LA-ESO spec-
trum also indicated that formation of new ester and
FIGURE 5 DSC analysis of LA-ESO polyurethane.
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SCHEME 2 Crosslinking structures of plant oil based polyurethane networks. (a) Noncontinuous crosslinking in previously reported
work and (b) Continuous crosslinking achieved through the introduction of lactic acid in this work.
hydroxyl groups as a result of the LA enabled ring-opening
reaction.
sented in Figure 3. The spectrum of soybean oil applied in
this work showed one major group of peaks around 879 m/
z and one minor group at 600 m/z, indicating the presence
of small amount of diglycerides in the soybean oil.12 Corre-
sponding to the major peaks of triglycerides, ESO showed a
major group of peaks with average Mw of 930, indicating the
addition of 3.2 oxygen atoms on average to each triglyceride
molecule. That is a moderate yield of epoxidation reaction
considering each triglyceride molecule of soybean oil con-
tains 4 to 5 C¼¼C double bonds. As can be seen in Figure 1,
the peak at 2.0 ppm corresponding to unsaturated double
bonds still presented in the spectrum of LA-ESO polyol prod-
uct, supporting the retention of C¼¼C double bonds in the
product. The LA-ESO product showed an average Mw at
1120, indicating the addition of 2.1 LA groups on average to
each epoxidized triglyceride molecule, in other words, a yield
of nearly 70% of the ring-opening by LA. That may be a
result of other side reactions such as epoxy
We further demonstrated that such a synthetic approach was
generally applicable to any unsaturated lipids. Following the
same procedure, EES was prepared and applied for ring-
1
opening reaction with LA. Figure 2 shows the H NMR spec-
tra of EES, and its ester product after reacting with LA. The
peak at 3.2 ppm of EES (H of epoxide ring, Peak 2 in Fig. 2)
almost disappeared after the ring opening reaction. Similar
observations were reported previously for ring-opening reac-
tion of epoxides by carboxylic groups in an oligomerization
reaction of epoxy stearic acid.16 Similar to the reaction with
ESO, the occurrence of the peaks at 4.9 and 3.6 ppm further
confirmed that, as expected, LA had opened the epoxy ring
efficiently to form ester bonds and hydroxyl groups.
The molecular weights of the products generated from ESO
and LA were determined by ESI-MS, and the results are pre-
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RAPID COMMUNICATION
homopolymerization that the epoxy rings may undergo.12,16
Accordingly, the LA-ESO hydroxyl content was 4.2 hydroxyl
groups per triglyceride molecule, a functionality of 4.2, as
the addition of each LA molecule would lead to the forma-
tion of 2 hydroxyl groups on the fatty acid chain (Scheme 1).
Preparation of PU Materials Based on LA-ESO
PU materials were then prepared by reacting the LA-ESO
polyol with MDI. Typically, 100 mg of LA-ESO was mixed
well with 10–90 mg of MDI and the molar ratio of isocyanate
group to hydroxyl group (NCO/OH) was controlled in the
range of 0.21:1–1.89:1. When MDI was less than 30 mg
(NCO/OH < 0.63:1), LA-ESO was not efficiently crosslinked
and remained in a liquid state. When NCO/OH > 1:1, the PU
product became rigid and adhered to the glass plates. The
synthesis process was monitored by using IR (Fig. 4). The
appearance of peak at 821 cmꢂ1 in the IR spectrum of ESO
confirmed the epoxidation of soybean oil, additional evidence
to the NMR analysis in Figure 1; the disappearance of this
peak in LA-ESO product indicated the complete consumption
of epoxy groups, also agreeing with the NMR analysis in Fig-
ure 1. The appearance of peaks at 1309 and 1525 cmꢂ1 in
the PU product confirmed the formation of urethane bonds
(the two peaks were due to the asymmetric stretching vibra-
tions and NAH deformation of the AOCONHA bond, respec-
tively).21 The IR spectrum of PU also showed several inter-
esting characteristics different from other types of PUs. First,
there were two peaks presented at 1217 and 1201 cmꢂ1
accounting for CAN stretching vibrations, whereas one peak
was reported for other types of PU materials.21,22 Second,
the peak of CAO stretching vibrations also accordingly
resulted in two peaks (1103 and 1060 cmꢂ1). Such observa-
tions were speculated to be the results of the presence of
two types of hydroxyl groups available in the LA-ESO poly-
ols, that is, the hydroxyl groups generated on the fatty acid
chains by ring-opening reaction and the pendant hydroxyl
groups on the LA moiety.
FIGURE 6 TGA chromatograms of LA-ESO-based poly-
urethanes.
case can only be realized through the hydroxyl group on a
second fatty acid chain of a triglyceride molecule. In the con-
trast, LA-initiated ring-opening reaction of epoxide group
lead to the introduction of two hydroxyl groups to each
epoxide group, thus making it possible to have continuous
linkage through one single fatty acid moiety of the triglycer-
ides [Scheme 2 (b)]. In addition, the carboxyl group intro-
duced by the addition of LA also increases the possibility of
formation of hydrogen bond, further enhancing the rigidity
of the materials.
The Tg of the PU products showed an interesting dependence
on the content of LA-ESO (Fig. 5). As the NCO/OH ratio
changed from 0.63, 0.84 to 1.05:1, the Tg of PU increased
from 31, 64 to 96 ꢀC correspondingly. As mentioned earlier,
PU from soybean oil reported previously exhibited much
8,10
ꢀ
lower Tgs in the range of 31–22 C.
The unique structure
of the polymer prepared in this work may be accounted for
such a difference. First, polyol LA-ESO has a higher function-
ality (4.2) than the previously reported soybean oil-based
polyols (3.5 or 2.5), attributing to the additional hydroxyl
groups introduced through LA molecules. Polyols with high
functionalities afford PU materials of higher Tg as has been
demonstrated previoiusly.10 Second, the PU prepared in this
study may possess a unique urethane bond crosslinking
structure different from previously reported materials
(Scheme 2). Previously reported synthetic routes generally
produced one hydroxyl group to the fatty acid chain through
each epoxide group [Scheme 2(a)],8,10,12 leading to discontin-
uous urethane bond connection as each fatty acid chain
mostly has only one hydroxyl group (as the functionality of
the triglyceride was mostly 2.5–3.5). Polymerization in this
Thermal stability of LA-ESO PU materials was evaluated via
TGA analysis as shown in Figure 6. A three-step thermal deg-
radation, evident from the appearance of three peaks on the
derivative weight loss curves, was observed in all the tested
samples. The first decomposition peak (Tmax peak) occurred in
ꢀ
the range of 292–320 C, corresponding to the decomposition
of weak urethane bonds. The second Tmax peak took place at
ꢀ
370–385 C, mostly due to decomposition and char formation
of crosslinked polymer networks. The third Tmax peak was
ꢀ
observed at 446–456 C, which could be related to the gasifi-
cation of any remaining components. TGA spectra of PUs gen-
erally have two decomposition temperature ranges,23 and the
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
5 Hoogenboom, R.; Thijs, H. M. L.; Fijten, M. W. M.; Schubert, U. S.
third weight loss peak observed in this work might be related
to the second breakdown temperature range of urethane
bonds. Similar to Tg, the thermostability of the PU materials
also increased as the NCO/OH ratio increased.
J Polym Sci Part A: Polym Chem 2007, 45, 5371–5379.
6 Kra¨mer, M.; Kopaczynska, M.; Krause, S.; Haag, R. J Polym Sci Part
A: Polym Chem 2007, 45, 2287–2303.
7 Lligadas, G.; Callau, L.; Ronda, J. C.; Galia`, M.; Ca´diz, V. J Polym Sci
CONCLUSIONS
Part A: Polym Chem 2005, 43, 6295–6307.
Soybean oil and LA are highly desired renewable feedstocks
for preparation of a broad range of polymeric materials
including PUs. The LA-ESO polyols prepared through this
work introduced two hydroxyl groups per epoxy group, lead-
ing to a unique crosslinking structure different from previ-
ously reported vegetable oil-based PU. The Tg of the LA-ESO-
based PU can be controlled in a broad temperature range,
up to 96 ꢀC, in contrast to Tgs < 40 ꢀC as have been
reported previously for plant oil-based PUs. The LA-ESO-
based PU materials contain both ester and urethane bonds
in the backbone chain of the polymer, resulting in a three-
step thermal decomposition property.
8 Petrovic, Z. S.; Zhang, W.; Javni, I. Biomacromolecules 2005, 6, 713–719.
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2007, 84, 55–63.
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Miao thanks a coeducational assistantship from University of
Minnesota for his visiting research at UMN. The authors thank
support from the National Natural Science Foundation of China
(Grant No. 20728607), Chinese Academy of Sciences (KSCX2-
YW-G-019), 973 Program (2009CB724705), and 863 Project
(2008AA10Z302).
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