Molecular architecture of electroactive and biodegradable copolymers composed of polylactide and carboxyl-capped aniline trimer

Article abstract of DOI:10.1021/bm9011248

Two-, four-, and six-armed branched copolymers with electroactive and biodegradable properties were synthesized by coupling reactions between poly(l-lactides) (PLLAs) with different architecture and carboxyl-capped aniline trimer (CCAT). The aniline oligo

Full text of DOI:10.1021/bm9011248

Biomacromolecules 2010, 11, 855–863
855
Molecular Architecture of Electroactive and Biodegradable
Copolymers Composed of Polylactide and Carboxyl-Capped
Aniline Trimer
Baolin Guo, Anna Finne-Wistrand, and Ann-Christine Albertsson*
Department of Fibre and Polymer Technology, School of Chemical Science and Engineering, Royal
Institute of Technology, SE-100 44, Stockholm, Sweden
Received October 2, 2009; Revised Manuscript Received January 27, 2010
Two-, four-, and six-armed branched copolymers with electroactive and biodegradable properties were synthesized
by coupling reactions between poly(L-lactides) (PLLAs) with different architecture and carboxyl-capped aniline
trimer (CCAT). The aniline oligomer CCAT was prepared from amino-capped aniline trimer and succinic anhydride.
FT-IR, NMR, and SEC analyses confirmed the structure of the branched copolymers. UV-vis spectra and cyclic
voltammetry of CCAT and copolymer solution showed good electroactive properties, similar to those of polyaniline.
The water contact angle of the PLLAs was the highest, followed by the undoped copolymer and the doped
copolymers. The values of doped four-armed copolymers were 54-63°. Thermal properties of the polymers were
g
studied by DSC and TGA. The copolymers had better thermal stability than the pure PLLAs, and the T between
4
8-58 °C and T between 146-177 °C of the copolymers were lower than those of the pure PLLA counterparts.
m
This kind of electroactive and biodegradable copolymer has a great potential for applications in cardiovascular or
neuronal tissue engineering.
Introduction
mers of pyrrole and thiophenes which were connected together
via degradable ester linkages and then concluded that this
polymer was conductive, degradable, and biocompatible. Re-
Research on conductive polymers for biomedical applications
expanded greatly with the discovery that these materials were
22
cently, Schmidt et al. incorporated alternating electroactive
1
,2
compatible with many biological molecules. Recent studies
have also shown that electrical signals can regulate cellular
activities, including DNA synthesis, cell adhesion, migration,
quaterthiophene units and biodegradable ester units into one
macromolecule; they demonstrated that the copolymer was
biodegradable and electroactive and that the copolymer was
3
,4
proliferation, and differentiation. Considerable interest has
been shown in conducting polymers and their derivatives, such
16
nontoxic to Schwann cells in vitro. Zhang et al. obtained
electrically conductive biodegradable composite from polypyr-
3
,5
6,7
8
as polypyrrole, polyaniline, polythiophene, and polyeth-
23
role nanoparticles and poly(D,L-lactide). Wei et al. incorporated
9
10
ylenedioxythiophene. For example, Hodgson assessed dif-
ferent oxidation states of polyaniline in vivo, and no obvious
aniline pentamer (AP) into polylactide and obtained linear
triblock copolymers that showed good electroactive, biodegrad-
able, and processing properties, but poor mechanical properties
3
inflammation was found. Langer showed with a culture of the
NGF-induced rat neuronal pheochromocytoma cells (PC-12) that
polypyrrole greatly enhanced the neurite outgrowth and spread-
ing of PC-12 cells.
2
4
because of their low molecular weights. Wei and co-workers
later synthesized linear multiblock copolymers of polylactide
and AP with higher molecular weights, which were electroactive
and biodegradable and which could accelerate the differentiation
Blends, composites, block copolymers, or graft copolymers
have been prepared from polyaniline, polypyrrole, and polymers
25
of PC-12 cells. Zhang et al. synthesized linear degradable
1
1
12,13
6,14
such as heparin, chitosan,
collagen,
poly (ethylene
electrically conducting copolymers of AP and polyglycolide.
Amino-capped aniline trimer (ACAT) has a well-defined
1
5
16,17
18,19
glycol), polylactide,
and oligopeptides,
which were
found more suitable in tissue engineering. However, there are
practical problems when these conductive polymers are used
in tissue engineering. The main drawbacks of the existing system
are poor polymer-cell interaction, absence of a cell interaction
site, hydrophobicity, poor solubility and processability, and
electroactive structure, good processing properties, and ease of
26,27
degradability.
It exhibits electronic and optical properties
28,29
that are remarkably similar to those of polyaniline.
We have
therefore incorporated the electroactive aniline trimer into
PLLAs to obtain biodegradable and electroactive copolymers
in this paper.
4
uncontrollable mechanical properties. One of the biggest
limitations for in vivo applications in tissue engineering of
conductive polymers is their inherent inability to degrade, so
that the incorporation of conducting polymers into biodegradable
polymer to obtain materials with both electroactive and biode-
gradable properties is still a challenge. Langer et al. prepared
erodible conducting polymers based on ꢀ-substituted pyrrole
monomers containing ionizable and/or hydrolyzable side groups.
Our group has worked on the synthesis of (co)polymers from
lactones for many years, and we have synthesized many
30–35
(co)polymers with different architectures.
Architecture plays
2
0
an important role on the performance of polymers. Lately, star-
shaped homo- and copolymers have been shown to be an
interesting alternative to their linear analogues in their interaction
with surrounding tissue. As a result, we used branched PLLAs
to couple with CCAT to obtain star-shaped electroactive and
biodegrable copolymers, which will have a better interaction
with the tissue. First, an electroactive CCAT was synthesized
2
1
Schmidt et al. synthesized polymers from conducting oligo-
*
To whom correspondence should be addressed. Tel.: +46-8-790 8274.
Fax: +46-8-20 84 77. E-mail: aila@polymer.kth.se.
10.1021/bm9011248
 2010 American Chemical Society
Published on Web 03/04/2010

8
56 Biomacromolecules, Vol. 11, No. 4, 2010
Guo et al.
Scheme 1. Schematic Synthesis of Carboxyl-Capped Aniline Trimer and the Copolymers
from ACAT, and characterized, then coupled with linear, four-
and six-armed star-shaped hydroxyl-capped polylactides to
generate a series of electroactive and biodegradable copolymers
with different architectures. Degradable polymers with different
architectures are used in tissue engineering and the architectural
design is a valuable tool for tuning the mechanical properties.
We hypothesize that, by adding electroactive properties to these
polymers, we obtain an additional parameter that can be utilized
for optimal cell culturing results.
(-NHCO-), 157.74 (Ar-C), 145.04 (Ar-C), 137.21 (Ar-C), 125.15
(Ar-C), 121.89 (Ar-C), 119.66 (Ar-C), 31.25 (-CH -), 29.03 (-CH -).
2 2
The synthetic route is shown in Scheme 1.
Synthesis of Branched PLLAs. Branched PLLA was prepared by
3
2
ring-opening polymerization (ROP) as follows: a silanized round-
bottomed flask was used as the reactor. The monomer (LLA), initiator
(
Sn(Oct)
and inositol (Ino)) were weighed and added into the flask in a glovebox
Mbraun MB 150B-G-I) purged with nitrogen. The mixture was then
2
), and co-initiator (ethylene glycol (EG), pentaerythritol (Pen),
(
immersed in an oil bath at 110 °C under a nitrogen atmosphere for an
appropriate time. The theoretical molecular weights of each kind of
PLLA were set to 80000, 40000, and 20000, respectively. After reaction,
Experimental Section
1
0 mL of chloroform was added in the flask to dissolve the mixture,
which was then precipitated in 300 mL of hexane/methanol (v/v )
5:5) solution. After filtration, the product was dried in a vacuum oven
Materials. L-Lactide (LLA; Aldrich) was purified by recrystallization
in dry toluene and subsequently dried under reduced pressure (10-2
9
mbar) at room temperature for at least 48 h prior to polymerization.
Toluene (Lab-Scan, 99.8%) was dried over a Na wire before use.
Aniline (Aldrich) was distilled twice under reduced pressure. Stannous
at room temperature for 24 h. Samples were coded as, for example,
EG-PLLA80000, which denoted a PLLA initiated by ethylene glycol
for which the theoretical molecular weight was 80000.
Synthesis of the Copolymers. The schematic routine for the
synthesis of copolymer with four-armed PLLA and CCAT as an
example is shown in Scheme 1. Purified Pen-PLLA40000 (0.436 g,
octoate, Sn(Oct)
under an inert atmosphere before use. p-Phenylenediamine, ammonium
persulfate ((NH ), ammonium hydroxide (NH OH), hydrochloric
acid (HCl), ethanol (EtOH), dimethyl sulphoxide (DMSO), succinic
anhydride (SA), chloroform (CHCl ), hexane, methanol, 1,4-dioxane,
N,N′-dicyclohexyl carbodiimide (DCC), 4-dimethylaminopyridine
DMAP), and camphorsulfonic acid (CSA) were all purchased from
2
(Aldrich), was dried over molecular sieves and kept
) S O
4 2 2 8
3
-
5
-5
1
1
.5 × 10 mol) and CCAT (0.03 g, 6 × 10 mol) were dissolved in
3
0 mL of 1,4-dioxane in a flame-dried flask with a magnetic stirrer,
-
4
-4
and DMAP (0.017 g, 1.5 × 10 mol) and DCC (0.034 g, 1.5 × 10
mol) were then added into the flask. The reaction was kept at 0 °C for
8 h. After the reaction, the mixture was filtered to remove dicyclo-
(
Aldrich and were used as received.
4
Synthesis of Carboxyl-Capped Aniline Trimer. ACAT was
3
6
hexylurea. The filtrate was precipitated in a mixture of hexane and
methanol (vol. (hexane)/vol. (methanol) ) 95:5). The product obtained
was dried in a vacuum oven for 48 h after filtration.
synthesized according to previous reports. Briefly, ACAT was
obtained by oxidative coupling of p-phenylenediamine and 2 equiv
amounts of aniline with equivalent amounts of (NH
as shown in Scheme 1. H NMR (400 MHz, DMSO-d
4
)
2
S
2
O
8
as oxidant,
) δ 6.96 (s, 4H,
Ar-H), 6.81 (d, 4H, Ar-H), 6.62 (d, 4H, Ar-H), 5.43 (s, 4H, -NH ).
) δ 155.21 (Ar-C), 147.69 (Ar-C),
39.35 (Ar-C), 135.18 (Ar-C), 124.33 (Ar-C), 114.13 (Ar-C). This
1
Characterization. FT-IR spectra of ACAT, CCAT, PLAAs, and
the copolymers were recorded on a “Perkin Elmer Spectrum 2000”
spectrometer (Perkin-Elmer Instrument, Inc.). Each spectrum was taken
6
2
13
C NMR (100 MHz, DMSO-d
6
-
1
1
as the average of 20 scans at a resolution of 4 cm .
1
13
result agrees well with ref 36.
For the synthesis of CCAT, ACAT (0.2280 g, 0.001 mol), and SA
H and C NMR spectra were recorded on a Bruker Avance 400
and 100 MHz NMR instruments, CDCl being used as the solvent for
all the PLLAs samples and as internal standard (δ ) 7.26 and 77.0
ppm). For aniline trimer and copolymer, DMSO-d was used as the
3
(0.3000 g, 0.003 mol) were added into a dry flask with magnetic stirrer,
and 15 mL of dry DMSO was then poured into the flask and the flask
was sealed. After a reaction for 24 h at room temperature, the solution
was dropped into 300 mL of distilled water and then precipitated in
HCl solution, pH ) 1. The precipitate was collected by filtration and
washed thoroughly with distilled water. Finally, the product was dried
6
solvent at room temperature and as internal standard (δ ) 2.50 and
39.5 ppm).
Number of average molecular weight, molecular weight distribution
w n
(M /M ) values, and R-parameter from Mark-Houwink equation of
1
in a vacuum oven at room temperature for 48 h. Characterization: H
polymers were determined by GPC experiments conducted in chloro-
form at 40 °C at a flow rate of 1 mL/min using a Spectra-Physics 8800
solvent delivery system with a set of two PLgel 5 µm MIXED-C
ultrahigh efficiency column, a Shodex SE 61 refractive index detector,
NMR (400 MHz, DMSO-d
6
) δ 10.05 (s, 2H, -COOH), 9.70 (s, 2H,
-
NHCO-), 7.63 (d, 4H, Ar-H), 7.38 (s, 4H, Ar-H), 6.87 (d, 4H,
1
3
6
Ar-H). C NMR (100 MHz, DMSO-d ) δ 174.14 (-COOH), 170.30

Electroactive and Biodegradable Copolymers
Biomacromolecules, Vol. 11, No. 4, 2010 857
Figure 1. H (A) and 13C NMR (B) spectra of carboxyl-capped aniline trimer.
1
and a viscometer. Polystyrene standards with narrow molecular weight
distributions were used as calibration.
The UV-vis spectra of ACAT, CCAT, and copolymer solutions
the number of molecular branches should be the same as the
number of the hydroxyl groups of the pentaerythritol. In
addition, the -CH - (δ ) 4.3 ppm) of the pentaerythritol
2
were monitored on a UV-vis spectrophotometer (UV-2401).
Glass transition temperature (T ) and melting temperature (T
1
appears in the H NMR spectra (Figure 2B), which is also
g
m
) were
13
confirmed in the C NMR spectra of Pen-PLLA40000 (Figure
measured by differential scanning calorimetry (DSC) using a Mettler-
Toledo DSC 820 module under a nitrogen atmosphere (nitrogen flow
rate of 80 mL/min). Measurements were made during the first heating
scan from -20 to 200 °C, the first cooling from 200 to -20 °C, and
2
D), the signal of pentaerythritol appearing at 66.9 ppm. Thus,
Pen-PLLA40000 with a four-armed branched structure is
certainly obtained. The hydroxyl group (δ ) 2.7 ppm) at the
1
PLLA chain end is also seen in the H NMR spectra. This will
the second heating scan from -20 to 200 at 10 °C /min. Data for T
and T were taken from the second heating scan.
m
be used for the coupling of the hydroxyl-capped PLLA and
CCAT in the next step. The same trends are found in the EG
and Ino systems.
g
The thermal stability of the PLLAs and copolymers was estimated
by thermogravimetric analysis (TGA) under a nitrogen atmosphere
1
(
nitrogen flow rate 50 mL/min) with a sample mass of 10 ( 1 mg and
Monomer conversion of the sample was determined by H
a heating rate of 10 °C/min. The scan range was from 30 to 800 °C.
Cyclic voltammetry (CV) of aniline trimer and copolymers was
performed on an Electrochemical Workstation interfaced and monitored
with a PC computer. A three-electrode system was employed, consisting
of a platinum disk working electrode (surface area 0.14 cm ), a
platinum-wire auxiliary electrode, and a reference electrode. The
NMR by comparing the peak integrals of methine protons of
PLLA (δ ) 5.2 ppm in Figure 2) with those of lactide (δ ) 5.0
ppm in Figure 2), and the results are summarized in Table 1.
The conversion of Ino as co-initiator is lower, but it exceeds
2
8
6% after 4 days of reaction. The monomer conversions of the
other co-initiators system are all higher than 92%, which
2
reference electrodes used were Ag/AgCl for DMSO/H O mixture. All
2
indicates that the low concentration of Sn(Oct) has good catalyst
the solutions were deoxygenated for 10 min with nitrogen prior to the
electrochemical measurements. The scan rate used was 60 mV/s.
Water contact angle measurements of the copolymer films were
measured using a contact angle and surface tension meter (KSV
instruments Ltd.). A drop of Milli-Q water was placed on the surface
of the sample and the images of the water menisci were recorded by a
digital camera. The KSV software was used to analyze the images and
gave the contact angle data. The contact angle of each sample was
taken as the average of five measurements at different points.
activity.
Molecular weights of the PLLA segment were calculated by
comparing the peak integrals of methine protons (δ ) 5.2 ppm,
CH in Figure 2) with those of the methine protons next to the
terminal hydroxyl groups (δ ) 4.4 ppm, CH (end) in Figure
2). The PLLA segment was equivalent to the PLLA molecular
chains of each branch; that is, the Mn-NMR of the PLLA segment
denoted the number average molecular weight of each branch.
The molecular weights calculated by NMR are also listed in
Table 1. We found that the molecular weight increase with the
increase of the ratio of the monomer to co-initiator. The
molecular weight determined by NMR was very close to the
GPC results as shown in Table 1, and the molecular weight
distribution was quite narrow. The relationship between intrinsic
viscosityandmolecularweightwasanalyzedbytheMark-Houwink
Results and Discussion
Synthesis of Carboxyl-Capped Aniline Trimer. The struc-
ture of CCAT was confirmed by NMR as shown in Figure 1.
The fact that there is no peak arising from the amino group (δ
)
5.4 ppm) in Figure 1A demonstrates that ACAT is completely
R
changed into CCAT. There are small peaks around 137 and
equation [η] ) kM ; the values of R is 0.5 at the theta state and
13
125 ppm in the C NMR spectrum in Figure 1B because CCAT
is in the range of 0.65-0.8 for linear random coils in the good
solvent. It is found that the R of the six- and four-armed
polymers are smaller than the linear counterparts with the similar
molecular weight, which indicates a smaller hydrodynamic size
and a more compact structure of the branched polymers. These
results further confirm the star-shaped structures of the polymers.
It is observed that some of the molecular weights are not in
accordance with the theoretical ones. This is because the
is in an emeraldine state and the NMR spectrum is affected by
2
3
the quinoid signals. CCAT contains carboxyl groups at both
end of the monomer, so it can be introduced into polymer by
graft or copolymerization, and the hydrophilicity should also
increase because of the introduction of carboxyl groups into
CCAT.
Synthesis of Branched PLLAs. Linear, four- and six-armed
PLLAs with different molecular weights were synthesized by
2
Sn(Oct) catalyzes transesterification reactions and the conver-
ROP. Sn(Oct)
2
was used as initiator, and the ratio of monomer
sion did not reach 100%, as shown in Table 1.
to initiator was 10000:1. The organic metal residuals in the
PLLA are therefore very low, and the polymers can thus be
Characterization of the Copolymers. Copolymers were
characterized by FT-IR and NMR. Figure 3 shows the IR spectra
of the copolymer together with those of its corresponding
precursors. Curves a, b, c, and d are the IR spectra of ACAT,
CCAT, Pen-PLLA20000, and Copo-Pen-PLLA20000, respec-
tively. In Figure 3a, the characteristic absorption bands at 3308
3
7
used in tissue engineering.
1
Figure 2B shows the H NMR of Pen-PLLA40000. No peak
is observed between 3.0 and 4.0 ppm corresponding to nonre-
acted hydroxyl groups of pentaerythritol, and this means that

8
58 Biomacromolecules, Vol. 11, No. 4, 2010
Guo et al.
Figure 2. H NMR spectra of EG-PLLA40000 (A), Pen-PLLA40000 (B), Ino-PLLA40000 (C), and 13C NMR spectrum of Pen-PLLA40000 (D).
1
and 3201 cm- arising from the terminal -NH
1
of ACAT are
new peak at 3246 cm-1 and peaks at 1722 and 1660 cm-1
2
-
1
observed. The characteristic peaks at 1597 and 1499 cm are
assigned to the stretching vibrational bands of quinoid rings
and benzenoid rings, respectively. Compared to curve a, the
corresponding to carbonyl groups (-CO-) in the -COOH and
2
-NHCO- groups reveal that the aromatic amine -NH in the
ACAT was successfully converted into the imine -NHCO-.

Electroactive and Biodegradable Copolymers
Biomacromolecules, Vol. 11, No. 4, 2010 859
Table 1. Properties of Branched PLLAs
reaction
time (h)
monomer
conversion (%)
molecular weight
determined by NMR
molecular weight
determined by GPC (M
R from
Mark-Houwink equation
sample code
w n
/M )
EG-PLLA80000
EG-PLLA40000
EG-PLLA20000
Pen-PLLA80000
Pen-PLLA40000
Pen-PLLA20000
Ino-PLLA80000
Ino-PLLA40000
Ino-PLLA20000
72
72
72
48
72
72
96
72
72
92
94
99
97
97
97
86
97
98
58000
32200
14700
59800
29100
14700
43600
42900
22000
67200 (1.17)
30000 (1.37)
18530 (1.29)
50050 (1.12)
20200 (1.15)
11230 (1.31)
40800 (1.12)
34000 (1.21)
21600 (1.30)
0.63
0.68
0.73
0.50
0.53
0.54
0.45
0.47
0.55
Due to the electron-withdrawing group at both ends of CCAT,
the characteristic absorption band of the quinoid unit shift to
in the NaOH solution (colorless solution). CCAT does not
dissolve in CHCl (colorless solution), but the copolymers
3
-1
1
575 cm is shown in Figure 3b. The peaks at 3308 and 3201
dissolved in CHCl3 easily (light red solution). All these
phenomena and data demonstrate that the copolymer was
successfully obtained. The good solubility of the copolymer in
-1
cm corresponding to -NH
2
of ACAT have completely
disappeared (Figure 3b), which confirms the total transformation
from ACAT to CCAT. Compared with curves b and c, curve d
contains peaks at 1670, 1593, and 1496 cm- from the CCAT
segment, and peaks at 1748, 1180, and 1078 cm- assigned to
CHCl also provides a convenient processing property of the
3
1
copolymer.
1
Electroactive Properties of ACAT, CCAT, and
Copolymers. Electrochemical properties of the monomer and
copolymers were studied by UV-vis spectra and cyclic
voltammetry.
-
CO- and C-O-C groups from the Pen-PLLA segment. All
these data demonstrate that the copolymer was obtained. Curve
d also shows a small peak at 3498 cm- arising from the -OH
groups at the end of the copolymer, which are very important
in tissue engineering, because they increase the hydrophilicity
of materials and improve cell adhesion to the copolymers.
1
The UV-vis spectra of ACAT, CCAT, and copolymers in
DMSO solution are shown in Figure 5. The UV absorption
spectra (Figure 5a,b) of ACAT and CCAT show a typical broad
quinoid absorption peak at about 520 nm (CCAT) and 590 nm
To further confirm the structure of the copolymer, the
1
copolymer was characterized by H NMR in DMSO-d
6
, and
(ACAT), and a narrower benzenoid peak at about 330-340 nm.
the result of the Copo-Pen-PLLA20000 copolymer is shown in
Figure 4 (see Supporting Information for the two- and six-armed
The quinoid peak is related to the excitonic transition of πb-πq
from the benzene unit to the quinone unit, while the benzenoid
peak is attributed to the π-π* transition in the benzene unit,
1
copolymer). Copo-Pen-PLLA20000: H NMR (400 MHz, DMSO-
d
4
2
6
) δ 10.07 (s, 1H, -COOH), 9.71 (s, 2H, -NHCO-), 7.63 (d,
H, Ar-H), 7.34 (d, 4H, Ar-H), 6.94 (d, 4H, Ar-H), 2.62 (t,
H, -CH CH -) for the CCAT segment, and 5.19 (t, 2H, poly
38
which is similar to that of polyaniline. The quinoid/benzenoid
intensity ratio decreases together with a slight blue shift caused
by enhanced conjugation arising from covalent modification by
imine groups that facilitates the delocalization of electrons in
the benzene unit, as shown in Scheme 2. Consequently, the
electronic concentration of quinone units is reduced, and the
intensity of the πb-πq transition is decreased. There is no new
absorption as a result of the synthesis of CCAT and copolymers,
and this indicates that the ACAT in the CCAT and copolymers
remains in the emeraldine base form.
2
2
-
CH), 4.24 (s, 2H, Pen), 1.45 (d, 3H, poly), 1.27 (d, 3H) for the
Pen-PLLA segment. The chemical shift of the end chain CH
group in the pure PLLAs is around 4.4 ppm, and it was shifted
to around 4 ppm in the copolymer as a result of the coupling
reaction of the hydroxyl groups at the PPLA chain end and the
carboxyl groups of CCAT, which confirms the successful
esterification in the copolymer.
In addition, two solubility tests demonstrated the successful
synthesis of the copolymer. CCAT dissolves easily in 1 mol/L
NaOH solution (red solution), but the copolymer did not dissolve
In Figure 5, it is evident that the UV-visible absorption
spectra of all the copolymers are similar to that of CCAT,
although the intensity of the UV absorbance decreases with
increasing molecular weight of the Pen-PLLA segment, that is,
with decreasing the CCAT content in the copolymer. There is
no obvious blue shift in Figure 5c-e. This is different from
23
previously reported data, in which the leucoemeraldine poly-
lactide-AP triblock copolymers in DMF solutions have a shift
that increases with decreasing AP content in the copolymer,
because the incorporation of the long and flexible PLA chains
into the triblock copolymer may restrict the planar conformation
of AP backbone in the copolymers, leading to decrease in the
effective conjugation length of the AP. We prepared the CCAT
segment at the end of the copolymer chain, so the copolymers
chains have little effect on the conjugation length of the CCAT
segment, which can retain its good electroactive properties.
Figure 6 shows the UV-visible spectra of Copo-Pen-
PLLA20000 in different solutions. The absorption peaks in
3
CHCl and 1,4-dioxane are somewhat lower, and there is a blue
shift compared to the peak in DMSO solution (Figure 6a). This
is related to the polarity of the solvents. A decrease in polarity
of the solvent leads to a blue shift of the absorbance peak. A
Figure 3. FT-IR spectra of ACAT (a), CCAT (b), Pen-PLLA20000
(
c), and Copo-Pen-PLLA20000 (d).

8
60 Biomacromolecules, Vol. 11, No. 4, 2010
Guo et al.
1
Figure 4. H NMR of the Copo-Pen-PLLA20000 copolymer.
Figure 5. UV spectra of ACAT (a), CCAT (b), Copo-Pen-PLLA20000
Figure 6. UV spectra of Copo-Pen-PLLA20000 in different solvents
at the same concentration: solvent a, DMSO; b, 1,4-dioxane; c, CHCl .
3
(
c), Copo-Pen-PLLA40000 (d), and Copo-Pen-PLLA80000 (e).
second reason is that the solubility of CCAT segment in CHCl
remains poor although the solubility of the copolymers in CHCl
3
3
the UV spectra of the copolymer solution is plotted in Figure
7. Figure 7 shows that two characteristic peaks at 330 and 520
nm corresponding to the benzene and quinoid gradually disap-
pear with increasing the doped ratio (curves afg) and that three
new peaks appear at 415, 638, and 818 nm, which are related
to the formation of polarons from the electron transition of
quinoid to benzenoid units (see Scheme 2). In Figure 7, the
peak at 638 nm increases with increasing doped ratio from 1:5
to 1:300 (curve e) and then begins to decrease with increasing
the doped ratio. This may be because the excess of HCl in the
solution will impair the hyperfine structure of the complex of
is greatly improved because of the PLLA segments. Good
solubility of PLLA segments in the copolymer in CHCl will
force the CCAT segment to disperse in CHCl . This pseudo-
solubility is confirmed by the H NMR spectrum of the
3
3
1
copolymer in CDCl
CCAT segment.
3
, in which there were no signals from the
We also studied the effect of the amount of dopant (HCl) on
the UV spectra of Copo-Pen-PLLA20000. In order to keep the
concentration of the copolymer constant in the solution, 4 mol/L
HCl was continuously added to the sample vial. The change in
39
Scheme 2. Proposed Mechanism for the HCl Doping of the Copo-Pen-PLLA20000

Electroactive and Biodegradable Copolymers
Biomacromolecules, Vol. 11, No. 4, 2010 861
Figure 7. UV spectra of doping process of Copo-Pen-PLLA20000
with HCl, (a) undoped copolymer, (b) doping ratio 1:5, (c) 1:10, (d)
1:100, (e) 1:300, (f) 1:1000, and (g) 1:4000.
aniline trimer radical cations paired with counterions and cause
the aggregates that form the bipolaron intermediates to dissociate
3
9
at the same time.
Figure 8 shows cyclic voltammetry data for (a) ACAT, (b)
CCAT, and (c) representative Copo-Pen-PLLA20000 obtained
in a DMSO/HCl aqueous solution mixture. The Vol. (1 M HCl)/
Vol. (DMSO) ratio was 1:1, and the concentration 0.0125% w/w
for ACAT and CCAT, and Vol. (1 M HCl)/Vol. (DMSO) ratio
was 1:7 and the concentration 0.00625% w/w for Copo-Pen-
PLLA20000. The copolymer cannot be tested at a higher HCl/
DMSO ratio because the copolymer precipitates in the high HCl
concentration solution. Under these conditions, the cyclic
voltammogram for ACAT (Figure 8a) showed two pairs of redox
4
0
peaks, similar to those of polyaniline. The first pair of the
well-defined redox peaks at 0.54 V can be attributed to the
reversible redox process from the “leucoemeraldine” to the
“emeraldine” form (Scheme 3). At higher potentials, a reversible
redox peak at 0.70 V is assigned to the oxidation/reduction of
the “emeraldine” form to the “pernigraniline” state. The cyclic
voltammogram for CCAT (Figure 8b) is similar to that for
ACAT, but the oxidization is higher than that of ACAT. This
is because the electron withdrawing imine group in CCAT
decreases the electron concentration of CCAT, so that it needs
higher voltage to oxidize. The cyclic voltammogram of Copo-
Pen-PLLA20000 (Figure 8c) also shows two pairs of peaks at
Figure 8. Cyclic voltammograms for (a) ACAT, (b) CCAT, and (c)
Copo-Pen-PLLA20000 in a DMSO/HCl mixture. The scan rate was
60 mV/s.
0
.42 and 0.60 V, which are assigned to the transition from the
leucoemeraldine oxidation state to the emeraldine oxidation state
and then to the “pernigraniline” state (Scheme 3). The oxidiza-
tion peaks of the copolymer are somewhat lower than those of
CCAT. This is attributed to the different test condition such as
PLLAs. This is due to more hydrophilic structure of CCAT,
which contains carboxyl groups and amide groups. With a higher
content of CCAT, a lower water contact angle was observed
indicating a more hydrophilic surface. The decrease to 63-54°
of water contact angles of copolymers doped with CSA can be
explained by the formation of emeraldine salt. The same trend
was also found for the linear and six-armed PLLAs and their
copolymers. These intermediately hydrophilic surfaces are more
suitable for cell adhesion and proliferation.
2
6
different HCl and copolymer concentrations. All these spec-
troscopic and electrochemical results demonstrate the good
electroactivity of the copolymers (see Supporting Information
for CVs of Copo-EG-PLLA20000 and Copo-Ino-PLLA20000).
Water Contact Angle of the Polymers. It is known that the
surface hydrophilicity is an important feature of polymeric
biomaterials. Surfaces with a moderate hydrophilicity (30-60°)
have been shown to be optimal for cell adhesion, proliferation,
Thermal Properties of the Polymers. The T
g
m
and T of the
polymers determined by DSC are listed in Table 2. The T
g
of linear
4
1
and function. Surfaces that are too hydrophilic or too
hydrophobic, like PLLA, are less cytocompatible. Water contact
angles of Pen-PLLAs and corresponding copolymers (undoped
or doped with CSA) were determined and the results are shown
in Figure 9. The water contact angles of Pen-PLLA80000, Pen-
PLLA40000, and Pen-PLLA20000 are all over 80°, and they
increase with increased Mn-NMR of the PLLA branches, although
the differences between them are small. The corresponding
copolymers have a lower water contact angle than the pure
and branched pure PLLAs and copolymers is between 48 and 58
°C, and they increase with increasing molecular weight of the
g
PLLA segment. The T of the pure PLLAs is somewhat higher
than that of the corresponding copolymer. This may be because,
on the one hand, CCAT at the chain end of the PLLAs hinders
the molecular segment movement of the PLLA chain because of
g
the rigid configuration of CCAT, which increases the T of the
copolymers; on the other hand, CCAT behaves as plasticizer for
PLLA as it can move freely in the PLLA chain end, and this

8
62 Biomacromolecules, Vol. 11, No. 4, 2010
Guo et al.
Scheme 3. Molecular Structure of CCAT in Various Oxidation States
decreases the T
effects, the T of the copolymers is slightly lower than that of the
corresponding PLLAs. The T of four- and six-armed branched
pure PLLAs and copolymers is less than that of their linear PLLA
counterparts, because the branched molecular architecture reduces
the interaction between the macromolecules.
g
of the copolymer. As a result of the two opposing
crystalline peak of Copo-Pen-PLLA20000 is at a lower temperature
and has a smaller area than that of Pen-PLLA20000. This is a result
of two counteracting effect of CCAT on the macromolecules. The
macromolecules begin to move and organize to rearrange and
crystallize at a lower temperature because of the plasticizing effect
of CCAT in the copolymer, but at the same time, the movement
of the molecular chain is restrained to some extent because of the
rigidness of the CCAT, which impedes the crystallization process.
It is clear in the figure that the plasticizing effect dominates. As a
consequence of these two effects, the crystalline peak is smaller,
g
g
The T
a pattern similar to that of the T
molecular weight of PLLA, and decreases as the number of arms
increases. The T of the pure linear and branched PLLA is from
78 to 150 °C, and the T of the copolymers decreases from 177
m
of pure PLLA and corresponding copolymers exhibits
g
. T increases with increasing the
m
m
1
m
m
and the T of the copolymer is slightly lower than that of the pure
PLLA counterpart.
to 146 °C, depending on the molecular weight and number of arms.
This is related to crystalline imperfections due to more free end
groups in the more branched polymers. Figure 10 shows that the
Thermal stability of the polymer is also important for the
application of electroactive polymers in many fields. To test
the thermal stability of the copolymers prepared, Copo-Pen-
PLLA80000, Copo-Pen-PLLA40000, Copo-Pen-PLLA20000,
and Pen-PLLA20000 were tested by TGA, and the results are
plotted in Figure 11. When the temperature was raised to
1
00-200 °C, there was a small weight loss from the PLLA
and copolymers. This is attributed to water evaporation and to
the loss of other solvent trapped in the copolymer. In case of
pure PPLA, there was a major weight loss between 280 and
3
10 °C, and the residual weight above 310 °C was almost zero,
indicating a degradation of the PLLA main chain. In the
copolymers, the first evident weight loss occurred between 320
and 360 °C, and 80∼90% of the weight of the copolymer was
lost depending on the CCAT content, due to the PLLA main
chain degradation. The thermal stability of the copolymer was
higher than that of the corresponding pure PLLA, due presum-
ably to the better thermal stability of CCAT. Between 360 and
6
00 °C, there was another obvious weight loss of the copoly-
mers, presumably related to the degradation of the CCAT
segment. All these results indicate that the copolymer has a
better thermal stability. It is also found that the stability is
Figure 9. Water contact angle of PLLAs and corresponding undoped
and doped copolymers.
Table 2.
T
g
and T
m
of the Polymers Determined by DSC
(°C)
58
sample code
T
g
T
m
(°C)
EG-PLLA80000
178
EG-PLLA40000
EG-PLLA20000
Pen-PLLA80000
Pen-PLLA40000
Pen-PLLA20000
Ino-PLLA80000
Ino-PLLA40000
Ino-PLLA20000
Copo-EG-PLLA80000
Copo-EG-PLLA40000
Copo-EG-PLLA20000
Copo-Pen-PLLA80000
Copo-Pen-PLLA40000
Copo-Pen-PLLA20000
Copo-Ino-PLLA80000
Copo-Ino-PLLA40000
Copo-Ino-PLLA20000
57
53
58
56
52
55
54
53
58
53
49
52
51
50
52
50
48
172
161
165
156
155
162
157
150
177
172
158
165
154
153
161
155
146
Figure 10. DSC traces of Pen-PLLA20000 and Copo-Pen-PL-
LA20000.

Electroactive and Biodegradable Copolymers
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Acknowledgment. The authors thank the Swedish Research
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