Enhanced electrical conductivity by macromolecular architecture: Hyperbranched electroactive and degradable block copolymers based on poly(ε-caprolactone) and aniline pentamer

Article abstract of DOI:10.1021/ma100530k

We present macromolecular architecture design as a useful tool to enhance the conductivity of degradable polymers. Linear and hyperbranched copolymers with electrical conductivity and biodegradability were synthesized by an 'A ₂ + B_n

Full text of DOI:10.1021/ma100530k

4472 Macromolecules 2010, 43, 4472–4480
DOI: 10.1021/ma100530k
Enhanced Electrical Conductivity by Macromolecular Architecture:
Hyperbranched Electroactive and Degradable Block Copolymers Based on
Poly(ε-caprolactone) and Aniline Pentamer
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 March 10, 2010; Revised Manuscript Received April 9, 2010
ABSTRACT: We present macromolecular architecture design as a useful tool to enhance the conductivity of
degradable polymers. Linear and hyperbranched copolymers with electrical conductivity and biodegrad-
ability were synthesized by an “A2 þ Bn (n=2, 3, 4)” strategy using carboxyl-capped aniline pentamer (CCAP)
and branched poly(ε-caprolactone)s (PCLs) by coupling reactions. A more hydrophilic surface and lower
crystallinity of the doped emeraldine state of aniline pentamer (EMAP) copolymer was achieved compared
with PCLs, and TGA results demonstrated that the CCAP contents in the copolymers were almost the same.
The structure of the polymers was characterized by FT-IR, NMR, and SEC. Good electroactivity of the
copolymers was confirmed by UV and cyclic voltammetry (CV), and CV showed three pairs of redox peaks.
The hyperbranched copolymers had a higher conductivity than the linear ones. It is suggested that the higher
conductivity of the hyperbranched copolymer is due to the ordered distribution of peripheral EMAP
segments that more easily form a conductive network. Therefore, the conductivity of the polymers is
improved and controlled by the macromolecular architecture.
Introduction
absorbable, and biodegradable, are some of the polymers widely
used in medical applications.24-26 PCL also possesses outstanding
The macromolecular architecture greatly effects the performance
of polymers. Polymers with the same molecular weight consisting of
the same monomer may possess different thermal, mechanical,
degradation, and biological properties because of their different
architectures.1-4 Our group has synthesized the linear, star-
branched, and cross-linked networks of degradable polymers and
has extensively investigated the effect of the architecture on the
degradation product, degradation rate, and thermal and mecha-
nical properties of these degradable polymers.5-9 As one of the
fourth class of polymer architecture after traditional types including
linear, cross-linked, and branched architectures, the hyperbranched
polymers (HBPs) usually exhibit properties such as multiple
terminal groups and globular shape similar to those of dendrimers.
HBPs are potentially promising materials because of their easy
synthesis and purification so that they represent a cost-effective
alternative for dendrimers in some applications.10,11 HBPs can be
synthesized by a one-step polymerization process, such as the step-
growth polycondensation of ABx monomer and the classical “A2 þ
Bn” (n g 3) approach.12-14 Hyperbranched or dendronized con-
ducting polymers exhibit interesting properties compared with their
linear counterparts and have recently attracted considerable atten-
tion as high-performance organic conductors,15,16 as magnetic,17
electrochromic, and 18 high dielectric materials,19 and as organic-
inorganic hybrid electrolytes.20
permeability, and the degradation product of PCL is nontoxic and
metabolizable.9,27 The disadvantages of PCL are its slow rate of
degradation and its hydrophobicity, but those can be overcome by
copolymerization or blending with other polymers.28-30 The third-
generation biomaterials are cell- and gene-activating materials
designed to stimulate specific cellular responses at the molecular
level.31 Resorbable polymer systems with molecular modifications
can arouse specific interactions between cell integrins and thereby
adjust cell behavior. Recent studies have shown that electrical
signals can regulate cellular activities, including cell adhesion, mig-
ration, proliferation, and differentiation of many kinds of cells.32-35
The degradable electrically conducting polymers (DECPs) with the
unique properties of being electrically conducting and degradable
are therefore a new exciting area and are attracting much attention.
For example, a novel electrically conducting and biodegradable
polymer was synthesized by oligomers of pyrrole and thiophene
connected together via ester linkages.36 The linear triblock and
multiblock copolymers were prepared using aniline pentamer (AP)
and polylactide, which are biodegradable and electroactive and are
also easily processed.37,38 Recently, a novel cytocompatible, bio-
degradable and electroactive polymer was synthesized by introducing
alternating quaterthiophene units and ester units into one macro-
molecule.39 These copolymers overcome to some extent the dis-
advantage of the conducting polymers, such as nonprocessability,
poor solubility, poor polymer-cell interaction, and hydrophobicity.
However, the synthesis of DECPs is still a challenge.
Degradable and resorbable synthetic polymers are widely used in
the biomedical field, scaffold preparation, bone fixation, sutures,
and drug delivery systems. Aliphatic polyesters of lactones and
lactides play an important role in this field because of their good
mechanical properties, hydrolyzability, and biocompatibility.21-23
Homopolymer and copolymers of PCL, which are biocompatible,
We have synthesized two-, four- and six-armed branched co-
polymers with electroactive and biodegradable properties based on
polylactides and carboxyl-capped aniline trimer (CCAT).40 How-
ever, the copolymers had quite a low conductivity, probably because
of their low CCAT content and the short conjugated length of
CCAT. Our object is thus to employ the macromolecular structures
to enhance and control the conductivity of the electroactive and
*To whom correspondence should be addressed. Tel: þ46-8-790 8274.
Fax: þ46-8-20 84 77. E-mail: aila@polymer.kth.se.
pubs.acs.org/Macromolecules
Published on Web 04/28/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 10, 2010 4473
degradable polymers, here exemplified by linear and hyperbranched
copolymers based on PCLs and CCAP. We hypothesized that
macromolecular architecture is a useful tool to improve the con-
ductivity of the degradable polymer: the hyperbranched copolymer
should exhibit a higher level of conductivity than the linear
copolymer with the same conductive segment content because the
ordered distribution of the AP segments on the periphery of the
hyperbranched copolymer should mean a better chance to form a
conductive network. It is anticipated that a higher level of con-
ductivity could thus be obtained with a lower CCAP content by
macromolecular design and thus reduce the toxicity of the degrad-
able copolymer. These copolymers have a promising prospect for
use as a neural or cardiovascular conducting tissue engineering
material, which can conduct bioelectrical signals in the body.
Experimental Section
Materials. The ε-caprolactone (CL) from Aldrich was dried in
CaH2 for 48 h and then distilled under reduced pressure. Ethylene
glycol (EG, Aldrich) was distilled under reduced pressure. Tri-
methylolpropane (TMP, Aldrich) was recrystallized from dried
acetone and then dried for 24 h in a vacuum oven. Pentaerythritol
(PEN, Aldrich) was sublimated under reduced pressure. Stannous
octoate (Sn(Oct)2, Aldrich) was dried over molecular sieves and
stored under a nitrogen atmosphere before use. p-Phenylenedi-
amine, succinic anhydride (SA), ammonium persulfate ((NH4)2-
S2O8), phenylhydrazine, ammonium hydroxide (NH3OH),
hydrochloric acid (HCl), N,N0-dicyclohexyl carbodiimide (DCC),
4-dimethylaminopyridine (DMAP), ethanol (EtOH), N-methyl-
2-pyrrolidone (NMP), dimethyl sulphoxide (DMSO), diethyl
ether, chloroform (CHCl3), hexane, methanol, and 1,4-dioxane
were all purchased from Aldrich and were used without further
purification.
Synthesis of Carboxyl-Capped Aniline Pentamer (CCAP).
CCAP was synthesized according to refs 38 and 41. CCAP in
the emeraldine base state (EM) was obtained from the oxidative
coupling of 4-oxo-4-(4-(phenylamino) phenylamino) butanoic
acid and p-phenylenediamine with two equivalent amounts of
(NH4)2S2O8 as oxidant. The leucoemeraldine state of aniline
pentamer (LMAP) was obtained by reduction of EMAP by
phenylhydrazine. 1H NMR (400 MHz, DMSO-d6, δ): 12.09
(s, 2H, -COOH), 9.70 (s, 2H, -NHCO-), 7.63 (d, 2H, -NH-),
7.52 (s, 2H, -NH-), 7.38 (d, 4H, Ar-H), 6.85-6.94 (m, 16H,
Ar-H). 13C NMR (100 MHz, DMSO-d6, δa) 173.91 (-COOH),
169.20 (-NHCO-), 140.81 (Ar-C), 138.27 (Ar-C), 137.06
(Ar-C), 135.60 (Ar-C), 130.74 (Ar-C), 120.41 (Ar-C),
119.52 (Ar-C), 118.33 (Ar-C), 117.46 (Ar-C), 115.34 m,
30.86 (-CH2-), 28.98 (-CH2-). All of these results agree well
with the refs 38 and 41.
Synthesis of Linear and Star-Shaped PCLs. PCLs with differ-
ent arms were prepared by ring-opening polymerization7,42
(ROP) as follows: the monomer (CL), initiator (Sn(Oct)2), and
co-initiator (EG, TMP, and PEN) were weighed and added to a
silanized round-bottomed flask in a glovebox (Mbraun MB
150B-G-I) purged with nitrogen. The mixture was then put in
an oil bath at 110 °C under a nitrogen atmosphere for 48 h.
After the reaction, 10 mL of chloroform was added to the flask
to dissolve the mixture, which was then precipitated in 300 mL
of hexane/methanol (v/v 95:5) solution. After filtration, the
product was dried in a vacuum oven at room temperature for
24 h. The polymer was then dissolved in CHCl3 and reprecipi-
tated three times. Finally, the copolymer was dried in an oven at
40 °C. Samples were designated 2a-PCL, 3a-PCL, and 4a-PCL,
which denote a PCL co-initiated with EG, TMP, and PEN,
respectively.
Figure 1. 1H NMR spectra of the (A) 2a-PCL, (B) 3a-PCL, and (C)
4a-PCL, and (D) 13C NMR spectrum of 4a-PCL.
-CH2O-), 4.00 (s, 2H, -CH2- from TMP), 3.63 (m, 2H,
-CH2OH), 2.30 (t, 2H, -CH2-), 1.64 (m, 2H, -CH2-), 1.37
(m, 2H, -CH2-), 0.88 (m, 2H, -CH3 from TMP). 4a-PCL: 1H
NMR (400 MHz, CDCl3, δ): 4.11 (s, 2H, -CH2- from PEN),
4.06 (t, 2H, -CH2-), 3.64 (m, 2H, -CH2OH), 2.31 (t, 2H,
-CH2-), 1.65 (m, 2H, -CH2-), 1.38 (m, 2H, -CH2-).
4a-PCL: 13C NMR (100 MHz, CDCl3, δ): 173.46 (-CO-),
64.07 (-CH2O-), 62.45 (-CH2- from PEN), 34.06 (-CH2-),
28.29 (-CH2-), 25.47 (-CH2-), 24.52 (-CH2-).
Synthesis of Linear and Hyperbranched Copolymers. The
synthesis of the copolymers using the “A2 þ Bn (n=2, 3, 4)”
approach is shown in Scheme 1. Consider the synthesis of
4a-PCL LMAP copolymer as an example: Purified 4a-PCL
(0.448 g) and CCAP (0.164 g) were dissolved in 10 mL of 1,4-
dioxane and 2 mL of NMP mixture in a flame-dried flask with a
magnetic stirrer under a nitrogen atmosphere, and DMAP
(0.054 g) and DCC (0.264 g) were then added to the flask. The
mixture was kept at 0 °C for 72 h. After the reaction, the mixture
was filtered to remove dicyclohexylurea. The filtrate was preci-
pitated in anhydrous diethyl ether. The copolymer was dissolved
in chloroform and then again precipitated in anhydrous diethyl
ether. The product obtained was dried in a vacuum oven for 48 h
after filtration.
The NMR spectra of the different PCLs are show in Figure 1.
2a-PCL: 1H NMR (400 MHz, CDCl3, δ): 4.26 (s, 2H, -CH2-
from EG), 4.03 (t, 2H, -CH2O-), 3.65 (m, 2H, -CH2OH), 2.28
(t, 2H, -CH2-), 1.62 (m, 2H, -CH2-), 1.37 (m, 2H, -CH2-).
3a-PCL: 1H NMR (400 MHz, CDCl3, δ): 4.05 (t, 2H,

4474 Macromolecules, Vol. 43, No. 10, 2010
Scheme 1. Synthesis Route of the Linear and Hyperbranched Copolymers of PCLs and CCAP
Guo et al.
Characterization. FT-IR spectra of PCL, LMAP, and their
copolymers were obtained with a Perkin Elmer Spectrum 2000
spectrometer (Perkin-Elmer Instrument) in the 4000-600 cm-1
range. Each spectrum was taken as the average of 20 scans at a
crystalline PCL. The value of ΔH0f used for the calculations was
139.5 J/g.43
Thermogravimetric analysis (TGA) was used to determine
the ratio of PCL to CCAP, and the thermal stability of the
polymers. TGA was carried out under a nitrogen atmosphere
(nitrogen flow rate 50 mL/min) and a heating rate of 10 °C/min.
The scan range was from 30 to 800 °C.
resolution of 4 cm-1
.
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were
obtained using a Bruker Avance 400 MHz NMR instruments
with CDCl3 as the solvent for all PCL samples and internal
standard (δ 7.26 and 77.0). DMSO-d6 was used as the solvent at
room temperature and as internal standard (δ 2.50 and 39.5) for
CCAP and all copolymer samples.
Cyclic voltammetry (CV) of CCAP and its copolymers was
carried out on an electrochemical workstation interfaced and
monitored with a PC computer. A three-electrode system with a
platinum disk as working electrode (surface area 0.14 cm2), a
platinum-wire as auxiliary electrode, and an Ag/AgCl as reference
electrode was employed. The scan rate was 60 mV/s. The sample
was dissolved in a DMSO/1 mol/L HCl mixture. All solutions were
deoxygenated for 10 min with nitrogen prior to the electrochemical
measurements. The electrical conductivity of the 1 mol/L HCl
doped EMAP copolymer films was determined by the standard
Van Der Pauw four-probe method. The conductivity of each
sample was measured four times at different current values, and
the average value was taken as the conductivity of the sample.
The surface hydrophilicity of the PCLs and copolymer films was
estimated by water contact angle measurements (CAMs) using a
contact angle and surface tension meter (KSV instruments). A drop
of Mini-Q water was placed on the surface of the sample, and the
image of the water drop was recorded by a digital camera. The
images were then analyzed with KSV software to obtain the contact
angle. The contact angle of each sample was taken as the average of
five measurements at different points on the samples.
Size exclusion chromatography (SEC) was carried out using a
TDA model 301 equipped with one or two GMHHR-M columns
with TSK-gel (Tosoh Biosep), a VE 5200 GPC autosampler, a
VE 1121 GPC solvent pump and a VE 5710 GPC degasser, all of
which were from Viscotek Corp. THF was used as the mobile
phase (flow rate 1.0 mL/min). The measurement was carried out
at 35 °C. The SEC apparatus was equipped with a triple detector
array, including a differential refractive index, differential visco-
meter, and RALLS detector. A linear polystyrene standard was
used for the calibration of the SEC apparatus.
The UV-vis spectra of oxidation process of copolymers
solutions with trace oxidant were recorded on a UV-vis spectro-
photometer (UV-2401).
The crystallization temperature (Tc), melting temperature
(Tm), and crystallinity (Xc) of the polymers were assessed by
differential scanning calorimetry (DSC) using a Mettler-Toledo
DSC 820 module under a nitrogen atmosphere (nitrogen flow
rate of 50 mL/min). Measurements were made during the first
heating scan from 25 to 100 °C, which was held for 2 min to erase
the thermal history, the first cooling from 100 to -70 °C and the
second heating scan from -70 to 100 at 10 °C/min. Data for
Tc and Tg were taken from the second heating scan. The
crystallinity of the copolymers was calculated using the equation
Xc =ΔHf/H0f , where Xc is the crystallinity, ΔHf is the heat of
fusion of the sample, and ΔH0f is the heat of fusion of 100%
Results and Discussion
Synthesis of PCLs. PCLs with different branched structures
(2a-PCL, 3a-PCL, and 4a-PCL) were synthesized by ROP.
Sn(Oct)2 was used as initiator, with monomer-to-initiator ratio
of 10 000:1, and the tin residuals were therefore very low after
1
purification.44 Figure 1 shows the H NMR of (A) 2a-PCL,

Article
Macromolecules, Vol. 43, No. 10, 2010 4475
Table 1. Properties of the PCLs
monomer
sample reaction conversion
Mn
theory
Mn
Mn
code
time (h)
(%)a
(g/mol)b (g/mol)c MWDc (g/mol)d
2a-PCL
3a-PCL
4a-PCL
48
48
48
98.3
99.3
99.6
2400
4300
5600
3250
4700
6200
1.09
1.14
1.12
2800
4200
5500
a Calculated from 1H NMR on crude reaction mixture. b Determined
by 1H NMR c Number-average molecular weight and distribution
determined by SEC d Theoretical number-=average molecular weight,
Mn=[M]/[I]co ꢀ MCL ꢀ Conversion
(B) 3a-PCL, and (C) 4a-PCL and (D) the 13C NMR spectrum of
4a-PCL. All 1H NMR data are listed in the experimental part.
It is noted that the spectra contained not only the CL peaks but
also the signal from the co-initiator. For example, the -CH2-
(δ 4.11) of the co-initiator PEN appears in the 1H NMR
spectrum (Figure 1C), and this is confirmed in the 13C NMR
spectrum of 4a-PCL (Figure 1D), the signal of PEN appearing at
64.2 ppm. Therefore, these polymers with star-shaped structures
are certainly obtained. The hydroxyl group (δ 3.65) at the PCL
chain end is used for the coupling reaction with CCAP in the next
step. The same trends were found for the EG and TMP systems.
We determined monomer conversion of the PCLs by com-
paring the peak integrals of methylene protons in PCL (about
δ 4.04 in Figure 1) with those of the monomer CL (δ 4.20 in
Figure 1), and the results are summarized in Table 1. The
conversion of all systems was quite high, exceeding 98% after
48 h of reaction, which indicated that the low ratio of Sn(Oct)2
to monomer had a high catalytic activity. We calculated
molecular weights of the PCLs by comparing the peak integrals
of methylene protons (δ 4.04, -CH2- in Figure 1) with those
of the methylene protons next to the terminal hydroxyl groups
(δ 3.65, -CH2OH in Figure 1). The molecular weights calcu-
lated by NMR are also listed in Table 1. These results are very
close to the theoretical values and agree well with the SEC
results. Furthermore, they also had a very narrow MWD. All
of these results indicated the successful synthesis of the PCLs
with different numbers of branches.
Figure 2. FT-IR spectra of (a) PCL, (b) LMAP, and (c) their copolymer.
Consider the 1H NMR spectrum of 2a-PCL LMAP copoly-
mer in DMSO-d6 as an example: 1H NMR (400 MHz,
DMSO-d6, δ): 1.38 (m, 2H, -CH2-), 1.65 (m, 2H, -CH2-),
2.31 (t, 2H, -CH2-), 4.06 (t, 2H, -CH2-), 4.24 (s, 2H,
-CH2- from EG) for the PCL segment and 2.58-2.67
(m, 4H, -CH2CH2-), 6.84-6.95 (m, 16H, Ar-H), 7.39 (d,
4H, Ar-H), 7.54 (s, 2H, -NH-), 7.62 (d, 2H, -NH-), 9.72
(s, 2H, -NHCO-) for the LMAP segment. In Figure 3a, it can
be seen that the methylene protons next to the terminal
hydroxyl groups in PCL, which were observed at ∼3.65
1
ppm in the H NMR spectrum of PCL in Figure 1, were
absent from the 1H NMR spectrum of the copolymer
(Figure 3a), indicating that the esterification reactions took
place between the hydroxyl-capped 2a-PCL and the carboxyl
group of CCAP. There was also a new peak at 172.2 ppm in
the 13C NMR spectrum of the linear copolymer in Figure 3b,
which also confirms that esterification occurred between the
PCL and CCAP. Compared with the single peak of the newly
formed esterification peak and the amide group peak of the
linear copolymer in Figure 3b, these newly formed esterifica-
tion group peaks and the amide group (-NHCO-) peaks
split into multiple peaks in Figure 3c,d of the hyperbranched
copolymer because these groups are in different chemical
environments of the subunit (“inner” or “outer” parts of the
hyperbranched copolymer),45,46 and this further demon-
strated that the hyperbranched copolymers had been suc-
cessfully synthesized.
Characterizations of the Linear and Hyperbranched Co-
polymers. The linear and hyperbranched copolymers were
prepared by the “A2 þ Bn (n = 2, 3, 4)” approach. The
coupling condensation polymerization took place between
the hydroxyl groups in the PCLs and the carboxyl groups in
CCAP with the DCC as water condensation agent and
DMAP as catalyst to form the different architectural copoly-
mers. The chemical structures of the copolymers obtained
were verified by FT-IR, NMR, and SEC. Figure 2 presents
the FT-IR spectra of the LMAP copolymer (c) together with
those of its corresponding precursors PCL (a), and LMAP
(b). In Figure 2a, the characteristic absorption band at
1722 cm-1 is assigned to the CdO stretching mode, and
the peak at 1166 cm-1 arising from the O-C-O of the PCL
is also observed. The characteristic peaks at 1692 and 1650
cm-1 are assigned to the CdO stretching vibration in LMAP
in Figure 2b, and the bands at 1610 and 1509 cm-1 are
characteristic of the benzenoid ring of AP. There is no peak
at 1587 cm-1 corresponding to the quinoid unit, indicating a
successful reduction from the EMAP to LMAP. All charac-
teristic absorption bands in both LMAP and PCL were
observed in Figure 2c demonstrated that the target copoly-
mer was obtained. There is a broad peak at ∼3355 cm-1
corresponding to hydroxyl groups and amide groups in the
copolymer, which can increase the hydrophilicity of the
polymer.
The molecular weight and molecular weight distribution
(MWD) values were recorded on a THF-SEC utilizing
universal calibration, a method in which the apparatus is
calibrated using both narrow and broad polystyrene stan-
dards. The molecular weight of the copolymers were 2.9 ꢀ
104 (MWD =1.6), 2.4 ꢀ 104 (MWD =3.4), and 2.7 ꢀ 104
(MWD =2.9) for the 2a-PCL copolymer, 3a-PCL copoly-
mer, and 4a-PCL copolymer, respectively. The relationship
between intrinsic viscosity and molecular weight was studied
using the Mark-Houwink equation [η]=kMR; the values of
R are equal to 0.5 at the θ state and between 0.65 and 0.8 for
linear random coils in the good solvent. However, it was
found that for the 3a-PCL hyperbranched copolymer
and the 4a-PCL hyperbranched copolymers, the R values
were ∼0.34, which is commonly below the value of 0.5 for
branched polymers47,48 and much lower than that for the
2a-PCL copolymers (R = 0.77). All of these results indi-
cate a much smaller hydrodynamic size and a highly bran-
ched compact and globular structure of the hyperbranched
copolymers.
Thermal Properties of the Polymers. The thermal proper-
ties were measured by DSC, and the Tc, Tm, and Xc of the
polymers are listed in Table 2. The Xc of 2a-PCL, 3a-PCL,
The 1H NMR and 13C NMR spectra were used to confirm
further the structure of the copolymers, as shown in Figure 3.

4476 Macromolecules, Vol. 43, No. 10, 2010
Guo et al.
Figure 3. 1H NMR of (a) 2a-PCL LMAP copolymer and the 13C NMR spectra of (b) 2a-PCL LMAP copolymer, (c) 3a-PCL LMAP copolymer, and
(d) 4a-PCL LMAP copolymer.
Table 2. Tc, Tm, and Xc of the Polymers
The Tc of the copolymers was a little lower than that of the
corresponding PCLs, indicating a lower crystallization ability
of the copolymer, probably because the PCL repeating unit
nearest to the branched core was unable to crystallize and the
chain folds were less tight in the hyperbranched copolymer,49
whereas the rigid AP segment hindered the PCL segment from
organizing and impeding the crystallization. The copolymer
therefore has a much lower Xc than the counterpart PCLs.
However, there seems to be no obvious difference between the
Tm of the PCLs and their corresponding copolymers, and this
can be explained by the better thermal stability of the CCAP
segment in the copolymer.
Thermal stability of electroactive polymers is important in
many applications. The thermal stability of the PCLs and
their corresponding copolymers were tested by TGA and are
shown in Figure 4. A small weight loss (∼4%) from the PCLs
and their copolymers occurred in the temperature range
100-220 °C, which was ascribed to water evaporation and
to the loss of other solvents trapped in the polymers. In the
case of the pure PCLs, there was a major weight loss between
230 and 450 °C, and the residual mass above 450 °C was
sample code
Tc (°C)
Tm (°C)
Xc
2a-PCL
3a-PCL
4a-PCL
35.0
31.9
25.0
34.6
27.8
23.2
52.2
49.5
41.3
52.7
49.5
40.9
52.3
43.0
37.2
34.8
29.3
25.7
2a-PCL EMAP copolymer
3a-PCL EMAP copolymer
4a-PCL EMAP copolymer
and 4a-PCL decreased sharply with increasing number of arms
of the PCLs. This may be because the star-shaped core imposed
restrictions on PCL crystallization.49 The Tm of the 2a-PCL,
3a-PCL, and 4a-PCL decreased accordingly because of the
difference in crystallinity and crystalline imperfections due to
more free end groups in the more branched polymers. The
parameter Tc is used to evaluate nucleation during polymer melt
crystallization. The higher the Tc, the easier the stable nucleus
can be formed via the regular arrangement of polymer segments,
and the higher crystallizing ability of the polymer.50 The 2a-PCL
has the highest Tc, so it has the higher crystallizing ability and
thus the highest Xc.

Article
Macromolecules, Vol. 43, No. 10, 2010 4477
almost 5%, indicating PCL main chain degradation. In the
case of the EMAP copolymers, the first evident weight loss
took place between 220 and ∼450 °C, 72-74% of the weight
of the copolymer being lost because of the PCLs main chain
degradation. The greater thermal stability of the copolymer
than of the pristine PCL is probably due to the greater
thermal stability of the EMAP segments. The EMAP content
in the copolymer can be calculated by using the TGA curves
because the PCLs segments were almost completely de-
graded after 450 °C; the results are listed in Table 3. It was
found that these results are close to the theoretical ones, and
this confirms the successful synthesis of the copolymers.
Between ∼450 and 800 °C, there was another obvious weight
loss of the copolymers related to the degradation of the
EMAP segments. The LMAP copolymers have a thermal
stability similar to that of the EMAP copolymers. All of
these results demonstrated that these copolymers have an
excellent thermal stability.
Figure 4. TGA curves of the PCLs and their EMAP copolymers.
Table 3. EMAP Content and Conductivity of the EMAP Copolymers
Electrochemical Properties of the Linear and Hyper-
branched Copolymers. The electroactivity of the biomaterials
means that they can transmit bioelectrical signals in vivo and
play an important role in life activities. The electrochemical
properties of the copolymer were characterized by UV and
CV. Figure 5 shows the chemical oxidation of the copoly-
mers in NMP solution. A trace amount of (NH4)2S2O8 was
added to the 3a-PCL LMAP copolymer NMP solution. The
solution gradually changed to blue and then to mauve
because of the oxidation of the copolymer. The oxidation
process was monitored by the UV absorbance spectrum and
is plotted in Figure 5. The copolymer exhibits only one
absorption peak at ∼330 nm, which is attributed to the
π-π* transition in the conjugated benzene unit. This also
confirms the full reduction from EMAP to LMAP, as
demonstrated by FT-IR. The absorption peak started to
decrease in intensity and gradually underwent a blue shift
during the oxidation. Meanwhile, a new absorption peak was
developed at ∼610 nm, which was assigned to an excitonic
transition from the benzoid ring to the quinoid ring appeared
in the spectrum and which continuously increased in inten-
sity. The absorption peak at 610 nm began to undergo a blue
shift (from 610 to 530 nm) after reaching its maximum
intensity. The intensity increase and the blue shift of the
peak at 610 nm during the oxidation process can be explained
by the three-step oxidation of the copolymer (Scheme 2).
sample name
EMAP content (theoretical)
22.4 (25.0%)
conductivity(S/cm)
2a-PCL EMAP
copolymer
3a-PCL EMAP
copolymer
4a-PCL EMAP
copolymer
5.01 ꢀ 10-6
2.42 ꢀ 10-5
8.02 ꢀ 10-6
21.8 (25.0%)
22.8 (25.0%)
Figure 5. Chemical oxidization process of the LMAP copolymers with
a trace amount of (NH4)2S2O8 in NMP solution.
Scheme 2. Molecular Structure of CCAP Segment in the Copolymer at Various Oxidation States

4478 Macromolecules, Vol. 43, No. 10, 2010
Guo et al.
During the continuous oxidation of copolymer in the leuco-
emeraldine oxidation state, the copolymer reached the first
emeraldine oxidation state (emeraldine I in Scheme 2), with
each CCAP segment containing only one quinoid ring, which
is why the UV spectrum showed the second absorption peak
at 610 nm. The copolymer was then oxidized to the second
emeraldine oxidation state (emeraldine II in Scheme 2), with
each CCAP segment containing two quinoid rings, which
increased the intensity of the absorption at 610 nm. Subse-
quently, the UV absorption exhibited a blue shift to ∼530 nm
after the absorption peak at 610 nm reached its maximum
intensity, indicating that the pernigraniline oxidation state
was formed.51
Figure 6 shows cyclic voltammograms for LMAP and
their linear and hyperbranched copolymers in a DMSO/HCl
mixture. The concentration was 0.25 wt % for LMAP and
for all of the copolymers, and the concentration of HCl was
1 mol/L. The cyclic voltammograms of 2a-PCL LMAP co-
polymer (Figure 6b), 3a-PCL LMAP copolymer (Figure 6c),
and 4a-PCL LMAP copolymer (Figure 6d) showed three
pairs of redox peaks (0.26, 0.42, and 0.65 V), which is
different from polyaniline, which typically shows two pairs
of redox peaks.52 The first well-defined oxidation peak at
∼0.26 V can be attributed to the redox process from the
“leucoemeraldine” to the ‘‘emeraldine I” form (Scheme 2),
and the second oxidation peak at ∼0.42 V is assigned to the
transition from the ‘‘emeraldine I” to the ‘‘emeraldine II”
state. At higher potentials, the oxidation peak at ∼0.65 V is
due to the transition from the “emeraldine II” state to the
“pernigraniline” oxidation state (Scheme 2). These cyclic
voltammograms for the copolymers are quite similar to that
for the LMAP in Figure 6a. These three oxidation processes
of the copolymers are in good agreement with UV absorp-
tion results, as shown in Figure 5. All of these UV spectro-
scopic and electrochemical results demonstrate the good
electroactivity of the copolymers.
Electrical Conductivity of the Copolymers. The PCL
EMAP copolymers were dissolved in chloroform and doped
with the same amount of 1 mol/L HCl. The electrical
conductivity values of the EMAP copolymer films were
determined by the standard four probe method and are
shown in Table 3. The conductivity of the copolymers was
between 2.42 ꢀ 10-5 and 5.01 ꢀ 10-6 S/cm, which is much
lower than that of AP films (10-2 S/cm). This is because of
the introduction of nonconjugated PCL segments to the
copolymers. However, this conductivity value is sufficient
to conduct bioelectrical signals in vivo because the micro-
current intensity is also quite low in human body.53
In Table 3, the conductivity of the hyperbranched co-
polymers is higher than that of the linear copolymer,
although these copolymers have almost the same EMAP
content as that tested by TGA. The conductivity of 3a-PCL
EMAP hyperbranched copolymer and 4a-PCL EMAP
hyperbranched copolymer is, respectively, 4.8 and 1.6 times
as high as that of the linear copolymer. In linear copolymer,
the EMAP segments are isolated by PCL chains, and it is
difficult for AP segments to form a network for electron
transport from one polymer chain to another (Scheme 3).
Furthermore, a microphase separation can occur as a
result of a self-assembly process among the linear block
copolymers.38 The soft PCL segments can easily aggregate
together, contributing to a continuous matrix, whereas the
hard AP segments form discontinuous domains. An AP
domain benefits electric conduction, but a tunnel effect
through the PCL matrix has to take place between two
neighboring domains to achieve electric conduction. There-
fore, the apparent conductivity is comparatively low in the
Figure 6. Cyclic voltammograms for (a) LMAP, (b) 2a-PCL LMAP
copolymer, (c) 3a-PCL LMAP copolymer, and (d) 4a-PCL LMAP
copolymer in a DMSO/HCl mixture.
case of the linear copolymer. In the hyperbranched copoly-
mers, the peripheral EMAP segments are distributed in an
orderly manner to some extent due to the special structure of
the hyperbranched copolymers. Because of the strong inter-
action between the EMAP segments in the solvent, the
EMAP segments can aggregate together and easily form an
intricate network of EMAP, which promote electric conduc-
tion (Scheme 3). There is thus a much greater possibility of
forming a conductive intricate network of EMAP segments
in the hyperbranched copolymer than in the linear copoly-
mer with the same EMAP content. The conductivity of the

Article
Macromolecules, Vol. 43, No. 10, 2010 4479
Scheme 3. Proposed Model for the Higher Conductivity of Hyperbranched Copolymers with That of the Linear Copolymers with the Same
EMAP Content
3a-PCL copolymer is higher than that of the 4a-PCL copoly-
mer, which may be because the 4a-PCL copolymer has more
defects than the 3a-PCL copolymer because of its over-
crowded branches. We can therefore use macromolecular
architecture to improve the conductivity of the polymers.
This is especially important in the biomedical field because a
high concentration of CCAP in the polymer is toxic. We can
now design a hyperbranched copolymer or dendrimer to give
a higher conductivity with a lower AP content and thus
reduce the toxicity of the polymers.
Wettability of the Polymers. Surfaces with a moderate
hydrophilicity (30-60°) have been shown to be optimal for
cell adhesion, proliferation, and function.54,55 The surface
wettability of the PCLs and copolymers in different oxida-
tion state was determined, and the results are shown in
Figure 7. Water contact angles of PCLs were ∼60°, and the
Figure 7. Water contact angles of the polymers.
differences between the different architectures are quite
small. The 2a-PCL has a higher water contact angle than
the 3a-PCL and 4a-PCL because of its higher crystallinity.
Conclusions
These values are a little lower than that of the ref 56, which is
66°, because of the lower molecular weight of the PCLs with
more hydroxyl groups in the polymers. The PCL LMAP
copolymers have somewhat higher water contact angle than
the pure PCLs, which may be because of the high hydro-
phobicity of LMAP segments. The contact angles of PCL
EMAP copolymers were lower than those of the correspond-
ing PCL LMAP copolymers because the hydrophilicity of
the AP segment in the EM state was higher than that in the
LM state. The dramatic decrease to ∼30° of the water
contact angle of the PCL EMAP copolymers doped with
1 mol/L HCl indicated that the polymer surface became
much more hydrophilic as a result of the formation of the
emeraldine salt. Increasing the hydrophilicity of the mate-
rial would involve lowering the surface energy, and this
would greatly influence the interactions between the cell
and material surfaces when in contact.57 These medium
hydrophilic surfaces are better for cell adhesion and cell
proliferation.
Macromolecular architecture design has been demonstrated as
a tool to increase the conductivity of polymers. The higher
electrical conductivity of the hyperbranched copolymers com-
pared with that of the linear copolymers with the same conductive
content was achieved by macromolecular architecture. This has
significant meaning in tissue engineering because the same level of
conductivity can be obtained by macromolecular engineering
with a lower concentration of the conductive segment and thus
reduces the toxicity of the DECPs. This is the first example of
hyperbranched copolymers with electroactive and degradable
properties. The wettability of the doped EMAP copolymer
increased dramatically, and the Xc of the copolymers decreased
sharply compared with the values for pristine PCLs, and this
means that the disadvantages of PCL, such as hydrophobicity
and low degradation rate can be overcome. Their structures and
properties have been fully characterized by FT-IR, 1H NMR, 13
C
NMR, SEC, DSC, TGA, UV, CV, and CAM. These copolymers
have excellent thermal stability with a two-step thermal degrada-
tion behavior and good electroactive properties with a three-step

4480 Macromolecules, Vol. 43, No. 10, 2010
Guo et al.
oxidation/reduction process. These linear and hyperbranched
electrically conductive copolymers with biodegradability have a
great potential for application in neural or cardiovascular tissue
engineering, and these results open the way to obtaining enhan-
ced electrical conductivity of the polymers by macromolecular
architectural design.
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