Crystalline structure and phase behavior of N-alkylated polypyrrole comb-like polymers

Article abstract of DOI:10.1039/c4ce00722k

We report the crystalline structure and phase behavior of a series of comb-like polymers (PPy-Cn), which were prepared via N-alkylation reactions between a pyrrole monomer and n-alkyl bromides (n = 18-26). Analysis by temperature-dependent X-ray diffraction and Fourier transform infrared spectroscopy (FTIR), as well as differential scanning calorimetry reveals that the crystal structure, thermal events and packing mode are affected by PPy backbones, which exhibit stronger rigidity than those of previously reported comb-like polymers. Remarkably, variable-temperature (VT)-FTIR proves the formation of orthorhombic phase at very low temperature, while VT-wide-angle X-ray diffraction only presents the hexagonal phase, illustrating that FTIR is a very sensitive tool for analyzing the micro-packing mode of molecular chains. The enhanced side-chain crystallizable carbon atoms indicate that PPy polymeric backbones play a negative role in the regular packing of nano-crystallites formed by alkyl groups. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ce00722k

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Crystalline structure and phase behavior of
N-alkylated polypyrrole comb-like polymers
Cite this: CrystEngComm, 2014, 16,
a
a
a
Haixia Wang, Xu Han, Haifeng Shi, Xingxiang Zhang,a Lu Qia and Dujin Wangb
7090
*
We report the crystalline structure and phase behavior of a series of comb-like polymers (PPy-Cn), which
were prepared via N-alkylation reactions between a pyrrole monomer and n-alkyl bromides (n = 18–26).
Analysis by temperature-dependent X-ray diffraction and Fourier transform infrared spectroscopy (FTIR), as
well as differential scanning calorimetry reveals that the crystal structure, thermal events and packing mode
are affected by PPy backbones, which exhibit stronger rigidity than those of previously reported comb-like
polymers. Remarkably, variable-temperature (VT)-FTIR proves the formation of orthorhombic phase at very
low temperature, while VT-wide-angle X-ray diffraction only presents the hexagonal phase, illustrating that
FTIR is a very sensitive tool for analyzing the micro-packing mode of molecular chains. The enhanced
side-chain crystallizable carbon atoms indicate that PPy polymeric backbones play a negative role in the
regular packing of nano-crystallites formed by alkyl groups.
Received 7th April 2014,
Accepted 27th May 2014
DOI: 10.1039/c4ce00722k
www.rsc.org/crystengcomm
mode of pended alkyl groups exhibiting the dependence upon
side-chain length is proven. For example, PEI16C and PEI18C
1. Introduction
Comb-like polymers have gained considerable attention in
recent years due to their structural features and hierarchical
self-assembly behavior, originating from the intermolecular
interactions between the polymeric main-chains and the flexi-
ble alkyl side-chains.1 The physical properties of comb-like
polymers are strongly affected by the rigidity of polymeric
main-chains and the length of flexible side-chains.2–9 There-
fore, understanding the interesting structural state of comb-
like polymers is very important for designing topological poly-
mers and strengthening knowledge of polymer crystallization.
Comb-like polymers present interesting crystals and phase
structures, and they exhibit dependence upon the side-chain
length and polymer backbones. For flexible backbones,
8 crystallizable carbon atoms are required to form the hex-
agonal phase (αH),10,11 while for the rigid ones 12 or more
carbon atoms are the minimum. Turner-Jones reported
orthorhombic (βO) phase structures in poly(α-olefin)s,12
while for poly(n-alkyl methacrylates),13 aromatic polyesters
with aliphatic side-chains,7,14 N-alkylated polyaniline15–17 and
poly(3-alkyl thiophene),2,18,19 αH phase was the only phase
structure. In our previous studies of N-alkylated poly-
ethyleneimine (PEI(n)Cs, 12 ≤ n ≤ 26),9,11,20–24 the packing
take αH and βO phase, respectively, while for PEI(n)Cs with
n ≥ 20 the coexistence phase of βO and αH is found. These
results indicate that the nanocrystal structure of comb-like
polymers, i.e., αH, βO or the two phases coexisting, is depen-
dent on the confinement alkyl side groups and the polymeric
backbones. Although some studies have discovered the ordered
structure composed by alkyl groups, understanding the interac-
tion of the polymeric backbone with side-chain crystallization
is still needed to clarify the nanoconfined crystallization behav-
ior of side-chains.
Polypyrrole (PPy) is an amazing polymer with high con-
ductivity and excellent chemical stability. To improve the pro-
cessability, chemical modifications have been explored.
Hamaide prepared N-substituted and N-oxyalkyl pyrrole
under solid triphase transfer catalysis conditions, and a low
alkylation degree was found.25 Sant'Ana et al. synthesized
1-dodecyl pyrrole derivative through alkylation reaction via
charge-transfer catalysis.26 To the best of our knowledge,
however, information on N-alkylated PPy containing the vari-
able side-chain length (n) from n = 18 to 26 is still not clear,
and the phase structure and crystallization behavior of
N-alkylated PPy derivatives also needs to be further investigated.
In this paper, a series of N-alkylated PPy comb-like poly-
mers (PPy-Cn, n = 18, 20, 22, 26) are prepared through
N-alkylated technique, as reported in our previous studies.
The phase behavior and crystalline structure of PPy-Cn
comb-like polymers is deeply characterized by differential
scanning calorimertry (DSC), variable-temperature wide-angle
X-ray diffraction (VT-WAXD) and Fourier transform infrared
a State Key Lab of Hollow Fiber Membrane Materials and Processes, Institute of
Art and Fashion, School of Materials Science and Engineering, Tianjin Polytechnic
University, Tianjin 300387, China. E-mail: hfshi@iccas.ac.cn
b Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of
Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
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spectroscopy (VT-FTIR). In particular, the ordered structure
and thermal behavior of PPy-Cn comb-like polymers are ana-
lyzed and compared with our previous studies in terms of
the molecular packing and phase transition process.
3–5 mg were encapsulated in aluminum pans and firstly
heated from −30 to 100 °C and kept at 100 °C for 10 min,
subsequently the samples were cooled to −30 °C and kept
for 10 min, and finally the samples were heated again
from −30 to 100 °C. The DSC thermogram in the second
heating process was recorded, and the heating/cooling rate
was 10 °C min−1.
2. Experimental section
2.1 Materials
Variable-temperature wide-angle X-ray diffraction. WAXD
measurements were performed on a Rigaku/max-2500 X-ray
diffractometer over the temperature range of −90 to 100 °C,
using the Cu Kα radiation of 1.542 Å.
Pyrrole (Py) (≥98%) was purchased from Aldrich and used
after refining. A series of n-alkyl bromides, (n = 18, 20, 22, 26)
were obtained from TCI and were used as received. Sodium
hydride (NaH), dimethyl sulfoxide (DMSO), ferric chloride
(FeCl3), CH3OH and chloroform were purchased from Tianjin
Sailboat Chemical Reagent Co., Ltd and used as received.
Variable-temperature Fourier transform infrared spectroscopy.
The FTIR measurements were performed on a Bruker
Equinox-55 spectrometer equipped with a temperature-
variable cell, and the spectra were processed with Bruker
OPUS program. The cell was kept in a vacuum by an oil
pump, and liquid nitrogen was used as the coolant. A resolu-
tion of 2 cm−1 was chosen and 32 scans were run. Infrared
spectra were recorded from −70 and 100 °C. At each tempera-
ture point, the samples were equilibrated for 5–8 min before
measurements.
2.2 Preparation of N-alkylated PPy-Cn comb-like polymers
For the synthesis of PPy-Cn comb-like polymers, two steps are
proceeded based on our studies.10,11,27–29 Firstly, the monomer
Py was substituted by n-alkyl bromide in a homogenous solu-
tion. 0.5 g NaH was dissolved in 50 ml anhydrous DMSO and
the suspension was stirred at 70 °C for 2 h under nitrogen gas.
9 mmol Py was added, and the system was continuously stirred
for 4 h, and then 9 mmol n-alkyl bromide was added under
stirring. The reaction was kept for another 5 h under stirring
and N2 gas atmosphere. After the reaction was finished, NaBr
was filtered off, and the solvent was removed. The Py-Cn deriva-
tives were rinsed alternatively with deionized water and CH3OH
three times, and finally dried under vacuum.
Secondly, PPy-Cn comb-like polymers were synthesized
under the catalysis of FeCl3 in chloroform solution at 0 °C
for 24 h. After that, the solution was dropped and washed
with CH3OH until no light green solid appeared. The final
PPy-Cn comb-like polymers were dried in a vacuum-oven for
3 h before use. Scheme 1 presents the detailed preparation
process of PPy-Cn comb-like polymers.
3. Results and discussion
1H NMR is used to analyze the structure of Py-Cn monomer
and the related PPy-Cn derivatives, and here Py-C22 and PPy-C22
are given as an example. As shown in Fig. 1a, the peaks at 6.65
and 6.13 ppm are characteristic of H atoms at the 2,5-position
and 3,4-position of the Py ring, respectively. Peaks of side alkyl
chains are located at 1.75, 1.56, 1.25, 0.88 ppm, respectively,26
while H atoms of –NCH2– are located at 3.88 ppm. The substi-
tution degree of C22 alkyl groups on the Py ring is calculated
according to the area ratio of H atoms of the Py-ring and CH2
connected by the N atom of the Py-ring. This result demon-
strates the successful substitution reaction of C22 side-chains
on the N site of Py. Thereafter, the Py-C22 monomer is used to
1
prepare the PPy-C22 derivative, and H NMR spectroscopy is
given in Fig. 1b, indicating that the PPy-C22 comb-like polymer
has been successfully prepared.
2.3 Characterization
Nuclear magnetic resonance spectroscopy. Nuclear magnetic
resonance (NMR) measurements were carried out on a Bruker
DMX 300 MHz NMR spectrometer at 25 °C. Deuterium
chloroform was used as the solvent and tetramethylsilane as an
internal reference.
Differential scanning calorimetry (DSC). A TA differential
scanning calorimeter Q2000, calibrated with indium, was
used to study the thermal behavior of PPy-Cn. Specimens of
Considering the influence of alkyl side-chains on the crys-
tallization behaviour of comb-like polymers, as reported in
previous studies,1,30 the thermal property of PPy-Cn comb-
like polymers is investigated by DSC, and the thermal events
also are compared with the previously reported comb-like
polymers. Fig. 2 presents the DSC curves of PPy-Cn comb-like
polymers in the heating and cooling process. From these
curves, the obvious endothermic and exothermic process
appears, originating from the contribution of side-chain
crystals.31–33 With increasing side-chain length from 18 to 26,
the melting (Tm) and freezing (Tc) temperature exhibit a lin-
ear change, changing from 19.8 to 70.0 °C and from 13.3 to
61.0 °C, respectively. Similarly, the melting enthalpy also
shows a linear increment tendency, followed by an increase
from 9.9 to 38.2 kJ mol−1, proving that the grafted side alkyl
chains formed the ordered crystal structures surrounded by
PPy backbones. To further analyze the ordered packing mode
Scheme 1 Synthesis process of N-alkylated polypyrrole comb-like
polymers.
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Fig. 2 DSC curves of PPy-Cn comb-like polymers at a scanning rate
of 10 °C min−1 in the heating process (a) and cooling process (b).
Fig. 1 1H NMR spectra of Py-C22 (a) and PPy-C22 (b) comb-like
polymer.
of side-chain crystals, the melt enthalpy of PPy-Cn vs. the
carbon number of the alkyl side-chain is plotted in Fig. 3
according to eqn (1), where k is the contribution of each
added CH2 group to enthalpy, and ΔHm,e is a constant
reflecting the contribution of chain end to enthalpy.10,34
ΔHm = nk + ΔHm,e
(1)
Through the fitted result of Fig. 3, the slope k is obtained
and its result is 3.56 kJ mol−1·CH2. According to previous
studies, it is well-known that the slope k reflects the crystal
structure of side alkyl groups. Usually, if the k value is
around 3.99 or 4.2 kJ mol−1·CH2, the alkyl side groups
constitute the rhombic or triclinic phase, respectively. How-
ever, if the k value is ca. 3.07 kJ mol−1·CH2, hexagonal phase
is the major form.34 For this estimated result, PPy-Cn comb-
like polymers should pack into a hexagonal phase, which is
similar to other comb-like polymers with long alkyl side-
chains, such as poly(n-alkyl methacrylate),30,35 N-alkylated
poly(p-benzamide),10 N-alkylated copolyamides,36 and
poly(n-alkyl itaconate).37
Fig. 3 Melt enthalpy of PPy-Cn vs. the carbon number of the alkyl
side-chain. The “■” values are experimental data, and the red line is a
linear fit with a slope of 3.56 kJ mol−1 per CH2 group.
only the part of side-chains away from the polymeric back-
bone, participate in the formation of crystals, and the size of
side-chain crystals is dependent upon the length of the side-
chains. To obtain the crystalline degree of side-chain crystals
and the crystallizable carbon atoms per side-chain, eqn (2)
and (3), as described in previous studies, are used.11 The
number of crystallizable CH2 groups (Nc) and the crystallinity
(Xc) of PPy-Cn comb-like polymers are listed in Table 1.
Additionally, it is well-known that only when the length of
the side alkyl groups is beyond a certain value does the side-
chain crystallization behavior appear. It is worth noting that
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Table 1 Calorimetric data for PPy-Cn comb-like polymers
The DSC results indicate that only the part of side-chains
away from the PPy backbone takes part in the formation of
side-chain crystals. Thus, XRD is used to analyze the crystalline
structures of side-chain crystals. Fig. 5 gives the XRD patterns
of PPy and PPy-Cn comb-like polymers at room temperature.
PPy shows a weak halo peak at 25.1°, while PPy-Cn derivatives
present one single sharp diffraction peak at 21.4° (d = 4.2 Å),
characteristic of αH phase according to the previous studies.13
Besides, an amorphous halo in PPy-C18 also appears, further
indicating its low crystallinity which is in good agreement with
the DSC results.40 The crystalline structure in PPy-Cn comb-like
polymers should originate from the contribution of side alkyl
groups because bulk PPy appears as a weak peak at 25.1°.
To further clarify the changes of side-chain crystals,
VT-WAXD is utilized to detect the phase transition behavior
(Fig. 6). With temperature increasing from −90 to 60 °C, the
intensity of the diffraction peak at 21.4° gradually decreases,
and once the temperature is above 80 °C this diffraction peak
disappears. Similarly, the peak position 21.4° also exhibits the
same variation with the intensity. The peak position almost
keeps constant at 21.4° before 60 °C, and when temperature
is above 80 °C the diffraction peak shifts to 19.6° (d = 4.6 Å),
Sample
Tm, °C
Tc, °C
ΔHm, kJ mol−1
Xc, %
Nc
2.7
4.6
7.0
PPy-C18
PPy-C20
PPy-C22
PPy-C26
19.8
45.1
55.3
70.0
13.3
33.4
46.0
61.0
9.9
17.1
25.7
38.2
11.9
18.8
26.3
34.0
10.4
Nc = ΔHm/k
Xc = (Nc × 14.026)/Munit
(2)
(3)
As shown in Table 1, Nc increases from 2.7 to 10.4 and Xc
increases from 11.9% to 34.0% with n changing from 18 to
26. It is concluded that only when the carbon atoms of side-
chains is over 16 does the side-chain crystallization behavior
for PPy-Cn comb-like polymers appear. To further understand
the thermal properties of comb-like polymers, the relation-
ship of Tm against the carbon atom (n) per side-chain in vari-
ous comb-like polymers is plotted in Fig. 4. As shown in
Fig. 4, poly(n-alkyl methacrylate) (PnAMA),38 PEI(n)C,11 poly
(styrene-co-maleic anhydride)-graft-1-alcohol (SMA-g-CnOH),31
and PPy-Cn exhibit higher Tm than those of n-alkanes, while
for N-alkylated poly( p-benzamide)s (PBA(n)C),10 the Tm is
lower than that of n-alkanes.39 Usually, n-alkane takes the
layer-by-layer stacking structure, while for comb-like poly-
mers, hexagonal phase is the major form. Such Tm changes
should be ascribed to the different stacking manner of alkyl
groups and the influence of polymer backbones. Note that,
rigid PPy and PBA backbones have an obvious influence on
Tm of confined side alkyl groups, indicating that the mobility
of alkyl chains is different from that of flexible backbones.
The PPy-Cn series requires more carbon atoms to realize
side-chain crystallization behaviour than that of PBA(n)C, i.e.,
2.7 for PPy-18 and 5.7 for PBA18C, indicating that the PPy back-
bone is more rigid than that of PBA. Interestingly, however, the
PPy-Cn series exhibits the higher Tm and Tc than that of
PBA(n)C, as shown in Fig. 4. The explanation is not clear, and
the reason needs to be further investigated in the future.
Fig. 5 XRD curves of PPy and PPy-Cn comb-like polymers at room
temperature.
Fig. 4 Melting temperature vs. carbon atoms of the side-chain in vari-
ous comb-like polymers and n-alkane.
Fig. 6 VT-WAXD curves of PPy-C26 with temperature changing from
−90 to 100 °C.
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characteristic of the amorphous state. The temperature transi-
tion range is in good agreement with that of the DSC result,
indicating that the melting process is attributed to the phase
transition from αH phase to amorphous state of the side-
chain crystals.10 It is worth noting that PPy-C26 shows the
asymmetric diffraction peak at 21.4° at low temperature. After
the curve-fitting of −90 °C diffraction peak, it further shows a
shoulder peak at 23.4° (d = 3.8 Å) close to the peak at 21.4°.
This is possibly influenced by the low temperature effect, and
then contributes to the regular packing of alkyl groups. Simi-
larly, PEI18C also gives the same behavior, and this shoulder
peak is assigned to βO packing.11
From the above XRD results, it is found that the crystalli-
zation behavior of the confined side alkyl groups in PPy-C26
is greatly decreased due to the limited mobility afforded by
rigid PPy.
FTIR is a powerful tool to detect microstructural informa-
tion and the packing mode of molecular chains from a
microscopic viewpoint. As a kind of side-chain crystallization
polymer, PPy-Cn comb-like polymers exhibit the obvious
phase transition process with temperature change, especially
at low temperature. Here, taking PPy-C26 comb-like polymer
as an example, we analyze the phase transition process and
the relevant packing mode of side alkyl groups distributed
along the PPy backbone. Fig. 7 presents the changes of
rocking band (γr(CH2)) of CH2 groups in PPy-C26 comb-like
polymer with temperature during the heating and cooling
process. Information on the packing mode of CH2 sequences
can be obtained from n-alkanes and our previous results on
PEI(n)Cs and PBA(n)Cs series comb-like polymers.9–11,20,22,23,41,42
A doublet at 719/730 cm−1 is correlated to γr(CH2) of βO packing,
while a single band at 720 cm−1 is attributed to αH packing. As
shown in Fig. 7a, it is found that the obvious double bands at
719/730 cm−1, characteristic of βO packing of CH2 chains, appear
below 0 °C. With temperature increasing from −40 to 20 °C,
730 cm−1 band gradually disappears followed by the decreased
absorption intensity, indicating that the βO packing has been
transformed into αH phase.22,23,27 With temperature further
increasing to 100 °C, the single 719 cm−1 band almost keeps
constant until the phase transition occurs at 70 °C, and then
shifts to around ~720 cm−1, characteristic of the crystalline C26
alkyl groups entering into the amorphous state.21 Similarly, the
reverse band shift and the phase transition process are also
found in the cooling process (Fig. 7b), demonstrating that the
phase transition from βO–αH phase to the amorphous state is
reversible with temperature. The similar phase transition pro-
cess is also observed in other series of comb-like polymers,
such as PEI(n)C,22,23 PBA(n)C,9 CS(n)Cs,27 etc. This proves that
FTIR is a sensitive technique for detecting the microstructural
changes and phase behavior of the confined side alkyl groups.
To clearly understand the phase transition behavior of C26 side
alkyl groups, the absorption intensity of 719 cm−1 band (Fig. 8)
is plotted as a function of temperature during the heating
and cooling process. As shown in Fig. 8, the intensity of the
719 cm−1 band gradually decreases with increasing tempera-
ture, and the temperature transition range appears between 40
Fig. 7 Rocking bands of methylene group of PPy-C26 vs. temperature
during the heating (a), and cooling (b) process.
Fig. 8 The changes of the absorption intensity of rocking band with
temperature.
and 70 °C, corresponding to αH–melt phase transition process,
as proved by the VT-WAXD and DSC results.
The phase transition from αH phase to amorphous state,
is directly observed in XRD and DSC result; while the FTIR
result, shows transition from βO–αH phase and then to the
amorphous state. It should be noted that the transition
behavior between WAXD and FTIR exhibits inconsistency,
which could be attributed to the experiments not being
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carried out on the same instrument. Additionally, it can be
concluded that FTIR is a sensitive tool to analyze the crystal
packing of molecular chains in the aspect of molecular view-
point. It is also worthy to note that PPy main chains are more
rigid than our previously reported polymer backbones in
comb-like polymers, such as PBA, chitosan and PPTA, etc.
PPy-Cn comb-like polymers show lower crystallinity and
smaller crystallizable carbon atoms than that of other series
of comb-like polymers, however, the appearance of regular
packing of the orthorhombic phase at low temperature
makes the role of the polymer backbone complicated. The
information is not clear now, and further characterizations
are still needed.
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Conclusions
In summary, N-alkylated polypyrrole (PPy-Cn) comb-like poly-
mers with the carbon number of alkyl side-chains ranging
from 18 to 26 exhibit typical side-chains crystallization behav-
ior, as observed in previously reported comb-like polymers.
PPy-Cn comb-like polymers show the obvious crystalline
structure, characteristic of βO and αH phase. Phase transition
from βO–αH phase to amorphous state is demonstrated by
FTIR, while XRD only proves the transformation from αH
phase to amorphous state in PPy-C26. This result indicates
that FTIR is a sensitive tool to analyze the microstructural
packing of confined alkyl side groups. Furthermore, the
phase transition and crystallinity of PPy-Cn comb-like poly-
mers is different from that of previously reported comb-like
polymers like PBA(n)Cs, CS(n)Cs and PPTA(n)Cs, which is
attributed to the rigid conjugated PPy backbone.
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Acknowledgements
This work was supported by the Program for New Century
Excellent Talents in University (NECT-13-0928), National Natu-
ral Science Foundation of China (grant no. 20904040 and
21174105), the Key Project of Tianjin Municipal Natural
Science Foundation (grant no. 12JCZDJC26800) and Beijing
National Laboratory for Molecular Sciences (BNLMS-2013016).
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