Supramolecular interactions of carbon nanotubes with biosourced polyurethanes from 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol

Article abstract of DOI:10.1016/j.polymer.2015.02.042

Biosourced polyurethanes (PU) were synthesized from a serinol derivative containing a pyrrole ring, 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol (SP), and stable supramolecular interaction with multiwalled carbon nanotubes (MWCNT) was established. Synthesis of SP was solvent free and with high atom efficiency: serinol was reacted with 2,5-hexandione, obtaining the tricyclic compound 4a,6a-dimethyl-hexahydro-1,4-dioxa-6b-azacyclopenta[cd]pentalene, whose aromatization led to SP. Solvent free polymerization of SP and 1,6-hexamethylene diisocyanate led to PU oligomers. High resolution transmission electron microscopy of CNT adducts with PU oligomers revealed individual CNT, with intact skeleton and PU oligomers tightly adhered to their surface. Suspensions of MWCNT-PU adducts in acetone were stable even after centrifugation. These results pave the way to composite material containing carbon nanofillers tightly bound to the polymer matrix, reducing their dispersion in the environment.

Full text of DOI:10.1016/j.polymer.2015.02.042

Polymer 63 (2015) 62e70
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Supramolecular interactions of carbon nanotubes with biosourced
polyurethanes from 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol
,
Maurizio Galimberti a *, Vincenzina Barbera a, Attilio Citterio a, Roberto Sebastiano a,
Ada Truscello a, Antonio Marco Valerio a, Lucia Conzatti c, Raniero Mendichi b
a Politecnico di Milano, Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Via Mancinelli 7, 20131 Milano, Italy
b National Council of Research, Institute for the Study of Macromolecules, Via Bassini 15, 20133 Milano, Italy
c National Council of Research, Institute for the Study of Macromolecules, Via De Marini 6, 16149 Genova, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Biosourced polyurethanes (PU) were synthesized from a serinol derivative containing a pyrrole ring, 2-
(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol (SP), and stable supramolecular interaction with multi-
walled carbon nanotubes (MWCNT) was established. Synthesis of SP was solvent free and with high atom
efficiency: serinol was reacted with 2,5-hexandione, obtaining the tricyclic compound 4a,6a-dimethyl-
hexahydro-1,4-dioxa-6b-azacyclopenta[cd]pentalene, whose aromatization led to SP. Solvent free poly-
merization of SP and 1,6-hexamethylene diisocyanate led to PU oligomers. High resolution transmission
electron microscopy of CNT adducts with PU oligomers revealed individual CNT, with intact skeleton and
PU oligomers tightly adhered to their surface. Suspensions of MWCNT-PU adducts in acetone were stable
even after centrifugation. These results pave the way to composite material containing carbon nanofillers
tightly bound to the polymer matrix, reducing their dispersion in the environment.
Received 19 December 2014
Received in revised form
20 February 2015
Accepted 23 February 2015
Available online 3 March 2015
Keywords:
Serinol
Pyrrole
Polyurethane
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
preferred. For example, glycerol carbonate is used for the synthesis
of polyurethanes, polycarbonates or polyethers [10].
The preparation of innovative macromolecular structures from
renewable sources is one of the hot research topic in the polymer
field [1,2]. Such a research activity becomes particularly meaningful
when there is no impact on the food chain, starting building blocks
are relatively available and reasonable economic perspective can be
envisaged.
Glycerol is a cheap, easily available raw material, non toxic and
biodegradable. It is the main co-product of bio-diesel production:
some 65% of glycerol came from the biodiesel industry in 2011, with
a total supply of 1.2 million ton in 2010 [3e8]. Glycerol is studied as
the building block for a C3 platform alternative to the oil based one
[3,4,8e10], but is not much used for the preparation of polymers
[10], mainly as a consequence of high viscosity and high hydro-
philicity and because of the presence of three hydroxyl groups with
similar reactivity, that could lead to side products. Hence, alterna-
tives routes that start from simple glycerol derivatives are
The present work started from 2-amino-1,3-propanediol,
known as serinol (S), with the objective to prepare innovative
polymeric structures and to establish supramolecular interactions
with carbon allotropes, exploiting the chemoselectivity of amino
and hydroxyl groups. Serinol is commercially available, is prepared
from glycerol or dihydroxyacetone, but can also be directly ob-
tained from renewable sources [11].
As first step of the research activity, the reaction of the amino
group of S with 2,5-hexanedione (HD), through the classical
PaaleKnorr reaction [12,13], was performed to prepare 2-(2,5-
dimethyl-1H-pyrrol-1-yl)-1,3-propanediol, whose chemical struc-
ture is shown in Fig. 1, serinol derivative containing a pyrrole ring,
hereinafter indicated as serinol pyrrole (SP) [14].
Report on PaaleKnorr reaction between S and HD is available in
the literature. S and HD were refluxed in toluene in the presence of
large amount of glacial acetic acid and of a catalytic amount of p-
toluensolfonic acid. SP was obtained in low yield, 13 as mol%,
together with polymeric products [15]. Objective of the present
work was thus to improve, substantially, reaction pathway and
efficiency, performing the neat reaction of S with HD. SP shows
what could be defined a dual reactivity. Thanks to the hydroxyl
* Corresponding author.
E-mail address: maurizio.galimberti@polimi.it (M. Galimberti).
http://dx.doi.org/10.1016/j.polymer.2015.02.042
0032-3861/© 2015 Elsevier Ltd. All rights reserved.

M. Galimberti et al. / Polymer 63 (2015) 62e70
63
the solubility of SWCNT in tetrahydrofuran [30] and stable solu-
tions of SWCNT and MWCNT in tetrahydrofuran were obtained
with aliphatic polyester dendrons bearing pyrene units at their
periphery [31]. A pyrene functionalized RAFT agent was used to
synthesize pyrene functionalized polymers that promoted the
preparation of stable solutions of MWCNT in tetrahydrofuran [32].
CNT were functionalized with a poly(benzoxazole) (PBO) pre-
cursor [33], enabling the CNT dispersion in dimethylacetamide.
Afterwards, a CNT-PBO adduct film with improved mechanical
properties was obtained by simple heating. Moreover, stable
concentrated dispersions of MWCNT and single-walled CNT in THF
and toluene were prepared by using poly(2,7-carbazole)s [34] and
poly(phenylacetylene)s with long alkyl tails on the side-chains [35].
In all cases, the absence of hydrophilic moieties allows the
dispersion of CNT only in aromatic solvents. The use of SP as
monomer could allow the preparation of polymers with tuned
amounts of aromatic rings in the backbone chain and with hydro-
philic moieties that could favour the CNT dispersion in aqueous and
environmentally friendly media. The structure of MWCNT-PU ad-
ducts was characterized through High Resolution Transmission
Electron Microscopy (HRTEM) and the stability of MWCNT-PU
suspensions in eco-friendly solvents, such as acetone and ethyl
acetate, was investigated. Preliminary results arising from centri-
fugation technique and UV spectroscopy are also reported.
Fig. 1. 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol (SP).
groups, it appears suitable monomer for stepwise polymerizations.
Moreover, the pyrrole ring could be exploited for establishing su-
pramolecular interactions that involve
p electrons.
Stepwise polymerizations were performed [16]. Those to pol-
y(ether)s, poly(ester)s, poly(carbonate)s and poly(urethane)s are
still under investigation. In this manuscript, are described poly(-
urethane)s (PU) prepared from SP and 1,6-hexamethylene diiso-
cyanate (HDI), through a solvent free synthesis. In the literature, the
use of serinol for polymers preparation is not yet carefully inves-
tigated. Only a couple of example have been reported in recent
papers. The reaction of the amino group of serinol with electro-
philes led to functional diol intermediates, that cyclisized to form
six-membered aliphatic cyclic carbonate monomers, converted into
polymers through organo-catalysed ring opening polymerization
[17]. In another report [18], water soluble biodegradable polyester
for selective drug release was prepared from adipoyl chloride and a
serinol derivative obtained from the reaction of serinol with suc-
cinimidyl ester of trimethyl-locked benzoquinone. Characterization
of PU prepared in this work was performed by means of Fourier
Transformed Attenuated Total Reflectance (FT-ATR), 1H and 13C
NMR spectroscopy, gel permeation chromatography (GPC), thermal
gravimetric analysis (TGA) and differential scanning calorimetry
(DSC).
2. Results and discussion
2.1. Synthesis of 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol
The serinol derivative 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-
propanediol was prepared by condensation of S and HD through
the classical KnorrePaal synthesis of pyrroles. As depicted in
Scheme 1, two-step synthesis was developed.
The neat reaction of equimolar amounts of S and HD was first
performed, at room temperature, obtaining the tricyclic compound
4a,6a-dimethyl-hexahydro-1,4-dioxa-6b-azacyclopenta[cd]penta-
lene (HHP), that was then isomerized to aromatic SP by simply
increasing the reaction temperature. Both reactions were thus
performed in the absence of solvents and catalysts. Quantitative
yield was obtained for the first step reaction and, after distillation of
SP, the overall yield of the two steps synthesis was calculated to be
about 95% by moles. Structural characterizations of tricyclic com-
pound and SP was performed by means of 1H NMR. It is worth
commenting here that the NMR spectra of the reaction mixtures
did not reveal the presence of products other than HHP, after the
first step, and SP, after the second step. Research is in progress to
further optimize reaction conditions and yields and to elucidate the
mechanism. At present, as far as the first step of the synthesis is
concerned, it can be commented that the absence of acids and the
mild reaction conditions favour hemiaminal formation and poly-
cyclization and prevent the aromatization to SP as well as the for-
mation of polymeric products. Water, that was not removed from
the products of the first step, could favour such isomerization,
bringing acidity, as a consequence of temperature increase, at a
level however not enough to promote the polycondensation
process.
PU based on SP were then used to establish supramolecular
interaction with Multiwalled Carbon Nanotubes (MWCNT) [19],
exploiting the synergy between pyrrole and carbonyl
p systems of
PU for establishing effective interaction with CNT. In fact, particular
objective of the research was to establish stable supramolecular
interactions with carbon nanotubes, that could allow the prepara-
tion of stable dispersions in liquid media and of composite mate-
rials with carbon nanotubes tightly bound to the polymer matrix,
preventing their dispersion in the environment. MWCNT are
concentric tubes of graphene sheets, with nanosize in two di-
mensions, characterized by exceptional mechanical properties and
with the ability of conducting electrons without dissipating energy
as heat. Despite the impressive research activity on both single-
[20,21] and multi-walled [22,23] CNT, efficient approaches to
convert CNT bundles into fully dispersed nanometric CNT having a
stable interaction with the matrix, remain a main research objec-
tive. Main concern that limits the large scale use of MWCNT is
indeed their dispersion in the environment, as a consequence of
faint interaction with their surrounding. To promote such interac-
tion, reviews are available in the scientific literature on carbon
nanotubes, reporting their covalent chemical modification [24e27]
and non-covalent interactions [27]. The latter ones do not alter the
electronic structure of nanotubes, thus preserving their exceptional
properties. Among them, polynuclear aromatic compounds car-
rying different substituents and polymers with aromatic structural
units are reported to promote the solubility of CNT in hydrophilic or
hydrophobic media. Stable dispersions of MWCNT in toluene,
tetrahydrofuran and chloroform were prepared thanks to co-
polymers of methyl methacrylate bearing as pendant pyrene
groups randomly distributed along the chains [28,29]. Styrene
based copolymers with a pyrene block were then shown to improve
Scheme 1. Reaction scheme for the preparation of the serinol derivatives HHP and SP.

64
M. Galimberti et al. / Polymer 63 (2015) 62e70
Table 1
As mentioned in the Introduction, a report is available in the
Synthesis of poly(urethanes): polymerization conditions and molecular mass
distribution.
literature on the KnorrePaal reaction between S and HD [16]. The
reaction mixture contained SP, the tricyclic compound (yield of
about 13% mol) and polymeric materials, whose amount was
enhanced when the conversion of the tricyclic compound into the
aromatic derivative was attempted with the help of acidic catalysts.
The novel two steps synthesis for the preparation of SP here pre-
sented appears to have relevant advantages: it is solvent free, it
does not use acid catalysis and it has high atom efficiency (78%),
thanks to the high atom economy (82%) and the high yield. The final
product is obtained by direct distillation, without the need of any
chromatography separation. SP appears thus a suitable monomer
for stepwise polymerizations.
b
c
Entry
HDI
(mmol)
SP
(mmol)
DMPa
(mmol)
T
(ꢀC)
t
Mn
Mw=Mn
(min)
(g molꢁ1
)
1
2
3
4a
4be
5a
5bf
6
2.96
2.96
2.96
2.96
0
2.96
2.96
3.87
3.87
3.87
2.96
2.96
2.96
2.96
2.96
2.96
0
0
0
0
0
0
0
0
0
0
0
3.87
3.87
0
39
39
90
90
90
90
90
50
90
90
30
150
120
120
60
120
60
30
1110
1070
2120
n.d.d
2410
n.d.d
2145
2100
n.d.d
10,935
2.2
2.7
2.4
n.d.d
3.0
n.d.d
2.7
2.3
7a
120
60
n.d.d
2.3
7bg
a
DMP: 2,2-dimethyl-1,3-propanediol.
b
c
Mn: number average molecular mass from GPC analysis.
w=Mn: polydispersity index from GPC analysis, Mw ¼ weight average mo-
2.2. Solvent free synthesis of poly(urethane)s
M
lecular mass.
d
Not determined.
Solvent-free synthesis of poly(urethane)s, from SP and HDI, was
performed. Scheme of the reaction and repeating unit are in
Scheme 2. Table 1 shows polymerization conditions and data of
molecular mass and molecular mass distribution obtained from
GPC analysis. Equimolar amounts of the comonomers were used,
varying temperature and time of polymerization.
e
f
SP was added at the end of 4a polymerization.
HDI was added at the end of 5a polymerization.
HDI was added at the end of 7a polymerization.
g
In both vibrational spectra, band at 3331 cmꢁ1 can be attributed
Polymerization reactions, performed at 39 ꢀC and different
polymerization times, 30 and 150 min (Entries 1 and 2 of Table 1),
led to products with low molecular mass (<1200 g/mol) and rela-
tively broad molecular mass distribution (Mw=Mn ranging from 2.2
to 2.7). Higher value of Mn was obtained performing the reaction at
higher temperature, 90 ꢀC (Entry 3 of Table 1), suggesting an effect
of the polymerization mixture viscosity on the propagation of the
polymerization reaction.
In order to have oligomers with defined end groups, Entry 3 was
repeated (Entries 4a and 5a of Table 1) and an equimolar amount of
either SP (Entry 4b) or HDI (Entry 5b) was added. To investigate the
role played by the pyrrole ring in preventing the formation of high
molecular mass products, polymerization was carried out by using
2,2-dimethyl-1,3-propanediol (DMP) as the diol instead of SP. The
polymerization reactions were carried out at 50 ꢀC and 90 ꢀC (En-
tries 6 and 7 of Table 1, respectively). PU obtained at the latter
(higher) temperature revealed appreciably higher molecular mass.
The lower polymerization degrees obtained with SP could be
tentatively ascribed to its larger steric hindrance as well as to the
to the NeH stretching of carbamate and the stretching of C]O
gives the band at 1696 cmꢁ1. The band of OH end group is not
visible in the spectrum in Fig. 2A, because it overlaps with the NeH
band. In the spectrum in Fig. 2B, the presence of the isocyanate end
group is evidenced by the band at 2267 cmꢁ1. The bands observed
at 1523, 1461 and 1418 cmꢁ1 are assigned to the pyrrole ring.
Stretching of CH group is revealed by peaks at 2931 e 2857 cmꢁ1
.
Oligomer microstructure was studied by means of 1H and 13C
NMR analysis. Fig. 3 shows the 1H NMR spectra of the reaction
products of Entry 4b (Fig. 3A) and of Entry 5b (Fig. 3B) of Table 1.
In both 1H NMR spectra of Fig. 3, chemical shifts are consistent
with the proposed polymer structure. Singlet at 7.10 ppm, assigned
pep interactions established by the pyrrole ring. This aspect is
under further investigation.
The nature of functional groups present in the oligomers ob-
tained from Entries 1e5 was investigated through FT-ATR spec-
troscopy. Fig. 2 shows the FT-ATR spectra of the reaction products
from Entry 4b (trace A) and Entry 5b (trace B) of Table 1, that should
have hydroxyl and isocyanate end groups, respectively.
Fig. 2. FT-ATR spectra (cmꢁ1) of PU samples from Entry 4 (A) and from Entry 5 (B) of
Scheme 2. Synthesis of PU from SP and HDI.
Table 1.

M. Galimberti et al. / Polymer 63 (2015) 62e70
65
Fig. 3. Structure and 1H NMR spectra 400 MHz in DMSO-d6 of sample from Entry 4b (A) and from Entry 5b (B) of Table 1.
to the carbamate NeH, is typical of urethane linkage. Methylene
protons (c) of the repeating serinol unit produce signals at 4.30 and
4.21 ppm. Peaks at 4.53 and at 2.94 ppm represent the resonance
respectively of the methine hydrogen (b) and of the methylene
ones (d) bonded to carbamate NeH. Absorptions at 3.76 and
3.66 ppm indicate the presence of CH2OH terminals (a, Fig. 3A), and
the one at 3.96 ppm is attributed to the hydrogens on the C carrying
the terminal isocyanate (a1, Fig. 3B). Both spectra also show the
characteristic signals of the heterocyclic unit: at 5.59 ppm, the
Instead, in the same area of the spectrum shown in Fig. 4B, the
signal at 42 ppm (a1) is relative to the methylene carbon adjacent to
the isocyanate nitrogen.
Thermal stability of SP-based oligomers was assessed by means
of thermal gravimetric analysis (TGA) under N2. As an example, TGA
scans of SP and of the PU sample obtained from Entry 4b of Table 1,
with hydroxyl end groups, are compared in Fig. 5.
Weight loss for the sample from Entry 4b in the temperature
range 25e150 ꢀC is likely due to evolution of adsorbed water. The
outset temperature for the degradation of the pyrrole based unit in
the oligomeric product can be detected around 300 ꢀC. No com-
parison is possible with SP, that achieves at lower temperature its
boiling point. Further weight losses are observed at about 350 ꢀC
and 385 ꢀC and are due to the degradation of carbamate linkage and
to the methylene sequences [36].
pyrrolic hydrogens in the
group in -position.
b-position, and at 2.17 ppm the methyl
a
13C NMR spectra of the reaction products obtained from Entries
4b and 5b of Table 1 are reported in Fig. 4A and B, respectively.
The presence of urethane carbonyl and aromatic pyrrolic car-
bons are revealed by peaks at 155, 127 and 106 ppm, respectively.
Most noticeable differences between the two samples are visible in
the area between 40 ppm and 63 ppm. In Fig. 4A, it is possible to
note the presence of the signal of terminal methylene (a) at 61 ppm.
The presence of thermal transitions was investigated through
DSC analysis. Glass transition was observed at about 17 ꢀC and 13 ꢀC
respectively, for PU samples from Entries 4b and 5b of Table 1, with
Fig. 4. Structure and 13C NMR spectra 100 MHz in DMSO-d6 of PU samples from Entry 4b (A) and from Entry 5b (B) of Table 1.

66
M. Galimberti et al. / Polymer 63 (2015) 62e70
Fig. 5. TGA curves under N2 of SP (A), PU (B), PU-MWCNT adduct (C), with PU sample
from Entry 4b of Table 1.
Fig. 6. XRD diffraction spectrum of: MWCNT (A), the MWCNT-PU adduct (B). PU was
from Entry 3 of Table 1.
hydroxyl and isocyanate end groups respectively. The sample with
2,2-dimethyl-1,3-propanediol as the diol (Entry 7b of Table 1)
showed glass transition at about 10 ꢀC. These values, clearly
affected by the low molecular mass of the oligomers, could how-
ever indicate that SP-based repeating unit lead to higher temper-
atures for the glass transition.
between two successive layers in CNT graphitic crystal, 8 was
estimated as the number of wrapped CNT layers.
Morphological analysis of MWCNT and MWCNT-PU adduct was
performed through HRTEM analysis at different magnifications. PU
treatment brought to a large CNT disentanglement, as shown by the
lower number of CNT micrometric bundles and by the presence of
individual tubes in a defined space, as observed in many HRTEM
images and represented in Fig. 7A. The micrograph in Fig. 7B shows
that the multiwalled CNT skeleton remained intact after the
treatment with PU oligomers. The inspection of such images
allowed to estimate the average number of CNT layers equal to 10, a
value close to the one calculated from the XRD pattern. The
coherence between the results obtained from XRD and HRTEM
indicates that the MWCNT-PU adduct shown in Fig. 7 (CNT with 10
walls) is representative of the whole sample. CNT surface was thus
decorated with PU chains, that form condensed polymer layer
tightly adhered to the CNT external surface, with a thickness from
about 3 to about 9 nm.
Acetone suspensions of MWCNT-PU adducts (1 mg/mL con-
centration) were prepared with PU from Entries 2, 3, 4b and 5b of
Table 1, with PU content of 46 0.5 as mass% (see Experimental
Section). Such suspensions were observed to be stable for at least
12 months. Fig. 8 shows the pictures of the acetone suspensions of
MWCNT (Fig. 8A) and of MWCNT-PU adducts after 12 months of
storage (Fig. 8B). The stable MWCNT-PU suspension (with PU from
Entry 3 of Table 1) was then centrifuged, at 5000 rpm for 5 min,
without observing any evident separation, as shown in Fig. 8C.
To confirm the dispersion of the MWCNT-PU in the solvent,
UVeVis absorption analysis was carried out on the suspensions
shown in Fig. 8, by carefully taking the upper part of the superna-
tant suspensions, obtaining the relation between absorbance and
MWCNT-PU concentration. Solutions with 0.1, 0.3, 0.5 and 1 mg/mL
as adduct concentrations were analyzed. In Fig. 9A, it is shown that
the absorbance monotonously increases with the adduct concen-
tration. Moreover, the stability of the solution with 1 mg/mL con-
centration of the PU/MWCNT adduct, prepared with PU from Entry
3 of Table 1, was investigated by taking UVeVis spectra after 12
months. In Fig. 9B, it is evident that the same absorbance was
detected for the suspensions (1 mg/mL) freshly prepared and
stored for 12 months (traces b and c), confirming that CNT treated
with PU oligomer did not settle down, even after long time.
To investigate the specific role of the aromatic ring, CNT adduct
was as well prepared with PU based on DMP in place of SP (Entry 7
2.3. Adducts of poly(urethane)s based on SP with MWCNT
Adducts of poly(urethane)s based on SP with MWCNT were
prepared, in order to obtain stable dispersions of disentangled
nanotubes in eco-friendly polar solvents, suitable for large scale
applications, such as those for coatings. The PaaleKnorr reaction
transforms the sp3 hybrids of the amino group in the sp2 ones of the
pyrrole ring. This allows to pursue the
hybridized carbon atoms of CNT.
p-p
interaction with the sp2
PU adducts with MWCNT were prepared as described in the
experimental part. Briefly, an acetone solution of a PU sample from
one of the Entries of Table 1 was added to a sonicated CNT sus-
pension in acetone, performing then a further sonication. The
concentration adopted for the MWCNT/PU adduct was 1 mg/mL.
Chemical composition and structure of the ensuing MWCNT-PU
adduct were investigated by means of TGA, XRD and HRTEM
analysis. The sample analysed was isolated by carefully taking the
upper part of the supernatant suspension obtained after centrifu-
gation, evaporating then the solvent. By using the PU sample from
Entry 3 of Table 1, the content of PU was calculated, from TGA, to be
about 46 as mass% (Fig. 5A). Fig. 6 shows the XRD patterns, in the
10ꢀe100ꢀ as 2
q range, of pristine MWCNT (Fig. 6A) and of the
MWCNT-PU adduct (Fig. 6B) with PU from Entry 3 in Table 1. XRD
pattern of a crystalline carbon allotrope, such as graphite, that has
many stacked graphene layers, shows, as the most intense peaks,
some (00[) and (hk[) reflections, that remain visible also when the
number of stacked layers is reduced (to few tenths) [37]: 002 and
004, at 26.2ꢀ and 54.3ꢀ as 2
0.34 nm, 100 and 110 reflections at 42.8ꢀ and 77.5ꢀ as 2
q
value, with a d002 interlayer distance of
value. In
q
the pattern in Fig. 6A of MWCNT that have a low number of layers
wrapped to formed the tubes, only the two most intense reflections
are visible: 002 at 25.3ꢀ and 100 at 42.8ꢀ. The same reflections can
be observed in the pattern of the MWCNT-PU adduct. The dimen-
sion of crystallites in direction orthogonal to CNT layers can be
estimated, by calculating the correlation length D00[, through the
Scherrer equation. A value of about 2.7 nm was obtained for both
pristine tubes and for the adduct. Taking into account the distance

M. Galimberti et al. / Polymer 63 (2015) 62e70
67
Fig. 7. HRTEM micrographs of: individual tubes (A), the MWCNT-PU adduct (PU from Entry 3 of Table 1) (B).
in the introduction, to exploit the synergy between pyrrole and
carbonyl systems for establishing stable interaction with CNT.
p
2.4. Interaction of SP with CNT
The above reported results suggest that SP promote stable
interaction of poly(urethane)s with CNT. Experiments were per-
formed to obtain at least a clue on the nature of such interaction. In
the literature, p-p interactions are hypothesized to occur between
aromatic molecule and carbon allotropes [38e41]. To investigate
such interaction, polynuclear aromatic molecules, such as pyrene
[42], coronene [42] and perylene [43], are used as model com-
pounds, essentially for the aromatic molecules. In this work, to
avoid any possible influence from by products present in CNT, such
as catalyst residues, perylene was used as model compound for CNT
and its interaction with SP was studied through NMR analysis. In
fact, it was shown that it is possible to obtain a direct evidence for
Fig. 8. Acetone suspensions of MWCNT (A), MWCNT-PU after 12 months storage (B)
and MWCNT-PU after centrifugation (C). In (B), vials from left to right contain PU
obtained from Entry 2, 3, 4b, 5b of Table 1, respectively. In (C), vial contains PU from
Entry 3 of Table 1.
the
p-p
interaction by means of 1H NMR spectral measurements
[40]. In brief, increasing amounts of perylene were added to a so-
lution containing SP, recording the NMR spectra. In Fig. 10, are re-
ported the 1H NMR spectra of perylene (Fig. 10A) SP (Fig. 10B) the
perylene adduct with SP (Fig. 10C). The chemical shifts of the
perylene-protons in the serinol pyrrole solution shifted to higher
magnetic field by ca. 0.007e0.003 ppm (Fig. 10D), compared to
those recorded for the perylene solution. The shift is due to the
effect of the electron rich system (pyrrole), in agreement with what
reported in the literature. It appears thus that the SP molecule is
of Table 1). Extraction tests with ethyl acetate at room temperature
were then carried out on both CNT adducts. TGA analysis of the
ensuing products showed weight loss of about 23 wt% and 1 wt% for
the adduct with PU based on DMP and SP respectively, revealing
the strong supramolecular interaction of PU based on SP with CNT.
These results are in line with the analysis of TEM micrographs that,
as shown in Fig. 7, revealed the intimate interaction of MWCNT
with the SP based PU. It was thus achieved the objective mentioned
Fig. 9. Dependence of UVevis absorbance on concentration of PU-MWCNT adduct in acetone (A); UV traces for acetone suspensions of: MWCNT (a), MWCNT-PU adduct (b),
MWCNT-PU adduct after twelve months storage (c) (PU from Entry 3 of Table 1) (B).

68
M. Galimberti et al. / Polymer 63 (2015) 62e70
Fig. 10. 1H NMR spectra in CDCl3 of perylene (A); SP (B); Perylene-SP adduct (C). Comparison of spectra C and A in the inset D: enlargement between 7.40 and 8.40 ppm.
able to interact with a polynuclear aromatic compound, thanks to
its electrons. However, further investigations are required to be
4. Experimental section
p
better elucidate the nature of such interaction.
4.1. Materials
Reagents and solvents commercially available were purchased
and used without further purification: 2,5-hexandione (Merck e
Schuchardt), 2-amino-1,3-propandiol (kindly provided by Bracco),
1,6-hexamethylene diisocyanate (Fluka e Analytical), perylene
(SigmaeAldrich), 2,2-dimethyl-1-propanol (Fluka e Analytical),
acetone (SigmaeAldrich), ethyl acetate (SigmaeAldrich). Purified
multi-walled nanotubes (NC7000 series) were purchased from
3. Conclusion
Innovative, green, efficient synthetic strategy was designed and
developed for the preparation of polyurethanes able to establish
stable supramolecular interactions with carbon nanotubes. Serinol
derivative containing a pyrrole ring, 2-(2,5-dimethyl-1H-pyrrol-1-
yl)-1,3-propanediol, was synthesized in
a two step reaction
NANOCYL™ Inc. They have average length of 1.5
mm, average
involving equimolar amount of serinol and 2,5-hexandione,
without the use of solvents and further chemicals. SP was obtained
from the aromatization of the trcyclic intermediate 4a,6a-dimethyl-
hexahydro-1,4-dioxa-6b-azacyclopenta[cd]pentalene, that is the
product of the first step of the synthesis. PU oligomers were syn-
thesized through the reaction of SP with HDI and their adduct with
CNT was prepared by sonication in an environmentally friendly
solvent such as acetone. Stability of the suspension was verified
over months, even after centrifugation. HRTEM analysis revealed
that CNT were prevailingly disentangled from the starting bundles.
They had intact skeleton and were decorated by PU oligomers,
tightly adhered to CNT surface. This work demonstrates that SP is a
versatile monomer for the preparation of polymers that allow to
diameter of 9.5 nm, surface area of 250e300 m2/g, 90% as carbon
purity and 10% as metal oxide content.
4.2. Synthesis of 4a,6a-dimethyl-hexahydro-1,4-dioxa-6b-
azacyclopenta[cd]pentalene (HD)
A mixture of hexan-2,5-dione (41.4 g; 0.36 mol) and serinol
(30.0 g; 0.33 mol) was poured into a 100 mL round bottomed flask
equipped with magnetic stirrer. The mixture was then stirred, at
room temperature, for 6 h. The resulting compound HD was char-
acterized through 1H NMR and the yield was estimated to be 99%.
1H NMR (400 MHz, DMSO-d6,
d in ppm): 1.28 (s, 6H); 1.77 (m, 2H);
1.93 (m, 2H); 3.60 (m, 4H); 3.94 (q, 1H).
exploit the p-p interaction between the pyrrole ring and a carbon
allotrope such as CNT, leading to the preparation of even and stable
CNT dispersions in a large variety of environments, from eco-
friendly (aqueous) liquids to polymeric matrices. A biosourced
molecule such as SP can thus pave the way for the preparation of
composite materials with carbon allotropes, particularly of nano-
size, tightly bound to the polymer matrix, preventing their
dispersion in the environment.
4.3. Synthesis of 2-(2,5-dimethyl-1H-pyrrol-1-yl)-1,3-propanediol
(SP)
The product mixture obtained from the synthesis of HD was
kept under vacuum for 2 h and then heated to 180 ꢀC for 50 min.
After distillation under reduced pressure at 130 ꢀC and 0.1 mbar, SP
was isolated as yellow oil with 96% yield. The global yield of the two
step synthesis was therefore about 95%. 1H NMR (400 MHz, DMSO-
The use of MWCNT-PU adducts for the preparation of PU based
nanocomposites is currently under investigation.

M. Galimberti et al. / Polymer 63 (2015) 62e70
69
d6,
d
in ppm): 2.16 (s, 6H, eCH3 at C-2,5 of pyrrole moiety); 3.63 (m,
and a relative calibration constructed with some narrow molecular
mass distribution polystyrene (PS) standards. The SEC system
consisted of: an Alliance 2695 separation module from Waters, a
differential refractometer (DRI) as concentration on-line detector,
2H, CH2OH); 3.76 (m, 2H, CH2OH); 4.10 (quintet, 1H, at C-3 of diol);
4.73 (t, 2H, CH2OH); 5.55 (s, 2H, C-3,4 of pyrrole moiety).
4.4. Polymerization reactions
two PLgel columns from Polymer Laboratories (5 mm of particle
size, 104 and 500 Å of pores size), 0.8 mL/min of flow rate, 100
ml of
A standard procedure was adopted for polymerization reactions.
Entry 3 of Table 1 is reported as an example. SP (0.500 g,
2.96 mmol) and HDI (0.498 g, 2.96 mmol) were added in sequence
in a round bottomed 100 mL flask equipped with a magnetic stirrer,
under nitrogen atmosphere. The resulting mixture was then stirred
at 90 ꢀC for 150 min. A sample was taken for determining by 1H
NMR spectroscopy the conversion of the reaction. After cooling at
room temperature, the polymer was isolated by dissolution in
CH2Cl2 (2 mL), precipitated from excess diethyl ether (50 mL), fil-
trated, and dryed in vacuo.
injection volume, about 3 mg/ml of sample concentration.
DSC analyses under N2 (80 mL/min) atmosphere were per-
formed with
a Mettler DSC 823e calorimeter. Each sample
(4.5 mg 0.05 mg) was kept at 50 ꢀC for 5 min, cooled to ꢁ85 ꢀC at
10 ꢀC/min, kept 10 min at this temperature, heated up to 50 ꢀC at
5 ꢀC/min.
TGA tests under flowing N2 (60 mL/min) were performed with a
Mettler TGA SDTA/851 instrument according to the standard
method ISO9924-1. Samples (10 mg) were heated from 30 to
300 ꢀC at 10 ꢀC/min, kept at 300 ꢀC for 10 min, and then heated up
to 550 ꢀC at 20 ꢀC/min. After being maintained at 550 ꢀC for 15 min,
they were further heated up to 650 ꢀC with an heating rate of 30 ꢀC/
min and kept at 650 ꢀC for 20 min under flowing air (60 mL/min).
The FT-ATR spectra were recorded between 450 and 4000 cmꢁ1
by using a Perkin Elmer FT-IR Spectrum One equipped with Uni-
versal ATR Sampling Accessory with diamond crystal.
HRTEM analysis of the MWCNT adduct with the PU sample
containing the pyrrole ring was performed with a Zeiss Libra® 200
FE microscope (Carl Zeiss AG, Oberkochen, Germany) operating at
200 kV and equipped with an in-column OMEGA filter for energy
selective imaging and diffraction. Few drops of acetone diluted
suspension of the sample were deposited on a holey carbon film
supported on a standard Cu grid and air-dried for several hours
before analysis.
4.5. Preparation of MWCNT-PU adducts
Dispersion of carbon nanotubes in acetone was prepared (1 mg/
mL) sonicating for 30 min in a 2 L ultrasonic bath (power 260 W). A
PU solution in acetone (1 mg/mL) was added to the previously
obtained instable suspension. Upon sonicating for 30 min, a sus-
pension of MWCNT/PU adduct with a concentration of 1 mg/mL
was obtained. To verify the stability of such suspension, centrifu-
gation at 5000 rpm was performed for 5 min. The MWCNT-PU
adduct powder was obtained by drying the stable suspension at
reduced pressure. The mass% of PU in MWCNT-PU adducts was
determined to be 46 0.5 for adducts prepared from Entries 2, 3, 4b
and 5b.
Wide-angle X-ray diffraction (WAXD) patterns were obtained in
reflection, with an automatic Bruker D8 Advance diffractometer,
4.6. Extraction tests with ethyl acetate
with nickel filtered CueK
a
radiation. Patterns were recorded in
the peak diffraction angle.
100 mg of the MWCNT-PU adduct powder were placed in a
round bottomed flask (50 mL) equipped with a magnetic stirring
bar and ethyl acetate (25 mL) was added. The ensuing suspension,
after being stirred overnight at room temperature, was centrifuged
at 5000 rpm for 30 min, and dried under vacuum. The so obtained
black powder was analyzed by TGA.
10ꢀe100ꢀ as the 2
q
range, being 2q
Distance between crystallographic planes was calculated from the
Bragg law. The Dhk[ correlation length, in the direction perpendic-
ular to the hkl crystal graphitic planes, was determined applying
the Scherrer equation
Dhk[ ¼ Kl=ðbhk[ cos qhk[
Þ
(1)
4.7. Investigation of the PU-CNT interaction: 1H NMR experiments
where: K is the Scherrer constant,
l
is the wavelength of the irra-
diating beam (1.5419 Å, Cu-Ka), bhk[ is the width at half height, and
An NMR tube was charged with 1.5 mL of a previously prepared
solution containing SP (10 mg) in CDCl3. The tube was inserted into
the probe of the NMR spectrometer. After having performed an 1H
experiment at 298 K, increasing amounts of perylene (1 mg ꢂ 5
times) were added to the solution containing SP. NMR spectra were
recorded each time after 5 min the addition of perylene.
qhk[ is the diffraction angle. The instrumental broadening, b, was
determined by obtaining a WAXD pattern of a standard silicon
powder 325 mesh (99%), under the same experimental conditions.
The width at half height, bhk[ ¼ (Bhk[ ꢁ b) was corrected, for each
observed reflection with bhk[ < 1ꢀ, by subtracting the instrumental
broadening of the closest silicon reflection from the experimental
Perylene (400 MHz, CDCl3,
d in ppm): 7.47 (t); 7.69 (d); 8.20 (d).
width at half height, Bhk[
.
Perylene-SP adduct (10 mg SP/5 mg perylene) (400 MHz, CDCl3,
in ppm): 2.27 (s, 6H, eCH3 at C-2,5 of pyrrole moiety); 3.97 (m, 4H,
UVeVis absorption measurements were made using a Hawlett
Packard 8452A Diode Array Spectrophotometer.
d
CH2OH); 4.41 (quintet, 1H, at C-3 of diol); 5.78 (s, 2H, C-3,4 of
pyrrole moiety); 7.46 (t); 7.68 (d); 8.19 (d).
Acknowledgements
4.8. Characterization
“PRIN Research Project 2010-2011” is acknowledged for the
financial support.
Authors are in debt with Dr. Davide Gentile, for donating the
picture for the background of graphical abstract.
1H NMR and 13C NMR spectra were recorded on a Bruker
400 MHz (100 MHz 13C) instrument at 298 K. Chemical shifts were
reported in ppm with the solvent residual peak as internal standard
(DMSO-d6: dH ¼ 2.50 ppm, CDCl3: dH ¼ 7.26 ppm). Centrifugations
were performed using an ALC e Centrifugette 4206.
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The number average molecular mass (Mn), weight-average
molecular mass (Mw) and polydispersity index (Mw=Mn) were
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