Imidazolium poly(butylene terephthalate) ionomers with long-term antimicrobial activity

Article abstract of DOI:10.1016/j.polymer.2012.03.003

Imidazolium poly(butylene terephthalate) ionomers with ionic groups located randomly along the polymer chain or selectively as end-groups (telechelic) have been prepared in order to determine their antimicrobial (AM) activity. Two different approaches hav

Full text of DOI:10.1016/j.polymer.2012.03.003

Polymer 53 (2012) 1823e1830
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Polymer
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Imidazolium poly(butylene terephthalate) ionomers with long-term
antimicrobial activity
,
a
a
a
a
Martino Colonna a , Corrado Berti , Enrico Binassi , Maurizio Fiorini , Simone Sullalti ,
*
Francesco Acquasanta a, Micaela Vannini a, Diana Di Gioia b, Irene Aloisio b
a Dipartimento di Ingegneria Civile, Ambientale e dei Materiali, Via Terracini 28, 40131 Bologna, Italy
b Dipartimento di Scienze e Tecnologie Agroambientali, Viale Fanin 42, 40127 Bologna, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Imidazolium poly(butylene terephthalate) ionomers with ionic groups located randomly along the
polymer chain or selectively as end-groups (telechelic) have been prepared in order to determine their
antimicrobial (AM) activity. Two different approaches have been followed for the linkage of the imida-
zolium to the polymer backbone: a covalent bond and an ionic aggregation to sulfonated groups cova-
lently bonded to the polymer. The ionic groups have been linked to the polymer in order to improve the
long-term AM activity since the low molecular weight additives commonly used tends to migrate toward
the surface during use. We have found that imidazolium ionomers present AM activity comparable with
that of commercial antimicrobial agents such as Triclosan. The AM activity depends on the polymer
architecture, the telechelic approach being more active compared to the random approach. We have
proved that imidazolium ionomers retain their high AM activity even after 6 days in water at 60 ꢀC while
Triclosan consistently loses his activity.
Received 3 January 2012
Received in revised form
27 February 2012
Accepted 1 March 2012
Available online 8 March 2012
Keywords:
Antimicrobial activity
Imidazolium salt
Terephthalate polyester
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
the deposition of a multilayer film containing cetrimide and silver
have been used [7]. Chen et al reported [8] the surface grafting
Terephthalate polyesters such as poly(ethylene terephthalate)
(PET) are widely used as fibers for the textile industry [1] and it is
clear the need of an antimicrobial (AM) treatment that perma-
nently inhibits the growth of microorganism that produces unde-
sired odors. Poly(butylene terephthalate) (PBT) is also used in
several applications, such as electrical and electronic devices used
in hospitals, in which AM activity is also needed [2]. The AM agents
commonly used (Triclosan and silver nanoparticles) [3,4] tends to
migrate during use and washing and therefore the AM activity
decreases during the lifetime of the polymer application. For this
reason, in the last years, several researchers have focused their
attention on the preparation of antimicrobial agents bonded to the
polymer backbone [5,6]. However, most of the works reported in
the literature [6] deals with vinyl polymers and the antimicrobial
agents are inserted in the polymer backbone by polyaddition
reactions at low temperature. In order to attach AM agent to
polycondensation polymers such as PET, complex methods such as
polymerization of N-vinyl-2-pyrrolidone onto PET by plasma
treatment with silver nanoparticles observing, only qualitatively,
an increased antimicrobial activity against Staphylococcus aureus
with respect to the untreated PET.
An alternative and straightforward route to impart permanent
AM conditions to a polycondensation polymer consists in the
linkage of a quaternary ammonium cation to the polymer back-
bone, since ammonium cations have proved to be particularly
active as antimicrobial agents [9] and are used for industrial AM
applications [10]. However, quaternary ammonium cations are not
stable enough to be blended or incorporated in a terephthalate
polyester chain since they degrade at the normal synthesis and
processing conditions of semi-aromatic polyesters [11]. On the
contrary, imidazolium salts are consistently more stable (up to
350 ꢀC depending on the counter-ion) [12,13] and can sustain the
processing and synthesis conditions of terephthalate polyesters
[14]. Moreover, imidazolium salts have proved to be very active as
microbicidal agents [15,16]. The effect of substituents of the imi-
dazolium moiety on the antibacterial activity of imidazolium salts
has been reported in the literature [16]. In particular, imidazolium
salts bearing long alkyl chains (with 15e18 methylene groups) can
more easily interact with the cell membrane of bacteria causing loss
* Corresponding author. Tel.: þ39 512090331; fax: þ39 512090322.
E-mail addresses: martino.colonna@mail.ing.unibo.it, martino.colonna@unibo.it
(M. Colonna).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.03.003

1824
M. Colonna et al. / Polymer 53 (2012) 1823e1830
of permeability with consequent cell death and therefore present
the strongest bactericidal properties [16]. Imidazolium salt deriv-
atives have also been used in the last few years in several biological
and material science applications and represent a very interesting
class of functional polymers for advanced applications. For
example, a recent review [17] reports that polyimidazoles can be
used as oxygen transport membranes [18] or as scaffold for
biomimetic applications [19]. Imidazolium ionomers can also be
used to create polyelectrolyte brushes [20], coat metal nano-
particles [21] and produce oriented liquid crystals [22]. Imidazo-
lium derivatives have been used in most applications as additives
blended with the polymer matrix [15].
We have recently reported [13] the synthesis and the study of
AM properties of imidazolium telechelic PBT (with ionic moieties
selectively located as chain ends). Imidazolium ionomers have
proved to have good AM properties [13] with a 97.5% reduction of
viable cells of S. aureus and a 48.5% of Escherichia coli in 24 h.
However, the synthesis of the monomer used to prepare the tel-
echelic imidazolium ionomer needs several steps with long reac-
tion times (over 70 h for the monomer synthesis) [13] and for this
reason we have started the study of alternative synthetic
strategies.
The synthesis of PBT ionomers with sulfonated groups is widely
reported in the literature [23e25] and consists in the addition of
a sulfonated aromatic salt containing one or two carboxylic acid or
ester moieties during a standard polycondensation process. The
cation of the sulfobenzoic acid derivative can be easily exchanged
with other cations such as imidazolium cations [26]. The prepara-
tion of the imidazolium ionic monomer in this case is easier and
requires less expensive reactants [23e25] with respect to the
monomer used for the covalent bond of the imidazolium that we
have recently reported [13]. With this approach, the imidazolium
group is ionically linked to the sulfonate group covalently inserted
in the polymer chain. However, since the ionic groups are not
covalently bonded to the polymer chain, a higher mobility with
respect to the covalent linking previously reported [13] of the AM
agent should be expected, especially in the presence of water and
moisture. For this reason, a study of long-term AM activity is also
crucial in order to verify the possible decrease of AM activity during
use. Moreover, the use of this new synthetic strategy can also allow
the preparation of random ionomers in which the ionic groups are
randomly distributed along the polymer chain. The comparison
between random and telechelic ionomers AM activities is also of
interest since permits to better understand the effect of the position
of imidazolium groups thus understanding the best polymer
architecture to be used in industrial AM applications.
2.2. Monomer synthesis
2.2.1. 1-hexadecyl-3-methyl imidazolium bromide synthesis
1-methylimidazole (8.21 g, 0.100 mol) and 1-bromohexadecane
(32.1 g, 0.105 mol) were dissolved in 100 mL of toluene. The solu-
tionwas put under vigorous stirring and refluxed for 6 h. The solvent
was removed under reduced pressure and the solid was washed
twice with ethyl acetate (200 mL each time) and dried under
vacuum. (yield ¼ 99%) 1H NMR (400 MHz, CDCl3/TFA, 4/1,
d, ppm):
0.88 (t, J ¼ 6.8 Hz, 3H, CH3-C15chain), 1.20e1.44 (m, 26H, CH2), 1.89
(m, 2H, CH2eCH2eN), 3.98 (s, 3H, CH3-N), 4.19 (t, J ¼ 7.5 Hz, 2H, CH2-
N), 7.28 (bs, 1H, CH in imidazolium ring), 7.29 (bs, 1H, CH in imida-
zolium ring), 8.70 (s, 1H, NeCHeN in imidazolium ring). 13C NMR
(100 MHz, CDCl3/TFA, 4/1, d, ppm): 13.92 (CH3-C15chain), 22.73,
26.22, 28.91, 29.34, 29.44, 29.51, 29.63, 29.70, 29.74, 29.76, 30.15,
32.01 (CH2), 36.59 (CH3-N), 50.68 (CH2-N), 122.30, 123.76, (CH in
imidazolium ring), 135.78 (NeCHeN in imidazolium ring).
2.2.2. 3-sulfobenzoic acid 1-hexadecyl-3-methyl imidazolium salt
(Im-SBA) synthesis
1-hexadecyl-3-methyl imidazolium
bromide
(5.00 g,
0.0129 mol) was dissolved in 65 mL of dichloromethane (DCM) and
added to a solution of sulfobenzoic acid sodium salt (3.18 g,
0.0142 mol) dissolved in 50 mL of water in a separating funnel. The
content of the funnel was vigorously shaken for 5 min until no
precipitate was present in the resulting two-phase mixture. The
organic layer was separated and a silver nitrate test was performed
to verify the complete exchange of the bromide counter-ion. If the
exchange was not complete, the organic layer was exchanged with
a new water solution containing the sulfobenzoic acid salt. The
organic layer was separated, dried over magnesium sulfate and the
solvent removed under reduced pressure. The yellow solid was
washed twice with ethyl acetate (100 mL each time). The product
was dried under vacuum (yield ¼ 95%). 1H NMR (400 MHz, CDCl3/
TFA, 4/1,
d, ppm): 0.88 (t, J ¼ 6.9 Hz, 3H, CH3-C15chain), 1.20e1.40
(m, 26H, CH2), 1.88 (t, J ¼ 7.1 Hz 2H, CH2eCH2eN), 3.97 (s, 3H,
CH3-N), 4.18 (t, J ¼ 7.5 Hz, 2H, CH2-N), 7.29 (dd, J1 ¼ 1.6 Hz,
J2 ¼ 1.7 Hz, 1H, CH in imidazolium ring), 7.31 (dd, J1 ¼ 1.6 Hz,
J2 ¼ 1.6 Hz, 1H, CH in imidazolium ring), 7.64 (dd, J1 ¼ 7.9 Hz,
J2 ¼ 8.0 Hz, 1H, CH in benzene ring, meta-position with respect to
COOH substituent), 8.17 (dd, J1 ¼ 7.9 Hz, J2 ¼ 1.6 Hz, J3 ¼ 1.2 Hz, 1H,
CH in benzene ring, ortho-position with respect to COOH substit-
uent), 8.26 (dd, J1 ¼7.9 Hz, J2 ¼ 1.6 Hz, J3 ¼ 1.1 Hz, 1H, CH in benzene
ring, ortho-position with respect to SO3 substituent), 8.52 (s, 1H,
NeCHeN in imidazolium ring), 8.56 (dd, J1 ¼1.6 Hz, J2 ¼ 1.5 Hz, 1H,
CH in benzene ring, ortho-position between COOH and SO3
In this paper we report for the first time the synthesis and the
characterization of two new imidazolium ionomers together with
the first study of the AM activity of different polymer architectures
(random or telechelic) and linkages (covalent or ionic) of the imi-
dazolium groups to the polymer chain. We also report the long-
term antimicrobial properties after immersion in water for 6 days
at 60 ꢀC of imidazolium PBT ionomers in comparison to a tradi-
tional AM agent.
substituents). 13C NMR (100 MHz, CDCl3/TFA, 4/1,
d, ppm): 13.91
(CH3-C15chain), 22.70, 26.15, 28.85, 29.30, 29.40, 29.47, 29.59,
29.66, 29.70, 29.72, 30.04, 31.97 (CH2), 36.30 (CH3-N), 50.52 (CH2-
N), 122.38, 123.87 (CH in imidazolium ring), 127.89 (CH in benzene
ring, ortho-position between COOH and SO3 substituents), 129.00
(C-COOH), 129.62, 131.85, 133.65 (CH in benzene ring), 135.38
(NeCHeN in imidazolium ring), 142.22 (C-SOꢁ3 ), 171.30 (COOH).
2.2.3. Dimethyl-5-sulfoisophthalate 1-hexadecyl-3-methyl
imidazolium salt (Im-DMSIP) synthesis
2. Experimental
1-hexadecyl-3-methyl
imidazolium
bromide
(10.0
g,
2.1. Materials
0.0258 mol) was dissolved in 130 mL of dichloromethane (DCM) and
added to a solution of dimethyl-5-sulfoisophthalate sodium salt
(8.02 g, 0.0271 mol) dissolved in 400 mL of water in a separating
funnel. The content of the funnel was vigorously shaken for 5 min
until no precipitate was present in the resulting two-phase mixture.
The organic layer was separated and a silver nitrate test was per-
formed to verify the complete exchange of the bromide counter-ion.
If the exchange was not complete, the organic layer was exchanged
Potassium hydroxide, 1-methylimidazole, 1-bromohexadecane,
1,4-butanediol (BD), dimethyl terephthalate (DMT), titanium tet-
rabutoxide, dichloromethane, 3-sulfobenzoic acid sodium salt,
dimethyl-5-sulfoisophthalate sodium salt and Triclosan (5-chloro-
2-(2,4-dichlorophenoxy)phenol) (all from Aldrich Chemicals) were
high purity products and were not purified before use.

M. Colonna et al. / Polymer 53 (2012) 1823e1830
1825
with
a
new water solution containing the dimethyl-5-
a CF3COOD/CDCl3 mixture (1/4, v/v) as solvent. The spectra have
been recorded just after dissolution in order to avoid the esterifi-
cation reaction of end-groups with trifluoroacetic acid.
Calorimetric measurements have been performed using a Perkin
Elmer DSC6 instrument equipped with a liquid sub ambient
accessory and calibrated with high purity standards (indium and
cyclohexane). Dry nitrogen was used as purge gas. The heating and
cooling rate were 20 ꢀC/min. All transitions have been measured
after a heating scan to 250 ꢀC and cooling down to room temper-
ature in order to delete previous thermal history.
sulfoisophthalate sodium salt. The organic layer was separated,
dried over magnesium sulfate and the solvent removed under
reduced pressure. The yellow solid was washed twice with ethyl
acetate (200 mL each time). The product was dried under vacuum
(yield ¼ 95%). 1H NMR (400 MHz, CDCl3/TFA, 4/1,
d, ppm): 0.88 (t,
J ¼ 6.9 Hz, 3H, CH3-C15chain), 1.20e1.38 (m, 26H, CH2), 1.88 (t,
J ¼ 7.3 Hz 2H, CH2eCH2eN), 3.98 (s, 3H, CH3-N), 4.04 (s, 6H, CH3-O),
4.18 (t, J ¼ 7.3 Hz, 2H, CH2-N), 7.29 (dd, J1 ¼1.6 Hz, J2 ¼ 1.8 Hz,1H, CH
in imidazolium ring), 7.32 (dd, J1 ¼ 1.6 Hz, J2 ¼ 1.8 Hz, 1H, CH in
imidazolium ring), 8.54 (bs, 1H, NeCHeN in imidazolium ring), 8.74
(d, J ¼ 1.6 Hz, 2H, CH in benzene ring, ortho-position with respect to
SO3 substituent), 8.81 (d, J ¼ 1.6 Hz, 1H, CH in benzene ring, ortho-
position between COOCH3 substituents). 13C NMR (100 MHz, CDCl3/
The thermogravimetric analyses (TGA) were performed using
a Perkin Elmer TGA7 apparatus in nitrogen (gas flow: 40 mL/min) at
10 ꢀC/min heating rate, from 25 ꢀC to 800 ꢀC.
DMTA analyses were performed with a Rheometrics dynamic
mechanic thermal analyzer DMTA 3E with a dual cantilever testing
geometry. Typical test samples were bars that were injection molded
at 275 ꢀC using a Minimax Molder (Custom Scientific Instruments)
equipped with a rectangular mold (30 ꢂ 8 ꢂ 1.6 mm3). The testing
was done at a frequency of 3 Hz and temperature range was
from ꢁ50 ꢀC to 200 ꢀC at a rate of 3 ꢀC/min.
TFA, 4/1, d, ppm): 13.90 (CH3-C15chain), 22.69, 26.15, 28.85, 29.29,
29.40, 29.47, 29.59, 29.66, 29.70, 29.72, 30.04, 31.97 (CH2), 36.30
(CH3-N), 50.52 (CH2-N), 53.63 (CH3-O), 122.39, 123.89 (CH in imi-
dazolium ring), 131.00 (C-COOCH3), 131.97 (CH in benzene ring,
ortho-position with respect to SO3 substituent), 133.77 (CH in
benzene ring, ortho-position between COOCH3 substituents),135.36
(NeCHeN in imidazolium ring), 143.34 (C-SOꢁ3 ), 167.18 (COOCH3).
Inherent viscosities (IV) were measured at 25 ꢀC with an Ubbel-
hode viscometerusinga 0.05M solution ofbenzyltriethylammonium
chloride in 1,1,2,2-tetrachloroethane/phenol 60/40 wt/wt. The salt
was used in order to suppress ionic aggregation between chains
which can induce an overestimation of molecular weight.
2.3. Ionomer synthesis
Polymerizations were carried out by using a two-stage process
in a 1.8 L stainless steel batch reactor equipped with a paddle stirrer
(driven at 60 rpm) and a strain-gauge sensor mounted on the stirrer
shaft in order to monitor the viscosity of the reaction melt (and
indirectly the increase of PBT molecular weight) during the poly-
merization. Two condensers in series (the first water-cooled and
the second liquid nitrogen-cooled) were connected to the reactor to
collect volatile products during the first and second stages. A typical
polymerization procedure is described below.
First stage: BD (742.0 g, 8.25 mol), DMT (800.0 g, 4.12 mol) and
the imidazolium salt (in different amounts in order to insert ionic
groups at a content of 0.6, 1 and 2 mol% with respect to DMT) were
loaded into the reactor and then heated at atmospheric pressure
under stirring to 180 ꢀC. At this temperature, titanium butoxide
(1.13 g, 3.31 mmol, 175 ppm as titanium with respect to the final
polymer weight) was introduced into the reactor. The reaction
temperature was increased from 180 to 215 ꢀC. Volatile products
(methanol and THF) were distilled off from the reactor, condensed
in the water-cooled condenser and collected in a graduated
cylinder. The start of the first stage was taken when the first drop of
liquid was collected in the water-cooled condenser. The tempera-
ture was then kept at 215 ꢀC until 95% of the theoretical amount of
methanol was distilled off. The distilled volume was recorded
versus time as an indicator of the catalytic activity during the first
stage and distillate samples were analyzed by 1H NMR in order to
measure the methanol/THF ratio.
2.5. Antimicrobial properties
A Gram-positive strain, S. aureus ATCC 6538, and a Gram-
negative strain, E. coli ATCC 25645, were used to evaluate the
antimicrobial properties of the polymers. The specimens employed
were of the same type used for the DMTA analyses (i.e. containing
ionic end-groups at 0.6, 1 and 2 mol% and Triclosan at 0.6, 1 and
2 mol%) plus a control specimen not containing any agent. Each
strain was grown aerobically in Nutrient Broth (NB, provided by
Becton, Dickinson and Company, Maryland, USA) for 18 h at 37 ꢀC.
The culture thus obtained was centrifuged at 7000 rpm for 10 min,
the pellet was washed in sterile saline (8.5 g NaCl/L) and resus-
pended in saline in order to obtain a cell concentration of 108 CFU/
mL. An agar slurry solution was prepared dissolving 0.3 g agar
(Biolife Italia, Milan, Italy) in 100 mL saline; the solution was
autoclaved at 121 ꢀC for 30 min, cooled at about 40 ꢀC and inocu-
lated with 1 mL of the prepared cell suspension. The specimens
were pre-wetted with sterile saline, placed into a Petri dish and
0.25 mL of inoculated agar slurry solution were pipette onto the
specimen. The agar slurry layer (thickness lower than 1 mm)
guarantees an optimal contact between the inoculum and the
specimen surface. The slurry layer was allowed to dry, and then the
Petri dishes containing the inoculated specimens were placed on
a capped box in which a 0.5 mm of water was layered in order to
avoid sample drying during incubation. Incubation was performed
at room temperature for 24 h. After incubation, the specimen was
placed in a StomacherÒ Bag (Seward, Worthing, United Kingdom)
with 10 mL of saline, vigorously shaken for 3 min in a Stomacher
Bag blender to allow the complete release of agar slurry from the
sample. Serial dilutions of the solution obtained after the shaking
procedure were performed, plated on Plate Count Agar (Biolife)
plates which were incubated for 48 h at room temperature. Each
specimen was processed in triplicate. The percentage of reduction
of viable cells was calculated using the following equation:
Second stage: the internal pressure was slowly reduced, first
from atmospheric pressure down to 10 mbar in 30 min, then from
10 mbar to 1 mbar in 20 min. At the same time the temperature of
the reaction melt was increased to 240 ꢀC and kept at this
temperature until the end of the polymerization. The start of
second stage was taken when the minimum pressure was reached.
The second stage was stopped when no further significant increase
in strain-gauge signal was detected.
Triclosan was blended with PBT in a Brabender Plasticorder
PL2000/W50 mixer for 15 min at a temperature of 260 ꢀC.
%Reduction ¼ ða ꢁ bÞ ꢂ 100=a
2.4. Instrumental
where a is the number of viable cells in the control specimen and
b is the number of viable cells in the specimens containing ionic
end-groups or Triclosan.
1H NMR spectra were recorded using a Varian Mercury 400
spectrometer (chemical shifts are downfield from TMS), using

1826
M. Colonna et al. / Polymer 53 (2012) 1823e1830
Fig. 1. Ionomers architectures and monomers used for the synthesis where Imþ is the imidazolium salt and Rꢁ is the organic anion and n is the degree of polymerization.
The antimicrobial activity of the PBT specimens containing 2%
Triclosan and 2% telechelic imidazolium was also evaluated via an
agar plate test. To perform this assay, 10 mL of NB containing 1.5%
w/v agar were poured on Petri dishes and allowed to solidify. The
specimen bars were then laid down on the agar surfaces, which
were covered with 10 mL of soft agar (i.e. agar with 0.7% w/v agar)
inoculated with 107 CFU/ml of S. aureus ATCC 6538. After solidifi-
cation of the soft agar, the plates were incubated at 37 ꢀC for 24 h.
Pictures of the plates were performed with a digital camera.
The evaluation of long-term AM activity has been conducted by
placing the molded bars of ionomers and of PBT with Triclosan on
the bottom of a 2 L glass beaker covered with a glass cap and
containing 1 L of distilled water, for 6 days at 60 ꢀC under stirring
and measuring the AM activity against S. aureus before and after the
exposure to water.
3. Results and discussion
The three different polymer architectures studied in this work
are reported in Fig. 1 along with the ionic monomers used for their
synthesis.
We have recently reported [13] the synthesis of the telechelic
covalent monomer. The synthesis of the other two architectures
have been performed by simple ioneexchange reactions between
1-methyl-3-hexadecyl imidazolium bromide and sulfobenzoic acid
or dimethyl-5-sulfoisophthalate sodium salts (Fig. 2).
Fig. 2. Synthesis of monomers for random ionic and ionic telechelic approaches.

M. Colonna et al. / Polymer 53 (2012) 1823e1830
1827
Fig. 3. TGA of the ionic monomers.
Imidazolium salts bearing an alkyl chain with 16 carbon atoms
have been used in all the experiments. The long alkyl chain is
necessary in order to have a good solubility in the monomers
mixture and for antimicrobial activity [16].
The TGA analysis (Fig. 3) shows that all of the three monomers
selected are stable up to 300 ꢀC and therefore do not degrade
during the polymerization. Bromine salts on the contrary are not
stable above 240 ꢀC and are not therefore suitable as monomers for
the synthesis of telechelic ionomers [27]. The choice of tosylate
salts for the covalent approach is due to the inhibiting effect of the
PF6, BF4, Br and Triflate salts on the titanium based catalyst used for
the polycondensation [13].
The ionic monomers have been added at the beginning of
a standard two stages polycondensation process. PBT ionomers
with 0.6, 1 and 2 mol% of ionic groups have been prepared in a lab
scale 1.8 L pilot plant for each type of the three different
approaches. The measure of the methanol distilled during first
stage (Fig. 4) shows that all the three salts used have no inhibiting
effect on the polycondensation catalyst and 95% conversion in first
stage can be achieved with similar reaction times with respect to
the standard polycondensation without ionic groups. The second
stage under vacuum was conducted for the same reaction time (2 h)
used for standard PBT polycondensation. The final polymer melts
were all clear and no discoloration was observed for all the ion-
omers prepared.
The 1H NMR analysis of the final ionomers (Figs. 5 and 6) shows
that almost no degradation occurs and no vinyl end-groups are
present. The incorporation of the ionic groups in the polymer chain
was confirmed by NMR analysis after dissolution in a chloroform/
hexafluoroisopropanol (95/5 v/v) solution and precipitation in
methanol (that is a solvent for the ionic monomers) for all the three
polymer architectures. The amount of ionic groups has been
calculated considering the signal at
random ionic approach, the signal at
telechelic ionic approach and using the method already reported in
literature [13] for the telechelic covalent approach.
d
¼ 8.80 ppm (1H) for the
d
¼ 7.60 ppm (1H) for the
The molecular weight has been measured using both inherent
viscosity and by end-groups analysis by 1H NMR. GPC has not been
used since ionic aggregation and interactions with the column can
occur during elution [28]. We have previously found that GPC
results are not reliable for PBT ionomers and depend on the sample
concentration [24,29]. On the contrary, adding
a benzyl-
triethylammonium chloride salt to the inherent viscosity solvent,
ionic aggregations are disrupted [29,30]. The amount of end-groups
Fig. 4. Methanol conversion during first stage polycondensation.

1828
M. Colonna et al. / Polymer 53 (2012) 1823e1830
CHCl3
CDCl3
+
l, n
O
O
O
n
O
f
O
m
l
O
n
i
e
SO3
a
c
N
N
13
h
g
b
d
methyl ester
end groups
h
hydroxy-
methylene
end groups
d
e, f
m
g, i
6.0
Fig. 5. 1H NMR of 2% telechelic ionic PBT.
5.0
9.0
7.0
4.0
8.0
8.
CHCl3
CDCl3
l
O
O
m
O
O
i
O
O
O
f
O
n
m
e
SO3
a
c
N
N
13
h
g
b
d
methyl ester
end groups
+ i
h
hydroxy-
methylene
end groups
e, f
d
m
g
l
9.0
9.
7.0
7.
6.0
6.
5.0
5.
4.0
4.
8.0
8.
Fig. 6. 1H NMR of 2% random ionic PBT.
has been calculated by 1H NMR taking into account methyl ester
end-groups (3H at 4.05 ppm), OH end-groups (2H at
d
¼
Table 1
Molecular and thermal properties of imidazolium ionomers.
d
¼ 3.80 ppm) and the ionic end-groups when present. The
molecular weight has been calculated comparing the total end-
groups content with the signal of the terephthalate group (4H at
Polymer
Inherent viscosity Mn NMR Tonset (ꢀC)a Tm (ꢀC)b
PBT control
0.96
0.96
0.92
0.91
0.89
0.80
0.67
31,800
34,300
30,100
31,600
29,100
22,500
15,200
20,800
20,100
13,200
399
402
397
397
400
391
391
403
399
396
220
219
219
219
223
222
220
222
221
223
d
¼ 8.15 ppm).
Random ionic 0.6%
Random ionic 1%
Random ionic 2%
Telechelic ionic 0.6%
Telechelic ionic 1%
Telechelic ionic 2%
Telechelic covalent 0.6% 0.77
Telechelic covalent 1%
Telechelic covalent 2%
Both the inherent viscosity measurements and end-groups
analysis have shown (Table 1) that the molecular weight of the
random ionomers is comparable to that of standard PBT while for
the telechelic is, as expected, lower depending on the amount of
ionic end-groups added.
The thermal stability of the ionomers (Table 1) is similar with
respect to PBT, slightly decreasing with ionic content and signifi-
cantly higher with respect to the processing temperature of PBT
(250e275 ꢀC).
0.75
0.60
a
Onset degradation temperature measured by TGA at 10 ꢀC/min.
Melting temperature measured during second DSC heating scan at 20 ꢀC/min.
b

M. Colonna et al. / Polymer 53 (2012) 1823e1830
1829
Fig. 7. DMTA analysis of ionomers and standard PBT.
The storage modulus of ionomers has been measured by DMTA
analysis (Fig. 7). The results show that the storage modulus of 2%
ionomers is similar below Tg and slightly higher over Tg compared
to that of standard PBT.
The AM activity of the prepared polymers containing ionic end-
groups and Triclosan at different concentrations was tested against
S. aureus and E. coli, a Gram-positive and a Gram-negative strain,
respectively. The choice of Triclosan as a reference AM agent is
related to its wide use as AM agent not only for health care products
but also for fabrics and plastic materials. In particular is reported in
the literature that Triclosan is among the 3 most used AM agents for
plastics [31].
The results are presented in Table 2. The AM activity is generally
higher against S. aureus, as already evidenced in a previous work
[13], reaching almost complete inhibition in the presence of 2%
telechelic ionic and telechelic covalent bound imidazolium salts. A
greater reduction of viable cells on the telechelic ionic and covalent
polymers was evidenced with respect to the random ionic speci-
mens. The lower activity of the random ionomers is probably due to
the fact that the imidazolium is more hindered by the polymer
backbone and therefore less available to exert its AM activity. No
significant difference has been observed between AM tests on the
telechelic ionic and on telechelic covalent polymers. The position of
the imidazolium (telechelic or random) seems to be the key factor
affecting the AM activity while the type of linkage with the polymer
backbone (ionic-covalent) has a minor effect. The activity of the
telechelic covalent and ionic polymers on the Gram-positive strain
is comparable to that of Triclosan.
surface, which correspond to the zone above the sample in Fig. 8.
This observation can allow us to speculate that the Triclosan
activity may be reduced with time due to diffusion of the molecule
in the surrounding environment, whereas the activity of the imi-
dazolium monomers can be more stable with time. In order to
verify this hypothesis, specimens containing Triclosan and imida-
zolium monomers were stirred in water for 6 days at 60 ꢀC and the
AM was tested after the treatment.
The results presented in Table 3 clearly show that the imida-
zolium ionomers retain their AM activity after 6 days of immersion
in water while PBT specimen containing Triclosan as the AM agent
loses more than 60% of its original AM activity. We have therefore
demonstrated that the AM activity of the telechelic covalent and
telechelic ionic PBT is stable after a 6 days washing treatment.
Toxicity studies described in the literature mainly regard ionic
liquids based on imidazolium cations, which evidence that the
cytotoxicity against human colon carcinoma cell lines is highly
dependent on the counter anion and on the length of any alkyl
chains on the nitrogens of the imidazole ring [33]. Imidazolium
salts were found to have anti-fibrotic properties in mouse hepatic
stellate cells and anti-metastic properties [17] making them
promising agents for the therapy of these pathologies. However,
imidazolium-based polymer toxicity studies represent a potential
area for future research.
Table 2
Antimicrobial activity against S. aureus and E. coli of imidazolium ionomers in
comparison with polymers with Triclosan as additive.
The AM activity of the imidazolium ionomers against E. coli is
lower with respect to S. aureus, reaching maximum values of 50%
reduction of viable cells. Considering the lower inhibition
percentages, it is more difficult to draw conclusions on the
importance of the position of the imidazolium salt within the
molecule using E. coli as the target organism. However, these results
are interesting considering the structure of Gram-negative cells
which makes them generally less susceptible to antimicrobial
agents [32].
The agar plate assay (Fig. 8) show significant difference in the
mode of action between the 2% Triclosan and 2% telechelic imida-
zolium specimens. Triclosan diffuses from the samples to the agar
and gives rise to an inhibition zone around the sample whereas the
imidazolium ionomers kill only the bacteria in contact with the
Sample
Reduction of viable cells (%)
Staphylococcus
aureus
Escherichia coli
Random ionic 0.60%
Random ionic 1.00%
Random ionic 2.00%
Telechelic ionic 0.60%
Telechelic ionic 1.00%
Telechelic ionic 2.00%
Telechelic covalent 0.60%
Telechelic covalent 1.00%
Telechelic covalent 2.00%
Triclosan 0.60%
45.2 ꢃ 3.8
44.0 ꢃ 2.9
40.0 ꢃ 2.5
85.3 ꢃ 5.4
82.5 ꢃ 3.8
97.5 ꢃ 4.2
89.4 ꢃ 5.7
75.0 ꢃ 6.8
97.5 ꢃ 6.4
83.5 ꢃ 3.6
91.2 ꢃ 3.0
99.4 ꢃ 2.7
48.4 ꢃ 5.2
41.9 ꢃ 5.8
48.8 ꢃ 3.8
56.4 ꢃ 4.3
47.7 ꢃ 2.7
47.5 ꢃ 3.4
26.3 ꢃ 2.9
24.5 ꢃ 3.2
48.5 ꢃ 5.4
42.9 ꢃ 3.8
75.5 ꢃ 5.4
74.6 ꢃ 2.3
Triclosan 1.00%
Triclosan 2.00%

1830
M. Colonna et al. / Polymer 53 (2012) 1823e1830
Fig. 8. Pictures obtained after the agar plate assay with PBT with 2% Triclosan (left) and PBT 2% telechelic ionic imidazolium (right).
Table 3
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Initial
After 6 days at 60 ꢀC
Telechelic covalent 2%
Telechelic ionic 2%
Random ionic 2%
Triclosan 2%
97.5 ꢃ 6.4
97.5 ꢃ 4.2
40.0 ꢃ 2.5
99.4 ꢃ 2.7
90.3 ꢃ 4.8
76.0 ꢃ 3.8
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4. Conclusions
Random and telechelic imidazolium ionomers have been
synthesized with ionic groups covalently and ionically bonded to
the polymer. The right choice of the counter-ion permits to obtain
high molecular weight and thermally stable ionomers. The pres-
ence of ionic groups has no significant effect on the thermo-
mechanical properties of PBT and therefore the ionic polymers
can be used for the same applications of traditional PBT.
Imidazolium ionic polymers present an antimicrobial activity
against S. aureus comparable with that of a commercial antimi-
crobial agent (Triclosan). We have found that imidazolium ion-
omers retain their high AM activity even after 6 days in water at
60 ꢀC while Triclosan additive loses most of its AM activity.
Although not comparable with the activity presented against
Gram-positive strains, the AM activity of the telechelic ionomers
against E. coli is also of great interest, considering that Gram-
negative strains are generally less susceptible to antimicrobial
agents.
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Acknowledgments
This work was financed by SABIC-IP.
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