Very fast CO2 response and hydrophobic properties of novel poly(ionic liquid)s

PAPER www.rsc.org/materials | Journal of Materials Chemistry

Very fast CO2 response and hydrophobic properties of novelpoly(ionic liquid)s†

Placido G. Mineo,a Letizia Livoti,b Marco Giannetto,c Antonino Gulino,a Sandra Lo Schiavob

and Paola Cardiano*b

Received 23rd June 2009, Accepted 18th August 2009

First published as an Advance Article on the web 19th October 2009

DOI: 10.1039/b912379b

A series of polymerizable tetraalkylammonium ionic liquids based on

[2-(methacryloyloxy)ethyl]dimethylheptyl ammonium cation and bis(trifluoromethylsulfonyl)imide,

nonafluoro-1-butanesulfonate, dodecylbenzenesulfonate, heptadecafluorooctanesulfonate,

4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecanoate anions, as well as their

corresponding homo- and copolymers, have been synthesized and characterized by means of 1H NMR,

TG-DTA, DSC, MALDI-TOF, viscosimetry and XPS investigations. Hydrophobic and CO2

sensing properties of the poly(ionic liquid)s have been explored by dynamic contact angle and quartz

crystal microbalance measurements. The CO2 sensing behavior of present polymers is very

remarkable as they are featured by extremely rapid and completely reversible response without any

memory-effect. Best results, in terms of sensitivity, have been obtained for [2-

(methacryloyloxy)ethyl]dimethylheptyl ammonium nonafluoro-1-butanesulfonate-based

homopolymer. Tensiometric data show good hydrophobic properties with qadv > 90� for all the

polymers under study except the one involving 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-

heptadecafluoroundecanoate anion (qadv ¼ 78.3�); receding contact angles, representative of the most

hydrophilic portions of the polymers surface, lie in the range 22–54� and can be further improved

by choosing the proper long-chained N-alkyl groups.

Introduction

Room temperature ionic liquids (ILs)1 are a class of compounds

exhibiting various unusual and attractive properties (i.e. very low

vapour pressure, large electrochemical window, wide liquid

phase range, thermal stability over a wide temperature range,

nonflammability, recyclability, high ion density, broad solvating

ability, environmentally friendliness, etc.). Their unique chemical

and physical properties, resulting from the proper combination

of organic cations and organic or inorganic anions, make them

absolutely unique and incomparable to other compounds, hence

their application is very broad as they have been used as reaction

media,2,3 catalysts,4 thermal fluids,5 energetic materials,6 fuel

cells,7,8 sensoristic applications,9–13 to name only a few. Despite

the huge amount of reports and patents concerning ILs, rela-

tively few papers have been published on ILs covalently bound to

a polymeric network and on their applications. These mainly

concern gas separation and sensoristic applications,14

polyelectrolyte15–17 and conductive flexible films synthesis,18 gels

aDipartimento di Scienze Chimiche, University of Catania, Viale A. Doria 6,95125 Catania, ItalybDipartimento di Chimica Inorganica, Chimica Analitica e Chimica Fisica,University of Messina, Salita Sperone 31 S. Agata, 98166 Messina, Italy.E-mail: [email protected] di Chimica Generale ed Inorganica, Chimica Analitica,Chimica Fisica, University of Parma, Viale G. P. Usberti 17/A, CampusUniversitario, 43124 Parma, Italy

† Electronic supplementary information (ESI) available: Syntheticprocedures and characterization data. See DOI: 10.1039/b912379b

This journal is ª The Royal Society of Chemistry 2009

and microgels synthesis for the entrapment of enzymes,19 nano-

particles stabilization.20,21 As a new application of poly(ionic

liquid)s (pILs), we recently reported on the synthesis

and hydrophobic properties of a series of pILs obtained by

free-radical polymerization of monomeric methacrylic trime-

thylethylammonium-based ILs.22 Dynamic contact angle inves-

tigations showed that most of them are featured by very

promising advancing contact angles (qadv > 110�), while the

receding ones were very low (qrec < 20�), indicating that on the

surface very low and high energy domains are present.

As a continuation of the cited work, in this paper we report on

the synthesis of a series of novel poly(ionic liquid)s involving the

long-chained dimethylethylheptyl ammonium cation and the

bis(trifluoromethylsulfonyl)imide, nonafluoro-1-butanesulfonate,

dodecylbenzenesulfonate, heptadecafluorooctanesulfonate,

4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecanoate

anions, entrapped into a methacrylic backbone. The new mate-

rials have been characterized by means of TG-DTA and DSC

analysis, as well as by solution NMR, MALDI-TOF and XPS

spectroscopy. The aim of this paper is twofold: on one hand, as

a continuation of the previous paper, to optimize the pILs

hydrophobic properties in terms of receding contact angles and

to clarify the role played by the cation on the hydrophobicity of

the resulting materials. Tensiometric measurements show that

the substitution of a methyl group on the quaternary nitrogen

with a heptylic one improves hydrophobic behavior of the

resulting films. The second aim, on the other hand, is to explore

the potentiality of the synthesized pILs as sorption materials for

CO2 sensing. It is well known that ionic liquids have remarkable

J. Mater. Chem., 2009, 19, 8861–8870 | 8861

CO2 sorption capacity and they have been found to behave as

non-volatile and reversible CO2 absorbents;9–13 in addition,

polymers prepared from ionic liquid monomers have shown even

higher CO2 sorption capability23 than the parent room temper-

ature ionic liquids, with faster response in terms of absorption/

desorption rates. It has been also found that ammonium-based

poly(ionic liquid)s have better CO2 sorption capacity than imi-

dazolium-based pILs.24,25 This paper reports also on quartz

crystal microbalance (QCM) investigations that clearly evidence

the remarkable CO2 sorption capabilities of the above new pILs

as well as the relationship between the sorption properties and

anion type.

Experimental

Materials

1-Bromoheptane, [2-(methacryloyloxy)ethyl]dimethylamine,

lithium bis(trifluoromethylsulfonyl)imide, potassium non-

afluoro-1-butanesulfonate, sodium dodecylbenzenesulfonate,

potassium heptadecafluorooctanesulfonate, potassium 4,4,5,5,

6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecanoate and

a,a0-azoisobutyronitrile (AIBN) were purchased from Sigma

Aldrich and used as received.

Measurement

1H NMR spectra were performed with a Bruker AMX R-300

spectrometer operating at 300.13 MHz. Thermogravimetric

analyses were performed by means of Perkin-Elmer Pyris Dia-

mond TG-DTA in the temperature range between 25 and 800 �C,

under nitrogen or air atmosphere (50 mL min�1) and with

a heating rate of 10 �C min�1. Differential scanning calorimetry

experiments were performed by using a TA Q100 instrument

equipped with a refrigerant cooling system (RCS) in the

temperature range between �90 and 50 �C, with a heating rate of

10 �C min�1, under an anhydrous N2 atmosphere (60 mL min�1).

The DSC measurements between 30 and 500 �C were performed

with a Mettler DSC 20S, under an anhydrous N2 atmosphere

(60 mL min�1), with a heating rate of 10 �C min�1. Inherent

viscosities of the polymers (hinh ¼ ln hr/c; c ¼ 0.5 g dL�1) were

measured in a Desreux–Bischoff suspended level viscometer, in

DMSO (samples P1, P2 and C1-5) or DMF (samples P3 and C1-4,

see later) at 50 �C. The positive MALDI-TOF mass spectra were

acquired by a Voyager DE (PerSeptive Biosystem) using a delay

extraction procedure (25 kV applied after 2600 ns with a poten-

tial gradient of 454 V mm�1 and a wire voltage of 25 V) with ion

detection in linear mode.26 The instrument was equipped with

a nitrogen laser (emission at 337 nm for 3 ns) and a flash AD

converter (time base 2). To avoid fragmentation of the polymers,

the laser irradiance was slightly above threshold (ca. 106 W cm�2).

The MALDI experiments were performed by loading on the

sample plate a 0.1 mmol sample and 40 mmol matrix trans-3-

indoleacrylic acid (IAA) or trans-2-[3-(4-tert-butylphenyl)-2-

methyl-2propenylidene]-malonitrile (DCTB), with DMSO

or DMF as solvent. Both 5,10-di(p-dodecanoxyphenyl)-15,20-

di(p-hydroxyphenyl) porphyrin (C68H78N4O4, 1014 Da), tetra-

kis(p-dodecanoxyphenyl)porphyrin (C92H126N4O4, 1350 Da)

and a PEG sample of known structure were used as external

standards for m/z calibration.27 Dynamic contact angle

8862 | J. Mater. Chem., 2009, 19, 8861–8870

measurements were performed on coated glass microscopy slides

(26 � 76 � 1 mm by Prestige) using the Wilhelmy method by

means of a KSV Sigma 700 tensiometer at the speed of 2 mm min�1

in ultra pure water. The glass slides were accurately cleaned by

sulfuric acid–K-dichromate cleaning solution before use, then

they were coated by dip coating method immersing the plates in

7.5 wt% acetone solution (for homopolymers P1 and P2),

7.5 wt% chloroform solution (for homopolymer P3), 3.75 wt%

acetone solution and 7.5 wt% acetone solution (for copolymers

C1-4 and C1-5, respectively) at a constant rate (20 mm min�1),

without delay time between immersion and withdrawn. After the

immersion cycles, the coated slides were aged for 10 days at 50 �C

and then contact angle measurements have been carried out. The

contact angles correspond to the average of the measurements

performed on five glass slides treated in the same way. X-Ray

photoelectron spectra (XPS) were measured at 45� take-off angle,

relative to the surface plane, with a PHI 5600 Multi Technique

System (base pressure of the main chamber 2 � 10�10 Torr).

The P1 sample was not suitable for XPS analysis because of

prolonged sample out-gassing. The spectrometer is equipped

with a dual anode X-ray source; a spherical capacitor analyzer

(SCA) with a mean diameter of 279.4 mm; an electrostatic lens

system Omni Focus III. The nominal analyzer resolution was set

to 400 meV. Samples were mounted on Pt stubs. Spectra were

excited with Al-Ka radiation. Structures due to satellite Ka2

radiation have been subtracted from the spectra before the data

processing. The XPS peak intensities were obtained after Shirley

background removal.28 Procedures to account for steady state

charging effect have been described elsewhere.29 Samples were

left overnight in the XPS antechamber for sample out-gassing

before spectra collection. Experimental uncertainties in binding

energies lie within �0.4 eV. Some spectra were deconvoluted by

fitting the experimental profiles with a series of symmetrical

Gaussian envelopes, after subtraction of the background. The

agreement factor, R ¼ [S(Fo � Fc)2/S(Fo)2]1/2, after minimization

of the function S(Fo � Fc)2 converged to R values #0.035.30

Quartz crystal microbalance (QCM) experiments were carried

out on quartz/chrome/gold PQCs (piezoelectric quartz crystals)

with a fundamental frequency of 7.98� 0.005 MHz, using a CHI

430 (CH Instrument, Texas) electrochemical workstation

supplied with QCM unit and managed by dedicated software.

The deposition of polymeric ionic liquids on the PQC units was

performed by simple drop-casting procedure. For this purpose

10 mL of the properly diluted solutions of P1–P3, C1-4 and C1-5

were deposed on the gold surface of the device and the solvent

was slowly allowed to evaporate under atmosphere saturated

with the solvent itself. The proper dilution factor of each casting

solution was determined starting from the concentrations

already employed for dip coating by progressive dilution until

reaching a suitable piezoelectric response. As a result, the dilu-

tion factors used for drop-casting solutions were 1 : 10 for P1 and

C1-4, and 1 : 20 for P2, P3 and C1-5. Retention properties of the

polymeric films towards CO2 were evaluated by exposing the

coated PQCs to a stream of a mixture of N2 (carrier gas) and

CO2. The composition of the stream conveyed to the detection

chamber was varied ranging from 100% of carrier gas to 100% of

CO2. For this purpose the inlet of the pure gases was set by

means of two flow regulators, while a flowmeter was employed in

order to assure a constant total flux (1.2 L min�1) in the detection


cell. After the deposition of each coating the PQC unit was placed

in the detection chamber and the frequencimetric signal was

allowed to equilibrate under a constant flow of N2 (1.2 L min�1).

When a satisfactory stability of the signal was reached, the

composition of the stream was progressively varied increasing

the percentage of CO2. The frequency shift was acquired for at

least 5 different levels of CO2, repeating the acquisition three

times for each explored concentration level. In order to evaluate

the sensitivity of the studied polymers the frequency shifts

were plotted versus the percent concentration of CO2 and linear

regression was carried out. The linearity of the response was

checked by means of Mandel test. Syntheses information as well

as 1H NMR and FT-IR data, elemental analyses and contact

angle results are available as ESI†.

Fig. 2 Synthesis of M1–M5 monomers.

Results and discussion

Bulk alkylation of [2-(methacryloyloxy)ethyl]dimethylamine by

1-bromoheptane affords, after 15 days, the expected product [2-

(methacryloyloxy)ethyl]dimethylheptylammonium bromide (M0)

with an acceptable yield (Fig. 1). The metathesis reactions of the

above precursor with the anions bis(trifluoromethylsulfonyl)imide,

nonafluoro-1-butanesulfonate, dodecylbenzenesulfonate, hepta-

decafluorooctanesulfonate and 4,4,5,5,6,6,7,7,8,8,9,9,10,10,

11,11,11-heptadecafluoroundecanoate, in water, lead to the ionic

liquids [2-(methacryloyloxy)ethyl]dimethylheptylammonium

bis(trifluoromethylsulfonyl)imide (M1), [2-(methacryloyloxy)

ethyl]dimethylheptylammonium nonafluoro-1-butanesulfonate

(M2), [2-(methacryloyloxy)ethyl]dimethylheptylammonium

dodecylbenzenesulfonate (M3), [2-(methacryloyloxy)ethyl]

dimethylheptylammonium heptadecafluorooctanesulfonate (M4)

and [2-(methacryloyloxy)ethyl]dimethylheptylammonium 4,4,5,5,

6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecanoate (M5),

with yields ranging from 46 to 98%, as described in ESI† (Fig. 2).

All the cited species have been characterized by means of

elemental analysis, 1H NMR solution spectroscopy, FT-IR,

DSC, and TG-DTA. 1H NMR investigations support the

alkylation as well as the exchange reaction; in particular the

spectrum of M0 clearly show the nitrogen quaternarization as

the signal at 3.37 ppm, ascribed to methylenic group adjacent to

Br in the parent compound 1-bromoheptane, is shifted to

3.58 ppm upon nitrogen alkylation. As expected, the resonances

of the other three N-alkyl groups are involved in a downfield shift

as well. As far as exchange reactions are concerned, all the

observed peaks strongly suggest the formation of M1–M5 species

although the proton signals attributable to the cations are poorly

Fig. 1 Synthesis of M0.


affected by the presence of a different anion, as already

observed;22 furthermore, the spectra of M3 and M5 are featured

by the presence of the additional arylic and alkylic resonances

due to the anions, with the correct integral ratio with respect to

the cation. FT-IR investigations carried out on M1–M5 defi-

nitely demonstrate that the exchange reactions have been

successful; in particular the anion exchange is clearly observed by

the appearance of new and typical bands corresponding to

bis(trifluoromethylsulfonyl)imide anion (1352, 1196, 1130 and

1057 cm�1), to nonafluoro-1-butanesulfonate anion (1269, 1143,

1052 and 904 cm�1), to dodecylbenzenesulfonate anion (2926,

2855, 1033 and 1007 cm�1), to heptadecafluorooctanesulfonate

anion (1261, 1151, 1036 and 875 cm�1) and to 4,4,5,5,6,6,7,7,8,8,

9,9,10,10,11,11,11-heptadecafluoroundecanoate anion (1571,

1405, 1370 and 875 cm�1).

Thermal properties of M2, M3 and M4, that are solids at room

temperature, and M1 and M5, that are viscous liquids, have been

Table 1 Thermal properties of monomers M1–M5

Sample Tg/�C Tmelt/�C Tonset/

�C Tmaxa/�C Residueb (%)

M1 �58.3 — 336 390 0M2 — 82.2 314 361 1.2M3 — — 276 343 1.5M4 — 95.7 308 372 2.5M5 �64.2 — 181 217 0

a Tmax: temperature of maximum rate of degradation. b Residue at 800�C under N2 flow.

J. Mater. Chem., 2009, 19, 8861–8870 | 8863

Fig. 3 DSC trace of M2.

Fig. 4 Thermogravimetric traces of M0–M5 under nitrogen flow.

explored by DSC and TG-DTA investigations. Thermal prop-

erties of monomers are summarized in Table 1. DSC scannings,

under nitrogen atmosphere, have been carried out on M1–M5

thus allowing us to explore their phase transitions. In particular,

in order to investigate the compounds already liquid at room

temperature, the scanning has been performed not only by

continuous heating (30 to 500 �C) but also by heating and

cooling cycles (�90 to 150 �C). In the latter temperature range

M0, M1 and M5 do not melt; on the contrary, at low tempera-

tures, they are featured by a solid amorphous phase that shows

a glass transition at �58.3, �64.2 and �34.5 �C, respectively. In

addition the observed glass transition temperatures (Tg) are not

followed by any other thermal transition indicating that the cited

compounds are not able to crystallize upon cooling. Straight

DSC scanning, starting from 30 �C, on M2 shows a melting at

92 �C. At the same time, whenever the sample is cooled and

heated from �90 �C up to 150 �C (Fig. 3), no Tg is detectable but

an exothermic effect, due to a reorganization of the solid in a crys-

talline material, is distinguishable at 63.4 �C (DHricr ¼ 13.69 J g�1);

the cited crystalline solid so-formed further undergoes melting at

82.2 �C (DHmelt¼ 21.54 J g�1). For this compound very likely the

glass transition occurs at a temperature less than �90 �C. The

ordered nature of solid M2 has been confirmed by means of

XRD (trace not reported) clearly showing sharp peaks due to

crystalline phases. Compound M3 does not show any

thermal transition in the studied range. Moreover, M4 displays

a melting endothermic signal at 95.75 �C corresponding to

a DHmelt ¼ 44.16 J g�1 but no Tg in the whole investigated

temperature range. XRD analysis on such a monomer confirmed

a high degree of crystallinity.

TG-DTA investigations allowed us to explore the thermal

behavior of M1–M5 either in air or under nitrogen flow. As

already observed, thermal behavior of M1–M5 is largely influ-

enced by the nature of the anion. In particular, thermogravi-

metric analyses under inert atmosphere show the first

degradation step, strictly connected to the thermal stability of the

studied species, whose temperature follows the M1 > M2 z M4

> M3 > M5 trend. The mentioned processes occur with differ-

ences in the rate of volatilization, being much faster for M1, M2

and M5. By comparing the above data with the thermal stability

of the parent M0, it appears that while M1–M4 are more stable

8864 | J. Mater. Chem., 2009, 19, 8861–8870

upon heating, M5 starts to decompose at 181 �C, at a tempera-

ture lower than the one displayed by M0 (Tonset ¼ 203 �C)

(Fig. 4). Thermal degradation of M1 and M2 is featured by two

main endothermic processes displaying a comparable rate of

volatilization herewith found to range between 12 and 18% min�1.

On the contrary, monomers M3 and M4 show a main decom-

position process (Tmax 343 and 372 �C, respectively) occurring at

a higher rate with a huge mass loss, while the other thermal

effects are quite slower. M5 decomposes under nitrogen with two

closely occurring endothermic effects; the main one, character-

ized by a high volatilization rate (22% min�1) with a weight loss

of 51%, culminates at 217 �C. From the comparison of the data

collected in Table 1, it can be stated that M5, among the studied

species, is the most vulnerable on heating; this evidence suggests

that the presence of the carboxylate anion can be responsible for

the above behavior. The hypothesis that M5 may undergo

decarboxylation during thermogravimetric scanning, as already

observed for other ionic liquids,31 has been confirmed by col-

lecting FT-IR spectra on the solid after heating at 120, 170 and

220 �C. These investigations confirm that decarboxylation occurs

as the signal at 1571 cm�1, due to the COO� moiety, disappears

after the heating at 220 �C. Moreover, thermogravimetric

investigations carried out in oxidative conditions do not show

notable differences with those recorded under nitrogen, as

already observed.32

Free-radical homopolymerization of monomers M1–M3 in

bulk or in CHCl3, following the work-up method described

in ESI†, leads to the formation of the homopolymers poly[2-

(methacryloyloxy)ethyl]dimethylheptylammonium bis(trifluoro-

methylsulfonyl)imide (P1), poly[2-(methacryloyloxy)ethyl]

dimethylheptylammonium nonafluoro-1-butanesulfonate (P2)

and poly[2-(methacryloyloxy)ethyl]dimethylheptylammonium

dodecylbenzenesulfonate (P3), with yields of about 75–80%.

As homopolymerization of M4 and M5 employing the above

experimental conditions led to insoluble materials, thus

hindering their structural characterization as well as the evalua-

tion of their hydrophobic properties and CO2 sorption capacity,

copolymerization reactions by employing 1 : 1 ratio of M1 and M4

as well as of M1 and M5 were carried out in order to obtain soluble

copolymers. In particular, the cited reactions in CHCl3 with AIBN

as free-radical initiator afford poly[2-(methacryloyloxy)

ethyl]dimethylheptylammonium bis(trifluoromethylsulfonyl)

imide-co-[2-(methacryloyloxy)ethyl]dimethylheptylammonium


heptadecafluorooctanesulfonate (C1-4) and poly[2-(methacryloy-

loxy)ethyl]dimethylheptylammonium bis(trifluoromethylsulfonyl)

imide-co-[2-(methacryloyloxy)ethyl]dimethylheptylammonium 4,4,

5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadecafluoroundecanoate

(C1-5) with good yields; the above copolymers display a slight

solubility in acetone only. Both homopolymers as copolymers

have been characterized by means of solution 1H NMR, FT-

IR, TG-DTA, DSC, MALDI-TOF, viscosimetry and XPS

measurements. In particular in NMR spectra the olefinic protons

due to the methacrylic moiety are hardly detectable after poly-

merization. In addition, in FT-IR spectra no signals at 988 and

1638 cm–1 due to C]C bonds are observable, while CO

stretchings of the polymers P1, P2, P3 and C1-4 are shifted

towards a higher frequency with respect to the corresponding

monomers. On the contrary, for C1-5 copolymer, CO stretching

is featured by an intermediate frequency between those displayed

by the monomers.

Thermogravimetric investigations for homo- and copolymers,

both under nitrogen and air flow, have been carried out. Fig. 5

shows the thermogravimetric traces of polymers in nitrogen

atmosphere. By comparing TGA data of M1–M3 with the

behavior of the corresponding homopolymers P1–P3, it appears

that the thermal stabilities of the discussed species do not show

significant variations upon polymerization (Table 2). For these

polymers PDT decreases following the trend P1 > P2 > P3 with

the corresponding Tmax of 378, 361 and 346 �C as well as weight

losses of 74, 84 and 52%, respectively. Thermogravimetric

patterns for P1–P3 are quite similar being characterized by two

main steps. Thermooxidative stability (data not shown) of P1–P3

Fig. 5 Thermogravimetric traces of P1–P3, C1-4 and C1-5 under

nitrogen flow.

Table 2 Properties of homopolymers P1–P3 and copolymers C1-4 andC1-5

Sample hinh Tg/�C Tonset/�C PDTa/�C Residueb (%)

P1 4.00 �44.0 333 378 1.4P2 4.06 — 291 361 1.8P3 1.17 — 284 346 3.3C1-4 0.76 — 317 386 0C1-5 0.46 �23.9 201 219 2.9

a PDT: polymer degradation temperature, temperature of maximum rateof degradation. b Residue at 800 �C under N2 flow.


is lower than the one displayed under nitrogen; P2 and P3 show

in fact the first degradation step at Tonset 234 and 204 �C,

respectively. As far as copolymers C1-4 and C1-5 are concerned,

their thermal behavior is mainly influenced by the presence of M1

meaning an increased stability of the cited copolymers towards

heating with respect to M4 and M5. The stabilization effect

against heating due to the presence of M1 occurs also for C1-5

that starts to degrade at Tonset 201 �C. The investigations carried

out in oxidative environment on C1-5 show a comparable

behavior to that displayed under nitrogen atmosphere while C1-4

starts to decompose at 219 �C, a temperature considerably lower

than the first thermal event occurring in nitrogen. DSC analysis

carried out on polymeric samples shows the presence of glass

transition for P1 (�44 �C) and C1-5 (�23.9 �C) only. The higher

Tg observed for C1-5 with respect to the one showed by P1 can be

ascribed to the presence of the repetitive unit M5 into the poly-

meric network that may induce a perturbation of the amorphous

structure of the homopolymer.

The poor solubility of the materials under study in THF

(solvent used as eluent in our GPC system) as well as the strong

ionic interactions between these products and the stationary

phase hinder the macromolecules elution thus hampering GPC

investigations. Therefore the macromolecular nature of the

homo- and copolymers has been ascertained by means of vis-

cosimetry, MALDI-TOF and XPS experiments. The viscosity

experiments suggest the polymeric nature of the studied samples.

In fact, homo- and copolymers show hinh values lying in the 4–0.5

range (Table 2). The characterization of polymers was also per-

formed through MALDI-TOF spectra that show signals

consistent with the presence of oligomeric poly(ionic liquid)s. In

particular, the positive MALDI-TOF mass spectrum of P1

(Fig. 6) is featured by two series of peaks: at m/z value of 1394 +

(n � 535) (+) with n ¼ 0–7, due to oligomers (from trimers to

decamers) bearing isobutyronitrile and vinyl (and/or hydrogen)

as end groups (arising from chain transfer phenomena); at m/z

value of 1462 + (n � 535) (++) with n ¼ 0–7, due to oligomers

(from trimers to decamers), detected as M+, with isobutyronitrile

fragments as end groups (deriving from chain coupling

phenomena). Furthermore, the ions of (+) and (++) are detected

as M+ species due to loss of a [N(SO2CF3)2]� fragment from the

corresponding oligomeric species.

The mass spectrum of P2 (Fig. S1, ESI†) consists essentially of

two series of peaks, detected as M+, at m/z values 1921 + (n� 555)

Fig. 6 MALDI-TOF mass spectrum of P1.

J. Mater. Chem., 2009, 19, 8861–8870 | 8865

Table 4 XPS derived atomic concentration analysis for C1-4

Shell, B.E./eVXPS atomic concentration (%), functionalgroup

C 1s, 285.0 23.0 18 (CH2, CH3)C 1s, 286.6 13.6 8 C–N+, 2 CO, 2 CORC 1s, 288.9 4.0 2 CF3–S, 1 CF2–SC 1s, 291.5 7.4 1 –CF2–CF2–CF2–CF2–CF2–CF2–C 1s, 293.7 1.5 1 –CF3

O 1s, 532.4 broad 13.1 11 ON 1s, 339.4 1.1 1 N�

N 1s, 402.7 2.2 2 N+

S 2p, 168.3 2.6 3 SF 1s, 689.6 31.5 23 F

Table 5 XPS derived atomic concentration analysis for C1-5


C 1s, 285.0 23.2 20 (CH2, CH3)C 1s, 286.6 11.7 8 C–N+, 2 CO, 2 CORC 1s, 288.9 3.2 2 CF3–S, –1 COO�

C 1s, 291.7 8.7 1 –CF2–CF2–CF2–CF2–CF2–CF2–CF2

C 1s, 293.6 1.8 1 –CF3

O 1s, 530.8–533.9 10.3 10 ON 1s, 339.3 1.4 1 N�

N 1s, 402.7 2.6 2 N+

S 2p, 168.5 1.8 2 SF 1s, 689.7 34.3 23 F

with n ¼ 0–6 and 1929 + (n � 555) with n ¼ 0–6, respectively.

Both the ion species are detected as M+ species due to loss of

a [C4F9SO3]� fragment from the corresponding oligomeric

species. It is worth noticing that the first series of peaks corre-

sponds to structures relative to oligomers (from tetramers to

decamers) having hydrogen and vinyl as end groups, while the

latter corresponds to chains (from tetramers to decamers) having

an isobutyronitrile and hydrogen as end groups. Little signals

due to oligomers having isobutyronitrile fragments as end groups

(deriving from chain coupling phenomena) are also present. The

presence of oligomeric species which do not contain iso-

butyronitrile as end groups, although unexpected, can be

ascribed to thermal polymerization of M2, in analogy with other

methacrylate systems,33 which undergo spontaneous self-poly-

merization via a biradical growth mechanism. In order to verify this

hypothesis a thermal bulk polymerization of M2 (�95 �C, 60 min)

without employment of chemical initiator was attempted.

Working up of the final mixture led to a low amount of product

(ca. 10%), which was analyzed by MALDI-TOF mass spec-

trometry. This affords spectra displaying a series of enveloped

signals indicative of oligomer formation (from dimer to pen-

tamer) and having the same structure of species at m/z values

1921 + (n � 555) and detected as M+ (spectrum omitted for

brevity).

The mass spectrum of P3 (spectrum not reported for brevity)

shows signals due to two series of peaks (from trimers to deca-

mers) assigned to oligomers bearing isobutyronitrile and vinyl

(and/or hydrogen) as end groups (m/z 1486 + (n � 581)), and

with isobutyronitrile fragments as end groups (m/z value of

1554 + (n� 581)). The ions are detected as M+ species due to loss

of a CH3(CH2)11Ph–SO3� fragment, from the corresponding

oligomeric species.

It is worth mentioning that the MALDI-TOF signals detected

for the pILs under study arise from the oligomeric species,

whereas the signals due to the rest of the polymeric chains cannot

be visible due to an intrinsic limitation of the technique

employed. In particular, the MALDI-TOF analysis of high

polydisperse polymers (MWD > 1.2) suffers from some instru-

mental and experimental limitations:34,35 (i) the decrease of

sensitivity and resolution by increasing the ion molecular mass;

(ii) the decrease of ion abundance when investigating higher

molecular mass distribution (i.e. MALDI-TOF technique gives

signals proportional to the molar abundance of the revealed

polymeric chain).

Table 3 XPS derived atomic concentration analysis for P2


C 1s, 285.0 25.8 9 (CH2, CH3)C 1s, 286.7 17.4 4 C–N+, 1 CO, 1 CORC 1s, 289.0 3.0 1 CF2–SC 1s, 291.3 5.8 1 –CF2–CF2–C 1s, 293.7 2.6 1 –CF3

O 1s, 532.2 11.2 4 OO 1s, 533.9 3.0 O (COR)N 1s, 402.7 3.0 N+

S 2p, 168.1 2.8 1 SF 1s, 689.6 25.4 9 F

8866 | J. Mater. Chem., 2009, 19, 8861–8870

Probably, the absence of oligomeric species as well as the

impossibility to be desorbed/ionized hampered the MALDI-

TOF analysis of C1-4 and C1-5 samples. Therefore, copolymeric

compositions were obtained by XPS experiments. Tables 3–5

collect XPS atomic concentration data for samples P2, C1-4 and

C1-5, once the relevant atomic sensitivity factors have been

accounted for.36–38 It is evident that concentration results always

match the polymer formula composition. P2 has been used as

a blank test and Table 3 shows the obtained XPS atomic

composition. It shows a N/S atomic ratio ¼ 1, a F/S ratio of 9 : 1

and a C/F ratio of 19 : 9, as expected on the basis of the formula

Fig. 7 Al-Ka excited XPS of P2 at 45� electron take-off angle: (a) in the

O 1s energy range; (b) in the C 1s energy range.


Fig. 9 Frequencygram recorded over progressive increase of the CO2

percentage in the mixture conveyed to the QCM cell for P2.

composition. In addition, on the basis of the binding energies, it

is also possible to distinguish between two kinds of oxygen and

five kinds of carbon atoms (Fig. 7). In particular, the three

sulfonate oxygens (–SO3�) and the carbonyl oxygen (–CO–) lie at

532.2 eV whilst the ester oxygen (–CO–O*–R) lies at 533.9 eV.38,39

Accordingly, deconvolution of the experimental profile gave

a 4 : 1 area ratio, in agreement with this assignment. Deconvo-

lution of the rich C 1s spectrum reveals five components with an

intensity ratio of 9 : 6 : 1 : 2 : 1. According to Table 3, both

binding energies and atomic concentration analysis allow for

a straightforward spectrum interpretation. In fact, ionizations of

the –CH2 and –CH3 backbone lie at 285.0 eV and add up for 9

carbon atoms; the –C–N+, –CO, and –COOR ionizations, in tune

with literature data, fall at 286.7 eV and count for 6 carbon

atoms; the –CF2–S group lies at 289.0 eV; ionization of the

–CF2–CF2– function lies at 291.3 eV and, finally, that of the CF3

group is at 293.7 eV. The sharp N 1s peak lies at 402.7 eV as

expected for the positive N+ function.30,38–40

Tables 4 and 5 show XPS atomic concentration analyses and

ionization binding energies for C1-4 and C1-5 copolymers. XPS

results evidence that the obtained copolymer compositions

parallel the starting equimolar monomers (M1 + M4 or M1 + M5)

reaction mixtures. In fact, the atomic F/S/N/O/C ratios are 23 : 3 :

3 : 11 : 40 and 23 : 2 : 3 : 10 : 43 for C1-4 and C1-5, in tune with the

expected 50 : 50 monomer composition. As already observed for

the P2 blank test, a careful deconvolution of the C 1s band

envelope (Fig. 8a) reveals for both C1-4 and C1-5 copolymers

five components: the first centered at 285.0 eV is due to

the aliphatic –CH2– and –CH3 backbone. The second at

286.6–286.7 eV is due to the carbon of the C–N+, –C]O and

–C*OOR groups. The third band centered at 288.9–289.0 eV is

consistent with ionization energies of CF2–S and, in the case of

the C1-5 sample, with the –COO� groups. The fourth component

at 291.3–291.7 accounts for the carbon centers of the CF2–

groups and, finally, the fifth component at 293.6–293.7 eV

belongs to the –CF3 moiety.38 Spectra of the nitrogen 1s core

level show binding energies in tune with the formal charge of the

nitrogen groups (Fig. 8b). Therefore, quaternized N+ gives an

XPS peak at 402.7 eV whilst negatively charged nitrogen shows

a peak at 339.3–339.4 eV. These features nicely account for the

two kinds of nitrogen depending on the particular sample. The

O 1s signal (Fig. 8c) lies at binding energies strongly dependent

Fig. 8 Al-Ka excited XPS of C1-5 at 45� electron take-off angle: (a) in the C


on the particular charge state thus ranging from 530.8 eV for the

–SO2– groups to 534.0 for the –COOR groups. The fluorine 1s

component shows a broad peak at 689.6–689.7 eV. Finally, the S

2p states show a peak in the 168.1–168.5 eV energy range.

CO2 sensing

On the basis of literature data, the study of the sensoristic appli-

cations of monomeric and polymeric ionic liquids based on tet-

ralkylammonium cations is particularly interesting considering

their capabilities for CO2 absorption.24,25 It is well established that

the affinity of ILs for CO2 is related to their structure in terms of

cations, anions and substituents, although the nature of the

anionic moiety seems to play a major role.41–44 Bearing this in

mind, the potentiality of the synthesized polymers as sensing

materials for CO2 has been explored by means of QCM

measurements. As the polymers under study are featured by

a common cationic part and five different anionic counterparts, we

have explored the variation of CO2 absorption by changing the

anion type. It is worth noticing that the studied anionic moieties

differ for functional group itself (imide, sulfonate and carboxylate)

as well as for the fluorination degree of the alkyl chain, if present.

The frequencimetric responses of the coating obtained by the five

studied polymers were compared in order to investigate the rela-

tions between structure and retention properties.

1s energy range; (b) in the N 1s energy range; (c) in the O 1s energy range.

J. Mater. Chem., 2009, 19, 8861–8870 | 8867

Fig. 10 Relationship between the frequency shift and the CO2

percentage, obtained by coating the PQC with pIL P2.

Table 6 Sensitivity values for homo- and copolymers exposed todifferent CO2 concentrations

SampleSensitivity/Hz (percentconcentration)�1

P1 0.629 � 0.007P2 1.56 � 0.02P3 0.88 � 0.02C1-4 0.40 � 0.01C1-5 0.53 � 0.01

In order to evaluate the repeatability and the reversibility of

the responses, the coated piezoelectric quartz crystals (PQCs)

were alternately exposed to N2–CO2 mixtures and to pure carrier

gas. All the polymeric coatings showed excellent responses

featured by quickness and reversibility without any memory-

effect. A quick and complete restoring of the baseline signal was

always obtained, with all coatings, when pure carrier gas was

conveyed to the detection chamber. It is worth mentioning that

the response time was, in all cases, lower than a second, a value

limited by the actual time required for adjusting the gas flows.

This behavior is easily visible as shown by the responses obtained

during the characterization of P2, reported as an example in

Fig. 9. The calibration line obtained by interpolating the

frequencimetric data is reported in Fig. 10. In order to estimate

the exceptional rapidity of the response, a comparison of the

performance of P2 with the literature data available for other

coatings used for the development of piezoelectric sensors,

whether or not based on ionic liquids, was performed. Baltus and

co-workers41 performed a QCM study on the CO2-philicity of 1-

n-butyl-3-methylimidazolium bis[trifluoromethylsulfonyl]amide

and other related compounds. From the data reported in the

paper it is possible to calculate both the detection limit, using

the regression line,45 and the sensitivity. The first value is

comparable to that obtained with P2 (6%) under similar exper-

imental conditions. The above detection limit values, in spite of

different sensitivities, indicate that, even if the frequency shifts

obtained by Baltus et al.41 are lower than those obtained with P2,

the signal-to-noise ratio is comparable. Conversely, a noticeable

advantage offered by P2 lies in the response time which, as

already stated, is in the order of a few seconds while in the case of

the above mentioned study the stability of the signal is achieved

about 3 min after adjusting the flow of CO2 and N2. In our

opinion this aspect is fundamental for the realization of real-time

responding sensors.

Concerning other materials employed for the realization of

CO2 responding piezoelectric devices, Gomes and co-workers

reported about the sensing properties of coatings based on

N,N,N0,N0-tetrakis 2-hydroxyethylethylenediamine46 or tetra-

methylammonium fluoride.47 A direct comparison between the

sensitivity of these sensors and the one displayed by the coatings

under study cannot be performed because of the different

experimental conditions employed for the calibration

8868 | J. Mater. Chem., 2009, 19, 8861–8870

procedures. Nevertheless the advantage in terms of response time

is even more evident, considering that, in the study concerning

the use of tetramethylammonium fluoride as sensing coating,

response times ranging from 15 min to 5 h are reported.

As far as the polymers under study are concerned, best results,

in terms of sensitivity, were obtained with P2, as established from

the comparison among the responses displayed by P1–P3 as well

as C1-4 and C1-5 (data not shown). The sensitivity values

obtained with all studied polymers are summarized in Table 6. It

is worth noticing that the best response of P2 was obtained

although the dilution of the casting solution (1 : 20) was higher

with respect to the factor employed for P1 (1 : 10). These results

indicate that the retention properties of polymer P2 are

remarkably higher if compared with the performance of the other

studied compounds. Furthermore, the comparison between the

responses shown by copolymers C1-4 and C1-5, only differing

for the co-monomers M4 and M5, does not evidence appreciable

differences in terms of CO2-philicity. Monomers M4 and M5

are associated by analogous polyfluorinated chains and differ for

the anionic functional group (carboxylate vs. sulfonate). The

comparability of the sensing properties evidenced by the corre-

sponding coatings indicates that the nature of the anionic func-

tional group does not influence the CO2-philicity of the

polymeric ionic liquids based on polyfluorinated chains.

Conversely, a comparison between the responses shown by

coatings P2 and P3 shows remarkable differences in terms of

sensitivity, referable to the CO2-philicity. Particularly, P2

showed a sensitivity almost double with respect to P3, so indi-

cating the fundamental role played by the nature of the alkylic

chain. Homopolymers P2 and P3 are, in fact, featured by the

same anionic moiety (sulfonate), with different alkylic chains,

unfluorinated for P3 and completely fluorinated for P2. This

observation is supported by the findings of other studies,42,48

aimed to investigate about the CO2-philicity of differently fluo-

rinated ionic liquids. As previously stated, the properties of

homopolymers P4 and P5 cannot be directly investigated

because of their insolubility. For this reason the coatings

obtained with copolymers C1-4 and C1-5 have been character-

ized. The role played by the extension of the polyfluorinated

chains (P2 vs. P4) cannot be investigated for the same reason. A

comparison between the retention properties of the polymers

with respect to the parent monomers was also attempted, but the

QCM measurements could not be performed since, after the

deposition of the monomer coatings, the stability of the fre-

quencimetric signal was not reached under a constant flow of N2

(1.2 L min�1). This behavior is probably ascribable to progressive

stripping of the monomeric coatings caused by the flow of the

carrier gas (bleeding).


Table 7 Dynamic contact angles measured by Wilhelmy balance oncoated glass slides

Sample qadv/� qrec/�

P1 97.3 � 0.7 47.6 � 1.4P2 108.8 � 0.4 49.0 � 1.3P3 100.0 � 0.6 54.0 � 0.9C1-4 113.2 � 0.8 42.0 � 1.5C1-5 78.3 � 0.9 22.2 � 0.3

Hydrophobic properties of homo- and copolymers

Hydrophobicity and wettability properties of polymeric

compounds P1–P3, C1-4 and C1-5 have been investigated by

means of dynamic advancing (qadv) and receding (qrec) contact

angle measurements, using Wilhelmy method. Advancing and

receding contact angles, as well as the difference between the two

cited values (hysteresis), are essential to evaluate the hydro-

phobic behavior of films. Dynamic contact angle values,

collected in Table 7, have been obtained from measurements

performed on microscope glass slides properly coated by films of

P1–P3, C1-4 and C1-5. From these data it can be inferred that

P1, P2, P3 and C1-4 display a hydrophobic behavior, being their

qadv > 90�, providing a trend that follows P1 < P3 < P2 < C1-4.

On the contrary, C1-5 shows a less hydrophobic behavior, with

a qadv of 78.3�. The most remarkable hydrophobic attitude has

been displayed by P2 and C1-4 coatings with advancing contact

angles of 108.8 and 113.2�, respectively. The tensiometric inves-

tigations on P2 and C1-4 also show receding contact angles of

49� and 42�, representative of the more hydrophilic portion of

the considered coatings. Molecular chains of C1-4, in particular,

are featured by an intrinsic heterogeneity, due to the monomer

differences so that the complexity of the organization of side

chains may rise leading to a growing hysteresis value. The high

hysteresis experienced for all the studied films may be explained

in terms of surfaces heterogeneity where hydrophobic and

hydrophilic microdomains coexist. As an example, for C1-5 the

low values of both qadv and qrec suggest the preferential orien-

tation of hydrophilic domains at the solid–air interface, with the

consequent reduction of the coating hydrorepellent activity. It is

worth mentioning that by comparing the contact angle values of

P1–P3 with the three [2-(methacryloyloxy)ethyl]-

trimethylammonium polymers already investigated,22 the

substitution of a methyl group with an heptylic one on the

quaternarized nitrogen does not affect qadv data to a great extent,

while considerably increasing the qrec with values ranging from

47.6� to 54�. This evidence supports the hypothesis previously

proposed where the tetraalkylammonium moiety was suggested

as the most responsible for low receding contact angle values; the

hydrophobic properties of such polymers can be then improved

by choosing the proper N-alkyl groups.

Conclusions

A series of poly(ionic liquid)s obtained by free-radical polymer-

ization of monomeric ionic liquids based on the polymerizable

cation [2-(methacryloyloxy)ethyl]dimethylheptylammonium and

on the anions bis(trifluoromethylsulfonyl)imide, nonafluoro-1-

butanesulfonate, dodecylbenzenesulfonate, heptadeca-


fluorooctanesulfonate, 4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-hep-

tadecafluoroundecanoate have been synthesized and characterized.

Our previous investigation on the evaluation of hydrophobic

properties of tetraalkylammonium poly(ionic liquid)s was very

promising; as a result, the current work has been focused on

assessing the variation of such hydrophobic behavior of ionic

liquid-based polymers obtained by substituting a methyl group

on the quaternary nitrogen with an heptylic one. This strategy

allowed us to improve the receding contact angles. Furthermore,

as ILs and pILs are featured by a considerable CO2-philicity, the

potential applications of the synthesized polymers as sensing

materials for CO2 have been explored by means of QCM

measurements. Regardless of the structure–property relation-

ship, the studied polymeric coatings showed very promising

properties for sensor applications. Frequencimetric responses

were very rapid, reversible and no memory-effect occurred. The

quickness of the response is a fundamental quality parameter of

chemical sensors: P2-based sensors showed a response time lower

than a second, limited by time required for adjusting the N2 and

CO2 flows. Considering also the cheapness of the materials and

the simplicity of the coating procedure, these materials are

excellent candidates for development of sensors.

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Very fast CO2 response and hydrophobic properties of novel poly(ionic liquid)s

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Transcript of Very fast CO2 response and hydrophobic properties of novel poly(ionic liquid)s