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Materials for energy and sustainability
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Konar and A. Srivastava, J. Mater. Chem. A, 2013, DOI: 10.1039/C3TA14470D.

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Transparent, Free-standing, Flexible and Selective CO2 Adsorbent
Films Fabricated from Homopolymer/Metal Salt Hybrid Gels
Qysar Maqbool, Amarendar Reddy M, Soumyabrata Goswami, Sanjit Konar, and Aasheesh Srivastava*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x5
This article details our efforts to prepare free-standing macroscopic films by optimized coordinative
crosslinking of poly(4-vinylpyridine) (P4VP) chains using transition metal ions. We encountered
homogenous gels (at <1% w/v polymer concentration) en route the fabrication of such films. The gels and
films were obtained through coordinative crosslinking of P4VP chains by Ni(ІІ) ions while use of other
metal ions (viz. Co(II), Cu(II) or Zn(II)) for this purpose resulted in either heterogeneous gels or10
gelatinous precipitates. Based on the gelation kinetics and the mechanical strength of the resulting films,
the most optimum coordination ratio was observed to be 1:1 molar ratio of 4VP:Ni(II) in methanol-water
mixed solvent system. Highly transparent, flexible, free-standing films of any dimension and shape could
be fabricated through controlled evaporation of solvent from the gels. The microscopic features of these
films were quite similar to those of xerogels. Magnetic susceptibility measurements, performed on the15
ground films showed ferromagnetic interactions between adjacent Ni(ІІ) centres in the temperature range
of 75 to 10 K. These coordinatively crosslinked pliable films exhibited selective, although moderate,
adsorption of environmentally-relevant CO2 at room temperature. Through variation of metal content in
the films, we further demonstrated that the metal centres present in the film are intimately involved in the
adsorption of CO2. We believe such coordinatively crosslinked polymer films can be a potential20
alternative to metal-organic frameworks (MOFs) in gas adsorption applications, with the added benefit of
flexibility and macroscopic dimensions.
1 Introduction
Designing and creating pliable macroscopic materials by utilizing
non-covalent interactions is fast gaining prominence amongst25
researchers. Amongst the various non-covalent interactions, H-
bonding has been exploited more widely in literature for creation
of viscoelastic materials.1-6
However, arguably, coordinative
crosslinking offers greater advantages in the preparation of
responsive materials due to the easy tunability of the strength of30
metal-ligand interactions through judicious choice of interacting
partners. Further, in contrast to the highly directional nature of
hydrogen bonding, coordinate crosslinking is more flexible
geometrically. As a result, while hydrogen bonding commonly
utilized for preparing thermoreversible materials,1
those prepared35
by employing metal-coordination interactions often exhibit
chemo-reversibility.7
The potential reversibility of metal-ligand interactions in such
systems is exhibited in response to chemical stimuli such as redox
reagents, competitive ligands, or change of pH. This reversibility40
allows researchers to create erodible bioinspired materials.8
This
reversible chemo-responsiveness also forms the conceptual basis
of many sensors or temporary gas-storage devices. Excited by
such potentials, researchers have created many macroscopic
materials by crosslinking polymers with metal ions. For example,45
thin films fabricated through ionotropic crosslinking of sodium
alginate with Ca2+
ions were employed for sensing metal ions
based on the structural colors generated by such films.9
Using the
same polymer and Fe3+
as the ionotropic crosslinker, Bracher et
al. fabricated films of various shapes by employing patterned50
paper as templates. When the paramagnetic cations such as Ho3+
or Gd3+
were used as crosslinking agents, the films produced
could be manipulated magnetically.10
A similar strategy was
employed by Winkleman et al. to prepare porous, low-κ dielectric
constant materials.11
Thus, literature is replete with examples of55
utilizing ionotropic gels to fabricate a variety of chemoresponsive
materials. However, examples of macroscopic free-standing films
constructed by coordinatively-crosslinking of macromolecules
are less prevalent. This is because the metal ions can exhibit
significant flexibility in their coordination number and geometry60
even for the same ligand and solvent pair, requiring optimization
of the coordination interactions to generate macroscopic
materials. On the contrary, this flexibility of metal center is useful
for reversible coordination of analytes for sensing and storage
applications. One such application is the selective adsorption of65
certain gases from a mixture, which has obvious advantages in
separation of gases. However, for re-usability of the materials,
such adsorption should be reversible upon change of conditions.
This article details our efforts on optimizing coordinative
crosslinking of a well-known and structurally-simple polymer,70
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2 | Journal Name, [year], [vol], 00–00 This journal is © The Royal Society of Chemistry [year]
poly(4-vinylpyridine) (P4VP), with appropriate transition metal
ions to fabricate gels and free-standing films from it. We were
aware of an extensive, early investigation by Agnew into the
transition metal complexes of P4VP.12
Further, there are several
reports where block copolymers of P4VP have been used to5
prepare gels13
or hybrid films14,15
utilizing metal-ligand
coordination. However, to the best of our knowledge, no study on
the P4VP homopolymer mentioned either gelation or film
forming capability through direct coordinative crosslinking with
added metal ion solutions, perhaps due to reasons mentioned in10
the previous paragraph. Craig and co-workers have crosslinked
P4VP by pincer complexes of Pt2+
and Pd2+
and have investigated
viscoelastic responses of the resulting organogels in detail.16-18
Even then, fabrication of gels and macroscopic films through
direct crosslinking of P4VP by transition metal ions is not15
reported. Only on sidelines is there an indication that addition of
cupric chloride to P4VP leads to the formation of crosslinked
insoluble gels.19
The current study thus aims to create
macroscopic films through direct coordinative crosslinking of
P4VP homopolymer, and to employ these films for selective20
uptake of an environmentally-relevant gas, carbon dioxide.
2 Experimental Section
2.1 Materials
4-vinylpyridine (4VP) was purchased from Alfa Aesar and
distilled before use. Azobisisobutyronitrile (AIBN) was obtained25
from Sigma Aldrich and was purified by recrystallization in
methanol. Nickel chloride hexahydrate was purchased from
Sigma Aldrich and used as received.
2.2 Synthesis of poly (4-vinylpyridine), P4VP
4-vinylpyridine (20 mmol) was dissolved in dry methanol (5 mL)30
and purged with Argon at room temperature with stirring for 60
min. AIBN (0.4 mmol) was added followed again by purging
with Ar for 60 min with stirring at room temperature. In order to
minimize the evaporation of methanol during purging, the
reaction vessel was capped with silicon septum and vented35
through an 18-guage needle . This reaction mixture was sealed
under positive argon pressure and heated at 60 ᵒ
C for 16 h and
subsequently cooled to room temp. The obtained brown viscous
mixture was concentrated and the polymer precipitated by
addition of diethyl ether. The solid thus obtained was dried in40
vacuo for 2 h. The dried product was dissolved in minimum
amount of 0.1 M HCl and dialyzed against distilled water for 48 h
using dialysis membrane with MWCO 10 kDa. The resulting
suspension was lyophilized, and a fine cottony solid was
obtained. The spectroscopic features of this material were in45
consonance with those of P4VP. The molecular weight of the
resulting polymer was found to be ~61 kDa by ESI-MS.
2.3 Gel formation
P4VP was dissolved in water-methanol mixture (6:4, v/v) at 10.5
mg/mL concentration and this was labeled as stock solution. In50
different glass vials, 1 mL of this solution was taken and varied
equivalents of aq. solution of MCl2 (M = Co, Ni, Cu and Zn) was
added dropwise. The final volume of the solvent was kept
constant at 1.2 mL. The final ratio of water to methanol was 2:1
(v/v). After mixing, the glass vials were left undisturbed at room55
temperature with closed caps for requisite amount of time. The
gelation was checked by standard stable-to-inversion method.
2.4 Thixotropic gels
P4VP was dissolved in DMSO at 10.5 mg/mL concentration. To
1 mL polymer solution, 200 µL of aqueous NiCl2 solution (0.560
M) was added. The final ratio of DMSO-to-water in this mixture
was 0.83:0.17, and that of metal-to-ligand was 1:1. After addition
of NiCl2 solution, the vial was left undisturbed on benchtop with
closed cap. Gelation occurred after at least 10 h of incubation.
2.5 Rheological studies65
Rheometric measurements were performed on a Rheoplus
MCR102 (Anton Par) rheometer using CP25-1 cone and plate
fixture (25 mm diameter, 1ᵒ
cone angle) at 25 ᵒ
C. The amplitude
sweep experiment at constant frequency (1 rad/s) was carried out
to know storage or elastic modulus, Gᵒ, and loss or viscous70
modulus, Gᵒ, from 0.1 to 100%. Frequency sweep measurements
were performed at 0.5% strain from 100 to 0.1 rad/s.
2.6 Film formation
Ni(ІІ)-crosslinked gels were made in polypropylene vials of
surface area ca. 5 cm2
in methanol-water mixture, as described75
above. The vials containing the gels were covered with
aluminium foil and numerous pores were punctured on it by an
18-gauge needle. The subsequent drying of gel under ambient
conditions through slow evaporation of solvent resulted in the
formation of transparent film. The drying time could be varied by80
choosing appropriate solvent system, e.g. if the solvent used was
water-methanol mixture (6:4, v/v) film formation took about 40 h
at 35 ᵒ
C. When the crosslinked gels were dried at slightly lower
temperatures (27 ᵒ
C), the film was obtained in ~45 h. Changing
the solvent system to 2:8 (v/v) water-methanol resulted in85
substantial decrease of this wait-period to 10 h. However, in each
case, mild heating of samples at 65 ᵒ
C for about 5 min ensured
easy detachment of the film from the substrate.
2.7 Atomic force microscopy
AFM images were obtained by using Scanning Probe Microscope90
(Agilent 5500). The AFM images were recorded in non-contact
mode using NSC-19 cantilever (Micromash) having length =
125±5 nm, force constant = 0.6 N/m and resonant frequency = 80
kHz.
2.8 Thermogravimetric analysis (TGA)95
Thermogravimetric experiments were carried out on TGA 4000
apparatus. The free-standing xerogel film was crushed and 10 mg
of it was loaded into silica crucible. The measurements were
performed under N2 flow (20 ml/min) with a constant
temperature ramp of 5 ᵒ
C/min from 30 to 700 ᵒ
C.100
2.9 Magnetization measurements
Temperature-dependent magnetic measurements were carried out
on crushed and ground films enclosed in gelatin capsule with
Quantum Design SQUID VSM magnetometer. The weight of
xerogel used in this study was 12.5 mg.105
2.10 Gas sorption measurements
The N2 and CO2 sorption properties of crushed and finely ground
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This journal is © The Royal Society of Chemistry [year] Journal Name, [year], [vol], 00–00 | 3
films prepared at 1:1 and 1:2 coordination ratio were investigated
at 77K and 298 K, respectively. The weight of the samples used
here was 134 mg. Before sorption experiments, the samples were
degassed at 80 ᵒ
C for 24 h.
3 Results and Discussion5
The presence of metal-coordinating N-atom in every repeat unit
of P4VP encouraged us to explore its ability to undergo
coordinative crosslinking with transition metal chlorides to yield
macroscopic films. We chose P4VP as a representative polymer
due to its structural simplicity, which would allow us to obtain10
molecular-level understanding on the coordinative interactions
involved. An additional advantage of employing such structurally
straight-forward polymers is the ready scalability of results
obtained. P4VP employed in this work was prepared through
standard free radical polymerization, and had average molecular15
weight ca. 61 kDa (as determined by ESI-MS). P4VP, as a
solution in water-methanol mixture (6:4 v/v) at a concentration of
10.5 mg/mL was mixed with aqueous solutions of various
transition metal chlorides, MCl2 (M = Co, Ni, Cu and Zn). The
final ratio of water-to-methanol was maintained at 0.66:0.33 v/v,20
and the final concentration of P4VP was 8.75 mg/mL, which was
kept invariable in each sample. The influences of metal ion and
pyridine-metal stoichiometry on the physical behavior of the
resulting mixtures were investigated initially.
Under similar conditions, as detailed in experimental section, we25
checked the physical constitution and the flow behavior of
mixtures containing P4VP and various first row transition metal
ions in methanol-water solvent system at various time points. We
observed that with CoCl2, a gelatinous precipitate was formed
instantaneously upon addition of metal ion to the polymer at all30
metal-to-vinylpyridine (4VP) ratios (this ratio is also referred to
as the coordination ratio). No further macroscopic changes were
observed for prolonged periods for these samples (Figure S1,
Table S1, ESI). On adding aqueous solutions of NiCl2 to P4VP
solution, we observed a rational change in the flow behaviour of35
P4VP solution as the molar concentration of metal ion was
increased (Table 1).
Table 1 Materials obtained at different coordination ratios with Ni(II)
NiCl2:4VP Inference
1:8 ms
1:4 lgp
1:2 hg
1:1 hg
ms: milky suspension, lgp: light-green ppt., hg: homogenous gel
A milky suspension was formed when Ni(II) to 4-vinylpyridine40
(4VP) molar ratio was 1:8, and a light green gelatinous
precipitate was obtained when this ratio was 1:4. As we increased
the amount of metal ion in relation to the 4VP repeat units, a
light-green physical gel was obtained at 1:2 coordination ratio.
The formation of a stable gel required ca. 90 min after the45
addition of Ni(II) ions. Upon further increasing the concentration
of NiCl2 to 1:1 coordination ratio, a stable, homogenous light-
green gel (NiP) could be obtained within 10 minutes of addition
of metal ions to the polymer (Figure S2, ESI). Since addition of
metal ions to P4VP solution was essential to achieve physical50
gelation (as well as film formation, discussed below), it was clear
that the added metal ions caused coordinative crosslinking of the
polymer chains.20
Upon employing CuCl2 as the coordinative
crosslinker, heterogeneous gels were obtained instantaneously at
1:4, 1:2 and 1:1 coordination ratios (Figure S3 and Table S1,55
ESI). Under similar conditions, ZnCl2 solutions afforded only
white precipitates at all the above coordination ratios. Thus, due
to the rapid formation of homogeneous gels by Ni(II) ions at 1:1
coordination ratio, we chose this system for further exploration.
3.1 Characterization and acid-response of NiP gels60
The mechanical and viscoelastic properties of the resultant NiP
gels were probed by oscillatory rheometry (Figure S4, ESI).
Mechanical robustness of the gels was reflected in the high G'
values of Ni(II) crosslinked gels (~4500 Pa, even at 8.75 mg/mL
P4VP concentration, Figure S4a). The G' value obtained in65
present study is superior to those reported recently by
Messersmith and coworkers at much higher polymer
concentrations (50 mg/mL in histidine containing PEG
polymers)21
due to the multiplicity of the coordinative crosslinks
that can be formed by P4VP. From Figure S4b, it can be seen that70
both G' and G" are nearly independent of frequency and no
crossover was seen at lower frequencies, which indicates
formation of crosslinked network of the polymer chains. Atomic
force microscopy (AFM) of the NiP xerogel showed nanoscale
globular features (Figure S5, ESI).75
We then investigated the mechanical and thermal stabilities of the
NiP gels. Even though the gel was obtained at rather low polymer
concentrations (<0.9 wt.% P4VP), it showed good thermal
stability. The gel mass was stable to inversion upto 90 ᵒC,
although it underwent sineresis at temperatures >75 ᵒC. No80
visible change was observed in the gel samples when left
undisturbed on benchtop for weeks. Acidification of the gel by
the addition of 100 L of 1 M HCl, however, resulted in complete
dissolution of the gel matrix, yielding a clear solution within 10
min of addition of the acid (Figure S6, ESI). This observation is85
ascribed to the protonation of 4VP repeat units (pKa = 5.6),22
which causes the decomplexation of the metal ion. Protonation of
the pyridines also introduces electrostatic repulsion between
polymer chains. These molecular-level changes result in a gel-to-
sol transformation upon acidification.90
3.2 Mechano-responsive, ‘self-healing’ gels in DMSO
The physical rigidification of DMSO/H2O mixture occurred
significantly more slowly compared to the rigidification of water-
methanol mixture under identical conditions (see section 2.4).
The mechanical strength of the DMSO-gel was also significantly95
lower than that of methanol-gel, with the G' value dropping to
~250 Pa (from a G' value of ~4500 Pa obtained in water-methanol
under the same conditions, Table 2).
Table 2 Influence of solvent on the mechanical strength of NiP gels
Solvent G (Pa) G -G (Pa)
H2O-MeOH 4500 3670
DMSO 250 212
100
However, the reversibility of the coordinative crosslinks was
reflected by the self-healing nature of these gels. Mechanical
agitation of the vial containing the DMSO gel for 2-3 min yielded
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a viscous sol indicating significant disruption of the gel-network.
On keeping this sol undisturbed overnight, the gel was
regenerated (Figure 1). These physical observations were
confirmed by rheological studies showing significant recovery of
the rheological behavior upon leaving the sample undisturbed for5
15 h (Figure S7, ESI). It indicates that the gels had recovered
their mechanical properties to a large extent, but not completely,
after they were left undisturbed for 15 h. Such regeneration of
mechanical strength upon removal of strain was also observed
recently by Messersmith’s group in their histidine containing10
PEG polymers.21
It has been ascribed to the reversible breaking
of the coordinative crosslinks upon exposure to physical stress,
and their steady reformation during the rest phase. In the
coordinatively crosslinked systems, the coordination bonds can
act as the sacrificial bonds due to their lower strength when15
compared to covalent bonds, and their reversibility allows the
system to exhibit such reversible mechano-response.
Figure 1. Demonstration of mechano-responsive ‘self-healing’ nature of
gel prepared in DMSO.
3.3 Formation of free-standing transparent films20
We found that the gelatinous materials obtained from Co(II) and
Cu(II), or the precipitates obtained from Zn(II), gave powders
upon drying. However, we could obtain transparent, macroscopic
self-supporting films (NiPF) through controlled drying of NiP
gels (Figure 2).25
Figure 2. Snapshots of free-standing NiPF films depicting transparency
and flexibility.
To explore this observation further, we attempted altering the
coordination ratio of the gel to 1:2 Ni(II):4VP (which had also30
resulted in formation of uniform gels after 90 min). However, we
could only obtain a mechanically weak film at 1:2 Ni(ІІ):4VP
ratio. Thus, increasing the density of coordinative crosslinks
improved the mechanical strength of the films. The NiPF films
could be handled by tweezers, and were flexible enough to be35
rolled. Further, these films can be cut into any desired shape by
using scissor or cutter and are writable. These films could be
stored in closed environments for weeks without any external
damage. However, prolonged drying of the film under ambient
conditions or at 100 °C resulted in development of brittleness in40
them. This could be reversed upon exposure of the film to humid
environments for 30 min, which made the films flexible and
highly transparent again. Thus, water seems to act as a plasticizer
for the NiPF films. We found out that almost 60 wt% of moisture
was present in the film having optimal suppleness, transparency45
and mechanical strength (data not shown).
3.4 Spectroscopic characterizations of the NiPF films
The UV-vis profiles of the NiPF films exhibited characteristic d-
d transitions due to the Ni(II) ions present in the film. The d-d
band for Ni(II) was considerably red shifted in the film (λmax50
~415 nm) compared to its value in aqueous medium (λmax ~392
nm) (Figure S8, ESI), indicating the coordination of Ni(II) ions
with the pyridine units of P4VP. Apart from this broad peak, the
UV-vis spectrum of the film was featureless in 400-700 nm
wavelength regime and had minimal scattering (Figure 3). These55
spectroscopic features are reflected in the high transparency and
the light-green colour of these films.
Figure 3. UV-visible absorption spectra of NiPF film showing
transparency. Inset: Digital image of such a film cut into a square shape.
For FTIR studies, the crushed film or solid P4VP was pelletized60
with KBr. As can be seen from Figure S9, while the CN stretch of
uncomplexed pyridine moieties was observed at 1599 cm-1
in
P4VP, this band is blue shifted by about 15 cm-1
in the Ni(II)-
crosslinked films, similar to the shifts reported earlier for metal-
P4VP complexes.23,24
65
3.5 TGA and Powder X-ray Diffraction studies on NiPF
The thermal stability of the films was gauged by TGA of the
crushed films. It indicates that the films retained their chemical
integrity upto 400 ºC, beyond which they underwent
decomposition (Figure S10, ESI). Powder X-ray diffraction70
(PXRD) studies indicated that both P4VP12
and its complexes
with Ni(ІІ) ions are predominantly amorphous, although the
metal complexes exhibited some crystallinity (Figure 4). A new,
broad diffraction peak emerges in the polymer metal complexes
(prepared at 1:2 and 1:1 coordination ratio), while the original75
peaks due to polymer at ca. 11° and 22° were not observed.
Importantly, the PXRD profiles at the two coordination ratios had
peaks at exactly the same 2θ values, indicating that the P4VP-Ni
complexes have same crystal structures at these two coordination
ratios. The complexes formed at either coordination ratio also80
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exhibited minor sharp peaks in addition to the broad peak
mentioned above. The only difference between the PXRD
patterns at the two coordination ratios was in the sharpness of the
peaks. However, we believe that the crystallinity in the films, as
elicited by the sharp peaks in the PXRD profiles, can be5
attributed to the inherent crystalline character of Ni(ІІ) salts
instead to any ordering induced by coordination.
Figure 4. X-ray diffraction patterns of P4VP (black) and crushed films
prepared at 1:2 (red) and 1:1 Ni(ІІ):4VP ratio (blue).
3.6 Microscopy of NiPF film10
AFM investigations were undertaken on these films to explore
the microscopic features of the NiPF film at 1:1 coordination
ratio. These studies revealed globular morphology of the film
(Figure 5), similar to those observed in the NiP gels (Figure S5)
that were precursors to the film.15
Figure 5. AFM images of the NiPF film in amplitude mode showing
network of channels. (a) Large area scan, and (b) zoomed in view.
When relatively larger areas were scanned, the film showed a
patchwork pattern throughout, with the individual patches
separated from each other by depressed channels. We surmise20
that these channels may have formed during the evaporation of
the entrapped solvent (Figure 5a). Due to the presence of Ni(II)
ions in the films, we further investigated their magnetic properties
and gas adsorption behavior, as detailed in the following sections.
3.7 Magnetic measurements on NiPF films25
Measurement of magnetic susceptibility of NiPF was carried out
with the finely powdered sample and the data was collected in the
temperature range 1.8–300 K at a magnetic field of 0.1 T (Figure
6). The product of the molar magnetic susceptibility (χM) with
temperature i.e. χMT value at 300 K was found to be 2.1130
cm3
mol-1
K, which almost matches with the theoretical
susceptibility value expected for two interacting Ni(II) ions (χMT
= 2.0 cm3
mol-1
K, g = 2). The susceptibility value remained nearly
constant as the temperature was decreased from 300 K to 75 K.
Below 75 K, the susceptibility value increased, signifying35
ferromagnetic interactions between the adjacent Ni(II) centers
within 75–10 K. At 10 K, the susceptibility reaches a maximal
value of 2.61 cm3
mol-1
K.
Figure 6. Temperature dependence of χMT measured at 0.1 T for ground
NiPF films.40
Furthermore, the value of magnetic moment calculated from the
data ( eff = 4.07 BM) agrees well with that expected for
tetrahedral (Td) Ni(II) complexes [ eff for Ni(II) ions with two
unpaired electrons and in Td environment ranges from 3.7-4.0
BM]. However, from the susceptibility-temperature curve, one45
cannot make a precise determination of the Td geometry.12
Hence,
based on the magnetic susceptibility studies along with the
magnetic moment value obtained, we propose that the film is
predominantly composed of tetrahedral complexes having
stoichiometry NiLCl2, as shown in the Figure 7.50
Figure 7. Proposed structure for the polymeric complex NiLCl2 in NiPF
films.
3.8 Selective adsorption of CO2 by NiPF films
Metal-organic frameworks (MOFs), which are essentially a
network of metal ions connected by multitopic organic ligands55
through coordination bonds, are well known for gas adsorption.25
However, the adsorption of gases by coordinatively crosslinked
polymer networks has not been studied till now, even though a
few covalently crosslinked polymeric systems have been reported
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in the recent past for this application. For example, Germain et al.
reported reversible uptake of hydrogen by hypercrosslinked
polymer gels prepared from poly(chloromethylstyrene-co-
divinylbenzene).26
We hypothesized that the metal ions in our
NiPF films may act as gas adsorbing centers, while the flexible5
framework of polymer chains will circumvent the powdery
texture that is routinely encountered with MOFs, even though a
few flexible MOFs have been recently reported.27-29
With this
proposition, we investigated the gas adsorption properties of the
NiPF films. We measured the adsorption of N2 (at 77 K) and CO210
(at 298 K) on dried and ground NiPF films. Before the sorption
measurements, the samples were pre-treated at 80 ºC with
vacuum to remove any kind of gases and vapours adsorbed.
While the adsorption of N2 was minimal at 1.14 cm3
/g, the
samples showed significantly more adsorption of CO2 gas (~6.515
cm3
/g) at room temperature (Figure 8). In other words, we can
say that these coordinative crosslinked films showed reasonable
CO2 uptake of 1.27 wt % at room temperature. Importantly, the
adsorption was quite reversible, and almost 90% of the adsorbed
CO2 was released upon decreasing the partial pressure of CO2.20
This nature is assigned to the reversible swelling of the polymer
matrix,30
and to the reversible-coordination of CO2 to the Ni(II)
centres present in the film. To test the latter hypothesis, CO2
adsorption measurement was also conducted on the films
obtained at the coordination ratio of 1:2. As is clear from the red25
curve of Figure 8, a substantial decrease was observed in the
amount of CO2 adsorbed (2.6 cm3
/g) by these films. Further, the
hysteresis observed in these films was much smaller, indicating
that decrease of metal centres in the film also affect their ability
to hold onto the adsorbed gas during the desorption process.30
Thus, optimized coordinative crosslinking not only affords
mechanically robust materials, but also endow them with superior
gas adsorbing properties.
Figure 8. Gas sorption curves for ground NiPF films formed at 1:135
coordination ratio; CO2 at 298 K (black) and N2 sorption at 77 K (blue).
CO2 adsorption at 298 K by films obtained at 1:2 coordination ratio are
shown by red curves. Filled symbols correspond to adsorption and empty
symbols correspond to desorption process.
Conclusions40
In conclusion, we have demonstrated that optimizing the
coordinative crosslinking of a well-known polymer, P4VP, offers
access to macroscopic materials with novel properties. In the
present case, optimized crosslinking of P4VP chains by Ni(II)
ions at the coordination ratios of 1:2 and 1:1 yielded homogenous45
gels, which were not obtained with other metal ions or at other
coordination ratios. AFM images of the gel samples prepared at
1:1 coordination ratio showed the presence of globular
nanostructures. However, the mechanical properties of the
resulting gels were intimately related to the solvent employed for50
their preparation. While water-methanol mixtures gave strong
gels (Gᵒ ~4500 Pa), those obtained from DMSO-H2O were quite
thixotropic. More importantly, we have further elaborated an easy
protocol to fabricate transparent, free-standing, flexible,
macroscopic films upon slow evaporation of solvent from such55
gels. These coordinatively crosslinked films exhibited moderate
and selective adsorption of CO2 at room temperature, which was
dependent upon to amount of metal present in the film. We
believe that the CO2 uptake by these films can be improved upon
by use of copolymers and by employing a combination of metal-60
ions. Magnetic susceptibility measurements were indicative of
ferromagnetic interactions between the adjacent Ni(ІІ) centres in
the films in the temperature range of 75 to 10 K. We believe that
the visual clarity, flexibility and self-supporting nature of these
films will further exhort researchers in exploring other such65
coordinatively crosslinked systems in the creation of new
materials such as transparent conductive films too.
Acknowledgements
Q. Maqbool, A. Reddy M, and S. Goswami thank IISER Bhopal
for institute fellowship. This work was supported by funds and70
facilities provided by IISER Bhopal.
Notes and references
Department of Chemistry, Indian Institute of Science Education and
Research Bhopal, Indore By-pass Road, Bhauri, Bhopal – 462066
Madhya Pradesh, India. Email: asri@iiserb.ac.in75
† Electronic Supplementary Information (ESI) available: [Supplementary
snapshots and figures, coordinative crosslinking with transition metal
chlorides, gel preparation, rheological data, AFM, UV-visible spectra,
FTIR spectra and TGA]. See DOI: 10.1039/b000000x/80
1. R. J. Thibault, P. J. Hotchkiss, M. Gray, and V. M. Rotello, J.
Am. Chem. Soc., 2003, 125, 11249.
2. R. F. M. Lange, M. van Gurp, and E. W. Meijer, J. Polym. Sci.,
Part A, Polym. Chem., 1999, 37, 3657.85
3. L. Brunsveld, B. J. B. Folmer, and E. W. Meijer, MRS Bull.,
2000, 49.
4. L. R. Rieth, R. F. Eaton, and G. W. Coates, Angew. Chem. Intl.
Ed., 2001, 40, 2153.
5. C. Hilger, M. Drager, and R. Stadler, Macromolecules, 1992, 25,90
2498.
6. M. Muller, A. Dardin, U. Seidel, V. Balsamo, B. Ivan, H. W.
Spiess, and R. Stadler, Macromolecules, 1996, 29, 2577.
7. S. Samai, and K. Biradha, Chem. Mater., 2012, 24, 1165.
8. A. Srivastava, N. Holten-Andersen, G. D. Stucky, and J. H.95
Waite, Biomacromolecules, 2008, 9, 2873.
9. M. D. Cathell, and C. L. Schauer, Biomacromolecules, 2007, 8,
33.
10. P. J. Bracher, M. Gupta, and G. M. Whitesides, Adv. Mater.,
2009, 21, 445.100
11. A. Winkleman, R. Perez-Castillejos, M. Lahav, M.
Narovlyansky, L. N. J. Rodriguez, and G. M. Whitesides, Soft
Matter, 2007, 3, 108.
12. N. H. Agnew, J. Polym. Sci. Polym. Chem. Ed., 1976, 14, 2819.
View Article Online
DOI: 10.1039/C3TA14470D

13. A. Noro, S. Matsushima, X. He, M. Hayashi, and Y. Matsushita,
Macromolecules, 2013, 46, 8304.
14. A. Noro, Y. Sageshima, S. Arai, and Y. Matsushita,
15. Y. Sageshima, S. Arai, A. Noro, and Y. Matsushita, Langmuir,5
2012, 28, 17524.
16. W. C. Yount, D. M. Loveless, and S. L. Craig, Angew. Chem.
Intl. Ed., 2005, 44, 2746.
17. W. C. Yount, D. M. Loveless, and S. L. Craig, J. Am. Chem.
Soc., 2005, 127, 14488.10
18. D. Xu, and S. L. Craig, Macromolecules, 2011, 44, 5465.
19. A. M. Lyons, M. J. Vasile, E. M. Pearce, and J. V. Waszczak,
20. R. M. Meudtner, and S. Hecht, Macromol. Rapid. Commun.,
2008, 29, 347.15
21. D. E. Fullenkamp, L. He, D. G. Barrett, W. R. Burghardt, and P.
B. Messersmith, Macromolecules, 2013, 46, 1167.
22. S. R. Narayanan, S. Yen, L. Liu, and S. G. Greenbaum, J. Phys.
Chem. B., 2006, 110, 3942.
23. L. A. Belfiore, A. T. N. Pires, Y. Wang, H. Graham, and E.20
Ueda, Macromolecules, 1992, 25, 1411.
24. M. P. McCurdie, and L. A. Belfiore, Polymer, 1999, 40, 2889.
25. J. Li, R. J. Kuppler, and H. Zhou, Chem. Soc. Rev., 2009, 38,
1477.
26. J. Germain, J. Hradil, J. M. J. Frechet, and F. Svec, Chem.25
Mater., 2006, 18, 4430.
27. A. Demessence, and J. R. Long, Chem. Eur. J., 2010, 16, 5902.
28. B. Zornoza, A. Martinez-Joaristi, P. Serra-Crespo, C. Tellez, J.
Coronas, J. Gascon, and F. Kapteijn, Chem. Commun., 2011, 47,
9522.30
29. J. T. Culp, L. Sui, A. Goodman, and D. Luebke, J. Colloid
Interface Sci., 2013, 393, 278.
30. J. Weber, M. Antonietti, and A. Thomas, Macromolecules, 2008,
41, 2880.
View Article Online
DOI: 10.1039/C3TA14470D

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