Halophilic polysulfabetaines-synthesis and study of gelation and thermoresponsive behavior

Article abstract of DOI:10.1039/c4ra00928b

Polysulfabetaines (PSBs) derived from zwitterionic sulfates (contrary to commonly used polysulfobetaines which are derived from zwitterionic sulfonates) were synthesized for the first time. PSBs dissolved in brine (halophilic), swelled in deionized water and also exhibited reversible and irreversible thermoresponsive behavior. Large differences in interaction with deionized water and brine solution as well as morphology of particles in dispersion was observed between poly[2-(dimethyl(4-vinylbenzyl)ammonio)ethyl sulfate] (PSB 1) and poly[3-(dimethyl(4-vinylbenzyl)ammonio)propyl sulfate] (PSB 2) even though a single methylene (-CH₂-) unit was the sole differentiating factor. The gelation and thermoresponsive behaviour observed in PSBs are similar to that of naturally occurring polymers. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra00928b

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
Halophilic polysulfabetaines – synthesis and study
of gelation and thermoresponsive behavior†
Cite this: RSC Adv., 2014, 4, 22596
a
a
a
*
*
Vivek Arjunan Vasantha, Satyasankar Jana, Anbanandam Parthiban
and Julius G. Vancsoab
Received 1st February 2014
Accepted 12th May 2014
DOI: 10.1039/c4ra00928b
www.rsc.org/advances
Polysulfabetaines (PSBs) derived from zwitterionic sulfates (contrary to nature of polymer in terms solubility and other characteristics.10
commonly used polysulfobetaines which are derived from zwitterionic There are possibilities to further inuence the characteristics of
sulfonates) were synthesized for the first time. PSBs dissolved in brine zwitterionic polymers. One such possibility is the zwitterionic
(halophilic), swelled in deionized water and also exhibited reversible polysulfates which can also be called as polysulfabetaines.
and irreversible thermoresponsive behavior. Large differences in Unlike the polysulfobetaines where sulphur atom of sulfonate
interaction with deionized water and brine solution as well as group is directly linked to carbon, in polysulfabetaines, carbon
morphology of particles in dispersion was observed between poly[2- atom is linked to sulphur through oxygen atom (Fig. 1). Theo-
(dimethyl(4-vinylbenzyl)ammonio)ethyl sulfate] (PSB 1) and poly[3- retically, this additional oxygen atom can be expected to intro-
(dimethyl(4-vinylbenzyl)ammonio)propyl sulfate] (PSB 2) even though duce more hydrophilicity to the polymer. The presence of
a single methylene (–CH2–) unit was the sole differentiating factor. additional heteroatom in the form of oxygen also increases the
The gelation and thermoresponsive behaviour observed in PSBs are distance between counterions. Such increase in distance could
similar to that of naturally occurring polymers.
ultimately inuence the nature of ionic interactions.11 Under
some circumstances, the interaction may become more of
intermolecular in nature than intramolecular.12 As reported
herein, this turned out to be true. Substantial difference in
properties were observed even between two polymers where a
single methylene (–CH2–) unit was the sole distinguishing
factor. Though there are many reports on the synthesis of poly-
sulfobetaines, to the best of our knowledge reports on poly-
sulfabetaines are non-existent. Polysulfabetaines exhibit many
interesting characteristics as discussed hereunder including
dissolution in concentrated sodium chloride solutions and thus
the term halophilic (“salt-loving”). It may be noted that micro-
organisms like bacteria living under harsher conditions such as
brine of varying concentration are commonly referred to as
halophilic in literature.13–17
Polymers bearing ionic groups such as polyampholytes and
zwitterionic polymers have great potential for various applica-
tions due to their unique interactions in aqueous solutions.1–3
Polymeric zwitterionic sulfonates also known as poly-
sulfobetaines4–9 are the predominant class of zwitterionic
polymers widely reported thus far because of ease of synthesis
as well as due to the commercial availability of wide variety of
starting materials. In the case of polysulfobetaines, property
variations are introduced by way of either changing the number
of methylene units between cationic species which is largely
ammonium ion and the sulphur atom or by changing the
nature of cation forming unit i.e. the tertiary amine or quar-
ternizable amino compound. The former approach is limited in
that the number of carbon atoms can only be varied between
three and four. However, by changing the nature of cationic
moiety, we recently introduced exceptional changes to the
aInstitute of Chemical and Engineering Sciences (ICES), Agency for Science, Technology
and Research (A*STAR), 1 Pesek Road, Jurong Island, Singapore 627833. E-mail:
vivek_vasantha@ices.a-star.edu.sg; aparthiban@ices.a-star.edu.sg
bMESA+ Research Institute for Nanotechnology, Faculty of Science and Technology,
University of Twente, P. O. Box 217, 7500 AE Enschede, The Netherlands
† Electronic supplementary information (ESI) available: Detailed experimental
description and results including 1H NMR, FT-IR DLS, TEM, detailed phase
studies etc. See DOI: 10.1039/c4ra00928b
Fig. 1 Typical chemical structure of (a) polysulfobetaine and (b)
polysulfabetaine.
22596 | RSC Adv., 2014, 4, 22596–22600
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
The polymerizable sulfabetaine monomers SBM 1 and 2
were prepared in a single step by ring opening reaction18 of
cyclic 1,2-ethylene sulfate and 1,3-propylene sulphate (Schemes
1 and S1†) by N-(4-vinylbenzyl)-N,N-dimethylamine, which led
to zwitterionic sulfabetaine monomers in good yields (83–87%).
To the best of our knowledge, SBM 1 and 2 have not been
reported before. SBM 1 and 2 were obtained as powders which
were highly hygroscopic and were thoroughly characterized.
The 1H-NMR spectrum of SBM 1 and 2 are given in Fig. 2. Table
S1† describes the melting point and mass spectroscopic char-
acterization of sulfabetaine monomers SBM 1 and 2.
SBM 1 and 2 were polymerized by free radical polymerization
ꢀ
using 4,40-azobis(4-cyanovaleric acid) (ACVA) at 90 C in 0.5 M
aqueous NaBr solution to yield polysulfabetaines, PSB 1 and 2
respectively. Scheme 1 also shows the polymerization of sulfa-
betaine monomers. PSB 1 and 2 were thoroughly characterized
by spectroscopic techniques. The polymerization conditions are
summarized in Table S2.† The complete disappearance of vinyl
protons aer polymerization can be noticed in Fig. 2 which
Fig. 2 1H-NMR spectra of sulfabetaine monomers (SBM) in D2O and
corresponding polysulfabetaines (PSB) in (CF3)2–CD–OD.
1
deionized water was negligible. Even in 6 M HCl acid solution
the extent of hydrolysis was about 60% thereby indicating that
polysulfabetaines are fairly stable.
displays the H-NMR spectra of PSB 1 and 2. PSB 1 and 2 were
also characterized by FT-IR (Fig. S1†) and UV-Vis (Fig. S2†)
spectroscopic techniques. The solubility characteristics of PSB 1
and 2 were determined in various solvents as detailed in Table
S3.† PSB 1 and 2 were found to be soluble in formic acid, hex-
auoroisopropanol, saturated NaCl solution, trichloroacetic
acid and triuoroethanol. Both PSB 1 and 2 were completely
insoluble in common organic solvents like acetic acid, chloro-
form, methanol as well as in dipolar aprotic solvents like DMF,
DMSO, etc.
The presence of directly linked heteroatoms in the from of
C–O–S linkage in the case of polysulfabetaines warranted us to
determine the hydrolytic stability of polysulfabetaines. Table 1
summarizes the hydrolytic stability of PSB 1 and 2. Poly-
sulfabetaines were heated in deionized water and aqueous
solution of 6 M HCl at 95 ꢀC for 24 h in two separate reactions to
evaluate hydrolytic stability. The sulphur content of the starting
polymers and that of those subjected to hydrolysis were deter-
mined by elemental analysis. Since the O–S linkage is expected
to cleave upon hydrolysis, the polysulfabetaines undergoing
hydrolysis would show lesser (partial hydrolysis) to no or near
zero sulphur (substantial to complete hydrolysis) content. As
can be noticed from Table 1, the extent of hydrolysis in
Polysulfabetaines, PSB 1 and 2 absorbed water to a varying
degree and formed hydrogels. The swelling tendency and the
lack of solubility in deionized water in spite of their zwitterionic
character are due to strong ionic interactions. The equilibrium
swelling and hydration studies of PSBs are summarized in Table
2. The water absorption of PSB 1 was found to be 0.95 g per g of
polymer. PSB 2 absorbed signicantly higher amount of water
(1.97 g per g of polymer) (Table S4†). The difference in water
absorbing tendency between PSB 1 and 2 is likely due to the
Table 1 Hydrolytic stability of polysulfabetaines
Sulfur
content
(calc.)
Sulfur
content
(observed)
Sulfur content
aer treatment
with DI watera
Sulfur content
aer treatment
with 6 M HCla
Polymer
PSB 1
PSB 2
11.16
10.64
10.31
10.2
10.31
9.92
4.3
4.01
a
Aer heating at 95 ꢀC for 24 h. The polymers were dialyzed and
lyophilized before elemental analysis.
Scheme 1 Polymerization of zwitterionic sulfabetaine monomers.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 22596–22600 | 22597

View Article Online
RSC Advances
Communication
Table 2 Swelling, thermal and solubility characteristics of PSB 1 and 2
Critical salt concentrationc
(CSC, wt%)
Swelling
ratioa (SR),
% DI H2O
DSCb
Sample
PSB 1
PSB 2
(Tmax
,
ꢀC)
Titration
Transmittance
21.71
95.08 ꢁ 4.8
58
72
21.04 ꢁ 0.2
(3.59 M)
197.86 ꢁ 6.7
12.93 ꢁ 0.2
12.09
(2.21 M)
a
SR ¼ [(weight of swollen polymer ꢂ weight of dry polymer)/weight of
b
dry polymer]
ꢃ
100.
Endothermic transition signifying the
temperature at which expulsion of water from the swollen polymers
occurred. For CSC, 1.11 wt% of PSB 1 and 0.87 wt% of PSB 2 were
c
dissolved in 22.62 wt% of NaCl solution and titrated with deionised
water at 25 ꢀC (Fig. S7†).
Fig. 3 Irreversible thermoresponsive behaviour of polysulfabetaine by
temperature sweep oscillatory measurement at a constant frequency
of 1 Hz and constant strain of 1% in a heating and cooling process
:
nature of interactions prevailing in the sulfabetaine polymers.
The presence of extra methylene unit (–CH2–) could aid in the
exibility as well as separate the charges farther apart. The
former effect could help in creating larger ion cages which could
accommodate more water. The latter could enhance the inter-
chain interactions which can also create larger void thereby
PSB 1 (51 wt%) and : PSB 2 (17.69 wt%) (a). Photograph of hydrogel
before and after heat treatment in presence of DI water and dissolution
upon addition of salts (b).
facilitating the binding of more water molecules. PSB 2 also conformational changes, resulting in the formation of network
showed increased absorption tendency in 0.9 wt% aqueous structures through strong intermolecular interactions.12,24,25
NaCl solution. In this case, the absorption was found to be 8 g
per g of polymer.
The rheological properties of PSB hydrogels were studied by
time, amplitude and frequency sweep measurements (Fig. S4†).
The hydrogel is formed by the polar interaction between Frequency sweep measurements of PSB hydrogels showed that
zwitterions and water as well as by the presence of water in voids the storage modulus (G0, the elastic response) and the loss
existing in the network formed through ionic interactions. Since modulus (G00, the viscous response) were dependent over the
heating the gel can disturb these interactions, the behavior of entire frequency range. The data suggested that the measured
hydrogels was studied by differential scanning calorimetry G0 exhibited characteristic linear viscoelastic behaviour.
(DSC) (Fig. S3†). In DSC analysis, PSB 1 hydrogel exhibited an However, large variations observed in the curve clearly indicate
endothermic transition centred around at 58 ꢀC, while PSB 2 the difference in nature of interaction between PSB 1 and 2. The
showed an endothermic transition at 72 ꢀC. As was also visually result reected the gel-like behaviour of PSB 1 and 2 and there
observed and will be described later, heating helped the chains was no break-up of the gel microstructure.
to align together closely which in turn enhanced the ionic
The irreversible changes that occurred in PSB hydrogels
interactions because of which association with water was upon heating due to the enhanced ionic interactions between
disturbed. As a consequence, water was expelled from the the charged sulfate, and ammonio groups were also evidenced
network and also induced contrasting morphological changes by temperature dependent oscillatory measurement and were
in the precipitated polymer as observed in a transmission consistent with the phase transitions observed visually as well
electron microscope (TEM).
as by DSC. Fig. 3a shows that upon increasing the temperature
In polysulfobetaines derived from methacrylate and acryl- from 25 to 55 C, a slight increase of G0 was observed. Above
amide19–23 the network was gradually destroyed at higher 55 ꢀC, G0 started to increase with temperature indicating that gel
temperature in water and became soluble (UCST). In contrast, network was broken which can be attributed to the expulsion of
PSB gels underwent temperature induced collapse and showed water. Above 60–75 ꢀC represented the temperature range where
some unexpected morphological changes. Visually, the expul- G0 modulus increased abruptly upon heating because of the
sion of water from PSB hydrogels, as indicated by the formation formation of self assembled 3D networks (Fig. 3) of varying
of deep milky appearance upon heating, occurred, in PSB 1 dimension and shape as dictated by the nature of basic ionic
around 55–60 ꢀC and in PSB 2 around 60–65 ꢀC which are interactions. Upon increasing the temperature above phase
comparable to the transitions observed in DSC. The polymers transition, higher values of G0 were reached, indicating the
began to coagulate and precipitate, when the gel was kept for formation of a more stable, irreversible network. Also, the
very short time (<1 min) above the cloud point temperature increase in G0 and G00 aer phase transition indicated the exis-
(Fig. 3b). This phenomenon is comparable to the situation tence of a stiffened network. Upon cooling, G and G00 showed
prevailing in some biomacromolecules. It is well known some deviation thereby showing the irreversible behaviour.
ꢀ
that upon thermal treatment biomacromolecules such as
gelatin, polysaccharides, collagen and lactoglobulin undergo was investigated by TEM and dynamic light scattering (DLS)
The self-assembly26,27 of heat-induced aggregates of PSB
22598 | RSC Adv., 2014, 4, 22596–22600
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
Fig. 5 Reversible UCST behavior by association and dissociation of
PSBs with respect to salt concentration after increasing the temper-
ature from 20 to 97 ꢀC; (a) the filled and open symbols represents the
phase boundaries of the PSB 1 and 2 in NaCl respectively. (b) Photo-
graphs of PSBs in NaCl solution before and after heat treatment; PSB 1
(1.03 wt% of PSB 1 in 21.04 wt% NaCl) and PSB 2 (0.49 wt% of PSB 2 in
12.93 wt% NaCl).
Fig. 4 TEM micrographs of PSB gel after heat treatment in the pres-
ence of water at 70 ꢀC for 15 min (a) spherical aggregates (PSB 1; scale
bar: 100 nm and inset 20 nm) and rod-like aggregates (PSB 2; scale
bar: 2 mm and inset 1 mm).
techniques. Fig. 4a and b show TEM images of the PSB gels
(1 wt%) aer heat treatment at 70 ꢀC for 15 min. It is clear from
TEM micrographs that PSBs formed self-assembled aggregates.
At 70 ꢀC, PSB 1 formed spherical, predominantly mono-
dispersed aggregates in the range of 70–90 nm (Fig. S5†). The
shape of the aggregates obtained for PSB 2, on the other hand,
was completely different, rod like aggregates of 300–600 nm
with the cross-section of around 100 nm. PSB 2 formed large
macro aggregates predominantly through interchain interac-
tions which prolonged to the extent of forming brils28 similar
to irreversible hydrocolloids like collagens25,29 or carrageen.30
Interestingly, the dendritic micellar microstructures observed
in PSB 2 was caused by the presence of one additional methy-
lene unit as compared to PSB 1.
DLS experiments further validated these observations
(Fig. S6†). Upon addition of salt to the heat treated dispersions,
deep milkiness of the dispersion changed to more translucent
in appearance thereby indicating that solvation of zwitterions
accompanied by the uncoiling of chains under the inuence of
charged interaction of the added electrolyte (Fig. 3b). PSB
polymer chains folded at higher temperatures due to the prev-
alence of intra- and interchain ionic interactions and near
complete unfolding occurred upon the addition of salt thus
showing unusual halophilic behaviour.
upon lowering the salt concentration (Fig. S8†). UCST of PSB 1
was higher than that of PSB 2. Furthermore, it was observed that
PSB 2 favoured less interaction when compared to its counter-
part thereby indicating the prevalence of interchain interac-
tions caused by the additional methylene unit. It may be noted
that similar trend was observed in sulphate based low molec-
ular weight surfactants as well.38,39
The reversibility of turbid-to-clear (globular-to-coil confor-
mations) transitions in brine solution upon increasing the
temperature was further studied using DLS (Fig. S9†) and
viscometry. At the elevated temperature, above the UCST, the
PSBs became clear without any observable aggregates (coil
form; 25–60 nm) due to charge screening (better solvation thus
greater solvent–polymer interaction and reduced polymer–
polymer interaction). As the temperature decreased, the poly-
mer conformation changed, resulting in the collapse of the
polymer coil which led to larger aggregates (like globular) with
the average size above 400 nm (greater polymer–polymer
interaction). This resulted in the formation of turbid solution.
Heating induces dissociation caused by the mobility of polymer
chains. Additionally, the tendency to form aggregates below
UCST was also supported by viscosity studies (Fig. S10†). Similar
to polysulfobetaines,10 polysulfabetaines also showed non-
Newtonian behaviour (Fig. S11†), due to the enhanced chain–
chain interactions caused by zwitterionic nature at higher shear
rate. Thus, polysulfabetaines exhibited electrolyte induced
thermoresponsive behaviour which is accompanied by confor-
mational changes such as globule-to-coil transition.
In summary, we have reported a novel class of halophilic,
thermoresponsive polysulfabetaines. The presence of one extra
methylene unit between the counterions of polysulfabetaines
induced more interchain interactions thereby inuenced
properties like water uptake, sorption of brine solution, con-
trasting rheological and thermoresponsive behaviour. Ionic
interactions increased with temperature which resulted in the
expulsion of water from swollen polysulfabetaine gels. The
nature of interactions (intra- vs. inter-) also determined the
shape of particles formed which varied from spherical to rod
like, dendritic aggregates. Furthermore, polysulfabetaines
showed a characteristic and reproducible thermoresponsive
The halophilic polysulfabetaines, PSB 1 and 2 also exhibited
temperature dependent phase separation in brine solution. The
characteristically different states were observed at low (turbid
phase; association) and high temperatures (clear phase; disso-
ciation). This association–dissociation behaviour depended on
the critical salt concentration31–33 (CSC) or critical electrolyte
concentration34 (CEC) (minimum salinity that can cause disso-
lution) and is summarized in Table 2 (Fig. S7†). Like poly-
sulfobetaines,10,35–37 polysulfabetaines also exhibited reversible
ꢀ
ꢀ
UCST behavior in salt solution between 20 C and 97 C. This
phase transition represents, to the best of our knowledge, the
rst example of a thermoresponsive UCST behavior in presence
of higher salt concentration. As expected, with decrease in salt
concentration the UCST increased. The three dimensional plot
of the UCST of the PSBs as a function of NaCl and polymer
concentrations with respect to temperature is shown in Fig. 5a.
As can be inferred from Fig. 5a, the UCST window shied from
low temperature region to high temperature region, simply
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 22596–22600 | 22599

View Article Online
RSC Advances
Communication
behavior in salt solution. UCST behaviour was inuenced by the 17 A. Oren, Environ. Technol., 2010, 31, 825–834.
concentration of salt as well as temperature. Such a subtle 18 D. A. Tomalia and J. C. Falk, J. Heterocycl. Chem., 1972, 9,
behavior of polysulfabetaines in brine solution is very inter- 891–894.
esting and may be suited for a range of applications such as 19 J. C. Salamone, W. Volksen, A. P. Olson and S. C. Israel,
enhanced oil recovery in the petroleum eld and low tempera-
ture precipitation of proteins in biomedical eld.
Polymer, 1978, 19, 1157–1162.
20 S. Liang, Q. M. Yu, H. Yin, Z. L. Wu, T. Kurokawa and
J. P. Gong, Chem. Commun., 2009, 7518–7520.
21 J. C. Galin and M. Galin, J. Polym. Sci., Part B: Polym. Phys.,
1995, 33, 2033–2043.
Acknowledgements
This work was funded by the Agency for Science, Technology
and Research (A*STAR), Singapore under Innovative Marine
Antifouling Solutions (IMAS) for high-value applications pro-
gramme. The authors thank Ms. Chen Junhui and Ms. Foo Ming
Choo for assisting in GPC and DLS studies and Ms. Tay Boon
Ying for assistance with TEM analysis.
22 J. Ning, G. Li and K. Haraguchi, Macromolecules, 2013, 46,
5317–5328.
23 Z. Zhang, T. Chao and S. Jiang, J. Phys. Chem. B, 2008, 112,
5327–5332.
24 M. Meyer, M. Antonietti and H. Schlaad, So Matter, 2007, 3,
430–431.
25 H. Hoermann and H. Schlebusch, Biochemistry, 1971, 10,
932–937.
26 T. Vermonden, R. Censi and W. E. Hennink, Chem. Rev.,
2012, 112, 2853–2888.
27 P. J. Roth, T. P. Davis and A. B. Lowe, Polym. Chem., 2012, 3,
2228–2235.
28 V. J. Bradford and B. L. Iverson, J. Am. Chem. Soc., 2008, 130,
1517–1524.
Notes and references
1 A. V. Dobrynin, R. H. Colby and M. Rubinstein, J. Polym. Sci.,
Part B: Polym. Phys., 2004, 42, 3513–3538.
2 S. E. Kudaibergenov and A. Ciferri, Macromol. Rapid
Commun., 2007, 28, 1969–1986.
3 S. Jana, V. A. Vasantha, L. P. Stubbs, A. Parthiban and
J. G. Vancso, J. Polym. Sci., Part A: Polym. Chem., 2013, 51,
3260–3273.
29 K. E. Kadler, D. F. Holmes, J. A. Trotter and J. A. Chapman,
Biochem. J., 1996, 316, 1–11.
4 A. B. Lowe and C. L. McCormick, Chem. Rev., 2002, 102,
4177–4189.
5 M. S. Donovan, A. B. Lowe, T. A. Sanford and
C. L. McCormick, J. Polym. Sci., Part A: Polym. Chem., 2003,
41, 1262–1281.
6 F. Q. Xuan and J. S. Liu, Polym. Int., 2009, 58, 1350–1361.
7 S. Y. Jiang and Z. Q. Cao, Adv. Mater., 2010, 22, 920–932.
8 J. Kuang and P. B. Messersmith, Langmuir, 2012, 28, 7258–
7266.
9 Z. Cao and S. Jiang, Nano Today, 2012, 7, 404–413.
10 V. A. Vasantha, S. Jana, A. Parthiban and J. G. Vancso, Chem.
Commun., 2014, 50, 46–48.
11 R. G. Laughlin, Langmuir, 1991, 7, 842–847.
12 B. Ozbas, J. Kretsinger, K. Rajagopal, J. P. Schneider and
D. J. Pochan, Macromolecules, 2004, 37, 7331–7337.
13 B. van den Burg, Curr. Opin. Microbiol., 2003, 6, 213–218.
14 P. DasSarma, J. A. Coker, V. Huse, S. DasSarma and
M. C. Flickinger, in Encyclopedia of Industrial Biotechnology,
John Wiley & Sons, Inc., 2009.
30 T. Funami, M. Hiroe, S. Noda, I. Asai, S. Ikeda and
K. Nishimari, Food Hydrocolloids, 2007, 21, 617–629.
31 V. M. Monroy Soto and J. C. Galin, Polymer, 1984, 25, 254–
262.
32 D. A. Z. Wever, F. Picchioni and A. A. Broekhuis, Prog. Polym.
Sci., 2011, 36, 1558–1628.
33 K. C. Khilar and H. S. Fogler, J. Colloid Interface Sci., 1984,
101, 214–224.
34 E. E. L. Kathmann, L. A. White and C. L. McCormick,
Macromolecules, 1997, 30, 5297–5304.
35 D. N. Schulz, D. G. Peiffer, P. K. Agarwal, J. Larabee,
J. J. Kaladas, L. Soni, B. Handwerker and R. T. Garner,
Polymer, 1986, 27, 1734–1742.
36 J. Seuring and S. Agarwal, Macromol. Rapid Commun., 2012,
33, 1898–1920.
37 Q. Zhang, J.-D. Hong and R. Hoogenboom, Polym. Chem.,
2013, 4, 4322–4325.
38 P. G. Faulkner, A. J. I. Ward and D. W. Osborne, Langmuir,
1989, 5, 924–926.
15 A. Oren, Microbiol. Mol. Biol. Rev., 1999, 63, 334–348.
16 J. M. Pastor, M. Salvador, M. Argandona, V. Bernal, M. Reina-
Bueno, L. N. Csonka, J. L. Iborra, C. Vargas, J. J. Nieto and
M. Canovas, Biotechnol. Adv., 2010, 28, 782–801.
39 P. G. Nilsson, B. Lindman and R. G. Laughlin, J. Phys. Chem.,
1984, 88, 6357–6362.
22600 | RSC Adv., 2014, 4, 22596–22600
This journal is © The Royal Society of Chemistry 2014