Non-covalent marriage of β-cyclodextrin-polyethylene glycol and functional polyacrylamide: Via host-guest interaction: Construction, characterization and performances

Article abstract of DOI:10.1039/c6ra07102c

This paper reports the non-covalent marriage of bridged β-cyclodextrin and functional acrylamide polymers via host-guest interactions. The structure of supramolecular polymeric systems (polymer/bis-CD) was determined by FT-IR, ¹H NMR and SEM. T

Full text of DOI:10.1039/c6ra07102c

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Nonꢀcovalent marriage of β-cyclodextrinꢀpolyethylene glycol and
functional polyacrylamide via hostꢀguest interaction: Construction,
Characterization and Performances
,
1
2
3
∗
Yanlin Zhang , Naidi Tan , Pinwen Zhao
1School of Chemistry and Pharmaceutical Engineering, Jilin Institute of
Chemical Technology, Jilin, Jilin 132022, PR China
2Department of Chemical Engineering, Northeast Dianli University, Jilin,
Jilin 132012, PR China
3China Petroleum Engineering Co., Ltd., Beijing Company (CPE Beijing
Company), Beijing 100085, PR China
∗Corresponding author at: No.45 Chengde Street, Jilin, Jilin Province
132022, PR China. Tel.: 0432ꢀ6218 5122 Fax: 0432ꢀ6218 5122
Eꢀmail address: tannaidi1970@163.com
1

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Abstract
This paper reports the nonꢀcovalent marriage of bridged
βꢀcyclodextrin and functional acrylamide polymer via host−guest
interactions. The structure of supramolecular polymeric system
1
(Polymer/BisꢀCD) was determined by FTꢀIR, H NMR and SEM. The
XRD and thermal analysis were carried out to further validate the
formation of the inclusion complex. This interesting hostꢀguest system
could significantly enhance the acrylamide polymer’s performance with
good viscosity property and shear resistance, high salt tolerance, thermo
responsive behavior, and surfactant compatibility, which indicate
potential applications in many fields.
Key words: Supramolecular; host–guest interactions; βꢀcyclodextrin;
functional polyacrylamide
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1. Introduction
In recent years, supramolecular polymeric system have attracted
considerable interests in various fields such as organic chemistry,
polymer chemistry [1, 2], biomedical applications [3] and pharmaceutical
applications [4ꢀ6] et al. The main reason is that the supramolecular
polymers have many unique properties such as being reversible,
adaptability, stability, selfꢀhealing ability and flexibility [7, 8]. Generally,
supramolecular polymers are constructed by noncovalent interactions，e.g.
van der Waals, hydrogen bonding, π–π stacking, hydrophobic interactions
and hostꢀguest interaction et al. [9ꢀ11], as well as the combination of
them. In particular, the construction of supramolecular polymeric system
forced by hostꢀguest interaction has received much more attention.
Particularly, the βꢀcyclodextrin (βꢀCD) and its derivatives have been
widely used as the host molecules to construct the supramolecular
polymeric systems because their excellent properties.
It is well known that βꢀCD is a kind of natural molecules derived
from starch [11], which is composed of 7 Dꢀ(+)ꢀglucose units linked by
1,4ꢀαꢀlinkages and exhibiting a torusꢀshaped ring structure [12]. The
βꢀCD has a unique structure, whose interior cavity is hydrophobic while
the external shell is hydrophilic [13]. Because of its special architectural
conformation, the βꢀCD can selectively incorporate hydrophobic
molecules of appropriate size as the guest into its cavity to generate
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hostꢀguest inclusion complexes. In addition, the βꢀCD contained
supramolecular systems exhibited different desirable properties according
to different researches. Brigitte Voit et al. [14] prepared a diblock
copolymer system via selfꢀassembly of cyclodextrin and adamantyl
endꢀfunctionalized polymer. It was found that the copolymer was
reversibly thermoresponsive and had potential use as drug delivery
system. Akira Harada and his coꢀworkers [15] used host−guest
interactions to prepare supramolecular system by βꢀCD and adamantane
derivative in aqueous solutions. This supramolecular hydrogel processed
highly elastic and shape recovery properties, which indicated its medical
and industrial material applications. Zhang et al. [16] presented a novel
supramolecular system, which was formed through host−guest
interactions between βꢀCD moiety and adamantyl groups, to control the
thermoꢀresponsive property of Poly(Nꢀisopropylacrylamide). Additionally,
many other papers focused on host/guest inclusion complexes with βꢀCD
have been reported to explore the unlimited potential applications in
various fields [17ꢀ22]. However, most of them used βꢀCD and
adamantane derivatives as the host and guest molecules, respectively.
Relatively limited reports have been found yet in the literature to use
polyacrylamide (PAM) derivatives with alkyl chains as guest molecules.
In fact, PAM occupies an important position in various fields of the
national economy. It is usually applied in water treatment, paper making,
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screen printing, medical science, or used as soil conditioner [23ꢀ27].
Another common use of polyacrylamide is in subsurface applications
such as Enhanced Oil Recovery (EOR) [28]. However, with the rapid
development of science and economy, the traditional polyacrylamide
cannot meet the needs of practical applications due to its following
disadvantages: (I) poor temperature resistance performance; (II) poor salt
tolerance; (III) lack of high shear resistance [29]. Therefore, how to make
a difference to overcome these shortcomings of PAM has become the
focus of domestic and international scientists. Consequently, the
preparation of ultrahigh molecular weight polyacrylamide and the
chemical modification of acrylamide polymer structure have received
worldwide attention from researchers [30]. However, to the best of our
knowledge, there is no report found in the literature to prepare
supramolecular system by using bridged bisꢀCD as host molecule and
functional acrylamide polymer as guest molecule.
Herein, in this paper, we report on the fabrication of a novel
supramolecular
polymeric
system
glycol
based
and
on
the
β-cyclodextrinꢀpolyethylene
[31],
functional
acrylamideꢀcontaining guest polymer via hostꢀguest interaction. We wish
to share new inspiration on designing and constructing noncovalently
connected supramolecular polymer through our present work.
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2. Experimental Section
2.1 Materials
The 4ꢀvinyl pyridine were obtained from Chengdu Best Reagent Co.,
Ltd., China. The 1, 3ꢀpropane sultone, octadecyl dimethyl allyl
ammonium chloride (C18DMAAC) were respectively supplied by Zibo
Shibang Chemical Co., Ltd., China and Sichuan Guangya Polymer
Chemical Co., Ltd., China. The rest of the reagents were purchased from
Chengdu Kelong Chemical Reagent Factory, China, including sodium
bisulfite (NaHSO3), anhydrous ethanol, acetone, ammonium persulfate,
bromine water, potassium iodide, sodium thiosulfate, tetramethylsilane
(TMS) and cetyl trimethyl ammonium chloride (CTAC). All of the above
materials were of analytical purity and were used without further
purification.
The bridged βꢀcyclodextrinꢀpolyethylene glycol (βꢀCDꢀPEG) was
selfꢀsynthesized according to the reported procedure [31].
2.2 Synthesis
4ꢀvinylꢀ1, 3ꢀsulfopropyl pyridinium (VPPS). 12.2g (0.1 mol) of 1,
3ꢀpropanesultonewas added into the flask with 60 mL of acetone and was
flushed for 20 min. Thereafter, 10 mL of 4ꢀvinyl pyridine was added and
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the solution was stirred at 30 °C for 12 h. The crude product was white
and precipitated in acetone. Then it was washed several times with
acetone and dried in a vacuum drying oven at 60℃ for 48 h. Yield: 19.8
g (87.2 %).
FTꢀIR (KBr): ν = 1043 (m, v(SꢀO)), 1197 (m, v(S=O), 1663 (s, v(C=N),
1
v(C=C)), 3081 (s, ν(CꢀH) on C=C), 910 (s, ν(C=H2)). H NMR (D2O):
8.653, 7.846 (m, protons in pyridine), 5.782 (s, protons in ꢀC=CH2), 2.664
(m, methylene protons in pyridine).
Functional acrylamide polymer P(AM/VPPS/C18DMAAC) as
guest polymer. 100 mL of aqueous solution containing AM (21.8 g),
C18DMAAC (1.2g) and VPPS (2g) was prepared in a three roundꢀneck
flask. Then, the oxygen dissolved in the reaction system was removed by
vacuumꢀpumping and nitrogenꢀfilling for 30min. Subsequently, 0.0375 g
of ammonium persulfate and NaHSO3 as initiator (molar ratio 2:1, 0.15
wt%), and 1 mL of EDTA as chelating agent were added into the flask
with stirring under the temperature of 45°C with argon and stirring for 5h.
The raw product was dissolved in deionized water and washed by acetone.
Then it was cut into pieces and dried in the oven for 48 h at 60°C. The
P(AM/VPPS/C18DMAAC) was obtained as a white solid. Yield: 21.4 g
(85.6 %).
FTꢀIR (KBr): ν = 3377 (b, ν(NꢀH)), 2940, 2839 (m, vs(CꢀH) and
vas(CꢀH)), 1647 (s, v(C=O), amide I), 1454 (m, ν(CꢀN) and δ(NꢀH),
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1
amide II), 1189, 1034 (m, v(SꢀO)), 588 (b, vs(CꢀH)). H NMR (D2O):
δ(ppm) = 2.064 (m), 1.031 (s) (ꢀCH2ꢀ in backbone), 2.212, 1.535 (m,
ꢀCHꢀ in backbone), 8.671, 7.830 (m, protons in pyridine), 3.111, 2.962,
1.641 (m, methylene protons in pyridine), 1.173 (m, ꢀCH2ꢀ in
C18DMAAC), 3.508(m, methyl protons in quaternary ammonium
connected with N+) 1.063 (s, CH3 in C18DMAAC).
βꢀcyclodextrinꢀpolyethylene glycol (βꢀCDꢀPEG) as host monomer.
The βꢀCDꢀPEG was selfꢀsynthesized according to the reported literature
[31] (also see Supporting Information). Yield: 18.2 g (77.4%).
FTꢀIR (KBr): ν = 3441 (b, ν(OꢀH)), 3049 (m, v(CꢀH)), 2932 (w,
vas(CꢀH)), 2867 (w, vs(CꢀH)), 1481 (s, δ(CꢀH)), 1198 (s, vas(CꢀO)), 870,
1
724, 597 (m, vibration absorption peaks of βꢀCD). H NMR (D2O):
δ(ppm) = 4.944 (s, C(1)H of βꢀCD), 3.756, 3.867, 3.533, 3.820 (m,
C(2)HꢀC(5)H of βꢀCD), 3.822 (m, C(6)H of βꢀCD), 3.441ꢀ3.497 (m,
protons in PEGꢀ400).
Fabrication
of
supramolecular
polymeric
system
(Polymer/BisꢀCD). Typically, the supramolecular polymer was formed
by mixing the aqueous solutions of host monomer βꢀCDꢀPEG and guest
polymer P(AM/VPPS/C18DMAAC) (mole ratio=1:1) with ultrasonic at
the room temperature.
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2.3 Methods
Fourier transform infrared (FTꢀIR) spectra were recorded on a Nicolet
Nexus 470 FTꢀIR spectrometer in the optical range of 400−4000 cm−1 by
1
averaging 32 scans at a resolution of 4 cm−1. The H NMR spectra were
recorded using a Bruker ASCENDꢀ400 NMR spectrometer (Bruker
Optics) at 500.13Hz. The solvent is D2O and the spectra recorded from
D2O
solutions
were
referenced
on
external
sodium
3ꢀ(trimethylsilyl)ꢀ1ꢀpropanesulfonate.
The scanning electron microscope (SEM) observations were carried
out on a Quanta 450 instrument (FEI, USA, Hillsboro). Powder Xꢀray
diffraction (XRD) was conducted in an X’pert PRO diffractometer
(PANalytical B.V., Netherlands), using CuꢀKα (λ= 1.5460 nm) with 40
mA, 40 kV. The scanning step size was 2θ = 0.02° between 2θ = 5°~70°.
The data of thermal analysis (thermoꢀgravimetry, TG and differential
scanning calorimetry, DSC) were recorded through a TG 209 F3 Tarsus
instrument (NETZSCH, Germany), with a 10 °C/min heating rate from
room temperature to 500 °C. Measurements of viscosities were conducted
on an HAAKE MARS III rheometer (Thermo Electron Corporation,
Karlsruhe, Germany), which was equipped with rheology software,
HAAKE RheoWin 3, to collect and store data. A thermostat HAAKE
Phoenix 2 was used to control the temperature with an accuracy of
0.1 °C.
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The cloud point temperature of the polymer solutions was measured
by using an 1800 UV/vis spectrometer from Shimadzu (Japan) with a
wavelength of 500 nm, circulating water bath in the temperature range of
10ꢀ70 °C. The measurements were carried out at least five times. The
polymer solutions were prepared in a concentration of 3000 mg/L using
distilled water.
3. Results and discussion
3.1 Preparation of supramolecular hydrogel
With the purpose of improving the properties of polyacrylamide and
enhancing its application ability, we synthetized the functional
acrylamide polymer P(AM/VPPS/C18DMAAC) as guest polymer. The
functional monomer VPPS was firstly synthetized, which has
antiꢀtemperature and antiꢀsalt properties. The C18DMAAC has highly
hydrophobic group which could be a good guest molecule for βꢀCD [29,
32]. The potential host component βꢀCDꢀPEG was prepared in two steps:
preparing of chlorinated polyethylene glycol monomer and bridging the
monomer between cyclodextrins [31]. The Fig. 1 depicts the preparation
process of monomers.
The supramolecular polymeric system (Polymer/BisꢀCD) could be
easily
constructed
by
mixing
the
βꢀCDꢀPEG
and
P(AM/VPPS/C18DMAAC) (mole ratio=1:1) in water, stirring until
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dissolved. Then the mixtures were dispersed continuously for half an
hour using an ultraꢀsonication homogenizer Sonifier250 (Branson
Ultrasonics, Danbury, USA). The final supramolecular polymeric system
was obtained after properly stewing via hostꢀguest interaction (see Fig.
2).
Figure.1 Synthetic routes employed for the preparation of
P(AM/VPPS/C18DMAAC) and βꢀCDꢀPEG.
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Figure.2 Schematic illustration of the formation of
supramolecular polymer.
3.2 Complexation Studies
In order to identify the interaction of βꢀCDꢀPEG and
P(AM/VPPS/C18DMAAC) in the aqueous solution, several methods were
conducted and discussed.
1
The FTꢀIR and H NMR spectra of the Polymer/BisꢀCD were shown
in Fig. 3. In Fig. 3(a), the absorption peaks observed at 934 cmꢀ1, 761
cmꢀ1 and 606 cmꢀ1 are the presence of typical structure of βꢀCD [31]. The
absorption peaks at 1673 cmꢀ1 and 1454 cmꢀ1 are assigned to the
stretching vibration of C=N bond and N－CH3 bond, respectively. In
addition, the wide scattered absorption peak at 3396 cmꢀ1 is attributed to
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the outꢀofꢀplane bending vibration of the hydroxyl group and amide
group. Therefore, it could be seen that the FTꢀIR spectrum of
Polymer/BisꢀCD is not
a
superposition of βꢀCDꢀPEG and
P(AM/VPPS/C18DMAAC). A new product was formed via hostꢀguest
interaction.
Generally, NMR spectroscopy is one of the most common used and
important method to confirm the structure of host/guest interactions. In
Fig. 3(b), the structure of Polymer/BisꢀCD could be further confirmed via
signal assignments. The characteristic signals of host and guest
monomers are identified again in the spectrum. However, the chemical
1
shift of some hydrogen atoms have been changed compared with the H
NMR spectra of βꢀCDꢀPEG and P(AM/VPPS/C18DMAAC) (see
Supporting Information). The results are listed in Table 1 and 2. In Table
1, C1~C6 represent the hydrogen atoms on βꢀCD, respectively. It can be
seen from Table 1 that ꢁδ of C3 and C5 in βꢀCD cavity was ꢀ0.023 and
ꢀ0.032 ppm, respectively, indicating an upfield shift of C3 and C5 protons
after complexation. That might be the results of the increase of electron
cloud density in βꢀCD cavity due to the complexation. On the other hand,
the protons of C1, C2, C4 and C6 are hydrogen atoms on the outside of the
cavity of βꢀCD, so the changes of δH were slight. Comparing the proton
chemical shift between P(AM/VPPS/C18DMACC) and Polymer/BisꢀCD
(see Table 2), there were almost no changes for pyridine ring, while the
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protons of octadecyl chain exhibited an upfield shift. It was suggested
that βꢀCDꢀPEG and P(AM/VPPS/C18DMACC) formed supramolecular
hostꢀguest complex, and the structure unit of guest in the host cavity was
longꢀchain alkyl.
1
Figure.3 (a) FTꢀIR spectra of the Polymer/BisꢀCD; (b) H NMR
spectra of the Polymer/BisꢀCD.
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Table 1 The change of chemical shift of Polymer/BisꢀCD compared
with βꢀCDꢀPEG
Chemical Shift (ppm) C1
C2
C3
C4
C5
C6
βꢀCDꢀPEG
Polymer/BisꢀCD
ꢀδ
4.944 3.756 3.867 3.533 3.820 3.822
4.947 3.751 3.844 3.539 3.788 3.824
+0.003 ꢀ0.005 ꢀ0.023 +0.006 ꢀ0.032 +0.002
Table 2 The change of chemical shift of Polymer/BisꢀCD compared
with P(AM/VPPS/C18DMAAC)
Chemical Shift(ppm)
P(AM/VPPS/C18DMAAC)
Polymer/BisꢀCD
ꢀδ
Ha
Hb
7.830
7.830
0
Hc
Hd
8.671
8.670
ꢀ0.001
1.173
1.165
ꢀ0.008
1.074
1.063
ꢀ0.011
The SEM images are shown in Fig. 4 to compare and characterize the
structure of Polymer/BisꢀCD. Both P(AM/VPPS/C18DMACC) and
Polymer/BisꢀCD exhibited obvious threeꢀdimensional net microstructures.
In the P(AM/VPPS/C18DMACC), the molecular chains were irregular
and they crossꢀlinked and twisted with each other. However, compared
with the SEM image of P(AM/VPPS/C18DMACC), the architecture of
Polymer/BisꢀCD was much more ordered, thicker and stronger. This
might because of the different construction methods between
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P(AM/VPPS/C18DMACC) and Polymer/BisꢀCD. Due to the random
association of hydrophobic groups in P(AM/VPPS/C18DMACC), the net
microstructure is messy. But in the Polymer/BisꢀCD, the βꢀCDꢀPEG
participated in the formation of the network structure. It could not only
shield the hydrophobic effect, but also acted as the “bridge” to make it a
clear and uniform grid morphology. Therefore, the results further
confirmed that the host−guest complexation was formed.
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Figure.4 SEM micrograph of (a) P(AM/VPPS/C18DMACC); (b)
Polymer/BisꢀCD.
Powder XRD patterns of the inclusion complex could confirm the
presence of host and guest monomers as isolated solids [33]. In contrast
to the characteristic peaks of βꢀCDꢀPEG, the diffractogram of
P(AM/VPPS/C18DMACC) exhibited that it was in amorphous state (see
Fig. 5). However, the diffractogram of Polymer/BisꢀCD showed both
βꢀCDꢀPEG peaks and the amorphous halo of P(AM/VPPS/C18DMACC).
This is another advance of the inclusion structure.
Figure.5
XRD
patterns
of
(a)
βꢀCDꢀPEG;
(b)
P(AM/VPPS/C18DMACC); (c) Polymer/BisꢀCD.
In order to further confirm the formation of inclusion complex, the
thermal properties of the Polymer/BisꢀCD were investigated by TG and
DSC. The results are shown in Fig. 6. According to the TG curves, the
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polymers firstly lost crystal water and became anhydrous substances at
40~170 °C and the decomposition process of βꢀCDꢀPEG ends at 400 °C,
and the P(AM/VPPS/C18DMACC) had a significant weightlessness peak
between 250ꢀ700 °C approximately. However, the Polymer/BisꢀCD had
better thermal stability, which exhibited the decomposition temperature at
735 °C. These results indicate that the thermal stability has been
improved because of the inclusion complexation. In addition, the DSC
curves offer further information about the enhanced thermal properties of
the Polymer/BisꢀCD complex. For the βꢀCDꢀPEG, it displays two
exothermic peak at 315 and 420 °C. But the P(AM/VPPS/C18DMACC)
does not show an obvious exothermic peak during its decomposition. In
contrast, in the DSC curve of the Polymer/BisꢀCD complex, there was a
largeꢀspan but unconspicuous exothermic peak at 364.5 °C, which might
correspond to the decomposition of free βꢀCDꢀPEG. In addition, the new
significant exothermic peak at 607.8 °C suggests that the
Polymer/BisꢀCD complex is more stable due to the higher decomposition
temperature.
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Figure.6 (a) The thermoꢀgravimetric curve of Polymer/BisꢀCD;
(b) The differential scanning calorimetry curve of Polymer/BisꢀCD.
3.3 Performances
3.3.1 Viscosity
Viscosity was firstly employed to characterize the viscosification
property of Polymer/BisꢀCD with the respect to the increasing polymer
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concentrations at room temperature. The results are shown in Fig. 7. For
comparison, the variation of apparent viscosity of
P(AM/VPPS/C18DMACC) is also presented. The apparent viscosity of
both polymers increased nonlinearly with the increasing of concentrations.
The viscosity of P(AM/VPPS/C18DMACC) was higher than that of
Polymer/BisꢀCD until the polymer concentration was reached at 2450
mg/L. In addition, the transitional concentration (TC, C*) [34] of
Polymer/BisꢀCD was C1*=2100 mg/L. When the concentration was
above C1*, the conformation transition behavior of supramolecular
system would happen. The phenomena could be explained by the
following reasons. In the low concentration range, the intramolecular
complexation occupied a dominant position, which had a shielding effect
on the intermolecular complexation. When the concentration of
Polymer/BisꢀCD reached a certain degree (2100 mg/L in this case), the
intermolecular complexation would gradually take over the position,
which lead to the formation of netꢀlike structure (see illustration in Fig.
7), then the viscosity of Polymer/BisꢀCD increased intensely. Contrarily,
the viscosity of P(AM/VPPS/C18DMACC) exhibited a limited increase
with the concentration.
In addition, the shear resistance of polymer solution was measured
(0−100 s−1) at room temperature and the concentration of polymers were
fixed at 3000 mg/L. Figure 8 presents the apparent viscosity versus shear
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rate for P(AM/VPPS/C18DMACC) and Polymer/BisꢀCD. At the low shear
rate (0−5 s−1), the P(AM/VPPS/C18DMACC) exhibited shearꢀthickening
behavior and then showed shear thinning behavior. The intraꢀentangled
molecular chains were stretched under the low shear force, which led to
the transition of intraꢀmolecular associations to interꢀmolecular
associations. Hence, the viscosity increased at the low shear rate. With the
increase of shear force, the hydrophobic domains were disrupted, so that
the viscosity decreased. On the other hand, the Polymer/BisꢀCD exhibited
obvious shear thinning behavior. This was mainly because that most of
the inclusion complexes changed conformations, some gradually
decomposed with the increased shear force. Interestingly, the viscosity of
P(AM/VPPS/C18DMACC) and Polymer/BisꢀCD were reversible on
gradually reduced the shear rate. However, the viscosity of
P(AM/VPPS/C18DMACC) was lower than the beginning. It might
because the molecular chain and chemical structure of
P(AM/VPPS/C18DMACC) were partially destroyed by the shear force,
and the hydrophobic interactions recovered with decreased shear force.
Hence, the viscosity recovered but cannot find the initial point. In the
Polymer/BisꢀCD solution, the dissociated βꢀCDꢀPEG and C18DMACC
units reformed the initial complex upon releasing the shear force, the
effects of electrostatic forces, hydrogen bonding and van der Waals forces
recovered and eventually the initial viscosity was restored. The diagram
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of this mentioned process is inserted in Figure 8.
In order to make a more clear comparison, the shear resistance of
Polymer/BisꢀCD at 1000 mg/L was also exhibited in Figure 8. As can be
seen in the picture, the viscosity of dilute Polymer/BisꢀCD solution is
below 50 mPa·s. In addition, the viscosity of dilute Polymer/BisꢀCD
solution decreased with the increase of shear rate. This is mainly because
the conformation transition behavior of supramolecular system did not
happen yet at the low concentration. And the intramolecular complexation
effect was still weak because the concentration was only half of the
transitional concentration (2100 mg/L). Therefore, the viscosity of
Polymer/BisꢀCD (1000 mg/L) was irreversible on gradually reduced the
shear rate.
Figure.7 Concentration dependence of the apparent viscosity.
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Figure.8 The shear resistance of P(AM/VPPS/C18DMACC) and
Polymer/BisꢀCD.
3.3.2 Thermoꢀresponsive behavior
The thermoꢀresponsive behavior of Polymer/BisꢀCD was also
studied in the present work. The temperature dependence of transmittance
(λ=500 nm) of P(AM/VPPS/C18DMACC) and Polymer/BisꢀCD are
shown in Fig.9. It can be seen that both polymers show remarkable
heatꢀinduced phase transitions at 35.2 and 42.6 °C, respectively. The
obviously low cloud point (CP) of P(AM/VPPS/C18DMACC) might
relate to the hydrophobic C18DMACC group [35]. On the other hand, the
CP of βꢀCDꢀPEG was 65 °C, approximately, according the literature [31].
Because the hydrophobic C18DMACC groups were embedded into the
hydrophilic βꢀCD cavities, the CP of βꢀCDꢀPEG would be elevated. In
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contrast, as compared to the CP of βꢀCDꢀPEG, the CP of Polymer/BisꢀCD
was reduced.
Figure.9 Temperature dependence of transmittance at 400 nm
recorded for aqueous solutions of P(AM/VPPS/C18DMACC) and
Polymer/BisꢀCD.
3.3.3 Salt tolerance
The salt tolerance of the inclusion complex solution was evaluated at
a temperature of 50 °C under the shear rate of 50 sꢀ1 and the concentration
of polymers were fixed at 3000 mg/L. The salt tolerance of hydrolyzed
polyacrylamide (HPAM) was also conducted as blank tests. The Fig. 10
reveals the relationship between apparent viscosity and salinity.
Apparently, the viscosity of Polymer/BisꢀCD was enhanced due to the
synergies between hydrophobe−hydrophobe interactions and cyclodextrin
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inclusion effects. The viscosity of P(AM/VPPS/C18DMACC) first
increased and then decreased with increasing NaCl concentration. The
similar tendency was found in the CaCl2 solution. Generally, the viscosity
of water soluble polymer will decrease under the high salinity [36].
However, the viscosity of hydrophobic associating polymer will increase
with the salinity in a certain range [29]. Therefore, the viscosity of
P(AM/VPPS/C18DMACC) initially increased. With the further increase in
salinity, the thickness of the electric double layer was compressed due to
the enhancing of electrostatic repulsion interaction, resulted in the
decrease of viscosity. On the other hand, the antiꢀpolyelectrolyte effect
was observed on the Polymer/BisꢀCD, whose viscosity increased with the
increasing concentrations of NaCl and CaCl2 solutions. The reason might
be that the addition of salt enhanced the solution polarity, which
contributed to the enhancement of host/guest interaction. In addition, the
electric double layer was difficult to be compressed owing to the presence
of rigid βꢀCD. The similar phenomenon of βꢀCD contained complex was
found by Li et al [37]. In conclusion, the Polymer/BisꢀCD inclusion
complex could be considered as a potential tackifier or soil conditioner in
highꢀsalinity circumstance.
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Figure.10 Effects of (a) NaCl solution and (b) CaCl2 solution on
apparent viscosity.
3.3.4 The influence of competitive molecules
The influence of competitive molecules on the structure of
Polymer/BisꢀCD was also investigated and studied. The experiment was
conducted at 50 °C and under the shear rate of 50 sꢀ1 while the
concentration of P(AM/VPPS/C18DMACC) and Polymer/BisꢀCD were
3000 mg/L. The cationic surfactant CTAC was chosen as the competitive
molecule in this study. The result is shown in Fig.11. It can be seen that
CTAC had an obvious effect on the viscosity of polymers, and an
interesting
phenomena
was
observed.
The
viscosity
of
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P(AM/VPPS/C18DMACC) increased first with the addition of CTAC
until the maximum value was reached, and then decreased with the
increase of CTAC concentration. In sharp contrast, the viscosity of
Polymer/BisꢀCD initially decreased with increasing CTAC concentration
and then increased until the maximum value was reached, thereafter the
viscosity decreased again. Attempts and assumption have been made to
explain the experimental phenomena.
At low CTAC concentrations, the mixed micelles of surfactant and
hydrophobic groups were formed in the pure P(AM/VPPS/C18DMACC)
solution, which led to an enhanced interꢀchain crosslink microꢀdomains.
In addition, the ionic surfactant CTAC could strengthen the interꢀchain
electrostatic interaction, which was beneficial for the hydrophobic
association. Hence, the viscosity increased. However, it would disrupted
the hydrophobic association with excess CTAC beyond the critical
micelle concentration (CMC), which led to a reduction in viscosity. For
the Polymer/BisꢀCD, when CTAC was added into the supramolecular
system, the free hydrophobic groups of CTAC played a competitive
molecules role to complex with the βꢀCD cavities of βꢀCDꢀPEG.
Therefore, the network structure of Polymer/BisꢀCD was broken, which
lead to the rapid decrease of viscosity. However, the replaced
hydrophobic groups of C18DMACC were then associated because of
hydrophobic association effect, contributing to the increase of viscosity.
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With the further increase of CTAC concentration, the CTAC molecules
were embedded into the βꢀCD cavities, the hydrophobic groups of
C18DMACC were shielded by CTAC, and the network structure was
gradually broken, which lead to the reduction of viscosity. Fig. 12 gives a
brief illustration of the process. Therefore, the viscosity of
supramolecular polymer is tunable by adjusting the concentration of
surfactant with strong hydrophobic group. This special property makes
the Polymer/BisꢀCD a potential material in many fields [38]. For example,
this property could certainly improve the mobility ratio of oil and water to
give a more efficient application for enhanced oil recovery as the
oilꢀdisplacing agent.
Figure.11 The relationship between viscosity of Polymers and
CTAC concentration.
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Figure.12 Schematic representation of the influence of
competitive molecules on the structure of Polymer/BisꢀCD.
4. Conclusion
In this paper, we successfully prepared a novel supramolecular
polymeric system via host−guest interactions by using bridged bisꢀCD
and functional acrylamide polymer as host and guest molecules,
1
respectively. The complexation structure was characterized by FTꢀIR, H
NMR, SEM, XRD, TG and DSC. The performance experiments
illustrated that the new supramolecular polymer could significantly
change the acrylamide polymer’s performance with good viscosity
29