Binding Interaction and Gelation in Aqueous Mixtures of Poly(N

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Binding Interaction and Gelation in Aqueous Mixtures of Poly(N-isopropylacrylamide) and Hectorite Clay Cuixia Lian, Enzhong Zhang, Tao Wang, Weixiang Sun, Xinxing Liu, and Zhen Tong J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp510526j • Publication Date (Web): 16 Dec 2014 Downloaded from http://pubs.acs.org on December 18, 2014

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The Journal of Physical Chemistry

Binding Interaction and Gelation in Aqueous Mixtures of Poly(N-isopropylacrylamide) and Hectorite Clay

Cuixia Lian, Enzhong Zhang, Tao Wang, Weixiang Sun, Xinxing Liu, and Zhen Tong*

Research Institute of Materials Science and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

*Corresponding author, Tel: (86)-20-87112886; Fax: (86)-20-87110273 E-mail: [email protected]

1

ACS Paragon Plus Environment


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ABSTRACT:

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The binding interaction between poly(N-isopropylacrylamide) (PNIPAm)

chains and hectorite clay platelets was directly detected by quartz crystal microbalance with dissipation (QCM-D) to explore the cross-linking mechanism in the strong nanocomposite hydrogel (NC gel), which is in situ polymerized with NIPAm in the clay suspension. PNIPAm chains were allowed to be adsorbed on the gold surface of the QCM electrode.

A

large frequency shift ∆f in the QCM as introducing the clay indicated that a large amount of clay platelets were adsorbed on the deposited PNIPAm layer.

The relationship between the

dissipation shift ∆D and ∆f revealed that the adsorption included two steps of fast initial buildup and following densification of the clay platelets.

In dilute aqueous mixtures, the

PNIPAm chain and clay formed aggregates as observed from the hydrodynamic diameter. Gelation state diagram was established for concentrated aqueous PNIPAm-clay mixtures. Raman spectrum pointed out that the conformation change of the PNIPAm chains in aqueous solutions when the clay was added, which would be caused by the adsorption of PNIPAm chains to the clay platelets.

Keywords: binding interaction, gelation, quartz crystal microbalance with dissipation (QCM-D), poly(N-isopropylacrylamide), hectorite

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1. INTRODUCTION Over the past decade, the hydrogel has been a very prevalent investigated topic for the actual and potential applications in the field of smart materials,1-5 which consist of a three-dimensional cross-linked network of hydrophilic polymer and plenty of water held by osmotic pressure.

In particular, the stimuli-response hydrogels have been widely expected

for the applications in soft actuators,6-10 biomaterials,11-15 and other systems. In order to improve mechanical property of the hydrogels, the composite gels have been designed based on hydrophilic polymers and inorganic particles, such as clay platelets, silica, graphene and so on.16-18

The nanocomposite hydrogel (NC gel) synthesized by in situ

polymerization of the acrylamide derivative monomers in the aqueous suspension of hectorite clay was most interesting due to its excellent strength, ultrahigh extensibility, high transparency, and fast stimuli-response.4,19-24

Here, hectorite clay is synthetic and

composed of a central magnesia sheet sandwiched by two silica sheets,25-27 with size of about 30 nm in diameter and 1 nm in thickness.28

The ultrahigh deformability of the NC gel was

attributed to their low effective network chain density with moderate relaxation in our previous study.29

Through various investigations, such as viscosity, mechanical property,

dynamic light scattering, small angle neutron scattering, were carried out,30-32 all of these methods cannot observe the cross-linking structure directly.

It was proposed that the

hectorite clay platelets acted as the multifunctional cross-linker and endowed the NC gel with high strength.23,31

However, the direct evidence for the cross-linking interaction in the NC

gel is still lacking.

The viscosity change in the clay suspension was caused by the aging,

which is promoted by adding salts and delayed by adsorbing polymer.33 The quartz crystal microbalance (QCM) has been used in monitoring the formation of ultrathin film based on the tiny change in oscillation frequency of the quartz crystal.34,35 Over the last decade, the QCM was also applied to monitor the real-time kinetics of the 3



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adsorption, conformation, and interaction of macromolecules as well as colloidal particles in solutions.36-41

Zhang et al. studied the temperature-induced dehydration and hydration in

water of thermal sensitive linear PNIPAm chains grafted on the gold electrode surface of the QCM.36,40

Physisorption of the PNIPAm on the gold surface was found and the adsorption

kinetics was monitored by the QCM.37,38

Zhang et al. recently reported the degradation of

copolymers in seawater detected with the QCM.42 clay

and

Ludox

silica

colloidal

Additionally, the adsorption of hectorite

nanoparticles

on

a

cationic

polymer

of

poly(diallyldimethylammonium chloride) was detected by the QCM.40 Raman spectroscopy is useful in studying the conformation and interaction of polymers through the molecular vibration.43-47

The temperature-induced evolution of the pore

structure in the macroporous PNIPAm hydrogel was investigated by directly monitoring the polymer chain density in lateral plane using Raman spectra.43

The hydrophilic and

hydrophobic transition of PNIPAm chains in aqueous solution was extensively investigated through the vibration change of the groups during the transition.45-47 In this work, we adopt the QCM with dissipation (QCM-D) to detect the adsorption of the hectorite clay platelets on the deposited PNIPAm layer to confirm the binding interaction directly for the first time.

Raman spectrum is also applied to reveal the interaction change

of the PNIPAm chains before and after adsorption on the clay.

Furthermore, the aggregate

is observed by dynamic light scattering from dilute aqueous mixture of PNIPAm and clay, and gelation state diagram is established from the concentrated mixtures.

2. EXPERIMENTAL METHODS 2.1. Materials.

N-Isopropylacrylamide (NIPAm, Acros, 1% stabilizer) was

recrystallized from a toluene/n-hexane mixture and dried in vacuum at 40 °C.

Potassium

peroxydisulfate (KPS, K2S2O8) was recrystallized from deionized water and dried in vacuum 4


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at room temperature.

N,N,N′,N′-Tetramethylethylenediamine (TEMED, Sinopharm Group

Chemical Reagent Co. Ltd.) was used as received.

Synthetic hectorite clay of sol-forming

grade LAPONITE® XLS (Rockwood Ltd., 92.32 wt% of Mg5.34Li0.66Si8O20(OH)4Na0.66 and 7.68 wt% of Na4P2O7) was used after dried at 125 °C for 2 h.

Pure water was obtained by

deionization and filtration using a Millipore purification apparatus (resistivity >18.2 MΩcm) and bubbled with argon gas for more than 1 h prior to use. different molecular weights were used in this work.

Two PNIPAm samples with

The low molecular weight sample

PNIPAm-69k (Sigma-Aldrich) with molecular weight Mw = 6.9 × 104 Da and PDI = 1.89 characterized by gel permeation chromatography (GPC, Waters Co.) was directly used. The high molecular weight sample PNIPAm-190k was radically polymerized in aqueous solution of 1 mol/L monomer, purified in deionized water (DI water) at 60 °C, and freezing dried.

Its molecular weight Mw and PDI were at 1.9 × 105 Da and 2.43, respectively.

2.2. QCM Measurements.

The QCM-D of Q-Sense AB with an AT-cut quartz

crystal of the fundamental frequency of 5.0 MHz and diameter of 14 mm was used to detect the adsorption of PNIPAm chains on the clay platelets.48

When small amount of mass ∆m

is adsorbed on the quartz electrode, the oscillation frequency is decreased by ∆f.49-51

∆f is

related to the mass change ∆m by the Sauerbrey equation ∆m = -C∆f/n,34 where C is the mass sensitivity constant 17.7 ng/(cm2Hz) at 5 MHz and n is the overtone number.

The

dissipation is defined as ∆D = Ed/2πEs, where Ed is the dissipated energy and Es is the stored energy during one oscillation in the oscillating system.

The change in frequency ∆f and

dissipation ∆D provides the information about the mass and structure of the adsorbed layer on the quartz crystal of the QCM. In this work, the frequency change ∆f in the 3rd overtone (n = 3) was chosen, for this overtone was more sensitive to the surface region than the others and had the highest

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signal-to-noise ratio.52

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During measurements, the quartz crystal with gold-plated electrode

was mounted in a fluid cell with one side exposed to the solution. roughness of the electrode on the quartz crystal was less than 3 nm.53 QCM-D cell was controlled to 25 ± 0.02 °C.

The root-mean-square Temperature of the

Before use, the quartz crystal electrode was

cleaned with a Piranha solution [H2SO4 (98%): H2O2 (30%) = 7:3] (Caution! Piranha solution is highly corrosive.) 30 min, rinsed with DI water, and dried with N2 stream.

The aqueous

solution of PNIPAm-190k was used with DI water as the reference in the QCM measurement.

2.3. Characterizations.

Hydrodynamic diameter and distribution of the clay,

PNIPAm in aqueous solution, and their mixture were acquired with dynamic light scattering (DLS) using a Zetasizer Nano ZS (Malvern instrument Co.).

The clay concentration in all

mixtures was kept at 1 mg/mL with various PNIPAm concentrations, and the temperature was 25 ± 0.1 °C. Rheology measurement was carried out by a stress-control rheometer of AR-G2 with a cone-plate fixture of diameter 60 mm and cone angle 1o under strain of 5% within the linear viscoelasticity regime.

Temperature was controlled at 25 ± 0.1 °C, and silicone oil was

applied on the edge of the fixture to prevent water evaporation. Raman spectrum was measured with a laser micro-Raman spectrometer (LabRAM Aramis, HORIBA Jobin Yvon Co.). temperature was 25 ± 1 °C.

The spectral resolution was 1 cm-1, and the

For investigation of the PNIPAm aqueous solution and its

mixture with clay, the spectrum of the solvent (water) was subtracted.

The peak position of

the characteristic vibration bands was derived from the smoothed spectrum, using the relevant minima of the second derivative.

3. RESULTS AND DISCUSSION 6


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3.1. Binding Interaction between PNIPAm and Clay.

Before investigating the

interaction between PNIPAm chains and hectorite clay platelets, the adsorption of PNIPAm on the gold surface of the QCM quartz electrode was monitored first.

Figure 1 shows the

frequency shift ∆f (A) and dissipation shift ∆D (B) varying with deposition time for the PNIPAm solutions at 25 °C.

As soon as the PNIPAm solution is introduced into the

electrode chamber, the frequency shift decreases immediately and then gradually levels off at an equilibrium value.

At the same time, ∆D increases abruptly and reaches a plateau.

Then, a small increase in ∆f and a decrease in ∆D are caused by rinsing the quartz surface with DI water.

This result indicates that the PNIPAm chains are adsorbed on the gold

surface of the quartz electrode and some weakly adhered PNIPAm chains are washed out. Finally, the strongly bound PNIPAm chains achieve the equilibrium adsorption on the electrode surface, causing an energy dissipation accompanying the adsorption. phenomena were reported in the literature.36,39

Similar

When increasing PNIPAm solution

concentration, ∆f decreases more significantly with a lower equilibrium value and ∆D achieves a higher plateau.

These ∆f changes are not induced by the solution viscosity

because the reference state is normalized to the DI water after rinse.

For example, a large

decrease in ∆D for the PNIPAm solution of 10 mg/mL after rinse implies a compact conformation for the adsorbed polymer chains with low energy dissipation.

By careful

comparison, the increase of ∆f and decrease of ∆D are not proportional to the PNIPAm concentration CPNIPAm, especially when CPNIPAm > 1 mg/mL, indicating that the adsorption becomes weaker at higher concentration.

Thus, the PNIPAm concentration of 1 mg/mL is

chosen in the present work for the all subsequent deposition experiments.

7



20 0

B

A PNIPAm

PNIPAm

rinse

15

rinse

0.01 mg/mL

-20

∆D / 10

-6

-10

∆f / Hz

0.1 mg/mL

10 mg/mL

10

-30

1 mg/mL 1 mg/mL

5 0.1 mg/mL

-40

10 mg/mL

0.01 mg/mL

0

-50 0

10

20

30

40

0

50

10

Figure 1.

20

30

40

50

Time / min

Time / min

Frequency shift ∆f (A) and dissipation shift ∆D (B) for the deposition of

PNIPAm chains with various concentrations (0.01, 0.1, 1, 10 mg/mL) onto the gold surface of quartz electrode at 25 °C.

After confirmation of the adsorption of PNIPAm chains on the gold surface of quartz electrode, the hectorite clay aqueous solution was introduced into the QCM.

Figure 2

presents the frequency shift ∆f and dissipation shift ∆D for adsorption of clay on the clean gold surface of quartz electrode.

After rinse, ∆f and ∆D approach to zero, indicating that

no clay platelets are adsorbed on the clean gold surface.

30

20

XLS: 3mg/ml

0

0

∆D / 10

10

rinse 10

-6

hectorite clay on gold electrode

20

∆f / Hz

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-10 -10 -20 -30

-20 0

5

10

15

20

25

30

35

Time / min

Figure 2.

The frequency shift ∆f (blue) and dissipation shift ∆D (red) for adsorption of

clay on the clean gold surface of quartz electrode at 25 °C.

8


Page 9 of 25

Figure 3 depicts the frequency shift ∆f (A) and dissipation shift ∆D (B) when the clay platelets are adsorbed to the deposited PNIPAm layer on the quartz electrode.

The first

rinse and second rinse with DI water are to eliminate the weakly adhered PNIPAm chains and clay platelets, respectively.

During the second rinse with DI water for 30 min, the change

of ∆f is small but the change of ∆D is large due to the formation of compact adsorption layer of the clay platelets on the PNIPAm adsorbed quartz electrode surface.

When the clay

concentration CXLS is 1 mg/mL, a small but measurable change in frequency and dissipation is observed, manifesting a small amount of clay platelets are adsorbed.

Furthermore, the

frequency and dissipation shifts become more obvious when CXLS increases to 3 mg/mL, indicating that substantial amount of clay is adsorbed to the PNIPAm deposited quartz electrode surface.

With increasing clay concentration from 3 to 10 mg/mL, the equilibrium

∆f value increases. PNIPAm: 1 mg/mL clay

0 -50

60 1 mg/mL

rinse

∆D / 10

-150 -200

3 mg/mL

-250

5 mg/mL

-300

10 mg/mL

B

50 -6

rinse

-100

∆f / Hz

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10 mg/mL 5 mg/mL

40

3 mg/mL 30 20 1 mg/mL

rinse 10

rinse clay

A

0

PNIPAm: 1 mg/mL

-350 0

50

100

150

200

0

50

Time / min

Figure 3.

100

150

200

Time / min

The frequency shift ∆f (A) and dissipation shift ∆D (B) for the clay adsorption

on the deposited PNIPAm layer at 25 °C.

The adsorbed amount ∆m of clay on the deposited PNIPAm layer was estimated by the Sauerbrey equation from the ∆f data in Figure 3A and plotted in Figure 4, where the clay was in the hydrated state. suspension.

∆m increases with increasing the clay concentration CXLS of the

As known from Figure 2, the clay does not adsorb on the net gold surface 9



without deposited PNIPAm layer.

Therefore, the hectorite clay platelets are bound strongly

by the PNIPAm chains, and this is the first direct evidence of this binding interaction.

This

seems to be essential for the extraordinary high strength and extensibility of the PNIPAm-hectorite clay NC gels.19,20

-2

2000

∆m / ng cm

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1500

1000

500

0 1

3

5

7

C XLS / mg mL

Figure. 4.

9

11

-1

Adsorbed mass ∆m of hydrated clay on the deposited PNIPAm layer at 25 °C.

For better understanding the binding interaction of the clay platelets and PNIPAm chains in the deposited layer, the QCM dissipation shift ∆D is plotted against the negative frequency shift -∆f in Figure 5.

This is a good method to study the adsorbed clay platelets

in terms of configuration, because it correlates the energy loss with the adsorbed mass change.36,37

The curve of ∆D vs. -∆f exhibits two regimes: fast increase in regime 1 and

slow increase in regime 2.

These two regimes correspond to the initial fast buildup of the

clay platelets bound on the deposited PNIPAm layer, and followed by a densification of the clay platelets in the adsorbed layer, as schematically illustrated in Figure 6. dissipation increases weakly during the adsorption in the second step.

10


Therefore, the

Page 11 of 25

65 CXLS = 10 mg/mL

-6

55

∆D / 10

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


45 35 25

2

1 15 5 50

100

150

200

250

300

350

-∆f / Hz

Figure 5.

The dissipation shift ∆D vs. the negative frequency shift -∆f for the clay

adsorption on the deposited PNIPAm layer.

deposited PNIPAm layer

Step1 + clay

Step2

electrode surface

Figure 6.

Schematic illustration for the dynamic process of the clay adsorption onto the

deposited PNIPAm layer by two steps.

3.2. Gelation in Aqueous PNIPAm and Clay Mixtures.

The QCM results reveal

the binding interaction between the PNIPAm chains and clay platelets.

The actual situation

in the aqueous mixture of the PNIPAm and clay was also investigated with hydrodynamic diameter distribution.

Flocculation happened in the PNIPAm-190k (used in the QCM

measurement) and clay mixture at the concentrations for this measurement.

Thus, the

PNIPAm-69k was used for this measurement with concentration CPNIPAm of 0.1 ~ 9 mg/mL and clay concentration CXLS of 1 mg/mL.

Figure 7A presents the average size of the

PNIPAm chains and clay platelets in aqueous solution as 16 nm and 32 nm, respectively.

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Page 12 of 25

Figure 7B displays the size distribution of the PNIPAm and clay mixture varying with composition.

When CPNIPAm:CXLS < 6:1, only one peak appears in the distribution with the

average size larger than that of either PNIPAm-69k or clay. towards larger size with increasing CPNIPAm:CXLS.

The distribution peak moves

This means that the PNIPAm chains and

clay platelets associate together forming aggregates in the mixture.

When CPNIPAm is much

high (≥ 6:1), two distribution peaks appear with the small peak for the PNIPAm-69k alone due to its excess. A

clay

PNIPAm

0

10

1

10

2

10

2

10

10

3

B CPNIPAm:CXLS 1:10 3:10 6:10 1:1 3:1 6:1 9:1

0

10

1

10

10

3

d / nm

Figure 7.

The hydrodynamic diameter distribution determined by DLS of the

PNIPAm-69k and clay in aqueous solutions with indicated composition.

As the concentration of the PNIPAm and clay mixture is increased, the microscopic aggregation will convert into a macroscopic gelation due to the binding interaction.

The

inverting bottle method is adopted to determine the sol or gel state, and Figure 8A demonstrates some examples, where PmSn denotes m w/v% of PNIPAm-69k and n w/v% of hectorite clay used in the aqueous mixture.

When n ≤ 2.6, the mixture behaves as a viscous

fluid, and when n ≥ 2.8, the mixture becomes a self-standing hydrogel.

12


In this way, the

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gelation state diagram of the aqueous mixture of PNIPAm and clay is established in Figure 8B. The upper part above the gelation boundary corresponds to the gel state and the low part corresponds to the fluid state. concentration is too low.

Therefore, no gelation is observed if the PNIPAm

Furthermore, increasing molecule weight of the PNIPAm used in

the mixture expands the gelation area in the diagram, i.e., for the mixtures with PNIPAm-190k (red), the gelation occurs at much lower clay concentrations.

However, when the

concentration of PNIPAm-190k is higher than 5 w/v%, no homogeneous mixture can be prepared by mixing with the clay. The dynamic mechanical response of the PNIPAm-clay aqueous mixture was detected along the gelation line of PNIPAm-69k concentration of 8 w/v%.

Figure 9 demonstrates

the storage modulus G’ and loss modulus G” as a function of angular frequency ω.

When

the clay concentration is 2 w/v%, G’ is smaller than G” and increases with ω, showing the viscoelastic fluid characteristics.

As the clay concentration increases to 3 w/v%, G’

becomes larger than G” and a plateau appears in G’ vs. ω curve, indicating the formation of cross-linked structure.

This behavior manifests clearly that the gelation occurs in the

aqueous PNIPAm-clay mixture as long as the concentration is beyond a certain value due to the binding interaction between the PNIPAm chains and clay platelets.

13



5

B

PNIPAm 69k PNIPAm 190k

CXLS / w/v%

4

3

2

1 0

5

10

15

CPNIPAm / w/v%

Figure 8.

A: The photos showing inverted bottles of the aqueous mixtures of PNIPAm-69k

(12 w/v%) and clay (2.4, 2.6, 2.8, 3.0, 3.5 w/v%); B: gelation state diagram of aqueous mixtures of PNIPAm-69k (black) and PNIPAm-190k (red) with clay at 25 °C. 2

10

P8S3 1

G ',G '' / Pa

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 25

10

0

10

P8S2 -1

10

-2

10

-1

10

0

1

10

10

2

10

-1

ω / (rad s )

Figure 9.

Storage modulus G′ (solid symbols) and loss modulus G” (open symbols)

plotted against angular frequency ω for the aqueous mixtures of 8 w/v% PNIPAm-69k with 2 and 3 w/v% of clay at 25 °C.

14


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3.3. Raman Spectra of PNIPAm and Clay Aqueous Mixtures.

For investigating

the interaction of PNIPAm chains in the aqueous mixture with hectorite clay, Raman spectra were measured on their solutions.

The Raman spectra of 12 w/v% aqueous PNIPAm-69k

solution, water and their difference are displayed in Figure 10. all the samples are used in the following analysis.

The subtractive spectra for

Figure 11 shows the C-H stretch of the

PNIPAm chain in aqueous solutions with different clay contents from 0 to 3.5 w/v%.

The

band of the CH3 antisymmetric stretch shifts of PNIPAm from 2975 cm-1 for solid to 2986 cm-1 for aqueous solution.

This is induced by the hydration of the polymer chains when

dissolved in water, because the wavenumber of the CH3 stretch becomes higher with larger number of the surrounding water molecules as reported in literature.45,46

Interestingly, with

addition of clay, the wavenumber of the CH3 antisymmetric stretch moves towards low value, close to the wavenumber in the solid state, implying that introduction of the clay platelets reduces the number of the water molecules surrounding the CH3 groups.

PNIPAm-69k

Water solution H2O Difference 3500

2500

1500

500 -1

Raman shift / cm

Figure 10.

Raman spectra of 12 w/v% aqueous solution of PNIPAm-69k, pure water, and

their subtractive spectrum at 25 °C.

15



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Page 16 of 25

νCH as

3

2986 P12 P12S2.4 P12S2.6 P12S2.8 P12S3 P12S3.5

2975 Solid PNIPAm 3000

2950

2900

Raman shift / cm

Figure 11.

2850 -1

Raman spectra of the C-H stretch of PNIPAm in 12 w/v% aqueous solutions

with clay of 0, 2.4, 2.6, 2.8, 3, and 3.5 w/v% compared with that of solid PNIPAm.

Figure 12 illustrates the C-H stretch bands within 2880 to 2970 cm-1, which can be separated into two components centered at 2943 (red) and 2923 cm-1 (blue), corresponding to the symmetric stretch of CH3 and the antisymmetric stretch of CH2, respectively.

When the

PNIPAm is in the solid state, the intensity and area of the CH3 symmetric stretch are much smaller than that of the CH2 antisymmetric stretch.

In contrast, when PNIPAm is in

aqueous solution, the band of the CH2 antisymmetric stretch is greatly reduced.

Addition

of the clay results in an increase in the intensity and area of the CH2 antisymmetric stretch, exhibiting the behavior close to the solid PNIPAm.

The band area change here would be

attributed to the adsorption of PNIPAm chains to the clay platelets, which causes the conformation change of the PNIPAm chains.

16


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PNIPAm in H2O

2970

P12S2.6

2940

2910

2880

2970

2940

2910

2940

2910

2880

P12S3

solid PNIPAm 2923

νCH as

2

2943

νCH s

3

2970

2940

2910

Raman shift / cm

Figure 12.

2880

2970

-1

Raman shift / cm

2880 -1

Raman spectra for the C-H stretch bands of PNIPAm in solid and aqueous

solutions with different clay contents.

The red and blue lines are the Gaussian components

of the CH3 symmetric stretch and CH2 antisymmetric stretch, respectively.

4. CONCLUSIONS PNIPAm in situ polymerized in hectorite clay suspension is known to form a tough and strong nanocomposite hydrogel (NC gel), but the cross-linking junction is still unclear up to now. In this research, the QCM and Raman spectrum were used to explore the binding interaction between PNIPAm chains and clay platelets.

PNIPAm chains were allowed to be adsorbed

on the gold surface of the QCM electrode.

A large decrease in the QCM frequency with

introducing the clay indicated strong adsorption of the clay on the deposited PNIPAm layer. The relationship between the dissipation shift and frequency shift revealed that the adsorption included two steps of initial fast buildup and following densification of the clay platelets on the deposited PNIPAm layer.

In dilute aqueous solutions, the PNIPAm chains and clay

platelets associated together to form aggregates, where the hydrodynamic diameter increased 17



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with increasing PNIPAm content in the mixture.

Page 18 of 25

The gelation state diagram was

established by inverting bottle method for concentrated aqueous PNIPAm-clay mixtures, and the gelation in the mixtures containing high-molecular-weight PNIPAm occurred at much lower clay concentrations.

The dynamic mechanical behavior of the PNIPAm-clay

mixtures verified the formation of the network accompanying with the gelation.

Raman

spectra suggested the conformation change of the PNIPAm chains in aqueous solutions when the clay was added, which would be caused by the adsorption of PNIPAm chains to the clay platelets.

AUTHOR INFORMATION Corresponding Author Tel: (86)-20-87112886. Fax: (86)-20-87110273. E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The financial support from the National Basic Research Program of China (973 Program, 2012CB821504) and the National Natural Science Foundation of China (51173052, 51203052) is gratefully acknowledged.

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TABLE OF CONTENTS IMAGE

Binding Interaction and Gelation in Aqueous Mixtures of Poly(N-isopropylacrylamide) and Hectorite Clay Cuixia Lian, Enzhong Zhang, Tao Wang, Weixiang Sun, Xinxing Liu, and Zhen Tong* Research Institute of Materials Science and State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China

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Binding Interaction and Gelation in Aqueous Mixtures of Poly(N

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