www.elsevier.com/locate/matlet

Materials Letters 59 (2

Preparation and characterization of poly(N-isopropylacrylamide) films on

a modified glass surface via surface initiated redox polymerization

Yun-Pu Wanga,T, Kun Yuana, Quan-Lian Lia, Li-Ping Wanga,

Sheng-Jiu Gua,b, Xiao-Wei Peia

aGansu Key Laboratory of Polymer Materials, Institute of Polymer, Northwest Normal University, Lanzhou 730070, P.R. ChinabPharmaceutical Department, Guilin Medical College, Gulin 541004, P.R. China

Received 27 September 2004; received in revised form 20 January 2005; accepted 24 January 2005

Available online 3 March 2005

Abstract

A functionalization with 3-aminopropyltriethoxysilane (APTES) monolayer of a hydroxylated glass surface, followed by the surface

initiated graft radical polymerization of N-isopropylacrylamide (NIPAm) using amino groups of APTES monolayer chemical bonded with

glass surface and Ce4+ as a redox initiating system. The microstructure of poly(N-isopropylacrylamide) (PNIPAm) film obtained from the

redox graft polymerization on the modified glass surfaces was examined by water contact angle, X-ray photoelectron spectroscopy (XPS),

and atomic force microscopy (AFM), and the results showed that about 60 nm thickness of thermosensitive polymer (PNIPAm) film

successfully formed.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Glass surface; Poly(N-isopropylacrylamide); Redox initiating system; Ceric ion

1. Introduction

Owing to its unique temperature sensitive properties, and

exhibiting the phase transition at approximately 34 8C in

water [1,2], poly(N-isopropylacrylamide) (PNIPAm) has

been extensively investigated in the field of preparing smart

materials, including release systems [3,4], chemical valves

[5,6], recyclable absorbents [7], and immobilization of

enzymes [8]. Among the research directions, PNIPAm

composite thin films and coating materials generated a

new area of applications [9–11]. These materials were

achieved by using several methods, such as the self-

assembly (SAM) technique, Langmuir–Blodgett (LB), and

the chemical initiated polymerization on the surfaces of

various inorganic materials.

In recent years, a great deal of attention was paid to

coating the surface of glass and related inorganic substrates

with a layer of polymers with special properties to prepare

0167-577X/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.matlet.2005.01.048

T Corresponding author. Tel./fax: +86 931 7970686.

E-mail address: wangyp@nwnu.edu.cn (Y.-P. Wang).

various functional materials [12–18]. These polymers

including polyaniline, poly(ethylene glycol), polystyrene,

poly(ethyleneimine), polystyrenesulfonate, poly(methyl

methacrylate), but there was few literature to report the

surface initiated graft polymerization of N-isopropylacryla-

mide (NIPAm) on a modified glass surface.

In this paper, we describe employing a self-assembly

silane monolayer formed by 3-amionpropyltriethoxysilane

(APTES), followed by the graft polymerization of NIPAm

on the glass surface. The method involves the initial

formation of a stable silane monolayer through its reaction

with the hydroxyl groups of the glass surface. This was

followed by the surface initiated radical graft polymerization

of NIPAm using a redox system consisting of ceric ion and

reducing groups (amino) on the glass surface as initiator.

This surface initiated polymerization offers a promising and

versatile method of preparing PNIPAm films, and these

kinds of glasses bonded with PNIPAm films have potential

applications as environmentally switchable materials, tem-

perature sensitive optical valve underwater engineering and

other bsmart windowsQ.

005) 1736–1740

Y.-P. Wang et al. / Materials Letters 59 (2005) 1736–1740 1737

2. Experimental section

2.1. Materials

The microscopic glass slides were purchased from

Aldrich Chemical Co., and were sliced into rectangular

strips of about 1.0 cm�2.0 cm in size, 3-aminopropyltrie-

thoxysilane (APTES) was obtained form Kanto Chemical

Co., N-isopropylacrylamide (NIPAm) (Aldrich Chemical

Co.) was purified by recrystallization from n-hexane/toluene,

ceric ammonium nitrate and nitric acid were used without

further purification. Toluene was washed with concentrated

sulfuric acid, refluxed over sodium, and distilled. The

solvents, such as acetone, ethanol and methanol and other

chemicals were of reagent grade and were used as received.

2.2. Glass substrate pre-treatment

To remove the organic residue from the surface, the glass

slides (1.0 cm�2.0 cm) were first soaked in a soap solution,

sonicated for 5 min, and then rinsed with a large amount of

distilled water. The substrates were then immersed in a

bpiranhaQ solution [a mixture of 70% volume concentrated

sulfuric acid (98 wt.%) and 30% volume of a hydrogen

peroxide solution (30 wt.%)] and boiled for about 50 min.

The cleaned glass slides were washed with a large amount

of distilled water and ethanol and then dried at 80 8C for 24

h for subsequent surface treatment. The contact angle of

water droplets on the glass surface was low, about 128, overthe entire surface, revealing the high cleanliness and

uniformity of the surface.

2.3. Silane treatment to produce SAM

The pre-treated hydrophilic glass slide was placed into a

2.5 wt.% toluene solution of 3-aminopropyltriethoxysilane

and refluxed in a stream of nitrogen for 24 h. After the

reaction, the glass was washed twice with dried toluene in the

same nitrogen atmosphere. Finally, the glass was removed

from the nitrogen atmosphere and cleaned in an ultrasonic

bath in toluene for 5 min, rinsed again successively with

toluene, acetone and ethanol, and finally dried in vacuo.

glass

Ce4+CH2

CH2

CH2

NH2

OO O

Si

OH OH OH

O

APTES

Fig. 1. Scheme for chemical strategy of the surface graf

2.4. Surface initiated graft polymerization of NIPAm

For the surface initiated graft polymerization with

NIPAm, the 3- aminopropyltriethoxysilane-SAM-glass sub-

strates were immersed in 4.0 mL of 4.7 mmol/mL aqueous

solution of NIPAm. After deaeration of the system by

bubbling nitrogen, 0.1 mL of 0.2 mol/mL solution of ceric

ammonium nitrate in 1 mol/mL nitric acid was added. The

polymerization was conducted at 30 8C with slow stirring by

a magnetic stirrer under dry nitrogen for 24 h. After the

reaction, the PNIPAm-An-APTES-glass was rinsed thor-

oughly under ultrasonic bath benzene, acetone, and distilled

water, respectively before being dried under reduced

pressure.

2.5. Characterization of the surface modified glass slides

The graft modified glass surface was characterization by

contact angle, XPS, and AFM measurements. The water

contact angles were measured in ambient air (relative

humidity 50%) using a CA-A contact angle measuring

system (Kyowa Kagaku Co., Ltd.). The values reported here

are the averaged values of at least five measurements on

different locations. XPS analysis was performed on a PHI-

5702 multifunction X-ray photoelectron spectrometer using

a pass energy of 29.35 eV and a MgKa ho=1253.6 eV line

excitation source, with the binding energy of contaminated

carbon (C1s: 284.6 eV) as the reference. The resolution for

the measurements of the binding energy is about 0.3 eV.

Tapping-mode atomic force microscopy (AFM) analysis

was carried out using the SPI 38001X scanning probe

microscope system (Seiko Instruments Inc.). The image was

acquired in air with standard silicon TESP probes (nominal

spring constant and resonance frequency respectively 50 N/

m and 300 kHz).

3. Results and discussion

The strategy for the PNIPAm functionalization of glass

hydroxyl-terminated surfaces consists of two basic steps,

depicted in Fig. 1: (1) the formation of a well-defined SAM

NIPAm CH2

CH2

CH2

NH

O O

Si

OO O

Si

C NH

CH(CH3)2

O()

n

CH2

CH2

CH2

NH

CH2

CH

t polymerization of PNIPAm on a glass substrate.

Table 1

Water contact angles after surface treatment

Samples Contact angle (8)

Glass surface 68

Glass surface after the treatment

with the piranha solution

12

Surface after reaction with

3-aminopropyltriethoxysilane

72

Surface after initiated graft

polymerization of PNIPAm

30

Y.-P. Wang et al. / Materials Letters 59 (2005) 1736–17401738

through the reaction of APTES with the hydroxyls of the

glass surface; (2) surface initiated graft polymerization of

NIPAm on the silane modified glass surface using the redox

initiated system consisting of ceric ion and reducing groups

(amino) on the glass surface.

At several steps during this procedure outlined in Fig. 1,

the contact angles of the glass substrates were measured. As

shown in Table 1, the contact angle changed after the

various treatments. After washing by the mixture of H2O2

and sulfuric acid, the contact angle was the same (128) over

295 290 285

Binding Ene(1)

(2)

Inte

nsi

ty

C1s

c

b

a

410 405 400 395

Binding Energy(eV)

Inte

nsi

ty

390

N1s

c

b

a

Fig. 2. C1s, N1s, O1s XPS spectra for 1: (glass-OH), 2: t

the entire surface, indicating that the substrate was

uniformly covered with hydroxyl groups. Further, the

hydroxylated surface was treated with APTES for 24 h,

the surface becomes covered with an APTES monolayer,

and the contact angle went up to 728. This demonstrates that

the hydrophobic APTES layer has replaced the hydrophilic

hydroxyl layer. When PNIPAm film formed by surface

initiated graft polymerization, the contact angle decreased to

308 because of the hydrophilic of PNIPAm. This indirectly

indicates the structure change of the glass surface with each

treatment.

To further investigate the composition of the film on the

glass surface, XPS measurement was used. Fig. 2 shows

C1s, N1s, O1s spectra for the bare glass (glass-OH), the

silane-SAM-glass and PNIAPm-An-APTES-glass. From

Fig. 2(1), for the glass-OH, the presence of the C1s signal

is attributed to the interference of unavoidable pollution of

glass substrate in analysis, and the signal is weak. After the

silane treatment, the C1s signal (285.0 eV) intensity was

enhanced greatly. As for PNIPAm-An-APTES-glass, careful

280

rgy(eV)

(3)

275

540 535 530 525

Binding Energy(eV)

Inte

nsi

ty

520

O1s

c

b

a

he silane-SAM-glass, 3: PNIAPm-An-APTES-glass.

0

[nm] 3.090.00

0.5 1[µm] [µm]

0

0.5

[nm]

5

00.5

0.5

0.0

1.01.0

[µm]

1

0

[nm] 9.280.00

0.5 1[nm] [µm]

0

0.5

[nm]

10

0.5

0.5

0.0

1.0

1.0

[µm]

1

0

[nm] 69.350.00

0.5 1 1.5[µm] [µm]

0

0.5

[nm]

50

0.5

0.5

0.0

1.0

1.51.5

1.0

[µm]

1

1.5

(a)

(b)

(c)

Fig. 3. Atomic force microscope (AFM) 2D and 3D images of (a) glass-OH, (b) APTES-SAM-glass, (c) PNIPAm-An-APTES-glass.

Y.-P. Wang et al. / Materials Letters 59 (2005) 1736–1740 1739

Y.-P. Wang et al. / Materials Letters 59 (2005) 1736–17401740

peak fitting on the C1s peak resolves three peaks

representing different carbons in PNIPAm: (1) aliphatic

hydrocarbon (C–C/C–H, at a binding energy of 285.0 eV),

(2) acylamino carbon (C–N, at 286.1 eV), and (3) the

carbonyl carbon (C=O, at 288.8 eV). These signals

indicated the presence of PNIPAm on the surface. Fig.

2(2) presents the N1s signal. For glass-OH, there was no

N1s signal to have been detected. After silane treatment, the

appearance of the N1s peak (N–H, at 399.2 eV) indicates

that the APTES monolayer was anchored to the glass

surface through a chemical bond. Comparing to N1s signal

of the APTES-SAM-glass, N1s signal of the PNIPAm-An-

APTES-glass shifted to 400.5 eV, this is attributed to the

acylamino nitrogen (N–C=O) of the PNIPAm chain. Fig.

2(3) is the O1s XPS scan spectra, the O1s signal of glass-

OH is similar to that of APTES-SAM-glass, both of them

are the O1s signal of the O–Si bond (532.0 eV). As for O1s

signal of the PNIPAm-An-APTES-glass, it shifted to a

higher binding energy of 534.2 eV, this corresponds to the

carbonyl oxygen (O=C) of PINPAm film. In general, the

wide scan XPS analysis indicates a functionalized surface

generated by the method outlined in Fig. 1.

To get a more quantitative and detailed impression of

the film morphology. AFM images (Fig. 3) of three

substrates, (a) glass-OH, (b) APTES-SAM-glass and (c)

PNIPAm-An-APTES-glass, were taken. The AFM image

of bare glass treated by bpirahanQ (Fig. 3(a)) shows a

relatively smooth surface, and its surface roughness is only

about 3 nm. From Fig. 3(b), we can see that the silane

molecules are densely arrayed, and the thickness of APTES

monolayer is about 7.0 nm. This value is higher than the

thickness of normal silane monolayer (3–5 nm), this is

probably because of the presence of the amido end of

APTES molecule with strong polarity, which makes the

incline angle of APTES molecule chains increase, and

APTES molecule chains become more out-of-order, thus,

the APTES molecules pile up easily in portrait [19].

Further more, in presence of minimum water, a few parts

of APTES molecules probably polymerized and deposited

on the glass surface, so the measurement value of thickness

is higher than the value of an ideal APTES monoalyer. The

polymerized substrate (Fig. 3(c)) shows more uniform

surface than that of APTES-SAM-glass, and the thickness

of the grafted PNIAPm film is about 60 nm. On the other

hand, it can be seen that the surface exhibits, in addition to

ordered arrangement of the molecules, a surface corruga-

tion due to the aggregation of PNIPAm chain.

4. Conclusion

Surface modification of glass substrates was carried out

via silanization with APTES (the APTES-SAM-glass sur-

face). It can be further functionalized by surface initiated

graft polymerization of NIPAm using amino group of

APTES anchored on the glass surface and Ce+4 as a redox

initiating system. The microstructure of the bare glass,

APTES monolayer and grafted PNIPAm were characterized

by contact angle, XPS and AFM measurements. The film of

PNIPAm grafted on glass surface is uniform and in a state of

corrugation, and the thickness of PNIPAm film reached

about to 60 nm under the polymerization condition.

Acknowledgment

This work was partly supported by the Natural Science

Foundation of China (No. 20074026).

References

[1] S. Hirotsu, Y. Hirokawa, T. Tanaka, J. Chem. Phys. 87 (1987) 1392.

[2] Y. Li, T. Tanaka, Annu. Rev. Mater. Sci. 22 (1992) 243.

[3] S.W. Chun, J.D. Kim, J. Control. Release 38 (1996) 39.

[4] D.E. Meyer, B.C. Shin, G.A. Kong, M.W. Dewhirst, A. Chilkoti, J.

Control. Release 74 (2001) 213.

[5] Y. Osada, H. Hori, H. Okuzaki, Nature 355 (1992) 245.

[6] Y.M. Sun, T.L. Huang, J. Membr. Sci. 110 (1996) 211.

[7] H. Kawaguchi, K. Fujimito, Y. Mizuhara, Colloid Polym. Sci. 270

(1992) 53.

[8] F. Liu, G.L. Tao, R.X. Zhuo, Polym. J. 25 (1993) 561.

[9] J.M. Weissman, H.B. Sunkara, A.S. Tse, A.S. Asher, Science 274

(1996) 959.

[10] S.A. Jeneke, X.L. Chen, Science 283 (1999) 283.

[11] J. Liu, R. Pelton, A.N. Hrymark, J. Colloid Interface Sci. 227 (2000)

408.

[12] Z.F. Li, E. Ruchenstein, Synth. Met. 129 (2002) 73.

[13] R.W.G. Rajapakse, A.D.L. Chandani, L.P.P. Lankeshwara, N.L.W.L.

Kumarasiri, Synth. Met. 83 (1996) 73.

[14] J. Piehler, A. Brecht, R. Vatlokas, B. Liedberg, G. Gauglitz, Biosens.

Bioelectron. 15 (2000) 473.

[15] L.K. Mulfort, J. Ryu, Q. Zhou, Polymer 44 (2003) 3185.

[16] C.O. Kim, S.J. Cho, J.W. Park, J. Colloid Interface Sci. 260 (2003)

374.

[17] Z.W. Li, J.F. Zhou, Z.J. Zhang, H.X. Dang, Acta Phys.-Chim. Sin. 18

(2002) 550.

[18] K. Urayama, S. Yamamoto, Y. Tsujii, T. Fukuda, D. Nether,

Macromolecules 35 (2002) 9459.

[19] Z. Feng, PhD thesis, Chinese Academy of Sciences, Lanzhou Institute

of Chemical Physics (2004) p. 93, 7.