Hydrogen-Bonding-Directed Layer-by-Layer Assembly of Poly(4-vinylpyridine) and Poly(4-vinylphenol):  Effect of Solvent Composition on Multilayer Buildup

1

Hydrogen-Bonding-Directed Layer-by-Layer

Assembly of Dendrimer and Poly(4-vinylpyridine)

and Micropore Formation by Post-Base Treatment

Hongyu Zhang,1 Yu Fu,1 Dong Wang,1 Liyan Wang,1 Zhiqiang Wang*2 and Xi Zhang*1

1Key Lab for Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun, 130023, P. R. China

2Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China

[*] Prof. Dr. Xi Zhang

Key Lab for Supramolecular Structure and Materials,

College of Chemistry, Jilin University, Changchun, 130023, P. R. China

Fax: 0086-431-8923907 or 8980729

E-mail: [email protected]

2

We reported a way to fabricate microporous films by post-base treatment of

hydrogen-bonding-directed multilayer films of poly(4-vinylpyridine) (PVP) and

carboxyl-terminated poly-ether dendrimer (DEN-COOH). The PVP/DEN-COOH

multilayer film was fabricated by layer-by-layer (LbL) assembly of PVP and

DEN-COOH from methanol solution. UV-vis spectroscopy revealed a uniform

deposition process. The interaction between PVP and DEN-COOH was identified as

hydrogen bonding through Fourier Transform Infrared (FT-IR) spectroscopy.

Meanwhile, the composition change of a PVP/DEN-COOH multilayer film in a basic

solution was detected by X-ray photoelectron spectroscopy (XPS), UV-vis

spectroscopy, and the morphology variation was observed by atomic force

microscopy (AFM). A two-step variation was observed: the dissolution of

DEN-COOH from the multilayer into the basic solution, and the gradual

reconformation of PVP polymer chains remaining on the substrate, which produced a

micropourous film. Interestingly, compared with our previous PVP/poly(acrylic acid)

(PAA) system, under the same conditions, the release of DEN-COOH from

PVP/DEN-COOH multilayer is slower than that of PAA, and the microporous

morphology is also different, which indicates that the molecular structure of a

building block has a remarkable influence on the variation of a

hydrogen-bonding-directed film in a basic solution.

3

Introduction

Self-assembly can offer rational design and construction of highly ordered meso-

and nanoscale structures with defined physical properties and chemical functions.

Various studies have been devoted to the realization of functionalized organic

materials by artificial supramolecular self-assembly.1 In the past decade, there has

been a tremendous surge towards the characterization, modification, and processing of

ultrathin films and multilayered structures constructed by self-assembly due to their

potential applications including catalysis, microelectronics, nonlinear optics, sensors,

and display technologies.2,3 The other reason for the intense interest in this field is that

multilayers can bridge the gap between monolayers and spun-on or dip-coated films.

A simple technique for ultrathin multilayer film assembly is the alternate

layer-by-layer (LbL) electrostatic deposition of oppositely charged polyelectrolytes.4,5

The fabrication of multicomposite films by the LbL procedure means literally the

nanoscopic assembly of different materials in a single device using environmentally

friendly, ultra-low-cost techniques. The materials can be small organic molecules6 or

inorganic compounds,7�12 macromolecules,13,14 biomacromolecules such as proteins,

15,16 DNA17,18 or even colloids.19-21 Although the ultrathin multilayers fabricated by

LbL method commonly cannot achieve a satisfactory well-defined layer structure,

because of the interfacial interpenetrating between neighboring layers, the versatile

method still challenges the traditional LB technique, and opens new avenues to

advanced materials with practical applications.

Although electrostatic interaction has been most widely used to construct

4

multilayer films,4-21 other weak interactions, such as hydrogen bonding, have also

been employed as driving forces for the LbL assembly. For example, Rubner et al.22

and Zhang et al.23 reported simultaneously the formation of ultrathin films via

H-bonding attraction by LbL assembly technique. One of the advantages of the

hydrogen-bonding-directed films is that the fabrication of the LbL film is allowed in

an organic solvent. Later, hydrogen-bonding-directed electroactive,24 photochromic,25

and photoreactive26 polyelectrolyte multilayers were successfully constructed.

Granick and co-workers prepared erasable hydrogen-bonded multilayers containing

weak polyacids, which could be assembled at low pH and subsequently dissolved at

higher pH as a consequence of increasing the ionization degree of the weak

polyacids.27,28 Lian et al. prepared polymer and nanoparticle composite multilayer

based on hydrogen bonding. 29 More noticeably, on the basis of the hydrogen-bonded

erasable system, Rubner et al. combined the light-initiated chemical reaction with the

dip-pen technique to fabricate a patterned surface.30 Very recently, Caruso et al.

reported on the preparation of multilayer films comprising alternate stacks of

hydrogen-bonded PVP and PAA and electrostatically formed poly(sodium

4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) layers via LbL

assembly technique, and their high pH sensitivity toward deconstruction.31

Microporous ultrathin films have received increasing attention recently due to

their numerous applications, including low dielectric constant and low refractive

index thin film coating, separation filters, biocompatible membranes for controlled

release and encapsulation systems and anti-reflection coating.3 For example, Rubner

5

et al.32 and Caruso et al.33 demonstrated that PAH/poly(acrylic acid) (PAH/PAA) films

could form microporous structures upon exposure to solutions with different pH

values or ionic strengths. In addition, Bruening and co-works reported that

poly(amidoamine) (PAMAM) dendrimer/PAH multilayers were also capable of

forming such microporous films by simply exposing multilayers to acidic aqueous

solutions.34 Obviously, with the proper choice of assembly conditions or treatment

conditions covering a wide range of pH and/or ionic strength, it should be possible to

induce microporosity in electrostatic assembly systems. In our previous study, Zhang

et al. investigated the structure variation of a hydrogen-bonding-directed

poly(4-vinylpyridine)/poly(acrylic acid) (PVP/PAA) LbL film in a basic aqueous

solution.35 In this case, a two-step variation was observed: the first step is the

dissolution of PAA from the film into the basic solution; the second is the gradual

reconformation of PVP polymer chains remaining on the substrate, which produces a

microporous film. The novel and unique mechanism of microporous film construction

is anticipated to have potential applications in materials science.

The aim of the present article is attempting not only to confirm the formation

mechanism of the microporous film as mentioned above, but also to find a new way to

control the fabrication of the microporous film. Hence, we have employed the

carboxyl-terminated poly-ether dendrimer (DEN-COOH) as a hydrogen donor and

constructed a multilayer film by alternating deposition of poly(4-vinylpyridine) (PVP)

and DEN-COOH via hydrogen bonding in a cyclic fashion. We are wondering if a

microporous film can be formed, when the multilayer film of PVP/DEN-COOH is

6

immersed in a basic solution. We anticipate that a comparison study between

PVP/DEN-COOH and PVP/PAA will be helpful and constructive for the forthcoming

discussion about the formation of microporous film.

Experimental Section

Materials. Poly(ethyleneimine) (PEI, Mw = 50,000), and

(4-aminobutyl)-dimethylmethoxysilane were obtained from Aldrich and used without

further treatment. Carboxyl-terminated poly-ether dendrimer (DEN-COOH), which

has been used as a building block to fabricate a hydrogen-bonding-directed multilayer

by self-deposition,36 was synthesized according to the literature.37

Poly(4-vinylpyridine) (Mw = 180,000) was synthesized as previously described.38

Film Preparation. The LbL film was assembled on a quartz slide or a calcium

fluoride (CaF2) plate. The quartz slide was used for UV-vis, XPS, and AFM

measurements, and the CaF2 plate for FT-IR. The quartz slide and CaF2 plate need to

be modified before LbL deposition. The quartz surface was modified with

(4-aminobutyl)-dimethylmethoxysilane, resulting in a NH2-tailored surface, and the

CaF2 surface was modified with a precursor layer of poly(ethyleneimine) (PEI). The

NH2-terminated substrate was first immersed in a PVP methanol solution (1 mg/mL)

for 10 min. In this way, the substrate was covered with a PVP layer, and thus a surface

tailored with hydrogen bonding acceptors (pyridine groups) was formed. After rinsing

with pure methanol and drying under a nitrogen stream, the resulting substrate was

transferred into a DEN-COOH methanol solution for 10 min, to add a DEN-COOH

7

layer. By repetition of the above two steps in a cyclic fashion, the LbL multilayer film

was fabricated. Figure 1 shows the schematic assembling process on a quartz slide.

The resulting multilayer films can be expressed as (PVP/DEN-COOH)n, where n is

the number of deposition cycles. To investigate the influence of basic aqueous

solution on the hydrogen-bonding-directed multilayer film, the resulting LbL film was

immersed in NaOH aqueous solution. After rinsing with water and drying by nitrogen,

the samples were stored under ambient conditions prior to measurement.

Methods. UV-vis spectra were obtained on a Shimadzu 3100 UV-vis-near-IR

recording spectrometer. FT-IR spectra of PVP/DEN-COOH multilayers were

collected on a Bruker IFS 66V instrument equipped with a DTGS detector at 4 cm-1

resolution. X-ray photoelectron spectroscopy (XPS) spectra were obtained on an

ESCALAB Mark II (VG company, UK) photoelectron spectrometer using a

monochromatic Mg Kα X-ray source. Atomic force microscopy (AFM) images were

taken with a Dimension 3100 (Digital Instruments, Santa Barbara, CA) under ambient

conditions. AFM was operated in the tapping mode with an optical readout using Si

cantilevers.

Results and discussion

UV-vis spectroscopy has proved to be a useful and facile technique to evaluate

the growth process of multilayers and was thus used in the present work to monitor

the LbL assembly process of PVP/DEN-COOH multilayer buildup. Figure 2 displays

8

the UV-vis absorption spectra of (PVP/DEN-COOH)n multilayers (with n = 1-12)

assembled on a NH2-tailored quartz surface. As shown in Figure 2, the DEN-COOH

absorption is clearly identified by the characteristic peaks at 234 and 281 nm due to

the π - π* transition of the benzene of DEN-COOH, substantiating the incorporation

of DEN-COOH molecules into the multilayers. Unfortunately, due to the strong

absorption of DEN-COOH in UV region, the overlapping spectra of the DEN-COOH

and PVP does not allow assignment of a unique absorption band of the multilayer film

solely to the PVP. The insert of Figure 2 shows the absorbance of quartz-supported

(PVP/DEN-COOH)n multilayer films at characteristic wavelength (234 nm) increases

proportionally with the number of deposition cycles, n. This nearly linear growth of

the absorption peaks indicates that an approximately equal amount of DEN-COOH is

deposited for each adsorption procedure and that the PVP/DEN-COOH LbL films

grow uniformly with each deposition cycle. However, the observed growth at 281 nm

is non-linear in Figure 2, which could be accounted for the formation of DEN-COOH

aggregates within the multilayer. Similar phenomena was observed in the electrostatic

LbL self-assembly of dye molecules.39,40 In addition, it is found that there is almost no

desorption of DEN-COOH during the multilayer buildup.

In order to understand the deposition process in more detail, we have studied the

physical adsorption kinetics. Figure 3 shows how the optical absorbance varies with

time during the process of adsorbing a single layer of DEN-COOH onto a quartz

substrate previously coated with a (PVP/DEN-COOH)2PVP precursor film. It is

shown that, under the conditions used, the deposition of a single DEN-COOH layer

9

onto a PVP surface is more than 90 % complete within the first 5 min of immersion

and it reaches a plateau of saturate adsorption after 10 min.

We also examined the dependence of the concentration of DEN-COOH and PVP

solutions on the adsorption behavior. It is found that the amount of DEN-COOH

adsorbed per bilayer grows with increasing the concentration of DEN-COOH from

0.08 (Figure 4b) to 0.16 mg/mL (Figure 4a). When increasing the concentration of

PVP from 0.5 (Figure 4c) to 1.0 mg/mL (Figure 4b), the adsorbed amount of the

DEN-COOH is also increased accordingly when using a fixed concentration of

DEN-COOH solution (0.08 mg/mL). We found that the amount of DEN-COOH

adsorbed is strongly dependent on the concentration of the dipping solution, with

higher concentrations resulting in a greater amount of adsorbed DEN-COOH at

equilibrium.

The driving force for the construction of the PVP/DEN-COOH multilayer film

was identified by FT-IR spectroscopy. Hydrogen-bonding formation between pyridine

and carboxylic acid leads to characteristic splitting patterns in the IR absorption of the

carboxylic acid OH group.41,42 Figure 5 a and b show the FT-IR spectra of the cast

films of PVP and DEN-COOH on CaF2 plates, respectively. For the cast film of PVP,

the peaks appearing at 1596, 1556, and 1450 cm-1 can be ascribed to the ring vibration

of pyridine groups of PVP. For the DEN-COOH, the bands at 1693 and 1720 cm-1 can

be separately assigned to the carbonyl vibrations of carboxylic acid groups in

associated and free states.43 The strong absorbance band appearing at 1161 cm-1 can

be attributed to the vibration of Ar-O bond. Figure 6 shows the FT-IR spectrum of an

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8-bilayer PVP/DEN-COOH film on a CaF2 plate. In this figure, we can find clearly

that a O-H stretching vibration appears at 2470 and 1934 cm-1, indicating a strong

hydrogen-bonding between the carboxylic acid of DEN-COOH and pyridine groups

of PVP. 41,42 Furthermore, in the region from 1660 to 1110 cm-1 in the FT-IR spectrum

of the PVP/DEN-COOH multilayer film, the absorption peaks could be assigned to

the ring vibration of PVP or DEN-COOH and the vibration of aryl-O band of

DEN-COOH, and no position change of which was observed in comparison with pure

PVP and pure DEN-COOH. These results further provide the evidence that the

multilayer film is assembled via the hydrogen bonding.

To investigate the influence of a basic aqueous solution on the PVP/DEN-COOH

multilayer, X-ray photoelectron spectroscopy (XPS) was used to detect the

composition variation of the LbL film in a NaOH solution. Prior to immersion in a

basic solution, there are two C 1s photopeaks at approximately 288.75 and 284.75 eV

as shown in Figure 7a, the former weak peak is assigned to the carbon of carboxylic

acid in DEN-COOH.44 Comparing the XPS spectra of (PVP/DEN-COOH)5PVP LbL

films before and after immersion in a pH = 12.5 NaOH aqueous solution for 2 min,

we can find that the distinct photopeak at 288.75 eV corresponding to the carbon of

carboxylic acid in DEN-COOH disappears in the spectrum of the film after the base

treatment as shown in Figure 7b. This change suggests that DEN-COOH is removed

from the multilayer film by the basic solution. Moreover, XPS also displays that the C

1s photopeak at 288.75 eV could be weaken after one-minute immersion in the basic

solution, which indicates that DEN-COOH partially releases from multilayer film for

11

a short immersion time. However, under the same condition used (pH = 12.5, 25 °C),

PAA can release thoroughly from multilayer films of PVP/PAA during immersion for

1 min in basic solutions.35 As for N 1s, no obvious difference between the films

before and after immersion was observed, which implies that the PVP still remains on

the substrate. From the above discussions, we demonstrate that when the

PVP/DEN-COOH LbL film is immersed in a basic aqueous solution, one of the film

components, DEN-COOH, dissolves away and the other component, PVP, remains on

the substrate.

In order to study the release kinetics of dendrimers from the PVP/DEN-COOH

multilayer film, we measured the change of film absorbance at 234 nm as a function

of pH of the basic solution and immersion time. Figure 8 shows the intensity change

of the DEN-COOH absorption with the immersion time in basic solutions with

different pH values. It indicates that in the basic solutions PVP/DEN-COOH

multilayer films are not stable and prone to deconstruction, and the deconstruction

process of the multilayer film depends sensitively on pH of basic solutions. From

Figure 8, it can be seen, with increasing pH of the basic solutions from 11.0 to 13.0,

the release rate of DEN-COOH increases greatly. For the (PVP/DEN-COOH)5PVP

multilayer films immersed in the basic solution of pH =11.0 for 180 min, no

DEN-COOH release from the multilayer was observed. While at pH =13.0, at the very

beginning of immersion, e.g. 1 min, approximately 80 % DEN-COOH was released,

and an equilibrium plateau was reached after 25-minute base treatment. The above

analysis indicates that, when the LbL film is dipped into a basic aqueous solution,

12

DEN-COOH can be removed from the film, and its releasing rate can be controlled by

changing the pH of the base solutions.

The above results indicate clearly that DEN-COOH is removed, and PVP

remains on the substrates. After the immersion of the multilayer film into the basic

aqueous solution, the carboxylic acid groups of DEN-COOH are ionized by the basic

solution, which leads to the destruction of hydrogen bonding between PVP and

DEN-COOH. After the hydrogen bonds are destroyed, DEN-COOH leaves the film

because of its solubility in the basic solution, while PVP remains due to its poor

solubility in the basic solution. One question is why DEN-COOH can release from

PVP/DEN-COOH multilayer even slower than PAA from PVP/PAA multilayer? Two

possible factors, the solubility and molecular shape, could be responsible for the

difference in the release rate. The first may be their solubility difference in a basic

solution. Although, both DEN-COOH and PAA are soluble in the basic solution, the

solubility of DEN-COOH should be less than that of PAA because of the existence of

benzene rings in the DEN-COOH, which must lower its release rate from the

multilayer. The second possible reason is the molecular shape. Because of the

branching structure, DEN-COOH would be anchored in the PVP matrix, and is harder

to escape out of the multilayer. However, in the case of PAA, the polymer chain is

linear, and should be easier to be drawn from the film. Therefore, the result that PAA

release faster than DEN-COOH is reasonable.

The morphology variation of the PVP/DEN-COOH multilayer film in the basic

aqueous solution was explored using AFM. The AFM image of the

13

(PVP/DEN-COOH)6PVP multilayer film prior to immersion in a basic solution is

shown in Figure 9. As can be seen from this figure, the LbL self-assembly film

containing PVP and DEN-COOH on a quartz plate exhibits a high coverage with

granular structures with the size from 150 to 300 nm. The AFM images of

(PVP/DEN-COOH)14 multilayer film after immersion in pH = 12.5 NaOH aqueous

solutions at 25 °C for different periods of time are shown in Figure 10. After

10-minute immersion in basic solutions, the surface of multilayer film is rougher than

that before immersion in basic solution, and no porous structure is observed (Figure

10A). While in the PVP/PAA system,35 nanosized pores emerge already after

10-minute immersion. When immersing the (PVP/DEN-COOH)14 multilayer film in

the basic solution for 30 min, the pores with about 200 nm in diameter and 16 nm in

depth appear. During the immersion time from 30 to 180 min, the diameter and depth

of the pores increase averagely from 200 to 380 nm and from 16 to 36 nm,

respectively. It is noted that the morphology of the film is different from the

microporous film resulting from PVP/PAA multilayer35 after base treatment under the

same treatment conditions (pH = 12.5, 25 °C). In contrast to the separate pores in

PVP/PAA system, for the same immersion time (180 min), the distribution and shape

of pores obtained in the present case is more uniform (Figure 10D). Moreover, the

surface pore coverage is significantly higher than that obtained in PVP/PAA system.35

The above analysis indicates that a time-controlled microporousity of multilayer film

can be obtained by immersion of the PVP/DEN-COOH film in a basic solution.

It is known that pH and ionic strength during or after the film construction can

14

not only anneal surface roughness45-47 but also lead to more dramatic structural

rearrangements, such as porosity32-35 in the multilayer structure. In this case, after

DEN-COOH is removed rapidly by the basic solution, at the beginning the remaining

PVP should retain that extended state. However, with prolonged immersion time the

extended PVP chains gradually rearrange due to their high surface tension in the basic

solution. As a result, in the lateral direction the film coverage decreases and in the

vertical direction the thickness increases, which results in the above-mentioned

morphology variation. Therefore, we propose that the morphology variation is a result

of the reconformation of PVP induced by the basic solution, after the escape of

DEN-COOH.

Conclusions

In this article, firstly we presented the fabrication and detailed characterization of

the PVP/DEN-COOH layer-by-layer film based on hydrogen bonding. Afterwards,

the variety behavior of such multilayer in a basic solution was investigated, which

indicated that a microporous film was formed by the rapid release of DEN-COOH and

slowed re-organization of remaining PVP on the substrate. Moreover, we compared

the varieties of the PVP/DEN-COOH and PVP/PAA mutilayers in basis solutions. An

interesting finding is that the release rate of DEN-COOH from PVP/DEN-COOH

multilayer is lower than that of PAA from PVP/PAA multialyer in a basic solution,

and the resulting microporous morphologies are remarkably different as well. We

presume that the phenomena could be accounted for the difference in the solubility

15

and molecular shape of DEN-COOH and PAA. We can conclude from the above

discussions that incorporating different building blocks as hydrogen-bonding donor

into multilayer assembly is an effective way to adjust the release process and

microporosity by immersion of layer-by-layer films into basic solutions. Our studies

on microporous films resulting from hydrogen-bonding-directed multilayer, combined

with other insights into hydrogen-bonded ultrathin films or porous thin films, may

pave the way for further theoretical researches and potential applications in the future.

Acknowledgment. This work is supported by the Major State Basic Research

Development Program (G2000078102), the National Natural Science Foundation of

China (20204003), and a key project of the Educational Ministry. The authors thank

Mr. Fengwei Huo for helpful discussions during the experiments.

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Figure captions

Figure 1 Schematics of the layer-by-layer assembly of PVP and DEN-COOH on a

quartz substrate based on hydrogen bonding: (I) adsorption of PVP and (II)

adsorption of DEN-COOH.

Figure 2 UV-vis spectra of (PVP/DEN-COOH)n multilayer films with n = 0~12 on

NH2-modified quartz substrates. The lowest curve corresponds to the baseline. (n

= 0) The other curves, from bottom to top, correspond to n = 1, 2, 3, 4, 5, 6, 7, 8,

9, 10, 11, and 12, respectively. Insert: absorbance at 234 nm vs the number of

deposition cycles.

Figure 3 UV absorbance at 234 nm recorded as a function of immersion time for

deposition of a single monolayer of DEN-COOH on a quartz substrate coated

with (PVP/DEN-COOH)3PVP precursor film.

Figure 4 Influence of concentration on the UV absorption (at 234 nm) against the

number of deposition cycles. (a, [PVP] = 1 mg/mL, [DEN] = 0.16 mg/mL; b,

[PVP] = 1 mg/mL, [DEN] = 0.08 mg/mL; c, [PVP] = 0.5 mg/mL, [DEN] = 0.08

mg/mL)

Figure 5 FT-IR spectra of cast films of (a) pure PVP and (b) pure DEN-COOH on

CaF2 plates.

Figure 6 FT-IR spectrum of a (PVP/DEN-COOH)n (n = 8) multilayer film on a

PEI-modified CaF2 plate. The insert shows a magnification of the FT-IR

spectrum in the range from 1300 to 1900 cm-1.

20

Figure 7 C 1s XPS spectra of (PVP/DEN-COOH)5.5 multilayer films before (a) and

after (b) immersion in pH = 12.5 NaOH aqueous solution at 25 °C for 2 min.

Figure 8 The decrease of absorbance at 234 nm of (PVP/DEN-COOH)5PVP

multilayer films vs. the immersion time in the NaOH aqueous solutions with

different pH values.

Figure 9 AFM height image (4.0×4.0 µm2) of a (PVP/DEN-COOH)6PVP multilayer

film.

Figure 10 AFM height images (4.0×4.0 µm2) of (PVP/DEN-COOH)14 multilayer

films on a quartz substrate after immersion in a pH = 12.5 NaOH aqueous

solution at 25 °C for 10 (A), 30 (B), 60 (C), and 180 min (D).

21

Figure 1

PVP DEN-COOH ⅡⅡⅡⅡ

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

N

N

N

N

N

N

N

N

N

N

N

OOH

O OH

OO

HOO

H

OO H

OOH O

O

H

OO

H

OOH

O OH

OO H

OOH

OO H

O OH

OO

H

OO

H

OOH

O OH

OO

HOO

H

OO H

OOH O

O

H

OO

H

OOH

O OH

OO H

OOH

OO H

O OH

OO

H

OO

H

ⅠⅠⅠⅠ

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

N

N

N

N

N

N

N

N

N

N

N

ⅠⅠⅠⅠ ⅡⅡⅡⅡ

Hydrogen bonding donor Hydrogen bonding acceptor

OOH

O OH

OO

HOO

H

OO H

OOH O

O

H

OO

H

OOH

O OH

OO H

OOH

OO H

O OH

OO

H

OO

H

N

CHCH2

n PVP DEN-COOH

22

Figure 2

200 250 300 350 400 450 500

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 2 4 6 8 10 120.0

0.3

0.6

0.9

1.2

1.5at 234 nm

Abs

orba

nce

Number of bilayers

Abs

orba

nce

Wavelength/nm

23

Figure 3

0 10 20 30 40 50 60

0.00

0.05

0.10

0.15

0.20

Abs

orba

nce

Immersion time/min

24

Figure 4

0 2 4 6 8 10 12 140.0

0.5

1.0

1.5

2.0

2.5

3.0

at 234 nm

cb

a

Abs

orba

nce

Number of bilayers

25

Figure 5

3500 3000 2500 2000 1500 1000

1720

1161

1693 15

96

145015

56

1596

b

a

Abs

orba

nce

(a. u

.)

Wavenumber/cm-1

26

Figure 6

3500 3000 2500 2000 1500 1000

0.01

0.02

0.03

0.04

0.05

0.06

0.07

1800 1700 1600 1500 1400 1300

1556

1596

1695

1720

Wavenumbers/cm-1

1159

1448

2470

1934

1695

1596

**

Abs

orba

nce

Wavenumber/cm-1

27

Figure 7

280 282 284 286 288 290 292

b

aDEN-COOH

*Inte

nsity

(a. u

.)

Binding Energy (eV)

28

Figure 8

29

Figure 9

30

Figure 10

A B

C D