Copyright © 2006 American Scientific Publishers All rights reserved Printed in the United States of America

Journal of Nanoscience and Nanotechnology Vol. 6, 1594–1601, 2006

Nanostructuring of Poly(diphenylamine) Inside the Galleries of Montmorillonite Organo Clay Through Self-Assembly Approach Anantha Iyengar Gopalan1
2
3 , Kwang-Pill Lee1
2
∗ , Mung-Hwa Hong1 , Padmanabhan Santhosh1 , Kalayil Manian Manesh1 , and Sang-Ho Kim2

RESEARCH ARTICLE

1

Department of Chemistry Graduate School, Kyungpook National University, Daegu 702-701, South Korea 2 Nano Practical Application Center, Daegu 704-230, South Korea 3 Department of Industrial Chemistry, Alagappa University, Karaikudi-630003, Tamil Nadu, India

Delivered Ingenta to:by confining PDPA in the galleries Hollow spheres of poly(diphenylamine) (PDPA)by was prepared Max-Planck-Institut of montmorillonite organo clay modified with organoammonium cations (MMT). At first instant, : 134.105.184.136 diphenylamine (DPA) was loaded intoIPthe galleries of MMT and subjected to subsequent oxidative Thu, 05 Aprsulfonic 2007 10:13:58 polymerization to form PDPA.
-naphthalene acid (NSA) was used as medium to influence self-assembly of DPA inside the galleries of MMT. Polymerization of self assembled structure resulted hollow spheres of PDPA inside galleries of MMT. X-ray diffraction analysis (XRD), field emission transmission electron microscopy (FETEM), Fourier transform infra-red spectroscopy (FTIR) and thermogravimetric analysis (TGA) were used to characterize the composites. Transmission emission microscopy of the composite shows the hollow spherical morphology of PDPA. FT-IR, UVVisible spectroscopy, conductivity measurement and X-ray photoelectron spectroscopy were used to characterize the PDPA extracted from MMT galleries. PDPA extracted from MMT galleries was found to have difference in electronic property than PDPA formed by the conventional method, due to the confinement effect. Keywords: Poly(Diphenylamine), Montmorillonite Organo Clay, Self-Assembly,
-Naphthalene Sulfonic Acid.

1. INTRODUCTION Recently, nanostructured conducting polymers synthesized by the template method1–3 or the self-assembly process4–9 have attracted attention because of their special properties and potential applications in nanodevices.10–13 The “template synthesis” method is proved to be an effective way of synthesizing microtubules of conducting polymers and it has been applied to synthesize microtubules of polypyrrole,14
15 poly(3-methylthiophene)15 and polyacetylene.16 Nanostructured poly(aniline) (PANI),17 polypyrrole,18 poly(diphenylamine) (PDPA),19
20 synthesized by self-assembly approach were also reported. Hard and soft templates were tried to nanostructure the conducting polymers.14–19 -naphthalene sulfonic acid (NSA) has been used as soft template for nanostructuring PANI.10
18 Polymer nanocomposites have received considerable attention owing to various engineering applications such ∗

Author to whom correspondence should be addressed.

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as enhanced mechanical property and thermal stability, reduced gas permeability, and self-extinguishing flame retardant characteristics.21–26 In particular, conducting polymer based nanocomposites provide the new synergistic properties, which cannot be attained from individual materials, such that the conductivity is more easily controlled, and the mechanical or thermal stability is improved through the synthesis of the nanocomposites. Many hybrid composites such as PANI/VOPO4 · 2H2 O,27 PANI/ FeOCl,28 PANI/MoO3 ,29 PANI/V2 O5 ,30 PANI/graphite oxide31 and PANI/clay32–37 etc. have been synthesized and studied. The combination of conducting polymers with host materials generates hybrid materials showing novel properties. Clays have been employed as host materials to confine conducting polymers. Several studies have been made using different synthetic strategies to prepare PANI between the layers of montmorillonite, a natural clay.32–37 Two different synthetic routes, in-situ and ex-situ polymerization were adopted for the preparation of conducting 1533-4880/2006/6/1594/008

doi:10.1166/jnn.2006.240

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Nanostructuring of Poly(diphenylamine) Inside the Galleries of Montmorillonite Organo Clay

polymer/clay nanocomposite.33
35 Intercalation polymerization (in-situ polymerization) affords a promising strategy to study the chain structure of conducting polymer and polymerization mechanism.33 PANI/clay hybrid nanocomposite with relatively high conductivity was prepared by intercalation polymerization.33 However, nanostructuring of conducting polymer inside the galleries of the clay host has not been reported so far. In the present study, we have used self-assembly approach to prepare PDPA inside the galleries of clay. -naphthalene sulfonic acid (NSA), an organic dopant that would assist self-assembly of diphenylamine (DPA) molecule inside the galleries, was selected as medium. On polymerization of self-assembled structure of DPA inside the MMT galleries, nanostructured PDPA with electronic properties that are different from the polymer prepared by the conventional method was resulted. The nanocomposites with PDPA present inside the galleries of MMT and PDPA extracted from the galleries of MMT were Delivered by characterized.

2. EXPERIMENTAL DETAILS

2.2. Polymerization of Diphenylamine (DPA) Inside the Galleries of MMT Through Self-Assembly of DPA

0.5 g of MMT was dispersed in a 10 mM solution of diphenylamine in 100 mM of NSA. The mixture was sonicated for 24 h with stirring. After sonication, the solid material (DPA loaded MMT) was filtered, washed several times with NSA and dried. This resulted in loading of self-assembled DPA inside the galleries of MMT. The selfassembled DPA loaded MMT powder (DPA(SA)-MMT) again was dispersed in 50 mL of 100 mM NSA. To this, 20 mL of 0.5 M ammonium persulfate (prepared in 100 mM NSA) was added slowly with constant stirring for 2 h at 5  C. A dark-green colored precipitate, PDPA loaded MMT nanocomposite (designated as PDPA-MMTNC) in which PDPA exists in the interior galleries of MMT, was obtained. PDPA-MMT-NC was filtered, washed with distilled water and dried at 60  C in a vacuum oven. Ingenta to: Also, PDPA-MMT-NC were synthesized by polymerizing Max-Planck-Institut DPA(SA)-MMT prepared with different concentrations of IP : 134.105.184.136 DPA (10 mM to 25 mM).

Thu, 05 Apr 2007 10:13:58

2.2.1. Extraction of PDPA from MMT Galleries

DPA (diphenylamine), APS (ammonium persulfate), NSA (-naphthalene sulfonic acid), were obtained from Aldrich. Montmorillonite clay modified with organoammonium cations (MMT) was obtained from Southern Clay Products, Inc. All the other chemicals were reagent grade and used as received. MMT belongs to the structural family of 2:1 phyllosilicates. The chemical formula of MMT is (Na,Ca) (Al,Mg)6 (Si4 O10 )3 (OH)6 -nH2 O. MMT is composed of layers stacked together periodically. Isomorphic replacement of trivalent metal ions (Al3+ ) by bivalent ions (Mg2+ ) within the layers causes a permanent net negative charge in the clay. The positive charge deficiency is usually balanced by the presence of cations such as Na+ or Ca2+ inside the galleries. These cations are usually hydrated and they are loosely held between the galleries by electrostatic force. The hydrated cations can be easily exchanged with other organic or inorganic ions through ion exchange reactions. Surfactants such as organoammonium salt are usually used to modify clay through cation exchange reaction with Na+ or Ca2+ and thereby impart clay with hydrophobicity. The clay used in this study is Cloisite
30B modified by an organoammonium salt (MeT2EtOH) having methyl, tallow (∼30% C16), bis-2-hydroxyethyl and quaternary ammonium with the cation exchange ability of 90 meq/100 g clay.

PDPA was extracted from the galleries of MMT as follows. PDPA-MMT-NC was placed in 10 mL aqueous ammonia for 10 h and stirred. The color of the solution changed from green to blue during this process. The blue color mass was filtered and dried. Then, the blue colored mass was stirred in DMF for 10 h and filtered. A white mass was obtained and this signified the removal of PDPA from MMT inter layers. The blue-colored filtrate contained the neutral PDPA extracted from galleries of MMT. The filtrate was evaporated to obtain neutral PDPA extracted from the galleries of MMT.

CH2CH2OH H3C

N+

T

CH2CH2OH

where T is tallow with chloride as anion. J. Nanosci. Nanotechnol. 6, 1594–1601, 2006

2.3. Characterization PDPA-MMT-NC and PDPA extracted from the galleries of MMT, PDPA(E-MMT) and PDPA prepared from conventional polymerization (PDPA-C) were characterized by UV-Vis spectroscopy (Shimadzu UV-2101 spectrophotometer) and FT-IR spectroscopy (Perkin-Elmer Lamda 9N-1062 spectrometer). The microstructure of PDPAMMT-NC was investigated by means of a field emission transmission electron microscope (FETEM) (JEOL, JEM2000EX) with a field emission electron gun operated at 200 kV. X-ray diffraction patterns of the samples were collected employing a D8-Advanced Bruker AXS diffractometer using Cu K radiation. Elemental compositions of the samples (% of carbon, nitrogen, hydrogen, and sulfur) were determined by Fision EA H10 elemental analyzer equipped with flash combustion furnace. Thermogravimetric analyses were carried out using a TA instrument 951. The room-temperature conductivity of the compressed pellets was determined using the conventional two-point 1595

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2.1. Materials

Nanostructuring of Poly(diphenylamine) Inside the Galleries of Montmorillonite Organo Clay

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probe method. The binding energy was determined by X-ray photoelectron Spectroscopy using an ESCA 210 and MICROLAB 310D spectrometer.

in the interlayer region causes an increase in the MMT basal spacing from 18.39 Å to 20.12 Å. Now, the increased d spacing in the organoclay upon loading DPA in it, is considered as follows; the self assembled DPA-NSA structure are intercalated in the galleries of MMT in a tilted 3. RESULTS AND DISCUSSION orientation (as the micelle size is more longer than the 3.1. Self Assembly of Diphenylamine Cations and gallery distance) relative to the layer and/or in a parFormation of Poly(Diphenylamine) Inside the allel disposition (flat orientation) in an interlayer region Gallery of Organic Clay (Scheme 1). A similar observation is reported previously.40 Further, the intercalation of DPA(SA) into the galleries 3.1.1. XRD Analysis of MMT was ascertained by determining the elemental compositions. Table I presets the elemental (% of carbon, Diphenylamine (DPA) was loaded into the galleries of nitrogen, hydrogen, and sulfur) compositions of MMT MMT in the medium of NSA. A comparison of XRD and DPA(SA)-MMT. The MMT has elemental composipatterns of DPA loaded MMT and pristine MMT clearly tion (C21 H49 N) which corresponds to MeT2EtOH, where informs that there is a change in d spacing on loading DPA T stands for steric acid. Upon loading of DPA-SA, the inside the MMT galleries. Also, DPA is expected to have organoclay showed a definite elemental composition of self-assembled structure with NSA inside the MMT galS (4.3%) which was virtually absent in pristine MMT leries (DPA(SA)-MMT). NSA with a hydrophilic naphtha− (Table I).to: This gives evidence that NSA molecules are lene unit and a hydrophobic SO3 group provides suitable by Ingenta Delivered intercalated inside the galleries of MMT. Interestingly, 1:1 environment for the self-assembly of DPA (Scheme 1). Max-Planck-Institut  elemental composition of S to N was found in DPA-SA Pristine MMT shows a crystalline peakIPat: 134.105.184.136 4.9 loaded MMT. This corroborates with self-assembly struc(Fig. 1(a)) that corresponds to the periodicityThu, in the05direcApr 2007 10:13:58 38 ture for DPA-SA in which N atom of –NH group in DPA tion of (001) of MMT with a d spacing of about is bound to S atom of sulfonate group of NSA (Scheme 1). 18.39 Å. MMT selected in this study is an organically Another interesting feature was noticed from the XRD modified one (MeT2EtOH) and has a higher gallery dispatterns recorded after polymerization of DPA molecules tance probably due to the larger size of the organoamthat are self-assembled inside the gallery of MMT. The monium cations intercalated inside the galleries compared basal spacing showed a decrease from 20.12 Å to 18.76 Å with pristine clay (Na+ clay) whose gallery distance is with a decrease in 2 theta from 4.59 to 4.52 after polymerrelatively low because of the size of sodium cations.39 ization of DPA between the clay layers (Fig. 1 curve c). We have specifically used organo clay (MeT2EtOH) so as A decrease in basal spacing of about 1.73 Å suggests that to replace the organocations by other protonated cations the polymer, PDPA, formed inside the MMT galleries is (protonated diphenylamine cations) and to have a higher present between the layers as given in Scheme 1. Similar gallery distance to load DPA. Curve b of Figure 1 observation was reported for the intercalated polyaniline in demonstrates that XRD pattern of DPA(SA)-MMT has an MMT.41 Elemental (C, H, N, and S) composition of PDPAincreased d-spacing in comparison to pristine MMT. The MMT-NC was determined (Table I). A 2  1 elemental replacement of organoammonium cations by DPA cations

Scheme 1. Schematic representation of intercalation of DPA ions and hollow sphere formation of PDPA inside MMT galleries.

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Nanostructuring of Poly(diphenylamine) Inside the Galleries of Montmorillonite Organo Clay Table I.

(A)

Elemental compositions.

Sample

d001

Carbon (%)

Hydrogen (%)

Sulfur (%)

1.59 4.23 4.42

27.95 65.13 59.19

5.63 4.86 5.71

— 4.30 2.71

MMT # DPA(SA)-MMT ∗ PDPA-MMT-NC

d

#

18.76 Å

∗

Counts

Nitrogen (%)

MMT: 0.5 g; DPA: 10 mM; NSA: 100 mM. MMT: 0.5 g; DPA: 10 mM; NSA: 100 mM; APS: 0.5 M.

PDPA loaded into MMT galleries (PDPA-MMT-NC (nanocomposite)). XRD patterns of a series of PDPAMMT-NCs synthesized with different DPA concentrations (15–25 mM) are presented in Figure 1B and the correb sponding d spacing and 2 theta values are presented in the Table II. It can be seen from Table II that there is a steady increase in d spacing with concentration of DPA a used for loading into the MMT galleries. An increase in the distance between galleries of MMT resulted upon Delivered by Ingenta formationto: of PDPA in the gallery. Also, the crystalline 32 36 40 Max-Planck-Institut peak of MMT (4.59 ) is shifted to the lower value, when IP : 134.105.184.136 PDPA was loaded in more amount. For example, the Thu, 05 Apr 2007 10:13:58 d-spacing in direction of (001) of PDPA-MMT-NC (prepared with 25 mM DPA) was 19.73 Å against pristine MMT (18.39 Å). Also, it is apparent from the XRD patterns that peak intensity of the nanocomposite increases with increasing DPA concentration. This is attributed to the formation of different size of micelles in the gallery of MMT. On increasing the molar concentration of DPA, it c is presumed that the size of ordered micelles may become larger and may cause an increase in crystallite size. A comparative analysis of field emission transmission electron b microscopy, FETEM images of PDPA-MMT-NC and pristine MMT (Fig. 2A) reveals that there is an increase in the distance between the galleries from 1.84 for MMT to 1.98 nm for PDPA-MMT-NC, which is in accordance with the XRD data. We have evaluated the morphology and a thermal properties of PDPA-MMT-NC. c

20.12 Å

18.39 Å

4

8

12

16

20

24

2 theta d001

(B)

19.41 Å

19.04 Å

3

6

9

12

15

2 theta Fig. 1. XRD pattern of (a) pristine MMT, (b) DPA(SA)-MMT, (c) PDPA-MMT-NC and (d) pristine PDPA. XRD pattern of PDPA-MMTNC with DPA (a) 15, (b) 20, and (c) 25 mM.

composition ratio of N to S was found for PDPA-MMTNC. This is consistent with the existence of PDPA in its emeraldine type form with protonation of imine units in the alternative amine and imine units in the backbone of PDPA. We further wanted to ensure the formation of PDPA inside the galleries of MMT and consequent variations in d spacing of MMT. Towards this purpose, we have prepared DPA loaded MMT, DPA(SA)-MMT with different concentrations of DPA (10–25 mM). After making of DPA(SA)-MMT, polymerization was carried out to prepare J. Nanosci. Nanotechnol. 6, 1594–1601, 2006

3.2. Characterization of PDPA-MMT-NC 3.2.1. Morphology The morphology of the PDPA-MMT-NC was investigated by TEM. The TEM images of PDPA-MMT-NC (Fig. 2B), prepared with 10 mM and 25 mM DPA reveal that PDPA-MMT-NC are having hollow spherical morphology. Table II.

Summary of XRD data.

Sample MMT ∗ PDPA-MMT-NC

∗

10 15 20 25

mM mM mM mM

d-spacing (Å) (d001 )

2-theta (d001 )

18.39 18.76 19.04 19.41 19.73

4.59 4.32 4.03 3.61 3.28

DPA DPA DPA DPA

MMT: 0.5 g; NSA: 100 mM; APS: 0.5 M.

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Counts

19.73 Å

28

Nanostructuring of Poly(diphenylamine) Inside the Galleries of Montmorillonite Organo Clay (A) (a)

(b)

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c

(B) (a)

(b)

% Transmittance

b

a

Fig. 2. (A) FETEM micrographs of (a) MMT and (b) PDPA-MMTNC prepared with DPA(SA) from 25 mM of DPA (Scale bar: 20 nm). to: (B) TEM micrographs of PDPA-MMT-NC prepared with Delivered DPA(SA) from by Ingenta 4000 Max-Planck-Institut (a) 10 mM and (b) 25 mM of DPA (Scale bar: 100 nm).

3000

2000

1000

Wavenumbers (cm–1)

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IP : 134.105.184.136 Fig. 10:13:58 3. FT-IR spectra of (a) pristine MMT, (b) PDPA and (c) PDPA2007

Hollow spheres with inner diameter in the range of 40– Thu, 05 Apr 90 nm and an outer diameter of about 50–100 nm are formed. We envisage the formation of hollow sphere morphology as due to the polymerization of self-assembled DPA units inside the galleries of MMT. We hypothecate the formation of self-assembly of DPA inside the MMT galleries as follows. Organoammonium ions modified MMT galleries are susceptible for exchange of other organic cations. Intercalation of DPA into galleries occurs through exchange of cations. The unshared electron pair present in the nitrogen atom of amine group in DPA acts as a donor site and exerts a donor-acceptor type interaction with electron-deficient Si–O groups of MMT. During sorption of the monomer, DPA is expected to be held within the gallery of MMT through such donor-acceptor interactions. Further, in presence of NSA, protonated DPA or DPANSA salt gets attached to MMT rather than the neutral DPA molecules. NSA is amphiphilic and provides possibility of micellar formation with DPA molecules inside the MMT gallery (Scheme 1). With micelles consisting of NSA-DPA salt, formation of spherical shell of micelles inside MMT gallery can be visualized (Scheme 1). This lowers the overall polarity inside the MMT gallery and shifts thermodynamic equilibrium to accommodate more DPA molecules inside the galleries of MMT. As a result, monomer-filled micellar structures inside the MMT galleries are formed (Scheme 1). Upon addition of oxidant, dissociation of APS into anions and cations occur which are hydrated in aqueous solution. These hydrated ions easily enter into the galleries of MMT due to the small sizes of the ions relative to the d-spacing of MMT. Self-assembled DPA units in the MMT galleries polymerize by the diffused S2 O− 8 ions to result hollow spherical PDPA. Hence, the self-assembled micellar structure acts as a template to form hollow sphere structured PDPA inside the MMT galleries (Scheme 1). 1598

MMT-NC.

3.2.2. FT-IR Studies Figure 3 shows the FT-IR spectra of PDPA-MMT-NC, pristine MMT and pristine PDPA. FT-IR spectrum of MMT show a doublet peak at 2854 and 2928 cm−1 due to asymmetric stretching and symmetric stretching of –CH2 present in the modifier quaternary ammonium ion, a sharp absorption band at 1054 cm−1 due to Si–O bond stretching, and another doublet at 450–600 cm−1 due to Si–O bond bending and Al–O stretching.42 The absorption band around 3400 and 1325 cm−1 correspond to N–H stretching mode of secondary amine. For PDPA, the bands around 1505 and 1612 cm−1 corresponding to oxidized form of PDPA consisting of diphenoquinodimine (quinoid) and diphenyl benzidine (benzenoid) structures,42 respectively were observed. The presence of band around 1175 cm−1 for PDPA is attributed to the presence of diphenoquinone type units.43 The band around 1316 cm−1 is assigned to stretching vibration of C–N groups with partial double bond characteristics. PDPA-MMT-NC composite shows prominent peaks at 1505, 1602, and 1171 cm−1 which are characteristics of doped PDPA. In addition, a sharp peak is observed at 1055 cm−1 for PDPA-MMT-NC that corresponds to the Si–O bond stretching in MMT. This indicates that the composites contain both PDPA and MMT. 3.2.3. Thermogravimetric Analysis Figure 4 shows the thermograms of PDPA/MMT-NC, pristine MMT and PDPA. The degradation of MMT (curve a) starts at 220  C.44 We wanted to know the degradation J. Nanosci. Nanotechnol. 6, 1594–1601, 2006

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Nanostructuring of Poly(diphenylamine) Inside the Galleries of Montmorillonite Organo Clay

100 a

a

60

% Transmittance

Weight (%)

80

c b

b

40

20 200

400

600

800

Temperature (ºC)

Delivered by Ingenta to: 4000

Fig. 4. Thermograms of (a) pristine MMT, (b) PDPA(ex-situ)-MMT and Max-Planck-Institut (c) PDPA-MMT-NC. IP : 134.105.184.136

3000

2000

1000

Wavenumbers (cm–1)

Fig. 10:13:58 5. FT-IR spectra Thu, 05 Apr 2007

3.3. Characterization of Hollow Spherical PDPA Extracted from MMT Galleries 3.3.1. FT-IR Spectroscopy FT-IR spectra of PDPA extracted from MMT galleries (PDPA(E-MMT)) and PDPA prepared through conventional polymerization (PDPA-C) are presented in Figure 5. J. Nanosci. Nanotechnol. 6, 1594–1601, 2006

of PDPA (a) prepared from conventional polymerization (PDPA-C) and (b) extracted from MMT (PDPA(E)-MMT).

PDPA(E-MMT) show variations in the band position in contrast to PDPA-C. The peaks which were noted around 1325, 1505, 1612, and 1171 cm−1 in the case of PDPA-C were found to shifted to 1350, 1512, 1618, and 1125 cm−1 . Also, the peaks around 3420, 1618, 1505, 1125 cm−1 in PDPA(E-MMT) show splitting with the appearance of a shoulder close to the main peaks. This indicates that PDPA formed inside the galleries of MMT may have few structural variations than PDPA prepared by conventional polymerization. 3.3.2. UV-Visible Spectroscopy UV-Vis spectroscopy was used to determine the electronic state of PDPA(E-MMT). UV-Vis spectra (Fig. 6) of PDPA(E-MMT) and PDPA-C are compared. For this, PDPA was prepared in NSA medium and neutralized by treating with aqueous ammonia. PDPA(E-MMT) showed prominent peaks at 320, 570, 890, and 995 nm. The peaks at 320 and 570 nm corresponds to the –* transition of aromatic ring and bipolaronic transitions of PDPA, respectively.45 The peaks at 890 and 995 nm are attributed to the free charge carriers.46 Interestingly, even after exhaustive neutralization, PDPA formed inside the MMT galleries exists in the bipolaronic state as evident from the appearance of band around 570 nm. The inset in Figure 6 shows the UV-Vis spectra of NSA doped PDPA-C and neutralized PDPA-C. UV-visible spectra of NSA doped PDPA-C shows peak at 570 nm characteristic of the bipolaronic form of PDPA47
48 and the neutralized PDPA-C shows peak around 610 nm. PDPA(E-MMT) retains the bipolaronic state even after neutralization with a band 1599

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characteristics of PDPA which is present in the galleries of MMT and to compare with degradation characteristics of mixture of PDPA and MMT. Hence, polymerization was also carried out without loading DPA inside the galleries of MMT. Polymerization of DPA in the presence of MMT resulted a composite consisting of mixture of MMT and PDPA. Here, PDPA was expected to form outside the galleries of MMT. We represent this composite as PDPA(ex-situ)-MMT composite. Here, ex-situ signifies the presence of PDPA outside the galleries of MMT. PDPA(exsitu)-MMT composite shows start of degradation around 180  C (Fig. 3, curve b). However, for PDPA-MMT-NCs in which PDPA is present inside the MMT galleries show the start of degradation around 220  C (Fig. 3, curve c). An enhancement of thermal stability for the PDPA is therefore noticed when it is formed inside the galleries of MMT. The increase in the thermal stability is indicative of the stabilization effect of the degradation of backbone units of PDPA when it is present inside MMT galleries. The characterization results of PDPA-MMT-NC clearly indicate that PDPA formed inside the galleries of MMT posses different thermal characteristics than PDPA formed ex-situ of MMT. We therefore focused our attention to analyze the structural and electronic properties of PDPA formed inside the galleries of MMT.

Nanostructuring of Poly(diphenylamine) Inside the Galleries of Montmorillonite Organo Clay

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0.25

Absorbance

0.15

0.2

(a)

0.1

=N+

b

–N+ 0.05

–N–

0.15

500

600

700

800

Wavelength (nm) =N– 0.1

CPS

Absorbance

a 0 400

a

(b)

=N+

b

0.05

–N+ –N – 0 400

600

800

Wavelength (nm)

1000

1200

=N–

Delivered by Ingenta to:

Fig. 6. Visible spectra of (a) NSA doped PDPA(E-MMT) and (b) neuMax-Planck-Institut tral PDPA(E-MMT) (Inset: (a) NSA doped PDPA-C and (b) neutral IP : 134.105.184.136395 PDPA-C).

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Thu, 05 Apr 2007 10:13:58

399

401

403

Binding Energy (eV)

around 570 nm. The confinement of PDPA inside the galleries of MMT is expected to alter the electronic states of PDPA. Such an effect is not observed for PDPA-C.

Fig. 7. XPS N 1s core level spectrum of (a) NSA doped PDPA-C and (b) neutralized PDPA(E-MMT).

3.3.3. Conductivity Measurement

Hollow spherical poly(diphenylamine) was synthesized by forming self-assembly of DPA in the presence of NSA inside the galleries of MMT. PDPA was extracted from the galleries of MMT and the electronic state was analyzed. The electronic state of PDPA extracted from MMT galleries shows the confinement effect which is different from PDPA formed by the conventional method. Hollow spherical conducting polymers are expected to find applications in sensors and separation analysis.

The room-temperature conductivity of PDPA(E-MMT) was found to be 0.25 mS · cm−1 . The conductivity of PDPA-C was 29.6 mS · cm−1 . The lower conductivity for PDPA(E-MMT) may be ascribed to the decreased possibility of doping of amine or imine units in PDPA by sulfonate anion. The formation of PDPA inside the MMT galleries could restrict the extent of doping and might result lower conductivity.49 3.3.4. XPS Measurement Figure 7 shows the nitrogen (N 1s) XPS core level spectra of NSA doped PDPA-C and neutralized PDPA(E-MMT). The N 1s main peak is decomposed into four lines. The lines of imine and amine nitrogen of the samples are centered at 398.3 and 399.3 eV, respectively. The lines at 401 and 403 eV are assigned to positive nitrogen (N+ ).50 The existence of imine nitrogen even after neutralization is apparent from the XPS results. The data confirm the existence of confinement effect for PDPA(E-MMT). Based on the analysis of the area ratio of the N 1s peaks, the area ratio of N+ component for PDPA(E-MMT) ([N+ ]/[N] = 22%) is relatively lower than that of PDPA-C ([N+ ]/[N] = 40%),50 which implies that the PDPA(E-MMT) are in low doping state. This result is in accordance with the lower value of conductivity for PDPA(E-MMT). 1600

4. CONCLUSIONS

Acknowledgments: This work was supported by Korean Research Foundation Grant (KRF-2004-00500009). The authors acknowledge the help of Korea Basic Science Institute, Daejon, Korea for recording the FETEM micrographs and Kyungpook National University Center for Scientific Instrument.

References and Notes 1. C. R. Martin, Science 266, 1961 (1994). 2. C. R. Martin, Chem. Mater. 8, 1739 (1996). 3. G. Che, B. B. Lakshmi, C. R. Martin, E. R. Fisher, and R. S. Ruoff, Chem. Mater. 10, 260 (1998). 4. M. Wan and J. Li, J. Polym. Sci. A Polym. Chem. 38, 2359 (2000). 5. J. Liu and M. Wan, J. Mater. Chem. 404 (2001). 6. H. Qiu, M. Wan, B. Mattews, and L. Dai, Macromolecules 34, 675 (2001). 7. Z. Zhang, Z. Wei, and M. Wan, Macromolecules 35, 5937 (2002). 8. K. Huang and M. Wan, Chem. Mater. 14, 3486 (2002).

J. Nanosci. Nanotechnol. 6, 1594–1601, 2006

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Nanostructuring of Poly(diphenylamine) Inside the Galleries of Montmorillonite Organo Clay

Received: 15 October 2005. Accepted: 30 November 2005.

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