General Introduction
1)
Over the last two decades a mammoth literature has accumulated on the preparation, characterization and application of polymer based hybrid nanocomposites with various types of inorganic nanocomponents. This area of research has involved polymer scientists, material scientists, physicists, and chemists, all over the world to fabricate new challenging nanocomposite materials for specialized applications. From time to time, several informative reviews have appeared [1-4] dealing with selective aspects of this area. The present review is aimed at highlighting the interesting and potential aspects of research on some selective conducting polymer/mixed polymer - inorganic hybrid nanocomposite materials. It is indeed difficult to discuss in one review of limited size, all relevant work accomplished in this vast area and naturally one has to be selective in focusing the lit- erature.
To whom all correspondence should be addressed.
(e-mail: mukul_biswas@vsnl.net )
In the above background, we intend to cover in this article the more important research accomplished during the past two decades on hybrid nanocomposites of speciality polymers, conventional polymers, and mixed polymer systems with (i) nanodimensional metal oxides, (ii) MMT-clay, (iii) zeolites, (iv) carbon black (CB)/
acetylene black (AB), (v) fullerene, and (vi) carbon nanotube (CNT). Interesting aspects of bulk properties including structural characteristics (FTIR, UV, and Raman), morphological characteristics (SEM, TEM, XRD), thermal stability, conductivity, and other distin- guished properties such as electrorheological properties in some selective systems will also be highlighted in the review.
Nanocomposites Based on Nanodimensional Metal Oxides
Conducting polymers are usually intractable materials particularly in the doped conducting state. Dispersions of the conducting polymers in aqueous/non aqueous media
Recent Progress in Conducting Polymer, Mixed Polymer-Inorganic Hybrid Nanocomposites
Arjun Maity and Mukul Biswas
Department of Chemistry, Presidency College, Calcutta-73, West Bengal, India Received March 20, 2006
Abstract: The article aims at reviewing the interesting and potential aspects of research accomplished in the area of conducting polymer, mixed polymer - inorganic hybrid nanocomposites during the past two decades. In specific, preparative aspects of hybrid nanocomposites of speciality polymers, conventional polymers and mixed polymer systems with nanodimensional metal oxides, MMT-clay, zeolites, carbon black, acetylene black, buckministerfullerene and carbon nanotubes will be discussed briefly. The mechanisms of polymerization vis-à-vis composite formation will also be discussed for a few typical systems. Notable features of the results of evaluation of structural characteristics by Raman spectroscopy, morphological characteristics by SEM, TEM, X-ray diffraction pattern analyses, thermal stability and dc conductivity behaviors in the various composite systems will also be described system-wise. Special properties like electrorheological characteristics in selective systems and the end use applications of these various composites will also be highlighted.
Keywords: polymer-inorganic hybrid nanocomposites, conductivity, thermal stability, morphology, conducting polymer, electrorheological fluid
Figure 1. Schematic representation of an isolated conducting polymer colloid particle.
have attracted considerable research interest because of case of processing. Among the conducting polymers, by far the largest amount of research thrust appears to have been directed to PANI and PPY based composite systems [5-30]. Since 1986, various research groups have de- scribed the preparation of sterically stabilized colloidal dispersions of air stable intrinsically conducting polymers such as PANI or PPY by several methods.
Preparative Aspects
Dispersion Polymerization in Presence of Polymeric Sta- bilizers
In this technique, the synthesis of sterically stabilized polymer particles via dispersion polymerization [4-30]
was achieved in aqueous medium.
PVA-stabilized particles of PPY have considerable potential as novel marker particles in immunodiagnostic strip assays [31]. Several workers synthesized [14-17] a wide range of copolymer stabilizers for the preparation of PANI colloids. 4-aminostyrene was copolymerized with various hydrophilic co-monomers such as 2-vinyl- pyridine or N-vinylpyrrolidone to produce reactive co- polymer stabilizers containing pendant ANI units [25].
Synthesis of colloidal PPY-particles was achieved using a tailor-made reactive copolymer stabilizer based on 2-(dimethylamino)ethylmethacrylate (PMAEMA). This water-soluble copolymer contained a small percentage of polymerized vinyl(bi)thiophene groups which led to the in-situ chemical grafting of the stabilizer onto the surface of the PPY particles (Figure 2) during dispersion polymerization [32]. Four different bithiophene based comonomers were used as potential graft sites: 2VT, 3VT, 2VBT or BTMA (Figure 3) [32].
In-situ Polymerization of Monomers in Presence of Nan- odimensional Metal Oxides
Incorporation of inorganic particles inside the core of organic polymers became a popular and interesting
Figure 2. Schematic representation of the synthesis of PPY colloids.
Figure 3. Chemical structure of the various comonomers.
method [33-45] for preparation of polymer based nano- composites during the early 1990s. These organic- inorganic composite materials differ from the pure poly- mers in regard to their physical and chemical properties and furthermore they differ from each other depending on the chemical nature of organic and inorganic coun- terparts.
In this technique, the preparation of colloidal nano-
Figure 4. Schematic of conducting polymer-coated SiO2 particles.
Figure 5. Schematic formation of polymer-inorganic oxide nanocomposite.
composites of intractable polymers was achieved by polymerizing respective monomers by conventional oxi- dants in an aqueous suspension of various nanodi- mensional metal oxides acting as particulate dispersants.
PPY and PANI coated SiO2[44], SnO2 [39] and Sb- doped SnO2 [39] suspensions were prepared by in-situ polymerization of water soluble ANI or PY monomers in stirred solutions containing FeCl3and the respective nano oxide. Schematic of the formation of conducting polymer coated SiO2particles is shown in Figures 4 [44] and 5.
Unlike ANI and PY monomers, NVC is insoluble in water and appropriate procedures for preparing PNVC- metal oxide nanocomposites were developed. PNVC- MnO2 [46] composite was prepared by in-situ bulk polymerization of NVC monomer in presence of nano- sized MnO2 without any extraneous oxidant. PNVC- ZrO2 [47] and PNVC-SiO2 [48] nanocomposites were prepared by the FeCl3 impregnated ZrO2 and SiO2 in benzene medium. PPY-MnO2[46], PANI-MnO2[46] and PANI-ZrO2 [47] composites were also prepared via polymerization of the respective monomers by using oxidants such as FeCl3 and APS in aqueous medium in which nanodimensional metal oxide was suspended.
Recently, in a simple procedure, PNVC-Al2O3nanocom- posite was prepared [49] by adding a THF solution of
Scheme 1. PTP-V2O5composite.
Scheme 2. PANI-V2O5composite.
preformed PNVC drop-wise into the aqueous dispersion of Al2O3. PANI-Al2O3[50], PPY-Al2O3[51], PTP-Al2O3 [52] and PF-Al2O3 [53] nanocomposites were syn- thesized via in-situ polymerization of ANI, PY (aqueous medium) and TP, F (CHCl3 medium) monomers using external oxidants (FeCl3) in presence of nanodimensional Al2O3.
More recently, a PNVC-MoO3 nanocomposite was obtained [54] from both in bulk and solvent polymeriza- tion of NVC monomer by MoO3oxidant. An interesting procedure was developed [55,56] for the preparation of PAN and PMMAcolloidal SiO2 based nanocomposites recently where the redox polymerization (K2CrO4- NaAsO2) of the corresponding monomers [57] was conducted in presence of nanodimensional SiO2suspen- sion to isolate the respective nanocomposites.
Intercalative Polymerization
With increasing attention on sol-gel derived materials for the development of advanced structure and electronic ceramics, a novel procedure was developed for polymer- metal oxide nanocomposite preparation. In this respect, V2O5 xerogels were regarded as unique among sol-gel derived materials by virtue of their possessing a porous structure with an interlayer distance of 11.55Å.
Intercalated PTP [58] (Scheme 1), PANI [59,60]
(Scheme 2) and PPY [61] in V2O5xerogel were prepared by in situ polymerization technique. A new family of polymermolybdenum bronze nanocomposites was devel- oped [62] with LixMoO3 (X = 0.31～0.40) as host material and polymers like PEO, PEG, PPG, PVP, MC,
Scheme 3. Head-to-tail oxidative coupling of aniline oli- gomers.
Scheme 4. PPY-FeOCl composite.
Scheme 5. PANI-FeOCl composite.
PAM and nylon-6.
In ANI-V2O5 xeroxel system, O2 acted as an electron acceptor both during insitu polymerization [60] and long after intercalation was complete. Notably, samples were more conducting than those in an inert atmosphere.
Aging of PANI-V2O5 composite in presence of O2 partially deoxidized the reduced V2O5framework and led the PANI chain inside the V2O5 gallery to couple oxidatively to form longer chain leading to enhanced conductivity (Scheme 3).
Layered (PPY)0.34FeOCl [63] (Scheme 4) and PANI- FeOCl [64] (Scheme 5) nanocomposites were prepared via polymerization and intercalation of PY or ANI monomers in the constraint of vander Waals gap in layered FeOCl. The PANI-FeOCl composite is report- edly the first intercalation compound in which substantial polymer ordering endotaxy inside a host material was realized. The freshly prepared (PANI)xFeOCl defined as theα-phase [64] was transformed into the β-phase [64]
Figure 6. PANI-MoS2composite.
upon standing for one month in presence of O2. It was suggested that in the intra lamellar space the PANI chains coupled further to form higher molecular weight polymer. IR spectra endorsed that the β-phase was in fact a mixture of PANI andβ-FeOOH. The α-phase [64]
consisted of alternating monolayers of conducting poly- mer and inorganic counterpart. NMR studies confirmed
“ring flips” (in contrast to bulk PANI) suggesting that in the FeOCl galleries PANI experienced host imposed motional restriction [64].
V2O5-PPY aerogel composites (ARG) were prepared [65] by in-situ polymerization of PY during sol-gel condensation process. The oxidative coupling of the PY units by V2O5 to form PPY was simultaneous with the polymerization of the inorganic oxide and was catalyzed by reaction with the PY monomer. The product was a monolithic inorganic-organic hybrid gel in which both components were mixed at the nanocomposite level.
The PANI-V2O5type of nanocomposites prepared [60]
as above was considered as eco-nonfriendly because of the hazardous benzene moiety in the polymer backbone.
However, development of PEDOT-V2O5 nanocompo- sites [66] by intercalation was regarded as more environ- ment friendly nanocomposite system.
Miscellaneous Systems
Layered metal phosphates like α-Ti(HOPO3)2․H2O, α- Zr(HOPO3)2․H2O and HUO2PO4. 4H2O are character- ized [67] by rich intercalation chemistry. The protons in the layer can be exchanged with cations or neutralized with bases. Intercalation of ANI and its dimer PPDA in layered metal phosphate followed by polymerization using O2led to interesting hybrid composites [67]. Inter- estingly, traditional oxidants like APS or FeCl3induced ion-exchange reactions, competing with the interlayer polymerization reaction and thereby forming PANI to form out-side the host crystallites. This ion exchange reaction was totally avoided by O2.
Transition metal chalcogenides have been extensively investigated for their electrical and other properties.
MoS2 is distinguished among the metal chalcogenides, for not intercalating inorganic molecules. However, in a novel observation intercalation into MoS2 was achieved
Figure 7. Schematic formation of a binary polymer-inorganic oxide nanocomposite.
[68] by dispersing MoS2into single layer by reaction of LixMoS2in water (Figure 6).
Reprecipitation of the layers in the presence of NMP solution of PANI produced a novel nanocomposite material containing well-ordered alternating monolayers of PANI and MoS2[68].
Composites with “Mixed Polymer” Systems
Numerous attempts appeared to have been made for the modification of one type of polymer (conducting or nonconducting) with another conducting polymer. The basic idea was to prepare a composite that would possess the distinctive properties of either polymeric compo- nents. However, most of these studies involved prepara- tion of polymer blends, grafts and copolymers [69].
In the background of the developments of water dispersible polymer - metal oxide nanocomposite systems described in the previous pages an interesting procedure was developed to modify the polymer - metal oxide dispersion further by loading them with another selective
polymer. A wide range of such binary polymer metal oxide based composite systems was prepared mainly using two types of procedures:
(a) Simultaneous Polymerization Technique
In this method, binary polymer composites were pre- pared by simultaneous polymerization of a mixture of water insoluble monomers in a solvent like THF in an aqueous metal oxide suspension in presence of an oxi- dant. Composites of PNVC-PPY [70], PNVC-PTP [70], PNVC-PANI [50], PNVC-PTP-Al2O3 [70], PNVC- PANI-Al2O3[50] were prepared by this method.
(b) Precipitation Technique
In this technique, the mixed polymer composite was subsequently obtained by adding non aqueous solution of a preformed polymer (II) with sonication or by polym- erization of monomer (II) in the medium-as explained schematically in Figure 7. Composites of PNVC-(PPY- Al2O3) [51], PPY-(PNVC-Al2O3) [51], PNVC-(PANI-
(A)
(B)
(C)
Figure 8. (A) TGA of (a) Al2O3(b) PTP-Al2O3Composite and (b) PTP homopolymer, (B) TGA of (a) PANI homopolymers (b) PANI-TiO2composite, (C) TGA of (a) SiO2(b) PANI-SiO2
Composite and (b) PANI homopolymer.
Al2O3) [50], PANI-(PNVC-Al2O3) [50], PANI-(PAN- SiO2) [71], PPY-(PAN-SiO2) [71], PANI-(PMMA- SiO2) [56] and PPY-(PMMA-SiO2) [56] were prepared in this technique.
Bulk Property Characteristics Thermal Stability Characteristics
Thermal Stability Order in Polymer - Metal Oxide
․
Nanocomposite Systems
In general, thermal stability of polymer - metal oxide
nanocomposite systems was higher than that of the respective base polymers and lower than that for the metal oxide counterpart. The reported trend in a variety of composite systems is shown below.
․SiO2 Based Composite Systems
SiO2 > PANI-SiO2> PANI [44], SiO2> PPY-SiO2 >
PPY [44], SiO2 > PAN-SiO2 > PAN [55], SiO2 >
PMMA-SiO2 > PMMA [56], SiO2 > PNVC-SiO2 >
PNVC [48].
․Al2O3 Based Composite Systems
Al2O3 > PNVC-Al2O3 > PNVC [49], Al2O3 > PANI- Al2O3 > PANI [50], Al2O3 > PPY-Al2O3 > PPY [51], Al2O3 > PTP-Al2O3 > PTP [52] (Figure 8a), Al2O3 >
PF-Al2O3> PF [53].
․ZrO2 Based Composite Systems
ZrO2> PNVC-ZrO2> PNVC [47], ZrO2> PANI-ZrO2>
PANI [47].
․MnO2 Based Composite Systems
MnO2 > PNVC-MnO2 > PNVC [46], MnO2 > PPY- MnO2> PPY [46], MnO2> PANI-MnO2> PANI [46].
An interesting trend was recently revealed [72,73] by the PANI-TiO2 nanocomposite system where upto ca., 650 oC, PANI was consistently stabler than the PANI- TiO2 composite (Figure 8b). Such a feature was thought to indicate a strong interaction at the surface of TiO2and PANI weakening the interactive force of PANI inter- chains and thereby enhancing thermal decomposition of the PANI in the composite.
The thermal stability of (PANI)x-V2O5, nH2O xerogel [60] was better than that of its aged counterpart under N2 or O2 despite the higher molecular weight in the latter system. The aging process would render the framework more oxidizing (higher V+5/V+4 balances) which would eventually degrade the polymer chain in a more facile manner.
Quantitative loading of the polymer in polymer - metal oxide nanocomposites systems was estimated for the PANI-SiO2 (Figure 8c) and PPY-SiO2 nanocomposites [40]. In ANI-SiO2/PY-SiO2 systems, the uncoated SiO2 particles produced a weight loss of 1.97 % due to dehydration of the hydrophilic group of the SiO2surface.
PANI-SiO2 and PPY-SiO2 composites exhibited [40]
weight losses of 5.70 and 7.70 % respectively. From these data the PANI and PPY layers in the composites were estimated to be 3.75 and 5.73 % by mass respec- tively. Taking the densities of SiO2(2.1 gcm-3), PANI or PPY (1.5 gcm-3), the average thickness of the conducting polymer overlayers was in the order of 9 and 14 nm respectively [40].
Table 1. Conductivity Data of Polymer Based Nanocomposite Systems
Entry no Materials Conductivity (S/cm) References
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
PNVC-Al2O3(FeCl3impregnated) PNVC-MnO2
PNVC-ZrO2(FeCl3impregnated) PNVC-SiO2(FeCl3impregnated) PNVC-MoO3
PPY-Al2O3 PPY-MnO2 PPY-ZrO2
PPY-SiO2 PPY-SnO2
PPY-SnO2(Sb doped) PPY-SnO2(p-tosylate-doped) PPY-V2O5
PPY-FeOCl PANI-MnO2 PANI-ZrO2
PANI-SiO2 PANI-Al2O3 PANI-Fe3O4
PANI-FeOCl PANI-V2O5 PANI-TiO2
PANI-TiO2(dedoped) PTP-Al2O3
PTP-V2O5
PF-Al2O3 PEDOT-V2O5
1×10-7 3.5-4.4×10-5 1-1.5×10-5 1×10-5 10-2 0.8×10-4 0.6-0.92×10-2 1-15 2×10-5 2×10-3 7 23 10-3 10-1 1-2.5×10-2 0.03-0.35×10-2 6.1×10-2 1.4-4.1×10-2 10-2 0.01-0.1 10-4-10-1 2.41×10-3 3.79×10-9 0.3-0.4×10-3 0.1 10-7 10-1-10-5
49 46 47 48 54 51 46 75 44 39 39 39 61 63 46 47 44 50 76 64 59 72,73 72,73 52 58 53 66
Table 2. Conductivity Data of Binary Polymer Based Com- posite Systems
Entry no Composites Conductivity
(S/cm) References 1
2 3 4 5 6 7 8 9 10 11 12 13
PNVC-PPY composite PNVC-(PPY-Al2O3) PPY-(PNVC-Al2O3) PTP-PNVC-Al2O3
PNVC-PANI-Al2O3 PTP-PNVC
PNVC-(PANI-Al2O3) PANI-(PNVC-Al2O3) PANI-(PAN-SiO2) PPY-(PAN-SiO2) PANI-(PMMA-SiO2) PPY-(PMMA-SiO2) PNVC-PANI
5×10-2 1.7×10-5 4.48×10-6 3×10-7 1.5×10-4 1-2×10-4 3.0×10-4 2.0×10-4 1.5×10-3 8×10-4 10-4 10-5 10-3
77 51 51 70 50 53 50 50 71 71 56 56 50 Thermal Stability Order in Mixed Polymer - Metal
․
Oxide Nanocomposite Systems
The overall thermal stability of the nanosized Al2O3 based composites of PNVC-PANI, PNVC-PPY and PNVC-PTP varied in the following order: (a) PNVC- Al2O3 > PNVC-PANI-Al2O3 > PANI-Al2O3 [50], (b)
PNVC-Al2O3 > PNVC-(PPY-Al2O3) > PPY-Al2O3[51], (c) PTP-Al2O3> PNVC-PTP-Al2O3> PNVC-Al2O3[70].
In the case of PNVC-PANI-Al2O3 [50] and PNVC- (PPY-Al2O3) [51] nanocomposite systems, the observed trend was reasonable because incorporation of less stable PANI or PPY moieties in the PNVC-PANI or PNVC- PPY network respectively would evidently cause a decrease in the thermal stability of PNVC-PANI-Al2O3 or PNVC-(PPY-Al2O3) nanocomposites. The lower ther- mal stability of the PNVC-PTP-Al2O3 nanocomposite compared [70] to that for PTP-Al2O3 composite was noteworthy and indicative of decreased bond strength in the composite matrix. A possible reason for this trend could be that the proposed loading of PTP on the PNVC chain produced a sterically loaded structure readily susceptible to bond breakage on heating.
Conductivity Characteristics
A survey of the literature data compiled in Table 1 and Table 2 for various polymer/binary polymer metal oxide based nanocomposite systems evidently points out to a lack of any consistent order in conductivity that could depend on the nature of polymer and metal oxide counterpart. Notably, however, PNVC-nanooxide based
composite systems, exhibited remarkably improved conductivity values compared to PNVC homopolymer [74] (essentially non conducting 10-12～10-16S/cm).
In the case of inherently conducting polymers like PANI or PPY, the conductivity values of the metal oxide based composites showed wide variations excepting in PPY-ZrO2, and PPY-SnO2[39] nanocomposite systems.
It would be expected that the conductivity in such com- posites would depend upon the amount of conducting polymer dispersions in the composite during preparation and structural purity of the polymer counterpart, both of which would depend upon the experimental conditions used.
The relatively high solid state conductivity exhibited by these nanocomposites indicate the possibility of inter particle charge transfer mechanism operative in the microscopic level. XPS observation in several systems confirmed the presence of conducting polymer compo- nent within top 2～10 nm of the particle surface in the PANI-SiO2and PPY-SiO2nanocomposite systems [40].
The nature of the oxidant also was seen to influence somewhat the conductivity of the nanocomposites. Use of FeCl3 oxidant led to the formation of a PPY-SiO2 nanocomposite of three orders higher conductivity com- pared to the same synthesized with APS oxidant. This effect was attributed to overoxidation of the conducting polymer and not to any conventional doping effect. A similar feature was also realized with PNVC-FeCl3 impregnated ZrO2 [47] and PNVC-FeCl3 impregnated SiO2, where higher orders of conductivity were observed [48].
The high conductivity values realized with PTP-Al2O3 [52] and PNVC-Al2O3nanocomposite [49] systems were due to the conventional doping action of I2 and FeCl3 used as a dopant in these systems. Consistently, dedoped PANI-TiO2composite showed a 106fold loss of conduc- tivity order compared to that for the PANI-TiO2-APS system [72].
Likewise, use of more conductive antimony doped SnO2yielded PPY-antimony doped SnO2composite [39]
with conductivity as high as 7 S/cm. A conductivity as high as 23 S/cm was realized for a p-tosylate doped PPY-SnO2 composite suggesting that the PPY com- ponent also made a contribution to the inter particle conductivity mechanism.
Intercalative polymerization of PY monomer via the reaction of FeOCl with an excess of neat PY monomer at 60 oC produced PPY-FeOCl composite [63] of con- ductivity higher than other FeOCl intercalative com- posites but lower than [(PPY)+y(X)-y]n, where X = ClO4-, BF4-, NO3-. For the PANI-FeOCl composite [64] system the conductivity of the β-phase (see earlier) was 10 times higher than that of α-phase at all temperatures suggesting presence of longer chains of PANI in the
oxidized product.
The in-situ intercalation/polymerization of 2,2' bithio- phene in layered V2O5, nH2O xeroxel produced a film of polymer-layered oxide bronze with 4 orders of higher magnitude with respect to pristine xerogel (0.1 S/cm) [58]. In 5～300 K range the conductivity decreased with falling temperature characteristic of a thermally activated behavior and at room temperature, the conductivity perpendicular to the films was 10 times lower than that parallel to them indicating anisotropy. TP measurements confirmed the p-type behavior with predominant charge careers being holes. Intercalated PANI-V2O5 xerogel bronze also showed [60] similar enhanced conductivity (0.5 S/cm) and other similar features.
Morphological Features
Scanning Electron Microscopic Analysis
SEM analyses were mostly used for morphological characterization of the various polymer-based composite systems. Table 3 summarizes the surface morphology patterns observed for various types of nanodimensional metal oxide based nanocomposite systems Figures 9(a～ g) present the SEM characteristics of some selective polymer/binary polymermetal oxide composite systems.
Transmission Electron Microscopic Analysis
․
Particle sizes of some selective polymer/binary polymer- nanodimensional metal oxide systems as computed from TEM analyses are collected in Table 4. Some repre- sentative TEM micrographs of a few systems are presented in Figures 10(a～g). Some distinctive features of the TEM patterns are (1) in general, TEM of PANI-based nanocomposites of several oxides revealed no apparent correlation between the particle sizes and nature of the oxides, (2) complete encapsulation was observed with in several systems like PANI-SiO2 [41]
(Figure 10a), PANI-Stringy SiO2 [42] (Figure 10b), PANI-ZrO2[47] (Figure 10c), PANI-MnO2[46] (Figure 10d), PPY-SnO2 [39] (Figure 10e), PTP-Al2O3 [52]
(Figure 10f) and PNVC-Al2O3 [49] (Figure 10g) com- posites (3) in PPY-stringy SiO2 nanocomposite system, clusters of original SiO2 particles with the conducting polymer binding component were visible in contrast to the near mono-disperse “raspberry” particles obtained [39]
with spherical SiO2particles.
Water Dispersibility
Speciality polymers such as PPY, PANI, and PTP display outstanding conductivity and other bulk prop- erties but suffer from possibility limitations because of their intractable nature. As discussed earlier considerable research attention was paid globally has been directed to obtain processable dispersion of these intractabl espe-
Table 3. Surface Morphology of Various Metal Oxide-Based Nanocomposite Systems
Materials Morphological features as revealed by SEM analyses Reference
PPY-SiO2 High magnification revealed PPY-coating with feature dimension in the order of 100 150 nm 44 PANI-SiO2stabilized
with Ludox S 30
Particles formed at room temperature had irregular shape, while particles formed at 0oC were smaller and spherical. Larger PANI-SiO2particles with typical “raspberry” patterns were formed when larger colloidal SiO2particles were used
44 PANI-TiO2 Exhibited mixed morphology, plate like for PANI and spheres for TiO2(Figure 9b) 72, 73 PANI-FeOCl Revealed singe crystal character with alternating monolayers of PANI and FeOCl. The insertion of
PANI caused the interlayer structnre to increase 6.54 (Figure 9a). 64 PTP-Al2O3 The composite particles were characterized by globular morphology and their tendency to form
clusters (Figure 9c). 52
PNVC-Al2O3 Formation of agglomerates of particles of non uniform sizes and shapes (Figure 9d). 49
PANI-Al2O3 Spherical particles (Figure 9e) 50
PPY-Al2O3 Non uniform sizes and shapes of aggregates 51
PAN-SiO2 Lumpy masses of irregular shapes and sizes 55
PMMA-SiO2 Nearly spherical particles 56
PNVC-MB Formation of lumpy aggregates with the tendency to form clusters of relatively larger sizes 78
PF-Al2O3 Formation of almost spherical particles 53
PNVC-PANI-Al2O3, PNVC-(PANI-Al2O3), PANI-(PNVC-Al2O3), PNVC-(PPY-Al2O3), PPY-(PNVC-Al2O3), PNVC-PTP-Al2O3, PANI-(PAN-SiO2), PPY-(PAN-SiO2), PANI-(PMMA-SiO2), PPY-(PMMA-SiO2)
The formation of lumpy aggregates with average sizes larger than those of the corresponding homopolymer based composites was a general feature.
50, 51, 70, 71
cility polymers using nanodimensional metal oxides such as SiO2, Al2O3, SnO2, ZrO2, MnO2 [39,49,49-53] as particulate dispersants in the polymerization medium.
Table 5 presents the conditions applied to obtain dispersions of various types composites in aqueous medium vis-à-vis the stabilities of suspensions.
MMT-Based Nanocomposite Systems
Clays are the most abundant minerals and available as inexpensive materials that have high physical and mechanical strengths as well as high chemical resistance [1,79-89]. Because of the small particle size and in- tercalation properties of clays, they afford an appreciable surface area for the adsorption of molecules. Clay polymer materials have received considerable interest because the interactions between them have effects on the properties of both the clay and polymer. A mammoth literature already has accumulated on the preparation and characterization of clay based polymer-nanoocmposites.
However, in this article discussions will be restricted to only conducting polymer clay based systems.
Preparative Aspects
Speciality Polymer-Based Nanocomposites
General preparative methods involved impregnation vis-à-vis intercalation of monomers with clay with or without any external oxidant. PNVC-MMT clay nano- composites were prepared [90] by direct interaction of NVC (solid monomer or in toluene solvent) with MMT clay. Monomers like PY [91], ANI [91], F [35] and TP [92] were also polymerized directly by refluxing with MMT, the reaction undergoing a distinctive colour change indicative of charge transfer reaction.
PANI-Na+MMT nanocomposite was synthesized [93]
via emulsion polymerization in presence of DBSA as an emulsifier as well as dopant. An inverted emulsion pathway was developed to prepare PPY-clay [94] and PANI-clay [95] nanocomposites by using DBSA. In this process DBSA was dissolved in iso-octane and mixed with APS in distilled water to form the inverted emul- sion. It was then mixed with Na+MMT dispersed in distilled water for 12 h at 0oC. PY monomer was added drop wise keeping the temperature at 0 oC. The polym- erization was finally terminated after 24 h by acetone.
A PANI-Cu fluorohectorite composite was synthesized
(a) (b)
(C) (d)
(e) (f)
(g)
Figure 9. (a) SEM image of the PANI-FeOCl composite, (b) PANI-TiO2composite, (c) PTP-Al2O3composite, (d) PNVC-Al2O3
composite, (e) PANI-Al2O3composite, (f) PTP-PNVC- Al2O3composite, (g) PNVC-(PANI-Al2O3) composite.
(a) (b)
(c) (d) (e)
(f) (g) (h) (i)
Figure 10. TEM image of the (a) PANI-colloidal SiO2composite, (b) PANI-stringy SiO2composite, (c) PANI-ZrO2composite, (d) PANI-MnO2 composite, (e) PPY-SnO2composite, (f) PTP-Al2O3composite, (g) PNVC-Al2O3composite, (h) PTP-PNVC-Al2O3
composite, (i) PNVC- PANI-Al2O3composite.
[96,97] by intercalative polymerization of ANI in fluorohectorite oriented film containing gallery Cu+2ion introduced by an ion exchange process.
A highly soluble PPY was produced [98] by an in-situ
polymerization method using Na+DEHS-in water at 0oC and an aqueous APS solution. The polymerization was allowed to proceed for 24 h at 0 oC. The resulting PPY was soluble in NMP, DMF and CHCl3 solvent. PPY-
Table 4. Particle Sizes of Various Metal Oxide-Based Nano- composite Systems
Entry no Materials Particle sizes References 1
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
PNVC-Al2O3 PNVC-SiO2 PNVC-ZrO2 PNVC-MnO2 PPY-Al2O3
PPY-MnO2
PPY-ZrO2
PPY-SiO2
PANI-Al2O3 PANI-ZrO2 PANI-MnO2 PANI-SiO2 PTP-Al2O3
PF-Al2O3
PAN-SiO2
PMMA-SiO2
PPY-MB
120 240 21 31 300 500 200 250 60 100 200 250
20 30 100 300
20 40 250 300 100 150 100 300 27 74
51 28 100 35 120
75
49 48 47 46 51 46 75 44 50 47 46 44 52 53 55 56 79
Scheme 6. Initiation of NVC by MMT clay.
OMMT composite was subsequently obtained [98] by mixing together in the same solvent PPY and OMMT and stirring the solution for 24 h at 0oC for intercalation.
Mixed Polymer-Based Nanocomposites
PANI-(PF-MMT), PANI-(PTP-MMT), PPY-(PF-MMT) and PPY-(PTP-MMT) composites were recently ob- tained [99] by adding ANI or PY monomers to preformed PTP-MMT and PF-MMT composites in 2(M)HCl solu- tion and in aqueous solution respectively.
Mechanistic Aspects
The PNVC-MMT composite was formed by the direct
Scheme 7. Initiation of TP by MMT clay alone.
initiation of NVC monomer (melt) with MMT clay presumably by soft cation centers present in MMT via formation of a II-complex [90] (Scheme 6). Direct polymerization of PY, ANI, TP and F monomers without any added oxidant was accompanied [91,92] by a char- acteristic colour change at high temperature indicative of a charge transfer interaction between sorbed cation cen- ters in MMT clay and the monomers (Scheme 7).
Initiation of ANI in Cu-fluorohectorite films was evi- dently due to the reaction [96,97] (Scheme 8).
Bulk Property Characteristics Thermal Stability Characteristics
In general, the trends in the thermal stability order in most clay based composite systems were: MMT clay >
Polymer-MMT clay composite > Homopolymer (Table 6).
This trend is consistent with the intercalation of the polymer in MMT layer since the intercalative matrix comprised polymers dispersed in the nanolayers of MMT.
The enhancement in TGA stability of PANI-DBSA-clay compared to PANI-DBSA was due to the attractive Coulomb interaction between the positive nitrogen of the intercalated PANI-DBSA layer and the negatively charged surface of the clay layer [90].
Table 5. Dispersibility of Homopolymers and Composites in Water Polymers/
Composites Conditions applied for obtaining aqueous dispersion Observations
PNVC PNVC homopolymer was sonicated in water for 1 h Immediate precipitation of polymer occurred PNVC-Al2O3 PNVC-Al2O3composite was sonicated in water for 1 h Dispersion stable upto 30 min
PANI PANI homopolymer was sonicated in water for 1 h Immediate precipitation of polymer occurred PANI-Al2O3 Nano-sized Al2O3powder was added to water and stirred for 2 h; PY
and FeCl3were added serially and stirred magnetically for 3 h
A green colored permanently stable dispersion was formed
PPY PPY homopolymer was sonicated in water for 1 h Immediate precipitation of polymer occurred PPY-Al2O3 Nanidimensional Al2O3was added to water and stirred for 2 h; PY
and FeCl3were added serially and stirred magnetically for 3 h Dispersion was stable up to 36 h PPY-Al2O3
Nanosized Al2O3 was added to an aqueous suspension of PVP and stirred for 2 h; PY and FeCl3 were added serially and stirred magnetically for 3 h.
A permanently stable dispersion resulted PAN PAN homopolymer was sonicated in water for 1 h Immediate precipitation of polymer occurred PMMA PMMA homopolymer was sonicated in water for 1 h Immediate precipitation of polymer occurred PAN-SiO2 SiO2 was added to water and then K2CrO4, AN and NaAsO2 were
added and the mixture was stirred magnetically for 6 h
A permanently stable dispersion (milky white suspension) was formed
PMMA-SiO2 SiO2was added to water and then K2CrO4, MMA and NaAsO2were added and the mixture was stirred magnetically for 6 h
A permanently stable dispersion (milky white suspension) was formed
These suspensions on precipitation and during produced materials which could be redispersed in water after sonication.
Table 6. Thermogravimetric Stability Trends in MMT-Based Systems
Entry no Materials Trends Reference
1 2 3 4 5 6 7
PNVC-MMT PANI-MMT PTP-MMT PF-MMT
PANI-DBSA-MMT PPY-MMT PPY-Na+MMT
MMT>PNVC-MMT> PNVC MMT>PANI-MMT> PANI MMT>PTP-MMT> PTP MMT>PF-MMT> PF
MMT>PANI-DBSA-MMT> PANI MMT>PNVC-MMT> PNVC MMT>PNVC-MMT> PNVC
90 91 92 35 93 91 94
Scheme 8. Initiation of ANI by Cu-fluorohectorite.
XRD Analysis
Intercalation in MMT lamellae would evidently be accompanied by an increase in this spacing. XRD pattern of unintercalated MMT lamellae exhibits a crystalline peak around 9.8 Å. Figure 11(a) indicates a crystalline peak at 14.6 Åin the PNVC-MMT nanocomposite due to intercalation of PNVC in the MMT lamellae [90].
Interestingly, this peak was missing when the same composite was prepared by FeCl3 impregnated MMT [98]. This was due to the presence of surface adsorbed PNVC which was formed much faster compared to that in intercalative polymerization. The XRD pattern of d001 plane of PPCL-MMT composite was reported (Figure 11b) at 16.0Åconsistent with intercalation of PPCL in MMT layer [99]. Similarly in the PANI-DBSA-clay composite the d001 spacing was enhanced to 15.2 Å
confirming intercalation in the nanolayers of MMT [93]
(Figure 11c).
Conductivity Characteristics
Polymer-MMT Clay Nanocomposite Systems
Table 7 collects the dc conductivity values of various MMT-based nanocomposites of speciality polymers.
Among the speciality polymers, PANI and PPY based nanocomposites of MMT showed variation of conduc- tivity from 10-3to 10 S/cm depending upon the procedure followed for polymerization and intercalation.
Interestingly, PTP-MMT composite showed a conduc- tivity value of 10-4 S/cm prepared without any FeCl3 as co-initiator [92]. On the other hand, PF-MMT composite prepared [35] with or without FeCl3 did not show any appreciable difference in conductivity although both values were improved compared to that of the PF homo- polymer.
The conductivity of PNVC-MMT composite likewise showed [90] low values though dramatically improved
(A)
(B)
(C)
Figure 11. (A) XRD of (a) MMT clay (b) PNVC- MMT clay, (B) XRD of (a) Na+MMT (b) PPCL (c) PPY, (C) XRD of (a) MMT clay (b) PANI-DBSA/Clay (c) PANI-DBSA.
from the conductivity of pure PNVC. It appears that in case of nonconducting polymers like PNVC and PF, inherent conductivity of MMT was displayed in the corresponding composites. In general, the conductivity values of the various composites appeared to be de- pendent of the preparation techniques and the extent of doping process in the clay layers which could introduce weak inter-chain and intra-chain interaction effecting the localization of charge centers. In this context, an inter- esting feature was revealed by comparing the conduc- tivity - temperature plot of PANI-DBSA and PANI-DBSA- Clay composite at 1.25(M) ratio [93]. The room tem- perature conductivity of the latter weakly increased with increasing molar ratio of the dopant due to interaction of
Table 7. Conductivity Values of MMT Based Nanocomposite Systems
Entry
no Materials Conductivity
(S/cm) Reference 1
2 3 4 5 6 7 8 9 10 11 12
PANI-Na+MMT-DBSA PPY-Na+MMT-
PANI-MMT (without oxidant) PPY-MMT (without oxidant) PTP-MMT (without oxidant) PF-MMT (without oxidant) PF-MMT-FeCl3
PNVC-MMT(without oxidant) PANI-MMT-(NH)4S2O8or FeCl3 PPY-MMT-FeCl3
PANI-Cu-fluorohectorite MEEP-MMT
101 10-1 1 10
10-5 10-6 10-4 10-8 10-7 10-6 10-1 10-3 10-1 10-5
0.05 10-1 10-3
93 94 91 91 92 35 35 90 110 111 1 1
Figure 12. Temperature dependence of σdc of PANI-DBSA and PANI-DBSA-clay composite.
effective doping by the clay. It was established [93] that dc conductivity-temperature variation of both the systems followed a quasi-one-dimensional variable range hopp- ing obeying the equation.
σdc(T)=σoexp[-(T0/T)1/2]
Here To = 16/[L L2 N(EF)kB],where L (L ) was the localization length in the parallel (perpendicular) direc- tion to the polymerization chain, N (EF) was the density of states at the Fermi level and kB was the Boltzmann constant. From the slope of conductivity-temperature curve (Figure 12), the values of T0for PANI-DBSA and PANI-DBSA-Clay composite were 900 and 6600 K respectively, implying that PANI-DBSA had higher conducting state than PANI-DBSA-Clay composite. The clay layer apparently, interrupted the effective doping process and induced weak inter-chain and intra-chain
Figure 13. Comparison of 2H NMR line shapes for bulk d- PEO and Li-fluorohectorite-intercalated d-PEO as a function of temperature.
Figure 14. Temperature dependent conductivities ( ) ofσ MEEP-NaMMT parallel to the composite layers, ( ) VTF fit;
(-) (MEEP), NaMMT perpendicular to the composite layers, ( ) VTF fit(-); Pristine Na-MMT ( ) measured perpendicular to the layers.
interaction.
Conductivity anisotropy in MMT based composite systems was observed in a variety of Na+-MMT smectite.
Smectites are essentially polyelectrolytes with fixed anions. The identity of current carriers is less ambiguous in this type of electrolyte than in salt solutions, where both anions and cations are mobile. The charge sites in MMT are well separated, so ion pairing with the mobile cation is attenuated. To reduce further the attractive forces between the cations and the aluminosilicate sheets, a variety of solvating species such as PEO [101], MEEP [106], cryptands [107] and crown ethers [107] were introduced into the clays. The cationic mobility of these composite electrolytes was highly anisotropic and greatly
Table 8. VTF Parameters for MEEP-Na MMT Clay Sample σo(Ω-1cm-1K1/2) β(J) T0(K)
σpara
σperp
2.2×10-3 2.1×10-1
1.2×104 1.3×104
218 204
Table 9. Conductivity Values of Some Binary Polymer-MMT Nanocomposite Systems
Entry no Materials Conductivity (S/cm) Reference 1
2 3 4
PANI-(PF-MMT) PANI-(PTP-MMT) PPY-(PF-MMT) PPY-(PTP-MMT)
10-2-10-3 10-2-10-3 10-2-10-3 10-2-10-3
100 100 100 100
enhanced in comparison with the parent clay. Conduc- tivity enhancement was attributed to increased layer separation and factors associated with relaxations of the polymer chain. The polymer decreased the interaction between the cation and the negatively charged clay surface and thereby increased the conductivity. Polymer dynamics and lithium ion transport in PEO-lithium- MMT composites were studied by solid state NMR techniques [108] (Figure 13).
At low temperatures, where the local segmental motion of the polymers was quiescent, a typical powder pattern was observed for both bulk d-PEO and the intercalated d-PEO than in the bulk d-PEO (270 K). The breadth of the central peak from the intercalated d-PEO was substantially narrower than that from the bulk d-PEO.
The central peak was believed to result from increased segmental motion causing temporal averaging of the signal.
Conductivity-temperature studies in intercalated MEEP- Na-MMT composite suggested (Figure 14) that the con- ductivity of the MEEP-Na MMT composite was substan- tially enhanced relative to pristine Na-MMT, with aσpara/ σperpratio about 100. Both the parallel and perpendicular conductivities of the MEEP-MMT satisfied the Vogel- Tammann-Fulcher (VTF) equation [109].
σ σ β
The VTF dependence was strongly suggestive of coupling between high amplitude segmental motion and long-range cation transport in these composites. The conductivity anisotropy arose from the variation in theσo
term.
Mixed Polymer MMT Clay Nanocomposite Systems
․
Table 9 presents the dc conductivity values of some recently studied MMT based binary polymer nanocom- posite systems. The conductivity values of PF-MMT and PTP-MMT composites were in the order of 10-7and 10-6 S/cm respectively. Even after doping with FeCl3 the
Figure 15. SEM image of the (a) PPCL composite, (b) PPY homopolymer.
conductivities improved 10 fold in either system.
However, upon incorporation of PANI and PPY moieties in the PF-MMT and PTP-MMT composites, the con- ductivity values were improved 105 and 103 fold re- spectively [100].
Morphological Characteristics
Scanning Electron Microscopic Analysis
SEM image of PPCL composite revealed an irregular shape and size within 10～20 µm and PPY was present in the outer clay layer [99] (Figure 15).
Transmission Electron Microscopic Analysis
TEM photograph of PANI-DBSA-clay composite (Figure 16) revealed [93] nano scale pattern of strips confirming. TEM pattern in several systems like PANI- MMT, PPY-MMT, PTP-MMT and PF-MMT, showed spherical shapes due to the inhabitable encapsulation of the clay as well as intercalated clay by the intractable polymers like PPY, PANI, PTP, and PF. It was difficult
Figure 16. TEM image of the PANI-DBSA-clay composite.
to identify the expected morphology of the nanocom- posites.
Zeolite-Based Nanocomposite Systems
Molecular sieve catalysts are characterized by high activity and good reactivity in a variety of reactions such as chelation, selective hydrogenation, dehydration, de- hydrohalogenation and the like. Another potential field of application of these synthetic zeolites is as polym- erization catalyst and by now there has been a huge accumulation of literature on this aspect [112]. However, in this section some selective recent developments in the area of zeolite based conducting composites of some speciality polymers and binary polymers will be high- lighted.
A list of some typical zeolites used in these studies is given in Table 10.
The importance of the zeolite based composites has increased following the discovery of mesoporous ma- terials comprising molecular sieves with ordered arrange- ment of uniform nanometer size pores. The SiO2 based mesoporous materials have been studied in various catalytic and other reactions including removal of heavy metal and contaminated solution and also as nanometer electronic materials. Furthermore, mesopores have been incorporated into various polymers to prepare nanocom- posites [122-125].
Preparative Aspects
Some selective aspects of such polymer-zeolite nano- composite synthesis are highlighted below.
AlMCM-41 and CuII-MCM-41 were prepared following an elaborate procedure [120]. The monomer adsorption into the CuII-MCM-41 was performed by exposing the zeolite to PY vapour in a vacuum reactor. The diffusion
Table 10. Some Typical Zeolites Used for Formation of Polymer Based Nanocomposite
Types of zeolite/molecular sieve Conducting polymer References
Y-Zeolite
FeIII-ion exchanged proton form Y-Zeolite CuII-ion exchanged proton form Mordenite MCM-41
CuII-MCM-41 MCM-41 13X-Zeolite
PANI
Oligomeric PPY PPY and PTP PANI PPY
PANI-co-MEPA PNVC, PANI and PPY
113 114 115-116 117-119
120 119 121
Table 11. Thermogravimetric Stability Data of 13X-zeolite, 13X-polymer Composites and Respective Homopolymers
Materials % weight loss (temperatureoC)
200 400 600 800
13X PNVC-13X PPY-13X PANI-13X PNVC PPY PANI
0 0 0 0 0 0 0
20 40 55 60 20 40 39
20 40 60 65 100
60 100
20 40 60 65 - 90
- of PY monomer through the interior of MCM-41 was followed by a distinctive colour change from light blue to black indicative of charge transfer interaction of PY [120].
PANI-zeolite pellet composites were obtained by dry mixing of synthesized maleic acid doped PANI with zeolite-Y, 13X and AlMCM-41 followed by compression to form the corresponding pellet composites [113].
An interesting preparative method was by direct polym- erization of NVC monomer in bulk at melting temper- ature and in toluene solution with 13X zeolite powder.
PANI-13X and PPY-13X zeolite composites were pre- pared by the polymerization of respective monomers in presence of CuCl2 catalyst in aqueous medium con- taining a suspension of 13X powder. PTP-13X and PF-13X zeolite composites were also obtained by the polymerization of TP and F respectively in presence of FeCl3oxidant in CHCl3[121].
Binary polymer based nanocomposites such as PANI- (PF-13X) and PPY-(PF-13X) were prepared by the polymerization of ANI and PY monomers by the conventional oxidants (APS for ANI in 2(M) HCl solution and FeCl3for PY monomer) in aqueous medium containing preformed PF-13X zeolite composite sus- pension [126].
Bulk Property Characteristics Thermal Stability Characteristics
Table 11 presents some typical % weight loss versus temperature (oC) data for 13X zeolite and for various polymer-13X zeolite composites including those of the
Figure 17. XRD scans of (a) 13X zeolite, (b) PNVC-13X zeolite composite, (c) PPY-13X zeolite composite, and (d) PANI-13X zeolite composite.
respective homopolymers. The trends in the thermogra- vimetric stability are as follows: 13X zeolite > PNVC- 13X composite > PNVC homopolymer [121]; 13X zeolite > PPY-13X composite > PPY homopolymer [11]
and 13X zeolite > PANI-13X composite > PANI homo- polymer [121].
This observation is in the line with those reported in the case of polymer-metal oxide and polymer-clay (MMT) composite systems.
XRD Analysis
The XRD patterns of AlMCM-41 and CuII-MCM-41 showed [120] no change implying that the framework of MCM-41 was not broken after CuII ion exchange. It would have been of interest to compare the XRD of PANI or PPY loaded composites of the zeolites but no data were available.
The XRD scans (2θversus intensity plot) of 13X and PNVC-13X (Figure 17) were almost similar in nature which suggested [121] the retention of 3D-network of 13X zeolite in the PNVC-13X composite. This could possibly be due to onset of polymerization of NVC monomer at the active sites present on the surface rather
Figure 18. ESR spectrum of PPY radicals in CuIIAlMCM-41.
than in the channels of 13X zeolite powder. In contrast, however, the XRD patterns of PANI-13X and PPY-13X composites were somewhat different from these of 13X and the PNVC-13X composite. Being small molecules unlike NVC, the polymerization of these monomers might occur in the channels of 13X zeolite and the growth of the polymers might damage the 3D network of 13X zeolite structure.
Conductivity Characteristics
The dc conductivity of PANI-MCM-41 composite [119]
was found to be in the order of 10-9 S/cm which was much lower than that of pristine doped PANI itself placed within the channels of MCM-41. This fact provided additional supporting evidence that most PANI synthesized was located only inside the MCM-41 channels without any PANI adsorption outside the MCM-41 surface. On the contrary, PANI or PPY-13X composites exhibited [121] dc conductivities in the range of 10-3 and 10-4 S/cm respectively characteristic of the presence of conducting polymers on the 13X zeolite surface.
The dc conductivity of the PNVC-13X zeolite com- posite [121] was low (10-8 S/cm), though it was 106fold higher than that of PNVC alone (10-12～10-16 S/cm). It was possible [121] to enhance the conductivity value to 10-5 and 10-6 S/cm by loading of conducting PPY and PANI moieties on the PNVC-13X composite respec- tively.
The presence of polaron in PPY-CuIIAl-MCM-41 composites [120] was confirmed by esr spectroscopy (Figure 18) which revealed [127-129] a signal at g=1.99.
Such a sharp esr signal with a g value close to that of free electrons was suggestive of the presence of polaron species in conductive polymers. Incidentally, no signal for CuII ions was visible suggesting that most of these ions were reduced to CuI by oxidation of N in PYring eventually leading to cation radical propagation of PY
(a)
(b)
(c)
Figure 19. TEM image of (a) a cross section of MCM-41, SEM images of (b) MCM-41 host and (c) PANI-MCM-41 composite.
monomer to PPY.
Morphological Characteristics
Regular hexagonal pore openings of the mesoporous channels and particle morphology of the synthesized pristine MCM-41 host are shown in Figure 19(a-c) via transmission electron and scanning electron micro-
(a) (b)
(c) (d)
(e) (f)
Figure 20. SEM image of (a) 13X zeolite (b) PNVC-13X zeolite composite (c) PPY-13X zeolite composite (d) PANI-13X zeolite composite (e) PPY-(PNVC-13X zeolite) composite (f) PANI- (PNVC-13X zeolite) composite.
graphic analyses, respectively. A SEM image of PANI- MCM-41 nanocomposite [118,119] particles is also given in Figure 19c. Even though some aggregates of MCM-41 particles were visible, the size of the primary
particles was less than 10 µm and they took the shape of a twisted rectangular bar. Virtually no difference in par- ticle surface morphology between the pristine MCM-41 host and the PANI-MCM-41 composite material was
