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Enhancement of adhesion between inorganic nanoparticles and polymeric matrix in nanocomposite by introducing polymeric thin film onto nanoparticles.

INTRODUCTION

The improvement of affinity and adhesion at interfaces between two or more components has been considered important in various fields of science, and thus numerous studies have been conducted concerning the issue [1, 2]. Especially in the last two decades, improvement of adhesion in nanosized matters has attained great attention proportional to the increasing interest on the potentials of nanocomposite materials [3-7], When the nanocomposite materials with inorganic fillers and polymeric materials are prepared, the homogeneous dispersion of nanoparticles in polymer matrix is critical for ensuring uniformities in properties of the composites. In most cases, unfortunately, inorganic fillers tend to aggregate in polymer matrix because of inherently low affinity between inorganic and organic matters, which results in the lack of unifonnity and poor performances in composite films. Enhancement of affinity, which is also directly associated with adhesion capability, is the key factor that should be addressed for preparation of high performance nanocomposite materials.

Broad range of methods to enhance affinity or adhesion between inorganic filler and polymeric matrix have been developed so far, and one of the most effective way is the surface treatment of inorganic materials with several coupling agents, such as silane or phosphoric acid coupling agents [8, 9]. Although the introduction of short organic chains on inorganic nanoparticles can increase the affinity, it is difficult to find a proper coupling agent for noncommon inorganic nanoparticles. To overcome this drawback, polymer-coated inorganic particles in nanometer scale might be suitable for the better dispersion of inorganic filler in polymer matrix through the improved interactions between the outer polymer shell of filler and polymer matrix [10, 11],

On the other hand, polymer-coated inorganic nanoparticles possess several advantages over the single component materials from the viewpoint of their potential applications. The uniform coating of inorganic particles with organic polymers can offer electrical, optical, and catalytic functions to the inorganic cores by proper choice of polymer coating [12-14], Therefore, extensive studies have been recently performed for the encapsulation of inorganic particles with various polymers, which include emulsion polymerization [15], layer-by-layer self-assembly of polyelectrolyte [16], and graft polymerization [17] on the particle surfaces. To date, most approaches have largely depended on the polymerization or adsorption process in solvent media as aforementioned. However, solution-based methods are relatively tedious to optimize several synthetic parameters for well-defined polymer shells, and often prefer different morphologies rather than the core-shells, such as raspberry or acorn types [18, 19].

In particular, in this report, we demonstrate the coating of indium tin oxide (ITO) nanoparticles having an irregular shape with not only vinyl polymers (polyfinethyl methacrylate) (PMMA), and polystyrene (PS)) but also conducting polymer such as polypyrrole (PPy) using a simple vapor deposition polymerization (VDP) method to improve the affinity and adhesion at interfaces [20], It has been generally accepted that the affinity and adhesion at interfaces between inorganic nanoparticles and encapsulating polymeric thin films is strongly dependent on the various parameters such as the structure of monomer, extensive properties of polymer (electrostatic, polymerization method, initiator, and viscosity), and surface properties of both polymer and inorganic particles [21], The inorganic nanoparticles and polymers are selected to prove the feasibility of VDP for the encapsulation of nanoparticles with an irregular morphology employing diverse polymers. In general, the fabrication of thin polymer films by vapor deposition is performed in an ultra-high vacuum and low temperature conditions initiated by E-beant or UV radiation on the desired flat substrates [22, 23], Vapor deposition has a number of advantages over the solution-based techniques such as the formation of thin polymer layer with highly uniform thickness, simple recovery of dry products, and elimination of the use of toxic organic solvents.

In addition, it can be expected that the presence of polymeric thin films results in the improvement in properties and performances, in case the encapsulated nanoparticles are incorporated into polymer matrix to generate nanocomposites. Herein, therefore, nanocomposites with polymer-coated-ITO nanoparticles and poly (vinyl alcohol) (PVA) are fabricated to substantiate the enhanced adhesion between ITO nanoparticles surface engineered by VDP and polymeric matrix.

EXPERIMENTS

Materials

The ITO powders with an average diameter of tens of nanometers were received from Advanced Nanoproducts. All the compounds used in this study including vinyl monomers (styrene and methyl methyl methacrylate), pyrrole (Py), azoisobutyronitrile (AIBN), iodine, and PVA with molecular weight 10,000-30,000 (g/mol) were purchased from Sigma Aldrich. The vinyl monomers were used after column purification and other compounds were used as received. Iodine played a role of dopant for PPy.

Sample Preparation and Characterization of ITOIVinyl Polymer

A simplified schematic diagram for VDP process is illustrated in Scheme 1. The ITO nanoparticles and AIBN powder were loaded into a reaction vessel equipped with a sealing apparatus and a monomer-loading reservoir. Then, the reaction vessel was evacuated until the inside pressure reached at 1CT2 torr. Variable amount of vinyl monomer was injected into the monomer-loading reservoir and VDP was performed for 12 h at 70[degrees]C. In general, the ITO powder was well-known materials as the reflection layer of infra-red region; therefore, the FT1R spectrum of PMMA/ITO was obtained after the etching of PMMA with solvent (acetone). The PMMA coated ITO powders were dissolved in acetone and precipitated. The upper solution were retrieved and poured onto polished-KBr powder. After evaporating acetone, residual KBr powder was palletized for FTIR analysis.

Sample Preparation and Characterization of ITOIConducting Polymer

To polymerize the Py monomer on the surface of ITO nanoparticles by VDP, ITO nanoparticles were pretreated by immersing in oxidant solution (Fe[Cl.sub.3] aqueous solution). Subsequently, the powder form of Fe[Cl.sub.3]-coated ITO nanoparticles was obtained by a drying process. The PPy and poly(3,4-ethylenedioxythiophene) (PEDOT) shell on the surface of ITO nanoparticles was formed by VDP of Py and 3,4-ethylenedioxythiophene (EDOT) monomer. The monomer was vaporized under the reduced pressure of about [10.sup.-2] torr at 70[degrees]C. Since Fe[Cl.sub.3] adsorbed on the ITO particle surface acted as an initiator for the polymerization of Py and EDOT, the polymerization occurred exclusively on the surfaces of the ITO nanoparticles. The conducting polymer-coated ITO nanoparticles were characterized using Raman spectroscopy and elemental analysis. To enhance electrical conductivity, the ppy-coated ITO nanoparticles were dispersed in an iodine/EtOH solution and stirred for 12 h. Final product was obtained ultracentrifuge (3000 rpm).

Preparation Methods of Nanocomposites with PVA

The ITO or PPy-coated ITO particles were dispersed in water and the PVA matrix was dissolved in the solution. The final mass fraction of PVA in the solution is fixed at 10 wt %. The composite film was produced by direct solvent casting method onto the polyfethylene terephthalate) (PET) substrate. The electrical conductivity of composite was measured by standard four probe method. All the measurements have been done by three times and averaged values are obtained and used.

RESULTS AND DISCUSSION

Preparation of ITOIPolymer Corel Shell Nanoparticles by VDP

Figure la shows TEM image of pristine ITO nanoparticles. The size of ITO nanoparticles is ~20 nm and irregular morphology can be observed. Fabrication of core-shell nanoparticles with the ITO is really challenging task, because it is difficult to coat these irregular shaped core with thin polymeric films using conventional techniques such as emulsion polymerization [20]. However, as displayed in Fig. lb-d indicating ITO nanoparticles coated with various polymers (b) PS, (c) PMMA, and (d) PPy, our VDP method makes it possible to obtain thin and highly uniform polymeric films on the surfaces of materials having diverse size and shapes. In addition, this VDP method is really versatile, so that various polymers can be adopted such as vinyl polymers and conducting polymers. The TEM images present that polymeric thin films with an average thickness of few nanometer are generated by the VDP process. According to the previous reports by Jang and Lim, the thickness of polymeric thin film is basically adjustable by controlling monomer feeding ratio and type of inorganic nanoparticles in the system [20].

Two types of polymerization methods have been adopted to coat inorganic nanoparticles with polymers; (1) radical polymerization for PMMA and PS, and (2) oxidation polymerization for PPy and PEDOT. Polymeric thin films with an average thickness of a few nanometers can be successfully introduced onto inorganic nanoparticles regardless of the polymerization technique. In general, radical polymerization methods result in the formation of relatively uniform coating on the nanoparticles than oxidation polymerization.

As we select ITO as inorganic nanoparticles, the surface properties of the inorganic particles are macroscopically fixed. Even if the shape of ITO nanoparticle is irregular, the effect of morphology on the surface properties of ITO nanoparticles is negligible. Therefore, the characteristics of polymeric thin films are strongly dependent on the properties of introduced polymers and the parameters for VDP process. The most influencing parameters are the structure of monomer, the surface tension of resulting polymeric layers, and the properties of polymers (such as [T.sub.g], viscosity, molecular diffusion, and spreading parameters). In addition, the experimental parameters for VDP process such as temperature, vacuum, polymerization time, and initiator are also critical.

The polymers were selected because PS and PMMA are representative thermoplastics showing significantly different surface energies and PPy is a typical conducting polymer showing a high charge density. Because the glass transition temperature ([T.sub.g]) of polymers is below 120[degrees]C, the molecular state of polymers is glassy under our VDP conditions. In addition, the mobility of deposited polymers is considered to be insignificant because the behavior of polymeric thin layers is governed by the ITO surface energy regardless of polymer.

Consequently, at this point, the control over parameters for VDP process is important. A relatively mild process window was employed for our experiment. Under the situation, the variation in experimental parameters does not affect the properties of resulting polymeric thin layers remarkably [20], Therefore, it was possible to prepare uniform thin layer of polymer onto ITO nanoparticles regardless of polymer.

The overall mechanism for formation of polymer layer is consisted of following steps: monomer deposition; surface diffusion; nucleation of polymeric chain; propagation; oligomer diffusion; entanglements of polymeric chain; loop formation [24]. Accordingly, the resulting exclusive and homogeneous deposition of polymer layer onto the surface of nanoparticles can be understood in terms of surface energy. As the monomer was supplied under vacuum, the atmosphere in the reaction vessel is entirely accommodated with monomer vapor. Here, the monomers can interact with the surface of nanoparticles, because the distance between vaporized monomer and surface is sufficiently close and there must be weak specific interaction such as electrostatic force between monomer and surface. In addition, the nucleation of monomer molecules on the surface of nanoparticles can be promoted by the initiator nearby, resulting in formation polymer layer on the surface. Finally, the generated polymer layer would be more uniform as the thickness increases gradually. In particular, additional experiments were conducted on the encapsulation of PMMA nanofibers with PPy using VDP method. It was found that only polymerization time exhibited a significant effect on the morphology of generated polymeric layer. That is, precise control over polymerization time is a prerequisite for formation of uniform polymer thin film. (Data not shown).

One of the simplest ways to confirm the generation of polymer layers produced by the VDP method is FTIR spectroscopy. However, it is intrinsically impossible to obtain FTIR spectrum because ITO nanoparticles strongly absorb infrared [25]. Therefore, Raman spectrum which is known as a powerful alternative for FTIR spectrum has been obtained to confirm the successful preparation of ITO/PPy core/shell nanoparticles as described in Fig. 2. Characteristic vibrational bands for PF'y, including 1600 [cm.sup.-1] for C=C backbone stretching, 1323 [cm.sup.-1] for C--N stretching, and 982 and 1046 [cm.sup.-1] for polaron ring deformation and C--H in-plane bending (arrows in Fig. 2) are observe respectively, implying the successful polymerization of PPy by the VDP method. As indicated in Fig. 2, no Raman signals have been detected from bare ITO nanoparticles [26, 27]. Additional analysis for the successful polymerization of PPy and PEDOT has been carried out using elemental analysis of ITO/conducting polymer core/shell nanoparticles. Elemental composition ratio of C/N and C/S for PPy and PEDOT was estimated about 4.2 and 6.5, respectively, which is consistent with the atomic ratio of PPy and PEDOT.

To obtain FTIR spectrum of PMMA. solvent extraction and re-precipitation method has been adopted. PMMA/ITO core/ shell nanoparticles prepared by the VDP method are additionally evacuated in the reaction vessel to remove unreacted monomer, and washed with hexane several times to remove oligomer-like species. Then, the refined PMMA/ITO nanoparticles were soaked into chloroform to extract PMMA from the core/shell nanoparticles, and nonsolvent such as heptane was introduced to obtain powder state of PMMA. The FTIR spectrum of extracted PMMA is illustrated in Fig. 3, which is perfectly matched with characteristic FTIR bands of PMMA such as C--H stretching at around 3000 [cm.sup.-1], C=0 stretching at 1730 [cm.sup.-1] and several C--O stretching at 1000-1100 cm 1 [28], These results implied that various polymeric thin films can be introduced into inorganic nanoparticles by the simple VDP method.

Preparation of Nanocomposite with PPy Coated ITO Nanoparticles and PVA

The most important issue associated with the preparation of nanocomposite materials is the effective dispersion of nanofil lers in the matrix [3-11]. It has been expected that large surface to volume ratio will extremely improve specific properties of nanocomposite. However, most attempts have failed to achieve desired final performances of nanocomposites, which were attributed to aggregation of nanoparticles in the matrix. Nanomaterials, by itself, have large surface energy and thus tend to form aggregation for stabilization. Enhancement of affinity between inorganic materials and organic matrix can decrease degree of aggregation. Surface treatment with several chemicals such as coupling agent has been generally employed to increase the affinity [8, 9]. However, for inorganic nanomaterials other than silica, alumina or carbons, it is generally difficult to find a proper coupling agent for ensuring a sufficient reactivity with inorganic matters. In addition, the short organic moiety in coupling agent, sometimes, is deficient to secure better dispersion. Therefore, modification of inorganic fillers with polymeric chains can be a strong alternative. The VDP method provides a versatile method to coat any inorganic nanoparticles with any polymers by simple procedures.

To examine the improved dispersion of polymer coated nanoparticles compared to bare ITO nanoparticles in matrix, a representative nanocomposite has been prepared. Figure 4 shows the electrical conductivity of PVA nanocomposites containing bare and PPy-coated ITO nanoparticles as a function of filler contents. The nanocomposite containing PPy-coated ITO nanoparticles shows lower value of percolation threshold (~4 wt %) than the nanocomposite with bare ITO.

In addition, when the PVA nanocomposite is prepared with bare ITO nanoparticles, most of fillers settle down to the bottom of the composite, and thus electrical conductivity cannot be measured on the top of composite. For the case of PPy-coated ITO nanoparticles, it was possible to measure the electrical conductivity even on the top side of films, implying minimized settlement of nanoparticles during film formation due to the enhanced affinity between matrix and fillers. Homogeneity of electrical conductivity in nanocomposite film has also been elevated in the nanocomposite with polymer coated nanoparticles.

A direct evidence for enhanced adhesion and affinity between fillers and matrix can be found on cross-sectional SEM images of both nanocomposites. Figure 5 presents the cross-sectional SEM images of PVA nanocomposites containing (a, b) bare ITO nanoparticles and (c, d) PPy-coated ITO nanoparticles. The crosssectional surface has been obtained by breaking PVA composite after immersing in liquid nitrogen. As presented in Fig. 5a and b, it is obvious that the bare ITO nanoparticles appear to be detached from matrix during the preparation of cross-sectional surface, implying that the adhesion between nanoparticles and matrix is relatively weak. On the other hand, the PPy coated ITO nanoparticles remain in the matrix even after film fracturing as described in Fig. 5c and d because of the improved adhesion.

The adhesion promotion can also be explained from the viewpoint of mechanical property. Strictly, the surface of ITO nanoparticles is not perfectly flat. Thus, there might be numerous defects on the bare surface. It can be expected that incorporation of a polymer layer onto the ITO surface can smoothen the rough surface profile [29]. In this case, a dramatic change can happen because of the presence of a buffering polymer layer. The existence of a viscoelastic polymer layer between polymer matrix and inorganic component can improve the tensile strength of the nanocomposite considerably, because the polymeric layer can damp or alleviate the transformation/disruption of nanocomposite by induction of shear viscosity upon strain or fracture [30],

The homogeneous dispersion of NPs is extremely important for the nanocomposite thin films to ensure uniformity in properties. In general, the homogeneous distribution of nanoparticles into a matrix has been achieved by tailoring the surface functional group to improve compatibility between nanoparticles and matrix. In our experimental condition, where the average diameter of ITO nanoparticles is tens of nanometer, the enthalpic advantage for dispersing the particles must be maximized while any entropy loss arising from packing the NPs in the polymer matrix must be minimized. The incorporation of polymer layer on the surface of ITO nanoparticles promotes the favorable interactions between nanoparticles and matrix owing to the presence of organic polymer layer. In addition, because the NPs were considerably small so that configurationally entropic losses of the polymer to incorporate the nanoparticles were also significantly small. Consequently, a uniform dispersion can be achieved and thus led to the adhesion promotion.

CONCLUSION

In summary, polymeric thin films can be introduced on the ITO nanoparticles having an irregular morphology by a simple and efficient VDP process. Various types of polymers such as vinyl polymers and conducting polymers can be adopted in this system. These thin polymeric layers play an effective role for improving adhesion and elevating compatibility between inorganic fillers and organic matrix for the preparation of nanocomposites. This technique can be expanded to prepare a series of polymer coated elegant nanostructures. In addition, it becomes clear that this work provides a novel breakthrough to engineer surface characteristics between inorganic matters and organic matrix for obtaining smart nanocomposites.

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Sang-Ho Cha, (1) Joonwon Bae, (2) Kyung Jin Lee (3)

(1) Department of Chemical Engineering, Kyonggi University, Suwon 443-760, Republic of Korea

(2) Department of Applied Chemistry, Dongduk Women's University, Seoul 136-714, Republic of Korea

(3) Department of Fine Chemical Engineering and Applied Chemistry, College of Engineering, Chungnam National University, Yuseong-gu, Daejeon 305-764, Korea

Correspondence to: Joonwon Bae; e-mail: joonwonbae@gmail.com or Kyung Jin Lee; e-mail: kjlee@cnu.ac.kr

DOI 10.1002/pen.24031

Published online in Wiley Online Library (wileyonlinelibrary.com).
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Author:Cha, Sang-Ho; Bae, Joonwon; Lee, Kyung Jin
Publication:Polymer Engineering and Science
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Date:Aug 1, 2015
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