Al2O3

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polystyrene (PS) latex and Al2O3 composites depending on PS particles size and Al2O3 content. ... dried latex films.7 Small angle neutron scattering has been.
Fluorescence study of effect of particle size in PS latex/Al2O3 nanocomposite films ¨ . Pekcan3 S¸. Ug˘ur*1, M. Selin Sunay2 and O Steady state fluorescence and BioUV-Visible techniques were used to study film formation of polystyrene (PS) latex and Al2O3 composites depending on PS particles size and Al2O3 content. Two film series (SmPS/Al2O3 and LgPS/Al2O3) were prepared covering PS sphere (SmPS: 203 nm; LgPS: 382 nm) templates with various layers of Al2O3 by dip coating method. The film formation behaviours of composites were studied by annealing them at a temperature range of 100–250uC and monitoring the scattered (Isc), fluorescence (IP) and transmitted (Itr) light intensities after each annealing step. The results indicate that LgPS/Al2O3 films underwent complete film formation independent of Al2O3 content while no film formation occurred above a certain Al2O3 content for SmPS/Al2O3. Extraction of PS produced ordered porous structures for high Al2O3 content in both series. Scanning electron microscopy images showed that the pore size and porosity could be easily tailored by varying PS particle size and Al2O3 content. Keywords: Nano composites, Porosity, Scanning electron microscopy (SEM), Annealing, Film formation

Introduction As a result of worldwide theoretical and experimental efforts, a very good understanding of the mechanisms of latex film formation has been achieved.1–16 Film formation from soft (low Tg) and hard (high Tg) latex dispersions can occur in several stages. In both cases, the first stage corresponds to the wet initial stage. Evaporation of solvent leads to the second stage in which the particles form a close packed array; here if the particles are soft they are deformed to polyhedrons. Hard latex, however, stays undeformed at this stage. Annealing of soft particles causes diffusion across particle–particle boundaries, which leads to a homogeneous continuous material. In the annealing of hard latex systems, however, deformation of particles first leads to void closure1–4 and then, after the voids disappear, diffusion across particle–particle boundaries starts, i.e. the mechanical properties of hard latex films evolve during annealing after all the solvent has evaporated and all voids have disappeared. Transmission electron microscopy has been the most common technique used to investigate the structure of dried films.5,6 Pattern of hexagons, consistent with face centred cubic packing, are usually observed in highly ordered films. When these films are annealed, complete disappearance of structure is sometimes observed, which is consistent with extensive polymer interdiffusion. Freeze fracture transmission electron microscopy has been used to study the structure of 1

Department of Physics, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey Faculty of Science and Letters, Piri Reis University, 34940, TuzlaIstanbul, Turkey 3 Kadir Has University, Cibali 34320, Istanbul, Turkey 2

*Corresponding author, email [email protected]

ß Institute of Materials, Minerals and Mining 2015 Published by Maney on behalf of the Institute Received 17 October 2014; accepted 30 December 2014 DOI 10.1179/1743289814Y.0000000120

dried latex films.7 Small angle neutron scattering has been used to study latex film formation at molecular level. Extensive studies using small angle neutron scattering have been performed by Kim and co-workers8 on compression moulded polystyrene (PS) film. Direct non-radiative energy transfer (DET) method has been employed to investigate the film formation process from dye labelled hard9 and soft10,11 polymeric particles. The steady state fluorescence (SSF) technique combined with direct nonradiative energy transfer has been used to examine healing and interdiffusion processes in the dye labelled poly(methyl methacrylate) latex systems.12–14 Recently UV–visible technique was used to study film formation from poly(methyl methacrylate) and PS particles15,16 where the transmitted light intensity was monitored during the film formation process. In the last decade, there has been growing interest in producing new materials by filling polymers with inorganic natural and/or synthetic compounds. These composite materials possess high heat resistance, mechanical strength and impact resistance or present weak electrical conductivity and low permeability for gases like oxygen or water vapour. Some efforts have been made to construct the microstructure and properties of coatings with composite ceramic–polymer microstructures, with the emphasis on composites in which a ceramic phase forms a connected network in a polymer matrix. Processing and microstructure development of ceramic and polymer coatings prepared by depositing a solution or through dispersion have been of interest in the last few years.17,18 Colloidal ceramics, sol–gel derived ceramics and polymers have been studied as coating systems. The organisation of monodispersed colloidal particles like latex and silica microspheres into higher order microstructures is attracting growing interest,19,20 since

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Effect of particle size in PS Latex/Al 2 O 3 nanocomposite films

1 Plot of Itr, Isc and IP intensities versus annealing temperature, T for SmPS/Al2O3 composite films with 0, 1, 5 and 10 layer of Al2O3. Numbers on each curve shows Al2O3 content

this provides unique structures suitable for various advanced devices and functional materials such as photonic crystals21 and porous polymers.22 Ordered arrays of polymer (e.g. polystyrene or poly (methyl methacrylate)) or silica nanospheres have been extensively studied in recent years for photonic crystal applications.23–27 Photonic crystals (i.e. spatially periodic structures of dielectric materials with different refractive indices) have been extensively investigated worldwide. Because the lattice constant of photonic crystals is in the visible or infrared wavelength range, they can control the propagation of photons in a way similar to the way a semiconductor does for electrons. Many studies have been carried out to predict and produce the threedimensional complete photonic band gap structures because of their wide potential applications in optics.28 Recently, they have attracted renewed interest, mainly because they provide a much simpler, faster and cheaper approach than complex semiconductor nanolithography techniques to create three-dimensional photonic crystals working in the optical wavelength range.29–31 Alumina is a very important inorganic material owing to its thermal, chemical, and mechanical stability, and the hierarchically porous alumina should be an attractive material which can potentially be used as catalyst supports, adsorbents, ion exchange materials, membrane substrates

etc.32–34 Al2O3 coatings have also become more popular for their high dielectric strength, exceptional stability, durability against hostile environments and high transparency down to 250 nm. During the last few years Al2O3 coatings have been widely used for their practical applications, such as in refractory materials, antireflection coatings, highly anticorrosive materials,35 microelectronic devices,36 capacitance humidity sensors37 and also in heat sinks in IC’s and for passivation of metal surfaces.38 In our previous study, we studied the film formation behaviour of PS/Al2O3 composites by mixing PS emulsion and Al2O3 sol at various compositions. We have found that, up to a certain Al2O3 content classical film formation occurs, however above this content no film formation is observed due to the complete coverage of the PS latexes with Al2O3. In addition, no porous structure was obtained after extraction of PS. In the present work, we report the preparation and characterisation of PS/Al2O3 films. These films were prepared using PS templates on glass substrates with different PS sizes and by covering them with different Al2O3 layers using the dip coating method. The film formation of these films was studied by annealing them at temperatures ranging from 100 to 250uC. After the completion of film formation, to prepare porous Al2O3 films, PS templates were extracted with toluene. The resultant structure was a replica of the

Table 1 Properties of the neat latexes.

130

Latex sample

Mwx103 (g.mol21)

PI (Mw/Mn)

Particle Size (nm)

SDS (%) (wt./vol.)

SmPS LgPS

90 90

3.5 4.3

203 382

0.12 0.03

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Effect of particle size in PS Latex/Al 2 O 3 nanocomposite films

2 Plot of Itr, Isc and IP intensities versus annealing temperature, T for LgPS/Al2O3 composite films with 0, 1, 5 and 10 layer of Al2O3. Numbers on each curve shows Al2O3 content

PS particles and the materials generated from this process exhibit remarkable ordering of the pores of different sizes. The shape of the templates can be controlled and the pore size can be adjusted by changing the PS latex size.

Experimental Materials Synthesis of polystyrene (latex) spheres

Fluorescent PS latexes were produced via emulsion polymerisation process.39 The polymerisation was performed batch wisely using a thermostated reactor equipped with a condenser, thermocouple, mechanical stirring paddle and nitrogen inlet. Water (50 mL), styrene monomer (3 g; 99% pure from Janssen) and 0?014 g of fluorescent 1Pyrenylmethyl methacrylate (PolyFluor 394) were first mixed in the polymerisation reactor where the temperature was kept constant at 70uC. The water soluble radical initiator potassium persulphate (1?6% wt/wt over styrene) dissolved in a small amount of water (2 mL) was then introduced in order to induce styrene polymerisation. Different surfactant sodium dodecyl sulphate concentrations (0?03 and 0?12% wt/vol) were added in the polymerisation recipe to change the particle size keeping all other experimental conditions the same. The polymerisation was conducted under 400 rev min21 agitation during 12 h under nitrogen atmosphere at 70uC. The particle size was measured using Malven Instrument NanoZS. The mean diameter of these particles is 203 nm (SmPS) and 382 nm (LgPS). The weight average molecular weights (Mw) of individual PS chain (Mw) were measured by gel permeation chromatography and found to be 906103 g mol21 for both 203 nm (SmPS) and 382 nm (LgPS). Glass transition

temperature (Tg) of the PS latexes were determined using differential scanning calorimeter and found to be around 105uC. Table 1 provides some characteristics of the two parent latex dispersions used in making the composite films. Al2O3 solution

Al2O3 sol was prepared in the following way: a total of 2 mL aluminium-tri-sec-butoxide (Aldrich; 97%) was dissolved in 45 cc water at 70uC. The solution was stirred for 30 min. A small amount of acid was continuously added as catalyst, until the solution became transparent and this was stirred for another 2 h. Oxide networks are formed upon hydrolytic condensation of alkoloxide precursors. Finally, a uniform and transparent Al2O3 sol was obtained for film fabrication.

Preparation of PS/Al2O3 Films Firstly, the glass substrates (0?862?5 cm) were cleaned ultrasonically in acetone and deionised water. LgPS and SmPS aqueous suspensions were dropped on clean glass substrates using the casting method and dried at room temperature. Upon slow drying at room temperature, powder LgPS and SmPS films were produced. The film thickness of both the powder films was determined to be 5 mm on average. As our aim is to study the particle size effect of PS latex and Al2O3 content on film formation behaviour of PS/Al2O3 composites, we prepared two series of films: (i) series 1: LgPS and Al2O3 (LgPS/Al2O3) (ii) series 2: SmPS and Al2O3 (SmPS/Al2O3). The LgPS and SmPS covered glass substrates are placed vertically into the Al2O3 sol for 2 min, and then taken out and dried at 100uC for 10 min. Here the Al2O3 content in the films could be adjusted by dipping cycle

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500 nm. A glass plate was used as a standard for all UVV experiments, and measurements were carried out at room temperature after each annealing processes. Scanning electron microscopy (SEM) measurements

Scanning electron micrographs of the PS/Al2O3 films were taken at 10–20 kV in a JEOL 6335F microscope. A thin film of gold (10 nm) was sputtered onto the surface of samples using a Hummer-600 sputtering system to help image the PS/Al2O3 films against the glass background.

Results and discussion

3 Plot of maxima of transmitted light intensities, (Itr)m from Figs. 1 and 2 versus Al2O3 layers for a SmPS/ Al2O3 and b LgPS/Al2O3 films

and therefore, to investigate the effect of Al2O3 content consecutive dipping was performed. When the templates were immersed into the Al2O3 sol, the Al2O3 precursor could permeate the close packed arrays of PS by capillary force and form a solid skeleton around the PS spheres. Seven different films for each series of films were produced with 0, 1, 3, 5, 8, 10 and 15 layers (dipping cycle) of Al2O3. In order to study the film formation behaviour of PS/Al2O3 composites, the produced films were separately annealed above Tg of PS, 105uC for 10 min at temperatures ranging from 100 to 250uC. The temperature was maintained within ¡2uC during annealing. After each annealing step, films were removed from the oven and cooled down to room temperature.

Methods Fluorescence measurements

After annealing, each sample was placed in the solid surface accessory of a Perkin–Elmer model LS-50 fluorescence spectrometer. Pyrene was excited at 345 nm and scattering and fluorescence emission spectra were detected between 300 and 500 nm. All measurements were carried out in the front face position at room temperature. Slit widths were kept at 8 nm during all SSF measurements. Photon transmission measurements

Photon transmission experiments were carried out using a Carry-100 BioUV-Visible (UVV) scanning spectrometer. The transmittances of the films were detected at

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Figures 1 and 2 show transmitted (Itr), scattered (Isc) and fluorescence (IP) light intensities versus annealing temperatures for both SmPS/Al2O3 and LgPS/Al2O3 composite film series. Upon annealing, Itr started to increase above a certain onset temperature, called the minimum film formation temperature T0, for all film samples. Isc showed a sharp increase at the single temperature named as the void closure temperature, Tv. Here we have to mention that Isc is scattered from below the surface as well as from the surface of the latex film; however, Itr goes through the film all the way. Fluorescence intensity, IP of all film samples first increases, reaches a maximum, and then decreases with increasing annealing temperature.40–44 The temperature where IP reaches the maximum is called the healing temperature, Th. The increase in Itr above T0 can be explained by evaluation of the transparency of the composite films upon annealing. Most probably, increased Itr corresponds to the void closure process;41 i.e. PS starts to flow upon annealing and voids between particles can be filled. Since higher Itr corresponds to higher clarity of the composite, then increase in Itr predicts that the microstructure of these films change considerably when annealing them, i.e. the transparency of these films evolves upon annealing. PS starts to flow due to annealing, and voids between particles can be filled due to the viscous flow. Further annealing at higher temperature causes healing and interdiffusion processes41–44 resulting in a more transparent film. The sharp increase in Isc occurs at Tv, which overlaps the inflection point on the Itr curve. Below Tv, light scatters isotropically because of the rough surface of the PS films. Annealing of the film at Tv creates a flat surface on the film, which acts like a mirror. As a result, light is reflected to the photomultiplier detector of the spectrometer. Further annealing makes the PS film totally transparent to light and Isc drops to its minimum. On the other hand, the increase in IP above T0 presumably corresponds to the void closure process up to the Th point where the healing process takes place. Decrease in IP above Th can be understood by interdiffusion processes between polymer chains.45,46 It is clear that all curves of SmPS/Al2O3 and LgPS/ Al2O3 films show similar behaviours indicating that these films undergo complete film formation.40–44 The behaviour of T0, Tv and Th support these findings. Minimum film formation (T0), void closure (Tv) and healing (Th) temperatures are important characteristics related to the film formation properties of latexes. T0 is often used to indicate the lowest possible temperature for particle deformation sufficient to decrease interstitial void diameters to sizes well below the wavelength of light.47 Below this critical temperature, the dry latex is

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Effect of particle size in PS Latex/Al 2 O 3 nanocomposite films

4 Images (SEM) of SmPS/Al2O3 composite films with a 0, b 1, c 5 and d 10 layer of Al2O3 annealed at 100uC. In every case, scale bar is 1 mm

opaque and powdery. However, at and/or above this temperature, a latex cast film becomes a continuous and clear film.48 Therefore, T0 has been considered in this study as the temperature above which the Itr starts to increase. Here Tv is the lowest temperature at which Isc becomes highest and defined as the maxima of the Isc curve. Here Tv is the minimum temperature at which Isc becomes highest. The healing temperature (Th) is the minimum temperature at which the latex film becomes continuous and free of voids. The healing point indicates the onset of the particle–particle adhesion.48 Here, Th

is defined as the maxima of the IP curves versus temperature. T0, Tv and Th, temperatures measured for two series are reported in Table 2. From Table 2, it should be noticed that T0 and Tv values increase with increasing Al2O3 content. This behaviour of T0 and Tv clearly indicates that the existence of Al2O3 delays the latex film formation process. However, the healing process is almost not affected by the presence of Al2O3, which is not surprising. As a result, film formations of PS latexes were strongly influenced by both the Al2O3 content and PS particle size.

Table 2 Minimum film formation (T0), void closure (Tv) and healing (Th) temperatures for two film series. SmPS/Al2O3

LgPS/Al2O3

Al2O3 layer

T0 (uC)

Tv (uC)

Th (uC)

T0 (uC)

Tv (uC)

Th (uC)

0 1 3 5 8 10 15

120 120 120 120 120 170 170

120 140 170 150 160 120 190

150 160 170 150 150 160 190

120 130 140 130 140 160 160

130 160 180 180 170 180 170

180 150 180 180 170 170 170

T0 ; minimum film formation temperature Tv ; void closure temperature T ; healing temperature

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Effect of particle size in PS Latex/Al 2 O 3 nanocomposite films

5 Images (SEM) of SmPS/Al2O3 composite films with a 0, b 1, c 5 and d 10 layer of Al2O3 annealed at 250uC. In every case, scale bar is 1 mm

It is also seen in Fig. 3a and b that the maxima of Itr, (Itr)m decreases with increasing Al2O3 content indicating that low transparency occurs for both series. Composite films prepared from large PS particles showed higher transmittance at all annealing temperatures compared to the films formed from the small PS latex. For 15 layers of Al2O3 content, LgPS/Al2O3 composite films transmit 60% of incident light at 500 nm, whereas SmPS/Al2O3 films are opaque to light and transmit only 35% of the incident light at the same wavelength. Since we measure the transmittances of the films at 500 nm (below the absorption wavelength of Al2O3) the absorption of the films is negligible at this wavelength. In addition, because the sizes of SmPS and LgPS particles are below 500 nm and also completely deformed at 250uC, they cannot contribute to the scattering and the scattering is only modified by the large scale structures of the composite films. Therefore, the interaction with the film structure determines the scattering. There exists two major factors to affect the transmittance, i.e. surface scattering and (PS–PS and PS–Al2O3) boundary scattering. Since at 250uC the film formation process is completed (disappearance of voids and PS–PS boundaries), scattering takes place predominantly from the PS–Al2O3 boundaries. As the Al2O3 content increases, the cluster size increases gradually and the screening performances of Al2O3 particles in the visible region are

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enhanced resulting in the lower transmittance. It is understood that this screening effect is most pronounced for SmPS/Al2O3 films. In order to support these findings, SEM micrographs of both composite series with 0, 1, 5 and 10 layers of Al2O3 were taken after annealing at 100 and 250uC. Figures 4 and 5 show SEM graphs of SmPS/Al2O3 films annealed at 100 and 250uC for 10 min, respectively. Here it has to be noted that the pure SmPS latex film with no Al2O3 in Fig. 4a presents perfect order whereas inclusion of Al2O3 destroys the order in the latex system as seen in Fig. 4b–d. As the Al2O3 content is increased SmPS spheres seem to attach to each other resulting in the agglomeration of particles (Fig. 4d). When the annealing temperature is 250uC (see Fig. 5), considerable change occurs. Particle– particle boundaries almost disappear and the film surface appears flat confirming our arguments related with IP and Itr data. However, in Al2O3 content films, some spherical nanoparticles are seen which can be attributed to the SmPS particles encapsulated by Al2O3. These highly encapsulated particles cannot be destroyed and contribute to the film formation process. Very similar behaviour can be observed for LgPS/Al2O3 composite films when Figs. 6 and 7 are compared with Figs. 4 and 5. Nevertheless, the nanoparticles in Fig. 7 seem larger in size than those in Fig. 5 indicating the dependence on PS particle size used as template.

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Effect of particle size in PS Latex/Al 2 O 3 nanocomposite films

6 Images (SEM) of LgPS/Al2O3 composite films with a 0, b 1, c 5 and d 10 layer of Al2O3 annealed at 100uC. In every case, scale bar is 1 mm

In order to determine the extent of film formation, films were soaked in toluene for 24 h to completely dissolve the PS template after the film formation is completed. Dissolution experiments were performed at room temperature (23uC). Here, toluene was used as the dissolution agent since PS is very soluble49 in this solvent and the Al2O3 films are stable. After extraction of PS, the morphology of SmPS/Al2O3 films with 1 and 5 layers of Al2O3 (Fig. 8a and b) do not change significantly. In these images, SmPS spheres highly coated with Al2O3 are clearly seen. However, some spherical pores which must belong to the Al2O3 encapsulated replica of SmPS latexes are also observed. These pores have a well pronounced circular shape and are isolated from each other. Scanning electron microscopy images of the film prepared with 10 layers of Al2O3 in Fig. 8c show an interconnected and open porosity with average pore size diameter of 203 nm, corresponding approximately to the SmPS template diameter. In all images, besides the pores there are also voids that might have been left by the interconnected SmPS aggregated spheres. It is understood that higher Al2O3 content and small PS size created a porous, disordered material after extraction of template. On the other hand, SEM images of LgPS/Al2O3 composites given in Fig. 9 present highly porous structures after the extraction process. The pores are uniformly distributed in space, but random pore

morphology in the films with 1 and 5 layers of Al2O3 (see Fig. 9a and b) destroy their spherical shape. These films show a poorly ordered pore structure and heterogeneous pore size distribution indicating that the Al2O3 particles might not be sufficient to fully cover the surface of the LgPS template. However, it could be seen from Fig. 9c that film with 10 layers of Al2O3 shows well defined spherical ordered pores. The pores are uniformly distributed in the sample and showed an ordered connected porous structure. The homogenous distribution of the pores in the Al2O3 framework indicates that the porous structure retains the periodicity of the LgPS template. In conclusion, SEM images of both film series showed that by varying the latex particle diameter, it is possible to systematically control the pore size in the final porous structure. There is also a close relationship between morphology and the Al2O3 content. Well defined open structure and interconnected porosity were obtained when Al2O3 content was increased. This behaviour can be explained by the removal of PS from the surface of the Al2O3 covered latex particles during the dissolution process. In other words, the film formation from SmPS and LgPS particles has occurred on top of the Al2O3 covered particles during annealing and, during dissolution, PS material is completely dissolved showing the microstructure of PS particles covered by the Al2O3 layer. This picture is now depicted in Fig. 10 where the

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Effect of particle size in PS Latex/Al 2 O 3 nanocomposite films

7 Images (SEM) of LgPS/Al2O3 composite films with a 0, b 1, c 5 and d 10 layer of Al2O3 annealed at 250uC. In every case, scale bar is 1 mm

behaviour of SmPS/Al2O3 and LgPS/Al2O3 composite films during annealing are presented.43,44 This is supported by the SEM pictures in Figs. 4–9. In Fig. 10a, the film possesses many voids, which result in short mean-free and optical paths of a photon yielding very low IP and Itr. Figure 10b shows a film in which interparticle voids disappear due to annealing, which gives rise to a long mean free and optical path in the film. At this stage, IP and Itr reach their maximum values. Finally, Fig. 10c presents a nearly transparent film with no voids but some Al2O3 background. At this stage, film has low IP but high Itr because the mean free path is very long but the optical path is short.

dr c 1 ~{ : dt 2g rðrÞ

(1)

where c is the surface energy, t is time and r(r) is the relative density. It has to be noted that here the surface energy causes a decrease in void size and the term r(r) varies with the microstructural characteristics of the material, such as the number of voids, the initial particle size and packing. Equation (1) is similar to one that was used to explain the time dependence of the minimum film formation temperature during latex film formation.51,52 If the viscosity is constant in time, integration of equation (1) gives the relation as

Film formation mechanisms Void closure

In order to quantify the behaviour of IP below Th and Itr above T0 in Figs. 1 and 2, a phenomenological void closure model can be introduced. Latex deformation and void closure between particles can be induced by shearing stress which is generated by surface tension of the polymer, i.e. polymer–air interfacial tension. The void closure kinetics can determine the time for optical transparency and latex film formation.50 In order to relate the shrinkage of spherical void of radius, r, to the viscosity of the surrounding medium, g, an expression was derived and given by the following relation50

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t~{

ðr 2g rðrÞdr c

(2)

r0

where r0 is the initial void radius at time t50. The dependence of the viscosity of polymer melt on temperature is affected by the overcoming of the forces of macromolecular interaction, which enables the segments of polymer chain to jump over from one equilibration position to another. This process happens at temperatures at which the free volume becomes large enough and is connected with

Ug˘ ur et al.

8 Images (SEM) of SmPS/Al2O3 films with a 1, b 5 and c 10 layer of Al2O3 after extraction of PS template with toluene. In every case, scale bar is 1 mm

the overcoming of the potential barrier. Frenkel–Eyring theory produces the following relation for the temperature dependence of viscosity53,54 g~

  N0 h DG exp V kT

(3)

where N0 is Avogadro’s number, h is Planck’s constant, V is molar volume and k is Boltzmann’s constant. It is known that DG5DH2TDS, so equation (3) can be written as   DH g~A exp (4) kT

Effect of particle size in PS Latex/Al 2 O 3 nanocomposite films

9 Images (SEM) of LgPS/Al2O3 films with a 1, b 5 and c 10 layer of Al2O3 after extraction of PS template with toluene. In every case, scale bar is 1 mm

where DH is the activation energy of viscous flow, i.e. the amount of heat which must be given to one mole of material to create the act of a jump during viscous flow; DS is the entropy of activation of viscous flow. Here A represents a constant for the related parameters that do not depend on temperature. Combining equations (2) and (4), the following useful equation is obtained

t~{

  ðr 2A DH exp rðrÞdr c kT

(5)

r0

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Effect of particle size in PS Latex/Al 2 O 3 nanocomposite films

11 ln (IP) versus T21 plots of data in Fig. 1 for SmPS/ Al2O3 composite film contain a 0, b 1, c 5 and d 10 layer of Al2O3. Slope of straight lines on right and left hand side of graph produce DHP and DE activation energies, respectively

where, ro22 is omitted from the relation since it is very small compared to r22 values after void closure processes started. Equation (7) can be solved for IP and Itr (5I) to interpret the results in Figs. 1–4 as   3DH (8) I(T)~S(t)exp { kT where S(t)5(ct/2AC)3. For a given time the logarithmic form of equation (5) can be written as follows   3DH (9) ln I(T)~LnS(t){ kT

a film possesses many voids that results in very low IP and Itr; b interparticle voids disappear due to annealing, IP reaches its maximum value; c transparent film with no voids but some Al2O3 background and has low IP but high Itr 10 Cartoon representation of SmPS/Al2O3 and LgPS/ Al2O3 composite films at several annealing steps

As it was already explained above, the increase in both IP and Itr originates due to the void closure process, then equation (9) was applied to Itr above T0 and to IP below

In order to quantify the above results, equation (5) can be employed by assuming that the interparticle voids are equal in size and the number of voids stays constant during film formation (i.e. r(r)