Journal of Colloid and Interface Science 295 (2006) 457–463 www.elsevier.com/locate/jcis
Ceramic encapsulated latex composites S. ¸ U˘gur a , E. Pehlivan a , F. Tepehan a , Ö. Pekcan b,∗ a Department of Physics, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey b Department of Physics, I¸sık University, 34398 Maslak, Istanbul, Turkey
Received 29 April 2004 Available online 26 September 2005
Abstract This work reports the encapsulation of latex particles in Al2 O3 –polystyrene (PS) composite films. These films were prepared from PS particles in Al2 O3 dispersion at room temperature in various latex contents. Composite films were annealed at elevated temperatures in 10 min time interval above the glass transition temperature (Tg ) of polystyrene. Transmitted photon intensities, Itr were monitored after each annealing step. AFM micrographs were also used to observe the physical changes of the composite films during annealing. It was observed that latex particles are encapsulated above a critical Al2 O3 content of 33 wt% which corresponds to the critical occupation probability of pc = 0.33 at which the film obey the site-percolation model with a critical exponent of 0.45. Below pc , it was seen that complete latex film formation process took place, where transparency of the film was increased by annealing. 2005 Elsevier Inc. All rights reserved. Keywords: Latex; Encapsulation; Percolation; Critical exponents
1. Introduction In last decade, there has been growing interest on producing new materials by filling polymers with inorganic natural and/or synthetic compounds. These composite materials posses high heat resistance, mechanical strength and impact resistance or present weak electrical conductivity and low permeability for gases like oxygen or water vapor. Since the inorganic particles display rather macroscopic dimensions and since there is mostly no interaction between the two mixed components at the interface between the two partners, the resulting composite materials can be seen as filled polymers. In general processing and structural development studies are coupled with investigations of coating properties including optical, electrical and mechanical properties [1–3]. Some efforts have been made to construct microstructure and properties of coatings with composite ceramic–polymer microstructures, where the emphasis in composites in which a ceramic phase forms a connected network in a polymer matrix. Processing and microstructure development of ceramic and polymer coating prepared by de* Corresponding author.
E-mail address:
[email protected] (Ö. Pekcan). 0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2005.08.053
positing a solution or dispersion have been of interest in last few years [4,5]. Colloidal ceramics, sol–gel derived ceramics and polymers have been studied as coating systems. Nano-sized antimony-doped tin oxide particles and poly(nbutyl methacrylate) lattices were combined to construct transparent conductive coating and a reduction in the percolation threshold was observed in the latex based matrix [6]. The reduction in the percolation threshold was attributed to the particle– particle interactions that favored microphase separation during film formation. Nano-sized carbon black particles were combined with latex particles to produce the conductive polymeric composites with segregated microstructures [7,8]. It was shown that there is a drastic decrease in the electrical conductivity at the percolation threshold which is very sensitive to the processing conditions such as drying temperature, dispersant content and latex particle characteristics. In the latex-based composites, a segregated microstructure is formed, i.e. the carbon particles segregate to the interstices and interfaces between coalescing latex particles. 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 second stage in which the particles
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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 system, however, deformation of particles first leads to void closure [9–12] 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 solvent has evaporated and all voids have disappeared. Transmission electron microscopy (TEM) has been the most common technique used to investigate the structure of the dried films [13,14]. Pattern of hexagons, consistent with face centered 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 TEM (FFTEM) has been used to study the structure of dried latex films [15]. Smallangle neutron scattering (SANS) has been used to study latex film formation at molecular level. Extensive studies using SANS have been performed by Sperling and co-workers [16] on compression-molded polystyrene film. Direct-nonradiative energy transfer (DET) method has been employed to investigate the film formation process from dye-labeled hard [17] and soft [18,19] polymeric particles. The steady state fluorescence technique combined with DET has been used to examine healing and interdiffusion processes in the dye labeled poly(methyl methacrylate) (PMMA) latex systems [20–22]. Recently UV– visible technique was used to study film formation from PMMA and polystyrene (PS) particles [23,24] where the transmitted light intensity was monitored during film formation process. In this work, the evolution of film formation from composites prepared with Al2 O3 ceramic and surfactant-free polystyrene (PS) latex, was studied. Nine different composites were casted with different latex contents (27, 37, 47, 57, 67, 78, 87, 89 and 100 wt%). Then films were prepared by annealing composites above the glass transition temperature of PS for 10 min intervals at temperatures ranging from 100 to 280 ◦ C. Transmitted photon intensity, Itr was monitored to study the evolution of transparency. It was observed that Itr increased dramatically above a certain onset temperature, T0 called minimum film formation temperature for the samples contained more than 67 wt% latex. Films with low latex content (less than 67 wt%) show no variation in Itr upon annealing, i.e. microstructure of these films present no change after annealing process has completed. 2. Theoretical considerations 2.1. Percolation model Initially the percolation theory has been associated with a paper of Broadbent and Hammersley which discussed the general situation of a fluid spreading randomly through a medium [25,26]. The most interesting feature of percolation phenomenon is the existence of a percolation threshold, pc below which spreading process is confined to be a finite region. The spread
of a blight from tree to tree in an orchard was discussed by Broadbent and Hammersley where the trees are planted on intersection of a square lattice. The percolation threshold, pc for this problem is around 0.6 for so called “site percolation” on a square lattice. In other example is the seepage of water in the cracks and fractures of a rock formation. A similar problem for practical interest is the spread of water displacing oil in porous rocks, where neighboring pores are connected by small capillary channels [27]. If no oil in the system injected water into any given pore may only invade another pore through capillary channels or “bonds.” The pores are “sites” connected to the chosen centre of injection form what is called a “cluster.” The largest cluster spans the lattice connecting the left and right edges to the bottom edge, which is called “percolating cluster.” The percolation probability, p∞ (p), is defined as the probability that water injected at a site, chosen at random, will wet infinitely many pores. Here one should note that the probability for having a pore at all the sites where water injection is attempted is p. Extensive simulations and theoretical work have shown that the percolation probability vanishes as a power-law near pc : p∞ (p) ≈ (p − pc )β ,
(1)
for p > pc , and p → pc . In a simple cubic lattice pc is found to be 0.31 for site-percolation and 0.249 for bond percolation models [28]. The exponent β for a simple cubic lattice is 0.45. 3. Experimental 3.1. Al2 O3 dispersion 2 ml aluminum-tri-sec-butoxide (Aldrich, 97%) was dissolved in 45 cm3 water at 70 ◦ C. The dispersion was stirred for 30 min. Small amount of acetic acid was continuously added as catalyst, until the dispersion became transparent and stirred for another 2 h. Oxide networks are formed upon hydrolytic condensation of alkoxide precursors. 3.2. PS latex Pyrene labeled polystyrene particles were produced via surfactant free emulsion polymerization process. The polymerization was carried out in a four-neck glass reactor equipped with a glass paddle type agitation, condenser and nitrogen inlet. The agitation rate was 300 RPM and the polymerization temperature was controlled at 70 ◦ C. Water (100 ml) and styrene (5 g) were first mixed in the polymerization reactor and when the temperature was constant (at 70 ◦ C), potassium peroxodisulfate (KPS) initiator (0.2 g) dissolved in small amount of water (2 ml) was introduced in order to induce styrene polymerization. The polymerization was conduced during 18 h. 3.3. Composite films Nine different films with 27, 37, 47, 57, 67, 78, 87, 89 and 100 wt% latex contents were prepared from the dispersion of PS/Al2 O3 mixtures by placing the same number of drops on a
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Fig. 1. AFM images of composite films prepared with (a) 100, (b) 67, (c) 57 and (d) 37 wt% of PS latex before annealing.
glass plates with the size of 1.0 × 1.0 cm2 and allowing the water to evaporate. The PS/Al2 O3 dispersion was stirred over night to be sure that complete mixing was achieved. Then samples were separately annealed above Tg of PS (105 ◦ C) for 10 min at temperatures ranging from 100 to 300 ◦ C. The temperature was maintained within ±2 ◦ C during annealing. 3.4. Photon transmission measurements Photon transmission experiments were carried out using model DU 530 Life Science UV–visible (UVV) spectrometer from Beckman. The transmittances of the films were detected between 300 and 400 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. 3.5. Atomic force microscopy (AFM) measurements Atomic force micrographs were taken with Shimadzu J33PM-9500J3 scanning probe microscope. Figs. 1a, 1b, 1c and 1d present the AFM micrographs of pure PS latex and composite films prepared with 67, 57 and 37 wt% PS latex before annealing, respectively. Here it has to be noted that the latex film with no Al2 O3 in Fig. 1a present perfect order. Par-
ticles in Fig. 1d are seen to be much larger than the particles in Figs. 1a–1c indicating that 37 wt% latex content particles are highly encapsulated with Al2 O3 ceramic during mixing and casting processes. However particles in films with 57 and 67 wt% PS are seen to be in the same size with the particles in pure latex film. It is also seen in Figs. 1b–1d that encapsulation of the latex particles by Al2 O3 destroys the order in the latex system. 4. Results and discussion Transmitted photon intensities from the films annealed at elevated temperatures are plotted in Figs. 2a, 2b, 2c, 2d, 2e and 2f for the composites contain 37, 47, 57, 67, 89 and 100 wt% PS latex, respectively. It is seen in Figs. 2a–2c that Itr presents no variation in its intensity by predicting that microstructure of these composites films shows almost no change. However annealing of the films prepared with 67, 89 and 100 wt% latex content, show a dramatic increase above a certain temperature called the minimum film formation temperature, T0 . In fact T0 moves slightly to the low temperature region by indicating, early film formation process takes place as PS content increased in these composites. Since higher Itr corresponds to higher clarity of the composite, increase in Itr predicts that microstructure
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Fig. 2. Plot of transmitted photon intensities, Itr versus annealing temperatures, T from the composite film contain (a) 37, (b) 47, (c) 57, (d) 67, (e) 89 and (f) 100 wt% PS latex.
of these films change considerably by annealing them above T0 , i.e. the transparency of these film evolve upon annealing. Polystyrene starts to flow due to annealing and voids between particles can be filled due to the viscous flow. However composites prepared with 37, 47 and 57 wt% latex do not show this behaviour, i.e. no viscous flow takes part and the voids cannot be filled in these films upon annealing. Most probably encapsulated latex particles can not be destroyed upon annealing. It is also seen in Figs. 2d–2f that the maxima of Itr , (Itr )m increase by increasing latex content in the composites and reaches its highest value in pure latex film. In order to support these findings AFM micrographs of the composites prepared with 37 and 57 wt% latex are compared in Figs. 3a, 3b and 3c, 3d before and after annealing at 280 ◦ C for 10 min. It is seen in Figs. 3a and 3c that microstructure of the composite film remain almost unchanged upon annealed at 280 ◦ C, indicating that PS latex is highly encapsulated by Al2 O3 which prevents PS particle from viscous flow even at 280 ◦ C temperature. Almost similar behaviour can be observed when Fig. 3b is compared with Fig. 3d, i.e. if 57 wt% latex content composite annealed at 280 ◦ C film still keeps its original microstructure form. Figs. 4a, 4b and 4c, 4d compare the composite films prepared with 67 and 89 wt% latex, before and after annealed at 280 ◦ C. It is seen that considerable change is occurred by annealing the 67 wt% latex content film at 280 ◦ C (see Figs. 4a and 4b), i.e. the microstructure of the composite film is changed
Fig. 3. AFM images of composite films prepared with (a) 37 and (b) 57 wt% of PS latex before and after annealed at 280 ◦ C ((c) and (d)).
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Fig. 4. AFM images of composite films prepared with (a) 67 and (b) 89 wt% of PS latex before and after annealed at 280 ◦ C ((c) and (d)).
and the particle–particle interfaces are beginning to disappear upon annealing. Figs. 4b and 4d compare the AFM micrographs of 89 wt% latex content films before and after annealing where it is seen that particle–particle boundaries are completely disappeared after annealing process is completed. From here, both Itr and AFM data suggest that 67 wt% latex content presents a critical percentage, below which composite films are in encapsulated form and the replica of the particles can not be destroyed upon annealing even at 280 ◦ C temperature. In order to quantify the findings, presented in Figs. 2d–2f, two types of treatment can be done for Eq. (1), at first it is assumed that |p − pc |β can be taken proportional to |T − T0 |β and then Eq. (1) becomes Itr (T ) ≈ (T − T0 ) , β
(2)
where P∞ (p) is considered to be proportional to the transmitted light intensity, Itr [29]. log Itr − log |T − T0 | plots are presented in Figs. 5a, 5b and 5c for the composites prepared with 67, 89 and 100 wt% PS latex. The slope of the linear relation produce the critical exponent β for the composite film formation process. β values and minimum film formation temperatures, T0 are plotted against wt% of latex content in Figs. 6a and 6b, respectively. It is interesting to note that the produced β value
corresponds to the β (= 0.45) value of a simple cubic lattice for the film contain 67 wt% PS. For the second treatment we define the following ratio mAl2 O3 , R= (3) mAl2 O3 + mPS where mAl2 O3 and mPS present the weight of Al2 O3 and PS in the composite film. If R is assumed to be the occupation probability, p of Al2 O3 in the whole lattice then the percolation threshold, pc becomes Rc and if once again P∞ (p) is considered to be proportional to the transmitted light intensity, Itr then Eq. (1) can be written as [30] Itr (R) ≈ (R − Rc )β .
(4)
Equation (4) describes the percolation model for Al2 O3 distribution in PS matrix. When R reaches Rc , the largest cluster of Al2 O3 appears by connecting the left and right edges to the bottom edge of the PS matrix. It is seen in Table 1 that R with 33 and 22 posses β with 0.45 and 0.48 values. These R values are equivalent to the pc for site-percolation and bond-percolation models in simple cubic lattice where β values are also satisfied. From here one may conclude that the composite films with 67 and 78 wt% PS content
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Fig. 6. Plots of (a) critical exponent, β and,1 (b) minimum film forming temperature, T0 versus PS latex content.
Fig. 5. log Itr versus log(|T −T0 |) plots of composite films prepared with (a) 67, (b) 89, (c) 100 wt% PS 1 latex content. Table 1 Relation between ratio R and critical exponents β Ra
33
22
16
11
0
βb
0.45
0.48
0.81
0.78
1.3
a R=m Al2 O3 /(mAl2 O3 + mPS ), where mAl2 O3 is weight of Al2 O3 , mPS is
weight of PS. b β—critical exponents, produced from Eq. (2).
obey percolation model, i.e. Al2 O3 percolate in the composite film during annealing above T0 . Composite films above pc (R = 33) present Al2 O3 encapsulated replica of latexes. However, composite films below pc (R = 22) show latex film formation behaviours with a little effect of Al2 O3 . In other word β values increase up to 1.3, indicating no percolation takes place and complete film formation can be accomplished. The produced β values and observed T0 values are plotted versus PS latex content in Figs. 6a and 6b, respectively. It is seen that as β exponent deviates from its percolation values as T0 temperatures drop to lower values. In other words, if Al2 O3 stops to percolate in PS matrix, then latex film formation can occur at low annealing temperatures.
Fig. 7. Plot of the maxima of transmitted light intensities, (Itr )max from Fig. 2 versus PS latex content.
On the other hand Fig. 7 presents the plots of the maximum values of (Itr )max versus wt% latex content. It is interesting to see that (Itr )max shows an increase starting from 67 wt% latex content in composite film, predicting a percolation threshold behaviour of Al2 O3 in polymer matrix. As we already know that 33 wt% Al2 O3 corresponds to the percolation threshold, pc for the latex composite system, above which latex particles encapsulated with Al2 O3 ceramic. At pc , Al2 O3 percolates among the latex film, below pc however latex film formation can be completed with the residues of Al2 O3 barriers. This picture is now depicted in Fig. 8 where the behavior of the composite film below and above pc are presented which are supported by the
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Fig. 8. Cartoon presentation of (I) encapsulated (p < pc ) and (II) transparent (p > pc ) composites. Is and Itr are the scattered and transmitted light intensities.
AFM pictures in Figs. 3 and 4. Here it is seen that when p > pc , encapsulated films shows no transparency to the light, however if p < pc then composite film becomes transparent to the light and (Itr )max increase dramatically. In conclusion this work has shown that the presence of Al2 O3 in PS matrix can affect the film formation processes. If the presence of Al2 O3 is higher than the critical value, Rc the PS latices are encapsulated by Al2 O3 . However, if the amount of Al2 O3 lower than Rc , the classical latex film formation process can take place. References [1] J. Sun, W.W. Gerberich, L.F. Francis, J. Polym. Sci. B Polym. Phys. 41 (2003) 1744.
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