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Hollow Silica as an Optically Transparent and Thermally Insulating Polymer Additive Lusi Ernawati, Takashi Ogi, Ratna Balgis, Kikuo Okuyama, Mario Stucki, Samuel C Hess, and Wendelin J. Stark Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.5b04063 • Publication Date (Web): 10 Dec 2015 Downloaded from http://pubs.acs.org on December 13, 2015
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Hollow Silica as an Optically Transparent and
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Thermally Insulating Polymer Additive
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Lusi Ernawati1, Takashi Ogi*,1, Ratna Balgis1, Kikuo Okuyama1,
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Mario Stucki2, Samuel C. Hess2, and Wendelin J. Stark2
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1
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Hiroshima University, 1-4-1 Kagamiyama, Hiroshima 739-8527, Japan
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2
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Department of Chemical Engineering
Institute for Chemical and Bioengineering
ETH Zurich, Wolfgang-Pauli-Strasse. 10, CH-8093, Zürich, Switzerland
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KEYWORDS: filler, thermal conductivity, additive, transmittance, energy efficient
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ABSTRACT
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We present an improved synthesis route to hollow silica particles starting from
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tetramethyl-orthosilicate (TMOS) instead of the traditionally used ethyl ester. The silica was
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first deposited onto polystyrene (PS) particles that were later removed. The here introduced,
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apparently minor modification in synthesis, however, allowed for a very high purity material.
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The improved, low density hollow silica particles were successfully implemented into
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polymer films and permitted maintaining optical transparency while significantly improving
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the heat barrier properties of the composite. Mechanistic investigations revealed the dominant
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role of here used methanol as a co-solvent and its role in controlling the hydrolysis rate of the
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silicic ester, and subsequent formation of hollow silica particles. Systematic experiments
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using various reaction parameters revealed a transition between regions of inhomogeneous
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material production at fast hydrolysis rate and reliable silica deposition on the surface of PS
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as a core-shell structured particle. The shell-thickness was controlled from 6.2 to 17.4 nm by
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increasing TMOS concentration and the diameter from 95 to 430 nm through use of the
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different sizes of PS particles. Hollow silica particle with the shell-thickness about 6.2 nm
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displayed a high light transmittance intensity up to 95 % at 680 nm (length of light path ~1
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cm). Polyether-sulfone (PES)/hollow silica composite films (35 ± 5 µm, thick) exhibited a
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much lower thermal conductivity (0.03 ± 0.005 W m-K-1) than pure polymer films. This
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indicates that the prepared hollow silica is able to be used for cost and energy effective
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optical devices requiring thermal insulation.
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1. INTRODUCTION Hollow nanostructured particles have been extensively studied as important
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nanomaterials. Recently, hollow silica particles has attracted great attention due to its unique
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structural properties and possible applications as efficient catalysts, in optical devices, drug
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delivery carriers, thermal and electrical insulators [1-8]. Among various methods for
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preparation of hollow silica particles, the templating method offers one of the most attractive
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and efficient approaches because of convenience and facile access to starting materials [9-14].
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Hollow silica is normally prepared in two steps: First, a core-shell (template-silica)
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structured particle is prepared and followed by removal of the inner template. Several groups
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have made significant advances to control the size and shell-thickness [15-17], and mainly used
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tetraethyl-ortho silicate (TEOS) as a starting material [18-20]. Reliable preparation of larger
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amounts of hollow silica and proper control is difficult due to (i) agglomeration of particles at
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any step; (ii) irregular shapes as a result of non-uniform silica deposition; (iii) detachment of
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shell material and/or formation of separated (solid) particles; and (iv) control of the 2 ACS Paragon Plus Environment
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hydrolysis and condensation reaction rate [21-23]. Therefore, the synthesis of hollow silica
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particle with controllable size and shell-thickness has remained a great challenge for the
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growth and expansion of industrial applications.
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In this paper, we pursue a presumably minor modification of the process by switching
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to the methyl ortho silicate instead of the broadly used ethyl ortho silicate. This apparently
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simple change, however, has far reaching consequences, both on the hollow silica quality and
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economics of production. We then investigated the resulting hollow silica as an optically
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transparent and insulating with polymer additive.
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Our motivation to investigate the smaller methyl derivative of silicic acid came from
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two observations: The methoxy residue (Me-O-Si) usually hydrolizes faster than the ethoxy
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residue (Et-O-Si). Faster reaction rates typically permit use of smaller equipment in
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production, which significantly reduces costs. Second, in large scale operation, the by-
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product ethanol (EtOH; from TEOS) or methanol (MeOH; present work, from tetramethyl-
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ortho silicate, TMOS) would be recycled. The here proposed simple switch from the ethyl to
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the methyl ester of silicic acid profoundly alters the amount of recycle streams and, as a
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consequence, energy and equipment costs (see Table 1, the details and all assumptions are
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given in the Supporting Information (SI.1.)).
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Table 1. Comparison of the energy required for the distillation of alcohols (MeOH and
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EtOH) generated from TMOS and TEOS as by-products.
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Silica source
Generation of alcohol* (i.e MeOH and EtOH)
TMOS TEOS
[kg h-1] 2133 3067
Reflux ratio
Required energy for alcohol distillation
Difference of required energy [MJ year-1]
0.14 2.44
[kJ h-1] 2939 10045
51163
*Production of 1000 kg h-1 of hollow silica particles was assumed in this calculation.
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Experimental tests confirmed that the production rate of silica particle from TMOS is
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about 2 times higher than that of TEOS at the same concentration of TMOS and TEOS (0.08
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mol L-1), respectively. This is in line with earlier reported hydrolysis rates of TMOS and
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TEOS in an ionic liquid [24-26]. Compared to the other tetra-alkoxysilane groups (e.g. propyl,
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butyl, isopropyl), TMOS has the highest reactivity [27, 28]. Unfortunately, a fast reaction is
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more difficult to control.
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Synthesis of hollow silica particles by using TMOS as silica precursor and polymeric
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micelle of poly (styrene-b-2-vinylpyridine-b-ethylene oxide) as a template has been reported
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by Nakashima et al. [29]. Different from previous work, here, PS particles are selected as a
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template for preparing hollow silica particles because of their excellent monodispersity,
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uniformly, narrow size distribution, and easy preparation [30-34]. MeOH was used as a co-
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solvent for controlling the hydrolysis and condensation of TMOS. Interestingly, we found
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that high MeOH concentration could serve as a co-solvent and permitted feasible formation
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of hollow silica.
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2. EXPERIMENTAL
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2.1 Materials. Styrene (99%, Kanto-Chemical, Japan) as a monomer and 2,2’-azobiz (2-
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methylpropionamide)-dihydrochloride (AIBA, Sigma-Aldrich, US) as an initiator were used
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to prepare PS particles. Tetramethyl-orthosilicate (TMOS 99%, Sigma-Aldrich, USA) was
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used as silica source. Ammonia solution (NH4OH 28%, Kanto Chemical, Japan) was used as
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a basic catalyst solution for silica formation, and high-purity methanol (MeOH 99.8%,
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Kanto-Chemical, Japan) as a co-solvent. Polyether-sulfone (PES) (Veradec, A-201) was used
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as a matrix of silica composite film and dimethylacetamide (DMAc) (Sigma Aldrich, US)
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was used as a solvent for PES.
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2.2 Synthesis of hollow silica particles. Details experimental method for the preparation of
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PS particle are given in Supporting Information (SI.2). The prepared PS particles were
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mixed with MeOH-water solution as a solvent under vigorous stirring of 400 rpm in a 100 ml
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two-necked glass reactor system. After 15 min of stirring, TMOS and NH4OH were added.
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The reaction was vigorously stirred for 3 h, and the temperature maintained at 35oC. The
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resultant, silica-coated PS particles, were isolated by centrifugation at 7000 rpm for 5 min
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then washed by using ethanol. As a last step, calcination at 550 °C for 2 h was maintained to
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remove the PS template and yielded hollow silica particle. Hollow silica particles were then
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re-dispersed in water prior to characterization. The experimental conditions for all
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preparations are shown in Table SI-4 and Table SI-5. In order to control the average
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diameter and shell-thickness of hollow silica particles, the sizes of PS particles and the
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concentrations of TMOS were varied from 72 to 380 nm and 0.025 to 0.1 mol L-1,
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respectively. Here, the molar ratio of NH4OH/MeOH was kept at 0.065 for all of the prepared
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samples. Therefore, the molar ratios of NH4OH/TMOS became varied from 0.068 to 0.017.
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2.3 Preparation of PES/hollow silica composite film. PES/hollow silica composite films
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were prepared according to previous work [35], after preparation of a dispersion, film coating
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and drying. Firstly, 0.05 g of hollow silica particles was mixed in 20 g of dimethyl-acetamide
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(DMAc) and this solution was mixed by ultrasonic homogenizer with 5 min. 2 g of PES
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pellet was added to this hollow silica-DMAc solution and vigorously stirred for overnight at
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room temperature. PES/hollow silica composite films were prepared using a spiral film
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applicator (Zehntner, ZAA-2300, 150 µm spiral). The prepared composite films then were
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dried in a ventilated oven and kept at 100 °C for 10 min to remove the solvent contained.
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2.4 Characterization. The size, morphology and distribution of the hollow silica particles
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were characterized using a scanning-(SEM, Hitachi S-5000, operated at 20 kV, Japan) and a
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transmission-electron microscopy (TEM, JEM-3000F, JEOL, operated at 300 kV, Japan). The 5 ACS Paragon Plus Environment
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morphology, average diameter and standard deviation of PS particles were obtained using
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SEM. More than 200 particles (Σni = 200) were statistically counted from SEM and TEM
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observations. The functional groups and chemical composition of the prepared samples were
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identified using Fourier-transform infrared spectroscopy in the range 500-4000 cm-1 (FT-IR,
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Shimadzu, IR-Affinity-1S, JIS C6802, operated at 150 V, Japan). The surface charge of PS
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particle was measured using a zeta potential analyzer (Malvern Zetasizer, Nano ZS, UK).
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Nitrogen adsorption and desorption analysis (BELSORP 28SA, Bel Japan, Japan) was used to
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determine the specific surface area and pore size. BET analysis was carried out at a relative
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pressure (P/Po) of approximately 0.6 to 1 at 77 K, where Po is the saturation pressure of N2.
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Each of samples was tested at least twice. The optical transmittance properties of hollow
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silica particles suspension (length of light path~1 cm) were analyzed using a UV-vis
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spectrophotometer (UV-3150, Shimadzu, Japan). The transmittance spectra of the sample was
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obtained through the cells prepared by mixture 0.01 g of hollow silica particle in 10 ml of
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ultra-pure water. All measured samples were observed in the wavelength region at 300 to 800
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nm. The thermal conductivity was measured with two thermocouples and a heat flux sensor
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(green TEG, g SKIN® KIT-2615C). Top (30°C) and bottom (29.9°C) temperature of the
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setup were kept constant by two individual water baths. The actual temperature difference
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was recorded by thermocouples (∆T, K) and the sensor measured the resulting heat flux (Øq;
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W m-2) through the sample. The thermal conductivity for the sample (λs) was obtained by
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dividing the layer thickness (ds) through the difference between the thermal resistance of the
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setup (Rsetup) and the measured total thermal resistance (Rm):
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Rm =
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λs =
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∆T
φq
ds Rm − Rsetup
(1)
(2)
Measurements per sample were repeated three times at a steady state period of at least 30 min 6 ACS Paragon Plus Environment
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each.
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3. RESULTS AND DISCUSSION
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3.1 Effect of MeOH concentration on the formation of hollow silica particle
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The synthesis condition of PS particle is shown in the Supporting Information
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(SI.2) and supplied in Figure SI-4 and Table SI-3. The hydrolysis and condensation
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mechanism of TMOS in MeOH-water is illustrated in the Figure 1.
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Figure 1. Hydrolysis and condensation of TMOS in a MeOH-water system. The addition of
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MeOH or presence of partially hydrolyzed methoxy-silane species (bottom) slow down the
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overall condensation to silica (right) as CH3-O-Si is less reactive than Si-O-H. The amount of
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MeOH can be used to control the condensation rate of TMOS for controlled deposition of
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silica onto a template.
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MeOH is able to substitute hydrogen bonding of water molecules in the silica
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network. The oxygen atoms of TMOS are easily attacked by either water or MeOH through
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protonation and/or formation of hydrogen bonds. The smaller methyl group leaves more room
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for an attach and thereby allows an intrinsically high reaction rate of TMOS compared to the
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one of TEOS. Methanol-group (Me-O-Si) further block condensation since only two Si-OH 7 ACS Paragon Plus Environment
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can form a Si-O-Si bond, the basic motive of silica. The presence of methanol therefore
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lowers formation of Si-OH moieties and a low concentration of free silanol groups slows
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down condensation. The methanol similarity to water in terms of size and its capability to
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slow down hydrolysis can therefore be used to control the hydrolysis reaction rate of TMOS.
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The effect of MeOH concentration on the nucleation rate of TMOS is shown in Figure 2.
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Figure 2. SEM images of the prepared hollow silica particles with different MeOH
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concentrations: (a) 0, (b) 30, (c) 50, (d) 75, and (e) 90 wt%. All the samples were prepared
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using PS particles of 116 nm in size. Hydrolysis of TMOS under various MeOH
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concentrations: At low concentration of MeOH, irregular gels and networks of particles are
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formed, (a-c). At higher concentration of MeOH, polystyrene template particles could be
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reliably coated with silica, (d,e).
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Highly agglomerated silica particles were observed from the sample prepared without
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MeOH as shown in Figure 2(a). Here, obviously, TMOS hydrolyzed fast, and intermediately
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formed Si-OH rapidly continuous reacting. As a result, a high nucleation occurred, so a lot of
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small, nm-size of silica particles are generated in a short period of time and grow into a large
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particle. Meanwhile, at a low MeOH concentration (30 and 50 wt%), homogeneous 8 ACS Paragon Plus Environment
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nucleation tends to occur rapidly and leads to the formation of aggregated silica particles
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(Figure 2(b and c)). It might be due to the fast formation of primary silica particles which are
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electrostatically unstable in water. Interestingly, Figure 2(d and e) shows that hollow silica
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particles were successfully produced at high MeOH concentrations of 75 and 90 wt%,
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respectively. Here, the formation of Si-OH and nucleation of new silica is sufficiently
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suppressed. All silica now deposits onto the polymer particles. This deposition as a function
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of MeOH concentration can be shown in Figure 3.
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Figure 3. Hollow silica is only formed at higher MeOH concentration (bottom) when the
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reaction rate is sufficiently low to suppress excessive nucleation (formation of free floating,
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new particles, (top)) and favoring deposition of silica onto surfaces or previously deposited
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silica nuclei on PS template surfaces. Here, surface growth fills the gaps between the porous
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shells and eventually leads to solid layers of silica (bottom).
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The hydrolysis reaction rate of TMOS is only fast in pure water, generating large
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numbers of very small silica particles in a short period of time. These very small silica
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particles become abundant collide and form highly agglomerated silica particles. The addition
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of TMOS. In this condition, silica particles growth better and surface growth now completes
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with nucleation of new silica particles. These primary particles eventually form
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interconnected silica network structures consisting of large aggregates. The hydrolysis
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reaction rate of TMOS decreases further when the MeOH concentration was increased up to
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75 wt%. Here, MeOH easy substitutes the Si-OH group of hydrolyzing TMOS. The
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hydrolysis reaction rate of TMOS was even slower at 90 wt% of MeOH concentration where
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we experimentally observe the formation of PS-silica in the form of core-shell structured
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particle. Proof of the composite nature of the particles and of the ability to remove the PS
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phase was provided by FT-IR spectra (the details are given in Figure SI-5, Supporting
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Information (SI.3.)).
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3.2 Effect of PS particle size on the size of hollow silica particle
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Figure 4. SEM and TEM images of hollow silica particles prepared with different PS
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diameter of: (a,b) 72, (c,d) 116, (e,f) 185, and (g,h) 380 nm. The molar ratio of
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NH4OH/MeOH and TMOS concentration were kept at 0.065 and 0.08 mol L-1, respectively.
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The coating process works well up to 180 nm. At 380 nm in size, homogeneous nucleation
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(i.e. independent of the template) leads to a second population of small particles (bottom) of
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20 to 40 nm in size.
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The size of hollow silica particles can be controlled by changing the size of PS
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particles, as shown in SEM and TEM images of Figure 4(a-h) and detail experimental
3
conditions were given in Table SI-4, (Supporting Information (SI.4)). Average diameters of
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hollow silica particle rapidly increased from 95 to 430 nm along with increase in PS particle
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sizes from 72 to 380 nm. Surprisingly, small silica particles are found in even larger samples
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(i.e. hollow silica particle with an average diameter of 430 nm), as shown in Figure 4(g and
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h). Here, the rapid homogeneous nucleation of free silica nanoparticles was higher than the
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capture rate of these large PS particles. The ratio of hollow and small silica particles was
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i =k n statistically calculated by ∑ i H = 0.72 , where niH and niS are the counted number of i=1 ni S
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hollow and small particles, respectively. The hydrodynamic diameter of hollow silica
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particles in aqueous medium which corresponds to the samples in Figure 4(a-h) are also
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measured using DLS, as shown in Figure SI-6, (Supporting Information (SI.5)).
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3.3 Effect of TMOS concentration on the shell-thickness of hollow silica particle
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Figure 5. The coating formation process becomes visible when using TEM (right side)
4
instead of SEM for increasing shell thickness. Hollow silica particles were prepared under
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various TMOS concentrations: (a,b) 0.025, (c,d) 0.04, (e,f) 0.06, (g,h) 0.08, and (i,j) 0.1 mol
6
L-1. Thin shells (top) appear grainy as a result of the growth mechanism (see Figure 3, where
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first a layer of small silica particles is deposited, and subsequently grows together through
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surface growth, (i.e. direct deposition of silica onto previously formed silica surfaces
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Hollow silica particles with different shell-thickness can be obtained by varying the
2
concentrations of TMOS, as shown in Figure 5(a-j) and summarized in Table SI-5,
3
(Supporting Information (SI.6)). The shell thickness of hollow silica particle increased
4
from 6.2 to 17.4 nm as initial concentrations of TMOS increased from 0.025 to 0.1 mol L-1.
5
This means that the shell-thickness of hollow silica particles can be controlled within a
6
certain range. To obtain the relationship between the shell-thickness of hollow silica (L) and
7
the initial TMOS concentration [TMOS]o, we used the following equation:
8
9
3 L = R i3 + 4π
[TMOS
]0 (1 − exp (− C h [OH
−
ρ SiO 2 N
))
] t MW
1
3 SiO 2 V solution − Ri
(3)
Here, Ri is an inner radius of the prepared hollow silica particle, Ch is a hydrolysis rate
10
constant of TMOS, [OH-] is a hydroxyl ion concentration in the solution, t is a reaction time,
11
Vsolution is total volume of precursor solution, MWSiO and ρ SiO are a molecular weight and a 2
2
12
density of silica, respectively, and N is a number of PS particle. (Detailed derivation of
13
equation (3) was described in Supporting Information (SI.7)). The experimental results
14
shown are in a good agreement with the theoretical analysis method, as shown in Figure 6(a).
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This is a simplified results to make quick prediction of shell-thickness of hollow silica
16
particles. It indicates that this method is greatly valid to estimate the shell-thickness of a thin
17
and thick spherical shell of the prepared hollow silica particles.
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Figure 6. (a) The shell-thickness of hollow silica particles can be controlled through the
3
TMOS concentrations. The layer thickness measured from TEM images well matches the
4
calculated values when assuming even distribution of the silica on all template surface. This
5
confirmation further underlines the mechanism given in Figure 3, and (b) Nitrogen (N2)
6
adsorption-desorption isotherms and BJH pore size distribution plots of hollow silica with 6.2
7
nm in shell-thickness. Adsorption and desorption volumes show a significant hysteresis,
8
typical for the presence of large cavities in a material.
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Representative nitrogen adsorption-desorption isotherms and the corresponding pore
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volume and size distribution of the hollow silica particles with 6.2 nm in shell-thickness was
11
shown in Figure 6(b). Hollow silica samples with different shell-thickness 13.2 and 7.1 nm
12
were also prepared (detail results are given in the Figure SI-8, Supporting Information
13
(SI.8)). Typical values for the surface area, total pore volume, pore size, core diameter and
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shell-thickness of the prepared hollow silica particles were summarized in Table SI-6 in the
15
supporting information. The specific surface area of the hollow silica particles was obtained
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to be 90 m2 g-1 (Figure SI-8 (a)), when the TMOS concentration of 0.08 mol L-1 was used. In
17
addition, along with the increasing of TMOS concentration 0.04 to 0.025 mol L-1, the specific
18
surface area of hollow silica was increased to 203 m2 g-1 (Figure SI-8 (b)) and 215 m2 g-1
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(Figure 6(b)), respectively. The pore volume and pore size also increased from 0.45 to 1.25 15 ACS Paragon Plus Environment
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cm3 g-1 and 19 to 23 nm with decreasing TMOS concentrations. The large pores were
2
obtained due to the formation of voids between the shells of particles.
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3 4
3.4 Optical properties of hollow silica particles with controllable shell-thickness
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Figure 7. (a) Hollow silica is nearly fully transparent and stable over weeks in dispersion:
7
photograph of the samples containing PS, silica-coated PS, hollow and dense silica particles
8
with different sizes, (b) Optical transmittance light spectra of the samples, (c) Transparent
9
stability of hollow silica particles (i.e. Dp =181 nm and L = 6.2 nm) as a function of
10
measuring time, and (d) Optical transmittance light spectra of the prepared hollow silica
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particles with different shell-thickness. Inset of Figure 7(d) shows the correlation between
12
shell-thickness and optical transparency of hollow silica at shorter (λ = 380 nm) and longer (λ
13
= 680 nm) wavelength, respectively. 16 ACS Paragon Plus Environment
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The digital photograph and optical transmittance light spectra of the sample solutions
2
containing PS particle, silica-coated PS particle, dense silica particle, and hollow silica
3
particle prepared at a concentration of 0.1 wt% are shown in Figure 7(a and b). Hollow silica
4
with diameter of 181 nm was transparent. In contrast, samples containing dense silica
5
particles with a similar diameter (i.e.172 nm) were opalescent. Sample of PS (i.e. 185 nm)
6
and silica-coated PS particles (i.e.195 nm) were completely opaque. The dispersion stability
7
and optical properties were measured over 5 days, as shown in Figure 7(c), where the
8
transmittance of the hollow silica remained around 90% in the visible region of 400 and 800
9
nm (length of light path ~1 cm), respectively. The correlation between hollow silica particle
10
and their shell-thickness were given in the inset of Figure 7(d). The transmittance intensity
11
mostly decreased at shorter wavelengths (λ = 380 nm), when the shell-thickness of hollow
12
silica was increased. In contrast, at longer wavelength, the thickness of the shell did not alter
13
significantly on the transmittance intensity. The highest transparency among all of the hollow
14
silica samples is 97.1% at longer wavelength (λ = 680 nm) and obtained from the prepared
15
particle with the thinner shell of 6.2 nm.
16 17
3.5 Thermal conductivity of PES/hollow silica composite film
18
Figure 8 (a) shows a digital photograph of a PES/hollow silica composite film
19
containing 2.5 wt% of hollow silica particles. The calculated percent volume of silica particle
20
using the corresponding material densities (i.e. hollow silica: 0.3 g cm-3, PES: 1.37 g cm-3) is
21
around 12 vol%. The film is transparent (i.e. absorption edge: 370 nm) and has a relatively
22
homogeneous thickness (35 µm). The morphology and structures of the composite film at the
23
top and bottom surface were shown in the Figure 8 (b) and (c). The film surfaces were
24
homogeneous and flat on both sides. Observation of the cross-section (Figure 8 (d))
25
confirmed the existence of hollow silica particles. 17 ACS Paragon Plus Environment
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1 2
Figure 8. Hollow silica particle can be implemented into polymer films. Photograph of the
3
succeed transparent composite film of PES/hollow silica (see image of (a), the film is
4
indicated with the dotted line), the results of SEM images are shown in: (b) view from the top
5
surface, (c) view from the bottom surface, and (d) cross sectional surface and the inset shows
6
high magnification image. The values of thickness and thermal conductivity of films are
7
given in Table 2.
8 9
The resulting thermal conductivity of pure PES and PES/hollow silica composites film were summarized in Table 2. The resulting thermal conductivity of PES/hollow silica
10
composite film (0.03 ± 0.005 W m-K-1) is obviously lower than that of pure PES (0.09 ± 0.02
11
W m-K-1) film. This result indicates that the hollow silica particles produced from TMOS
12
have a potential to become a cost and energy effective low thermal conductivity additive even
13
in application where transparency is a key material attributed.
14 15
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Table 2. Thermal conductivity of PES/hollow silica composite films. Sample
Particle Hollow silica/PES Thickness of Thermal conductivity size mass ratio composite film of composite film
Polyether-sulfone (PES) only PES/hollow silica*
a
[W m-K-1]
b
[nm]
[wt%]
-
0
33 ± 1
0.09 ± 0.02
200
2.5
35 ± 5
0.03 ± 0.005
[µm]
2 3 4 5 6 7
*Transparent composite film containing 2 g of PES and 0.05 g of hollow silica was tested for three times at a steady state period of at least 30 min. Hollow silica with 181 nm in size was used for film preparation, (See Table SI-4 for the synthesis condition). Additionally, a pure PES also was measured for comparison. The error a b values of and are calculated by the standard error mean and measured three times.
8
4. CONCLUSIONS
9
Hollow silica particles from TMOS with controllable size and shell thickness were
10
successfully prepared in MeOH-water system via liquid-phase synthesis using PS particles as
11
a template. The size and shell thickness of hollow silica particles were controlled by changing
12
the size of PS particle and the concentration of TMOS. A high concentration of MeOH (i.e.75
13
to 90 wt%) was required to produce hollow silica particles. The measured shell-thickness of
14
hollow silica particles as function of the TMOS concentrations was in a good agreement with
15
theoretical calculation. Hollow silica particle with the shell-thickness of 6.2 nm displayed a
16
high light transmittance intensity of 97.1%, in the wavelength at 680 nm. The thermal
17
conductivity of the PES/hollow silica composite film (silica composition, 2.5 wt%, film
18
thickness, 35 ± 5 µm) shows a low value (0.03 ± 0.005 W m-K-1) if compared to the pure
19
PES film. These results indicate that the prepared hollow silica particle has a high potential to
20
be used for flexible optical devices, thermal insulator, yet visible-transparent composite film
21
applications.
22
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ASSOCIATED CONTENT
2
Supporting Information
3
The Supporting Information is available free of charge on the ACS publication website.
Page 20 of 25
4 5
AUTHOR INFORMATION
6
Corresponding Author
7
*E-mail:
[email protected] 8 9
ACKNOWLEDGMENTS
10
This work was supported by JSPS KAKENHI Grant Numbers 26709061 and ETH Zürich.
11
We also gratefully acknowledge the Ministry of Education, Culture, Sports, Science and
12
Technology (MEXT) of Japan for providing a scholarship (L.E.).
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Table of Contents (TOC) Graphic
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