Research of Materials Science June 2013, Volume 2, Issue 2, PP.16-22
Study on Dispersion Polymerization Process of Silica Aerogel/Polystyrene Core-Shell Composite Particles Gangqiang Geng a#, Suqing Wen b, Huan Wang c Department of Materials Science and Engineering, Chang,an University, Xi’an 710064, China Email: #agengangqiang@163.com,bsuqingwy@163.com,cwanghuan@163.com
Abstract Silica aerogel/polystyrene core-shell composite particles have been synthesized by dispersion polymerization. In the chemical synthesis microwave irradiation heating method is used to induce the polymerization. And study on the influence of the mass ratio of the silica aerogel and styrene monomer on the coating rate of the composite particles. Then study on the content of the initiator AIBN on the density of the composite particles and the conversion rate of the styrene monomer. Finally composite particle is confirmed to be a kind of strawberry shaped core-shell composite structure by SEM and energy spectrum analysis. Keywords: Silica Aerogel; Polystyrene; Core-shell Structure; Dispersion Polymerization
1 INTRODUCTION Now, polystyrene insulation board is widely used in the external wall insulation system [1]. But its inherent disadvantages, such as high thermal conductivity and poor flame retardant performance, can not be ignored. Silica aerogel [2] is a highly porous material with pore diameters in the range of 1~100nm. The porosity is up to 80% to 99.8%, which combined with the nanometer pore size makes the aerogel a highly insulating material with a thermal conductivity lower than of 0.020W/m•K. So it is a super thermal insulation material. However it is the defects such as low structural strength, high water absorption, high price that limit its application in the thermal insulation field. The core-shell composite materials[3,4], due to its distinctive physical and chemical properties，have a promising applications in the biomedical, optical, electrical, magnetic, catalytic and other fields[5-9]. In view of the above-mentioned facts about polystyrene and silica aerogel, based on organic-inorganic hybrid principle, silica aerogel/polystyrene core-shell composite particles have been synthesized by dispersion polymerization in order to achieve the effect of complementary advantages. In the chemical synthesis microwave irradiation heating method[10-14] is used to induce the polymerization of silica aerogel and styrene monomer.
2 EXPERIMENTAL 2.1 Material Absolute ethanol, silica aerogel, distilled water, standard sodium hydroxide (NaOH, 0.1N), the monomer, styrene, the initiator,2,2’-azobis (isobutyronitrile) (AIBN), and the stabilizer, PVP K30, the coupling agent, KPS.
2.2 Characterization 1) Conversion rate of the styrene monomer measurement [15] The conversion rate of styrene monomer is determined by weight method. Firstly a weighing bottle is placed in a vacuum drying oven drying to constant weight. Then put the liquid after polymerization reaction in the weighing bottle, which is put in the vacuum drying oven at 75 ℃ drying to constant weight. And obtain the total weight of polymerization reaction system. The conversion rate is calculated by the formula (2.1). - 16 http://www.ivypub.org/rms
TPS
A (B C D) 100% E
（2.1）
TPS - conversion rate of the styrene monomer; A- total weight of the polymerization reaction system after drying; Bcontent of the silica aerogel in polymerization reaction; C- content of the dispersant; D- content of the initiator ;Econtent of the styrene monomer in polymerization reaction. 2)Coating rate of the composite particles measurement [16] Put the liquid after polymerization reaction in the centrifuge for removing upper solution (unconverted styrene monomer and uncoated silica aerogel) and getting lower precipitation. Centrifugal process is 5000rpm, 30 min. The precipitation is washed with distilled water for 3~5 times and put in muffle furnace for sintering processing, in which sintering temperature is 350℃, sintering time is 120 min. Weigh out the content of the residual powder, namely the content of the silica aerogel coated in the composite particles .The coating rate is calculated by the formula(2.2).
m1 100% m2
（2.2）
η- coating rate of the composite particles; m1- content of the silica aerogel coated in composite particles; m2 - content of the silica aerogel in polymerization reaction. 3)Tap density of the composite particles Composite particles powder is sufficiently dried in a vacuum drying oven. Then put a certain amount of the powder in a standard container which is placed in the density tester, adjust vibrational frequency and time. After some time, we can get compact volume. The density is calculated by the formula (2.3). ρ= m/v
（2.3）
4)Morphology of the composite particles Hitachi S-4800 Field Emission Scanning Electron Microscope is used to check morphology of the composite particles. Energy spectrum analysis is used for composition distribution of the composite particles. 5)Thermal conductivity of the composite particles Hot Disk heat constant analyzer, Sweden TPS2500 type, is used to measure thermal conductivity.
2.3 Synthesis of the silica aerogel/polystyrene core-shell composite particles 1) A certain amount of styrene is washed with 5% NaOH solution for 3~4 times and then washed with deionized water for 2~3 times. Then a certain amount of initiator AIBN is added to it and stir well; 2) The dispersant PVP- K30, EtOH and silica aerogel are mixed with ultrasonic mixing for 15～20min. Then a certain amount of KPS was added to it and stir well; 3) The above two systems were poured into a 500ml vessel under stirring at 450 rpm and adjust the parameters of the microwave heating; 4) Put the liquid after polymerization reaction in the centrifuge with the speed of 4000rpm to separate the lower product; 5) The lower product is washed with EtOH for 2~3 times and then washed with deionized water for 2~3 times; 6) Silica aerogel/polystyrene composite particles can be got after drying for 24h at 70℃ in the vacuum drying oven.
3 RESULTS 3.1 Influence of the mass ratio of the silica aerogel and styrene monomer Experimental design is shown in Table 1. - 17 http://www.ivypub.org/rms
TABLE 1 EXPERIMENTAL DESIGN ABOUT MASS RATIO OF THE SILICA AEROGEL AND STYRENE MONOMER Test number
1
2
3
4
5
6
7
8
Mass ratio
1:7
1:9
1:10
1:15
1:20
1:25
1:28
1:30
Note: In the experiment the content of the silica aerogel is 0.2g. 70
coating rate /%
65 60 55 50 45 1:7
1:9 1:10 1:15 1:20 1:25 1:28 1:30 mass ratio
FIG.1 INFLUENCE OF THE MASS RATIO OF THE SILICA AEROGEL AND STYRENE MONOMER ON THE COATING RATE OF THE COMPOSITE PARTICLES
Figure 1 shows that the mass ratio of the silica aerogel and styrene monomer has a certain influence on the coating rate of composite particles. The coating rate of the composite particles increases as the mass ratio increases from 1:7 to 1:20. When the mass ratio reaches to 1:20, the coating rate tends to be stable.
a-1:10
b-1:20
c-1:30
FIG.2 SEM MICROGRAPHS OF THE COMPOSITE PARTICLES UNDER THE CONDITIONS OF DIFFERENT MASS RATIO OF THE SILICA AEROGEL AND STYRENE MONOMER
When the mass ratio reaches 1:10, Figure 2(a) shows that composite particles have small particle size and uneven distribution phenomenon. This indicates that silica aerogel is not well coated by PS. Most of silica beads, due to a large specific surface area and higher interfacial energy, are still in the state of aggregation. So the coating rate is low. When the mass ratio reaches 1:20, it can be seen from Figure 2(b), particle size of the composite particles with smooth and uniform surface become larger .Coating rate is significantly improved. When the mass ratio reaches 1:30, it can be seen from Figure 2 (c), the particle size of the composite particles becomes small. It is because with the increase of the styrene monomer a large number of styrene monomer was dissolved in the system. After formation and stability of the composite particles large quantity of initiator and stabilizer have been consumed. Now the remaining styrene monomer occurs self-polymerization, forming seedless structure. So the particle size becomes small. Compared with Figure 2(b), particle size in the Figure 2(c) becomes smaller. This is because polystyrene - 18 http://www.ivypub.org/rms
particles with small particle size partly cover composite particles with large particle size and SEM just shows material surface morphology.
3.2 Influence of the initiator AIBN Experimental design is shown in Table 2. TABLE 2 EXPERIMENTAL DESIGN ABOUT THE CONTENT OF THE INITIATOR AIBN Test numbers
1
2
3
4
5
6
AIBN/g
0.1
0.15
0.2
0.25
0.3
0.35
Note: the content of the silica aerogel is 0.2g in the polymerization reaction. conversion rate density 76
0.29 0.28
74
0.26 0.25
72
0.24 0.23
70
3 density/g/cm
conversion rate/%
0.27
0.22 0.21
68 0.10
0.15
0.20
0.25
0.30
0.35
AIBN/g
FIG.3 INFLUENCE OF THE CONTENT OF THE INITIATOR AIBN ON THE CONVERSION RATE OF THE STYENE MONOMER AND THE DENSITY OF THE COMPOSITE PARTICLES
As shown in Figure 3, the conversion rate of the styrene monomer increases gradually as the content of the initiator AIBN increases. When the content of the initiator AIBN increases to a certain degree, the conversion rate tends to be stable. It is because with the increase of the content of the initiator AIBN, the number of free radicals increases, namely active centers increase, which lead to the conversion rate of styrene monomer continuously being improved. As the reaction goes onďźŒthe number of polymer particles in the reaction system has been fixed. So the conversion rate tends to be stable; As shown in Figure 3, with the increase of the content of the initiator AIBN, the density of the composite particles firstly decreases and then increases. When the content of the initiator AIBN is 0.2 g, the density reaches a minimum. As the content of the initiator AIBN increases, the growth rate of free radicals increases, then polymer particles in the reaction system are too centralized in an instant and system stability fails. So the average molecular weight of polymer reduces, the viscosity of the system reduces accordingly, the density of composite particles also decreases. For the same initiator, different adding manners can lead to different results. The initiator AIBN in the experiment is added once and in two times[17]. The results show that the higher conversion rate of the styrene monomer can be got when the initiator AIBN is added in two times. Compared with adding initiator AIBN in two times, adding it once can make its initial content increases and decomposition rate accelerates, which lead to it prematurely being consumed. So polymerization reaction stops in a low conversion rate stage. Adding initiator AIBN in two times is that adding part of the initiator AIBN in the initial stage of the reaction for inducing polymerization reaction. After a period of time add the rest of the initiator in order to make residual monomer continue reacting, which make the conversion rate further improved and reach above 80%. - 19 http://www.ivypub.org/rms
3.3 Characterization of the composite particles Verification experiment was made using the optimal process parameters of the dispersion polymerization (the mass ratio of the silica aerogel and styrene is 1:20; ethanol/water ratio is 10:1; initiator AIBN (added in two times)is 0.2 g) . The properties of the composite particles are shown in table 3. TABLE 3 THE PROPERTIES OF THE COMPOSITE PARTICLES Thermal conductivity
particle size
Conversion rate
Coating rate
/(g/cm )
/(W/(m路K))
/渭m
/%
/%
silica aerogel
0.140
0.024
0.1~1
-
-
polystyrene
0.791
0.0920
10~100
79.4
-
composite particles
0.250
0.0506
1~20
78.5
70
Items
Density 3
Table 3 shows that the density of the composite particles is 0.250 g/cm3, higher than that of silica aerogel of 0.140 g/cm3, less than that of polystyrene of 0.791 g/cm3. From the side we can say that the composite particles are an aggregation of silica aerogel and PS; The thermal conductivity of the composite particles is 0.0506 W/(m路K), less than that of polystyrene of 0.0920 W/(m路K), decreased by about 50%, greatly improving the performance of the composite particles. The morphology of silica aerogel/ polystyrene composite particle was checked by SEM. As shown in Figure 4, it is similar to the strawberry shaped core-shell composite structure. The particle size of the composite particle is about 100~200nm. The surface of the composite particles is rough, which is because silica aerogel is porous structure with three-dimensional network. From Figure 4 we can observe the morphology of the composite particles, but not enough to characterize the composition distribution of the composite particles. So energy spectrum analysis is made in order to simulate the composition distribution of the composite particles by line scanning.
FIG.4 SEM MICROGRAPH OF THE COMPOSITE PARTICLE
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FIG.5 ENERGY SPECTRUM ANALYSIS OF THE COMPOSITE PARTICLE
From Figure 5 it can be seen that the composite particle is composed of three kinds of elements C, O, Si. C element appeared peak in the middle position, which is coating layer structure formed by polystyrene in the surface of the composite particles. The peaks of Si and O are not obvious. The relationship of their average content is approximately 1:2, which make further explanation to the fact that the "nuclear" of the composite particle is silica aerogel. Therefore composite particle is silica aerogel/ polystyrene core-shell structure.
4 CONCLUSIONS In this paper silica aerogel/polystyrene core-shell composite particles have been synthesized by dispersion polymerization. In the chemical synthesis microwave irradiation heating method is used to induce the polymerization. Draw the following conclusions: (1)When the mass ratio of the silica aerogel and styrene monomer is approximately 1:20, the highest conversion rate appears, about 68% ~ 75%, and the coating rate tends to be stable. (2)When the content of the initiator AIBN is 0.2g, that is the mass ratio of the initiator AIBN and silica aerogel is 1:1, we can get composite particles with low density and high conversion rate. Especially the initiator AIBN is added in two times, we can get the higher conversion rate of the styrene monomer. (3)SEM micrograph and energy spectrum analysis show that silica aerogel/ polystyrene composite particle is a kind of strawberry shaped core-shell composite structure.
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AUTHORS Gangqiang Geng was born in 1954, and received her Master degree in hot-working of matals from Xi'an highway institute, China in 1987. Now, he is a Professor at the Department of Materials Science and Engineering, Chang,an University, His research interests include materials surface engineering and nano materials research. Email: gqgeng@chd.edu.cn
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