Article pubs.acs.org/Langmuir
Improved Dielectric and Mechanical Properties of Polystyrene− Hybrid Silica Sphere Composite Induced through Bifunctionalization at the Interface Thottunkal S. Sasikala, Bindu P. Nair, Chorappan Pavithran, and Mailadil T. Sebastian* Materials Science and Technology Division, National Institute for Interdisciplinary Science and Technology (NIIST), Council of Scientific and Industrial Research (CSIR), Thiruvananthapuram-695019, Kerala, India S Supporting Information *
ABSTRACT: Hybrid silica spheres (HS) of size 270−350 nm with vinyl and aminopropyl surface groups were incorporated in polystyrene (PS), and its effect on dielectric properties, coefficient of thermal expansion (CTE), and strength of PS−HS composite was studied. Incorporation of HS in PS followed a decrease in the dielectric constant from 3.2 for PS to 2.6 for composite with 7.5 vol % HS. The decrease in the dielectric constant was attributed to (i) increased interfacial porosity, (ii) formation of anhydrous HS having low dielectric constant, during hot processing of the composites, and (iii) dispersion and preservation of the anhydrous HS in the hydrophobic matrix. The dielectric constant of the composites with HS content up to 7.5 vol % does not vary much with temperature in the range from −20 to 65 °C. These composites also exhibited reduced CTE and improved flexural strength/stiffness due to good interfacial bonding through HS vinyl groups and dispersion of the filler in the matrix. The dielectric loss increased with HS content, and the loss measured for 7.5 vol % PS−HS composite was 6 × 10−3, as compared to 10−4 for PS. At HS loading above 7.5 vol %, the tendency of HS to agglomerate and form percolated structure lead to an increase in the dielectric constant and decrease in the mechanical properties of the composites.
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dielectric polymer composites.1,7,18 POSS exhibits a very low dielectric constant of ∼2 due to the nanoporosity (∼0.5 nm) of the cube−cage structure and high thermal stability.7 In a previous paper, we reported the synthesis of HS spheres of POSS−siloxane composition by poly-co-condensation of vinyland (aminopropyl)triethoxysilanes in an ethanol/water mixture.19 The hybrid silica spheres (HS) thus obtained is amphiphilic in nature due to the presence of hydrophilic amino groups and hydrophobic vinyl groups. While the vinyl group, which is compatible with hydrophobic polymers like polystyrene, would provide good interfacial adhesion/bonding in their composites, incompatibility with the hydrophilic groups would lead to interfacial porosity. As a result, the composite is likely to show improved mechanical properties and reduced dielectric constant. The reduction in dielectric constant can also be expected from the internal porosity of HS. This instigated us to probe the possibilities. During the present investigation, polystyrene−hybrid silica composites have been prepared and the variation of dielectric properties of the composites with hybrid silica content is discussed in this paper. Thermal and flexural properties of the composites are also presented.
INTRODUCTION The rapidly developing telecommunication industry demands low dielectric constant materials for microelectronic packaging and also as radio and microwave frequency substrates.1−4 The speed of the signal passing through the dielectric medium is inversely proportional to the square root of the dielectric constant, and signal strength in the substrate material weakens with frequency. Hence, the dielectric constant and loss tangent of the base substrate play a vital role when designing high frequency circuits.2 Low dielectric constant (εr) materials have been known to decrease power dissipation, resistance− capacitance (RC) delays, and cross-talk noise when incorporated in the device systems.1 Varieties of materials including polymers,5,6 polysilsesquioxanes,7,8 and organic−inorganic hybrids9−13 have been investigated for this purpose. The dielectric constant of polymers can be lowered by techniques such as incorporation of fluorinated substituents,14 thermal degradation of labile blocks in copolymers,15 and introduction of porosity taking advantage of the low dielectric constant of air.16,17 The porosity introduced in the case of polymer− inorganic hybrids can be the filler−polymer interfacial porosity or the structural porosity obtained through cross-linking of polymer segments with the reactive functional groups of the filler or the internal porosity of the filler itself. Polyhedral oligomeric silsesquioxane (POSS) and POSS− siloxane hybrids have been widely explored for making low© 2012 American Chemical Society
Received: February 21, 2012 Revised: May 12, 2012 Published: May 22, 2012 9742
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EXPERIMENTAL SECTION
Materials. (3-Aminopropyl)triethoxysilane (99%, AS) and vinyltriethoxysilane (97%, VS) were purchased from Aldrich Chemicals. Absolute ethanol (spectroscopic grade) was from S. D. Fine Chem Limited. Polystyrene was purchased from Nikunj Industries, Mumbai, India, and toluene from Fischer Chemicals. Millipore-grade water was used. Methods. HS of size in the range of 250−370 nm were synthesized by the method reported elsewhere.19 Typically, a mixture of VS and AS in 3:1 mol ratio was diluted with ethanol−water mixture (v/v = 14/1) to a solution concentration 0.45 M, and the solutions were aged at ambient conditions for a minimum of 7 days. HS of desired size were obtained by diluting the siloxane solution with ethanol to a silane concentration of 0.045 M and then drop-casting and drying on a glass plate. The spheres thus obtained were postcured at 100 °C for 12 h in an air oven. The method adopted for making PS−HS composite was solution-blending of PS and HS in toluene initially, followed by hot pressing the blend, with a view to preserve the hydrophobic vinyl and hydrophilic aminopropyl groups on HS and also its spherical morphology. PS−HS blends for composites were prepared by mixing solutions of PS and dispersions of HS in toluene by stirring, evaporating the solvent by heating under stirring, and then postdrying in a hot air oven at 110 °C for 24 h. HS content was varied in the range of 0−11 vol %. Composites were prepared by hot pressing the blends in a suitable steel mold at a temperature of 170 °C and pressure of 2 MPa for about 1 h and ejecting the samples after cooling the mold to room temperature. Test samples of pure HS and PS were also prepared by the above process. Dielectric properties up to 1 MHz were measured using the parallel plate capacitor method at different temperatures varying from −20 to 65 °C. This method involves sandwiching a thin sheet of the material between two metal electrodes to form a parallel plate capacitor and measuring the dielectric properties with the aid of an LCR meter (LCR HiTESTER, HIOKI 3532-50). Fractographs of the samples were taken in a scanning electron microscope (JEOL-JSM 5600 LV). The samples were prepared by breaking the composite after dipping in liquid nitrogen and coating the fractured surfaces with gold. The density of HS was measured using a pycnometer (Micromeritics Accupyc 1330 gas pycnometer) and that of composites by the Archimedes method using liquid paraffin. Moisture absorption by HS and the composites was studied by measuring the weight gain by moisture-free samples (kept at 110 °C overnight) upon exposure to the atmosphere (ambient temperature of ∼30 °C and relative humidity of ∼80%) for 24 h, using a chemical balance of accuracy of ±0.1 mg. The coefficient of thermal expansion was measured in a thermomechanical analyzer (TMA-60 H, Shimadzu) using cylindrical samples of diameter 6 mm and height 8 mm and heating them in the range of 25−120 °C. Flexural tests were performed on rectangular samples (30 × 10 × 2 mm3) using an INSTRON (5500R) and the test was run under displacement control mode at a cross-head speed of 1 mm/min.
Figure 1. SEM image of hybrid silica spheres.
Figure 2. Variation of dielectric constant and dielectric loss with HS loading.
composites with HS loading in the range of 0−11 vol %. The dielectric constant initially decreased from 3.2 for PS to 2.6 for the composite with HS loading of 7.5 vol % and then increased to 3.1 at 11 vol % loading. On the other hand, the dielectric loss gradually increased from the order of 10−4 to the order of 10−3 with the HS content in the range of 0−11 vol %. The loss measured for the composite having the lowest dielectric constant of 2.6 was 6 × 10−3. Pure PS as well as 7.5 vol % HS-filled PS composites were also characterized at 5 GHz using the split post dielectric resonator (SPDR) method. For PS, the dielectric constant and loss measured at 5 GHz were 2.8 and 2 × 10−4, and for the composite, the values were 2.3 and 3 × 10−3, respectively. Fractographs of the composites with HS loading of 7.5 and 11 vol % revealed that at 7.5 vol % loading the spheres were well-dispersed in the PS matrix, whereas at 11 vol % loading they were seen to be agglomerated and formed a percolated structure (see the Supporting Information). Generally, the dielectric constant of polymer−ceramic composites shows a sharp increase at the onset of forming a percolated structure.20 The variation of dielectric constant with HS loading along with the fractographs of the composites suggests that HS tends to agglomerate and form a percolated structure at loading above 7.5 vol %. In nanocomposites of anisotropic fillers such as silica tube21 and mesoporous silica particles,22 percolation was observed at a very low filler loading of about 3 wt %. Through the initial solution blending, the HS can be distributed uniformly in the PS matrix without agglomeration up to a critical loading of 7.5 vol %, and any method that prevents this initial uniform distribution of HS can lead to a percolated
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RESULTS AND DISCUSSION HS were formed by coprecipitation and self-assembly of POSS and incompletely condensed siloxanes to form layered structure within the sphere.19 Figure 1 shows the SEM image of the spheres. HS possessed a density of 1.25 g cm−3. The postdried spheres were stable in solvents and to sonication, indicating that the incompletely condensed siloxanes undergo further condensation and form stable network. The spheres were dispersible in nonaqueous solvents like toluene due to a high vinyl to amino (lipophilic/hydrophilic) ratio of 3, so that preparation of PS−HS blends for composites of desired composition was possible by the solution blending process. HS compacts by hot pressing showed a dielectric constant of 3.5 and dielectric loss of 0.03 at 1 MHz. Figure 2 shows the variation of dielectric constant and dielectric loss of PS−HS 9743
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where εp, εc, εs, and εa represent dielectric constant of PS, composite, HS (experimental dielectric constant of 3.5 for HS was used for calculating the theoretical dielectric constant of the composites), and air, respectively, and Vp, Vs, and Va represent volume fractions of PS, HS, and interfacial air, respectively. Interfacial air volume was calculated using the equation23
structure at a low loading itself. Percolation at a higher level of filler loading in PS−HS composite can also be due to the close packing made possible by the HS spheres. The dielectric constant measured for HS alone (3.5) was higher than that of PS (3.2). Accordingly, the composites were expected to show an increase in dielectric constant with an increase in HS content. The decrease in dielectric constant indicated the presence of substantial amounts of porosity in the composites. Figure 3 shows the variation of experimental and theoretical
ρct (1 − V ) + ρa V = ρce
(3)
where V is the volume fraction of air in the composite and ρct and ρce respectively are the theoretical and experimental densities of composite. Figure 4 shows the variation of experimental and theoretical dielectric constant as a function of HS loading. It can be seen
Figure 3. Variation of density and interfacial porosity of PS−HS composites with HS loading. Figure 4. Plot of experimental and theoretical dielectric constant of PS−HS composites.
densities of the composites with increasing HS content. The theoretical density was calculated using the rule of mixture equation ρc = ρp Vp + ρs Vs
that the plot of experimental values against HS loading does not match with that of theoretical values. The experimental values of the dielectric constant were significantly lower than the theoretical values. POSS−siloxane hybrid systems have been reported to show low dielectric constant in the range of 2.1−2.6.24 However, a higher dielectric constant of 3.5 was found for HS. It was observed that HS absorbed moisture on exposure to atmosphere due to the presence of hydrophilic amino groups. At an ambient temperature of ∼30 °C and relative humidity of ∼80%, HS was saturated with a relatively high amount of moisture (3 wt %) within a few hours. It is well-known that the presence of moisture (εr = 80) increases the dielectric constant of materials.25 However, PS−HS composites were prepared at an elevated temperature of 170 °C, which favored evaporation of moisture absorbed by HS and formation of anhydrous HS. Analysis of moisture absorption characteristics of the composites showed interesting results (See Supporting Information). The composites with HS loading up to 7.5 vol % did not show appreciable moisture absorption, whereas those with higher loading showed moisture absorption, which increased with an increase in HS content. This implies that, in a well-dispersed state, the hydrophobic matrix acts as a barrier to moisture absorption by the anhydrous HS that was formed during hot-pressing. In contrast, the tendency of HS to agglomerate and form percolated structure favors the moisture absorbed by HS spheres at the composite surfaces being diffused into the composite through HS−HS interaction. Calculation of the dielectric constant of HS by Bruggeman’s equation and using experimental dielectric constant of the composites yielded a value of 2.45, which is well within the
(1)
where ρc, ρp, and ρs respectively are densities of PS−HS composite, PS, and HS, and Vp and Vs respectively are volume fractions of PS and HS. PS showed a density of 1.03 gcm−3 and the density measured for HS was 1.25 g cm−3. While the theoretical density linearly increased with increase in HS content, the experimental density initially decreased to a value of 0.90 g cm−3 at 7.5 vol % HS loading and then increased to 0.99 g cm−3 at 11 vol % HS loading. The corresponding porosity values were 14.6% and 7.3%, respectively. The porosity could be attributed to interfacial porosity, since the porosity increased proportionately with HS content for the well-dispersed systems. Interfacial area increases as the filler loading increases; hence, there was an increase in interfacial porosity and decrease in density of the composite up to 7.5 vol % HS loading. At higher loading, the tendency of HS to form agglomerates favored fusion of the spheres during hot-pressing. This resulted in a reduction in interfacial area and hence a reduced interfacial porosity and increased density. Knowing that the composites contained three components, i.e., PS, HS, and pores (interfacial air), the dielectric constant of the composites (dispersed systems, i.e., up to 7.5 vol % HS loading in the present case) were calculated using the Bruggeman’s equation22 ⎛ εp − εc ⎞ ⎛ ε − εc ⎞ ⎛ ε − εc ⎞ ⎟⎟ + Vs⎜ s Vp⎜⎜ ⎟ + Va⎜ a ⎟=0 ⎝ εs + 2εc ⎠ ⎝ εa + 2εc ⎠ ⎝ εp + 2εc ⎠
(2) 9744
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range reported for POSS−siloxane hybrids.24 This may also confirm the formation of anhydrous HS during hot-pressing and its preservation in the composites with HS loading below percolation threshold. Thus, the decrease in dielectric constant of the composites with HS loading up to 7.5 vol % can be attributed to the following: (i) increased interfacial porosity, (ii) in situ formation of anhydrous HS of low dielectric constant, and (iii) dispersion and preservation of the anhydrous HS in the matrix. Figure 5 shows the frequency dependence of dielectric constant of PS−HS composites in the frequency range from 1
temperature, which may be due to moisture absorption. It has been reported that the dielectric constant of epoxy nanocomposites increased with increasing temperature due to the rapid thermal motion of dipolar polarization at elevated temperatures,28 and the increase in dielectric constant was more significant for samples after water treatment. It has also been observed that at a particular frequency, the value of dielectric constant continued to increase as the relative water content increased in the epoxy laminates.29 Variation of the coefficient of thermal expansion (CTE) of PS−HS composites with HS content in the temperature range of 30−120 °C was studied (see Supporting Information). The temperature range was chosen so that the test covers the Tg of polystyrene (94 °C). Incorporation of HS resulted in sharp reduction of CTE from 100 ppm/°C for PS to 55 ppm/°C for the composite with 7.5 vol % HS loading and thereafter at a lower rate to 50 ppm/°C at 11 vol % loading. Generally, CTE of composites lowers with an increase in filler content, and the composites with strong interface exhibit an additional reduction of CTE.30 Good interfacial bonding due to interaction between HS vinyl groups and the PS matrix along with increased interfacial area due to dispersion of HS in the matrix causes sharp reduction in CTE of composites with HS loading up to 7.5 vol %. Figure 7 shows the flexural strength and modulus of PS−HS composites. Both strength and modulus increased with HS
Figure 5. Variation of dielectric constant of PS−HS composites with frequency.
kHz to 1 MHz. The dielectric constant of the composites do not vary much with frequency, except at very low frequencies, where dielectric constant of composites are slightly higher due to interfacial polarization of amino and residual hydroxyl groups.26 It is well-known that the dielectric constant of materials tends to decrease gradually with increasing frequency, because the response of the electronic, atomic, and dipolar polarizable units varies with frequency.27 This behavior can be attributed to the frequency dependence of the polarization mechanisms. At a fixed temperature, the dielectric constant of PS−HS composite decreases only slightly with increasing frequency. Figure 6 shows the temperature dependence of the dielectric constant in temperature range from −20 to 65 °C. It can be seen that the composites with HS loading up to 7.5 vol % show dielectric constant independent of temperature. Beyond 7.5 vol %, the dielectric constant increased with
Figure 7. Variation of flexural strength and modulus of PS−HS composites with HS loading.
loading up to 7.5 vol % and then decreased at higher loading. The strength increased from 38 to 49 MPa and modulus from 3310 to 3879 MPa by loading PS with 7.5 vol % HS. The dispersion of filler in the matrix and the filler−polymer interface play important roles in determining the properties of composites.31 Generally, interfacial porosity adversely affects the mechanical properties of composites. The increased strength/modulus of the composites suggests coexistence of good interfacial bonding and interfacial porosity, and that improvement by interfacial bonding overcomes the adverse effect of porosity. Agglomeration of HS leads to reduction in strength/modulus of the composites with HS content above 7.5 vol %. In PS−HS composites, coexistence of interfacial porosity and interfacial bonding is attributed to the presence of hydrophilic amino groups and hydrophobic/lipophilic vinyl groups at HS surfaces. The hydrated amino group retards interfacial contacts
Figure 6. Variation of dielectric constant of PS−HS composites with temperature. 9745
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CONCLUSIONS Polystyrene-bifunctionalized hybrid silica sphere composites were prepared (0−11 vol %) and the composite with 7.5 vol % HS exhibited a low dielectric constant of 2.6, a low coefficient of thermal expansion of 55 ppm/°C, a good flexural strength of 49 MPa, and a flexural modulus of 3.9 GPa as compared to that of PS (εr = 3.2, CTE = 100 ppm/°C, flexural strength and modulus of 38 MPa and 3.3 GPa, respectively). The decrease in dielectric constant is attributed to (i) high interfacial porosity, (ii) in situ formation of anhydrous HS of low dielectric constant of 2.45 during hot-pressing of the composites, and (iii) dispersion and preservation of the anhydrous HS in the hydrophobic matrix. Good interfacial bonding is also observed in the composites and it improved the strength and modulus. The coexistence of large interfacial porosity and good interfacial bonding originated from the dual functionality of HS having hydrophilic amino groups and lipophilic vinyl groups.
between HS and PS and, along with evaporation of the absorbed moisture during hot-pressing of composite, promotes interfacial pore formation. On the other hand, the vinyl group which is compatible with PS promotes interfacial bonding. In order to prove this, pure silica (mean particle size ∼0.4 μm and density 2.6 g cm−3 from Aldrich Chemicals) was surface modified with varying compositions of vinyl and aminopropyl silanes, and its effect on interfacial porosity, dielectric constant, and mechanical property of PS−silica composites was studied. Silica was silane-treated by dispersing calculated amounts of the powder in silane solutions of desired AS/VS compositions in alcohol−water mixture (1:1 v/v) and stirring for overnight. The treated powder, collected by repeated centrifugation and washing with alcohol−water mixture, was dried at 80 °C in a vacuum oven. Silica particles were treated with solutions of VS, silane compositions of VS:AS ratio of 3:1, 1:1, 1:3, and AS. The total silane used for the treatment was 1% by weight of silica. Polystyrene−silica composite samples with 3.7 vol % silanetreated silica were prepared for the analysis. The variation in density and porosity, dielectric constant and loss, and diametrical compressive strength of the composites with variation in silane composition was analyzed (see Supporting Information). As expected, the composite prepared using VS-treated silica exhibited a density close to the theoretical value. The density of the composites with VS/AS-treated silica decreased with an increase in AS content so that AS-treated silica showed the least density. The porosity increased from 0.9 to 16.6% by varying the VS:AS ratio from 1:0 to 0:1, confirming that AS promotes interfacial porosity. AS-treated silica resisted dispersion in PS matrix, since the treatment more favored silica−silica interaction than silica−PS interaction. Its composite showed the highest porosity due to the combined effect of interfacial porosity and formation of porous agglomerates in the composite. As a result, the dielectric constant of the composite decreased from 3.2 to 2.8 by varying the VS:AS ratio from 1:0 to 1:3 and then increased to 2.9 for AS-treated silica composite. A significant effect was not observed on dielectric loss; however, the loss slightly increased from 3 × 10−3 to 8 × 10−3. Composites with VS-treated silica showed a maximum compressive load at break of 1903 N, which decreased to 1645 N by varying the VS:AS ratio from 1:0 to 1:3, confirming that the vinyl group promotes interfacial bonding. A drastic reduction in compressive load at break to 1210 N was observed for composites with AS-treated silica due to high porosity and agglomeration of the particles. The results undoubtedly support the arguments made for PS−HS composites. The test using surface-treated silica was carried out mainly to prove the results obtained for PS−HS composites, and the subject is under detailed investigation by the authors. Also, HS of POSS− siloxane composition is seen more effective than surfacemodified silica for incorporating porosity and reducing the dielectric constant of the composites due to contribution of the internal porosity of POSS. This was observed by comparing the air volume and dielectric constant of the composites containing an identical volume fraction of HS and silica surface treated with the same composition of silanes that was used for preparing HS (see Supporting Information). The results of the present study also suggest that low dielectric constant polymer−ceramic composites with improved mechanical properties can be obtained by appropriate surface modification of the ceramic particles, which promotes coexistence of interfacial porosity and interfacial bonding.
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ASSOCIATED CONTENT
S Supporting Information *
SEM fractographs of PS−HS composites with 7.5 and 11 vol % HS loading, moisture absorption of PS−HS composites with HS loading, variation of CTE of PS−HS composites with HS loading, plot of density and porosity of PS−silica composites vs VS/AS composition for silica surface modification, dielectric constant and dielectric loss of PS−silica composites vs increase in AS content in VS/AS composition, diametrical compressive load of PS−silica composites vs increase in AS content in VS/ AS composition, and comparison of air-volume fraction and dielectric constant of PS−HS and PS−silica composites with the same volume fraction of inorganic loading. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +91471 2515294. Fax: 0091-471-2491712. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Mr. M. R. Chandran, NIIST-Trivandrum, for SEM. Thanks are also due to CSIR, Delhi, India, for providing a Senior Research Fellowship to B.P.N. and T.S.S.
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