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. ■ 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 (ε r ) materials have been known to decrease power dissipation, resistance- capacitance (RC) delays, and cross-talk noise when incorpo- rated in the device systems. 1 Varieties of materials including polymers, 5,6 polysilsesquioxanes, 7,8 and organic-inorganic hybrids 9-13 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- 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 vinyl- and (aminopropyl)triethoxysilanes in an ethanol/water mix- ture. 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. Received: February 21, 2012 Revised: May 12, 2012 Published: May 22, 2012 Article pubs.acs.org/Langmuir © 2012 American Chemical Society 9742 dx.doi.org/10.1021/la300743e | Langmuir 2012, 28, 9742-9747