High refractive index films of polymer nanocomposites
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Solutions of PbS particles and gelatin were used for the preparation of nanocomposites by a spin-coating process. This allows for the preparation of nanocomposite films with controlled thickness, e.g., between 40 nm and 2 /mm for a film containing 45 wt. % PbS. Surface roughness and film thickness were investigated by surface profilometry and scanning electron microscopy (SEM). The refractive index at 632.8 nm can be expressed by a linear function of the volume fraction of PbS in the range of 0 to 55 vol. % PbS. In this range, the refractive index increases from 1.5 to 2.5 with increasing PbS ratio and belongs, therefore, to the highest refractive indices known for polymeric composite materials.
I. INTRODUCTION Composites between polymers and inorganic particles in the micrometer range are often opaque.1'2 Light scattering, responsible for the opacity, can be suppressed either by using materials with nearly matching refractive indices3 or by decreasing the filler's dimensions to a range below ca. 50 nm.1'2'4'5 Therefore, nanocomposites of polymers and inorganic colloids can act as optically homogeneous materials with modified optical properties.1'2'4'6'7 Nanocomposites containing lead sulfide, for instance, can act as optical filters,4 nonlinear optical,6 or "ultrahigh" refractive index materials. 7 The refractive index of polymers is usually between 1.3 and 1.7.8 Examples of higher refractive index are, e.g., 2.12 for poly(thiophene)9 and 1.7245 for [CH2CH = CHCH2 N + (CH 2 CH3)2(CH 2 ) 6 N + (CH 2 CH 3 )2]« • n-Hgl4 .10 Aromatic polyamides can exhibit (isotropic) refractive indices up to 2.05 11 or 2.07.12 The refractive index of polymers can be enhanced by the addition of colloidal particles with high refractive index,7 e.g., PbS that exhibits (at least in crystalline form) a refractive index on the order of 4 in a wide wavelength range. 13 The highest refractive index for polymeric materials, i.e., pure polymers or polymer composites, has been obtained to date, to our best knowledge, for nanocomposites of lead sulfide and poly(ethyleneoxide).7 They exhibit refractive indices of up to 2.9 at 632.8 nm and of up to 3.0-3.4 in the whole range of 1000—2500 nm. These nanocomposites were prepared by adding a hydrogen sulfide solution to a solution containing the polymer and Pb(n) ions. The genesis of colloidal PbS particles and the precipitation of the nanocomposite material take place simultaneously and immediately after addition of hydrogen sulfide. This method allows for the preparation of nanocomposites 1742
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J. Mater. Res., Vol. 8, No. 7, Jul 1993
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filled to ca. 50 vol. % PbS; however, attempts to prepare nanocomposites with a markedly lower PbS-content, e.g., by variation of the initial Pb2+/pctly(ethyleneoxide) ratio, failed. Also, experiments aimed at preparing nanocomposite films were inconclusive because of poor film quality. In this report, we describe the preparation of polymer nanocomposite films with controlled layer thickness and refractive index,
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