| 520 |
|
|
|a Semiconductor/polymer nanocomposites are of increasing importance with their tunable properties being used as dielectric materials. This thesis focused on cadmium sulfide (CdS)/polymer nanocomposites. CdS has been combined with three polymer matrices, i.e. poly(styrene-divinylbenzene) [P(S-DVB)], poly(methacrylic acid-ethyleneglycoldimethacrylic acid) [P(MAA-EGDMA)] and sulfonated poly(styrene-divinylbenzene) [SO3H-P(S-DVB)]. CdS/P(S-DVB) nanocomposite was synthesized by in-situ polymerization in a miniemulsion system using monomer as oil-phase. CdS/P(MAA-EGDMA) nanocomposite has been synthesized by ion exchange and precipitation processes. While, the CdS/SO3-P(S-DVB) nanocomposite has been prepared by sulfonation, ion exchange and precipitation. Agglomerated nanoclusters of CdS were obtained from the above in-situ preparation methods. The structure-dielectric property relationship of the nanocomposites is elucidated by various techniques such as UV - Vis, FTIR, UV-Vis DR, TEM, SEM, XRD, impedance analyzer, AAS, EDX, thermal conductivity analyzer and thermogravimetric analysis. Dielectric properties of the CdS/polymer nanocomposites have been studied at frequencies of 0.1 - 1,000 kHz. The decrease in dielectric constant was found in CdS/SO3H-P(S-DVB) nanocomposite. Considering that the SO3H-P(S-DVB) has very high dielectric constant due to its proton mobility, the replacement of the proton of SO3H-P(S-DVB) with the CdS particles caused the decrease in dielectric constant of the nanocomposite. Interestingly, an increase in the dielectric constant was also observed in CdS/P(S-DVB) and CdS/P(MAA-EGDMA) nanocomposites compared to that of CdS nanoparticles or pure polymers. It is also demonstrated that the unusual enhancement of dielectric constant of CdS/P(MAAEGDMA) depended on the concentration of CdS nanoparticles. The occurrence of strong interfacial interaction between CdS nanoparticles with P(MAA-EGDMA) polymer has been proved by FTIR spectra. One explains that an increase in dielectric constant is due to the increase of interfacial interaction among CdS nanoparticles and also CdS nanoparticles with polymer. These interactions increase the mobility of charge carriers and polarizability of electron. Based on the results of this study, it can be suggested that dielectric properties of CdS/polymer nanocomposites can be explained by the following unique properties i.e. nanosize of CdS particles, semiconductor property of CdS, the interfacial interaction between CdS nanoparticles and polymer and intrinsic properties of polymer. These conclusions lay the foundation for developing new synthetic strategies for designing new dielectric materials by varying the size, concentration and distribution of CdS nanoclusters in various polymer matrices.
|
