Polymer/TiO2 Hybrid Materials Prepared via in-situ Sol-Gel Reaction: Synthesis and Characterization
Date Issued
2006
Date
2006
Author(s)
Chen, Hung-Jen
DOI
en-US
Abstract
Abstract
In this research, various functional organic polymer/nano-sized inorganic particle hybrids were prepared via in-situ sol-gel reaction. The hybrid would have best mutuality, and then optimal performance, if the synthesis was operated in a suitable recipe and there were interactions among organic, inorganic and solvent. In this study, poly [2-methoxy-5-(2’-ethylhexyloxy)-p-phenylenevinylene] (MEH-PPV) and poly(acrylic acid) (PAA) were chosen as organic polymer components, TiO2 was chosen as inorganic component, and 2-chlorophenol and 1-butanol were utilized as solvent for the MEH-PPV and PAA system, respectively. There are three major parts in this dissertation.
In the first part, nanosize-hybrid colloids of poly(acrylic acid) (PAA)-titania (TiO2) were synthesized based on an in-situ sol-gel technique using titanium isopropoxide (TIP) as precursor in the butanol solution of PAA. Two synthetic pathways to the fabrication of hybrids were utilized, involving adding titanium precursor after or before the PAA was fully dissolved. Different phenomena of phase separation and gelation were observed in these two synthetic pathways, but both yielded stable colloidal solutions after further heating. Various molar ratios of PAA/titanium isopropoxide/water and synthetic routes were employed to examine the interaction between PAA and titania. The hybrid materials were characterized by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electronic microscopy (SEM). The FTIR results verified the successful formation of the chelation bond between PAA and titania. TGA demonstrated that the thermal stability of pure PAA was improved by chelating with titania. DSC showed that the PAA strongly interacted with titania, leading to the confinement of the thermal motion of PAA chains. The SEM results revealed that the preparation method and the molar ratios of PAA/titanium precursor/water considerably affected the size and shape of PAA-titania hybrid aggregates, and the extent of aggregation. As the molar ratio of PAA to TIP increased, the extent of aggregation would decrease. Moreover, if the preparation method was carried out under the adding of TIP before the PAA was fully dissolved, and then nanorod-like titania could be synthesized by adding additional water.
According to the result of SEM photographs, the size of hybrid aggregates would be smaller as increasing the molar ratio of PAA to titania because the higher amount of PAA would decrease the extent of aggregation and inhibit the growth of titania. A new sol-gel kinetic model for partially chelated titanium alkoxide by PAA was proposed. Since the reaction rate of hydrolysis and condensation of titanium alkoxide was too fast to get the rate constant from experimental measurement, the proposed rate equations and rate constants in this part were based on the rate constants of silica and all discussed chemical reaction of titanium alkoxide from literatures, and then some reasonable assumptions and neglects were made to obtain the complete and appropriate rate equations and rate constants. Based on this sol-gel kinetic model that considered the effect of chelation, the concentration of water, Ti-OR, Ti-OH, and (Ti-O)-Ti bonds during sol-gel reaction could be calculated. The variation of number of particles at different degree of chelation showed that the particle number increased with increasing the degree of chelation. Finally, the size of hybrid aggregates at different degrees of chelation could be qualitatively described; thus, higher degree of chelation resulted in lower size of hybrid aggregates. The simulated results from this model were in agreement with the observation from SEM photographs.
In the second part, the influence of polarity and cohesive energy of solvents on the interaction between inorganic particles and solvents in the synthesis of stable TiO2 colloid was investigated. Because TIP must be chelated before sol-gel reaction to obtain a stable TiO2 colloid for all selected solvents, the starting material of TIP was choosen to be acetyl acetone (acac) chelated; then, the sol-gel reaction proceeded in different selected solvents after slowly adding water. In this part, the selected solvents were tetrahydrofuran (THF), acetone, 1-butanol, chloroform, toluene and hexane. A clear and stable TiO2 colloid could be obtained from THF, acetone and 1-butanol; turbid but stable from chloroform; precipitated from toluene and hexane. The solvent effect on the colloidal particle size and stability were explained according to Hansen Solubility Parameters (HSP). Finally, the Hansen Solubility Parameters of acetyl acetone modified TiO2 were estimated and a useful correlation chart could be plotted to predict the other suitable solvents for the synthesis of stable TiO2 colloid.
In the third part, homogenous organic (MEH-PPV)/inorganic (nanosized titania) hybrids were successfully synthesized via a one-step in-situ sol-gel technique using TIP as precursor. Interactions among titanium isopropoxide, 2-chlorophenol and MEH-PPV were identified to be one of the key points for decreasing the degree of aggregation and phase separation in hybrids. Moreover, the solvent, 2-chlorophenol, was benefit for the electrical property of MEH-PPV film because it was an aromatic solvent. In this part, three subjects were in discussion. First and second, various weight ratios of MEH-PPV/TIP/H2O/2-chlorophenol with different molecular weight of MEH-PPV were employed to examine the optical and physical properties of MEH-PPV/TIP(titania) hybrid. The observation of MEH-PPV gelation, FTIR and XPS results verified that the interaction existed in MEH-PPV and TIP. UV-vis absorption showed that the addition of TIP would decrease the conjugation length of MEH-PPV. Photoluminescence (PL) revealed that almost all hybrids had red-shift in solution state owing to the aggregation of MEH-PPV. But after spinning, the aggregation of MEH-PPV would be destroyed and separated by the centrifugal force. The XRD measurement exhibited that if the conversion efficiency of TIP to titania was higher, the crystal patterns were clearer. The TGA results indicated that the TIP(titania) had well mutuality with MEH-PPV from the thermal degradation behavior of hybrids became a single characteristic curve. The SEM results revealed that the weight ratios of MEH-PPV/TIP/H2O/2-chlorophenol and molecular weight of MEH-PPV considerably affected the morphology of MEH-PPV/TIP(titania) hybrid aggregates. In the condition of 4 equivalent moles ratio of water and 5 mg/ml concentration, the amount of TIP could be as higher as 89 wt% and still had homogenous MEH-PPV/TIP(titania) hybrid, whatever molecular weight of MEH-PPV was 600,000 or 50,000.
Except for the adjustment of composition, the annealing treatment could be also helpful for the preparation of a homogenous MEH-PPV/titania film by the conversion of MEH-PPV/TIP film to MEH-PPV/titania film. So, in the third subject, the effects of annealing treatment on the optical properties and morphology of synthesized hybrid film were also investigated. Blue shift in UV-vis absorption was observed after annealing treatment at 2300C for 1 hr. It was because higher temperature and time of annealing treatment would give rise to the thermal decomposition and a coil-like structure of MEH-PPV chain. In contrast, the annealing treatment would promote the movement and migration of MEH-PPV chains and induce the chains to pack into a low-energy structure, thus lowering the energy band-gap and red-shifting the PL emission of hybrids after annealing treatment.
Subjects
有機/無機混成材料, 二氧化鈦, 原位溶膠-凝膠法
organic/inorganic hybrid, titania, in-situ sol-gel reaction
Type
thesis
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