Polyimide/silica/titania nanohybrids via a novel non-hydrolytic sol–gel route

Abstract

Polyimide/silica/titania hybrid films were prepared via a non-hydrolytic sol–gel route. Silicic acid and titanium tetrachloride were used as the precursors of silica and titania, respectively. The absorption band of Si–O–Ti bonds in FTIR spectra of the hybrid films revealed the formation of the hybrid inorganic network between SiO₂ and TiO₂. Scanning electron microscopy results indicated that the nanometer-scaled inorganic domains were homogeneously dispersed in polyimide matrix due to the introduction of silica-stabilized TiO₂ and the interactions between organic and inorganic phases. The studies on the optical properties of the hybrid films indicated the red-shift of the absorption band increased with increasing TiO₂ content, while all the hybrid films maintained their transparencies. The surface resistances of the hybrid samples decreased with increasing TiO₂ content. The thermal decomposition temperature of the ternary hybrid films decreased slightly with increasing TiO₂ content. This kind of hybrid materials may have potential application in the preparation of opto-electronic devices.

Introduction

Currently, many research efforts have been focused on the organic/inorganic nanocomposites with unique mechanical, photoelectric and thermal properties [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. There are plenty of literatures on polymer/silica hybrid materials with improved mechanical and thermal properties [12], [13], [14], [15]. On the other hand, polymer/titania nanohybrids have also attracted lots of interests due to their adjustive refractive indices and good UV shielding properties which have application in the photoelectric fields [1], [2], [7], [8], [9], [10], [11], [16], [17], [18], [19], [20]. However, the reports on polyimide/titania nanohybrid materials are limited though the polyimides (PI) have excellent thermal and photoelectric properties [16], [17], [18], [19], [20]. There lies some problems for the preparation of PI/titania nanohybrids: first, the decrease of thermal stabilities of the hybrids caused by the decomposition catalytic activity of TiO₂ and the incomplete imidization of polyamic acid (PAA), the PI precursor, induced by TiO₂ and second, the difficulty with the control of the hydrolytic reaction of TiO₂ precursors due to their high reactivity, which often results in the accumulation of inorganic particles [17], [18], [20]. It is necessary to use some chelating agents such as allyl acetylacetone to stabilize the fast condensation of titanium alkoxides, the titania precursor, and the addition of overmuch chelating agents may do harm to the resulting hybrids because the chelating agents are found difficult to be completely removed from the titanium center [21], [22].

Recently, Liu and Qiu prepared PI/silica/titania hybrid materials via a sol–gel process using tetraethoxysilane and tetrabutyl titanate as precursors for silica and titania, respectively [19], [23]. Liu's results indicated that the introduction of TiO₂ led to a decrease in the thermal stability of PI hybrid materials, and these results are in accordance with some previous reports [1], [12]. But they found that this adverse effect could be compensated by the addition of SiO₂. However, Qiu's results showed that the thermal stabilities of all the obtained PI hybrids containing SiO₂, TiO₂ and SiO₂/TiO₂ are better than pure PI samples.

In this paper, we present a new non-hydrolytic sol–gel method to prepare the PI/silica/titania nanohybrids in which the reactivity of titania precursors can be well controlled without using any chelating agents. The more abundant and affordable titanium tetrachloride and silicic acid are used as titania and silica precursors, respectively. Meanwhile, the interactions between PAA main chains and TiO₂ components decrease, which is favorable for the complete imidization of PAA. Further more; the addition of SiO₂ components can also compensate the decrease of the thermal stabilities of the nanocomposites because of the introduction of TiO₂. The structure, and thermal, optical and electrical properties of the resulting ternary nanohybrids have been studied. The results show that PI/SiO₂/TiO₂ nanocomposites have potential application in the opto-electronic devices [7], [8], [9], [10], [11], [15], [16], [17], [18], [19], [20].