Organic-inorganic hybrid materials have been recognized as a new class of advanced materials because of their versatile synthetic approaches and molecular tailing properties.1-10 The present inventors are particularly interested in the hybrid materials for optical applications, such as high refractive index materials, optical waveguides, antireflection films, etc. For such optical applications, the inorganic domains must be well controlled around 20 nm or less to maintain the optical transparence besides their high refractive index characteristics.
Polymer-titania hybrid materials have been extensively studied as high refractive index materials, including poly(silsesquioxanes),7-8 poly(methyl methacrylate) (PMMA),9-15 and polyimide (PI)16-18, etc. For controlling the titania domain and maintaining good miscibility with polymer moiety in the hybrid materials, sol-gel processing is commonly employed strategy to prepare such hybrid materials. In these systems, the major challenge is to generate specific intermolecular interaction with each other in order to get homogeneous hybrid optical films. The present inventors have successfully prepared trialkoxysilane-capped PMMA-titania hybrid optical thin films by an in situ sol-gel process.9-10 The prepared hybrid thin films exhibit tunable high refractive index in the range of 1.505-1.876 and very high optical transparence in the visible region. Wang et al further investigated the crystallinity of titania and nonlinear optical behavior after further hydrothermal treatment.11,13 However, the thermal stability of the PMMA moiety is limited and restricted the applications on optoelectronic devices. Replacing the PMMA moiety in the hybrid materials with a highly thermal stable polymer, such as polyimide, may resolve the problem.
The formation of polyimides containing titania by the incorporation of titanium alkoxide compounds into the precursor polyamic acid (PAA) has been reported.19 For titanium alkoxide compounds (Ti(OR)4), however, they are very reactive due to the presence of highly electronegative OR groups that render titanium very susceptible to nucleophilic attack and result uncontrolled aggregation. Therefore, a nonhomogeneous distribution of the titania clusters with markedly high concentration and large titania particles with sizes>100 nm, are often observed. In addition, when titanium alkoxide compounds are blend with multifunctional acids of PAA, a fast gelation would occur due to the coordination reaction. To overcome these shortcomings, various approaches to prevent the aggregation and phase separation of titania in the evolving polyimides have been investigated. Chemical modification of titanium with chelating ligands, such as acetylacetone (acac), is one of the commonly employed methods to control the condensation pathway of titanium alkoxide compounds. Various coupling agents, such as 3-aminopropyl trimethoxysilane and 3-methacryloxypropyl trimethoxysilane, are the other ways to stabilize the titanium precursors. These agents are desired to generate the covalent bonding force to connect the organic-inorganic moieties using the heterogeneous condensation (Ti—O—Si). Although polyimides-titania hybrid materials with well-controlled morphologies have been successfully fabricated from the above studies, several drawbacks exist. The additional coupling agents and chelating ligands would still remain in these materials after curing to affect important thermal/mechanical/optical properties. Besides, polyimide-titania materials could be also synthesized using the concept of site isolation.19 The alkoxides of titanium are known to react with carboxylic acids, leading to the replacement of one or more alkoxides by carboxylate groups. Thus, it is reasonable to expect that the titanium precursors would bind to the polymeric backbones. The coordination of polyimide carbonyl groups to the titanium particles could prevent the aggregation of titania. However, note that there is an upper limit (14%) for the titanium concentration due to the multifunctional acid groups of PAA and the coordination number of titanium is greater than one. Over the limited value, a tridimensional gel is formed. In acrylic-titanium polymers, the polymer containing a well-know ratio of acidic functions is used to get the organotitanium acrylate polymers.20-22 An excess of titanium alkoxide compounds should be used to obtain the esterified organotitanium acrylate polymers, which are soluble in the organic solvents.
Thus in the present invention, a new synthetic method was developed to prepare synthesize polyimides-nanocrystalline titania hybrid materials with a relatively high titania content.