In situ formation of nanosized TiO₂ domains within poly(amide–imide) by a sol–gel process

Abstract

Poly(amide–imide)/TiO₂ (PAI/TiO₂) composite films were prepared by an in situ sol–gel process. These composite films exhibited high optical transparency, having nanosized TiO₂ rich domains well dispersed within the PAI matrix. The size of the domains increased from 5 to 50 nm when the TiO₂ content was increased from 3.7 to 17.9% by weight. Hydrogen bonding interactions between the amide groups in the PAI and the hydroxyl groups on the inorganic oxide were observed. In comparison to the pure PAI polymer, the PAI/TiO₂ composite films exhibited higher glass transition temperature, an increase and flattening of the rubbery plateau modulus, and a decrease in crystallinity.

Introduction

In recent years much effort has been devoted to developing new materials created by modifying known polymers via the incorporation of a variety of inorganic additives by a sol–gel process. The resulting hybrid materials have had potential applications as abrasive-resistant coatings, catalysts, electronic and optical materials, and absorbents [1], [2], [3], [4], [5], [6], [7], [8], [9]. Hybrid organic–inorganic materials can be formed by reacting an inorganic alkoxide directly with an organic polymer or an oligomer having the appropriate functional groups, thus providing covalent linkages between the organic phase and the inorganic network [9], [10], [11], [12], [13], [14], [15], [16], [17]. In particular, polyimide–silica hybrid materials have been sucessfully prepared via this approach [13], [14], [15], [16], [17]. Alternatively, in situ polymerization of an alkoxide within a swollen polymer network can be used to form organic/inorganic composite materials without covalent crosslinks [18], [19], [20], [21], [22], [23], [24], [25], [26]. The size of the inorganic phase then depends upon the sol–gel conditions, the type of alkoxide, as well as on the nature of molecular interactions between the polymer and the inorganic oxide. High level of mixing can be achieved on a nano scale, particularly with polymers which contain functional groups such as carbonyls, hydroxyls or ether oxygens, which can form hydrogen bonds with the inorganic network [22], [23], [24], [25]. This latter method is especially interesting because it can be employed to directly modify commercially available polymer systems.

The in situ formation of an inorganic network within a polymer matrix is governed by several parameters. The composite system is prepared by co-dissolving a precursor, a tetra-functional metal alkoxide with the polymer in a common solvent. A small amount of catalyst is added to catalyze the sol–gel reactions, which can be generalized into hydrolysis, alcoxolation, and oxolation [27], [28] reactions. The sol–gel reactions of tetraethyltitanate (TET), for example, include the following reactions,

Hydrolysis

Ti(OEt)₄+H₂O → TiOH+EtOH;

Alcoxolation

Ti–OH+EtO–Ti → Ti–O–Ti+EtOH;

Oxolation

Ti–OH+HO–Ti → Ti–O–Ti+H₂O.

These reactions are concurrent and their relative rates are governed by pH, solvent, water to alkoxide ratio, concentration, type of catalyst and temperature [1], [28]. In the case of polymer matrices, whose glass transition temperature is above ambient temperature, the final structure of the inorganic network and morphology of the composite is further influenced by the relative rates of vitrification and the rate of inorganic network formation [21]. In particular, pronounced differences in structure can occur, depending on whether the processing is carried out under acidic or basic conditions [1], [27]. Based catalyzed systems with high water content, tend to consist of highly branched non-interpenetrating clusters, while acid catalyzed systems with low water content have linear or random branches. This structure results because under acidic conditions, the hydrolysis is faster than either of the two condensation reactions [1], [27].

We are particularly interested in forming homogeneous polyimide/TiO₂ or polyimide/SiO₂ composites, which could ultimately be used for gas separation applications. As we wish to use the blend approach without covalent linkages between the organic and inorganic components, we have to rely on physical interactions to achieve a highly dispersed, homogeneous system. For this reason we had to incorporate an amide repeat unit into the polyimide structure, in order to facilitate hydrogen bonding with the inorganic component. Further, aromatic poly(amide–imide)s, (PAIs), combine superior mechanical properties, high thermal stability and chemical resistance [29], [30] into one polymer. In the case of PAI/TiO₂ or PAI/SiO₂, we need to create highly dispersed, ceramic nanoclusters within the polymer matrix in order to achieve a high performance gas-separation membrane [31]. Because the point of zero charge (PZC) for TiO₂ metal oxide is at a higher pH (6.0) than that of SiO₂ (pH=2.5), and the reaction kinetics of the Ti-based alkoxide are relatively fast, at low pH conditions there are many nucleating sites with highly positively charged surfaces, leading to repulsive forces between the TiO₂ particles [28]. This means that TiO₂ domains should be much smaller than SiO₂ domains formed under similar conditions. In fact, SiO₂ domains formed within PAIs and Torgamid by the sol–gel method are typically of the order of microns [32]. Hence, in order to satisfy our objectives, we have chosen to focus on the development and characterization of the PAI/TiO₂ composite system.