Preparation and Characterization of Silica/Polyamide-imide Nanocomposite Thin Films
© The Author(s) 2010
Received: 28 June 2010
Accepted: 26 July 2010
Published: 7 August 2010
The functional silica/polyamide-imide composite films were prepared via simple ultrasonic blending, after the silica nanoparticles were modified by cationic surfactant—cetyltrimethyl ammonium bromide (CTAB). The composite films were characterized by scanning electron microscope (SEM), thermo gravimetric analysis (TGA) and thermomechanical analysis (TMA). CTAB-modified silica nanoparticles were well dispersed in the polyamide-imide matrix, and the amount of silica nanoparticles to PAI was investigated to be from 2 to 10 wt%. Especially, the coefficients of thermal expansion (CET) continuously decreased with the amount of silica particles increasing. The high thermal stability and low coefficient of thermal expansion showed that the nanocomposite films can be widely used in the enamel wire industry.
Polyamide-imide (PAI) is a kind of thermoplastic resin, has good high-temperature resistance, outstanding mechanical properties and excellent oxidative stability, all of which have led PAI to have been widely used with electronic materials, adhesives, composite materials, fiber, and film material [1–8]. Comparing with the polyimide and polyamide, the PAI own the better process ability and heat-resistant properties. The basic studies of the synthesized PAI have attracted increasing interest for a wire-coating material with a thermal resistant property. However, an increasing number of breakdown cases have been reported when a higher surge voltage exists in some devices. Therefore, the development of an inorganic/organic nanocomposite insulating material should be widely discussed to improve the electrical life of enameled wires [9–11].
Actually, there are so many factors affecting the properties of hybrid composites, such as the particle size, size distribution, and filler content. In addition, the inorganic particle shape, surface structure, and mechanical properties of a filler (stiffness, strength, etc.) all play an important role in the synthesizing of inorganic/organic composite materials. Especially, the bond strength between inorganic particles and polymer matrix should be improved, which always was influenced by the type of dispersion aid or coupling agent used [12–18].
Silica nanoparticles as a very important inorganic material have emerged as an area of intense current interest motivated because of their special physical and chemical properties, such as their small size, strong surface energy, high scattered performance, and thermal resistance [19–23]. However, the applications of silica nanoparticles are largely limited because of their high energetic hydrophilic surface, which causes the silica nanoparticles to be easily agglomerated. Fortunately, this problem could be resolved by using some surface modification methods with different surfactant agents. In other words, the strong interface adhesion between the organic matrix and silica nanoparticles is a key to the application of silica nanoparticles as fillers.
Jadav et al.  had successfully synthesized the silica/polyamide nanocomposite film via interfacial polymerization process using two types of silica nanoparticles of size about 16 and 3 nm, respectively. The nanocomposite films exhibit superior thermal stability than the pure polyamide membranes. In this work, the authors observed that silica nanoparticles loading could significantly modify the polyamide network structure, pore structure and transport properties. The excellent membrane performance in terms of separation efficiency and productivity flux was discussed. Zhang et al.  prepared a novel isometric polyimide/silica hydrid material through sol–gel technique. At the beginning, 3-[(4-phenylethynyl) phthalimide]propyl triethoxysilane (PEIPTES) was synthesized to be used for modifying nano-silica precursor. Then the isomeric polyimide/silica hybrid material was produced by using isomeric polyimide resin solution and the modified nano-silica precursor after heat treatment process. The isomeric polyimide/silica composite has much better thermal properties and nano-indenter properties than those of isomeric polyimide.
In this work, cationic surfactant CTAB was chosen to modify the silica nanoparticles and the amount of CTAB to silica nanoparticles was changed from 0 to 3 wt%. After the surface modification process, the CTAB-modified silica nanoparticles had the better compatibility with PAI polymer matrix and the CTAB-modified silica nanoparticles could be well dispersed into the PAI matrix, even when the amount of silica to PAI reached to 10 wt%. The thermal properties were obviously improved, and the decomposition temperature was increased with the amount of silica increasing.
Silica nanoparticles were purchased from Sigma–Aldrich Company, and the size of the nano-silica was 10–20 nm. Commercial polyamide-imide powder (PAI, Torlon AI-10) was purchased from Solvay Company (USA). Cetyltrimethyl Ammonium Bromide (C16, CTAB, 99% purity, ACROS) was used as a surface modification agent. N,N-Dimethylformamide (DMF) was anhydrous and was purchased from Sigma–Aldrich Company. Sodium hydroxide (NaOH) was purchased from Sam Chun Co (Korea). Ethanol was a chemical reagent and was purchased from DUKSAN Pure Chemicals. Distilled and deionized water was used throughout the work.
Bar coater 08, coating rods are used primarily to apply a variety of coatings or emulsions to a multitude of substrates. Stainless steel rods are wrapped very tightly with stainless steel wire and the diameter of wire are 0.2 mm. In this work, the silica/PAI composite films were cast by bar coater to make the films more uniform.
Preparation and Surface Modification of Silica Nanoparticles
In a typical experiment, 100 ml deionized water and 1.000 g silica particles were added to the flask, and the solution was adjusted to pH 8 by the addition of 0.1 M NaOH. Then the silica nanoparticles were modified at 65°C under constant stirring with CTAB added into the system. In order to improve the dispersal state of the silica nanoparticles in the PAI matrix, the amount of CTAB to silica nanoparticles was changed from 0 to 3.0 wt%. After the surface modification process, the modified nano-silica particles were collected by suction filtration and then dried at 90°C for 6 h.
Preparation of Silica/Polyamide-imide Nanocomposite Films
The fracture surface of composites films were determined by a Hitachi Co. S-4700 scanning electron microscope (SEM). Before SEM imaging, the samples were sputtered with thin layers of Pt–Pd. A Scinco STA S-1500 simultaneous thermal analyzer was applied to analyze the thermal stability of the nano-silica/PAI composite films. The samples were heated from 30 to 800°C at 10°C/min in an air atmosphere. The coefficients of thermal expansion (CET) of silica/PAI composites films were evaluated by a Q 400 EM (USA) thermomechanical analysis (TMA) (5°C/min from 25 to 300°C, 50 mN). All the samples were of 3 × 16 mm, which were cut from original films using a razor blade.
Results and Discussion
FT-IR Spectra of the Silica/PAI Composite Films
SEM Micrographs of Silica/PAI Composite Films Fracture Surface
Thermal Stability of Silica/PAI Composite Films
Coefficient of Thermal Expansion (CTE) of Silica/PAI Composite Films
The silica/PAI inorganic/organic nanocomposite films were successfully synthesized by the simple ultrasonic blending, after the silica nanoparticles were modified by CTAB. In the fracture surface micrographs of composite films, the silica nanoparticles were found well dispersed in the PAI polymer matrix. The silica nanoparticles were still monodispersed without any agglomerations when the amount of silica nanoparticles to PAI reached 10 wt%. The thermal stability of PAI was improved, and the decomposition temperature was increased when the amount of silica nanoparticles was increased. The lower coefficient of thermal expansion of composite films could reduce the peeling and cracking at the interface between the polymer film and copper. In this system, the high thermal stability and low coefficient of thermal expansion showed that the silica/PAI nanocomposite films can be widely used in the enamel wire industry.
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