Synthesis and characterization of maleimide-functionalized polystyrene-SiO2/TiO2 hybrid nanocomposites by sol–gel process
© Ramesh et al.; licensee Springer. 2012
Received: 10 May 2012
Accepted: 9 June 2012
Published: 27 June 2012
Maleimide-functionalized polystyrene (PSMA-SiO2/TiO2) hybrid nanocomposites were prepared by sol–gel reaction starting from tratraethoxysilane (TEOS) and titanium isopropoxide in the solution of polystyrene maleimide in 1,4-dioxane. The hybrid films were obtained by the hydrolysis and polycondensation of TEOS and titanium isopropoxide in maleimide-functionalized polystyrene solution followed by the Michael addition reaction. The transparency of polymer (PSMA-SiO2/TiO2) hybrid was prepared from polystyrene titanium isopropoxide using the γ-aminopropyltriethoxy silane as crosslinking agent by in situ sol–gel process via covalent bonding between the organic–inorganic hybrid nanocomposites. The maleimide-functionalized polystyrene was synthesized by Friedel-Crafts reaction from N-choloromethyl maleimide. The FTIR spectroscopy data conformed the occurrence of Michael addition reaction between the pendant maleimide moieties of the styrene and γ-aminopropyltriethoxysilane. The chemical structure and morphology of PSMA-SiO2/TiO2 hybrid nanocomposites were characterized by FTIR, nuclear magnetic resonance (NMR), 13 C NMR, SEM, XRD, and TEM analyses. The results also indicate that the inorganic particles are much smaller in the ternary systems than in the binary systems; the shape of the inorganic particles and compatibility for maleimide-functionalized polystrene and inorganic moieties are varied with the ratio of the inorganic moieties in the hybrids. Furthermore, TGA and DSC results indicate that the thermal stability of maleimide-functionalized polystyrene was enhanced through the incorporation of the inorganic moieties in the hybrid materials.
KeywordsPolystyrene maleimide Optical transparency Nanocomposites
Nanostructure hybrid organic–inorganic composites have attracted considerable attention recently, both from the perspectives of fundamental research and their technological applications[1–3]. One approach for preparing these materials is via sol–gel process. In the inorganic matrix, components are formed in situ through hydrolysis and condensation of metal oxide precursors, while the organic matrix undergoes simultaneous polymerization. However, the sol–gel approach is limited by the evaluation of volatile biproducts and concomitant shrinkage when the hybrid is processed at elevated temperature[4, 5]. The sol–gel method is one of the most suitable methods to prepare the silica gel through siloxane linkages by the hydrolysis and condensation reactions. The silica hybrid materials greatly depend on the interaction between the organic polymers and inorganic alkoxides and their homogeneous distribution through hydrogen bonding, covalent bonding, formation of stereo regular complex, and π-π and ionic interactions[6–8]. The Michael addition reaction in polymer synthesis and applications in emerging technologies, composites, coatings, and optical coatings are outlined in the article. Organic–inorganic hybrid nanocomposites are a new category of high-performance materials which is currently an area that has received extensive interests for other matrix of polyimide (PI)-SiO2[10, 11], PI-Al2O3, and PI-TiO2[13–15]; these composites have been successfully synthesized. Many researches have been focusing on developing the PI-inorganic hybrid nanocomposites, such as the use of dianhydride and diamine to synthesize the PI matrix and the use of metal alkoxides to provide the inorganic network. Nanocomposites can be prepared through different processes. Among those successful ones[16–19], the in situ polymerization and gelation reaction is a type of processing in which the inorganic phase was generated from the metal alkoxide precursors through hydrolysis and condensation reactions that took place simultaneously with the polymerization reaction. Wang and Chang and other researchers[20, 21] prepared the hybrid nanocomposite films of TiO2 in the PI matrix from 2,5-bis (4-aminophenyl)3,4-oxadiazole, 4,4′-oxydiaphthalic anhydride, and titanium precursors by an in situ sol–gel process. The titanium precursor was prepared by mixing tetraethyl titanate (TET) and acetyl acetone (acac) in the solution of alcohol and water. These nanocomposites exhibited fairly good optical transparency at 40 wt.% of TiO2 content. The transmission electron microscope (TEM) results showed that the size of the TiO2 particle increased from 10 to 40 nm[20–22]. Several successful examples of in situ polymerization and gelation reaction processes could be found in the literature. The polymer nanocomposites were prepared using poly(amic acid) solution by the condensation of 3,3′,4,4′-benzonphenone tetra carboxylic dianhydride and 4,4′-oxydianhydride 4,4′-oxydianiline (ODA) then added TET followed by thermal imidization from PI/TiO2 hybrid nanocomposites. They reported that nanosized inorganic TiO2 network dispersed in PI films at an average diameter of 1.5 nm at TiO2 content of 12 wt.%[22–24]. The poly(amide-imide)/TiO2 (PAI/TiO2) nanocomposite films obtained 4,4′-oxy(phenyl trimellimide) and ODA using tert-butyl benzoic acid as the mono functional endcapper; with TET, these composite films exhibited high transparency and had well-dispersed nanosized TiO2 in the PAI matrix[25–27]. The size of the TiO2 network increased from 5 to 50 nm when the TiO2 content was increased from 4% to 18% by weight. While large sized, nanosized inorganic particles made the nanocomposite films, transparent in such the particle size effect, hydrogen bonding between the amide group in the PAI and the hydroxyl groups on the inorganic oxides played an important role in making such small particle size possible[28–30]. According to these reports, the sol–gel process is one of the most commonly used processes of preparing titanium dioxide. However, in the process for preparing titanium dioxide, there are a few technical problems that must be resolved. First of all the titanium alkoxide is a highly reactive compound when it is exposed to moisture, and white precipitate will form rapidly. In order to prepare nanosized TiO2 suspension solution, the pH values and the use of chelating agents are crucial in the reaction steps. It is reported elsewhere in the literature that[32–34] prepared high refractive index thin films of pyrometallic dianhydride titania hybrid materials from dianhydride, γ-aminopropyltriethoxy silane (γ-APS), and titanium isopropoxide via sol–gel process followed by spin coating and multi-step backing; through adjustment in the concentration and reaction time, they were able to produce thin films of hybrid inorganic content at 59.2%. Therefore, the transparent polymer hybrid used for lenses has about 90% transmittance/cm. Typical materials PMMA, PC, polystyrene, and styrene-MMA copolymers have been used for the lenses in projection television and lenses for using compact disk. The plastic materials in optical disk circuit need to be highly transparent, resistant to heat, low impurities, and must have a low double refractive index and low fluidity. However, no materials with these properties have yet been found or developed. Semiconductor-mediated photocatalytic oxidation has been accepted as a promising method for the removal of organic contaminants from waste water. Among the semiconductors employed, TiO2 is known to be a good photocatalyst because of its high photosensitivity, non-toxicity, easy availability, strong oxidizing power, and long-term stability[35, 36]. Existing bulk semiconducting materials possess low surface area, less absorption property, and fast electron–hole recombination. In order to circumvent such problems, researchers are interested in recent days in the synthesis of nanomaterials for environmental applications[34–36]. The polymer metal oxide hybrid nanocomposites contained bimetallic dopants of the titanium and barium oxides. The precursors of metal oxides were formed from tetrabutyl titanate (TBT) and barium carbonate, which were then mixed with poly(amic acid) solution followed by thermal imidization. The synthesized hybrid nanocomposites with inorganic particles are smaller than 50 nm, and the dielectric constant increased from 3.5 to 4.2 when the inorganic content increased from 1 to 10 wt.%. Polyimides are considered to be one of the most important super-engineering materials due to their thermal stability as well as the superior mechanical properties at elevated temperature[37, 38]. Since the polyimide/silica hybrid materials have been prepared successfully through the sol–gel process, more attention were given to the field[34–40]. Recently, metal-containing polyimide/titania hybrids were also prepared. The key challenge for the preparation of the hybrid materials is how to control the phase separation between the organic and inorganic moieties. The phase behavior is connected with the interaction between the organic segment and the inorganic network in the hybrids. Hydrogen bonding or covalent bonding is usually used to prevent phase separation. Recently, the sol–gel process is a novel technique for the preparation of nanocrystalline TiO2. It has been demonstrated that through the sol–gel process, the physico-chemical and electrochemical properties of TiO2 can be modified to improve its efficiency. It provides a simple and easy means of synthesizing nanoparticles at ambient temperature under atmospheric pressure, and this technique does require simple setup. Since this method is a solution process, it has all the advantages over other preparation techniques in terms of purity, homogeneity, felicity, and flexibility in introducing dopants in large concentrations, stoichiometry control, ease of processing, and composition control. Through the sol–gel process, the growth of TiO2 colloids in sub-micrometer range can be effectively controlled by hydrolysis and condensation of titanium alkoxides in aqueous medium. Nanosize TiO2 used so far in photocatalytic applications has been prepared by hydrolysis of titanium precursors followed by annealing, flame synthesis, and hydrothermal and sol–gel processes. In most studies, attempts have been made to enhance the photocatalytic activity of TiO2 only by varying the calcination temperature and, in a few cases, aging period and drying conditions[40–43]. Among the various techniques under the development, the sol–gel process has been found to be extremely suitable as it enables good control of composition and optical behavior of the final nanomaterials. In recent years, silica-titania hybrid organic–inorganic materials have been studied as a promising system for photonic applications[41–43], and low loss wave guide based on the organically modified alkoxides has been fabricated by the sol–gel process. Therefore, the sol–gel integrated optics is beginning to show potential applications, and it stimulates the studies on optical wave guide material which have been explored for a long time and such sol–gel materials used for optical applications. In the area of advanced oxidation technology, titanium dioxide semiconductor photocatalysis has been widely studied because of its potential application in air clean-up and water purification. TiO2 is largely used as photocatalyst due to its beneficial characteristics: high photocatalytic efficiency, physical and chemical stability, low cost, and low toxicity[40–42]. TiO2/SiO2 composites are very promising in the field of heterogeneous photocatalysis since they could provide simultaneously enhanced photocatalytic and thermal properties compared to pure TiO2 photocatalyst. It has been reported that photocatalytic reactivity of TiO2/SiO2 nanocomposites is highly dependent on the Ti/Si ratios. The photocatalytic activity and mechanical stability were reported to improve by the addition of about 50% SiO2[40–48]. Moreover, in the present work, an attempt has been made to develop the silica/titania-incorporated transparent maleimide-functionalized polystyrene to improve the thermochemical and optical characteristics with γ-APS, TEOS, and titanium isopropoxide through the Michael addition reaction at relatively low temperature through the sol–gel method. Furthermore, the organic–inorganic transparent hybrid nanocomposite materials were characterized by FTIR, nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), differential scanning calorimeter (DSC), optical images, scanning electron microscope (SEM), and TEM analyses.
Maleimide, phosphorous trichloride, stannic chloride, γ-APS, TEOS, titanium tetraisopropoxide, and acetylacetone, obtained from Aldrich chemical company (Sigma-Aldrich Corporation, St. Louis, MO, USA), were used as received. Polystyrene obtained from Supreme Petrochemical Ltd, Mumbai, India. The 1,4-dioxane was purified by distillation in vacuum, and other reagents were used as received from SRL, India. FTIR spectra was taken using a PerkinElmer (Model RX1, Waltham, MA, USA) spectrometer (cured samples were ground with solid KBr). 1 H (400 MHz) and 13 C (200 MHz) NMR spectra were recorded on a JEOL GSX 400 spectrometer (JEOL Ltd., Akishima, Tokyo, Japan) operating at 298 K with tetramethylsilane as an internal standard. Differential scanning calorimetry was performed using both Netzsch (Erich Netzsch GmbH and Co., Bavaria, Germany) and TA instruments (TA 2000 analyzer, New Castle, DE, USA). Cured samples (50 mg) were analyzed in open (silicon) pans at 20 K/min in N2 atmosphere. X-ray diffraction analysis (XRD) was characterized by Rigaku Corporation D/max-3 C (Tokyo, Japan). Scanning electron microscopy of fractured surfaces was performed using a JEOL JSM model 6360 microscope. The fractured surfaces of the specimens were coated with platinum and were exposed to accelerating voltage of 20 kV. The particles in solution are characterized by placing a drop of the homogeneous suspension in a TEM copper grid with a lacy carbon film and then using a JEOL 2010-F TEM at an accelerating voltage of 200 kV.
Synthesis of N-chloromethyl maleimide
Synthesis of maleimide-functionalized polystyrene
Maleimide-substituted polystyrene-silica-titania hybrid nanocomposites
Preparation of the hybrid films
Experimental details of organic–inorganic polymer hybrid sol–gel nanocomposites
0.1-M HCl in aqueous
Results and discussion
Preparation of organi-inorganic hybrid nanocomposites controlled by Michael addition reaction
In the present study, the PSMA-γ-APS and TEOS were used to prepare the transparent polymer hybrid PSMA which was synthesized by mild, Friedel-Crafts reaction between the N-chloromethyl maleimide and polystyrene in the presence of stannic chloride or TiCl4 as a catalyst. The amount of maleimide substitution in polystyrene depends on the percentage weight of N-chloromethyl maleimide added to the polystyrene. The maximum time required to achieve 16% of maleimide substitution on polystyrene is about 20 h. The percentage maleimide substitution was calculated using the integration values obtained from 1 H NMR spectrum. Figure1 illustrates the sequence of reaction involved in the preparation of transparent organic–inorganic polystyrene maleimide silica hybrid using γ-APS as cross linking agent by Michael addition reaction through in situ sol–gel process. Table1 summarizes the composition of reactants for the preparation of hybrid polymer. In order to achieve the homogeneity and transparency of the resulting hybrid polymer, suitable experimental conditions have been adopted by adjusting the concentration of both TEOS and γ-APS in the reactions. It was ascertained that the experiments conducted in the absence of γ-APS result in only turbid materials, while the homogeneous polymer hybrids were obtained in the presence of γ-APS as per experiments in Table1 which shows the optical and SEM images of the polystyrene-silica hybrids. In case of the absence of γ-APS, phase separation was clearly observed, and the silica gel domain was larger than 5 μm. On the other hand, no phase separation was observed when the experiment was carried out using γ-APS[30–35]
Control of the SiO2/TiO2 nanoparticle in the PSMA matrix
The silicic acid was prepared according to the procedure described previously. A silicic acid THF solution and a TiCl4 ethanol solution were mixed and stirred at room temperature for 3 h. The mole ratio of silica to titania was 1. Appropriate amounts of silica/titania precursor were introduced into a THF solution of poly (MMA-co-MSMA) copolymer, and the mixture was stirred mechanically at room temperature for 4 h to obtain a uniform and transparent solution. The solution was cast on a glass substrate and dried at 80°C for 5 h. Then, the samples were thermally treated at 100°C for 10 h under vacuum after drying the resulting mixture for the preparation of PMMA/silica/titania nano composites[20–25]. In this study, synthesis and characterization of polystyrene maleimide-SiO2/TiO2 hybrid nanocomposites with the formation of covalent bonding between the siloxane segment of γ-APS containing PSMA matrix and the network through the introduction of the coupling agent (γ-APS) were done. On the basis, it was expected that these interfacial covalent bonds would render the material's different properties, which will be discussed shortly. For the example, large solubility parameter difference between PI and PDMS interfacial gap that existed will lead to micro-phase separation to minimize the interfacial gap; the titanium alkoxide, such as titanium (IV) tetra ethoxide Ti(OEt)4, TET, titanium isopropoxide (Ti(ipro4)), is generally used as the starting material of TiO2 in the sol–gel process[12–14, 30–36].
FTIR spectra of the obtained polymer hybrids
Differential scanning calorimeter
Thermal properties of organic–inorganic PSMA-SiO 2 /TiO 2 hybrid sol–gel nanocomposites
T g (°C)
First degradation temperature (°C)
Second degradation temperature (°C)
Characteristic yield at 999.1 °C (%)
Optical transparency of SiO2/TiO2 hybrid nanocomposites
X-ray diffraction analysis
Scanning electron' microscope
Transmission electron microscope
In this study, the PSMA/TiO2 hybrid nanocomposite films with covalent bonding at the organic–inorganic interface have been successfully synthesized by sol–gel process. The chelating agent acetyl acetone was used to reduce the gelation rate of titanium alkoxide. The FTIR, optical transparency, and surface morphology analysis confirm the Si-O-Ti and Ti-O-Ti bondings and provided the evidence of the presence of TiO2 in the PSMA matrix. The PSMA/TiO2 hybrid nanocomposites have good transparency even at high content (up to 30%); therefore, the maleimide-functionalized polystyrene-titania-incorporated nanocomposites have been prepared by Michael addition reaction through the sol–gel process using γ-APS as a cross-linking agent. The polymer hybrid shows optical transparency and improved thermal stability compared with those of functionalized polystyrene. In the present investigation, an attempt has been made to develop hybrid nanocomposites with improved optical transparency and thermal stability through controlled hydrolysis of metal alkoxides. The hybrid materials developed with present expected materials to find the applications in optoelectronics, solar cells, and hybrid coatings for performance and longevity.
The authors would like to thank the support from the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2011–0005781) and also supported by the MKE (The Ministry of Knowledge Economy), Korea, under the ITRC (Information Technology Research Center) support program supervised by the NIPA (National IT Industry Promotion Agency)(NIPA-2012-H0301-12-2008).
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