Interface modification of clay and graphene platelets reinforced epoxy nanocomposites: a comparative study

The interface between the matrix phase and dispersed phase of a composite plays a critical role in influencing its properties. However, the intricate mecha-nisms of interface are not fully understood, and polymer nanocomposites are no exception. This study compares the fabrication, morphology, and mechanical and thermal properties of epoxy nanocomposites tuned by clay layers (denoted as m-clay) and graphene platelets (denoted as m-GP). It was found that a chemical modification, layer expansion and dispersion of filler within the epoxy matrix resulted in an improved interface between the filler mate-rial and epoxy matrix. This was confirmed by Fourier transform infrared spectroscopy and transmission electron microscope. The enhanced interface led to improved mechanical properties (i.e. stiffness modulus, fracture toughness) and higher glass transition temperatures (Tg) compared with neat epoxy. At 4 wt% m-GP, the critical strain energy release rate G1c of neat epoxy improved by 240 % from 179.1 to 608.6 J/m2 and Tg increased from 93.7 to 106.4 �C. In contrast to m-clay, which at 4 wt%, only improved the G1c by 45 % and Tg by 7.1 %. The higher level of improvement offered by m-GP is attributed to the strong interaction of graphene sheets with epoxy because the covalent bonds between the carbon atoms of graphene sheets are much stronger than silicon-based clay

Correlation of catalytic activity with infrared, Si-29 MAS NMR and acidity data for HCL-treated fine fractions of montmorillonites

The <2 mu m fractions of SAz-1 (Cheto, Arizona, USA) and JP (Jelsovy Potok, Slovakia) montmorillonites were treated with 6 M HCl for 30, 300 and 900 minutes at 95 degrees C. The materials obtained were investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), thermogravimetry (TG), infrared (IR) spectroscopy, and Si-29 MAS NMR spectroscopy. The number of acid sites was determined from the thermal desorption of cyclohexylamine and the catalytic activity was evaluated by reacting 2,3-dihydropyran with methanol to yield the tetrahydropyranyl ether. All the investigative methods utilised, which each provided discrete evidence for the depopulation of the octahedral sheet, confirmed that treatment of SAz-1 for 300 and 900 minutes caused complete destruction of the original structure. In contrast JP was more resistant to acid attack and the treatments resulted in materials with different levels of structural decomposition. Samples of JP and SAz-1 treated for 30 minutes exhibited Bronsted acidities commensurate with the exchange capacities of the parent materials and this was reflected in their catalytic activity. A reduction in both acidity and catalytic activity was observed after longer treatment times and this was attributed to the presence of fewer exchange sites due to the depopulation of the octahedral sheet. The Ca back-exchanged samples did not catalyse the ether forming reaction

Characterization of moderately acid-treated, size-fractionated montmorillonites using IR and MAS NMR-spectroscopy and thermal-analysis

The <2 mu m fractions of Cheto (Arizona, USA) and Jelsovy Potok (Slovakia) montmorillonites have been treated with 1 mol dm(-3) H2SO(4) for periods up to 6h at 95 degrees C. The materials obtained were investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), infrared (IR) spectroscopy, and Si-29 and Al-27 magic-angle spinning (MAS) NMR spectroscopy. Both clays were partially decomposed under the conditions utilized, but the Cheto montmorillonite, which contained a higher proportion of octahedral magnesium, was more susceptible to acid attack. XRD was insensitive to this level of acid attack but XRF, IR and Si-29 MAS NMR revealed the depopulation of the octahedral sheet. The number of acid sites was determined from the thermal desorption of cyclohexylamine and the catalytic activity was evaluated by reacting 2,3-dihydropyran with methanol to yield the tetrahydropyranyl ether. The test reaction was a more sensitive indicator of changes in clay acidity than the values determined by desorption of cyclohexylamine from acid sites

Correlation of catalytic activity with infrared, Si-29 MAS NMR and acidity data for HCL-treated fine fractions of montmorillonites

The <2 mu m fractions of SAz-1 (Cheto, Arizona, USA) and JP (Jelsovy Potok, Slovakia) montmorillonites were treated with 6 M HCl for 30, 300 and 900 minutes at 95 degrees C. The materials obtained were investigated by X-ray fluorescence (XRF), X-ray diffraction (XRD), thermogravimetry (TG), infrared (IR) spectroscopy, and Si-29 MAS NMR spectroscopy. The number of acid sites was determined from the thermal desorption of cyclohexylamine and the catalytic activity was evaluated by reacting 2,3-dihydropyran with methanol to yield the tetrahydropyranyl ether. All the investigative methods utilised, which each provided discrete evidence for the depopulation of the octahedral sheet, confirmed that treatment of SAz-1 for 300 and 900 minutes caused complete destruction of the original structure. In contrast JP was more resistant to acid attack and the treatments resulted in materials with different levels of structural decomposition. Samples of JP and SAz-1 treated for 30 minutes exhibited Bronsted acidities commensurate with the exchange capacities of the parent materials and this was reflected in their catalytic activity. A reduction in both acidity and catalytic activity was observed after longer treatment times and this was attributed to the presence of fewer exchange sites due to the depopulation of the octahedral sheet. The Ca back-exchanged samples did not catalyse the ether forming reaction

Characterization and catalytic activity of acid-treated, size-fractionated smectites

Five layered silicates in which the octahedral sheet contained differing amounts of Al, Mg, Fe, and Li were acid leached using acid concentrations and treatment temperatures selected to produce materials in which the octahedral sheet was depopulated in a controlled, stepwise (yet comparable) manner. SAz-1 and JP are dioctahedral smectites with octahedral compositions rich in Mg and Al, respectively. SWa-1 is a ferruginous smectite and ST an iron-rich beidellite. The fifth mineral was a trioctahedral hectorite which contains almost exclusively octahedral Mg. The Bronsted acidity and catalytic activity of the resulting materials were highest for the samples prepared with the mildest acid treatments but decreased as the octahedral sheet became increasingly depleted. Only the hectorite exhibited no catalytic activity despite the proven existence of Bronsted acid sites. The elemental composition of the starting material did not appear to make a significant contribution to the catalytic activity for the chosen test reaction although it does play a key role in determining the severity of the activation conditions required for the optimization of catalytic activity. Fourier transform infrared (FTIR) spectroscopy was found to be as sensitive to structural acid attack as Si-29 magic angle spinning NMR and the octahedral depletion (measured using FTIR) correlated well with the acidity determined from thermal desorption of cyclohexylamine

Acidity and catalytic activity of mildly acid-treated Mg-rich montmorillonite and hectorite

XRD, FTIR spectroscopy, cyclohexylamine desorption, potentiometric titrations and catalytic activity in an ether-forming test reaction have been used to investigate mildly acid-treated montmorillonite (SAz-1) and hectorite. The number of acid sites obtained from the cyclohexylamine desorption agreed with the accepted cation exchange capacity values for all samples. No evidence of any free protons was found in the potentiometric titration curves, which proved that complete autotransformation of the acid-treated samples had occurred over a much shorter period than previously recorded. Acid sites associated with Al3+ (Fe3+) released from the SAz-1 structure during acid treatment and subsequent autotransformation were of sufficient strength to cataryse the test reaction and produce tetrahydropyrstnyl ether in 80% yield. The complete absence of catalytic activity in the acid-treated hectorite samples confirmed that acid sites associated with interlayer Mg2+ and Li+ were unable to catalyse the test reaction

Acid-activated organoclays: Preparation, characterization and catalytic activity of acid-treated tetraalkylammonium-exchanged smectites

The catalytic activity of acid-treated clays for reactions involving polar substrates is optimized when the acidity of the surface and the swelling ability of the catalyst are at a maximum. In contrast the activity for reactions using nonpolar substrates is best when the surface area is maximized and the catalyst presents an essentially hydrophobic surface which serves to attract the nonpolar reagents. We have investigated the catalytic ability of acid-activated organoclays (AAOCs) which should provide reasonable levels of acidity, hydrophobicity, and swelling ability for use with nonpolar reagents. A range of organoclays containing tetramethylammonium, dodecyltrimethylammonium, or octadecyltrimethylammonium cations at the 25, 50, or 100% exchange level were prepared and subjected to selected acid-leaching procedures at either 20 or 95 degrees C. The activity of these AAOCs for the conversion of a-pinene to camphene was investigated. The conditions used for acid leaching seldom removed extensive amounts of organocation, and the yields (40% conversion to camphene) compared favorably with those reported for pillared clays. Acid-leached tetramethylammonium clays were the most active with yields four times higher than those for the corresponding parent clay. Acid-leached dodecyltrimethylammonium and octadecyltrimethylammonium clays were only active when the organocations occupied 25% of the exchange sites