Synthesis of linear low density polyethylene with a nano-sized silica supported Cp2ZrCl2/MAO catalyst

A nano-sized silica supported Cp₂ZrCl₂/MAO catalyst was used to catalyze the copolymerization of ethylene/1-hexene and ethylene/1-octene to produce linear low-density polyethylene (LLDPE) in a batch reactor. Under identical reaction conditions, the nano-sized catalyst exhibited significantly higher polymerization activity, and produced copolymer with greater molecular weight and smaller polydispersity index than a corresponding micro-sized catalyst, which was ascribed to the much lower internal diffusion resistance of the nano-sized catalyst. Copolymer density decreased with the increase of polymerization temperature, probably due to the decrease of reactivity ratio r ₁ and ethylene solubility with increasing temperature. Polymerization activity of the nano-sized catalyst increased rapidly with increasing comonomer concentration. Ethylene/1-octene exhibited higher polymerization activity and had a stronger comonomer effect than ethylene/1-hexene.     

Study on the phase structures and toughening mechanism in PP/EPDM/SiO2 ternary composites

In this paper, EPDM rubber and nano-SiO₂ particles were employed to modify PP simultaneously. Our goal was to control the distribution and dispersion of EPDM and nano-SiO₂ particles in PP matrix by using an appropriate processing method and adjusting the wettability of nano-SiO₂ particles toward PP and EPDM, so as to achieve a simultaneous enhancement of toughness and modulus of PP. With regard to this, two kinds of nano-SiO₂ particles (with hydrophilic or hydrophobic) as well as two processing methods (one-step or two-step) were employed to prepare PP/EPDM/SiO₂ ternary composites. A unique structure with the majority of EPDM particles surrounded by SiO₂ particles was first observed by using hydrophilic SiO₂ and two-step processing method, resulting in a dramatically increase of Izod impact strength as the rubber content in the range of brittle-ductile transition (15-20 wt%). The observation that poor adhesion and poor compatibility between particles and PP matrix could result in a significant increase in Izod impact strength was unusual and needed further investigation. This could be tentatively understood as a consequence of the overlap of the ‘stress volume’ between EPDM and SiO₂ particles due to the formation of the unique structure. Our work provided a deep understanding of the toughening mechanism and a new way for the preparation of high performance polymer composites.     

Rates and product properties of polyethylene produced by copolymerization of 1-hexene and ethylene in the gas phase with (n-BuCp)2ZrCl2 on supports with different pore sizes

The effects of catalyst support pore size and reaction conditions (T=40-100 °C; ethylene pressure=1.4 MPa; 1-hexene concentration=0-47 mol/m³) on gas-phase polymerization rates and product properties were studied. Catalysts were prepared by impregnation of mesoporous molecular sieves (pore sizes of 2.5-20 nm) with methylaluminoxane and (n-BuCp)₂ZrCl₂. Temperature rising elution fractionation, differential scanning calorimetry and size exclusion chromatography were used to characterize the products. The results showed that these catalysts contained multiple types of catalytic sites and that the types of sites were a strong function of the support pore size. Ethylene polymerization and 1-hexene incorporation rates were strong functions of support pore size, 1-hexene concentration, and temperature. Large-pored catalysts had higher 1-hexene incorporation rates and the rate of 1-hexene incorporation was a function of polymerization time. Highest polymerization rates were obtained at 80 °C and 1-hexene concentration of 4-12 mol/m³; high 1-hexene concentrations resulted in large decreases in polymerization rates.     

Solid acidities of SiO2–TiO2/montmorillonite composites synthesized under different pH conditions

SiO₂–TiO₂/montmorillonite composites were prepared under acidic, neutral and basic conditions and the solid acidity of the resulting composites were determined. All the SiO₂/TiO₂ ratio of the colloidal particles was set at 10 but the resulting SiO₂/TiO₂ ratios were significantly richer in TiO₂. The XRD patterns of the acidic composite showed expanding and broadening of the (001) reflection by intercalation of colloidal SiO₂–TiO₂ particles, but the neutral and basic composites showed only broadening of the reflections and no intercalation. The specific surface areas of the acidic, neutral and basic composites (375, 237 and 247 m²/g, respectively) were much larger than of montmorillonite (6 m²/g). The average pore sizes were about 4, 15 and 50 nm, and the amounts of solid acidic sites measured by the NH₃-TPD were 178, 95 and 86 µmol/g for the acidic, neutral and basic composites, respectively. The solid acid amount of the acidic composite was twice that of a commercial catalyst, K-10, (85 µmol/g) and much higher than the guest phase SiO₂–TiO₂ gel (16 µmol/g) or the host phase montmorillonite (6 µmol/g). The TPD peak temperatures reflect the acid strength, and were similar in all the samples, ranging from 175° to 200 °C.     

Pickering emulsion polymerization: Preparation of polystyrene/nano-SiO2 composite microspheres with core-shell structure

Polystyrene/nano-SiO₂ composite microspheres (PS/nano-SiO₂) with core-shell structure were successfully synthesized in a Pickering emulsion route using nano-SiO₂ particles as stabilizers, which were organically modified by methacryloxypropyltrimethoxysilane (MPTMS) which containing a reactive CC bond. The products were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), fourier transformation infrared spectrum (FTIR), thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). In addition, bare PS spheres could also be obtained by changing synthetic conditions. It was found that the morphology of the resulting products depends on particle concentration, particle wettability and pH value of particle dispersion. A possible mechanism for the formation of the composites with different morphologies is proposed.     

Organic/inorganic composite membranes based on polybenzimidazole and nano-SiO2

Organic/inorganic composite membranes based on polybenzimidazole (PBI) and nano-SiO₂ were prepared in this work. However, the preparation of PBI/SiO₂ composite membrane is not easy since PBI is insoluble in water, while nano-SiO₂ is hydrophilic due to the hydrophilicity of nano-SiO₂ and water-insolubility of PBI. Thus, a solvent-exchange method was employed to prepare the composite membrane. The morphology of the composite membranes was studied by scanning electron microscopy (SEM). It was revealed that inorganic particles were dispersed homogenously in the PBI matrix. The thermal stability of the composite membrane is higher than that of pure PBI, both for doped and undoped membranes. PBI/SiO₂ composite membranes with up to 15 wt% SiO₂ exhibited improved mechanical properties compared with PBI membranes. The proton conductivity of the composite membranes containing phosphoric acid was studied. The nano-SiO₂ in the composite membranes enhanced the ability to trap phosphoric acid, which improved the proton conductivity of the composite membranes. The membrane with 15 wt% of inorganic material is oxidatively stable and has a proton conductivity of 3.9 × 10⁻³ S/cm at 180 °C.     

Inhibited transesterification and enhanced fibrillation of TLCP by nano-SiO2 in polycarbonate matrix

Hybrid composites composed of a thermotropic liquid crystalline polymer (TLCP), nano-SiO₂ and polycarbonate (PC) were prepared by melt blending in a twin-screw extruder. Infrared spectroscopy analysis indicated that the transesterification between PC and TLCP molecules during melt blending was significantly reduced in TLCP/PC blends filled with nano-SiO₂, compared to the unfilled TLCP/PC one. Scanning electron microscopy (SEM) observation showed that better compatibility and finer TLCP dispersion were reached in the unfilled blend, which made the fibrillation of TLCP difficult in capillary flow even at high shear rate. In contrast to this, well-developed TLCP fibrils were formed by capillary flow in nano-SiO₂ filled TLCP/PC blends. By increasing the nano-SiO₂ concentration and shear rate, the fibrillation of TLCP was significantly enhanced. Thermodynamically the interfacial tension between these components and dynamically the viscosity ratio of TLCP to PC were used to investigate the mechanism of nano-SiO₂ in inhibiting the transesterification and enhancing the fibrillation of TLCP droplets in these hybrid composites.     

Meldola blue immobilized on a new SiO2/TiO2/graphite composite for electrocatalytic oxidation of NADH

Meldola blue immobilized on a new SiO₂/TiO₂/graphite composite was applied in the electrocatalytic oxidation of NADH. The materials were prepared by the sol-gel processing method and characterized by several techniques including scanning electronic microscopy coupled to energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electronic microscopy (HRTEM). Si and Ti mapping profiles on the surface showed a homogeneous distribution of the components. Ti2p binding energy peaks indicate that the formation of Si-O-Ti linkage is presumably the responsible for the high rigidity of the matrices. The good electrical conductivity presented by the composites (5 and 11 S cm⁻¹) can be related to a homogeneous distribution of graphite particles observed by TEM. After the materials characterization, a SiO₂/TiO₂/graphite electrode was prepared and some chemical modifications were performed on its surface to promote the adsorption of meldola blue. The resulting system presented electrocatalytic properties toward the oxidation of NADH, decreasing the oxidation potential to −120 mV. The proposed sensor showed a wide linear response range from 0.018 to 7.29 mmol l⁻¹ and limit of detection of 0.008 mmol l⁻¹. SiO₂/TiO₂/graphite has shown to be a promising material to be used as a suitable support in the construction of new electrochemical sensors.     

改性纳米SiO2/聚氨酯-含氟丙烯酸酯无皂乳液合成及其胶膜性能表征

引言水性聚氨酯相对溶剂型聚氨酯具有不燃、气味小、不污染环境等优点[1-2],从而广泛用于涂料[3]、胶黏剂[4]、油墨[5]等领域。目前,常用于软包装领域的薄膜主要是表面能很低的非极性膜,而水性聚氨酯胶黏剂具有较高的表面自由能,对非极性膜的润湿性差,因此需要降低水性聚氨酯的表面张力,达到润湿非极性膜的目的。     

Comparative study of propylene polymerization using Me2Si(RInd)2ZrCl2/SiO2-SMAO/AlR3 and Me2Si(RInd)2ZrCl2/MAO (R = Me, H)

A mechanistic analysis of propylene polymerization was performed, in which the catalyst system was Me₂Si(R¹Ind)₂ZrCl₂/SMAO/AlR₃² (in situ supported catalyst onto MAO-modified silica) or Me₂Si(R¹Ind)₂ZrCl₂/MAO (homogeneous), where R¹ = H or CH₃, cocatalyzed by AlR₃² = TEA (triethylaluminum), IPRA (isoprenylaluminum), or TIBA (triisobutylaluminum). The catalyst activity of the homogeneous system Me₂Si(2-Me-Ind)₂ZrCl₂/MAO was almost 8 times higher than that observed for Me₂Si(Ind)₂ZrCl₂/MAO (38 vs 4.6 kg PP/g cat h), while the polypropylene molar mass was 3 times higher (M_w: 93 vs 34 kg/mol). Conversely, the in situ supported systems Me₂Si(Ind)₂ZrCl₂/SMAO/AlR₃ and Me₂Si(2-Me-Ind)₂ZrCl₂/SMAO/AlR₃ showed similar activities, ranging from 0.2 to 1.5 kg PP/g cat h. The molar mass of the resulting polymers prepared using the in situ procedure was dependent on the AlR₃ nature and on the Al/Zr ratio. Generally, the heterogeneous catalysts produced PP with higher molecular weights than that obtained with homogeneous ones. The influence of the alkylaluminum, used as the cocatalyst, on the chain-transfer termination reaction to the alkyl compound was evident from the activity and the molecular weight of the produced polymers.     

Comparative study of homogeneous and heterogeneous styrene polymerizations with Ni(acac)2/MAO catalytic system

Styrene polymerization was carried out with Ni(acac)₂/MAO and Ni(acac)₂/SiO₂/MAO. The influence of reaction parameters (Al/Ni mole ratio, catalyst concentration, temperature and time polymerization) on styrene polymerization was evaluated. It was observed that both catalytic systems were affected by reaction parameters and that the heterogeneous catalyst presented higher activity than the homogeneous one. Polystyrenes with different molecular weight, stereoregularity and polydispersity were obtained. These results suggest that different active catalyst species could have been present. In addition, two types of methylaluminoxane (MAO) with different molecular weights were also evaluated as cocatalyst. As a result, the catalyst activity and stereospecificity were strongly affected by the MAO type.     

Characterization of Pd catalysts supported on USY zeolites with different SiO2/Al2O3 ratios for the hydrogenation of naphthalene in the presence of benzothiophene

A series of ultra-stable Y-type (USY) zeolites with different SiO₂/Al₂O₃ ratios in the range of 10–80 were used as supports for preparing Pd/USY at 2 wt% Pd loading. The FT-IR of hydroxyl groups of USY zeolites, the n-butylamine chemisorption and the temperature-programmed desorption were used in combination to characterize the zeolite acidity. TPR, H₂-TPD and chemisorption using H₂ were used to characterize the Pd reduction and dispersion. The hydrogenation of naphthalene was conducted at 200 °C in the presence of benzothiophene at different sulfur/metal ratios. The hydrogenation activity, selectivity, and the sulfur tolerance strongly depended on the SiO₂/Al₂O₃ ratio (thus the acidity) of the zeolites. The activity decreased with increasing SiO₂/Al₂O₃ in this range. The IR and n-butylamine TPD showed that both the amount and strength of Brönsted acidity decreased with the increase of the SiO₂/Al₂O₃ ratio. The good relationship between the acidity modification and catalytic performance suggests that the sulfur tolerance of Pd/USY zeolite might be due to the desired metal-support interaction, which resulted in larger amount of electron-deficient Pd. However, as shown in TGA and TPO-IR studies, the higher hydrogenation performance on more acidic zeolite also caused higher amount of carbonaceous species on the catalyst.     

Sintering of glass-added BaZn1/3Nb2/3O3 ceramics

The complex perovskite oxide Ba(Zn_1/3Nb_2/3)O₃ (BZN) has been studied for its attractive dielectric properties which place this material interesting for applications as multilayer ceramics capacitors or hyperfrequency resonators. This material is sinterable at low temperature with combined glass phase–lithium salt additions, and exhibits, at 1 MHz very low dielectric losses combined with relatively high dielectric constant and a good stability of this later versus temperature. The 2 wt.% of ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of LiF-added BZN sample sintered at 900 °C exhibits a relative density higher than 95% and attractive dielectric properties: a dielectric constant ?_r of 39, low dielectrics losses (tan(δ) < 10⁻³) and a temperature coefficient of permittivity τ_? of 45 ppm/°C⁻¹. The 2 wt.% ZnO–SiO₂–B₂O₃ glass phase and 1 wt.% of B₂O₃-added BZN sintered at 930 °C exhibits also attractive dielectric properties (?_r = 38, tan(δ) < 10⁻³) and it is more interesting in terms of temperature coefficient of the permittivity (τ_? = −5 ppm/°C). Their good dielectric properties and their compatibility with Ag electrodes, make these ceramics suitable for L.T.C.C applications.     

Particle size effect of Li[Ni0.5Mn0.5]O2 prepared by co-precipitation

Spherical (Ni_0.5Mn_0.5)(OH)₂ with different secondary particle size (3 μm, 10 μm in diameter) was synthesized by co-precipitation method. Mixture of the prepared hydroxide and lithium hydroxide was calcined at 950 °C for 20 h in air. X-ray diffraction patterns revealed that the prepared material had a typical layered structure with space group. Spherical morphologies with mono-dispersed powders were observed by scanning electron microscopy. It was found that the layered Li[Ni_0.5Mn_0.5]O₂ delivered an initial discharge capacity of 148 mAh g⁻¹ (3.0-4.3 V) though the particle sizes were different. Li[Ni_0.5Mn_0.5]O₂ having smaller particle size (3 μm) showed improved area specific impedance due to the reduced Li⁺ diffusion path, compared with that of Li[Ni_0.5Mn_0.5]O₂ possessing larger particle size (10 μm). Although the Li[Ni_0.5Mn_0.5]O₂ (3 μm) was electrochemically delithiated to Li_0.39[Ni_0.5Mn_0.5]O₂, the resulting exothermic onset temperature was around 295 °C, of which the value is significantly higher than that of highly delithiated Li_1−δCoO₂ (∼180 °C).     

NiP^O and [Cp2ZrCl2/MAO] as a versatile dual-function catalyst system for in situ polymerization of ethylene to linear low-density polyethylene (LLDPE)

[Ni(Ph₂PCHCOPh)(Ph)(PPh₃)] (NiP^O) and Cp₂ZrCl₂/methylaluminoxane (MAO), well known as ethylene oligomerization and polymerization catalysts, respectively, are combined to produce LLDPE by in situ polymerization. Melting temperature (T_m), degree of crystallinity (χ_c), intrinsic viscosity, average molecular weight and ¹³C NMR analysis of copolymers confirm the insertion of α-olefins into the polyethylene chain. Variations in α-olefin concentration and ethylene pressure during the polymerization stage lead to changes in the degree of branching. The resulting copolymers have χ_c and T_m in the 25.8–65.2% and 114–132 °C ranges, respectively. Experimental results show the versatility of the dual-function catalyst.     

Thermo-mechanical properties of LLDPE/SiO2 nanocomposites

Two series of linear low density polyethylene (LLDPE)/SiO₂ nanocomposites were prepared. They were based on two types of commercial LLDPE, one prepared by metallocene (mLLDPE) and the other by traditional Ziegler-Natta (zLLDPE) catalysts, and silica nanoparticles surface treated with dimethyldichlorosilane. The silica nanonparticles used have an average diameter of 16 nm, and their weight fraction varied from 2 up to 10%. The structure and thermal-mechanical features of the nanocomposites were characterized by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), dynamic mechanical spectroscopy (DMA) as well as tensile tests. The effect of nanoparticles on crystallinity, and hence to the morphology of the materials was studied. The secondary transitions were also affected by the filler presence, while the tensile properties were reinforced with varying the nanoparticle weight fraction. The addition of the nanofillers brought up an increase in the elastic modulus and the tensile strength of mLLDPE accompanied by an unusual dramatic increase in the elongation at break. The same trend, although to a lesser extent, was observed for the zLLDPE/SiO₂ composites. The increment of the elastic modulus of the composites with increasing filler content was simulated with three micromechanical models developed in previous works. The model which assumes an effective interface between the matrix and the nanoparticles provided the best fitting with the experimental data of mLLDPE/SiO₂.     

Effect of CeO2 addition on densification and microstructure of Al2O3-YSZ composites

Alumina (Al₂O₃) and alumina-yttria stabilized zirconia (YSZ) composites containing 3 and 5 mass% ceria (CeO₂) were prepared by spark plasma sintering (SPS) at temperatures of 1350-1400 °C for 300 s under a pressure of 40 MPa. Densification, microstructure and mechanical properties of the Al₂O₃ based composites were investigated. Fully dense composites with a relative density of approximately 99% were obtained. The grain growth of alumina was inhibited significantly by the addition of 10 vol% zirconia, and formation of elongated CeAl₁₁O₁₈ grains was observed in the ceria containing composites sintered at 1400 °C. Al₂O₃-YSZ composites without CeO₂ had higher hardness than monolithic Al₂O₃ sintered body and the hardness of Al₂O₃-YSZ composites decreased from 20.3 GPa to 18.5 GPa when the content of ZrO₂ increased from 10 to 30 vol%. The fracture toughness of Al₂O₃ increased from 2.8 MPa m^1/2 to 5.6 MPa m^1/2 with the addition of 10 vol% YSZ, and further addition resulted in higher fracture toughness values. The highest value of fracture toughness, 6.2 MPa m^1/2, was achieved with the addition of 30 vol% YSZ.     

Effect of molar ratio of TiO2/SiO2 on the properties of particles synthesized by flame spray pyrolysis

Titania (TiO₂)–silica (SiO₂) nanoparticles were synthesized from sprayed droplets of a mixture of TEOS and TTIP by flame spray pyrolysis (FSP). The effect of molar ratio between TEOS and TTIP in the mixture on the particle properties such as particle morphology, average particle diameter, specific surface area, crystal structure, etc., were determined using TEM, XRD, BET, and FT-IR. A UV-spectrometer was also used to measure the absorption spectrum and the band gap energy of the product particles. As the molar ratio of TEOS/TTIP increased by increasing TEOS concentration at the fixed TTIP concentration, the average particle diameter of the mixed oxide nanoparticles increased with maintaining uniform dispersion between TiO₂ and SiO₂, and crystal structure was transformed from anatase to amorphous. The band gap energy of the TiO₂–SiO₂ nanoparticles increased with respect to the increase of the molar ratio due to the decrease of width of UV-absorption spectrum. Photocatalytic activity of TiO₂–SiO₂ composite particles decreased with the concentration of TEOS.     

Rapid synthesis of Ti3AlC2/TiB2 composites by the spark plasma sintering (SPS) technique

Dense Ti₃AlC₂/TiB₂ composites were successfully fabricated from B₄C/TiC/Ti/Al powders by spark plasma sintering (SPS). The microstructure, flexural strength and fracture toughness of the composites were investigated. The experimental results indicate that the Vickers hardness increased with the increase in TiB₂ content. The maximum flexural strength (700 ± 10 MPa) and fracture toughness (7.0 ± 0.2 MPa m^1/2) were achieved through addition of 10 vol.% TiB₂, however, a slight decrease in the other mechanical properties was observed with TiB₂ addition higher than 10 vol.%, which is believed to be due to TiB₂ agglomeration.