Nanocomposites based on polyethylene and Mg-Al layered double hydroxide. Part II. Rheological characterization

The rheological properties of Mg-Al based layered double hydroxide (LDH)/polyethylene nanocomposites have been presented in details. Both unmodified and maleic anhydride (MAH) grafted polyethylene based composites are characterised. The X-ray diffraction and electron microscopic investigations reveals the primary structures of the dispersed LDH particles exists as thin platelets. The rheological analysis carried out under different modes show behaviors typical of those reported extensively in literature for various cationic clay based polymer nanocomposites. The dispersed LDH particles show stronger interaction with maleic anhydride grafted matrix. The influence of LDH loading increases the shear thinning nature of the composite melt. The storage modulus vs. frequency plots of the composites show apparent plateau behaviors in low frequency region in a dynamic frequency sweep experiment. The appearance of stress overshoot peak in flow reversal experiment indicates that the dispersed LDH particles form structural aggregation, which are ruptured by shearing, but reform with time when shearing is stopped.     

A solution blending route to ethylene propylene diene terpolymer/layered double hydroxide nanocomposites

Ethylene propylene diene terpolymer (EPDM)/MgAl layered double hydroxide (LDH) nanocomposites have been synthesized by solution intercalation using organically modified LDH (DS-LDH). The molecular level dispersion of LDH nanolayers has been verified by the disappearance of basal XRD peak of DS-LDH in the composites. The internal structures, of the nanocomposite with the dispersion nature of LDH particles in EPDM matrix have been studied by TEM and AFM. Thermogravimetric analysis (TGA) shows thermal stability of nanocomposites improved by ≈40 °C when 10% weight loss was selected as point of comparison. The degradation for pure EPDM is faster above 380 °C while in case of its nanocomposites, it is much slower.     

Preparation and characterization of poly(vinyl chloride)/layered double hydroxide nanocomposites with enhanced thermal stability

Poly(vinyl chloride)/layered double hydroxide (PVC/LDH) nanocomposite has been prepared via solution intercalation process using dodecyl sulfate (DS⁻) or stearate anion modified LDH. XRD and TEM results give evidence that the inorganic LDH particles were dramatically exfoliated into nanoscale and homogeneously dispersed in PVC matrix. The enhanced thermal stability was confirmed by conventional Congo Red test. The nanocomposite containing Mg₃Al-LDH-stearate had the maximum of increased dehydrochlorination time, 15 times of that of neat PVC, and more than twice of that of PVC/LDH-NO₃ composite and PVC/LDH-DS nanocomposite at 5 wt% loading. Furthermore, its thermal stability time increased with the LDH content. At 10 wt% loading of Mg₃Al-LDH-stearate, it reached 20 times of that of PVC, and obviously larger than that of the previously reported nanocomposite using alkyl phosphonate (AP) grafted LDH. The reason may lie on the absorption of the released HCl during degradation by both nanoscale dispersed LDH particles and stearate anions.     

Preparation of new layered double hydroxide,Zn-Mo LDH

Layered double hydroxide (LDH) is prepared conventionally with bivalent and trivalent cations. We recently reported that the preparation of Zn-Ti LDH consisting of bi and tetravalent cations is possible through the decomposition of urea. In this study, Zn-Mo LDH consisting of bi and hexavalent cations were prepared and reacted with organic monocarboxylic and dicarboxylic acids. Interlayer spacing of the prepared LDH (Zn-Mo-CO₃) contained carbonate anions between the layers was 0.72 nm. The spacing was small compared to usual LDH (Zn-Al-CO₃) of 0.76 nm in the case of carbonate anion as the guest. Also, TG analysis indicated that the electrostatic force between the Zn-Mo layers and carbonate anions was larger than those of Zn-Al LDH. Certainly, the carbonate anions in Zn-Mo LDH decomposed at 260°C while they in usual LDH decomposed at 230–240°C. ESCA showed that Mo⁵⁺ had changed to Mo⁶⁺ through the preparation procedure. These results showed the preparation of layered double hydroxides consisting of bivalent and hexavalent cations. By the intercalation of Zn-Mo LDH with suberic acid at 60°C, a sharp peak was observed at 1.06 nm and the peak of LDH itself (0.72 nm) disappeared. This result has suggested that the intercalation of organic acid into new LDH was performed completely.     

Mechanochemical route for synthesizing nitrate form of layered double hydroxide

This paper introduces a mechanochemical process for synthesizing a nitrate compound, Mg-Al-(NO₃) (Mg:Al = 3:1), which is a layered double hydroxide (LDH) from three kinds of starting materials, Mg(OH)₂, Al(OH)₃ and Mg(NO₃)₂.6H₂O. The operation was attempted by one-and two-step milling operations, and the former is to grind the three kinds of samples simultaneously, while the latter is to mill two kinds, Mg(OH)₂ and Al(OH)₃, followed by further milling with Mg(NO₃)₂.6H₂O. Characterizations by XRD, FT-IR, and TG-DTA indicate that the latter step operation is superior to the former one in terms of the formation of the target material, nitrate form of LDH. This form could be taken as a potential candidate for slow-release fertilizer, where the release of the nitrate from the synthesized LDH has been held back due to the intercalation into the LDH structure.     

Strengthening and toughening effects of layered double hydroxide and hyperbranched polymer on epoxy resin

The effects of organically modified layered double hydroxide (O-LDH) and epoxy functionalized hyperbranched aliphatic polyester (E1), on the mechanical strength, thermal property and toughness of diglycidyl ether of bisphenol A (DGEBA) epoxy resin were studied in detail. The crystalline structure and morphology of the nanocomposites composed of DGEBA/O-LDH and DGEBA/E1/O-LDH were investigated by wide angle X-ray diffraction analysis and transmission electron microscopy observation. Both results showed that the LDH nanosheets were sufficiently exfoliated and randomly dispersed in the epoxy matrix. The thermal and mechanical properties were compared with the corresponding neat polymer matrix. The enhancement in strength and toughness was achieved by the addition of O-LDH and E1, respectively, and confirmed in terms of fracture surface analysis by scanning electron microscopy.     

The effect of the interlayer anions on the electrochemical performance of layered double hydroxide electrode materials

Both high energy density and high power density are vitally required for new applications such as electric vehicles. Here we present a comparison of two well-crystallised layered double hydroxides, [Ni₄Al(OH)₁₀]OH and [Ni₄Al(OH)₁₀]NO₃, which shows that the former can maintain a better discharge capacity, 294-299 mAh g⁻¹, than the latter, 233-287 mAh g⁻¹ at a current density of 2000 mA g⁻¹ within about 300 cycles, although both electrodes deliver a similar capacity of 326 mAh g⁻¹ at 200 mA g⁻¹ initially. It is believed that both the more watery interlayer space in [Ni₄Al(OH)₁₀]OH than in [Ni₄Al(OH)₁₀]NO₃ and the morphologic changes induced by anion exchange of NO₃⁻ by OH⁻ during electrochemical cycles play key roles in their behaviour.     

Preparation and nonisothermal crystallization behavior of polypropylene/layered double hydroxide nanocomposites

Polypropylene (PP)/layered double hydroxide (LDH) nanocomposites were prepared via melt intercalation using dodecyl sulfate anion modified LDH and maleated PP as compatibilizing agent. Evidently the interlayer anions in LDH galleries react with maleic anhydride groups of PP-g-MA and lead to a finer dispersion of individual LDH layers in the PP matrix. The nanostructure was characterized by XRD and TEM; the examinations confirmed the nanocomposite formation with exfoliated/intercalated layered double hydroxides well distributed in the PP matrix. The nonisothermal crystallization behavior of resulting nanocomposites was extensively studied using differential scanning calorimetry (DSC) technique at various cooling rates. In nonisothermal crystallization kinetics, the Ozawa approach failed to describe the crystallization behavior of nanocomposites, whereas the Avrami analysis and Jeziorny method well define the crystallization behavior of PP/LDH nanocomposite. Combined Avrami and Ozawa analysis (Liu model) also found useful. The results revealed that very small amounts of LDH (1%) could accelerate the crystallization process relative to the pure PP and increase in the crystallization rates was attributed to the nucleating effect of the nanoparticles. Polarized optical microscopy (POM) observations also support the DSC results. The effective crystallization activation energy was estimated as a function of the relative degree of crystallinity using the isoconversional analysis. Overall, results indicated that the LDH particles in nanometer size might act as nucleating agent and distinctly change the type of nucleation, growth and geometry of PP crystals.     

Effect of photofunctional organo anion-intercalated layered double hydroxide nanoparticles on poly(ethylene terephthalate) nonisothermal crystallization kinetics

Very recently, we report a facile preparation and strong UV-shielding function of poly(ethylene terephthalate) (PET) nanocomposites using 4,4′-diaminostilbene-2,2′-disulfonic acid anion-intercalated layered double hydroxide (LDH_DDA). Herein, the effect of the photofunctional organo anion-intercalated LDH nanoparticles on nonisothermal crystallization kinetics of PET is reported by differential scanning calorimetry technique. First, the nonisothermal crystallization behaviour is discussed by several basic parameters including crystallization peak temperature, relative degree of crystallinity with temperature or time, and half-time of crystallization. Then, Avrami and Jeziorny method, as well as Mo model were applied for the PET/LDH_DDA nanocomposites. Finally, the crystallization activation energy was investigated by Kissinger method and Flynn conversion. The results reveal that the incorporation of LDH_DDA nanoparticles acted as nucleating agent and significantly accelerated the PET nonisothermal crystallization process, whereas had little effect on the three-dimensional growth pattern of spherulites.     

Second staging of tartrate and carbonate anions in Mg–Al layered double hydroxide

The intercalation of tartrate was performed through ion exchange using carbonate Mg–Al layered double hydroxide (LDH) as precursor. The intermediate crystalline phase (so-called second-staging phase) was observed during the intercalation process. The pure second-staging LDH, in which alternate interlayer regions are occupied by carbonate and tartrate anions, was obtained by the deintercalation of interlayer tartrate with carbonate. It was also found that the interlayer tartrates are so stable that they are difficult to exchange by carbonate at room temperature.     

Engineered morphologies of layered double hydroxide nanoarchitectured shell microspheres and their calcined products

CoFe^IIFe^III-layered double hydroxide (LDH) hollow spheres with two distinctly different hierarchical morphologies—flower-like and raspberry-like, involving edge-on and face-on oriented LDH platelets as building blocks, respectively—can be fabricated by tuning the rate of addition of NaOH solution in a coprecipitation process with sulfonated polystyrene spheres as a template. Ex situ pH-dependent microscopic observations of the formation of the flower-like LDH shells shows the details of non-classic nucleation and growth underlying the edge-on orientation. After calcination, the resulting CoFe₂O₄ materials retain the hierarchical morphology of the corresponding LDH precursor. The hollow CoFe₂O₄ spheres with raspberry-like morphology exhibit magnetic properties comparable to a reference material obtained by calcination of a conventional LDH powder precursor. Similar materials with tunable magnetic properties can be prepared by virtue of the flexibility in LDH layer composition. The CoFe^IIFe^III-LDH spheres with a raspberry-like morphology may be also be prepared by using a scalable procedure involving separate ultra-rapid nucleation and aging steps in a modified colloid mill reactor. The results reported here may open up the possibility of producing shell structures with controllable morphologies and properties on a large scale.     

Synthesis of multi-walled carbon nanotubes over tungsten-doped cobalt-based catalyst derived from a layered double hydroxide precursor

Multi-walled carbon nanotubes (CNTs) were prepared over a series of W-doped Co-based catalysts derived from layered double hydroxide precursor by catalytic chemical vapor deposition (CCVD) of acetylene. The materials were characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption–desorption experiments and Raman spectroscopy. The effect of the proportion of W in the Co-based catalysts on the carbon yield, diameter uniformity and quality of CNTs was investigated. The results demonstrated that with the increasing W/Co molar ratio from 0 to 1.0, both the mean number of walls and the average diameter of CNT produced over catalysts increased from about 8 to 28 nm and from about 12.1 to 23.7 nm, respectively. A small amount of tungsten added to the catalyst with the W/Co molar ratio of 0.3 could facilitate the dispersion of catalytically active Co species on the surface of support, and thus uniform and high-quality CNTs with a remarkably high yield of ca. 1600% were obtained.     

New sulfur adsorbents derived from layered double hydroxides: I: Synthesis and COS adsorption

Mixed oxides, prepared via the thermal decomposition of layered double hydroxides (LDHs), were screened gravimetrically for their ability to adsorb carbonyl sulfide (COS). Based on promising results obtained for Ni/Mg/Al, Ni/Mg/Fe and Co/Mg/Al mixed oxides, a study was undertaken to optimize the composition of these materials for COS adsorption. To investigate the effect of the M(II):M(III) ratio, LDHs of the type [M_zMg_yAl_x(OH)₂](CO₃)_x/2·0.5H₂O (where M = Ni or Co, and x + y + z = 1) were prepared at values of x corresponding to 0.33 and 0.20. Simultaneously, the elemental ratio of transition metal to magnesium (z/y) was varied. Mixed oxides obtained from the resulting LDHs were tested in fixed bed mode with a feed of 100 ppm COS in N₂ to determine breakthrough capacity. In general Ni/Mg/Al mixed oxides showed the best performance, a composition with Ni/Mg/Al = 0.32/0.48/0.20 showing the best adsorption capacity. Treatment of the spent adsorbent under an atmosphere of 5% H₂ in N₂ at 450 °C was found to provide an effective means of restoring the adsorption capacity over two cycles of adsorption and regeneration, although after three such cycles, adsorption capacity decreased.     

Preparation and characterization of polyimide/modified layered double hydroxide nanocomposites

Polyimide (PI)/modified layered double hydroxide (m‐LDH) nanocomposites were prepared in this study. For this work, m‐LDHs were prepared from layered double hydroxides (LDHs) through an anionic exchange reaction with pyromellitic dianhydride (PMDA), succinic acid or terephthalic acid. PMDA and 4,4′‐oxydianiline were used to make the poly(amic acid) precursor for PI. X‐ray diffraction and transmission electron microscopy measurements confirmed that the PMDA‐modified LDH (PMH) and terephthalic acid‐modified LDH (TMH) were well dispersed in the PI matrix. For the succinic acid‐modified LDH, some of the LDH was intercalated with the succinic acid molecules but most maintained its original structure. Thus, the PI/PMH and PI/TMH nanocomposites exhibited improved mechanical, thermal and electrical properties compared to pure PI. The PMH has aromatic groups and is expected to have better π–π interactions with the PI chains than the other m‐LDHs. Thus, the PI/PMH nanocomposites exhibited the best properties among the nanocomposites investigated. Copyright © 2010 Society of Chemical Industry     

Effect of interlayer anions on chromium removal using Mg–Al layered double hydroxides:Kinetic,equilibrium and thermodynamic studies

The influence of interlayer anions such as NO3-, SO42-and Cl-on Mg–Al hydrotalcites for Cr(VI) removal from aqueous solution was studied. The structure of the prepared LDHs was characterized by XRD, SEM, FTIR, TGA, BET surface area and p Hzpc. The sorbent ability and sorption mechanisms were also investigated. The LDHs exhibit high removal for Cr(VI), and the sorbed amount depends on the nature of interlayer anion, which decreased in the following order: NO3-N Cl-N SO42-. Nitrate-containing LDH reached a Cr(VI) sorption equilibrium within only 30 min. The effects of operating conditions, including initial concentration, solution p H, agitation time and sorbent amount have been studied in batch mode. The optimum conditions were observed at an initial concentration of 100 mg·L-1, p H = 6, agitation time of 60 min and a sorbent dose of 2 g·L-1. The equilibrium data were fitted to the Langmuir, Freundlich and Dubinin–Radushkevich isotherm models. The Langmuir model was found to sufficiently describe the sorption process, offering a maximum sorption capacity of 71.91 mg·g-1. The sorption kinetic follows pseudo-second-order reaction with high accuracy. Thermodynamic parameters suggested that the sorption process is spontaneous and endothermic in nature.     

Preparation and properties of layered double hydroxide/poly(ethylene terephthalate) nanocomposites by direct melt compounding

Thermal, rheological and mechanical properties of layered double hydroxide (LDHs)/PET nanocomposites were investigated. To enhance the compatibility between PET matrix and LDHs, organic modification of parent LDH having carbonate anion was carried out using various anionic surfactants such as dodecylsulfate (DS), dodecylbenzenesulfonate (DBS), and octylsulfate(OS) by rehydration process. Then, PET nanocomposites with LDH content of 0, 1.0, and 2.0 wt% were prepared by direct melt-compounding. The dispersion morphologies were observed by transmission electron microscopy and X-ray diffraction, indicating that LDH-DS were exfoliated in PET matrix. From the rheology study, there are some network structures owing to filler-filler and/or filler-matrix interactions in nanocomposite systems. Consequently, DS intercalated LDH provided good compatibility with PET molecules, resulting in exfoliated LDH-DS/PET nanocomposites having enhanced thermal and mechanical properties as compared to other nanocomposites as well as homo PET.     

A facile synthetic strategy for Mg-Al layered double hydroxide material as nanocarrier for methotrexate

Mg-Al layered double hydroxide nanopowders were synthesised by a facile coprecipitation technique at different pH conditions. LDH nanoparticles of higher aspect ratio with an average particle size of 26 nm were obtained at pH 9 whereas a pH of 11.3 resulted in LDH nanoparticles of average size 50 nm with lower aspect ratio and narrower size distribution. LDH-MTX organo-inorganic nanohybrid was produced with an average particle size of 53 nm after intercalation of MTX into the interlayer space of LDH, as evident from the shift of (0 0 3) peak in X-ray diffraction. This was corroborated by the transmission electron micrograph, which showed an increase in average interlayer spacing from 8.00 Å in pristine LDH to 21.4 Å in LDH-MTX nanohybrid. Thermogravimetric analyses showed ∼33.2 wt% MTX loading in the LDH structure. The MTX release profile from Mg-Al LDH-MTX nanohybrid in phosphate buffer saline at pH 7.4 follows Ritger-Peppas kinetics model which demonstrates that the release kinetics is diffusion controlled. An attempt has been made to explain the above observations based on the effect of electrical double layer repulsions on the growth of LDH nuclei, primarily considering significance of the particle morphology in drug delivery application.     

Desorption of Cl- from Mg-Al layered double hydroxide intercalated with Cl- using CO2 gas and water

Mg-Al layered double hydroxide intercalated with CO_3~(2-)(CO_3·Mg-Al LDH) is effective for treating HCl exhaust gas.HCl reacts with CO_3~(2-) in CO_3·Mg-Al LDH, resulting in the formation of Cl·Mg-Al LDH.We propose that CO_2 can be used for the desorption of Cl~-from Cl·Mg-Al LDH to regenerate CO_3·Mg-Al LDH.Herein,we studied the desorption of a from CI-Mg-Al LDH by adding water to Cl·Mg-Al LDH and blowing CO_2 into it.We also analyzed the effects of temperature and water addition speed on the desorption of CI~-from Cl·Mg-Al LDH.Our results show that the added water adhered to CI·Mg-Al LDH and that CO_2 in the gaseous phase was dissolved in this adhered water,thus generating CO_3~(2-).Therefore,anion exchange occurred between CO_3~(2-) and Cl~-in the Cl·Mg-Al LDH,thus desorbing Cl~-.     

Polydimethylsiloxane based polyurethane and its composite with layered double hydroxide: Synthesis and its thermal properties

This investigation reports the synthesis of a new class of polyurethane (PU) based on bis(hydroxyalkyl) polydimethylsiloxane (PDMS) as diol and isophorone diisocyanate as diisocyanate followed by the preparation of PU/layered double hydroxide (PU/LDH) nanocomposite via ex-situ technique. Nuclear magnetic resonance and Fourier-transform infrared spectroscopy studies confirm the formation of PU and incorporation of PDMS into the PU backbone. Thermogravimetry analysis revealed that thermal stability of the composite improves significantly with incorporation of LDH into the PU matrix. This may be accredited to the barrier effect rendered by the LDH layers. Differential scanning calorimetry study reveals that with the incorporation of LDH, glass transition temperature (T_g) of the composite increases for an optimum level of loading beyond which it remains constant.     

Nanocomposites formed from linear low density polyethylene and organoclays

Polyethylene-clay nanocomposites were prepared by melt compounding various combinations of a maleic anhydride grafted linear low density polyethylene (LLDPE-g-MA), a linear low density polyethylene (LLDPE), and two organoclays. The two types of organoclay were selected to show the effect of the number of alkyl groups attached to the nitrogen of the organic modifier on exfoliation and improvement of mechanical properties. Nanocomposites derived from the organoclay having two alkyl tails, M₂(HT)₂, exhibited better dispersion and improvement of mechanical properties than nanocomposites based on the organoclay having one alkyl tail M₃(HT)₁. This result is the opposite of what is observed for nylon-6 nanocomposites. In addition, the rheological properties and gas permeability of the nanocomposites derived from the organoclay having two alkyl tails, M₂(HT)₂ were investigated. Both melt viscosity and melt tension (melt strength) increased with increased content of clay (MMT) and LLDPE-g-MA. Gas permeability was decreased by the addition of MMT.