New inorganic nanotube polymer nanocomposites: Improved thermal, mechanical and tribological properties in isotactic polypropylene incorporating INT-MoS2.

Environmentally friendly molybdenum disulfide (INT-MoS2) inorganic nanotubes were introduced into an isotactic polypropylene (iPP) polymer matrix to generate novel nanocomposite materials through an advantageous melt-processing route. The effects of INT-MoS2 content on the thermal, mechanical and tribological properties were investigated. The incorporation of INT-MoS2 generates notable performance enhancements through reinforcement effects, highly efficient nucleation activity and excellent lubricating ability in comparison with other nanoparticle fillers such as nanoclays, carbon nanotubes, silicon nitrides and halloysite nanotubes. It was shown that these INT-MoS2 nanocomposites can provide an effective balance between performance, cost effectiveness and processability, and should be of some interest in the area of multifunctional polymer nanocomposite materials.

Opportunities and challenges in the use of inorganic fullerene-like nanoparticles to produce advanced polymer nanocomposites

Polymer/inorganic nanoparticle nanocomposites have garnered considerable academic and industrial interest over recent decades in the development of advanced materials for a wide range of applications. In this respect, the dispersion of so-called inorganic fullerene-like (IF) nanoparticles, e.g., tungsten disulfide (IF-WS2) or molybdenum disulfide (IF-MoS2), into polymeric matrices is emerging as a new strategy. The surprising properties of these layered metal dichalcogenides such as high impact resistance and superior tribological behavior, attributed to their nanoscale size and hollow quasi-spherical shape, open up a wide variety of opportunities for applications of these inorganic compounds. The present work presents a detailed overview on research in the area of IF-based polymer nanocomposites, with special emphasis on the use of IF-WS2 nanoparticles as environmentally friendly reinforcing fillers. The incorporation of IF particles has been shown to be efficient for improving thermal, mechanical and tribological properties of various thermoplastic polymers, such as polypropylene, nylon-6, poly(phenylene sulfide), poly(ether ether ketone), where nanocomposites were fabricated by simple melt-processing routes without the need for modifiers or surfactants. This new family of nanocomposites exhibits similar or enhanced performance when compared with nanocomposites that incorporate carbon nanotubes, carbon nanofibers or nanoclays, but are substantially more cost-effective, efficient and environmentally satisfactory. Most recently, innovative approaches have been described that exploit synergistic effects to produce new materials with enhanced properties, including the combined use of micro- and nanoparticles such as IF-WS2/nucleating agent or IF-WS2/carbon fiber, as well as dual nanoparticle systems such as SWCNT/IF-WS2 where each nanoparticle has different characteristics. The structure–property relationships of these nanocomposites are discussed and potential applications proposed ranging from medicine to the aerospace, automotive and electronics industries.

Bio-based polymer nanocomposites based on nylon 11 and WS2 inorganic nanotubes

Tungsten disulphide nanotubes (INT-WS2) have been successfully dispersed in a bio-based polyamide matrix (nylon 11) by conventional melt processing. The effect of INT-WS2 content on the morphology, thermal stability, crystallization behaviour and dynamic mechanical properties is investigated. The results indicate that these inorganic nanotubes can be efficiently incorporated into the bio-based polymer matrix without the need for modifiers or surfactants. Additionally, it is found that the non-isothermal crystallization behaviour of nylon 11/INT-WS2 depends on both the cooling rate and INT-WS2 concentration. In particular, crystallization kinetics results demonstrate that the nucleating activity of INTs plays a dominant role in accelerating the crystallization of nylon 11. This fact leads to the appearance of the more-disordered phase at higher temperature. More significantly, it was shown that these INT-WS2 nanocomposites can facilitate a good processability and cost efficiency, and will be of interest for many eco-friendly and medical applications.

Morphology and properties of thermoplastic polyurethane nancomposites incorporating hydrophilic layered silicates

Hydrophilic layered silicate/polyurethane nanocomposites were prepared via twin screw extrusion and solvent casting. Good dispersion and delamination was achieved-regardless of processing route, illustrating that the need for optimised processing conditions diminishes when there is a strong driving for de for intercalation between the polymer and organosilicate. Evidence for altered polyurethane microphase morphology in the nanocomposites was provided by DMTA and DSC. WAXD results suggested that the appearance of an additional high temperature melting endotherm in some melt-compounded nanocomposites was not due to the formation of a second crystal polymorph, but rather due to more well-ordered hard microdomains. Solvent casting was found to be the preferred processing route due to the avoidance of polyurethane and surfactant degradation associated with melt processing. While tensile strength and elongation were not improved on organosilicate addition, large increases in stiffness were observed. At a 7 wt% organosilicate loading, a 3.2-fold increase in Young's modulus was achieved by solvent casting. The nanocomposites also displayed higher hysteresis and permanent set. (C) 2004 Elsevier Ltd. All rights reserved.

The effect of Yb addition in Bi2Sr2Ca1-x,YbxU2Oy partial melted thick films

To study the phase relations in the Bi-2212 and Yb2O3 system, Bi2Sr2Ca1-xYbxCu2Oy thick films are prepared by partial melt processing via an intermediate reaction between Bi-2212 and Yb2O3. When Bi-2212 and Yb2O3 are partially melted and then slowly cooled, solid solutions of Bi2Sr2Ca1-xYbxCu2Oy form by reactions between liquid and solid phases which contain Yb. Following these reactions, Ca is partially replaced in Bi-2212 matrix and participates in the formation of secondary phases, such as Bi-free, (Ca, Sr)O-x and CaO. Variation of the Bi-2212-Yb2O3 ratios and processing parameters changes the balance between the phases and leads to different Yb:Ca ratios in the Bi-2212 matrix of processed thick films. When the partial melting process is optimized for each sample to minimize the growth of secondary phases, x = 0.42-0.46 for the samples prepared at pO(2) = 0.01 atm, x = 0.24-0.29 for the samples prepared at pO(2) = 0.21 atm, x = 0.18-0.23 for the samples prepared at pO(2) = 0.99 atm are obtained regardless to the starting compositions. It is found that superconducting properties of Bi2Sr2Ca1-xYbxCu2Oy thick films strongly depend on the processing conditions, because the conditions result in different Yb content in the Bi-2212 matrix and the volume fraction of the secondary phases. The highest T-c(0) of 77, 90 and 91 K were obtained for the samples processed at 0.01, 0.21 and 0.99 atm of O-2, respectively.

Thermal stability analysis of organo-silicates, using solid phase microextraction techniques

An analysis of thermal degradation products evolved during the melt processing of organo-layered silicates (OLS) was carried out via the use of a solid phase microextraction (SPME) technique. Two commerical OLSs and one produced in-house were prepared for comparision. The solid phase microextraction technique proved to be a very effective technique for investigating the degradation of the OLS at a specific processing temperature. The results showed that most available OLSs will degrade under typical conditions required for the melt processing of many polymers, including thermoplastic polyurethanes. It is suggested that these degradation products may lead to changes in the structure and properties of the final polymer, particularly in thermoplastic polyurethanes, which seem significantly succeptable to the presence of these products. It is also suggested that many commercially available OLSs are produced in such a way that results in an excess of unbound organic modifier, giving rise to a greater quantity of degradation products. All OLSs where compared and characterised by TGA and GC-MS. (c) 2004 Elsevier B.V. All rights reserved.

Synthesis and evaluation of novel polymer modifiers for biodegradable polymers

The effects of ester plasticizers and copolymers on the mechanical properties of the natural biodegradable polymers, poly(3-hydroxybutyrate) [PHB] and poly(lactic acid) [PLA] have been studied after subjecting to melt processing conditions. Ester plasticizers were synthesized from citric, tartaric and maleic acids using various alcohols. A variety of PLA copolymers have also been prepared from poly(ethylene glycol) derivatives using stannous octanoate catalysed ring opening polymerisations of DL-lactide. A novel PLA star copolymer was also prepared from an ethoxylated pentaerythritol. The structures of these copolymers were determined by NMR spectroscopy. The plasticizing effect of the synthesised additives at various concentrations was determined. While certain additives were capable of improving the mechanical properties of PLA, none were effective in PHB. Moreover, it was found that certain combinations of additives exhibited synergistic effects. Possible mechanisms are discussed. Biotic and abiotic degradation studies showed that the plasticizers (esters and copolymers) did not inhibit the biodegradability of PHB or PLA in compost at 60°C. Simple toxicity tests carried out on compost extract and its ability to support the growth of cress seeds was established. PLA was found to be susceptible to limited thermal degradation under melt processing conditions. Conventional phenolic antioxidants showed no significant effect on this process, suggesting that degradation was not predominantly a free radical process. PLA also underwent photo-oxidative degradation with UV light and the process could be accelerated in the presence of a photoactivator such as iron (III) diisononyl dithiocarbamate. The mechanisms for the above processes are discussed. Finally, selected compounds were prepared on a pilot plant scale. Extruded and blown films were prepared containing these additives with conventional polymer processing equipment. The mechanical properties were similar to those obtained with laboratory produced compression moulded films.

Reinforced thermoplastics for engineering applications

The effect of mechano-chemically bound polypropylene modifiers on the mechanical performance and thermal-oxidative stability of polypropylene composites has been studied. The mechanical performance of unmodified polypropylene containing silane coupled glass and Rockwool (mineral) fibre was poor by comparison with a similar commercially produced glass reinforced composite; this was attributed to poor fibre-matrix adhesion. Mechano-chemical binding with unsaturated additives was obtained in the presence of a free radical initiator (di-cumyl peroxide). This process was inhibited by stabilisers present in commercial grades of polypropylene composites by chemical bond formation between the chemically bound modifier and the silane coupling agent on the fibre surface, resulting in a dramatic improvement in the mechanical properties, dimensional stability and retention of mechanical performance after immersion in fluids typically found in under-bonnet environments.A feature unique to some of these modifiers was their ability not only to enhance the mechanical properties of polypropylene composites to levels substantially in excess of currently available commercial materials, but their ability to act as effective thermal-oxidative polypropylene stabilisers. The mode of action was shown to be a chain-breaking mechanism and as a result of the high binding levels achieved during melt processing, these modifiers were able to efficiently stabilise polypropylene in the most severe volatilising and solvent-extracting environments, thus giving much better protection to the polymer than currently available commercially stabilised grades of polypropylene.

Effect of extrusion and photo-oxidation on polyethylene/clay nanocomposites

Polyethylene (a 1:1 blend of m-LLDPE and z-LLDPE) double layer silicate clay nanocomposites were prepared by melt extrusion using a twin screw extruder. Maleic anhydride grafted polyethylene (PEgMA) was used as a compatibiliser to enhance the dispersion of two organically modified monmorilonite clays (OMMT): Closite 15A (CL15) and nanofill SE 3000 (NF), and natural montmorillonite (NaMMT). The clay dispersion and morphology obtained in the extruded nanocomposite samples were fully characterised both after processing and during photo-oxidation by a number of complementary analytical techniques. The effects of the compatibiliser, the organoclay modifier (quartenary alkyl ammonium surfactant) and the clays on the behaviour of the nanocomposites during processing and under accelerated weathering conditions were investigated. X-ray diffraction, transmission electron microscopy (TEM), scanning electron microscopy (SEM), rheometry and attenuated reflectance spectroscopy (ATR-FTIR) showed that the nanocomposite structure obtained is dependent on the type of clay used, the presence or absence of a compatibiliser and the environment the samples are exposed to. The results revealed that during processing PE/clay nanocomposites are formed in the presence of the compatibiliser PEgMA giving a hybrid exfoliated and intercalated structures, while microcomposites were obtained in the absence of PEgMA; the unmodified NaMMT-containing samples showed encapsulated clay structures with limited extent of dispersion in the polymer matrix. The effect of processing on the thermal stability of the OMMT-containing polymer samples was determined by measuring the additional amount of vinyl-type unsaturation formed due to a Hoffman elimination reaction that takes place in the alkyl ammonium surfactant of the modified clay at elevated temperatures. The results indicate that OMMT is responsible for the higher levels of unsaturation found in OMMT-PE samples when compared to both the polymer control and the NaMMT-PE samples and confirms the instability of the alkyl ammonium surfactant during melt processing and its deleterious effects on the durability aspects of nanocomposite products. The photostability of the PE/clay nanocomposites under accelerated weathering conditions was monitored by following changes in their infrared signatures and mechanical properties. The rate of photo-oxidation of the compatibilised PE/PEgMA/OMMT nanocomposites was much higher than that of the PE/OMMT (in absence of PEgMA) counterparts, the polymer controls and the PE–NaMMT sample. Several factors have been observed that can explain the difference in the photo-oxidative stability of the PE/clay nanocomposites including the adverse role played by the thermal decomposition products of the alkyl ammonium surfactant, the photo-instability of PEgMA, unfavourable interactions between PEgMA and products formed in the polymer as a consequence of the degradation of the surfactant on the clay, as well as a contribution from a much higher extent of exfoliated structures, determined by TEM, formed with increasing UV-exposure times.

Novel organo-modifier for thermally-stable polymer-layered silicate nanocomposites

A new novel approach for the stabilisation of polymer-clay nanocomposites has been investigated based on reacting chemically an antioxidant function, a hindered phenol moiety, with an organic modifier based on a quaternary ammonium salt. The chemically linked antioxidant-containing organic modifier (AO-OM) was then introduced into natural montmorillonite (MMt) through a cation-exchange reaction resulting in antioxidant-containing organo-modified clay (AO-OM-MMt). The new antioxidant-containing modified clay, along with other organo-modified clays having a similar organo-modifier but without the reacted antioxidant, were characterised by spectroscopic, thermogravimetric and x-ray diffraction techniques and tested for their thermo-oxidative stability. PA11-based clay nanocomposites samples containing the AO-OM-MMt and the other organo-modified clays, both without and with an added (i.e. not chemically reacted) hindered phenol antioxidant (similar to the one used in the AO-OM) were prepared by melt processing and examined for their processing and long-term thermal-oxidative stability at high temperatures. It was shown that although the new organo-modifier, AO-OM, was also susceptible to the Hoffman elimination reaction, the nanocomposites containing this newly modified clay (PA11/AO-OM-MMt) showed higher melt processing and long-term thermo-oxidative stability, along with excellent clay dispersion and exfoliation, compared to the other PA11-nanocomposites examined here (with and without the conventionally added antioxidant). It is suggested here that the excellent overall performance observed for the PA11/AO-OM-MMt nanocomposites is due to an in-situ partial release of low molecular weight antioxidant species having stabilising functionalities that are capable of acting locally at the interface between the inorganic clay platelets and the polymeric matrix which is a critical area for the onset of degradation processes.

The effect of polypropylene-graft-maleic anhydride on the morphology and dynamic mechanical properties of polypropylene/polystyrene blends

Polypropylene (PP) and polystyrene (PS) blends were prepared by melt processing in a haake at 180 °C. PP/PS blends are immiscible and the blend morphologies were characterized by scanning electron microscopy. The viscoelastic properties were characterized using dynamic mechanical analysis (DMA) with reference to blend ratio. The blend morphologies such as matrix droplet and phase inverted morphologies were observed. The storage modulus of the blends increased with increase in PS content and the value was maximum for neat PS. DMA showed changes in the polystyrene glass transition temperatures (Tg) over the entire composition range. There was a sharp increase in the Tg of PS with increasing PP content in the blend and a 12 °C elevation in Tg was observed. The increase in Tg was explained by proposing a new model based on the physical interaction between the blend components. It is assumed that the different effects by the PP phase resulted in the formation of constrained PS chains leading to high Tg values. The addition of PP-g-MAH has a positive effect on the morphology, increases the storage modulus, and decreases the Tg till 80/20 blends. However, for PP/PS blends with higher concentrations of PS, the PP-g-MAH has little effect or adverse effect on the morphology, and storage modulus, but decreases the Tg.

Liquid encapsulated-melt zone processing of GaAs

A liquid encapsulated melt Bone process has been developed for single crystal growth of GaAs. Single crystals of 40 mm long have been grown with this technique. To avoid unwanted nucleation events and maintain a constant crystal diameter, from top to bottom growth using a short zone with a convex zone surface was found to give the best results. An arsenic overpressure was used to in conjunction with a B2O3 encapsulant in order to suppress arsenic dissociation from the melt and maintain the stoichiometry of the crystal.