Nikolaos E. Zafeiropoulos

A study of transcrystallinity and its effect on the interface in flax fibre reinforced composite materials

Abstract Cellulose fibres have long been used in the plastics industry as cost-cutting materials. Nowadays they are recognised as a potential replacement for glass fibres for use as reinforcing agents in composite materials. They have a number of certain advantages over glass fibres, such as low cost, high strength-to-weight ratio, biodegradability and ease of processing. In this study crystallisation from the melt of two different isotactic polypropylene matrices (iPP) in the presence of flax ( Linum usitatissinum ) fibres of four different types (green flax, dew retted flax, Duralin ® treated flax and stearic acid sized flax) was examined. The effect of processing parameters such as crystallisation temperature and cooling rate was investigated using hot stage optical microscopy. Differential scanning calorimetry (DSC) was used to investigate the inner morphology of the transcrystalline (TC) layer. Scanning electron microscopy (SEM) and X-ray diffraction were used in an attempt to identify the origin of the TC layer in connection with the structural characteristics of the fibres. The effect of transcrystallinity upon the mechanical properties of the interface was assessed using the single fibre fragmentation test. It was found that the interfacial adhesion is improved by the presence of a TC layer.

Engineering and characterisation of the interface in flax fibre/polypropylene composite materials. Part II. The effect of surface treatments on the interface

Abstract Natural fibres have attracted much attention recently for use as reinforcing agents in composite materials. However, even though natural fibres possess many advantages over glass fibres, such as lower density, lower cost and recycleability, they are not totally free of problems. Natural fibres are comprised mostly of cellulose, a highly hydrophilic macromolecule with strong polarity and, as a result, problems of compatibility with very apolar matrices (e.g. polyolefins) almost certainly arise. Surface treatments, although having a negative impact on economics, may improve the compatibility and strengthen the interface in natural fibre composite materials. In Part I of the present study two such surface treatments, acetylation and stearation, have been developed and applied to flax fibres. In this second part, the effect of these treatments upon the interface of flax fibre/polypropylene composites is assessed by means of fragmentation tests. It has been found that both treatments led to improvement of the stress transfer efficiency at the interface, and both applied treatments were optimised, accordingly.

Synthesis of novel tantalum oxide sub-micrometer hollow spheres with tailored shell thickness.

Sub-micrometer-sized hollow tantalum oxide (Ta2O5) spheres with tunable shell thickness and void size have been fabricated exploiting beta-diketone-functionalized polystyrene (PS) beads as sacrificial templates in a sol-gel process. First, a controlled precipitation of Ta2O5 nanoparticles was carried out on the template surface by hydrolyzing tantalum ethoxide (Ta(OEt)5) at room temperature, and subsequently, the polymer core was removed either via chemical treatment with toluene or calcination at 650 degrees C. The thickness of the tantala shell precipitated on the PS core during the coating process was tuned between 100 and 142 nm by varying the concentration of tantala precursor in the reaction media. The obtained Ta2O5-coated PS particles and hollow microspheres were characterized by scanning electron microscopy, transmission electron microscopy, infrared spectroscopy, X-ray diffraction, and thermogravimetric analysis. Due to the unique optical and dielectric properties, these nanostructured materials are envisaged to be used in applications such as novel building blocks for the fabrication of advanced materials, surface coatings, catalysts, and drug delivery systems.

Switchable photoluminescence of CdTe nanocrystals by temperature-responsive microgels.

In the present study, we report a method for preparing a fluorescent thermosensitive hybrid material based on monodisperse, thermosensitive poly( N-isopropyl acrylamide) (PNIPAM) microgels covered with CdTe nanocrystals of 3.2 nm diameter. The CdTe nanocrystals were covalently immobilized on the surface of PNIPAM microgels. The chemical environment around the CdTe nanocrystals was modified by changing the temperature and inducing the microgel volume-phase transition. This change provoked a steep variation in the nanocrystal photoluminescence (PL) intensity in such a way that when the temperature was under the low critical solution temperature (LCST) of the polymer (36 degrees C) the PL of the nanocrystals was strongly quenched, whereas above the LCST the PL intensity was restored.

Temperature sensitive hybrid microgels loaded with ZnO nanoparticles

We report on a generic approach for the preparation of ZnO loaded temperature sensitive hybrid microgels with structural hierarchy. The synthetic process involves controlled hydrolysis of Zn(CH3COO)2·2H2O salt in the presence of poly(N-vinylcaprolactam-co-acetoacetoxyethylmethacrylate) (PVCL-AAEM) microgel templates in 2-propanol. Transmission electron microscopy (TEM) and electron mapping image (EMI) analysis have been employed to prove the presence of ZnO nanoparticles inside the polymeric templates. Dynamic light scattering (DLS) measurements reveal that hybrid microgels display a temperature sensitivity similar to the pure template particles, even at a high loading of ZnO nanoparticles. Changes in size, morphology and physical properties of hybrid microgels as a function of the loading amount of ZnO nanoparticles have been discussed. We demonstrate that these nanostructured materials can effectively be used as transparent UV-shielding materials. In addition, these submicrometre-size hybrid microgels could be applied in the fields of optoelectronic devices, UV-detectors, and photocatalysts.

Highly ordered arrays of magnetic nanoparticles prepared via block copolymer assembly

In the present study we report a simple and reproducible method to prepare highly ordered arrays of Fe3O4 superparamagnetic nanoparticles (MNPs) via block copolymer (BCP) self assembly. Pre-synthesized MNPs with a mean diameter of 6.1 nm are selectively segregated within the lamellae or hexagonally packed cylinders composed of PVP blocks in poly(styrene-b-vinylpyridine) (PS-b-PVP) block copolymers without any additional surface modification. The density of the stabilizing shell in the MNPs as well as the position of pyridine nitrogen in the PVP block of the BCPs are found to be crucial for selective incorporation of MNPs into the PVP domains. The obtained results suggest a key importance of a mutual affinity between active blocks and the nanoparticles which should be maximized in order to attain high nanoparticles loadings and long-range structural orders.

Template-Assisted Fabrication of Magnetically Responsive Hollow Titania Capsules

This study reports on the fabrication of magnetically responsive hollow titania capsules by confining the superparamagnetic Fe(3)O(4) nanoparticles within a hollow and porous titania (TiO(2)) shell. The employed protocol involves precipitation of titania shell on the magnetite (Fe(3)O(4)) encapsulated polystyrene beads followed by the calcination of resulting composite particles at elevated temperature. Scanning electron microscopy and transmission electron microscopy reveal the presence of a thick, complete but irregular titania shell on the magnetic polystyrene beads after the templating process. Electron energy loss mapping image analysis has been employed to investigate the spatial distribution of titania and magnetite phases of magnetic hollow titania capsules (MHTCs). Magnetic characterization indicates that both titania-coated magnetic polystyrene beads (TMPBs) and MHTCs are superparamagnetic in nature with the saturated magnetizations of 5.6 and 8.1 emu/g, respectively. X-ray diffraction (XRD) analysis reveals that titania shell of these capsules is composed of photoactive anatase phase. Nitrogen adsorption-desorption analysis has been employed to estimate the specific surface area and the average pore diameter of the fabricated hollow structures. Photocatalytic activity of the fabricated MHTCs for the photodegradation of rhodamine 6G dye has been demonstrated and compared with that of bulk titania nanoparticles.

Investigation of the relationship between hydrogen bonds and macroscopic properties in hybrid core-shell gamma-Fe2O3-P(NIPAM-AAS) microgels.

We investigate in a hybrid material the interactions existing between magnetic nanoparticles of gamma-Fe(2)O(3) and the polymer matrix constituted by core-shell poly(N-isopropylacrylamide-sodium acrylate) microgels. These interactions provoke the shifting of the microgel volume phase transition to higher temperatures when the amount of gamma-Fe(2)O(3) increases. The study was performed using different techniques such as incoherent quasi-elastic neutron scattering (IQNS), infrared spectroscopy (FTIR-ATR), and dynamic light scattering (DLS). Below the low critical solution temperature (LCST) of the polymer, the IQNS data confirm that the presence of inorganic nanoparticles affects the PNIPAM chain motions. Thus, in the swollen state both the mean-square displacement of the polymer segments and the diffusive motion of the polymer chains decrease as the iron oxide content increases. The FTIR-ATR study indicates that the reduction of vibrational and diffusional motions of the polymer chains is due to the formation of hydrogen bonds between the amide groups of the polymer matrix and the OH groups of the magnetic nanoparticles. The creation of this hybrid complex would explain the reduction of the swelling capacity with increasing the iron content in the microgels. Furthermore, this interaction could also explain the shift of the polymer LCST to higher temperatures as due to the extra energy required by the system to break the hydrogen bonds prior to the PNIPAM collapse.

On the use of single fibre composites testing to characterise the interface in natural fibre composites

In recent years, natural fibre composites have received considerable attention as a serious contender to replace glass fibres in composite material applications. One of the key aspects in composite materials is the interface between the reinforcing fibres and the matrix and a critical assessment of the interfacial bond is needed for a successful design of the final component. Natural fibres possess many intriguing advantages over man-made fibres such as glass, but they also present serious difficulties, especially in terms of material heterogeneity and more specifically in terms of fibre diameter. In this sense, most of the traditional methods for interfacial characterisation are difficult to apply, since the required data reduction involves the use of stress analysis or fracture mechanics approaches in which the fibre diameter is a critical parameter. In the present study, interfacial characterisation is discussed for flax fibre/polypropylene composites and a sensitivity analysis is presented for the single fibre fragmentation test. The results indicate that traditional stress analysis fails to correctly assess the interface, whilst a statistical based data analysis can overcome the fibre heterogeneity problem.

Nanoparticle directed domain orientation in thin films of asymmetric block copolymers

We investigated the thin film morphology of two different asymmetric block copolymers (BCP), polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and poly(n-pentyl methacrylate)-block-poly(methyl methacrylate) (PPMA-b-PMMA), loaded with pre-synthesized iron oxide nanoparticles (NP). The chemical composition of the BCP constituents determines the strength of the interaction between polymer chains and nanoparticles. In the case of NP/PS-b-P4VP system, the nanoparticles interact preferentially with the P4VP block and hence localize selectively in the P4VP cylindrical microdomains. However, for the NP/PPMA-b-PMMA system, the nanoparticles have no significant preference for the copolymer blocks and segregate at the polymer/substrate interface. Interestingly, this changes the effective substrate surface energy and hence leads to a remarkable change in domain orientation from parallel to perpendicular with respect to the substrate. These results clearly demonstrate the importance of both enthalpic and entropic factors which determine spatial distribution of NP in BCP films and influence domain orientation.