Observation of transcrystalline growth of PLA crystals on sisal fibre bundles and the effect of crystal structure on interfacial shear strength

Hot-stage microscopy was used to characterise crystal growth at the interface between sisal fibre bundles and a polylactic acid (PLA) matrix in order to better understand the mechanical properties of sisal fibre–PLA composites. Cooling rates and crystallisation temperatures and times were varied to influence crystalline morphology at the interface. Single sisal fibre bundles were evaluated in their as received state or treated with 6 wt.% caustic soda solution for 48?h at room temperature. A microbond shear test was used to characterise the shear strength of the interface as a function of fibre surface treatment. These tests were performed on sisal fibre bundles carefully embedded in flat films of PLA supported on card mounts. Fibre bundles in a PLA matrix were cooled from 180?°C at rates from 2 to 9?°C/min and then crystallised isothermally. For as received fibre bundles uneven growth of PLA spherulites occurred at all cooling rates and crystallisation temperatures. For caustic soda treated fibres, uneven spherulitic growth was observed at crystallisation temperatures at and above 125?°C. In contrast, transcrystalline growth was observed for samples cooled to 120?°C at cooling rates from 2 to 6?°C/min and then allowed to crystallise. The microbond shear strengths of untreated and caustic soda treated fibre bundles were evaluated using Weibull statistics and the caustic soda treated fibres exhibited higher interfacial shear strengths in comparison to untreated fibres, reflecting the development of a transcrystalline layer at the fibre to matrix interface.     

Electrospun cerium nitrate/polymer composite fibres: synthesis, characterization and fibre-division model

Cerium (Ⅲ) nitrate/poly(vinylpyrrolidone) (Ce(NO3)3/PVP) composite fibres have been prepared by electrospinning. After calcining the composite fibres in air at 500℃,CeO2 nanowires were obtained. The characterizations of the as-spun composite fibres and resultant nanowires have been carried out by a scanning electron microscope (SEM),an infrared spectrometer,an x-ray diffractometer and a fluorescence spectrophotometer. Interestingly,some unusual ribbon-like or twin fibres were observed besides the common fibres with circular or elliptic cross sections. We developed a fibre-division model resulting from Coulomb repulsion and solvent vaporization to interpret the formation of the ribbons or twin fibres,which has been confirmed by the SEM studies. Our results also indicate that the formation of the ribbons or twin fibres is less dependent on operation voltage and work distance.     

Study of 1-3 PZT fibre/epoxy composite force sensor

Lead zirconate titanate (PZT) fibres were prepared by a powder-based extrusion method. Pre-sintered PZT powder mixed with poly(acrylic acid) was spun in a spinnerette to produce fibres. The fibre of ∼400 μm diameter was used to fabricate 1-3 PZT fibre/epoxy composite discs with different volume fractions (ϕ) of PZT. Since the ceramic fibres are rather brittle, their elastic properties cannot be measured directly. In order to determine the properties of the ceramic fibres, effective properties of the fibres/epoxy 1-3 composite were measured. By using a modified series and parallel model, the properties of 1-3 composites can be calculated. Then, the elastic coefficient s_33,fibre^E, relative permittivity ε_33,fibre^T and piezoelectric strain coefficient d_33,fibre of the ceramic fibre could be found. Ring-shaped PZT fibre/epoxy materials composites with different ϕ were fabricated to be used as the sensing material in force sensor applications. The ring-shape composite with ϕ=0.5 was installed into a housing and the sensor was calibrated by different methods and its sensitivity was found to be 144 pC/N within the frequency range of 0.5–6.4 kHz which is much higher than that of a quartz force sensor with a similar structure. PACS 07.07.Df; 72.80.Tm; 77.84.Dy     

Electrospun PLA: PCL composites embedded with unmodified and 3-aminopropyltriethoxysilane (ASP) modified halloysite nanotubes (HNT)

Electrospinning is a simple and versatile fiber synthesis technique in which a high-voltage electric field is applied to a stream of polymer melt or polymer solution, resulting in the formation of continuous micro/nanofibers. Halloysite nanotubes (HNT) have been found to achieve improved structural and mechanical properties when embedded into various polymer matrices. This research work focuses on blending poly(ε-caprolactone) (PCL) (9 and 15 wt%/v) and poly(lactic acid) (PLA) (fixed at 8 wt%/v) solutions with HNT at two different concentrations 1 and 2 wt%/v. Both unmodified HNT and HNT modified with 3-aminopropyltriethoxysilane (ASP) were utilized in this study. Fiber properties have been shown to be strongly related to the solution viscosity and electrical conductivity. The addition of HNT increased the solution viscosity, thus resulting in the production of uniform fibers. For both PCL concentrations, the average fiber diameter increased with the increasing of HNT concentration. The average fiber diameters with HNT-ASP were reduced considerably in comparison to those with unmodified HNT when using 15 wt%/v PCL. Slightly better dispersion was obtained for PLA: PCL composites embedded with HNT-ASP compared to unmodified HNT. Furthermore, the addition of HNT-ASP to the polymeric blends resulted in a moderate decrease in the degree of crystallinity, as well as slight reductions of glass transition temperature of PCL, the crystallization temperature and melting temperature of PLA within composite materials. The infrared spectra of composites confirmed the successful embedding of HNT-ASP into PLA: PCL nanofibers relative to unmodified HNT due to the premodification using ASP to reduce the agglomeration behavior. This study provides a new material system that could be potentially used in drug delivery, and may facilitate good control of the drug release process.     

Fabrication and Optimization of Poly(vinyl alcohol)/Zirconium Acetate Electrospun Nanofibers Using Taguchi Experimental Design

This paper presents an investigation regarding poly(vinyl alcohol)/zirconium acetate (organic–inorganic) (PVA/Zrace) nanofibers prepared by electrospinning which could be used as a precursor for fabricating ceramic metal oxide nanofibers. The effect of some processing variables, including polymer solution concentration, tip to collector distance and applied voltage of electrospinning, and the amount of Zrace and their interactions, on the diameter of the nanofibers were studied. Taguchi experimental design and a statistical analysis (ANOVA) were employed and the relationship between experimental conditions and yield levels determined. It was concluded that to obtain a narrow diameter distribution as well as maximum fiber fineness, a polymer concentration of 10 wt%, tip to collector distance of 18 cm and applied voltage of 20 kV variables were the optimum. Furthermore, it was also concluded that the ratio of Zrace (6 g) to PVA solution (10% wt) played an important role for achieving the minimum fiber diameter. Under these optimum conditions, the diameters of the electrospun composite fibers ranged from 86 nm to 381 nm with a diameter average of 193 nm. The experiments were done with Qualitek-4 software with “smaller is better” as the quality characteristics. The optimized conditions showed an improvement in the fibers diameter distribution and the average fibers diameter showed good resemblance with the result predicted using the Taguchi method and the Qualitek-4 software. The ANOVA results showed that all factors had significant effects on the fibers diameter and distribution, but the effect of PVA concentration and zirconium acetate were more significant than the other factors.     

Nonisothermal Crystallization Kinetics of Poly(lactic acid)/Nanosilica Composites

Poly(lactic acid) (PLA)/nanosilica composites were prepared by blending the PLA and nanosilica in chloroform and then evaporating the solvent to form the composite films in a dish. The Ozawa and Mo equations were used to characterize the nonisothermal cold crystallization kinetics of the PLA/nanosilica composites. The results indicated that the Ozawa equation was not successful while the Mo equation was successful to describe the nonisothermal crystallization kinetics of PLA/nanosilica composites. The values of crystallization activation energy (E _c) of the samples were calculated by the Kissinger method. Although the sample crystallization rates were enhanced with the increase of nanosilica content, the samples exhibited increased E _c in the presence of nanosilica. The results showed that nanosilica had an effect on both the nucleation and the crystal growth of PLA, promoting the nucleation but interfering with the molecular motion of PLA in the crystallization process.     

Thermal and Mechanical Properties of Poly(lactic acid)/Modified Carbon Black Composite

Poly(ethylene glycol) (PEG) was added as a plasticizer to the composite of poly(lactic acid) (PLA) and a modified carbon black (MCB). Among the three different molecular weight (M_n = 1000, 2000, 6000) PEGs used, PEG2000 promoted crystallization of PLA and enhanced the nucleation activity of MCB more efficiently than the other two. The crystallization rate of PLA/PEG2000/3 wt% MCB composite was three times that of PLA. Although a small decrease in tensile strength and modulus of elasticity of the composite was found as the PEG content increased, the elongation at break of the PLA/PEG/MCB composites significantly improved. When the PEG2000 content was 15 wt%, the elongation at break of the blend was 90%, 4.5 times that of the neat PLA.     

Preparation and Properties of Polylactide/Poly(ethylene-co-octene)/nano-SiO2 Ternary Composites

Polylactide (PLA)/poly(ethylene-co-octene)(POE) blends with various contents of nano-SiO₂ were prepared via melt mixing. The structure and properties of the PLA/POE/nano-SiO₂ ternary composites were studied by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), rheometry, and tensile testing. The particle size of the dispersed POE phase first decreased with increasing nano-SiO₂ content and then remained constant. Nano-SiO₂ played an important role in the heterogeneous nucleation of PLA, which resulted in an increase of the crystallinity of PLA. The synergistic effect of both POE and nano-SiO₂ can significantly improve the toughness, strength, and modulus of PLA. When the ratio of PLA/POE/nano-SiO₂ was 90/10/0.5, PLA/POE/nano-SiO₂ composite had the best comprehensive properties.     

The effect of bias voltage and working pressure on S/Mo ratio at MoS2-Ti composite films

S/Mo ratio has a crucial effect on the tribological properties of MoS₂-Ti composite films. The deposition parameters as such bias voltage and working pressure play a dominant role on the change of this ratio value. To determine the effect of working pressure and bias voltage on S/Mo ratio, MoS₂-Ti composite films were deposited on glass wafers by pulsed-dc magnetron sputtering (PMS). The deposition process was performed for nine different test conditions at various levels of target current, working pressure, and substrate voltage using the Taguchi L₉(3⁴) experimental method. It was observed that the chemical composition of MoS₂-Ti composite films was significantly affected by sputtering parameters. It was also observed that S/Mo ratio decreased as the bias voltage increased at a constant working pressure and S/Mo ratio increased with increasing working pressure at a constant bias voltage.     

Nucleation of the D1a type ordered phase in the near-surface of A4 B alloy: I. Nucleation in bulk

Nucleation in the D1_a type A₄B ordering alloy was studied by means of an atomistic calculation based on the Bragg-Williams approximation, as a first step in the understanding of ordering phenomena in the near-surface. A detailed investigation was made for bulk Ni₄ Mo on the free energy (ΔF) surface of the system containing a nucleus as a function of the size and the degree of order of the nucleus. The nucleation rate, and the degree of order and the size of the nucleus at temperatures above the instability temperature T₀ were given as a function of temperature by employing a phenomenological nucleation theory. Above T₀ (= 0.844 T_c) the Δ F surface is dam-shaped and has a saddle point, while it has neither dam shape nor saddle point below T₀. The nature of nucleation is different at higher and lower temperatures: at temperatures higher than T₀ a critical nucleus must have a high degree of order although the value is not so large as the equilibrium degree of order, while the nature of nucleation has a continuous mode at lower temperatures. By examining the calculated results, it was concluded that the use of this nucleation theory was valid at least qualitatively.     

Fibre matrix adhesion of natural fibres cotton,flax and hemp in polymeric matrices analyzed with the single fibre fragmentation test

In this research the adhesion and the resulting interfacial shear strength (IFFS) between the natural fibres flax, hemp and cotton and the polymer matrices polypropylene with coupling agent (MAPP) and polylactide acid (PLA) was surveyed with the single fibre fragmentation test (SFFT). The adhesion between MAPP and the fibres was good enough to produce fragments, whereas the adhesion between PLA and flax was too weak to transmit enough tension for fibre cracks which is clearly visible on SEM-photographs. Comparing the IFFS values of the fibres in MAPP with an equal fibre diameter shows that the IFFS value of flax is highest with 7.09 N/mm² followed by hemp 6.13 N/mm². The IFFS of cotton is a lot smaller (0.664 N/mm²). The critical fragmentation or fragmentation length of the bast fibres flax (3.16 mm) and hemp (3.20 mm) in MAPP is smaller than the critical fragmentation length of cotton (5.03 mm). The adhesion between the lignocellulosic fibres and MAPP is much better than between the lignin and pectin free cellulose fibre and MAPP. Possible reasons for this — the surface structure of the cotton fibre and its different chemical composition being made up of only cellulose, hemi-cellulose and wax with no pectin or lignin present — are discussed.     

Methods to determine surface energies of natural fibres: a review

To tailor the interaction across composite interfaces especially for the development of green composites, i.e. composites made completely from renewable materials, information about the fibre surfaces is required. We review the current state of the art of methods to determine the surface tension of natural fibres and discuss the advantages and disadvantages of techniques used. Although numerous techniques have been employed to characterise surface tension of natural fibres, it seems that commonly used wetting techniques are very much more affected by the non-ideal character of natural fibres. Inverse Gas Chromatography (IGC) is a much better suited technique to determine the surface energetic properties of natural fibres than wetting techniques. The surface tension of natural reinforcements, determined using IGC, was reported for nanosized bacterial cellulose as well as bamboo, cornhusk, flax, hemp and sisal, covering a wide range of cellulose content. The effect of methods to separate/extract fibres from the plants as well as of a few surface modification procedures on the fibre surface properties is also reviewed. The dispersive part of the natural fibre surface tension γ ^d _S varies from 32 to 61 mJ/m². The fibre surface tension increases with increasing cellulose content of natural fibres. We also found that a higher basicity (Donor Number, K _B to Acceptor Number, K _Aratio) was observed for fibres containing more cellulose. This may be reflective of higher crystalline cellulose content in the surfaces of the fibres, as only the ether linkage of the cellulose is labile for hydrogen bonding.     

On the mechanisms of photogeneration of charge carriers in PEPC-C60 composites

Photogeneration of electric charge carriers at high strengths of the external electric field and the temperature of transition to the viscous-flow state (T _visc) of thin PEPC-C₆₀ composite films obtained by casting from a toluene solution have been studied. The rheology of the composite films has been investigated by the nondestructive optical method. The consistent correlated change in the values of T _visc and the effective temperature in the expression for the photogeneration quantum yield (T ₀) in the Meltz representation with variations in the C₆₀ concentration has been established. The difference between T _visc and T ₀ in thin composite films does not exceed 2–3% of T _visc (T ₀).     

Nucleation of the D1a type ordered phase in the near-surface of A4B alloy: II. Nucleation at surface

Nucleation in the D1_a type A₄B ordering alloy was investigated by means of an atomistic calculation based on a phenomenological nucleation theory and the Bragg-Williams approximation. The nucleation rate at the Ni₄Mo surface is presented as a function of temperature, surface orientation, and type of the plane to which the surface is parallel (fundamental plane or superlattice plane). The radius, degree of order and activation free energy of the critical nucleus are also given as a function of temperature. The value of the nucleation rate at the surface is compared with that in the bulk. The results are as follows: (i) at temperatures near T_c. the nucleation rate is higher at the surfaces than in the bulk and the {200}_FCC surfaces are the highest in nucleation rate; (ii) however, at temperatures near the instability temperature t₀, the nucleation rate is lower at the surface than in the bulk; (iii) the present calculated results give reasonable explanations to our experimental results on the Ni₄ Mo alloy: (a) preferential surface ordering at high temperatures near T_c; (b) surface disordering at intermediate temperatures near the nose temperature.     

Microstructure of a composite with a quasicrystalline phase fraction obtained by directional solidification of Al61Cu27Fe12 alloy

The microstructure of a composite containing a quasicrystal phase, i.e. so-called crystal–quasicrystal (CQ) composite, was studied. The CQ composite was obtained by the Bridgman method via solidification of Al₆₁Cu₂₇Fe₁₂ alloy (numbers indicate at%). The process was conducted at a pull out rate of v = 0.07 mm/min. The average temperature gradient in the heating zone was 43 K/cm. The composite matrix consisted of cubic β phase Al(Fe, Cu), with reinforcement of λ-phase rod-shaped fibres surrounded by a quasicrystal icosahedral ψ phase, which also existed in the fibre core. The fibres were rhomboidal in cross-section. The composite was studied using X-ray and electron diffraction, light-optical and scanning electron microscopy (SEM), X-ray topography and Laue diffraction.     

Spontaneous generation of the electrical voltage in charge-ordered manganite Pr0.6Ca0.4MnO3

Spontaneous generation of the electrical voltage in a Pr_0.6Ca_0.4MnO₃ single crystal has been found. The charge and orbital ordering in the crystal takes place at T _CO = 240 K, while the antiferromagnetic ordering occurs at T _N = 174 K. As the temperature lowers, spontaneous voltage U increases initially slowly (in the temperature range from 300 K to T _CO) and then more rapidly (in the range T _CO-T _N ). Starting from T _N , the voltage U increases exponentially and, at 85 K, reaches 115 mV (in the ab plane) and 6.5 mV (along the c axis). The magnetic field differently affects the voltage U in different temperature ranges: it decreases the value of U in the temperature range of 85–130 K and increases in the range of 130–240 K. It is assumed that the spontaneous voltage is associated with the existence of ferromagnetic and charge orbital ordered clusters of different topologies in the crystal.     

Microstructure and mechanical properties of continuous Al2O3 fibre reinforced Ni45Al45Cr7.5Ta2.5 alloy (IP75) matrix composites

Single crystalline Al₂O₃ fibres (sapphire), coated with the NiAl alloy IP75 by physical vapour deposition (PVD), were assembled to fabricate composites by means of diffusion bonding. The microstructure and chemistry of both as-coated fibre and as-diffusion bonded composites were investigated by electron microscopy and microanalysis. The interface shear stress for complete debonding was measured by fibre push-out tests at room temperature, and the composite tensile strength was measured at 900°C and 1100°C. An amorphous layer with a thickness of about 400?nm formed between the fibre and the matrix during the PVD process and was maintained during diffusion bonding. A Laves phase precipitated along NiAl grain boundaries in the IP75 matrix. This caused a lower tensile strength of the IP75/Al₂O₃ composite at high temperatures compared to as-cast monolithic IP75 and rendered the composite useless for structural applications.     

A model for high-spin/low-spin transitions with an interpretation of thermal hysteresis effects

A model for high-spin/low-spin transitions is discussed. It can explain the temperature dependence of the high-spin fraction of the compound Fe(4, 7-(CH₃)₂-1,10-phenanthroline) (NCS)₂ and allows to interpret hysteresis effects at T_c = 120 K as a result of the existence of two minima for the free energy. The transition is assumed to take place by nucleation. The smoothness of the transition (1.5 K wide) is due to a distribution of the nucleation rate within the microcrystals of the powder sample. The distribution of the T_c is smaller 0.1 K.     

Toughening of Poly(L-Lactic Acid) by Annealing: The Effect of Crystal Morphologies and Modifications

Poly(L-lactic acid) (PLA) samples prepared by hot pressing were treated by two different processes: process a—quenching of the molten specimens in water, then annealing at high temperatures; process b—direct crystallization at high temperatures from the molten specimens. The crystal modification and morphology of PLA were investigated by differential scanning calorimetry, dynamic mechanical analysis, polarized light microscopy, and wide-angle X-ray diffraction. In the case of process a, the α′ (disordered α) crystal modification was formed at relatively low annealing temperature (T _a < 100°C), while the ordered α phase was formed in the case of process a at high T _a (> 100°C) and process b. Furthermore, in process a, the nucleation density of spherulites was higher and the radius of the spherulites was much smaller compared with that of the spherulites formed by process b. The effects of crystal modification and morphology on the impact behaviors of PLA were investigated by notched Izod impact testing. The macroscopic fracture toughness was discussed in terms of the microscopic structures. Finally, we suggest an alternative approach for the preparation of high-performance PLA.     

Kinetics of isothermal nucleation in a supercooled iron melt

An isothermal kinetic diagram for the beginning of homogeneous nucleation is constructed using the molecular-dynamics model of an instantaneously supercooled iron melt near the icosahedral percolation transition temperature identified with the glass transition temperature T _g . This diagram is compared with the theoretical one calculated using quantitative information obtained by analyzing the kinetics of the initial stage of growth of supercritical nuclei at temperatures higher than T _g . A satisfactory coincidence of the theoretical curve with computer simulation data at temperatures higher than T _g and substantial disagreement with these data below T _g , where crystallization is necessarily preceded by the formation of an icosahedral percolation cluster, demonstrate the substantive influence of an icosahedral substructure on the nucleation rate predicted by the classical theory.