Two-phase Morphology Research Articles

Application of the Digital Material Representation to strain localization prediction in the two phase titanium alloys for aerospace applications

In the present work Digital Material Representation (DMR) approach was utilized to simulate the deformation behavior of the two phase Ti-6Al-4V alloy. DMR models of the two phase structure, containing different morphologies of alpha grains within a beta matrix – lamellar and equiaxed, were created. Each phase was then separated and different mechanical properties were assigned. Subsequently, their response to loading was tested using simple shear numerical simulations with special focus on strain inhomogeneities, as the main driving force for spheroidization is considered to be the formation of intense shearing within alpha lamellae. The proposed modeling approach combining Finite Element Method (FEM) with DMR allowed for much more detailed numerical analysis of deformation behavior of two phase titanium alloys at the micro scale and provided information such as strain localization and stress distributions within the alpha and beta phases. It was showed that presented model offers a new and powerful tool to study the physical bases of microstructure evolution processes such as spheroidization or recrystallization of Ti alloys. It shows good potential in simulation of deformation processes of complex two-phase morphologies that is a crucial step towards optimization of process parameters during hot forming of Ti-6Al-4V alloys.

Improved mechanical properties of ATBN-toughened epoxy networks by controlling the phase separation scale

abstractAmine-terminated butadiene acrylonitrile (ATBN) rubber-toughened epoxy networks with different phase separation scale were formulated by adjusting acrylonitrile content in ATBN, and the effect of phase separation on stress–strain behavior, impact strength, dynamic mechanical thermal properties and thermo-gravimetric performance of the toughened epoxy networks were studied in detail. Scanning electron microscope analysis demonstrated that phase separation occurred in the toughened networks, forming a two-phase morphology, and the size of rubber particles was highly dependent on the acrylonitrile content in ATBN. Toughened epoxy network with larger phase separation scale exhibited longer ultimate elongation, and smaller phase separation scale was proved to be more effective in improving impact strength. Furthermore, bimodal rubber-particle distributed epoxy networks were obtained by addition of two types of ATBN simultaneously. The impact strength of the bimodal rubber-particle distributed epoxy network showed a great increase of 47% without sacrificing other mechanical properties, as compared to those of unimodal rubber-particle distributed epoxy networks.

An investigation of epoxy/thermoplastic blends based on addition of a novel copoly(aryl ether nitrile) containing phthalazinone and biphenyl moieties

In this paper, a novel soluble copoly(aryl ether nitrile) containing phthalazinone and biphenyl moieties (PPBEN) was synthesized for the first time to improve the impact resistance of tetraglycidyl 4,4'-diaminodiphenylmethane epoxy resin cured with 4,4-diaminodiphenylsulfone. Then a series of blends were prepared via solution blending with different contents of PPBEN. The thermal and mechanical properties and the micromorphology of the cured blends were investigated by differential scanning calorimetry, dynamic mechanical analysis (DMA), parallel plate rheometry, mechanical property tests and SEM analysis, respectively. The results indicated that the incorporation of thermoplastic PPBEN delayed the epoxy curing reaction, and the crosslinking density of epoxies was also reduced. The no-notch impact strength of the cured blend with 15% PPBEN was up to 16.7 kJ m−2, higher by about 104% than that of pure epoxy resin without sacrificing the modulus due to a specific sea-island structure. All the blends showed two-phase morphology characterized by DMA and SEM. The size of the thermoplastic morphology was only 70−80 nm, much less than that of commonly used thermoplastics, due to the special segment structure of PPBEN. © 2015 Society of Chemical Industry

Simulation of liquid thin film evaporation and boiling on a heated hydrophilic microstructured surface by Lattice Boltzmann method

Understanding heat transfer process of liquid thin film evaporation in porous media or on microstructured surfaces is the key for optimal designs of phase-change based thermal management devices, such as vapor chambers or flat heat pipes. In this paper, a newly developed 3D thermal Lattice Boltzmann method (LBM), incorporating double distribution functions (DDF), pseudo-potential model and equation of state (EOS) of a real gas, is used to simulate dynamic evaporation along the vapor/liquid interface and boiling in the liquid thin film on a microstructured surface consisting of micro pillars. Simulations are conducted under constant heat fluxes for four solid hydrophilic surfaces with contact angles of 10°, 23°, 37° and 63° respectively. Residual liquid volume, two-phase interface morphology, wall temperature distribution, as well as effective heat transfer coefficient and CHF under different conditions are discussed in details. With increasing heat flux, it is found that the liquid thin film recedes gradually into the wick structure, with the lowest liquid level at the center of the heating area, to maintain a continuous liquid supply via capillary pumping. The receding of the liquid thin film can normally enhance evaporation heat transfer because it not only reduces the thermal conduction resistance of liquid thin films, but also elongates the evaporating liquid meniscus along the height direction of micro pillars. For larger contact angles of 37° and 63° for example, dry-out initiates from the heating center at a sufficiently high heat flux, leading to serious deterioration of heat transfer with expansion of the dry-out area. However, for smaller contact angle of 10° and 23°, fixed and periodical bubble nucleation appears sequentially before dry-out heat flux. At small heat fluxes, there is no obvious difference in effective heat transfer coefficient for four hydrophilic surfaces. On the other hand, considerable increase of effective heat transfer coefficient can be obtained at higher heat flux on a stronger hydrophilic surface because of the receding of the liquid film can sustain a higher dry-out heat flux, or CHF.

Compatibilization by Olefin Block Copolymer (OBC) in Polypropylene/Ethylene-Propylene-Diene Terpolymer (PP/EPDM) Blends

The compatibilization by olefin block copolymer (OBC) in the blends of polypropylene (PP)/ethylene-propylene-diene terpolymer (EPDM) and the phase morphology of the ternary blends were investigated by rheology, scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD) measurements. It was found that the PP/EPDM blends exhibited enhanced mechanical properties in the presence of OBC. The addition of OBC had a significant influence on the phase separation behavior of the blends. For the PP/EPDM-50/50 heterogeneous blends, the addition of 15 phr OBC enabled the two-phase morphology to change from a droplet-matrix structure to a co-continuous one. In the temperature range of 150 to 200 °C, OBC was shown to have a better compatibility with PP than EPDM. The changes in viscosity ratio of the dispersed phase to matrix phase caused by adding OBC might be the dominant factor in controlling the coalescence of the dispersed phase domains. For the crystallization behavior of PP/EPDM/OBC ternary blends, OBC was found to have an induction effect on the formation of β-crystals of PP that was not proportional to the volume of OBC addition. In addition, DSC results showed that PP could induce the OBC crystallization and improve the crystallization temperature of OBC. The existence of simultaneous crystallization behavior between PP and OBC was also observed. A possible mechanism of phase evolution induced by crystallization was proposed.

Two-domain morphology in viscoelastic polyurethane foams

The morphology of viscoelastic polyurethane (PU) foams is examined and compared with flexible, elastic PU foam. Hard and soft domains for both foam types are found with atomic force microscopy, with good signal to noise. TD-NMR spectroscopy is sensitive to the presence of multiple phases through the contrast in the segmental mobility of the domains. Results on the viscoelastic foams, as with the flexible foams, indicate two-phase morphology, contrary to the conventional wisdom in the industry that viscoelastic polyurethane foams are phase-mixed. Segmental mobility is correlated to tan delta at the experimental temperature. The segmental mobility of the soft domain is correlated to the viscoelastic recovery time of the memory foams. The example of viscoelastic foams demonstrate the detection sensitivity of TD-NMR, where domains with similar composition/density but different mechanical strength are distinguished through segmental mobility contrast.

Two-phase separation in a lithiated spinel Li4Ti5O12 crystal as confirmed by electron energy-loss spectroscopy

The coexistence state of two phases (Li4Ti5O12 and lithiated Li7Ti5O12) in spinel lithium titanium oxide (LTO; Li4Ti5O12) is examined using scanning transmission electron microscopy with electron energy-loss spectroscopy (STEM–EELS). A single crystalline LTO specimen is prepared from a TiO2 wafer and is then partially lithiated. STEM–EELS spectrum imaging of the lithiated specimen reveals that the two phases exist separately inside the specimen with phase boundaries, and no apparent misfit strains or misorientations are detected. The phase interface appears to be preferentially formed parallel to the (100) plane owing to preferential Li diffusion along the <011> and equivalent directions in the LTO crystal. The observed two-phase morphology can be explained by the fact that relative to solid solutions, the Li7Ti5O12 phase can accommodate inserted Li atoms with negligible lattice distortions or volume changes in the common Ti–O bond framework.

Synthesis and Characterization of Liquid Natural Rubber as Impact Modifier for Epoxy Resin

Liquid natural rubber (LNR) with a molecular weight Mn =16×103 was prepared by the depolymerization of deproteinized natural rubber latex (DPNR). The liquid natural rubber (LNR) was characterized by FTIR and H’NMR spectroscopic analysis. LNR was premixed with the epoxy resin (EP) and cured with a diamine curing agent for 1 h at 100 °C and post cured at 110 °C, for 2 h in air oven. The modified EP containing different contents of LNR (5, 10, 15 and 20 phr) were evaluated. Thermal, mechanical and morphology properties were determined. The fracture toughness (KIC) of both unmodified and modified EPs were determined on static loaded single edge notched (SEN-B) specimens at room temperature. The glass transition temperatures (Tg) of the modified EPs were decreased with increasing LNR content. The strengths and modulus of EPs were slightly reduced with the incorporation of LNR. The effect was also reflected in the significant increase in the tensile strain of modified EP. Fracture toughness of the EP was observed to increase with the presence of LNR. The toughening effect became more apparent as the testing speeds were increased from 1 to 500 mm/min. Fracture surface analysis by scanning electron microscopy (SEM) revealed the presence of a two-phase morphology.

Study on thermal stability of polyurethane-urea based on polysiloxane and polycaprolactone diols

Different grades of segmented polyurethane-urea were synthesized through two-step solution polymerization of polydimethylsiloxane (PDMS) and polycaprolactone (PCL) polyols with methylene diphenyl diisocyanate (MDI) and mixture of 1,4-buthanediol and 4,4-methylene bis (3-chloro 2,6-diethylaniline) in toluene/tetra-hydrofuran media. Structural characterization of the synthesized samples was conducted using Fourier transform infra-red spectroscopy. X-ray diffraction, dynamic mechanical thermal analysis, thermal gravimetric analysis, and differential scanning calorimetry techniques were utilized to assess material characteristics. The results showed a relationship between PDMS content and thermal stability, morphology and mechanical properties of the urethane-urea samples. Onset degradation temperature was increased by increasing the PDMS content in the polyurethane backbone where the crystallinity was varied versus PDMS content. Strong interaction established between hard and soft segments resulted in a positive shift in PCL glass transition temperature. Tracking E′, E″ and damping factor in DMTA measurements confirmed the two-phase morphology. Hydrophobicity of polymer surfaces was traced by contact angle measurement.

Properties of modified ethylene terpolymer/poly(lactic acid) blends based films

In order to explore an alternative method instead of plasticization for improving the toughness, flexibility and processability of PLA based packaging films, two different kinds of modified polyethylene based elastomers, such as glycidyl methacrylate or maleic anhydride functionalized ethylene-acrylate based elastomers, were melt blended with PLA. Their properties were compared with conventional PEG plasticized PLA. The chemical interaction between end groups of PLA and epoxide or maleic anhydride functional groups of elastomers was shown by FTIR. Scanning electron microscopy showed that up to 20 % PEG loading, one phase morphology was achieved, however beyond this point, a phase separation was observed for plasticized PLA. For PLA/elastomer blends, a two-phase morphology was obtained as a result of immiscible nature of PLA and elastomers. Tensile and dynamic mechanical properties indicated that elastomer based blends were better than plasticized PLA independently from elastomer type. Differential scanning calorimeter (DSC) analysis exhibited that the Tg value was remarkably lowered in the plasticized PLA; however, it did not change in the case of elastomers. In terms of oxygen permeability and biodegradability, plasticized PLA was found to be better than elastomer based blends.

Photopolymerisation kinetics and electro-optical properties in mixtures of dichroic dye-doped nematic liquid crystal and photocurable polymer

This work reports the effect of dye on the photopolymerisation and electro-optical properties of polymer dispersed liquid crystal (PDLC) composite films. Dichroic PDLC (DPDLC) films based on a photocurable polymer and nematic liquid crystal (LC) with an azo dichroic dye were prepared by photopolymerisation-induced phase separation method. Polarising optical microscopy has been used for monitoring the phase separation kinetics and two-phase morphology evolution in the DPDLC system. LC domains with radial structures during initial period of phase separation adopted a resultant morphology of bipolar configuration over the course of polymerisation. The phase separation and morphology of LC domains was found to be dependent on the amount of dye used. Moreover, the addition of small amount of dye reduced the switching voltage, and enhanced the contrast ratio with improved switching time in the PDLC films. It was shown that, under the application of an electric field, the molecular orientation and absorbance of dichroic dye can be controlled in DPDLC to induce non-linearity and colour contrast without the use of polarisers.

Mechanical, thermal and electrical properties of epoxy modified with a reactive hydroxyl-terminated polystyrene-butadiene liquid rubber

This study investigated the modification of epoxy resin with a hydroxyl-terminated polystyrene-butadiene liquid rubber copolymer. The Fourier transform infrared analysis evidenced the occurrence of a chemical reaction between the oxirane ring of epoxy resin and the hydroxyl functional group of hydroxyl-terminated polystyrene-butadiene using benzyldimethylamine as a catalyst. The mechanical results indicated that the 10 phr hydroxyl-terminated polystyrene-butadiene-modified epoxy resin exhibited the maximum impact strength of 16.5 J/m2, which is about 91.8% higher than that for the neat epoxy; the best mechanical performance in terms of tensile and flexural properties was achieved at 10 phr hydroxyl-terminated polystyrene-butadiene content; higher concentration of hydroxyl-terminated polystyrene-butadiene resulted in the cured networks lower impact and strength values. The investigation on the microstructure of hydroxyl-terminated polystyrene-butadiene–modified epoxy resin showed a two-phase morphology where the hydroxyl-terminated polystyrene-butadiene domains were uniformly dispersed in epoxy resin matrix. The addition of hydroxyl-terminated polystyrene-butadiene into epoxy resin reduced the thermal stability, whereas, it obviously improved the volume, surface resistivity; and the dielectric breakdown strength of 20 phr hydroxyl-terminated polystyrene-butadiene–modified epoxy resin was 36 kV/mm compared to 31.2 kV/mm of the neat epoxy resin.

Soybean protein films. Characterization and potential as novel delivery devices of Duddingtonia flagrans chlamydospores

This study was aimed at assessing the potential use of soybean protein concentrate (SPC) – based film as vehicle for the delivery of Duddingtonia flagrans chlamydospores for the biological control of gastric nematodes in ruminals. Glycerol and dialdehyde starch (DAS) were used as plasticizer and cross-linking agent, respectively. Films were obtained by casting and characterized in terms of their physico-mechanical properties. The impact of cross-linking extent on moisture absorption, swelling and tensile properties of the resultant films was evaluated. The adequate tensile properties and stability in wet environment of SPC films cross-linked with 10wt.% DAS was counterbalanced with the two-phase morphologies developed, irrespective of glycerol content, limiting their potential application as delivery devices. SPC films cross-linked with 5wt.% DAS and plasticized with 30wt.% glycerol exhibited the best compromise between solubility (only 29%), homogeneous morphology, adequate tensile strength (2.50±0.43MPa) and elongation at break (18.72±2.34%) and swelling profile. The preliminary results of the release of D. flagrans chlamydospores in ruminal fluid revealed a slow release profile attaining 4.9% in a period of 24h. Overall, these results substantiated the potential use of DAS-cross-linked SPC films as viable carrier matrix for D. flagrans release applications.

Characterization of two phase distribution in electrochemically-lithiated spinel Li4Ti5O12 secondary particles by electron energy-loss spectroscopy

Spinel lithium titanate, Li4Ti5O12 (LTO) is attracting much attention as an alternative material replacing carbon based anodes in lithium ion batteries, due to its high rate properties as well as its durability and safety. The coexistence of the two phases, spinel Li4Ti5O12 and rock salt Li7Ti5O12, during the charge–discharge reactions is the key issue for the understanding of reaction mechanism and the development of LTO electrodes with improved performance. However, the two-phase distribution morphology in real LTO electrodes has not yet been reported, due to the difficulty in identifying the Li-inserted Li7Ti5O12 phase with high resolutions. Thus we apply the scanning transmission electron microscopy with electron energy loss spectroscopy (STEM–EELS) using the latest equipment with high energy and spatial resolutions. We have successfully identified the presence of the Li7Ti5O12 phase in Li-inserted LTO secondary particles, by the EELS analysis of Li composition, Ti oxidation state, and local configurations around oxygen atoms, compared to the Li4Ti5O12 phase. We have also obtained the two-phase distribution image in 50% Li-inserted LTO secondary particles, where separate presence of both the phases is clearly revealed. This indicates the particle-by-particle reaction mechanism, where the rate determining process exists in the boundary between primary particles.

Toughening of dicyandiamide-cured DGEBA-based epoxy resins by CTBN liquid rubber

Toughening of a diglycidyl ether of bisphenol-A (DGEBA)-based epoxy resin with liquid carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer has been investigated. For this purpose six blend samples were prepared by mixing DGEBA with different concentrations of CTBN from 0 to 25 phr with an increment of 5 phr. The samples were cured with dicyandiamide curing agent accelerated by Monuron. The reactions between oxirane groups of DGEBA and carboxyl groups of CTBN were followed by Fourier-transform infrared (FTIR) spectroscopy. Tensile, impact, fracture toughness and dynamic mechanical analysis of neat as well as the modified epoxies have been studied to observe the effect of CTBN modification. The tensile strength of the blend systems increased by 26 % when 5 phr CTBN was added, and it remained almost unchanged up to 15 phr of CTBN. The elongation-at-break and Izod notched impact strength increased significantly, whereas tensile modulus decreased gradually upon the addition of CTBN. The maximum toughness of the prepared samples was achieved at optimum concentration of 15 phr of CTBN, whereas the fracture toughness (KIC) remained stable for all blend compositions of more than 10 phr of CTBN. The glass transition temperature (Tg) of the epoxy resin significantly increased (11.3 °C) upon the inclusion of 25 phr of CTBN. Fractured surfaces of tensile test samples have been studied by scanning electron microscopic analysis. This latter test showed a two-phase morphology where the rubber particles were distributed in the epoxy resin with a tendency towards co-continuous phase upon the inclusion of 25 phr of CTBN.

Self-assembled Janus-like multi-ionic lithium salts form nano-structured solid polymer electrolytes with high ionic conductivity and Li+ion transference number

A solid polymer electrolyte with high ambient temperature conductivity, 4 × 10−4 S cm−1, and transference number, tLi+ = 0.6, is formed from blends of polyethylene oxide (PEO) and a multi-ionic polyoctahedral silsesquioxane lithium salt, POSS-phenyl7(BF3Li)3, with Janus-like properties. A two-phase morphology is proposed in which the hydrophobic phenyl groups cluster and crystallize, and the three –BF3− form an anionic pocket, with the Li+ ions solvated by the PEO phase. The high ionic conductivity results from interfacial migration of Li+ ions loosely bonded to three –BF3− anions and the ether oxygens of PEO. Physical crosslinks formed between PEO/Li+ chains and the POSS clusters account for the solid structure of the amorphous PEO matrix. The solid polymer electrolyte has an electrochemical stability window of 4.6 V and excellent interfacial stability with lithium metal.

Development of lignin-based polyurethane thermoplastics

Here we report utilizing a tailored feedstock to synthesize mechanically robust thermoplastic polyurethanes at very high lignin contents (65–75 wt%). The molecular weight and glass transition temperature (Tg) of lignin were altered through cross-linking with formaldehyde. The cross-linked lignin was coupled with diisocyanate-based telechelic polybutadiene as a network-forming soft segment. The appearance of two Tg's, around −35 and 154 °C, for the polyurethanes indicates the existence of two-phase morphology, a characteristic of thermoplastic copolymers. A calculated Flory–Huggins interaction parameter of 7.71 also suggests phase immiscibility in the synthesized lignin polyurethanes. An increase in lignin loading increased the modulus, and an increase in crosslink-density increased the modulus in the rubbery plateau region of the thermoplastic. This path for synthesis of novel lignin-based polyurethane thermoplastics provides a design tool for high performance lignin-based biopolymers.

Monte Carlo Simulations of Charge Transport in 2D Organic Photovoltaics.

The effect of morphology on charge transport in organic photovoltaics is assessed using Monte Carlo. In isotopic two-phase morphologies, increasing the domain size from 6.3 to 18.3 nm improves the fill factor by 11.6%, a result of decreased tortuosity and relaxation of Coulombic barriers. Additionally, when small aggregates of electron acceptors are interdispersed into the electron donor phase, charged defects form in the system, reducing fill factors by 23.3% on average, compared with systems without aggregates. In contrast, systems with idealized connectivity show a 3.31% decrease in fill factor when domain size was increased from 4 to 64 nm. We attribute this to a decreased rate of exciton separation at donor-acceptor interfaces. Finally, we notice that the presence of Coulomb interactions increases device performance as devices become smaller. The results suggest that for commonly found isotropic morphologies the Coulomb interactions between charge carriers dominates exciton separation effects.

Clicked (AB)2 C-type miktoarm terpolymers: Synthesis, thermal and self-assembly properties, and preparation of nanoporous materials

A well-defined (PEO-PS)2-PLA miktoarm terpolymer (1) was synthesized by stepwise click reactions of individually prepared poly(ethylene oxide) (PEO), polystyrene (PS, polymerized by atom transfer radical polymerization), and polylactide (PLA, polymerized by ring-opening polymerization) blocks. As characterized by differential scanning calorimetry and small-angle X-ray scattering techniques, the terpolymer self-assembled into a hexagonal columnar structure consisting of PLA/PEO cylindrical cores surrounded by PS chains. In contrast, the ion-doped sample (1-Li+) with lithium concentration per ethylene oxide = 0.2 exhibited a three-phase lamellar structure, which was attributed to the microphase separation between PEO and PLA blocks and to the conformational stabilization of the longest PLA chain. The two-phase columnar morphology before the ion doping was used to prepare a nanoporous material. PLA chains in the cylindrical core region were hydrolyzed by sodium hydroxide, producing nanopores with a pore diameter of about 14 nm. The resulted nanoporous material sank to the bottom in water, because of water-compatible PEO chains on the walls. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

EVA reactive blending with Si–H terminated polysiloxane by carbonyl hydrosilylation reaction: From compatibilised blends to crosslinking networks

A new and original method based on carbonyl hydrosilylation was developed to prepare ethylene-vinyl acetate (EVA)/polysiloxane polymer blends. This focused on the addition of hydrogenosilane groups (SiH) from polysiloxane to the carbonyl groups of EVA. The influence of the nature of the polysiloxane on blend properties was investigated by rheology and scanning electron microscopy. Mixing of a low viscosity polysiloxane with a high viscosity EVA matrix produced a two-phase morphology. The occurrence of the hydrosilylation reaction at the EVA/polysiloxane interface promoted a homogenisation of the blend depending on the molar ratio SiH/vinyl acetate groups, [SiH]/[VA], and the viscosity ratio of the blend. Two distinct behaviours were observed. The formation of a crosslinked network under shear was obtained for a low viscosity ratio between polysiloxane and EVA (λpolysiloxane/EVA = 4.0 × 10−6) with a high concentration of SiH groups ([SiH]/[VA] = 0.5), while the formation of a compatibilised blend was observed for high molar mass polysiloxanes (Mn > 15,000 g mol−1) with a low concentration of SiH ([SiH]/[VA] < 4.0 × 10−3).