Phase-separated Microstructure Research Articles

Functional PDMS enhanced strain at fracture and toughness of DGEBA epoxy resin

The thermo-mechanical properties of diglycidyl ether of bisphenol-A (DGEBA) epoxy/silyl-diglycidyl ether terminated polydimethyl siloxane (PDMS) composites have been investigated; PDMS concentration was increased up to 15wt%. Differential scanning calorimetry (DSC) evidenced a phase separated microstructure as two glass transition temperatures, Tg, were detected: a high Tg at ca. 103°C corresponding to DGEBA phase, and a lower Tg at ca. 40°C. The lower Tg suggests interpenetration of PDMS into the DGEBA molecular network. SEM and AFM analyses confirmed the phase separated, droplet morphology, with well dispersed PDMS domains into the epoxy matrix. The PDMS droplets had diameters as small as 0.6μm, and the droplet size increased up to ca. 1.8μm at 15% PDMS content. Detailed EDS elemental mapping of the fractured composites evidenced siloxane residue in epoxy cavities suggesting incorporation of siloxane into the epoxy network, in agreement with DSC. Flexural testing showed a monotonic reduction of modulus (E) as PDMS content increased, as usually observed in rubber reinforced epoxy resins. However, strain at fracture and toughness increased twofold at about 10wt% PDMS content. Water contact angle increased and then decreased as PDMS content increased, reaching maxima at ca. 10wt% concentration. This behavior was apparently driven by surface roughness. The elastic mechanical modulus was found to scale with the droplet size of the rubbery phase.

Shape stability enhancement of PVDF electrospun polymer electrolyte membranes blended with poly(2-acrylamido-2-methylpropanesulfonic acid lithium)

The purpose of this study is to overcome the poor dimensional stability of poly(vinylidene fluoride) (PVDF)-based electrospun membranes for polymer electrolytes, a new type of composite fibrous membranes based on PVDF/poly(2-acrylamido-2-methylpropanesulfonic acid lithium) (PAMPSLi) blend systems with different blend ratios were fabricated by electrospinning method. Morphology of the composite fibrous membranes was evaluated by scanning electron microscopy. Average diameters of the membranes were less than 250 nm, which were far less than that of pure PVDF fibrous membrane (400 nm). Fourier transform infrared spectroscopy and Raman scattering were used to characterize the interactions of two polymers. Wide-angle X-ray diffraction and differential scanning calorimetry techniques were applied to investigate the crystal structure of composite fibrous membranes. Owning to the good miscibility between PVDF and PAMPSLi, no phase-separated microstructure was observed in composite fibrous membranes. The membranes possessed a good wettability by liquid electrolytes and exhibited an excellent dimensional stability even at high loading of electrolytes. The polymer electrolyte showed the ionic conductivity of 3.45 × 10−3 S/cm at room temperature and electrochemical stability up to 5.4 V for the blend ratio of 5/1. PVDF/PAMPSLi (5/1)-based polymer electrolyte was observed much more suitable than polymer electrolytes with other ratios of PVDF/PAMPSLi for application in high-performance lithium rechargeable batteries.

Influence of hydroxyl-terminated polydimethylsiloxane on high-strength biocompatible polycarbonate urethane films

The present study describes a series of novel polycarbonate urethane films that were fabricated via the solution-casting method from 4,4′-methylenebis(cyclohexyl isocyanate) (H12MDI) and 1,4-butanediol (BDO) chain extender as hard segments, poly(1,6-hexanediol)carbonate diols (PCDL) and hydroxyl-terminated polydimethylsiloxane (PDMS) as soft segments, with dibutyltin dilaurate as the catalyst. Varied molar ratios of PDMS (less than 30%) were utilized to enhance the mechanical properties and biocompatibilities. The microstructure and degrees of phase separation were characterized using atomic force microscopy. The chemical structure and surface morphology of the materials were further confirmed by attenuated total reflectance Fourier transform infrared spectroscopy, 1H NMR and 13C NMR, water droplet contact angle and scanning electron microscopy. Thermal properties were measured by differential scanning calorimetry. MTT assay and hemolytic tests were studied for evaluating cellular viability and hemocompatibility of fabricated films using L929 fibroblast cells and adult rabbit blood. The results demonstrated polyurethane films with soft segments partially replaced by PDMS could remarkably improve the biocompatibility while maintaining relatively stable mechanical behavior, making them exciting potential candidates for artificial vessels or other tissue engineering applications.

Powder metallurgical processing of equiatomic AlCoCrFeNi high entropy alloy: Microstructure and mechanical properties

Phase formation, microstructural evolution and the mechanical properties of novel multi-component equiatomic AlCoCrFeNi high entropy alloy synthesized by high energy ball milling followed by spark plasma sintering have been reported here. The microstructure of the mechanically alloyed (MA) powder and sintered samples were studied using X-ray diffraction, scanning electron and transmission electron microscopy, whereas the detailed investigation of the mechanical properties of the sintered samples were measured using micro and nano hardness techniques. The fracture toughness measurements were performed by applying single edge V notch beam (SEVNB) technique. The MA powder shows the presence of FCC (τ) and BCC (κ) solid solution phases. Extended ball milling (up to 60h) does not change the phases present in MA powder. The sintered pellets show phase-separated microstructure consisting of Al-Ni rich L12 phase, α′ and tetragonal Cr-Fe-Co based σ phase along with Al-Ni-Co-Fe FCC solid solution phase (ε) for sample sintered from 973 to 1273K. The experimental evidences indicate that BCC (κ) solid solution undergoes eutectoid transformation during sintering leading to the formation of L12 ordered α′ and σ phases, whereas FCC (τ) phase remains unaltered with a slight change in the lattice parameter. The hardness of the sample increases with sintering temperature and a sudden rise in hardness is observed 1173K. The sample sintered at 1273K shows the highest hardness of ~8GPa. The elastic modulus mapping clearly indicates the presence of three phases having elastic moduli of about 300, 220 and 160GPa. The fracture toughness obtained using SEVNB test shows a maximum value of 3.9MPam1/2, which is attributed to the presence of brittle nanosized σ phase precipitates. It is proposed that significant increase in the fraction of σ phase precipitates and eutectoid transformation of the τ phase contribute to increase in hardness along with better densification at higher sintering temperatures.

Small Molecule/Polymer Blend Organic Transistors with Hole Mobility Exceeding 13 cm(2) V(-1) s(-1).

A ternary organic semiconducting blend composed of a small-molecule, a conjugated polymer, and a molecular p-dopant is developed and used in solution-processed organic transistors with hole mobility exceeding 13 cm(2) V(-1) s(-1) (see the Figure). It is shown that key to this development is the incorporation of the p-dopant and the formation of a vertically phase-separated film microstructure.

The influence of the molecular weight of the water-soluble polymer on phase-separated films for controlled release

Hydroxypropyl cellulose (HPC) and ethyl cellulose (EC) can be used for extended release coatings, where the water-soluble HPC may act as a pore former. The aim was to investigate the effect of the molecular weight of HPC on the microstructure and mass transport in phase-separated freestanding EC/HPC films with 30% w/w HPC. Four different HPC grades were used, with weight averaged molecular weights (Mw) of 30.0 (SSL), 55.0 (SL), 83.5 (L) and 365 (M) kg/mol. Results showed that the phase-separated structure changed from HPC-discontinuous to bicontinuous with increasing Mw of HPC. The film with the lowest Mw HPC (SSL) had unconnected oval-shaped HPC-rich domains, leaked almost no HPC and had the lowest water permeability. The remaining higher Mw films had connected complex-shaped pores, which resulted in higher permeabilities. The highest Mw film (M) had the smallest pores and very slow HPC leakage, which led to a slow increase in permeability. Films with grade L and SL released most of their HPC, yet the permeability of the L film was three times higher due to greater pore connectivity. It was concluded that the phase-separated microstructure, the level of pore percolation and the leakage rate of HPC will be affected by the choice of HPC Mw grade used in the film and this will in turn have strong impact on the film permeability.

Oxyfluoride Glass-Ceramics for Transition Metal Ion Based Photonics: Broadband Near-IR Luminescence of Nickel Ion Dopant and Nanocrystallization Mechanism

Nickel ion doped transparent bulk glass-ceramics containing K2SiF6, ZnF2, and KZnF3 nanocrystals were elaborated to show the prospect that this novel group of oxyfluoride glass-ceramics is promising for transition metal ion based photonics. These new oxyfluoride glass-ceramics exhibit a new broadband luminescence of Ni2+ ions in the intriguing near-IR spectral region ranging from 1200 to 2400 nm. Ni2+ ions are incorporated into the precipitated K2SiF6, ZnF2, and KZnF3 crystals, which provide octahedral sites for Ni2+ and make it optically active. Taking into account the microstructure of phase separation observed by replica TEM, a detailed mechanism of phase-separation-assisted nanocrystallization was described for these oxyfluoride glasses. These results allow one to establish a full physical model of the nanocrystallization mechanism. For the precipitation of fluoride crystallites, not only can the nanometric size be explained but also the formation of different crystal phases can be controlled by the i...

Spontaneous Formation of an Ideal-Like Field-Effect Channel for Decay-Free Polymeric Thin-Film Transistors by Multiple-Scale Phase Separation.

We demonstrate semiconducting polymer-based thin-film transistors (PTFTs) with fast switching performance and an uncommon nondecaying feature. These PTFTs based on widely studied poly(3-hexylthiophene) are developed by incorporating the insulating polymer into the active channel and subjecting the compound to specific, spontaneous multiple-scale phase separation (MSPS). An in-depth study is conducted on the interfacial and phase-separated microstructure of the semiconducting/insulating blending active layer and its effect on the electrical characteristics of PTFTs. The polyblends exhibit a confined crystallization behavior with continuously semiconducting crystalline domains between scattered insulator-rich domains. The insulator-rich domains can block leakage current and strengthen the gate control of the channel. A small amount of the insulating polymer penetrates the bottom of the active channel, resulting in effective interface modification. We show specific MSPS morphology of the present blending films to reduce charge trapping effects, enhance charge accumulation, and create a high-seed switching channel. The findings enable us to develop the required morphological conceptual model of the ideal-like field-effect-modulated polymer-based active channel. The polyblend-based PTFTs with MSPS morphology also have promising sensing functions. This study offers an effective approach for overcoming the major drawbacks (instability and poor switching) of PTFTs, thus allowing such transistors to have potential applications.

Microstructure and rheology design in protein–protein–polysaccharide composites

The microstructure and rheology of protein–protein–polysaccharide composites were explored, with wheat or soy protein used to modulate the microstructure and rheological properties of both gelatin and gelatin + starch gels. The extent of phase separation typically associated with protein–polysaccharide mixtures was greatly curtailed in the presence of either plant-based protein. The negatively-charged soy protein generally increased the elastic modulus of the gelatin + starch gels whereas the positively-charged wheat protein decreased it. Unique rheological behaviour was observed when both starch and 6 wt% plant-based protein were present, as the phase-separated gels became soft and flowable. Microstructurally, addition of soy protein to gelatin led to the presence of coacervates whereas the wheat protein aggregated within the continuous gelatin phase. In gelatin + soy + starch mixtures, the gelatin and soy remained mixed as the continuous phase and starch existed as the discontinuous phase. In gelatin + wheat + starch mixtures, a complex multiple phase-separated microstructure was evident as the three biopolymers appeared distinct. Overall, these results demonstrated that plant-based proteins may be used as structure modifiers to generate novel phase-separated gels with diverse rheological properties.

Triple-shape memory properties of polyurethane/polylactide-polytetramethylene ether blends

A facile method to prepare triple-shape memory polymers was developed by blending polyurethane and polylactide–polytetramethylene with well-separated glass transition temperatures. The thermal properties of the blends were characterized using modulated differential scanning calorimetry and differential scanning calorimetry. Field emission scanning electron microscopy, Fourier transform infrared spectroscopy and wide-angle X-ray diffraction were used to characterize the microstructures and crystal structures of the blends. The mechanical properties were also evaluated. The versatile triple-shape memory effect and quantitative shape memory response were evaluated by consecutive thermal mechanical experiments based on a two-step programming process and subsequent progressive thermal recovery. The results show that the blends have phase-separated microstructures resulting in an ability to fix two temporary shapes independently and can recover to their original shapes sequentially. The blends have excellent triple-shape memory properties and may have some applications in multi-shape coatings, adhesives, films and temperature sensing or actuating elements. © 2015 Society of Chemical Industry

Liquid phase separation microstructure and properties of mixed Cu-Sn and Co-base alloys without or with molybdenum addition processed by plasma transferred arc hardfacing

In this study, two types of blended alloys were processed by plasma transferred arc (PTA) hardfacing to form coatings. In the first case, a pre-alloyed Cu-Sn alloy and a pre-alloyed Co-base alloy were blended at a weight ratio of 85:15 (Cu-Sn/Co-base). In the second case, the pre-alloyed Cu-Sn and Co-base alloys were mixed with the molybdenum (Mo) element at a weight ratio of 81:15:4 (Cu-Sn/Co-base/Mo). Metastable liquid phase separation formed in the Cu-Sn/Co-base and Cu-Sn/Co-base/Mo coatings. The Co-rich spheroids as the minor precipitating alloy particles distributed on the Cu-rich matrix. The Co-rich spheroids consisted of α(Co) and M7C3 phases, and the Cu-rich matrix included α(Cu, Sn) and Cu10Sn3 phases. The relative content of α(Co)+M7C3 phases in the Cu-Sn/Co-base/Mo coating was higher than that in the Cu-Sn/Co-base coating. The numbers of the Co-rich spheroids in the cross section of the Cu-Sn/Co-base coating increased with increasing the distance from the substrate, and an obvious macroscopic segregation could be found in the coating close to its substrate. When Mo as additive was further added, the numbers of the Co-rich spheroids in the cross section of the coating increased and the macroscopic segregation could not be seen when compared to that in the Cu-Sn/Co-base coating. The wear rate of the Cu-Sn/Co-base and Cu-Sn/Co-base/Mo coatings was lower than that of the Cu-Sn coating, and a distinct improvement of the wear rate could be obtained in the Cu-Su/Co-base/Mo coating, attributing to mainly the increasing hardness of the coatings by sequence of the Cu-Sn coating, Cu-Sn/Co-base coating, and Cu-Sn/Co-base/Mo coating. The friction coefficient of the coatings has an increasing trend by sequence of the Cu-Sn coating, Cu-Sn/Co-base coating, and Cu-Sn/Co-base/Mo coating, but an obvious difference could not be seen in the coatings.

Hansen solubility parameters of surfactant-capped silver nanoparticles for ink and printing technologies.

Optimal ink formulations, inclusive of nanoparticles, are often limited to matching the nanoparticle's capping agent or surface degree of polarity to the solvent of choice. Rather than relying on this single attribute, nanoparticle dispersibility was optimized by identifying the Hansen solubility parameters (HSPs) of decanoic-acid-capped 5 nm silver nanoparticles (AgNPs) by broad spectrum dispersion testing and a more specific binary solvent gradient dispersion method. From the HSPs, solvents were chosen to disperse poly(methyl methacrylate) (PMMA) and nanoparticles, give uniform evaporation profiles, and yield a phase-separated microstructure of nanoparticles on PMMA via film formation by solvent evaporation. The goal of this research was to yield a film that is reflective or transparent depending on the angle of incident light (i.e., optically variable). The nanoparticle HSPs were very close to alkanes with added small polar and hydrogen-bonding components. This led to two ink formulations: one of 90:10 vol % toluene/methyl benzoate and one containing 80:10:10 vol % toluene/p-xylene/mesitylene, both of which yielded the desired final microstructure of a nanoparticle layer on a PMMA film. This approach to nanoparticle ink formulation allows one to obtain an ink that has desirable dispersive qualities, rheology, and evaporation to give a desired printed structure.

Organic Semiconductor/Insulator Polymer Blends for High-Performance Organic Transistors

We reviewed recent advances in high-performance organic field-effect transistors (OFETs) based on organic semiconductor/insulator polymer blends. Fundamental aspects of phase separation in binary blends are discussed with special attention to phase-separated microstructures. Strategies for constructing semiconductor, semiconductor/dielectric, or semiconductor/passivation layers in OFETs by blending organic semiconductors with an insulating polymer are discussed. Representative studies that utilized such blended films in the following categories are covered: vertical phase-separation, processing additives, embedded semiconductor nanowires.

Acid-induced gelation properties of heated whey protein−pectin soluble complex (Part II): Effect of charge density of pectin

As a direct continuation of the first part of our study, here we investigate the effect of charge density of pectin on acid-induced gelation properties of heated whey protein–pectin soluble complex which was formed by heating protein and pectin together at pH 7.0 and 85 °C for 30 min. The gelation properties of complex were also compared with polymer/pectin, where whey protein and pectin were heated separately and then mixed together before acid-induced gelation. Four different types of pectin with charge density ranging from low to high were chosen. For pectin with high charge density, complex gels showed enhanced gel strength and water holding capacity compared to polymer/pectin gels. For those with low charge density pectin, polymer/pectin could not form gel network at highest pectin concentrations, whereas complex still formed gel at high pectin concentrations and showed improved water holding capacity at low pectin concentrations. Confocal micrograph showed pronounced difference in gel microstructure between complex and polymer/pectin gels. An obvious increase in the coarsening of the protein network was observed as pectin concentration increased from 0.1% to 0.5% for polymer/pectin gels. Complex gels showed much smoother network with less phase separation. At 0.25% pectin a change from a homogeneous to a coarse strand, phase separated microstructure upon decreasing charge density of pectin was observed for polymer/pectin gels, while complex gels were homogeneous even with low charged pectin. Results from scanning electron microscopy confirmed that complex gels were composed of larger protein strands and higher interconnectivity between biopolymers due to higher degree of interaction between protein and pectin.

New insights on how to adjust the release profile from coated pellets by varying the molecular weight of ethyl cellulose in the coating film

The major aims of this work were to study the effect of the molecular weight (Mw) of ethyl cellulose (EC) on the drug release profile from metoprolol succinate pellets coated with films comprising EC and hydroxypropyl cellulose (HPC) with a weight ratio of 70:30, and to understand the mechanisms behind the different release profiles. A broad range of Mws was used, and the kinetics of drug release and HPC leaching followed. The higher the Mw of EC, the slower the HPC leaching and the drug release processes. Drug release occurred by diffusion through the pores created in the coating by the HPC leaching. A novel method was used to explain the differences in the release profiles: the effective diffusion coefficient (De) of the drug in the coating film was determined using a mechanistic model and compared to the amount of HPC leached. A linear dependence was found between De and the amount of HPC leached and, importantly, the value of the proportionality constant decreased with increasing Mw of EC. This suggests that the Mw of EC affects the drug release profile by affecting the phase separated microstructure of the coating and the hindrance it imparts to drug diffusion.

Influence of Dangling Chains on Molecular Dynamics of Polyurethanes

The effect of dangling chains on phase-separated microstructure and molecular dynamics for polyurethanes (PUs) was investigated. PUs with different dangling chain lengths and polar groups were prep...

Microstructure of an Immiscible Polymer Blend and Its Stabilization Effect on Amorphous Solid Dispersions

This study proposes use of the phase separation of immiscible polymer blends as a formulation approach to improve the stabilization and solubilization of amorphous molecular dispersions of poorly soluble drugs. This approach uses the phase separation and different drug solubilization properties of the two immiscible polymers in the blend to optimize drug loading and stabilization. The model system tested in this study is a EUDRAGIT E PO-PVP-VA 50/50 (w/w) blend loaded with felodipine via hot melt extrusion. The phase separation behavior of the polymer blend and drug loaded polymer blend formulations were characterized using a range of thermal (MTDSC), spectroscopic (ATR-FTIR), and imaging (AFM and thermal transition mapping) techniques. The polymer blend formulations demonstrated superior performance in drug release as well as stabilization against stressed temperature, stressed humidity, and mechanical milling in comparison to the drug-polymer binary systems. This is attributed to the configuration of the phase separated microstructure of the polymer blend formulations where the hydrophilic polymer domains host high concentrations of molecularly dispersed drug which are protected from moisture induced recrystallization on aging by the hydrophobic polymer domains. Additionally drug incorporation as a molecular dispersion in different polymer phases reduces the drug recrystallization tendency on aging under high temperatures and during milling.

Growth, microstructure, and luminescent properties of direct-bandgap InAlP on relaxed InGaAs on GaAs substrates

Direct-bandgap InAlP alloy has the potential to be an active material in nitride-free yellow-green and amber optoelectronics with applications in solid-state lighting, display devices, and multi-junction solar cells. We report on the growth of high-quality direct-bandgap InAlP on relaxed InGaAs graded buffers with low threading dislocation densities. Structural characterization reveals phase-separated microstructures in these films which have an impact on the luminescence spectrum. While similar to InGaP in many ways, the greater tendency for phase separation in InAlP leads to the simultaneous occurrence of compositional inhomogeneity and CuPt-B ordering. Mechanisms connecting these two structural parameters are presented as well as results on the effect of silicon and zinc dopants on homogenizing the microstructure. Spontaneous formation of tilted planes of phase-separated material, with alternating degrees of ordering, is observed when InAlP is grown on vicinal substrates. The photoluminescence peak-widths of these films are actually narrower than those grown on exact (001) substrates. We find that, despite phase-separation, ordered direct-bandgap InAlP is a suitable material for optoelectronics.

Molecular Dynamics of Segmented Polyurethane Copolymers: Influence of Soft Segment Composition

Dielectric relaxation spectroscopy was used to investigate the molecular dynamics of model segmented polyurethane copolymers having identical hard segments and hard segment weight fractions, but with four different soft segment chemistries of particular interest in biomedical devices. All soft segments have molecular weight ∼1000 g/mol and are composed of either tetramethylene oxide, hexamethylene oxide, aliphatic carbonate, or dimethylsiloxane (PDMS) segments. These microphase-separated materials exhibit rich dielectric relaxation behavior: up to two relaxations in the glassy state, a segmental α relaxation (two for the polymer with predominately PDMS soft segments), and three slower relaxations. The slowest process arises from interfacial (MWS) polarization, and its strength decreases significantly with increasing temperature (over a few tens of degrees) and disappears at a temperature similar to that at which the small-angle X-ray scattering from the phase-separated microstructure disappears.

The Role of Soft Segment Molecular Weight on Microphase Separation and Dynamics of Bulk Polymerized Polyureas

The influence of poly(tetramethylene oxide) (PTMO) soft segment length on the phase-separated microstructure, state of hydrogen-bonded associations, and molecular dynamics was investigated in polyureas polymerized from the bulk. For higher PTMO molecular weights (1000 and 650 g/mol) hard segments self-assemble into ribbon-like domains, while incorporation of a 250 g/mol soft segment leads to a predominately mixed segment material. The degree of microphase separation of the hard and soft segments, however, is rather incomplete for polymers synthesized from 1000 and 650 g/mol PTMO and decreases with decreasing soft segment molecular weight. Broadband dielectric relaxation spectroscopy reveals two segmental relaxations: a soft segment rich (α) and slow segmental (α2) process. When the molecular weight is reduced from 1000 to 650 g/mol the mobility of these processes is reduced, consistent with findings from differential scanning calorimetry and dynamic mechanical analysis.