Reduction Of Interfacial Free Energy Research Articles

Enhanced nucleation of bimodal molecular weight distribution polymers: A molecular dynamics study

We perform coarse-grained molecular dynamics (CGMD) simulations to study the homogeneous nucleation of bimodal and unimodal molecular weight distribution polymers with equivalent average molecular weight. First, a statistical method is proposed to determine the critical nuclei and thus calculate the free energy barrier of nucleation. From the temperature dependence of diffusion coefficient, we also determine the activation energy of diffusion. Then we calculate the nucleation rate and find that it is consistent with the classical nucleation theory for homogeneous nucleation in semi-crystalline polymers. Compared with unimodal system, the bimodal system exhibits lower interfacial free energy and consequently lower free energy barrier for nucleation, while the two systems have similar activation energy for diffusion. This suggests that the promoted nucleation rate of bimodal molecular weight distribution polymer is a result of the reduction of interfacial free energy, which is eventually a consequence of chain-folding nucleation of long chain component.

Solubilization Behavior of Homopolymer in Its Blend with the Block Copolymer Displaying the Feature of Lower Critical Ordering Transition.

Blending with homopolymer offers a facile approach for tuning the microdomain morphology of block copolymer, provided that the homopolymer chains are uniformly solubilized in the corresponding microdomain to swell the junction point separation. Here we studied the solubilization behavior of poly(4-vinyl pyridine) homopolymer (h-P4VP) in the lamellar microdomain formed by its blends with a poly(ethylene oxide)-block-poly(4-vinyl pyridine) (PEO-b-P4VP) showing the feature of lower critical ordering transition (LCOT) in terms of weaker segregation strength at lower temperature. We revealed that, while the conventional criterion of homopolymer-to-block molecular weight ratio for attaining uniform solubilization was applicable to LCOT blend, there was an excess swelling of junction point separation upon the addition of homopolymer, leading to a decrease of interdomain distance with increasing homopolymer composition. This anomalous phenomenon was attributed to the reduction of interfacial free energy due to the incorporation of P4VP homopolymer into the microdomain interface.

Calculation of Solid-Fluid Interfacial Free Energy with Consideration of Solid Deformation by Molecular Dynamics Simulations.

Fluid-fluid interfacial free energy can be measured accurately and can also be calculated from molecular simulations. However, it is challenging to measure solid-fluid interfacial free energy directly. Accurate computation has not yet been advanced by molecular simulations. In this study, we derive working expressions for estimating solid-fluid interfacial free energy based on the free-energy perturbation method with consideration of solid deformation. A Lennard-Jones solid-fluid system is simulated. Our derivations indicate that the effect of solid deformation is pronounced on solid-fluid interfacial free energy, and the results may be significantly different from the conventional test area method. Our results reveal that the contribution of the solid deformation highly depends on the stress conditions in the solid, which can be either positive or negative. Adsorption of fluids onto the solid surface has a significant effect on interfacial free energy. In weak adsorption, the interfacial free energy is close to the solid-vacuum surface free energy. Strong adsorption results in a significant reduction in interfacial free energy.

The Element Segregation Between γ/γʹ Phases in a Ni-Based Single Crystal Superalloy Studied by 3D-APT and Its Potential Impact on Local Interfacial Misfit Strain

Three-dimensional atom-probe tomography was used to characterize the γ/γʹ interface structure in a third-generation Ni-based single crystal superalloy with Re addition. It was found that an element-segregation layer with Re, Co and Cr was formed in the γ phase close to the γ/γ′ interface, resulting in a more negative local interface misfit (− 0.29%) compared to the measured result (− 0.16%) from high-resolution X-ray diffraction. Furthermore, the total reduction of interfacial free energy due to the solute atom segregation based on the Gibbsian interfacial excess was calculated to indicate that Re element was the most beneficial element in producing this more negative local misfit with the largest interfacial free energy reduction (13.67 ± 0.21 mJ/m2). Simultaneously, because of the co-segregation of Re, Co and Cr, and the depletion of Ni in the γ phase close to the γ/γ′ interface, it was also deduced that some harmful topologically close-packed phases might be easier to nucleate and grow in the γ phase close to the γ/γ′ interface in service. The element-segregation layer across γ/γ′ interface resulted in a more negative local interface misfit (− 0.29%), compared to the measured result (− 0.16%) from highresolution X-ray diffraction, and the total reduction of interfacial free energy due to the solute atom segregation was calculated to indicate that Re element was the most beneficial element in producing this more negative local misfit with the largest interfacial free energy reduction (13.67 ± 0.21 mJ/m2).

Strategies to hydrophilize silicones via spontaneous adsorption of poly(vinyl alcohol) from aqueous solution

It is challenging to achieve long-lasting hydrophilicity by surface modification of polydimethylsiloxane (PDMS), principally due to the hydrophobic recovery that occurs. This involves the migration of low molecular weight species from the bulk to the surface and is driven by the reduction of interfacial free energy. In this study, spontaneous adsorption of poly(vinyl alcohol) was carried out on Sylgard PDMS films and their modified derivatives. PDMSox1s, PDMSox60s, and PDMSox60s+2k were prepared by 1-s oxygen plasma, 60-s oxygen plasma, and 60-s oxygen plasma followed by covalent attachment of linear PDMS of 2 kDa molecular weight on PDMS films, respectively. Surface morphology was characterized by optical and atomic force microscopy and hydrophilicity was monitored by dynamic water contact angle measurements. It was found that negligible PVOH adsorption takes place on PDMSox60s due to the lack of hydrophobic driving force and that extensive PVOH thin film dewetting on PDMS and PDMSox1s results in insignificant improvement in hydrophilicity. However, a continuous PVOH thin film albeit with some small holes was obtained on PDMSox60s+2k. PDMSox60s+2k-PVOH exhibits advancing and receding contact angles of 80–90°/16 ± 2°, which are significantly lower than 123 ± 5°/97 ± 2° on unmodified PDMS. A range of static contact angles were also measured, some of which are lower than those reported in the literature. The PDMSox60s+2k-PVOH system demonstrates superior long-term and hydrolytic stability, which are attributed to the removal of the driving force for hydrophobic recovery by inserting a hydrophobic PDMS layer between a hydrophilic, plasma-oxidized, PDMS bulk and a hydrophilic PVOH exterior. This is a new concept in addressing hydrophobic recovery of hydrophilized silicones. The spontaneous nature of the adsorption process and the crystallinity of the PVOH barrier layer are the other advantages demonstrated in this study.

Synthesis and Characterization of Chitosan-Grafted BPPO Ultrafiltration Composite Membranes with Enhanced Antifouling and Antibacterial Properties

In this study, chitosan was successfully grafted onto the top surface of a bromomethylated poly(phenylene oxide) (BPPO) ultrafiltration membrane without pretreating the membrane at harsh conditions and/or using other cross-linkers. Due to the grafting of polar groups of chitosan onto the membrane top surface, the hydrophilicity of the membrane top surface and the polar component of the total surface energy are improved compared to the pristine BPPO membrane. Theoretical calculation shows that the polar component of surface energy is a major contributor to the reduction in interfacial free energy of the membrane surface and the interaction strength between foulants and membrane surface. Therefore, the foulants adsorbed onto the top surface of chitosan/BPPO composite membranes are much easier to desorb during the cleaning process and as a result, a higher flux recovery was obtained compared to the pristine BPPO membrane. Moreover, due to the antibacterial nature of chitosan, such composite membranes show a ...

Effect of anionic surfactants on grafting density of gelatin modified with PDMS-E

The effect of anionic surfactants on the interfacial compatibility in mono epoxy terminated polydimethylsiloxane (PDMS-E) macromonomer and gelatin mixed system was studied by Gibbs free energy (ΔGM), which played a crucial role in deciding the grafting density of immiscible polymer in heterogeneous system. Aggregation behavior of gelatin chains at boundary between gelatin phase and solvent phase was investigated using viscosity, surface tension and conductivity measurements. Viscosity analysis showed a regular increase in viscosity with the increasing alkyl chain length from C7 to C16 of the homologous alkyl sulfate surfactants. Changes of surface tension exhibited the regular curves of polyelectrolyte–anionic surfactant for alkyl sulfate surfactant systems. The results demonstrated that aggregate structure of gelatin–sulfate surfactants was dominated by electrostatic and hydrophobic interactions, which resulted in a self-assembly process of the hydrophobic segments and hydrophilic segments among gelatin chains and surfactant molecules. However, the interactions between gelatin and alkyl sulfonate surfactants were mainly governed by hydrophobic interactions, which induced conformation change of gelatin molecules. Well-ordered arrangement of gelatin chains at a fluid interface has observed by high-resolution transmission electron microscopy (HR-TEM). It is a key factor to contribute to the reduction of interfacial free energy, which mainly depends on the hydrophobic interaction between gelatin and alkyl sulfate/sulfonate surfactants. MD simulations conclusions are great agreement with our experimental results.

The influence of middle B-block polydispersity on the molecular configurations and lamellar structure characteristics of lamella-forming ABA triblock copolymers

A recent experimental study (Widin J. M., Schmitt A. K., Schmitt A. L., Im K. and Mahanthappa M. K., J. Am. Chem. Soc., 134 (2012) 3834) showed that polydispersity of the middle B-block has large effects on the phase behavior of ABA triblock copolymers. It is well known that the intriguing properties of ordered structures are associated with the molecular configurations and their distribution at microscale. By using a comprehensive dissipative particle dynamics (DPD) simulation method, we study the effect of middle B-block polydispersity of lamella-forming ABA triblock copolymers on the molecular configurations and the characteristics of lamellar structures. The results show that molecules with short B-blocks in a polydisperse system mainly adopt a looped configuration and preferably locate at the A/B interfaces. On the contrary, the long blocks can adopt either looped or bridged configurations with an equal probability. The bridging fraction of triblock copolymers will decrease with the increase of middle B-block polydispersity, which is due to the fact that there will be more short triblock copolymers adopting looped configurations. Such triblock copolymers with short B-blocks accumulating at the interface region can act as compatibilizers and reduce the interfacial free energy. On the other hand, the domain spacing is mainly determined by the long blocks filling in the domain center. With an increase in middle B-block polydispersity, the domain spacing will expand due to the molecular stretching of the long blocks in the domain center as well as to the reduction in interfacial free energy.

The influence of one block polydispersity on phase separation of diblock copolymers: The molecular mechanism for domain spacing expansion

Comprehensive dissipative particle dynamics simulations are performed to investigate the effect of A-block polydispersity on the phase behavior of AB-diblock copolymers. The experimental results of Lynd and Hillmyer on polydispersity induced domain spacing expansion at different segregations (Macromolecules 2005, 38, 8803) are well reproduced and explained in terms of interfacial free energy and molecular stretching in domain center. Short block copolymers in polydisperse system are found to preferably accumulate at A/B interface as compatibilizers causing a reduction in interfacial free energy. At the same time, domain centers are found mainly occupied by long blocks which in turn determining the spacing of the corresponding domain. More importantly, increase in polydispersity will introduce more long blocks into system and therefore will enhance the molecular packing and stretching in the domain center. Such chain stretching in domain center and reduction in interfacial free energy are found to be more severe at stronger segregation, which will lead to more severe domain spacing expansion with increasing polydispersity.

Self-Assembly and Self-Tiling: Integrating Active Dies Across Length Scales on Flexible Substrates

This paper reports on recent progress in the field of directed self-assembly, wherein discrete inorganic semiconductor device components are assembled on flexible substrates, and compares these results with prior work. The research aims to develop self-assembly-based chiplet assembly processes that can extend minimal die sizes and throughput beyond what is currently possible with robotic pick and place methods. This manuscript concentrates on self-assembly that is driven by the reduction of surface free energy between liquid solder-coated areas on a substrate and metal-coated contacts on semiconductor dies that act as binding sites. Scaling prior results to sub-100 micrometer-sized components has required a transition to a new self-assembly platform. Specifically, recent work has moved from a drum delivery concept to a new scheme that uses a stepwise reduction of interfacial free energy at a triple interface between oil, water, and a penetrating solder-patterned substrate to introduce components. Finally, this paper also discusses design rules to produce highly periodic “self-tiled” domains on rigid, flexible, and curved substrates. We describe discrete, self-tiled, and microconcentrator-augmented solar cell modules as applications that are fault tolerant and reduce the amount of Si material used by up to a factor of 22 when compared to conventional cells.

Anomalous Decay of Electronically Stabilized Lead Mesas on Ni(111)

With their low surface free energy, lead films tend to wet surfaces. However, quantum size effects (QSE) often lead to islands with distinct preferred heights. We study thin lead films on Ni(111) using low energy electron microscopy and selected area low energy electron diffraction. Indeed, the grown lead mesas show distinct evidence for QSE's. At about 526 K metastable mesas reshape into hemispheres within milliseconds, driven by a huge reduction in interfacial free energy. The underlying diffusion rate is many orders of magnitude faster than expected for lead on bulk lead.

Self‐Tiling Monocrystalline Silicon; a Process to Produce Electrically Connected Domains of Si and Microconcentrator Solar Cell Modules on Plastic Supports

A method for self-tiling and electrically connecting microscopic semiconducting tiles with high speed and yield is presented. Assembly is driven by a stepwise reduction of interfacial free energy at a triple interface. We discuss design rules to produce highly periodic domains on rigid, flexible, and curved substrates. A fault-tolerant microconcentrator solar cell is demonstrated.

Self-assembly of microscopic chiplets at a liquid–liquid–solid interface forming a flexible segmented monocrystalline solar cell

This paper introduces a method for self-assembling and electrically connecting small (20-60 micrometer) semiconductor chiplets at predetermined locations on flexible substrates with high speed (62500 chips/45 s), accuracy (0.9 micrometer, 0.14 degrees), and yield (> 98%). The process takes place at the triple interface between silicone oil, water, and a penetrating solder-patterned substrate. The assembly is driven by a stepwise reduction of interfacial free energy where chips are first collected and preoriented at an oil-water interface before they assemble on a solder-patterned substrate that is pulled through the interface. Patterned transfer occurs in a progressing linear front as the liquid layers recede. The process eliminates the dependency on gravity and sedimentation of prior methods, thereby extending the minimal chip size to the sub-100 micrometer scale. It provides a new route for the field of printable electronics to enable the integration of microscopic high performance inorganic semiconductors on foreign substrates with the freedom to choose target location, pitch, and integration density. As an example we demonstrate a fault-tolerant segmented flexible monocrystalline silicon solar cell, reducing the amount of Si that is used when compared to conventional rigid cells.

Correlating Linactant Efficiency and Self-Assembly: Structural Basis of Line Activity in Molecular Monolayers

Surfactants exhibit characteristic phenomena, including the reduction of interfacial free energy, self-assembly into aggregates, and even the formation of lyotropic liquid crystalline phases at high concentrations. Our research has shown that a semifluorinated phosphonic acid can act as the two-dimensional analogue of a surfactant-a linactant-by reducing the line tension between hydrocarbon-rich and fluorocarbon-rich phases in a Langmuir monolayer. This linactant can also self-assemble into nanoscale clusters in a monolayer. Here, we explore the dependence of linactant behavior on molecular structure. Members of a homologous series of partially fluorinated phosphonic acids were synthesized and tested as linactants: CF(3)(CF(2))(n-1)(CH(2))(m)PO(3)H (abbreviated as FnHmPO(3)). The tests revealed that linactants with longer hydrophobic chains were most efficient in lowering line tension. For linactants with the same overall chain length, the length of the fluorocarbon block was correlated with efficiency. Thus, the linactant efficiency was ranked in the order F8H8PO(3) < F10H6PO(3) < F8H11PO(3) < F10H9PO(3). In all cases, linactant-containing Langmuir-Blodgett monolayers exhibited nanoscale molecular clusters with characteristic dimensions of 20-30 nm; enhanced linactant efficiency was systematically correlated with larger clusters.

A Theoretical Model for Digestive Ripening

Recently, gram quantities of monodisperse gold or silver nanoparticles were reported to be produced through a digestive ripening process, in which colloidal particles of size 2 to 40 nm are transformed to nearly monodisperse particles of 4 ~ 5 nm diameter. Digestive ripening, an example for an inverse Ostwald ripening process, is a puzzling phenomenon since it appears to go against the usual capillary effect, i.e., reduction of interfacial free energy. A theoretical model is presented, which accounts for the monodisperse state of such nanoparticles by considering the effect of charges on the particles and thus electrostatic energy during particle size evolution.

A theoretical model for digestive ripening

Recently, gram quantities of monodisperse gold or silver nanoparticles were reported to be produced through a digestive ripening process, in which colloidal particles of size from 2 to 40 nm are transformed to nearly monodisperse particles of 4–5 nm diameter. Digestive ripening, an example for inverse Ostwald ripening, is a puzzling phenomenon since it appears to avoid the usual capillary consequence, i.e., reduction of interfacial free energy. This paper presents a theoretical model which accounts for the self-assembled monodisperse state of such nanoparticles by considering the effect of charges, and therefore electrostatic energy, in the coarsening behavior of the particles. An appropriate Gibbs–Thomson equation is first derived, and then particle growth rates are calculated. The results show that success of a monodisperse state depends on, among other things, the initial particle distribution, and the size distribution at equilibrium follows some of the thermodynamic principles observed in binary phase diagrams of alloy systems.

Phase Separation Dynamics and Reaction Kinetics of Ternary Mixture Coupled with Interfacial Chemical Reaction

In this paper, computer simulation results of phase separation dynamics of ternary mixture (A, B, and C) coupled with an interfacial chemical reaction A + BdC in two-dimension are presented. The effect of reduction of interfacial free energy due to the presence of species C along the interface is taken into consideration in our study. In the case of fixed domain size, it is shown in simulations that for both reversible and irreversible reactions the generation of species C is not affected by the reaction rate constants, and it is a diffusioncontrolled process. Also, the simulation reveals that for reversible chemical reaction in the case of fixed domain size, the equilibrium of concentrations is established. In the cases of coupling between domain growth and reversible reaction kinetics, it is shown in simulations that the domain growth eventually freezes regardless of the magnitude of the reaction rate constants. The higher the reaction rate constants are, the slower the evolution to reach the reach steady state is, the larger the steady state domain size is, and the smaller the average concentration of species C is.

Calcite inclusions in forsterite

Forsterite in metamorphosed siliceous dolomites commonly contains calcite inclusions that are significantly more magnesian than even the most-magnesian calcite grains in the matrix of the same rock. Calcite inclusions in forsterite from dolomite-bearing rocks therefore have an unexploited potential for providing more-accurate estimates of the peak T of metamorphism than do matrix calcite grains. Some consequences for estimating fluid pressure and for the development of models of heat and fluid transport during contact metamorphism are reviewed. Measured compositions of calcite inclusions in forsterite from five contact aureoles indicate that significant growth of forsterite grains continues over a 50–200 °C range in T after the initial nucleation and growth of the mineral during heterogeneous mineral-fluid reaction. The continued growth may occur by an annealing mechanism driven by reduction in interfacial free energy.

Discontinuous grain growth in recrystallised vein quartz — implications for grain boundary structure, grain boundary mobility, crystallographic preferred orientation, and stress history

Grain growth is a process driven by the reduction of interfacial free energy, leading to an increase in grain size and a reduction of the number of grains in a given volume of a single phase polycrystalline material. Discontinuous grain growth (DCGG), as opposed to normal grain growth (NGG) involves just a few grains (blasts) growing at the expense of a matrix, that itself undergoes much slower modification in grain size, hence producing a bimodal grain size distribution as a transient state prior to impingement of the blasts. Such a transient state has been found in a quartz layer in metamorphic rocks of the Sesia Zone, Western Alps. There, large strain-free quartz grains occur that reveal concave outward grain boundaries against a matrix of small grains with a pronounced crystallographic and a weak dimensional preferred orientation. The misorientation of the blasts with respect to the matrix grains systematically exceeds 30°. This finding suggests that the mobility of high-angle grain boundaries in a natural quartzite depends on the degree of their misorientation (among other factors), as known for metals and alloys. The width of the grain boundaries must be sufficiently small to allow for this effect. DCGG is promoted by a pronounced crystallographic preferred orientation (CPO), that leaves only a few grain boundaries with sufficient misorientation to be mobile. In contrast to NGG, DCGG wipes out any pre-existing CPO and information on activated glide systems or kinematics during the preceding stage of flow is lost after DCGG has gone to completion. A complementary new CPO could even develop, that could be difficult to interpret. With respect to tectonic history, interfacial free energy driven grain growth reflects a period of low stress annealing and thus constitutes a first order signal.

Numerical simulation of grain growth in liquid phase sintered materials-I. Model

The simulation technique based on the Potts model, originally applied to microstructural coarsening by Srolovitz et al. [Scripta metall., 1983, 17, 241] has been extended to study grain growth by Ostwald ripening in liquid phase sintered materials. The model, which makes no assumptions about solid fractions, grain shapes or diffusion fields around grains, has been developed and characterized in this investigation. A two-dimensional, square lattice is used to digitize the microstructure. The representation of the two phases, solid grains in a liquid matrix, were achieved by populating the lattice with a two-component, canonical ensemble, where the two components were designated as A and B. Grain growth was driven by the reduction in interfacial free energy, which was defined by the bond energies between neighboring sites. The solution-reprecipitation mechanism was simulated by allowing neighboring sites to exchange places via the classical Metropolis algorithm.