Macrophase Separation Research Articles

Hydrogen bonding induced microphase and macrophase separations in binary block copolymer blends

This study investigated the macrophase and microphase transitions of blends comprising polystyrene-block-poly(ethylene oxide) (PS-b-PEO) and polystyrene-block-poly(acrylic acid) (PS-b-PAA). PEO and PAA molecules formed hydrogen bonds that enabled the two block copolymers to organize into a single microphase-separated morphology composed of two PS block chains in a common PS microdomain and PEO and PAA block chains in a coexisting PEO/PAA microdomain. Increasing temperature weakened the hydrogen-bonding interaction through the formation of PAA monomers and anhydride, causing the blends to macrophase separate into PS-b-PEO-rich and PS-b-PAA-rich phases. Further increasing the temperature weakened the repulsive interactions between both the PS and PEO segments and the PS and PAA segments, inducing order-to-disorder transitions in the PS-b-PEO-rich and PS-b-PAA-rich phases. However, the increasing repulsive interaction between the PEO and PAA segments with increasing temperature prevented the blends from forming an isotropic phase. The unique phase transformation of each block copolymer in the blend with an upper critical ordering transition (UCOT)-type interaction parameter, which caused the formation of a lower-critical-solution-temperature-type phase diagram, was attributed to the hydrogen-bonding interaction between PEO and PAA, which outweighed the UCOT behaviors of PS-b-PEO and PS-b-PAA and reversed the phase behaviors of their blends.

Semiflexible polymer nanocomposites: Role of stiffness on structure and macrophase separation

Polymer nanocomposites are lighter in weight as compared to the traditionally bigger size filler-loaded composites. Moreover, polymer nanocomposites show enhancement in properties with small fractions of nanoparticles. In certain cases, a high fraction of nanoparticles is needed to incorporate the required properties significantly. In most of the theoretical and simulation studies of polymeric systems, the polymer chain is modeled as ideal freely-jointed chains, thereby neglecting the effects of chain stiffness. We study a system of semiflexible polymers containing 50% volume fraction of nanoparticles using Polymer Reference Interaction Site Model (PRISM) theory. In the absence of chemical anisotropy, these systems undergo a disordered fluid phase to a macrophase-separated state. The phase transition of these systems is favored by an increase in the persistence length of the polymer chain. At lower values of persistence lengths, a slight increase in the persistence length results in a sharp decrease in the critical packing fraction. However, for longer persistence lengths, an increase in the persistence length barely affects the critical packing fraction. Studies of the local structure of these systems show that an increase in persistence length increases the polymer–polymer contacts and polymer-nanoparticle contacts. On the contrary, nanoparticle-nanoparticle contact initially increases suddenly, then decreases slightly at longer persistence lengths.

Tuning Diblock Copolymer Particle Packing Symmetry with Variable Molecular Weight Core-Homopolymers

We report the sensitivity of diblock copolymer particle packing symmetry to the ratio α = Nhomo/Ncore, where Nhomo and Ncore are the respective homopolymer and core-block degrees of polymerization. Three poly(ethylene oxide)-b-poly(2-ethylhexyl acrylate) (OA) diblock copolymers with O volume fractions fO = 0.15, 0.16, and 0.20 were blended with O homopolymers characterized by α = 0.23, 0.42, 0.80, and 1.00. Small-angle X-ray scattering revealed rich phase behavior as a function of α, the overall particle core volume fraction (fcore), and temperature. BCC, HCP, and Frank–Kasper σ, C14, C15, and dodecagonal quasicrystal phases, were all identified. “Dry-brush” homopolymer distribution (α = 0.80 and 1.00) favor C14 and C15 phases with macrophase separation at high homopolymer loadings. “Wet-brush” homopolymer distributions in the particle cores (α = 0.42) lead to narrow σ or C15 windows and cylinders at greater O concentrations. Solvent-like swelling (α = 0.23) favors widespread σ phase formation.

Microphase and macrophase separations in discrete potential fluids


 
 
 In this paper, we studied the liquid-vapor phase diagram and structural properties of discrete potential fluids using Gibbs ensemble simulations and integral equations theory. For this, we considered three discrete fluids, namely, the square well, square well-barrier, and square well-barrier-well. They represent simple models for fluids with competing interactions that exhibit a rich microscopic and macroscopic phase behavior depending on both the strength and range of the attractions and repulsions in the potential. Here, we emphasized a structural behavior near the liquid- vapor coexistence. For the square well-barrier fluid, we observed a possible scenario of a microscopic phase separation associated with a cluster-like formation near the critical region, which could be interpreted as a frustration mechanism of the liquid-vapor transition when either the strength or range of repulsion increases. This microscopic- like separation can be inhibited by suppressing the repulsion or adding an extra well to the interaction potential. However, for the square well fluid with long-range potential, we found evidence of a microscopic aggregation driven solely by attractions.
 
 

Binary Co-Gelator Strategy: Toward Highly Deformable Ionic Conductors for Wearable Ionoskins.

Stretchable ionic conductors have been actively developed due to the increasing demand for wearable electrochemical platforms. Herein, we propose a convenient and effective strategy for tailoring the mechanical deformability of ionic conductors. The mixing of poly(methyl methacrylate) (PMMA, polymer gelator) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BMI][TFSI], ionic liquid) produces mechanically stiff ionic conductors. To reduce the chain entanglement of polymer gelators and induce effective dissipation of applied stresses, flexible poly(butyl acrylate) (PBA) with a low glass-transition temperature is additionally doped into the ionic conductor. An extremely stretchable (∼1500%) homogeneous ternary ionic conductor is obtained without a notable change in electrochemical characteristics, unless the content of PBA exceeds the macrophase separation limit of 3 wt %. In addition, the mechanical elasticity (1.8 × 105 Pa) and durability (e.g., recovery ratio of ∼86.3% after 1000 stretching/releasing cycles) of the conductor further support its suitability as a strain sensory platform. In contrast to conventional ionoskins that have to fit the area of target body parts, even a small piece of the ternary ionic conductor successfully monitors human motion over large areas by taking advantage of its superior deformability.

Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends

Visualization of Macrophase Separation and Transformation in Immiscible Polymer Blends

Polar flocks with discretized directions: The active clock model approaching the Vicsek model

We consider the off-lattice two-dimensional q-state active clock model (ACM) as a natural discretization of the Vicsek model (VM) describing flocking. The ACM consists of particles able to move in the plane in a discrete set of q equidistant angular directions, as in the active Potts model (APM), with an alignment interaction inspired by the ferromagnetic equilibrium clock model. We find that for a small number of directions, the flocking transition of the ACM has the same phenomenology as the APM, including macrophase separation and reorientation transition. For a larger number of directions, the flocking transition in the ACM becomes equivalent to the one of the VM and displays microphase separation and only transverse bands, i.e., no re-orientation transition. Concomitantly also the transition of the q → ∞ limit of the ACM, the active XY model (AXYM), is in the same universality class as the VM. We also construct a coarse-grained hydrodynamic description for the ACM and AXYM akin to the VM.

On a system of coupled Cahn–Hilliard equations

We consider a system which consists of a Cahn–Hilliard equation coupled with a Cahn–Hilliard–Oono type equation in a bounded domain of Rd, d=2,3. This system accounts for macrophase and microphase separation in a polymer mixture through two order parameters u and v. The free energy of this system is a bivariate interaction potential which contains the mixing entropy of the two order parameters and suitable coupling terms. The equations are endowed with initial conditions and homogeneous Neumann boundary conditions both for u,v and for the corresponding chemical potentials. We first prove that the resulting problem is well posed in a weak sense. Then, in the conserved case, we establish that the weak solution regularizes instantaneously. Furthermore, in two spatial dimensions, we show the strict separation property for u and v, namely, they both stay uniformly away from the pure phases ±1 in finite time. Finally, we investigate the long-time behavior of a finite energy solution showing, in particular, that it converges to a single stationary state.

The pH-responsive phase separation of type-A gelatin and dextran characterized with static multiple light scattering (S-MLS)

Macromolecular phase separation has attracted increasing attention due to its wide applications in food industry, such as modulating food texture and delivering bioactive compounds. The phase separation kinetics is important but rarely reported due to the limitation of current methods. This work provided an effective method, static multiple light scattering (S-MLS), for the study of phase separation kinetics, and investigated the effects of pH on the phase separation of type-A gelatin and dextran mixtures (GE/DE) using S-MLS technique, fluorescence microscopy and visual assessment. Phase diagrams of GE/DE (1.0–6.0 wt%) at pH 3.00–11.0 were measured based on the micro-phase separation. Interestingly, GE/DE mixtures were likely to be miscible at low and high pHs. GE/DE (4.0 wt%/4.0 wt%) mixture at pH 3.00–10.0 was selected for following study. Phase separation of GE/DE (4.0 wt%/4.0 wt%) mixture was pH-responsive, e.g. no phase separation at pH 3.00–4.75 and pH 10.0, only microphase separation at pH 5.00 and 5.25, and both micro- and macro-phase separation at pH 5.50–9.00. The order of phase separation rates was pH5.50 > pH6.00 > pH9.00 > pH7.00 > pH8.00. Based on the results of S-MLS, micro- and macro-phase separation images, phase separation process was proposed to be divided into four steps, including droplet coalescence, droplet migration, primary and stable phase separation. The variation of the GE/DE interaction and the self-aggregation of GE as a function of the pH might explain the pH-response of GE/DE phase separation. S-MLS is an effective method to study the phase separation kinetics of macromolecular systems.

Influence of dodecyl surfactants on the cross-linking, plasticization and damping behavior of epoxy novolac resins.

A series of modified epoxy novolac resins (ENRs) were prepared by incorporating dodecyl chain surfactants with polar groups such as amine, carboxylic acid, phenol, resorcinol and benzene sulfonic acid. Except for the case of benzene sulfonic acid, no macrophase separation is seen in any of the modified ENRs. The addition of these dodecyl surfactants to ENR hinders the cross-linking reaction as revealed from their dynamic and isothermal DSC curing measurements and reduced glass transition temperatures (Tg). The cross-link density evaluated from the storage modulus (E') in the rubbery region using DMTA measurements is found to be low with the addition of surfactants in agreement with their plasticization behavior. The stiffness of the materials obtained at low temperatures showed a moderate increase for ENRs with carboxylic acid and phenol surfactants. Upon heating, their storage modulus drops at low temperatures compared to ENR supporting the mechanism of plasticization in them. This high value of E' for the plasticized material in the glassy phase is different from the generally known behavior. Such unusually high storage modulus together with increased 'd' spacing from the XRD results seems to indicate 'partial segmental confinement' of epoxy chains. It is believed that high damping obtained in these two materials is due to 'partial segmental confinement' of epoxy chains because of interaction with lauric acid and phenolic surfactants and the associated internal and interfacial friction in them. Dielectric relaxation measurements support the plasticization process based on the high dielectric loss and ionic conductivity in the surfactant modified ENR, as compared to the pristine ones.

Scalable preparation of high-strength hierarchically porous carbon beads with bicontinuous macroporous network by solvent induced phase separation technique for NOx removal

The potential of hierarchically porous carbons is drastically limited by their intrinsic fragility, tedious preparation, high cost, and poor processability. Here we develop a novel solvent-induced phase separation technique for fast and continuous preparation of hierarchically macro-/microporous carbon beads (MMCBs). The as-prepared MMCBs show spherical shape with millimeter size, which give the carbons many value-added physical properties such as regular geometry, good mechanical properties, low dust, and good flowability. The MMCBs have a high BET surface area of 1099 m 2 g −1 , and percolating macroporous superstructure with axial-oriented macrovoid channel and sponge-like bicontinuous network issued from solvent-induced macrophase separation and microphase separation, respectively. In addition, the MMCBs have a high nitrogen doing of 4–5 wt% originated from their nitrogen-rich polyacrylonitrile precursor. The MMCBs could directly sever as the metal-free carbon catalysts for room-temperature oxidation of NO to NO 2 with over 80% conversion, and the urea-loaded MMCBs could use for selective catalytic reduction of NO to N 2 with long-time NO x removal. This convenient synthesis to porous carbons with the engineered morphology and percolating macropores has great potential in practical applications where both rapid mass transport and high surface areas are required. Scalable preparation of high-strength porous carbon beads with hierarchically macroporous network by solvent induced phase separation for NOx removal. • Macro/microporous carbon beads with bicontinuous macroporous network were prepared. • The MMCBs have highly spherical shape and millimeter size. • The MMCBs have high strength, high wear resistance, low dust, and good flowability. • The MMCBs could sever as the metal-free catalysts for room-temperature oxidation of NO to NO 2 . • The urea-loaded MMCBs could use for selective catalytic reduction of NO to N 2 .

Tuneable phase behaviour and glass transition via polymerization-induced phase separation in crosslinked step-growth polymers.

Once limited to chain-growth polymerizations, fine control over polymerization-induced phase separation (PIPS) has recently been demonstrated in rubber-toughened thermoset materials formed through step-growth polymerizations. The domain length scales of these thermoset materials can be elegantly tuned by utilizing a binary mixture of curing agents (CAs) that individually yield disparate morphologies. Importantly, varying the composition of the binary mixture affects characteristics of the materials such as glass transition temperature and tensile behavior. Here, we establish a full phase diagram of PIPS in a rubber-toughened epoxy system tuned by a binary CA mixture to provide a robust framework of phase behaviour. X-Ray scattering in situ and post-PIPS is employed to elucidate the PIPS mechanism whereby an initial polymerization-induced compositional fluctuation causes nanoscale phase separation of rubber and epoxy components prior to local chain crosslinking and potential macrophase separation. We further demonstrate the universality of this approach by alternatively employing binary epoxy or binary rubber mixtures to achieve broad variations in morphology and glass transitions.

Density Functional Theory for Polymer Phase Separations Induced by Coupling of Chemical Reaction and Elastic Stress

AbstractA density functional theory is applied to phase separation dynamics influenced by crosslinking reactions in dense polymer solutions. The crosslinking reaction is modeled by a change from non‐crosslinked polymers comprising transient network (TN) to crosslinked polymers participating in the percolated permanent network (PN). Deformed TN polymers are considered to relax to the isotropic equilibrium state according to the Maxwellian linear viscoelastic constitutive equation, which is used in the modeling of viscoelastic phase separations. The PN is modeled by a linear elastic constitutive model. When TN polymers are taken into the PN by crosslinking reaction, the instantaneous deformation of the TN polymers are frozen, and such frozen deformations are accumulated as time goes on. A series of simulations is performed using this model, so that two specific features of the viscoelastic and reactive phase separation are obtained, i.e., 1) two‐stage phase separation process that leads to a domain structure with two different characteristic length scales, and 2) fixing the phase‐separated structure before reaching the macrophase separation.

Lewis Adduct-Induced Phase Transitions in Polymer/Solvent Mixtures.

Functionalization-induced phase transitions in polymer systems in which a postpolymerization reaction drives polymers to organize into colloidal aggregates are a versatile method to create nanoscale structures with applications related to biomedicine and nanoreactors. Current functionalization methods to stimulate polymer self-assembly are based on covalent bond formation. Therefore, there is a need to explore alternative reactions that result in noncovalent bond formation. Here, we demonstrate that when the Lewis acid, tris(pentafluorophenyl) borane (BCF), is added to a solution containing poly(4-diphenylphosphino styrene) (PDPPS), the system will either macrophase-separate or form micelles if PDPPS is a homopolymer or a block in a copolymer, respectively. The Lewis adduct-induced phase transition is hypothesized to result from the favorable interaction between the PDPPS and BCF, which results in a negative interaction parameter (χ). A modified Flory-Huggins model was used to determine the predicted phase behavior for a ternary system composed of a polymer, a solvent, and a small molecule. The model indicates that there is a demixing region (i.e., macrophase separation) when the polymer and small molecule have favorable interactions (e.g., χ < 0) and that the phase separation region coincides well with the experimentally determined two-phase region for mixtures containing PDPPS, BCF, and toluene. The work presented here highlights that Lewis adduct-induced phase separation is a new approach to functionalization-induced self-assembly (FISA) and that ternary mixtures will undergo phase separation if two of the components exhibit a sufficiently negative χ.

Phase-separation of cellulose from ionic liquid upon cooling: preparation of microsized particles

Cellulose is an historical polymer, for which its processing possibilities have been limited by the absence of a melting point and insolubility in all non-derivatizing molecular solvents. More recently, ionic liquids (ILs) have been used for cellulose dissolution and regeneration, for example, in the development of textile fiber spinning processes. In some cases, organic electrolyte solutions (OESs), that are binary mixtures of an ionic liquid and a polar aprotic co-solvent, can show even better technical dissolution capacities for cellulose than the pure ILs. Herein we use OESs consisting of two tetraalkylphosphonium acetate ILs and dimethyl sulfoxide or γ-valerolactone, as co-solvents. Cellulose can be first dissolved in these OESs at 120 °C and then regenerated, upon cooling, leading to micro and macro phase-separation. This phenomenon much resembles the upper-critical solution temperature (UCST) type thermodynamic transition. This observed UCST-like behavior of these systems allows for the controlled regeneration of cellulose into colloidal dispersions of spherical microscale particles (spherulites), with highly ordered shape and size. While this phenomenon has been reported for other IL and NMMO-based systems, the mechanisms and phase-behavior have not been well defined. The particles are obtained below the phase-separation temperature as a result of controlled multi-molecular association. The regeneration process is a consequence of multi-parameter interdependence, where the polymer characteristics, OES composition, temperature, cooling rate and time all play their roles. The influence of the experimental conditions, cellulose concentration and the effect of time on regeneration of cellulose in the form of preferential gel or particles is discussed.Graphical abstractRegular micro-sized particles regenerated from a cellulose-OES mixture of tetrabutylphosphonium acetate:DMSO (70:30 w/w) upon cooling

Formation of biomimetic hierarchical nanostructure in homopolymers and block copolymer ternary blend particles

In order to mimic hierarchical nanostructures in nature, particles of polymer blends consisting of poly(4-butyltriphenylamine) (PBTPA), poly(methyl methacrylate) (PMMA) and PBTPA-block-PMMA were fabricated by a solvent evaporation method. Effects of the molecular weight and the chemical composition of PBTPA-b-PMMA, molecular weights of homopolymers, and the composition of the blend on the morphology were investigated. The polymer blend particle consisting of PBTPA and PMMA homopolymers exhibited thermodynamically favored core-shell structure, in which more hydrophilic PMMA-shell surrounded PBTPA-core. The addition of 10 wt% of PBTPA-b-PMMA caused the morphological transition from core-shell to Janus or inversed core-shell, in which PBTPA-shell surrounded PMMA-core, depending on the molecular weight of PBTPA segment in PBTPA-b-PMMA. When the molecular weight of PMMA segment was higher than that of PMMA homopolymer, watermelon-like particles in which small PBTPA domain less than 80 nm dispersed in the PMMA domain surrounded by PBTPA shell were observed. As the ratio of PBTPA-b-PMMA increased, the interface of the macrophase separation became obscure. At 50 wt% of the PBTPA-b-PMMA, only microphase separation was observed. The measurement of interfacial tension by pendant drop method demonstrated that PBTPA-b-PMMA lower the interfacial tension between PBTPA and the aqueous phase to the value similar to that of PMMA with the aqueous phase.

Polymer Interface-Dependent Morphological Transition toward Two-Dimensional Porous Inorganic Nanocoins as an Ultrathin Multifunctional Layer for Stable Lithium-Sulfur Batteries.

Two-dimensional (2D) porous inorganic nanomaterials have intriguing properties as a result of dimensional features and high porosity, but controlled production of circular 2D shapes is still challenging. Here, we designed a simple approach to produce 2D porous inorganic nanocoins (NCs) by integrating block copolymer (BCP) self-assembly and orientation control of microdomains at polymer-polymer interfaces. Multicomponent blends containing BCP and homopoly(methyl methacrylate) (hPMMA) are designed to undergo macrophase separation followed by microphase separation. The balanced interfacial compatibility of BCP allows perpendicularly oriented lamellar-assembly at the interfaces between BCP-rich phase and hPMMA matrix. Disassembly of lamellar structures and calcination yield ultrathin 2D inorganic NCs that are perforated by micropores. This approach enables control of the thickness, size, and chemical composition of the NCs. 2D porous and acidic aluminosilicate NC (AS-NC) is used to fabricate an ultrathin and lightweight functional separator for lithium-sulfur batteries. The AS-NC layer acts as an ionic sieve to selectively block lithium polysulfides. Abundant acid sites chemically capture polysulfides, and micropores physically exclude them, so sulfur utilization and cycle stability are increased.

Investigating Nanoparticle Organization in Polymer Matrices during Reaction-Induced Phase Transitions and Material Processing.

Controlling nanoparticle organization in polymer matrices has been and is still a long-standing issue and directly impacts the performance of the materials. In the majority of instances, simply mixing nanoparticles and polymers leads to macroscale aggregation, resulting in deleterious effects. An alternative method to physically blending independent components such as nanoparticle and polymers is to conduct polymerizations in one-phase monomer/nanoparticle mixtures. Here, we report on the mechanism of nanoparticle aggregation in hybrid materials in which gold nanoparticles are initially homogeneously dispersed in a monomer mixture and then undergo a two-step aggregation process during polymerization and material processing. Specifically, oleylamine-functionalized gold nanoparticles (AuNP) are first synthesized in a methyl methacrylate (MMA) solution and then subsequently polymerized by using a free radical polymerization initiated with azobis(isobutyronitrile) (AIBN) to create hybrid AuNP and poly(methyl methacrylate) (PMMA) materials. The resulting products are easily pressed to obtain bulk films with nanoparticle organization defined as either well-dispersed or aggregated. Polymerizations are performed at various temperatures (T) and MMA volume fractions (ΦMMA) to systematically influence the final nanoparticle dispersion state. During the polymerization of MMA and subsequent material processing, the initially homogeneous AuNP/MMA mixture undergoes macrophase separation between PMMA and oleylamine during the polymerization, yet the AuNP are dispersed in the oleylamine phase. The nanoparticles then aggregate within the oleylamine phase when the materials are processed via vacuum drying and pressing. Nanoparticle organization is tracked throughout the polymerization and processing steps by using a combination of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). The resulting dispersion state of AuNPs in PMMA bulk films is ultimately dictated by the thermodynamics of mixing between the PMMA and oleylamine phases, but the mechanism of nanoparticle aggregation occurs in two steps that correspond to the polymerization and processing of the materials. Flory-Huggins mixing theory is used to support the PMMA and oleylamine phase separation. The reported results highlight how the integration of nonequilibrium processing and mean-field approximations reveal nanoparticle aggregation in hybrid materials synthesized by using reaction-induced phase transitions.

3D models of the dynamics of cancer cells under external pressure.

Skin cancer is one of the most frequent cancers worldwide. Recently, it has been shown that the tumor proliferation rate in skin and its dynamics can be changed by an osmotic pressure. However, these findings are rather unstructured. A weak pressure can slow down the tumor growth, while a very high pressure can, on the contrary, lead to accelerated growth and metastases. The magnitude and spatial distribution of osmotic pressures in tumors at present cannot be measured experimentally. Therefore, it is of particular interest to find appropriate models that would simulate the effects of additional osmotic pressures in skin and assess the features of its implementation. In this paper, we suggest an improved model based on the principles of the conventional hydrodynamic model for macrophase separations, which allows one to include not only the properties of healthy and cancer cells but also the microenvironment. We study and analyze the proliferation of cancer cells in 3D models of the epidermal layer of skin under an osmotic pressure. There are two suggested 3D models that are based on the same principles: (1) cellular cubic lattice and (2) cell spheroid. This allows expanding the application of the model depending on a given task. Here, we are focused on the study of melanoma at an early stage when there are not many cancer cells. Additional compressive and expansive pressures are added to the central part of the system. Both systems demonstrate similar results in slowing down the rate of tumor growth with a small pressure.

High-level hierarchical morphology reinforcing covalent adaptable networks

High-level hierarchical morphology reinforcing covalent adaptable networks