Reaction-induced Phase Separation Research Articles

Kinetic Monte Carlo modeling of reaction-induced phase separation in Au/Ni(111) surface alloy

Kinetic Monte Carlo (KMC) simulations of Au–Ni phase separation in Au/Ni(111) surface alloy during nickel carbonyl formation reaction were performed at room temperature by taking into account realistic rates of Au and Ni adatom diffusion and CO adsorption and desorption, while keeping the rate of nickel carbonyl formation reaction as a free parameter. We also obtained pair and three-body interaction constants between Au and Ni adatoms using the ab initio calculations and demonstrated that their proper choice is crucial for understanding the Au–Ni separation process. Three regimes with qualitatively different Ni–CO reaction propagation kinetics in Au/Ni(111) surface alloy were found by varying the constants of trio-triangle interactions within the limits of their accuracy. The sensitivity of the proposed model to interaction parameters leads to the regimes that differ by step flow rate, Au islands formation mechanism (channel-type or homogenous flow), Au island contamination level by Ni impurities and reaction damping tendency at the reaction front. Nucleation of nickel-free Au islands was observed for a homogeneous step flow pattern, and the corresponding step flow rate is in a good quantitative agreement with existing experimental data.

Reinforcing thermosets using crystalline desoxyanisoin structures

ABSTRACTThis article presents results on the potential of using crystalline flame retardants for thermoset reinforcement. The approach involves introducing reinforcement in thermosetting polymers through low molecular weight crystallizable additives. Thermally induced phase separation (TIPS) and crystallization of desoxyanisoin in diglycidylether of bisphenol‐A (DGEBA) epoxy monomer were investigated. Small angle light scattering and polarized optical microscopy were utilized to monitor phase separation and the crystallization of desoxyanisoin in DGEBA at different concentrations. Reaction induced phase separation (RIPS) with polyetheramine was carried out under isothermal and temperature gradient curing conditions. Altering the cure schedule resulted in a rich range of morphologies due to the competition between TIPS and RIPS. During isothermal cure, straight fiber‐like anisotropic crystals on a centimeter length scale developed. In contrast, thermal gradients frustrated the crystal growth and resulted in complex and rich morphologies. Desoxyanisoin provided marginal epoxy thermoset reinforcement at 10 vol %. However, the additive did not increase the thermoset flammability retardancy. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39853.

Heterogeneous polycondensation for composition control of poly(p -mercaptobenzoyl-co-p -benzamide) by shearing

Composition control of aromatic poly(thioester-amide) was examined by the reaction-induced phase separation during polymerization of S-acetyl-4-mercaptobenzoic acid (AMBA) and p-acetylaminobenzoic acid (AABA) in aromatic solvent. The poly(thioester-amide)s were obtained as precipitates and their yields became lower at the middle range of the content of AMBA in feed (χf). The contents of p-mercaptobenzoyl (MB) moiety (χp) in the precipitates prepared without shearing were in good agreement with the χf values. In contrast to this, the χp values of the precipitates prepared at χf of 50–70 mol % under shearing were much lower than the χf values. The reaction rate of AMBA increased with shearing, whereas that of AABA was unchanged by shearing. This shearing effect on the reaction rates accelerated to form the homo-oligomers. The solubility of MB oligomers enhanced by shearing, whereas that of p-benzamide oligomers did not enhance owing to the strong interaction through hydrogen bonding. The MB oligomers were inhibited to be precipitated, resulting in the lower χp values than the χf values. The composition could be controlled by the application of the shearing to the heterogeneous polymerization. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4301–4308

Morphological and Mechanical Characterization of Nanostructured Thermosets from Epoxy and Styrene-block-Butadiene-block-Styrene Triblock Copolymer

Styrene-block-butadiene-block-styrene (SBS) triblock copolymers epoxidized at several epoxidation degrees by hydrogen peroxide in water/dichloroethane biphasic system were blended with epoxy based on diglycidyl ether of bisphenol A (DGEBA) and DDM (4,4′-diaminodiphenyl methane) as curing agent. The incorporation of epoxidized block copolymers in epoxy resulted in the formation of nanostructured blends. The morphologies of the blended polymers were studied using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS) analysis. The key factor controlling the morphology is the mole percentage of epoxidised butadiene units in SBS. In fact, the morphologies changed from macroscopic phase separated domains in unmodified SBS to nanostructured domains in epoxidised SBS and the resulting morphologies were fixed by the cross-linking reaction. Nanostructured morphologies such as worm-like and spherical micelles (radius 8 nm) were generated due to reaction induced phase separation of PS phase followed by the self-assembly of PB sub chains. The mechanical properties such as fracture toughness (stress field intensity factor (KIC)), and impact strength of these blended systems were measured. It was established that nanostructured blends significantly improved fracture toughness and impact strength. Field emission scanning electron micrographs of fractured surfaces were examined to understand the toughening mechanism.

Epoxy/poly(ɛ‐caprolactone) nanocomposites: Effect of transformations of structure on crystallization

ABSTRACTThe influence of morphology of the epoxy/poly(ɛ‐caprolactone) (PCL) system and corresponding nanocomposites with organophilized layered silicate on PCL crystallization was studied by differential scanning calorimetry, scanning, and transmission electron microscopy. The results obtained indicate a significant affecting of nonisothermal PCL crystallization by phase morphology brought about by the reaction‐induced phase separation (RIPS) influenced either by various nanoclay contents or the epoxy/PCL ratio. Dispersed morphology of PCL matrix with epoxy globules induces crystallization at higher temperatures. The inverse dispersed morphology of epoxy matrix with PCL inclusions causes crystallization at lower temperature. The co‐continuous morphology induces crystallization in both steps. Rate of the second crystallization step is substantially higher than that in the first step. No nucleation effect has been found in the nanocomposites with the added nanofiller. Multicomponent samples show retarded crystallization, i.e., lower crystallinities and lower overall crystallization rate compared with neat PCL. The results obtained suggest that it is primarily morphological/interfacial effects that play a decisive role in the crystallization behavior of PCL in the epoxy/PCL/clay system. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3197–3204, 2013

Reaction induced phase separation in a bisphenol A-aniline benzoxazine/N,N′-(2,2,4-trimethylhexane-1,6-diyl)bis(maleimide)/imidazole blend: the effect of initial curing temperature on morphology and properties

The effect of initial curing temperature on final morphology, the influence of different morphologies on the properties and the reason for the initial curing temperature's effect on phase separation in a bisphenol A-aniline benzoxazine (BA-a)/N,N′-(2,2,4-trimethylhexane-1,6-diyl)bis(maleimide) (TBMI)/imidazole blend were studied in this paper. The blend showed different morphologies at different initial curing temperatures and the morphologies changed from a clear bi-continuous structure to an obscure structure, followed by an interpenetrating network, with increasing initial curing temperature. This may be mainly caused by the amount of co-reaction of two monomers and the relative rates of phase separation and reaction. At the same time, the Flory–Huggins parameter of blends became smaller and smaller as the initial curing temperature increased, which led to a difficulty in phase separation due to the thermodynamics. Finally, phase separated products showed improved toughness and thermal properties.

Effect of an organoclay on the reaction-induced phase-separation in a dynamically asymmetric epoxy/PCL system

The addition of layered silicates can significantly affect the phase behaviour of both immiscible thermoplastic blends and partially miscible thermoset systems that undergo reaction-induced phase separation (RIPS) during curing. This study focuses on the phase behaviour of polycaprolactone (PCL)/epoxy in the presence of organically modified montmoril- lonite (oMMT). Due to the high dynamic asymmetry caused by the differences in the molecular weights and viscosities of the PCL and the uncured epoxy, the critical point is localised at low PCL concentrations, as indicated by the pseudophase diagram. The addition of oMMT to the system led to the marked shift of the critical point towards higher concentrations of PCL, with an increase in the oMMT content occurring as a consequence of the preferential localisation of the clay in the epoxy phase, making this phase more dynamically slow. Significant changes in morphology, including phase inversion of the PCL/epoxy systems caused by the presence of oMMT, were recorded for PCL concentrations ranging from 10 to 30%.

Photocrosslinking-induced phase separation in evaporative solvents: formation of skin layers and microspheres

We report the structure formation of films obtained via photocrosslinking of precursors during the evaporation of solvents. Although most precursor/solvent systems result in uniform dense films after the process, reaction-induced phase separation can occur in solvents with a unique combination of solubility, evaporation rate and ratio of latent heat to heat capacity. Most significantly, for such solvents, the forced convective evaporation under controlled N2 flow results in a highly hierarchical film morphology, featuring a skin layer on top of a layer of microspheres formed via a nucleation and growth mechanism. For the first time, the skin layer formed during the evaporation was directly observed after the complete evaporation of the solvent. The thickness of the skin layer is dependent on the processing parameters including the N2 flow rate, UV intensity and precursor concentrations. The skin layer formation could be suppressed by addition of non-solvent, in which case the characteristic morphology resulted from dominant spinodal decomposition. A model is presented that can qualitatively describe the skin layer formation and its dependence on the processing parameters, providing a mechanistic understanding of the photocrosslinking-induced phase separation under evaporative environments.

Clay nanostructure and its localisation in an epoxy/liquid rubber blend

Different nanoclay concentrations (1–3 phr) were incorporated into an epoxy/carboxyl terminated poly(butadiene-co-acrylonitrile) liquid rubber (CTBN) blend to obtain epoxy/clay/CTBN nanocomposites. Morphological analysis (X-ray diffraction and electron microscopy techniques) of epoxy/clay/CTBN nanocomposites revealed that the clay platelets were largely exfoliated at lower (1 phr) clay concentration. This highly exfoliated portion of the clay platelets was found to easily get inside the CTBN phase during the reaction induced phase separation process in 1 phr clay loaded epoxy/clay/CTBN nanocomposite. However, the intercalated clay platelets in epoxy/clay/CTBN nanocomposites with higher (2, 3 phr) clay concentration, locate in epoxy phase with a tendency to align along the epoxy–CTBN interface which suppresses the coalescence of the rubber particles during phase separation. It was found that the clay nanostructure and localisation among the blend components affect both viscoelastic and mechanical properties of the epoxy/clay/CTBN nanocomposites. The quantitative estimation of constrained epoxy volume confirmed the morphological findings. The thermal degradation behavior of the epoxy/clay/CTBN nanocomposites was found to depend mainly on the dispersion of clay rather than its distribution among the blend components.

Synthesis of Janus particles via kinetic control of phase separation in emulsion droplets

A reaction-induced phase separation strategy in emulsion droplets has been developed to fabricate non-spherical Janus particles. The final morphology of the resulting Janus particles is controlled by a reaction sequence. These Janus particles are amphiphilic and show regio-selective adsorption orientation at oil/water interfaces.

Effect of Attapulgite Nanorods and Calcium Sulfate Microwhiskers on the Reaction-Induced Phase Separation of Epoxy/PES Blends

The influence of two kinds of mesoscale inorganic rod fillers, nanoscale attapulgite and micron-sized CaSO4whisker, on the reaction-induced phase separation of epoxy/aromatic amine/poly- (ether sulfone) (PES) blends has been investigated by optical microscopy (OM), scanning electron microscopy (SEM), and time resolved light scattering (TRLS). By varying the PES concentration and curing temperature, we found that the incorporation of attapulgite and CaSO4had dramatic impact on the phase separation process and the final phase morphology of blends. In blends at higher content than critical concentration, the process of phase separation was retarded by the incorporation of nanoscale fillers but accelerated by that of the micron-sized fillers, mainly due to the enhanced viscoelastic effect and the preferential wettable effect, respectively. Meanwhile both mesoscale fillers could change the cocontinuous phase structure of blends with lower PES content than critical concentration into PES-rich dispersed structure due to the surface affinity of fillers to epoxy matrix.

Hollow spheres of aromatic polyamide prepared by reaction‐induced phase separation

AbstractHollow spheres of aromatic polyamide are obtained by the reaction‐induced phase separation during polymerization of 5‐hydroxyisophthalic acid and 1,4‐phenylene diamine in an aromatic solvent at a concentration of 1–2% at 320 °C without stirring. The hollow sphere has a dimple hole and the diameters of the hollow spheres are 3–4 μm. The droplets are initially generated via liquid–liquid phase separation and then rigid cross‐linked network structure formed the rigid skin layer on the surface of the droplets. The solidification of the droplets occurred owing to the further polymerization in them with maintaining the morphology to form the hollow spheres. The hollow spheres exhibit outstanding thermal stability. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2013

Modelling the morphology evolution of polymer materials undergoing phase separation

This contribution models the morphology evolution of hetero-phase polymers that undergo phase separation and inversion during their formation. High impact polystyrene (HIPS) is a selected representative of hetero-phase polymers with double-emulsion (or ‘salami’) morphology, because it consists of the continuous polystyrene (PS) phase with dispersed micron-sized polybutadiene (PB) particles containing sub-micron occlusions of the partially grafted copolymer (PB-g-PS). Our modelling effort builds on the work of Nauman and He (2001) [1], but addresses the weakest point of the Cahn–Hilliard model applied to HIPS evolution: the Ostwald ripening, which is in reality suppressed or reduced by grafting. Phase inversion is the most critical but so far least-modelled step in double-emulsion morphology evolution. Therefore we demonstrate the modelling of all three experimentally observed mechanisms of double-emulsion formation: (i) encapsulation of PS particles into PB domains by shear forces, (ii) thermodynamically consistent reaction-induced phase separation, and (iii) preservation of graft-stabilized PS particles through the phase inversion. We stress out the requirement of proper setting of three-component (polystyrene–polybutadiene–styrene) thermodynamics as the basis for the realistic description of the phenomena occurring during the evolution of hetero-phase morphology. We thus present the first phenomenological model capable to describe all principal steps in the HIPS double-emulsion morphology evolution and can thus conclude with the discussion of future efforts aimed at its quantitative refinement and modelling of industrial reactors.

Epoxy resin/polyetherimide/carbon black conductive polymer composites with a double percolation structure by reaction-induced phase separation

A novel double-percolation structural conductive polymer composite based on epoxy/polyetherimide/carbon black was prepared for the first time. The electrical property, phase structure of the composite and the selective distribution of carbon black were investigated. The results show that a reaction-induced phase separation between epoxy and polyetherimide can be observed in epoxy/polyetherimide/carbon black composites. In the blend with 16 wt% polyetherimide, polyetherimide phase changes from a dispersed phase to a three-dimensionally continuous phase, while epoxy converts to a globular dispersed phase. The selective distribution of carbon black in the blends is that almost all carbon black particles are located in polyetherimide phase. Accordingly, a double-percolation structure is formed when carbon black content comes to the percolation threshold in polyetherimide phase. In fact, the percolation threshold of carbon black reduces to 0.5 wt% in the blend with 16 wt% polyetherimide.

Effect of nanoclay and carboxyl-terminated (butadiene-co-acrylonitrile) (CTBN) rubber on the reaction induced phase separation and cure kinetics of an epoxy/cyclic anhydride system

The effects of nano clay, carboxyl-terminated (butadiene-co-acrylonitrile) (CTBN) liquid rubber and the combination of both on the cure kinetics of diglycidyl ether of bisphenol-A (DGEBA)-based epoxy resin/nadic methyl anhydride were studied. Cure kinetics studies were carried out by performing dynamic and isothermal differential scanning calorimetric (DSC) experiments. The dynamic DSC experiments were carried out at four different heating rates. Dynamic kinetic modeling was performed using Kissinger and Ozawa approaches. Since these methods are based exclusively on the maximum rate of cure, which occurs approximately at the beginning of the cure reaction, the activation energy calculated using these methods is valid only for the initial stage of the cure. The clay (3 phr) filled epoxy system has an activation energy 24 % lower than the unfilled system. The role of the surfactant chemistry on the initial stage of the cure reaction was also studied. A plausible reaction mechanism which involves the effect of the nanoclay surfactant as an accelerator of the cure reaction was proposed. The phase separated CTBN rubber hindered the cure reaction and has 3 % higher activation energy for epoxy/CTBN system than the unfilled system. In the ternary epoxy/3 phr clay/15 phr CTBN system, the accelerating effect of clay on cure was highlighted. The cure activation observed in the presence of clay overshadows the hindrance created by the phase separated CTBN. Isothermal DSC scans were carried out at five different temperatures. The experimental datas showed an autocatalytic behavior of the reaction, and the isothermal modeling was carried out by Kamal autocatalytic model. The results showed a very good agreement within the whole conversion range for the unfilled and all the filled systems. The evolution of the morphology and phase separation was also studied using optical and scanning electron microscope. Faster cure reaction resulted in smaller phase-separated CTBN particles in epoxy/clay/CTBN ternary system as compared with those observed in epoxy/CTBN binary blend.

Preparation of Monolithic Epoxy Resin-2-methylimidazole Porous Material for Adsorption/Electrosorption of Cu(II)

A new monolithic porous polymer was successfully prepared by using epoxy resin (bisphenol A diglycidyl ether, EP), 2-methylimidazole (2-MZ) as curing agent and polyethylene glycol (PEG-1540) as porogen. The reaction-induced phase separation of this system was successfully controlled by the weight ratio of EP-MZ-PEG (2.0:0.27:2.8) and other reaction conditions. The stepwise polymerization completed at 100 °C for 11 h without disturbance mode. The porogen was removed and the monolithic material with macro-through-pores was obtained. The synthesized polymer was characterized by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM). The prepared material was found to be more efficient for Cu(II) adsorption as compare to Co(II) and Ni(II) in aqueous water. Furthermore, it was then used as the working electrode connected with a direct high-voltage power supply, and a platinum wire was used as counter electrode. The experimental conditions including the adsorption/desorption time, pH value, adsorption/electrosorption mode, and voltage were investigated and optimized, and a new method for Cu(II) removal was established. After adsorption at pH 6, the stirring speed of 400 rpm and the voltage below 120V for 1 h, the loaded Cu(II) was then eluted with 3 mL of 5% HNO3 solution for 10 min, and determined with inductively coupled plasma atom emission spectrometry (ICP-AES). The results showed that the adsorption rate was improved significantly by the electrically enhancing means, and the recovery was > 95%.

Viscoelastic effects in thermoplastic poly(styrene-acrylonitrile)-modified epoxy–DDM system during reaction induced phase separation

The viscoelastic phase separation of a poly(styrene-co-acrylonitrile) (SAN) modified epoxy system based on the diglycidyl ether of bisphenol A (DGEBA) cured with 4,4′-diaminodiphenylmethane (DDM) has been monitored in situ using rheometry, optical microscopy (OM) and small angle laser light scattering (SALLS). The amount of SAN in the epoxy blends were 3.6, 6.9, 10, and 12.9 wt%. The relationship between rheological properties and phase separation was carefully explored. The evolution of storage modulus, loss modulus, and tan δ were found to be closely related to the evolution of complex phase separation. From the rheological profile, two gel points are identified, corresponding to physical gelation and chemical gelation, the first one because of viscoelastic phase separation and the second one related to crosslinking of the epoxy resin, these depend on the cure temperature and amount of thermoplastic. Further SALLS investigations investigated the mechanism of phase separation. The time-dependent peak scattering vector was simulated with a Maxwell-type viscoelastic relaxation equation. Relaxation times obtained at different temperatures for the blends could be described by the Williams–Landel–Ferry equation. Moreover, the development of light scattering profile follows the Tanaka model of viscoelastic phase separation.

Ubiquitous nature of the three-layered structure formation in the asymmetric phase separation of the epoxy/thermoplastic blends

The reaction induced phase separation behavior was investigated in various epoxy/thermoplastic (TP) blends. Several morphological structures, including the sea-island, dual morphological (epoxy-rich particles may disperse in the TP-rich domains, while TP-rich particles can also have some dispersions in the epoxy-rich phase) or three-layered (TP-rich continuous domains mainly located in the middle part of the sample while outer layers are mostly composed of epoxy-rich matrix) and nodular structure, were observed with the increase of the TP concentrations. In the middle TP concentration ranges, when the TP with low glass transition temperature ( T g) was used, dual morphological structures were formed. While for the epoxy/TP blends with high T g thermoplastics, three-layered structures were observed. High T g and the large molecular weight of the TP was considered to cause the dynamic asymmetry, which dominated the phase separation mechanism and formed such an unusual structure. Through the in-situ investigation on time evolution of phase separation process, ubiquitous nature of this formation mechanism of the three-layered structure were discussed, which may have some important implications on the applications of the epoxy/TP blends.

Reaction-induced phase separation of bis(triethoxysilyl)ethane upon sol–gel polymerization in acidic conditions

While organically bridged alkoxysilane precursors such as bis(triethoxysilyl)ethane (BTESE) find increasing use in materials synthesis, their polymerization still has not been subject to detailed kinetic investigations. One factor complicating the measurement and interpretation of the behavior of these monomers is their tendency to aggregate during polymerization into small clusters or particles. Here, the phase behavior and kinetics of BTESE during polymerization in acidic water–ethanol solutions are investigated in situ using 29Si NMR spectroscopy. Based on macroscopic observation of the colloidal stability of the reacting solutions, a ternary pseudo-phase diagram is constructed and solutions that seem from a macroscopic point of view to be favorable for kinetic investigation are probed in situ using 29Si NMR. However, even when the solutions remain optically clear, the polymerization mixtures are sometimes prone to gradual loss of NMR signal. The rate of signal loss is found to be dependent on the initial composition as well as the pH of the reacting mixtures. We speculate that this phenomenon is caused by microphase separation of reaction intermediates formed early in the polymerization process. This phenomenon is likely to affect the formation and distribution of oligomers in the solution that eventually react together to form a material.

Interfacial reaction induced phase separation in LaxHfyO films

Amorphous LaxHfyO films containing La at concentrations (x) of 50 and 20% were prepared by atomic layer deposition on ultrathin SiO2 films (1 nm). We examined the electronic structures and microstructures of the LaxHfyO films by x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). Phase separation into La2O3 and HfO2 was observed in the LaxHfyO films subjected to annealing temperatures over 900 °C, although the mixture of La2O3 and HfO2 is thermodynamically stable. The structural changes that occurred as the result of phase separation were dependent on the concentrations of La and Hf in the films. During the annealing treatment, silicate was produced due to interfacial reactions and the interfacial reactions were found to be dependent on the La2O3 content in the LaxHfyO films, which has a significant influence on the phase separation process and resulting film structure.