Castor-oil-modified epoxy resins as model systems of rubber-modified thermosets. 1: Thermodynamic analysis of the phase separation

Castor oil (CO) was used to replace polydisperse commercial rubbers (carboxy- or epoxy-terminated butadiene-acrylonitrile random copolymers, CTBN or ETBN) in model systems developed to analyse the origin of the phase separation process in rubber-modified thermosets. Mixtures of CO with an epoxy resin based on the diglycidyl ether of bisphenol A (DGEBA) showed a higher miscibility than a typical CTBN (18% acrylonitrile)-DGEBA system. Thermodynamic factors accounting for this behaviour are discussed using the Flory-Huggins lattice model. Addition of a stoichiometric amount of ethylenediamine (EDA) to a DGEBA-CO solution increased the miscibility. A pseudo-binary approach was used to fit the cloud point curve up to 30% volume fraction of CO. Percentage conversions at the cloud point during the DGEBA-EDA polymerization were experimentally determined and compared with theoretical predictions using the pseudo-binary approach in the framework of the Flory-Huggins lattice model. Reasonable agreement was found, giving direct evidence of the fact that phase separation results from the decrease in the entropic contribution to the free energy of mixing during polymerization.     

Phase diagram of different epoxy-amine precursors modified with a thermoplastic: Effect of structure of epoxy-amine system on miscibility

The miscibility of a thermoplastic with the precursors of different epoxy-amine systems was analyzed thermodynamically in which the dependence of interaction parameter on temperature and composition χ (T, ?) and the polydispersity of components were considered. The epoxy-amine precursors were different only in the nature of amino groups, which were provided by a monoamine and a diamine in different proportions. Cloud-point curves were measured for five unreacted modified systems resulting that miscibility of the system increased with the proportion of monoamine. The thermodynamic analysis was realized in two steps: first, the model was applied to each system individually and secondly, a general equation for χ (T, ?) depending on the monoamine-diamine proportion was searched and used to analyze all systems together. Theoretical calculations of cloud-point curves, shadow curves, spinodal curves, critical points, vitrification curves and species distributions were realized and discussed for those systems.     

Structure development via reaction-induced phase separation in tetrafunctional epoxy/polysulfone blends

For the cure process of tetrafunctional epoxy resin/polysulfone(EP/PSF) blends, we investigated the effect of cure temperature and blend composition on the phase separation behavior by light scattering and the structure development during cure by an optical microscope. The EP/PSF blend without the curing agent was shown to exhibit an LCST-type phase behavior (LCST = 241°C). At the early stage of curing, the EP/PSF blend was homogeneous at the cure temperature. As the cure reaction proceeded, the blend was thrust into a two-phase regime by the LCST depression caused by the increase in a molecular weight of the epoxy-rich phase, and the phase separation took place via a spinodal decomposition (SD) or nucleation and growth (NG) mode, depending on the blend composition and the cure temperature. When cured isothermally at 220°C, the blend exhibited a sea-island morphology formed via the NG mode below 5 wt % PSF content, while the SD mode prevailed above 20 wt % PSF content. At the intermediate composition range, combined morphology with both sea-island and cocontinuous structure was observed. On the other hand, by lowering the cure temperature and/or increasing the content of PSF component, a two-phase structure with a shorter periodic distance was obtained. It seems that the rate of the phase separation is considerable reduced, while that of the cure reaction is not as much. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2233–2242, 1997     

Porous epoxy monolith prepared via chemically induced phase separation

Macroporous epoxy monolith was prepared via chemically induced phase separation using diglycidyl ether of bisphenol A (DGEBA) as a monomer, 4,4′-diaminodiphenylmethane (DDM) as a curing agent, and epoxy soybean oil (ESO) as a solvent. The morphology of the cured systems after removal of ESO was examined using scanning electron microscopy, and the composition of epoxy precursors/solvent for phase inversion was determined. The phase-separation mechanism was deduced from the optic microscopic images to be spinodal decomposition. The pore structure of the cured monolith was controlled by a competition between the rates of curing and phase separation. The ESO concentration, content of curing agent, and the curing temperature constituted the influencing factors on the porous morphology. The average pore size increased with increasing ESO concentration, increasing curing temperature, and decreasing the content of curing agent.     

Organically modified layered-silicates facilitate the formation of interconnected structure in the reaction-induced phase separation of epoxy/thermoplastic hybrid nanocomposite

We incorporated organic modified layered silicates (OLS) into the mixture of epoxy and poly(ether imide) (PEI) to obtain a ternary hybrid nanocomposite and investigated its reaction-induced phase separation behavior. We found that OLS had dramatic impact to the phase separation process and the final phase morphology. The onset of phase separation and the gelation or vitrification time were greatly brought forward and the periodic distance of phase-separated structure was reduced when OLS was incorporated. Phase separation of the unfilled specimens was greatly suppressed at temperatures higher than 190 °C, and no etch hole of PEI-rich phase could be observed in the SEM images. An interconnected, or bicontinuous morphology could only be observed at cure temperatures lower than 140 °C. On the contrary, the OLS-filled hybrid nanocomposites carried out obvious phase separation at cure temperatures ranging from 120 to 220 °C. Even at cure temperatures higher than 190 °C, the hybrid nanocomposites had an interconnected phase-separated microstructure. These phenomena were related to the preferential wettability, chemical reaction of OLS with epoxy oligomer and the enhanced viscosity of the mixture.     

Visualizing phase separation in polystyrene/polystyrene homo‐IPNs via sulfonation

BACKGROUND: Polystyrene/polystyrene (PS/PS) interpenetrating polymer networks (IPNs) represent ideal homo‐IPNs. Whether phase separation occurs in this system has been a long‐standing problem, which is closely related to the self‐organization mechanism in IPN formation and is important to the exploration of new polymer morphologies and properties by topological isomerism. RESULTS: A series of bead samples of PS/PS sequential IPNs with the same nominal divinylbenzene contents were synthesized by suspension polymerization, followed by sulfonation. Scanning electron micrographs and energy‐dispersive X‐ray mapping show unique distinctive topography on both surfaces and fractured surfaces and large heterogeneity in sulfonation of the PS/PS IPN beads, which for the first time provide visual evidence for dual‐phase continuity in PS/PS IPNs. CONCLUSION: The phase separation behavior is proposed to be due to hydrodynamic screening, architectural asymmetry and excluded volume interactions between network I and the precursor chains of network II. This is considered to represent pure IPN effects in sequential formation and may shed light on the general constitution mechanism and molecular design of IPN materials. Copyright © 2009 Society of Chemical Industry     

Reaction induced phase separation in semicrystalline thermoplastic/epoxy resin blends

The phase separation behaviour and phase morphology of blends of 4,4′-diaminodiphenyl sulphone cured diglycidyl ether of bisphenol-A with poly(ε-caprolactone) were investigated by means of scanning electron microscopy, small angle light scattering and optical microscopy. The components are miscible prior to curing. High-temperature isothermal curing induces phase separation. Blends with near to critical concentrations demix via spinodal decomposition. The associated co-continuous morphology is only preserved in the actual critical compositions whereas for off-critical compositions it rapidly breaks up into spherical particles. The proceeding reaction in the separated phases induces a secondary phase separation. Occasionally, tertiary phase separation is observed as well. Off-critical compositions that are further away on either side from the critical point, phase separate via the direct formation of spherical particles, most likely as a result of the dynamic asymmetry of these blends. The influence of the amount, the molar mass of PCL and the cure temperature is discussed.     

Fluctuation-assisted nucleation in the phase separation/crystallization of iPP/OBC blends

This work mainly focused on the nucleation behavior in iPP/OBC (isotactic polypropylene/polyolefin block copolymers) blends with two distinct OBCs. The influence of composition and molecular structure of the OBC component on the crystallization kinetics of the blends was investigated systematically with the aim to better understand the interplay between the two coupled phase transitions in the blends: macrophase separation and crystallization. The isothermal crystallization kinetics showed component and composition dependence in iPP/OBC blends. All the blends in the studied range have enhanced nucleation ability of iPP than the pure iPP under identical conditions. Furthermore, the distinct macrophase separation morphology resulting from the different compatibility between the various OBCs and iPP caused remarkable diversity between the blends: the nuclei density is qualitatively higher (or the nucleation rate is qualitatively faster) in the more compatible blends, and this enhancement of nucleation can be depressed by imposing a macrophase separation process before crystallization. The crystal nuclei from the phase separated matrix were preferentially formed at the interface of the phase domains, and then grew toward and into the iPP-rich phase. It is postulated that the increased nuclei density and/or nucleation rate followed the fluctuation-assisted nucleation mechanism: the enhanced concentration fluctuation at the interfacial area created by the spinodal decomposition played an important role in the nucleation behavior of iPP/OBC blends. The decreased interface areas with increased domain sizes after deeper phase separation, coupled with a more depressed concentration fluctuation, are responsible for lower nuclei density after long time annealing for phase separation.     

Composition dependence of polymerization-induced phase separation of 2-chlorostyrene/polystyrene mixtures

Dynamics of phase separation induced by polymerization of 2-chlorostyrene in the presence of polystyrene was studied by an scanning electron microscope and a time-resolved light scattering instrument with varying initial monomer composition θ in the range from 0.4 to 0.6. Phase-separation rate increased with increasing θ or temperature. Phase being rich in poly(2-chlorostyrene) formed droplets initially and these droplets coagulated to form continuous domains at low temperatures. A maximum was found in the initial-composition dependence of the time when morphological change from droplets to continuous domains occurred. Qualitative discussion was made for the effects of initial composition change on morphological change of poly(2-chlorostyrene)-rich phase.     

Phase separation in epoxy resins containing polyethersulphone

Scanning electron microscopy and dynamic mechanical spectroscopy have been used to study phase separation of dissolved polyethersulphone (PES) from trifunctional and tetrafunctional epoxy resins during curing. No phase separation was observed at high concentrations of the tetrafunctional resin. Observations of nodules on fracture surfaces, and of multiple peaks in the dynamic mechanical spectra provided evidence for a separate, crosslinked, PES-rich phase in the remaining materials. Despite the variety of morphologies obtained in mixtures of PES with different hardeners and resins, modulus and fracture toughness showed little dependence upon composition.     

Phase separation of polystyrene-b-(ethylene-co-butylene)-b-styrene (SEBS) deposited on polystyrene thin films

Phase separation of a triblock copolymer, polystyrene-b-(ethylene-co-butylene)-b-styrene (SEBS) on the thin films of a homopolymer, polystyrene (PS), was studied by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The final morphology after phase separation was found to be greatly dependent on the relation between the molecular weight of the PS block and homo-PS. Dispersed spherical and worm-like micelles of SEBS were observed when the molecular weight of homo-PS is smaller than the PS block in SEBS, while large structures with inner micro-phase separation of SEBS was found when the molecular weight of homo-PS was much higher than that of the PS block. The origin of such a change in morphology is attributed to the difference of structure and interfacial tension at the interface between the matrix homo-PS and the PS block in SEBS triblock copolymer assembly.     

Epoxy/amine systems with monodisperse modifiers: thermodynamic analysis of the phase separation process taking the polymer polydispersity into account

Phase separation during polymerisation was studied by taking into account in the thermodynamic analysis the generated distribution of polymer species. A model system consisting of a diepoxide based on diglycidylether of bisphenol A, a diamine and castor oil as modifier was considered. One-and two-step polymerisation processes were simulated (in the two-step process a first step was carried out with an epoxy excess up to a particular conversion, stoichiometry was balanced and the reaction continued in a second step). A thermodynamic analysis for a monodisperse modifier-epoxy/amine system was performed taking the polydispersity of the polymer into account. It was shown that the cloud point conversion was decreased when the polymerisation was carried out in two steps. This effect was enhanced for lower stoichiometric ratios (higher epoxy excesses) and higher epoxy conversions in the first step. The dispersed phase became richer in low molecular weight species than the continuous one. In particular both monomers increased their concentration in the dispersed phase.     

Layered structure formation in the reaction-induced phase separation of epoxy/polysulfone blends

In an epoxy/polysulfone blend, the reaction-induced phase separation behavior and the final morphology were investigated. Three distinct morphological structures were obtained. Sea-island and nodular structures were observed at lower and higher polysulfone contents, respectively. A three-layered structure was obtained in the middle polysulfone concentration range. In order to understand the formation of three-layered structure, phase separation process was studied using time-resolved light scattering, phase-contrast optical microscope and scanning electron microscope. Bicontinuous structure formed uniformly in the whole sample at the beginning of phase separation. After the phase structure grew for a certain time, large domains formed and developed. Then, the large epoxy-rich domains gradually flew to the outer space of the sample film. This process assisted the formation of the three-layered structure. The mechanism of the formation of the three-layered structure was discussed based on the different viscoelastic properties of the components.     

Double phase separation in preparing polyimide/silica hybrid films by sol-gel method

In this paper, the phase separation process of the polyimide/silica hybrid films made from polyamic acid (PAA) and precursor (TEOS-A) hydrolyzed tetraethoxysilane under acidic condition in N-methyl-2-pyrrolidone (NMP) through sol-gel method was investigated by the scanning electron microscope (SEM). A double phase separation was discovered for the preparation of the hybrid films. With evaporation of the solvent NMP at lower than 100 °C, the component miscibility of TEOS-A and PAA decreases so that the first phase separation took place and a larger particle phase of TEOS-A precursor with size around 2.0 μm was formed. The second phase separation from the matrix phase appeared, as PAA was imidized at elevated temperature, which destroyed the interaction between carboxyl group of PAA and hydroxyl group of TEOS-A, and a nanoscale SiO₂ particle phase formed. The formation mechanism of the double phase separation was explained by the “capture-release” model. According to the model, the second phase separation can be controlled by synthesizing amic acid-imide copolymer with different contents of carboxyl group.     

Phase distribution and separation in poly(2-acetoxyethyl methacrylate)/polystyrene latex interpenetrating polymer networks

A series of latex interpenetrating polymer networks (LIPNs) were prepared via two-stage soap-free emulsion polymerization of styrene on cross-linked poly(2-acetoxyethyl methacrylate) (PAEMA) seed latexes, using potassium persulfate as initiator. It was found that a compositional gradient was present when PAEMA seeds cross-linked either lightly or highly were used. The polystyrene (PS) phase is localized near the particle center in the former case, while it is segregated near the surface in the latter case. A uniform distribution of PS phase in LIPN was formed, if moderately cross-linked PAEMA seed was used. All the LIPNs appeared to be microphase-separated, and increase of cross-linking degree in seed latexes decreased the PS-rich domain size. The results were explained by the particle growth mechanism that involved the formation of surface-active oligomeric radicals in water phase, adsorption of the radicals onto monomer-swollen particle/water interface, and chain propagation in the interface with subsequent phase migration dominated by the competitive effects of thermodynamics and kinetics.     

Time-resolved light scattering of the phase separation in polymer-dispersed liquid crystals formed by photo-polymerization induced phase separation

Time-resolved light scattering is utilized to monitor the phase separation of photo-initiated polymer-dispersed liquid crystals. At the lowest cure intensities studied, the system undergoes spinodal decomposition and the results are analyzed with Cahn-Hilliard theory. As the cure intensity increases, the rate of phase separation increases such that the early stages of spinodal decomposition are no longer observable. These systems are analyzed using the Debye-Bueche model, which provides the time evolution of the number and size of LC domains. These results indicate that an increase in cure beam intensity initially increases the rate of domain growth, but this effect is overwhelmed by the fast vitrification and cross-linking that can occur at highest cure beam intensities.     

Investigation of the phase separation of PNIPAM using infrared spectroscopy together with multivariate data analysis

The use of vibrational spectroscopy to investigate complex structural changes in polymers yields chemically rich data, but interpretation can be challenging and subtle but meaningful spectral changes may be missed through visual inspection alone. Multivariate analysis is an efficient approach to gain an oversight of small but systematic spectral differences anywhere within the spectra, providing further insight into structural changes and associated transformation mechanisms. In this study, the novel analytical approach of infrared spectroscopy combined with principal component analysis and Gaussian peak fitting was used to investigate the structural changes in aqueous solutions of a polymer, using poly(N-isopropyl acrylamide) (PNIPAM) in the atactic form and with controlled tacticity as a model system. Subtle spectral changes associated with the dehydration and phase separation upon heating included peak shifts, an area ratio change of the amide I band to the amide II band and formation of a new peak in the amide I band were efficiently detected. Dehydration and phase separation of PNIPAM occurred in two temperature ranges, one for the atactic and one for isotactic rich part, both involving a complex re-organization of the hydrogen bonds and change of the hydration layer. The changes agreed with existing results from other techniques, and new insights were gained into the effect of controlled tacticity on phase transformation behaviour. The study demonstrates that infrared spectroscopy combined with the multivariate analytical method principal component analysis and Gaussian peak fitting is an efficient approach to probing structural change in polymers during heating. The simplicity of the presented approach could find excellent use in analysing and understanding the molecular environment of a range of stimuli-responsive polymers, for instance block or grafted types of polymers, as well as those with controlled tacticity.     

Facile strategy and mechanism of greatly toughening epoxy resin using polyethersulfone through controlling phase separation with microwave-assisted thermal curing technique

Toughening has been the essential issue for developing high-performance thermosetting resins. Herein, starting from polyethersulfone (PES) and bisphenol A epoxy resin (EP), a facile strategy is developed to prepare tough resins through controlling phase structure with microwave-assisted thermal curing. PES/EP resins cured with the assistance of microwave curing (m-PES/EP) have two major differences compared with those using traditional heat curing (t-PES/EP), one is high degree of phase separation, and the other is phase separation mechanism. Initial phase separation of all t-PES/EP resins follows spinodal decomposition mechanism, while those of m-PES/EP systems with 8 wt% and 18 wt% of PES follow nucleation and growth mechanism. These differences are derived from the fact that microwave curing has much faster phase separation rate and curing reaction rate than thermal curing owing to the higher thermal effect. Differences in phase structure bring different macro performances, and m-PES/EP systems have higher impact strengths, fracture toughnesses, and flexural strengths than t-PES/EP resins. This investigation provides a facial and effective way of developing higher performance thermoplastic/thermosetting resin system through controlling phase structure. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48394.     

New hyperbranched polyether containing cyclic carbonate groups as a toughening agent for epoxy resin

Hyperbranched polyglycerol containing terminal five-membered cyclic carbonate groups has been obtained by anionic polymerization of glycidol and then in the reaction of its terminal vicinal hydroxyl groups with dimethyl carbonate in the presence of potassium carbonate. The remaining OH groups of the polymer were protected in the reaction with acetic anhydride to reduce polymer hydrophilicity and increase miscibility with the epoxy resin. A strong decrease in viscosity was observed after esterification, from 73 to 3.6 Pa s. The product (HBPG 3) was used for modification of the bisphenol A based epoxy resin. The epoxide-cyclic carbonate compositions were cured using a polyamine hardener (TETA) in a one-step procedure. Thermal and mechanical properties of the cured compositions were characterized and compared with the parent epoxy resin. The optimal mechanical properties were obtained for the compositions containing HBPG 3 when phase separation takes place. The mechanical properties are discussed in terms of the morphology observed by SEM.