Polymerization-induced phase separation in polyether-sulfone modified epoxy resin systems: effect of curing reaction mechanism

Polyethersulfone (PES)-modified epoxy systems with stepwise reaction were studied throughout the entire curing process by using optical microscopes, time-resolved light scattering (TRLS), and a rheolometry instrument compared with that of chainwise polymerization. The results suggested that the phase separation process is mainly controlled by the diffusion of epoxy oligomers for stepwise mechanism system and by that of epoxy monomers for chainwise mechanism system. In case of high PES content (SPES-20%) light-scattering results showed a viscoelastic phase separation and the characteristic relaxation time of phase separation can be described well by the WLF equation. However, in the case of low PES content (SPES-14%) secondary phase separation phenomenon was observed by Optical Microscope and further demonstrated by rheological study.     

Polymerization-induced phase separation in silica sol-gel systems containing formamide

Silica gels with well-defined pores both in micrometer and nanometer ranges were obtained by acid-catalyzed hydrolysis and polymerization of tetramethoxysilane in the presence of formamide. The micrometer-range structures of these gels are studied in terms of the phase diagram of the quasi two-component system, namely solvent-rich and silica-rich end compositions. The resulting interconnected structures and aggregates of particles are related to the occurrence of spinodal phase separation. The composition region that gave interconnected structures for the present system was much more limited and their characteristic sizes were much smaller than those for the previously reported systems containing an organic polymer. These results could be explained qualitatively by the effect of the degree of polymerization on the Flory-Huggins' type free energy change of mixing.     

Rheological behaviors and structural transitions in a polyethersulfone-modified epoxy system during phase separation

The rheological behaviors and gelation transitions in a polyethersulfone (PES)-modified epoxy system during phase separation were studied by rheometry, time-resolved light scattering, and differential scanning calorimetry. Two separate structural transitions in the curing process of the blend were identified as the first one because of phase separation and the second one related to cross-linking reaction of epoxy resin. Both the times of the two structural transition at different temperatures could be described well by the Arrhenius type equation. The complex viscosity exhibits an exponential growing process during phase separation at various temperatures, correlating to the light-scattering results. The exponential behavior of complex viscosity could be attributed to the viscoelastic flow of epoxy-rich escaping from PES-rich during phase separation process.     

Polymerization-induced phase separation in gradient copolymers

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Reaction-induced phase separation in polyethersulfone-modified epoxy-amine systems studied by temperature modulated differential scanning calorimetry

In epoxy-amine systems with a thermoplastic additive, the initially homogeneous reaction mixture can change into a multi-phase morphology as a result of the increase in molecular weight or network formation of the curing matrix. Temperature modulated DSC (TMDSC) allows the real-time monitoring of this reaction-induced phase separation. A linear polymerizing epoxy-amine (DGEBA–aniline) and a network-forming epoxy-amine (DGEBA–methylene dianiline), both with an amorphous engineering thermoplastic additive (polyethersulfone, PES), are used to illustrate the effects of phase separation on the signals of the TMDSC experiment. The non-reversing heat flow gives information about the reaction kinetics. The heat capacity signal also contains information about the reaction mechanism in combination with effects induced by the changing morphology and rheology such as phase separation and vitrification. In quasi-isothermal (partial cure) TMDSC experiments, the compositional changes resulting from the proceeding phase separation are shown by distinct stepwise heat capacity decreases. The heat flow phase signal is a sensitive indication of relaxation phenomena accompanying the effects of phase separation and vitrification. Non-isothermal (post-cure) TMDSC experiments provide additional real-time information on further reaction and phase separation, and on the effect of temperature on phase separation, giving support to an LCST phase diagram. They also allow measurement of the thermal properties of the in situ formed multi-phase materials.     

Gelation behavior of thermoplastic-modified epoxy systems during polymerization-induced phase separation

The rheological behavior and gelation characteristics of epoxy blends are of critical importance to property study and industrial application. In this work, we studied the rheological behavior and structural transition of different thermoplastics, including polyetherimide, polymethylmethacrylate, and polyethersulfone (PES), modified epoxy systems by using rheometry instrument, differential scanning calorimetry, time-resolved light scattering, and scanning electronic microscopes. At the same molecular weight level of thermoplastics, different epoxy blends show profound diversities on the rheological and gelation behavior due to the large differences in phase separation and curing process. For early phase-separation systems of PES-modified epoxy blends, two gel points are identified, which correspond to physical gelation and chemical gelation, respectively. With the variation of the PES molecular weight and curing rate, dramatic changes in gel time and critical exponent were observed. As the molecular weight of thermoplastics is increased, the gelation time becomes shorter and the gel strength gets lower, while the faster curing rate would increase the physical gel strength significantly.     

Polymerization-induced phase separation as a one-step strategy to self-assemble alkanethiol-stabilized gold nanoparticles inside polystyrene domains dispersed in an epoxy matrix

The rational design of nanoparticle (NP)/polymer composites with advanced functional properties is based on controlling the distribution and self-assembly of NPs in the polymer matrix. In this study we report a new one-step strategy to produce the self-assembly of alkanethiol-stabilized Au NPs in one of the phases generated by polymerization-induced phase separation. The polymerization of a formulation composed of stoichiometric amounts of diglycidylether of bisphenol A (DGEBA) and m-xylylenediamine (mXDA), containing polystyrene (PS) and dodecanethiol-stabilized Au NPs as modifiers, produced the phase separation of PS and Au NPs into microdomains dispersed in the epoxy matrix. A subsequent phase separation and self-assembly of Au NPs took place inside the PS domains leading to an increase in their concentration in a region close to the interface as revealed by TEM images. SAXS spectra showed that NPs self-assembled as colloidal crystals with a body-centered cubic (bcc) structure. By an adequate selection of the amount of PS and the nature of the epoxy precursors, different morphologies of the final blend could be generated. This brings the possibility of controlling the dispersion and self-assembly of NPs in the final material.     

The influence of polyetherimide on gelation and phase separation in epoxy systems

In the process of curing under thermostatic conditions, the time dependence of active and reactive components of the dielectric permittivity of epoxy amine compositions that contain a thermoplastic substance is studied in a wide range of electrical frequencies. The times of gelation and vitrification are calculated from the dielectric data and the onset of the phase separation is identified. The curing behavior established by dielectric spectroscopy is confirmed by viscometry and optical microscopy. During the phase separation, the morphology of the precipitating phase differs between samples depending on the chemical nature of the curing agent.     

Reaction-induced phase separation in rubber-modified epoxy resin

The phase separation mechanism,and structure development during curing of epoxy with a novel liquid rubber-ZR were investigated by time-resolved light scattering,optical microscope and differential scanning calonmetry (DSC) The mixture loaded with curing agent was a single-phase system in the early stage of curing.When the cure reaction proceeded,phase separation took place via the spinodal decomposition induced by polymerization of epoxy resin.This was supported by the characteristic change of light scattering profile with curing time.Cure reaction plays an important role in the progress of phase separation.The bigger the cure reaction rate is,the longer periodic distance will be.The overall two-phase structure was basically locked in when the conversion approached 80% estimated by DSC,and finally the co-continuous two-phase structure was successfully obtained.     

PVT behavior of thermoplastic poly(styrene-co-acrylonitrile)-modified epoxy systems: relating polymerization-induced viscoelastic phase separation with the cure shrinkage performance

The volume shrinkage during polymerization of a thermoplastic modified epoxy resin undergoing a simultaneous viscoelastic phase separation was investigated for the first time by means of pressure-volume-temperature (PVT) analysis. Varying amounts (0-20%) of poly(styrene-co-acrylonitrile) (SAN) have been incorporated into a high-temperature epoxy-diamine system, diglycidyl ether of bisphenol A (DGEBA)-4,4'-diaminodiphenyl sulfone (DDS) mixture, and subsequently polymerized isothermally at a constant pressure of 10 MPa. Volume shrinkage is highest for the double-phased network-like bicontinuous morphology in the SAN-15% system. Investigation of the epoxy reaction kinetics based on the conversions derived from PVT data established a phase-separation effect on the volume shrinkage behavior in these blends. From subsequent thermal transition studies of various epoxy-DDS/SAN systems, it has been suggested that the behavior of the highly intermixed thermoplastic SAN-rich phase is the key for in situ shrinkage control. Various microscopic characterizations including scanning electron microscopy, atomic force microscopy, and optical microscopy are combined to confirm that the shrinkage behavior is manipulated by a volume shrinkage of the thermoplastic SAN-rich phase undergoing a viscoelastic phase separation during cure. Consequently, a new mechanism for volume shrinkage has been visualized for the in situ polymerization of a thermoplastic-modified epoxy resin.     

Polymerization-induced phase separation III. Morphologies and contrast ratios of polymer dispersed liquid crystals

Polymerization induced phase separation in mixtures of liquid crystals (LCs) and acrylates (Merck TL205/PN393) proceeds by liquid-gel demixing, in most cases of practical interest. At high LC content or low temperature of polymerization liquid-liquid separation cannot be excluded. Depending on the elasticity and homogeneity of the polymer network at the onset of phase separation, spherical or non-spherical LC domains are observed; non-spherical domains reflect an inhomogeneous gel structure. The change from spherical to non-spherical occurs in a very narrow range of LC concentrations and curing temperatures. The transition between these two morphologies can be explained using conversion phase diagrams obtained from the Flory-Huggins-Dusek theory. The contrast ratio of PDLCs made from the Merck mixture passes through a maximum when the droplet shape at the onset of phase separation changes from spherical to non-spherical. Lowering the LC content or increasing the temperature leads to smaller LC domains which scatter less efficiently. The reverse changes lead to early phase separation and large LC domains which also scatter inefficiently. It is speculated that the maximum of the contrast ratio is related to secondary phase separation, leading to subdomains of an appropriate size.     

GPU accelerated numerical simulations of viscoelastic phase separation model

We introduce a complete implementation of viscoelastic model for numerical simulations of the phase separation kinetics in dynamic asymmetry systems such as polymer blends and polymer solutions on a graphics processing unit (GPU) by CUDA language and discuss algorithms and optimizations in details. From studies of a polymer solution, we show that the GPU-based implementation can predict correctly the accepted results and provide about 190 times speedup over a single central processing unit (CPU). Further accuracy analysis demonstrates that both the single and the double precision calculations on the GPU are sufficient to produce high-quality results in numerical simulations of viscoelastic model. Therefore, the GPU-based viscoelastic model is very promising for studying many phase separation processes of experimental and theoretical interests that often take place on the large length and time scales and are not easily addressed by a conventional implementation running on a single CPU.     

Preparation of epoxy monoliths via chemically induced phase separation

Epoxy porous monoliths were prepared from a commercial epoxy resin, D.E.R. 331, that cured with a tertiary amine, 2,4,6-tris-(dimethylaminomethyl) phenol, in the presence of a solvent, diisobutyl ketone (DIBK). During the curing process, polymers were formed and a decrease in its solubility in DIBK; the solution thus phase-separated, usually referred to as chemically induced phase separation. The phase separation formed interconnected polymer-poor phase that then became interconnected pores after the removal of DIBK. By varying the content of DIBK from 32 to 40 vol.%, epoxy monoliths with interconnected pores were prepared, with surface pore size ranging from 0.20 to 2.33 μm, overall porosity from 0.41 to 0.60, and ethanol permeability from 10 to 4,717 L/(m²?h^?1?bar^?1). The glass transition temperatures of the epoxy monoliths, measured with differential scanning calorimetry, were all higher than 100 °C, and temperatures of 5 % weight loss, analyzed by thermal gravimetry, were higher than 350 °C, evidencing the monoliths’ high thermal stability. Also, the monolith morphology was found to be strongly related to the reaction mechanism of polymerization. The results indicate that the mechanism of chain initiation and propagation associated with the tertiary amine can effectively form monoliths with interconnected pores, which cannot be easily prepared with a stepwise polymerization mechanism associated with using primary amine as the curing agent.     

Phase separation and rheological behavior in thermoplastic modified epoxy systems

The relationship between rheological behavior and phase separation in polyesterimide modified epoxy systems was studied by rheometry, time-resolved light scattering (TRLS), and Differential Scanning Calorimetry (DSC). The rheological behaviors of blends during phase separation showed an exponential grow of complex viscosity, while the phase separation was inhibited by the vitrification of the polyesterimide-rich matrix phase rather than gelation of dispersed epoxy-rich particles. The characteristic relaxation time obtained by the simulation of complex viscosity could be described well by the Williams–Landel–Ferry equation, which corresponded well with the light scattering results. Therefore, this work would further provide the experimental proofs that the exponential relaxation behavior of complex viscosity could be attributed to the viscoelastic flow of epoxy-rich escaping from polyesterimide-rich matrix during phase separation.     

Chemically induced phase separation in the preparation of porous epoxy monolith

A methodology for preparing porous epoxy monolith via chemically induced phase separation was proposed. The starting system was a mixture of an epoxy precursor, diglycidyl ether of bisphenol‐A (DGEBA), a curing agent, 4,4′‐diaminodiphenylmethane (DDM), and a thermoplastic polymer, polypropylene carbonate (PPC). As DGEBA was cured with DDM, the system became phase‐separated having PPC particles dispersed in epoxy matrix. After PPC particles were removed by thermal degradation, a porous structure was obtained. The phase separation mechanism was determined by the initial composition and illustrated by a pseudophase diagram. The pore size increased with increasing the concentration of PPC and raising the curing temperature. The intermediate and final morphologies of the system were studied using optical and scanning electron microscopy, respectively. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Polymerization-induced microphase separation of polymer-polyoxometalate nanocomposites for anhydrous solid state electrolytes

Solid-state electrolytes (SSEs) with high ionic conductivity, mechanical stability, and high thermal stability, as well as the stringent requirement of application in high-temperature fuel cells and lithium-ion batteries is receiving increasing attention. Polymer nanocomposites (PNCs), combining the advantages of inorganic materials with those of polymeric materials, offer numerous opportunities for SSEs design. In this work, we report a facile and general one-pot approach based on polymerization-induced microphase separation (PIMS) to generate PNCs with bi-continuous microphases. This synthetic strategy transforms a homogeneous liquid precursor consisting of polyoxometalates (POMs, H₃PW₁₂O₄₀, Li₇[V₁₅O₃₆(CO₃)]), poly(ethylene glycol) (PEG) macro-chain-transfer agent, styrene and divinylbenzene monomers, into a robust and transparent monolith. The resulting POMs are uniformly dispersed in the PEG block (PEG/POM) to form a conducting pathway that successfully realizes the effective transfer of protons and lithium ions, while the highly cross-linked polystyrene domains (P(S-co-DVB)) as mechanical support provide outstanding mechanical properties and thermal stability. As the POM loading ratio up to 35 wt%, the proton conductivity of nanocomposite reaches as high as 5.99 × 10^-4 S/cm at 100 °C in anhydrous environment, which effectively promotes proton transfer under extreme environments. This study broadens the application of fuel cells and lithium-ion batteries in extreme environments.     

Electronic phase separation in transition metal oxide systems

Electronic phase separation is increasingly getting recognized as a phenomenon of importance in understanding the magnetic and electron transport properties of transition metal oxides. The phenomenon dominates the rare-earth manganates of the formula Ln(1-x)A(x)MnO(3)(Ln = rare earth and A = alkaline earth) which exhibit ferromagnetism and metallicity as well as charge-ordering, depending on the composition, size of A-site cations and external factors such as magnetic and electric fields. We discuss typical phase separation scenarios in the manganates, with particular reference to Pr(1-x)Ca(x)MnO(3)(x= 0.3-0.4), (La(1-x)Ln(x))(0.7)Ca(0.3)MnO(3)(Ln = Pr, Nd, Gd and Y) and Nd(0.5)Sr(0.5)MnO(3). Besides discussing the magnetic and electron transport properties, we discuss electric field effects. Rare-earth cobaltates of the type Pr(0.7)Ca(0.3)CoO(3) and Gd(0.5)Ba(0.5)CoO(3) also exhibit interesting magnetic and electron transport properties which can be understood in terms of phase separation.     

Effects of phase separation on network and viscoelastic properties in SBS thermoplastic elastomers

Stress relaxation of poly(styrene-b-butadiene-b-styrene) thermoplastic elastomers is studied in dependence of molecular weight and degree of hydrogenation in the temperature range between ?30° and +80 °C. The influence of these parameters on the structure of the physical network and the degree of partial mixing in the domain boundary is investigated by separating the stress-relaxation modulus into a viscoelastic term and an equilibrium network modulus calculated from the relaxation-time spectrum. The temperature dependence of the one-second relaxation modulus is quantitatively described by use of a modified Kerner model for the simulation of the viscoelastic term. The modification allows the estimation of the volume fraction of interfacial material and its correlation to the parameters which govern phase separation.