Thermoset Blends Research Articles

Nanostructured thermoset epoxy resin templated by an amphiphilic poly(ethylene oxide)‐block‐poly(dimethylsiloxane) diblock copolymer

AbstractAn amphiphilic poly(ethylene oxide)‐block‐poly(dimethylsiloxane) (PEO–PDMS) diblock copolymer was used to template a bisphenol A type epoxy resin (ER); nanostructured thermoset blends of ER and PEO–PDMS were prepared with 4,4′‐methylenedianiline (MDA) as the curing agent. The phase behavior, crystallization, hydrogen‐bonding interactions, and nanoscale structures were investigated with differential scanning calorimetry, Fourier transform infrared spectroscopy, transmission electron microscopy, and small‐angle X‐ray scattering. The uncured ER was miscible with the poly(ethylene oxide) block of PEO–PDMS, and the uncured blends were not macroscopically phase‐separated. Macroscopic phase separation took place in the MDA‐cured ER/PEO–PDMS blends containing 60–80 wt % PEO–PDMS diblock copolymer. However, the composition‐dependent nanostructures were formed in the cured blends with 10–50 wt % PEO–PDMS, which did not show macroscopic phase separation. The poly(dimethylsiloxane) microdomains with sizes of 10–20 nm were dispersed in a continuous ER‐rich phase; the average distance between the neighboring microdomains was in the range of 20–50 nm. The miscibility between the cured ER and the poly(ethylene oxide) block of PEO–PDMS was ascribed to the favorable hydrogen‐bonding interaction. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3042–3052, 2006

Phase behavior, crystallization, and nanostructures in thermoset blends of epoxy resin and amphiphilic star‐shaped block copolymers

AbstractThis article reports thermoset blends of bisphenol A‐type epoxy resin (ER) and two amphiphilic four‐arm star‐shaped diblock copolymers based on hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(propylene oxide) (PPO). 4,4′‐Methylenedianiline (MDA) was used as a curing agent. The first star‐shaped diblock copolymer with 70 wt % ethylene oxide (EO), denoted as (PPO‐PEO)4, consists of four PPO‐PEO diblock arms with PPO blocks attached on an ethylenediamine core; the second one with 40 wt % EO, denoted as (PEO‐PPO)4, contains four PEO‐PPO diblock arms with PEO blocks attached on an ethylenediamine core. The phase behavior, crystallization, and nanoscale structures were investigated by differential scanning calorimetry, transmission electron microscopy, and small‐angle X‐ray scattering. It was found that the MDA‐cured ER/(PPO‐PEO)4 blends are not macroscopically phase‐separated over the entire blend composition range. There exist, however, two microphases in the ER/(PPO‐PEO)4 blends. The PPO blocks form a separated microphase, whereas the ER and the PEO blocks, which are miscible, form another microphase. The ER/(PPO‐PEO)4 blends show composition‐dependent nanostructures on the order of 10−30 nm. The 80/20 ER/(PPO‐PEO)4 blend displays spherical PPO micelles uniformly dispersed in a continuous ER‐rich matrix. The 60/40 ER/(PPO‐PEO)4 blend displays a combined morphology of worm‐like micelles and spherical micelles with characteristic of a bicontinuous microphase structure. Macroscopic phase separation took place in the MDA‐cured ER/(PEO‐PPO)4 blends. The MDA‐cured ER/(PEO‐PPO)4 blends with (PEO‐PPO)4 content up to 50 wt % exhibit phase‐separated structures on the order of 0.5–1 μm. This can be considered to be due to the different EO content and block sequence of the (PEO‐PPO)4 copolymer. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 975–985, 2006

Morphology and cure kinetics of unsaturated polyester resin/block copolymer blends

Thermoset blends based on unsaturated polyester and triblock copolymers containing poly(ethylene oxide) and poly(propylene oxide) blocks were investigated. No evidence of self-organization was found. Under our experimental conditions, the block copolymers behaved like classical thermoplastic additives used for shrinkage compensation. According to the type and content of the copolymer, initial systems with one phase or two phases were generated. During curing, except in one case in which thermally induced phase separation was found, reaction-induced phase separation was observed, leading to various types of cured morphologies (cocontinuous or dispersed). For a cocontinuous, two-phase morphology, experimental observations revealed phase separation proceeding via spinodal decomposition frozen in the early stage by gelation. These triblock copolymers appear to be interesting new additives that could be used as good shrinkage compensator additives for industrial applications because it is known that a cocontinuous morphology is needed for this purpose. As for the final morphologies containing dispersed phases, the interpretation of the phenomena occurring upon polymerization requires further investigation. No clear discrimination between spinodal decomposition and nucleation and growth was achieved; in some cases, various successive phase separations were observed. The curing kinetics were examined: an increase in the molar weight and/or copolymer content led to a lower reaction rate. Kinetics curves were modeled by the semiempirical autocatalytic reaction model of Kamal and Sourour with a diffusion-control function. The gelation and vitrification of the system played major roles in the reaction kinetics. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 149–165, 2006

Micro- or nanoseparated phases in thermoset blends of an epoxy resin and PEO–PPO–PEO triblock copolymer

Diaminodiphenylmethane (DDM) curing at several temperatures of a diglycidyl ether of bisphenol A (DGEBA) epoxy resin modified with a poly(ethylene oxide)- block-poly(propylene oxide)- block-poly(ethylene oxide) (PEO–PPO–PEO) block copolymer has been investigated in order to characterize the miscibility and morphological features. Two distinct phases are present for every blends studied except for DGEBA/DDM modified with 10 wt% PEO–PPO–PEO and cured at low temperature. Depending on the curing condition, phase separation takes place at micro or nanoscale due to competition among kinetic and thermodynamic factors. The mechanistic approach used for modeling the curing reactions shows that the formation of epoxy–hydroxyl complex and the auto catalytic process are slightly decreased whilst the noncatalytic process is favoured upon copolymer addition. Modifier addition delays curing process as the influence of both formation of epoxy–hydroxyl complex and catalytic process on reaction rate is higher than the influence of noncatalytic process. A thermodynamic model describing a thermoset/block copolymer considered as only one entity system is proposed. The LCST behaviour allows to elucidate nano or micro separated structures obtained at low and high curing temperatures, respectively.

Mobility, Miscibility, and Microdomain Structure in Nanostructured Thermoset Blends of Epoxy Resin and Amphiphilic Poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) Triblock Copolymers Characterized by Solid-State NMR

Solid-state NMR methods were used to characterize the heterogeneous dynamics, miscibility, and microdomain structure in nanostructured thermoset blends of epoxy resin (ER) and amphiphilic poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymers (PEO−PPO−PEO). NMR experiments show that there is a distinct dynamic difference between the block copolymer (both PEO and PPO) and cured-ER matrix indicating the presence of phase separation, which also confirms the existence of the interphase region including a considerable amount of immobilized PEO and mobilized partially cured ER. An improved method based on spin-diffusion experiments enabled a quantitative determination of the interphase thickness. It is shown that a large percentage of PEO were intimately mixed with ER in the interphase region of the blends, while the rest mobile PEO and all PPO segregated from the ER network. It is observed that the domain size and long period depend strongly on the PEO fraction in the c...

Investigation of phase separation mechanisms of thermoset polymer blends by time-resolved SAXS

Phase separation induced by crosslinking was studied by time-resolved small angle X-ray scattering in thermoset blend based on unsaturated polyester, styrene and low molar weight saturated polyester as low profile additive. The scattering intensities were analyzed using the Cahn–Hilliard–Cook theory. Combining DSC and DMA experiments allowed us to conclude that phase separation proceeded via spinodal decomposition frozen in the early stage. Apparent diffusion coefficients D app were estimated.

Phase‐separation mechanism and corresponding final morphologies of thermoset blends based on unsaturated polyester and low‐molar‐weight saturated polyester

AbstractThe final morphology of cured blends based on unsaturated polyester, styrene, and low‐molar‐weight saturated polyester as a low profile additive (LPA) was investigated with atomic force microscopy and scanning electron microscopy. The observed structure was compared to those obtained with widely used poly(vinyl acetate) (PVAc). On the surface and in the bulk, a network of particles, ranging in size from 50 to 60 nm, was observed with saturated polyester as an LPA. The influence of the molar weight and LPA content was investigated. To determine the mechanism of formation of such a morphology, in situ experiments were carried out to elucidate the phase‐separation mechanism. Small‐angle laser light scattering and small‐angle neutron scattering experiments were performed on ternary blends containing PVAc and saturated polyester, respectively. The first stage of spinodal decomposition was observed in both cases. Within our experimental conditions, gelation froze further evolution and led to a two‐phase cocontinuous structure that imposed the final morphology characteristics. In particular, the period and amplitude of the concentration fluctuations generated during the phase separation played essential roles. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 95: 1459–1472, 2005

Study of the porous network developed during curing of thermoset blends containing low molar weight saturated polyester

Polymerisation of unsaturated polyester/styrene blend added with a thermoplastic additive leads to the formation of a porous biphasic polymer network. In this paper, the porous medium was specifically studied by TEM associated with images analysis by chord length distribution and small angle X-ray scattering (SAXS). When the molar weight and/or the amount of low profile additive (LPA) are sufficient to ensure porosity higher than 2%, chord length distribution can be performed and is characteristic of a Debye random medium and Porod's law was observed in SAXS. The lengths deduced from Porod's law are similar to those observed for Debye random regime and are lower than 50nm. These characteristics are closely related to the processes happening during curing and especially to the phase separation. For less porous samples, deviation from Porod's law was observed and could be attributed to additional scattering intensity coming from concentration fluctuations within polymer matrix. For nonporous samples, scattering intensity was only due to concentration fluctuations.

Nanostructures, Semicrytalline Morphology, and Nanoscale Confinement Effect on the Crystallization Kinetics in Self-Organized Block Copolymer/Thermoset Blends

This work reports the first instance of self-organized thermoset blends containing diblock copolymers with a crystallizable thermoset-immiscible block. Nanostructured thermoset blends of bisphenol A-type epoxy resin (ER) and a low-molecular-weight (Mn = 1400) amphiphilic polyethylene-block-poly(ethylene oxide) (EEO) symmetric diblock copolymer were prepared using 4,4‘-methylenedianiline (MDA) as curing agent and were characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), small-angle X-ray scattering (SAXS), and differential scanning calorimetry (DSC). All the MDA-cured ER/EEO blends do not show macroscopic phase separation but exhibit microstructures. The ER selectively mixes with the epoxy-miscible PEO block in the EEO diblock copolymer whereas the crystallizable PE blocks that are immiscible with ER form separate microdomains at nanoscales in the blends. The PE crystals with size on nanoscales are formed and restricted within the individual spherical micelles in the nanostructured ER/EEO blends with EEO content up to 30 wt %. The spherical micelles are highly aggregated in the blends containing 40 and 50 wt % EEO. The PE dentritic crystallites exist in the blend containing 50 wt % EEO whereas the blends with even higher EEO content are completely volume-filled with PE spherulites. The semicrystalline microphase-separated lamellae in the symmetric EEO diblock copolymer are swollen in the blend with decreasing EEO content, followed by a structural transition to aggregated spherical micellar phase morphology and, eventually, spherical micellar phase morphology at the lowest EEO contents. Three morphological regimes are identified, corresponding precisely to the three regimes of crystallization kinetics of the PE blocks. The nanoscale confinement effect on the crystallization kinetics in nanostructured thermoset blends is revealed for the first time. This new phenomenon is explained on the basis of homogeneous nucleation controlled crystallization within nanoscale confined environments in the block copolymer/thermoset blends.

Phase Behavior, Crystallization, and Hierarchical Nanostructures in Self-Organized Thermoset Blends of Epoxy Resin and Amphiphilic Poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) Triblock Copolymers

Nanostructured thermoset blends of bisphenol A-type epoxy resin (ER) and amphiphilic poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (PEO−PPO−PEO) triblock copolymers were successfully prepared. Two samples of PEO−PPO−PEO triblock copolymer with different ethylene oxide (EO) contents, denoted as EO30 with 30 wt % EO content and EO80 with 80 wt % EO content, were used to form the self-organized thermoset blends of varying compositions using 4,4‘-methylenedianiline (MDA) as curing agent. The phase behavior, crystallization, and morphology were investigated by differential scanning calorimetry (DSC), transmission electron microscopy (TEM), atomic force microscopy (AFM), and small-angle X-ray scattering (SAXS). It was found that macroscopic phase separation took place in the MDA-cured ER/EO30 blends containing 60−80 wt % EO30 triblock copolymer. The MDA-cured ER/EO30 blends with EO30 content up to 50 wt % do not show macroscopic phase separation but exhibit nanostructures on the order of 10−30 nm as revealed by both the TEM and SAXS studies. The AFM study further shows that the ER/EO30 blend at some composition displays structural inhomogeneity at two different nanoscales and is hierarchically nanostructured. The spherical PPO domains with an average size of about 10 nm are uniformly dispersed in the 80/20 ER/EO30 blend; meanwhile, a structural inhomogeneity on the order of 50−200 nm is observed. The ER/EO80 blends are not macroscopically phase-separated over the entire composition range because of the much higher PEO content of the EO80 triblock copolymer. However, the ER/EO80 blends show composition-dependent nanostructures on the order of 10−100 nm. The 80/20 ER/EO80 blend displays hierarchical structures at two different nanoscales, i.e., a bicontinuous microphase structure on the order of about 100 nm and spherical domains of 10−20 nm in diameter uniformly dispersed in both the continuous microphases. The blends with 60 wt % and higher EO80 content are completely volume-filled with spherulites. Bundles of PEO lamellae with spacing of 20−30 nm interwoven with a microphase structure on the order of about 100 nm are revealed by AFM study for the 30/70 ER/EO80 blend.

Surface morphologies of composites based on unsaturated polyester pre-polymer

We report the study of surfaces of bulk molding compounds (BMC) based on miscible polymeric thermoset blends (TB)—unsaturated polyester, styrene and low profile additive (LPA)—containing fillers and glass fibers. In contrast to scanning electron microscopy (SEM) that identified a continuous organic layer at the BMC surface, atomic force microscopy (AFM) showed the existence of aggregates linked together to form a network at the micrometric scale. This indisputably demonstrated that phase separation took place at the surface of the BMC. The influence of TB was examined by comparing the surface morphologies of BMC and corresponding TB. Several cases were distinguished as a function of the TB composition. (1) Without LPA, the surface of the TB was continuous (no phase separation took place during curing) and the surface of BMC revealed the presence of aggregates resulting from a phase separation induced by the fillers. (2) For very low molecular weight LPA, aggregates randomly spread on islands surrounded by large holes were observed on the TB surface. These holes were shown to result from surface deformations induced by absence of shrinkage compensation. The corresponding BMC presented particles randomly spread on the surface. (3) The general case (higher molecular weight LPA) corresponded to similar TB and BMC surfaces morphologies with aggregates randomly spread on the surface. In this case, BMC roughness and morphology reflected the TB roughness and morphology. These observations led to the proposal of some considerations concerning the control of surface aspects of BMC.

Modeling domain mixing in semi‐interpenetrating polymer networks composed of poly(vinyl chloride) and 5% to 15% of crosslinked thermosets

AbstractThe nature of phase mixing in semi‐interpenetrating polymer networks (SIPNs) of poly(vinyl chloride) (PVC)/thermoset blends was studied by using both the glass transition temperature third power composition equation and DMTA modeling. From 5% to 15% by weight of an oligomeric MDI isocyanate or a low viscosity epoxy were blended separately with PVC to make two series of SIPNs. The DMTA‐derived Tg data were modeled by the Tg third power composition equation to characterize “energy interaction” features of PVC/thermoset phase mixing. Fitting experimental Tg values gave estimations of the characteristic parameters, K1 and K2 of the Tg power equation. K1 and K2 were each positive for the PVC/epoxy (K1 = 1.1, K2 = 7.2) and the PVC/isocyanate (K1 = 29.9, K2 = 38) blends, showing that binary hetero‐interactions (enthalpic effects) and conformational redistributions (entropic effects) during the binary hetero‐interactions both contributed favorably to phase mixing. Negative K1 ‐ K2 values for both groups of blends indicate entropic contributions predominate. The thermoset dilution produced a lightly crosslinked thermoset network, which was locked into the amorphous PVC, forming a mixed thermoset/PVC SIPN domain. Conformational redistributions of PVC and thermoset segments continue to occur within the developing SIPN domain before phase separation can occur. The combined series‐parallel Takayanagi coupling model, which assumed that a PVC phase and a mixed PVC/thermoset SIPN phase coexisted, was employed to describe the viscoelastic behavior of PVC/thermoset blends. Reasonable fits between the experimental DMTA plots and modeling predictions were found. The predictions were not sensitive to the degree of series‐parallel coupling. The modeled DMTA plots, like the experimental results, exhibited only a single E″ peak in the glass transition temperature range for both the PVC/PAPI and PVC/epoxy systems. Thus, very small PVC/thermoset SIPN domains (< 20 nm diameter) that are dispersed in separate domains of a PVC‐rich phase (despersive phase mixing characteristics) provide a reasonable explanation of the blends' phase structures.

Monitoring phase separation and reaction advancement in situ in thermoplastic/epoxy blends

Frequency-dependent dielectric measurements have been used to monitor and characterize the phase separation process in high-performance thermoplastic–thermoset blends of 2,6-dimethyl-1,4-phenylene ether (PPE) with an epoxy diglycidylether of bisphenol A (DGEBA) and 4,4-methylene bis(3-chloro-2,6-diethylamine) (MCDEA). The systems studied are 30, 45 and 60% PPE/DGEBA–MCDEA blends. The results are compared with dynamical mechanical measurements and the developing morphology. Both dielectric and mechanical measurements are shown to be good techniques to monitor the phase-separation process and the reaction advancement. Dielectric measurements monitor the buildup in T g in both the PPE-rich continuous phase and in the epoxy-rich occluded phases. Dielectric measurements are advantageous as they can be made in situ continuously on a single sample throughout the entire cure process. The results show that the phase separation process initially occurs rapidly involving a large amount of the epoxy–amine diffusing into the occluded phase. The rate of the epoxy–amine reaction in the epoxy-rich 30% PPE mixture is approximately equal to that in the neat epoxy–amine system due to two opposing effects, a slower reaction rate due to dilution and a lower level of conversion at vitrification due to the presence of high T g PPE. In the 60% PPE mixture, the dilution effect of the PPE has a large affect on the decreasing the reaction rate and achievement of vitrification. The continuous thermoplastic-rich phase is observed to vitrify first, followed by vitrification of the thermoset as occluded particles. Finally, the results show as evidenced by the size of the occluded particles and the composition of the continuous phase that the morphology is strongly influenced by the kinetics, diffusion, and viscosity conditions during phase separation.

Cure behavior and thermal stability of an epoxy: new bismaleimide blends for composite matrices

AbstractA new bismaleimide (BMI) resin was synthesized to formulate epoxy(tetraglycidyl diaminodiphenyl methane; TGDDM) – bismaleimide thermoset blends for composite matrix applications. 4,4′‐diaminodiphenyl methane (DDM) was used as an amine curing agent for the TGDDM. A Fourier transform infrared (FTIR) spectroscopy was employed to characterize the new BMI resin. Cure behavior of the epoxy–BMI blends was studied using a differential scanning calorimeter (DSC). DSC thermograms of the thermoset blends indicated two exothermic peaks. The glass transition temperature of the thermoset blends decreased with BMI content. Thermogravimetric analysis (TGA) was carried out to investigate thermal degradation behavior of the cured epoxy–BMI thermoset blends. The new BMI resin reacted partially with the DDM and weak intercrosslinking polymer networks were formed during cure of the thermoset blends.

Characteristics of the polyamide‐epoxy resin system

AbstractPolyamide resins, from a dimeric acid mixture, have been interacted with low‐molecular‐weight resins with terminal epoxy groups. The reaction mechanisms of the primary and secondary amino groups of the polyamide resins with the epoxy groups are discussed. Comparison is made with the reaction conditions and the properties of the resultant blends when epoxy resins are interacted with amine hardening agents.The polyamide‐epoxy thermoset blends are highly adhesive, partly due to the high concentration of polar groups, and have unusual flexibility because of the high degree of internal plasticization due to the fatty character of the polyamide resin. The physical properties of various blends are tabulated, and their current and prospective uses discussed.