Preparation and characterization of bismaleimide (N,N′‐bismaleimido‐4,4′‐diphenyl methane)–unsaturated polyester modified epoxy intercrosslinked matrices

An intercrosslinked network of unsaturated polyester–bismaleimide modified epoxy matrix systems was developed. Epoxy systems modified with 10, 20, and 30% (by weight) of unsaturated polyester were made by using epoxy resin and unsaturated polyester with benzoyl peroxide and diaminodiphenylmethane as curing agents. The reaction between unsaturated polyester and epoxy resin was confirmed by IR spectral studies. The unsaturated polyester toughened epoxy systems were further modified with 5, 10, and 15% (by weightt) of bismaleimide (BMI). The matrices, in the form of castings, were characterized for their mechanical properties. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) of the matrix samples were performed to determine the glass transition temperature (T_g) and thermal degradation temperature of the systems, respectively. Mechanical properties, viz: tensile strength, flexural strength, and plain strain fracture toughness of intercrosslinked epoxy systems, were studied by ASTM methods. Data obtained from mechanical and thermal studies indicated that the introduction of unsaturated polyester into epoxy resin improves toughness but with a reduction in glass transition, whereas the incorporation of bismaleimide into epoxy resin improved both mechanical strength and thermal behavior of epoxy resin. The introduction of bismaleimide into unsaturated polyester‐modified epoxy resin altered thermomechanical properties according to their percentage concentration. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 2853–2861, 2002     

Study of morphological and mechanical performance of amine‐cured glassy epoxy–clay nanocomposites

The effect of three different alkylammonium‐modified montmorillonite on morphological and mechanical properties of glassy epoxy‐amine nanocomposites is reported. Small amounts of clays <10 phr (part per hundred of resin) were used in each system of nanocomposite. The morphology of the prepared nanocomposites was performed by means of X‐ray diffraction and transmission electron microscopy. Differential scanning calorimetry (DSC) was used to investigate the glass transition temperatures (T_g). Mechanical properties were based on tensile characteristics (Young's modulus), impact strength, and fracture toughness. The measured moduli were compared to theoretical predictions. Scanning electron microscopy was used to study the morphological structure of the fracture surfaces of impacted specimens. It was found that at a low content of 2 phr (1.2 wt %) of nanoclays, the impact strength and the fracture toughness were improved by 77 and 90% respectively, comparatively to the neat epoxy, whereas DSC revealed a reduction of the T_g of nanocomposites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

A new strategy for controlling shrinkage of DGEBA resins cured by cationic copolymerization with hydroxyl‐terminated hyperbranched polymers and ytterbium triflate as an initiator

We report a novel strategy for preparing epoxy thermosetting systems with low shrinkage and improved flexibility and degradability. Diglycidyl ether of bisphenol A (DGEBA) resin was cured with different proportions of hydroxyl‐terminated hyperbranched polymer (HBP), using ytterbium triflate as a cationic initiator. The curing process was studied using differential scanning calorimetry and thermomechanical analysis. Characterization of the resulting materials was evaluated using DSC, thermogravimetric analysis, and dynamic mechanical thermal analysis, and the fracture surface was studied using scanning electron microscopy (SEM). When DGEBA is modified with HBP, it shows a homogeneous morphology and the HBP is incorporated chemically into the network, because hydroxyl groups can react with epoxides under cationic conditions. Higher proportions of HBP reduce the glass transition temperature (T_g) and thermal stability and increase the flexibility. When the proportion of HBP in the curing mixture is increased, the degree of shrinkage is reduced significantly and expansion can be observed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Curing behavior and thermal and mechanical properties enhancement of tetraglycidyl‐4,4′‐diaminodiphenylmethane/4,4′‐diaminodiphenylsulfone using a liquid crystalline epoxy

A thermosetting resin system, based on tetraglycidyl‐4,4′‐diaminodiphenylmethane, has been developed via copolymerization with 4,4′‐diaminodiphenylsulfone in the presence of a newly synthesized liquid crystalline epoxy (LCE). The curing behavior of LCE‐containing resin system was evaluated using curing kinetics method and Fourier transform infrared spectroscopy. The effect of LCE on the thermal and mechanical properties of modified epoxy systems was studied. Thermogravimetric analysis indicated that the modified resin systems displayed a high T_0.05 and char yield at lower concentrations of LCE (≤5 wt%), suggesting an improved thermal stability. As determined using dynamic mechanical analysis and differential scanning calorimetry, the glass transition value increased by 9.7% compared to that of the neat resin when the LCE content was 5 wt%. Meanwhile, the addition of 5 wt% of LCE maximized the toughness with a 175% increase in impact strength. The analysis of fracture surfaces revealed a possible effect of LCE as a toughener and showed no phase separation in the modified resin system, which was also confirmed by dynamic mechanical analysis. © 2016 Society of Chemical Industry     

Novel dibutyltin oxide‐tributyl phosphate condensate as accelerator for the curing of the epoxy resin/4,4′‐diamino diphenyl sulfone system. II. Toughening of the resin with hydroxyl‐terminated poly(tetramethylene) glycols

When a crystalline Bu₂SnO‐Bu₃PO₄ condensate was used as a catalyst for the curing of the Epon 828/DDS system, the addition of hydroxyl group to epoxy group took place. On the basis of this reaction, direct employment of poly(tetramethylene) glycols (PTMG) as toughener for the epoxy resin system was successful. Morphology of the modified resin depended on the molecular weight and the concentration of PTMG. With the incorporation of a small amount of PTMG, the critical fracture energy of the cured resin was improved greatly, while the flexural strength and the modulus were less influenced. A slight enhancement in glass transition temperature (T_g) of the modified resin was found up to the PTMG concentration of 5 phr; further increase of the PTMG concentration caused a significant lowering of T_g. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 1237–1242, 2001     

Bis[4‐(4‐maleimidephen‐oxy)phenyl]propane/N,N‐4,4′‐bismaleimidodiphenylmethyene blend modified with diallyl bisphenol A

In this article, 2,2′‐bis[4‐(4‐maleimidephen‐oxy)phenyl)]propane (BMPP) resin and N,N‐4,4′‐bismaleimidodiphenylmethyene (BDM) resin blends were modified by diallyl bisphenol A (DABPA). The effects of the mole concentration of BMPP on mechanical properties, fracture toughness, and heat resistance of the modified resins were investigated. Scanning electron microscopy was used to study the microstructure of the fractured modified resins. The introduction of BMPP resin improves the fracture toughness and impact strength of the cured resins, whose thermal stabilities are hardly affected. Dynamic mechanical analysis shows that the modified resins can maintain good mechanical properties at 270.0°C, and their glass transition temperatures (T_g) are above 280.0°C. When the mole ratio of BDM : BMPP is 2 : 1(Code 3), the cured resin performs excellent thermal stability and mechanical property. Its T_g is 298°C, and the Charpy impact strength is 20.46 KJ/m². The plane strain critical stress intensity factor (K_IC) is 1.21 MPa·m^0.5 and the plane strain critical strain energy release rate (G_IC) is 295.64 J/m². Compared with that of BDM/DABPA system, the K_IC and G_IC values of Code 3 are improved by 34.07% and 68.10%, respectively, which show that the modified resin presented good fracture toughness. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40395.     

Development of high Tg epoxy resin and mechanical properties of its fiber‐reinforced composites

A new epoxy resin with high glass transition temperature (T_g) (～ 180°C) and a viscosity low enough for infiltration into dry reinforcements at 40°C was developed for the vacuum‐assisted resin transfer molding process. To study the curing behavior and viscosity, several blends were formulated using multifunctional resin, aromatic hardener, and reactive diluents. Effects of these components on the viscosity and T_g were investigated by thermomechanical analysis, dynamic scanning calorimetry, and rheometer. Experimental results showed that a liquid aromatic hardener and multifunctional epoxy resin should be used to decrease the viscosity to <1 Pa·s at 40°C. Moreover, the addition of a proper reactive diluent decreased the viscosity and simultaneously minimized the deterioration of T_g. Mechanical properties of the composite produced with the optimized blend were evaluated at both room‐temperature and high‐temperature conditions. According to the results, the composite showed comparable mechanical properties with that of the current commercial resin. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Investigation on the effect of the presence of hyperbranched polymers on thermal and mechanical properties of an epoxy UV‐cured system

The use of hydroxyl‐functionalized hyperbranched polymers (HBPs) (Boltorn^® H20, H30 and H40) was investigated with respect to a UV‐cured epoxy system. Their presence induced an increase of the final epoxy conversion, which was interpreted on the basis of a chain‐transfer reaction. A decrease of the T_g values in the photocured films was observed when the amount of HBP additive in the photocurable formulation was increased. When the amount of HBP in the photocurable resin was increased, the density of photocured films increased, indicating a decrease on the free volume. Moreover, a clear increase in toughness was observed and attributed to the plasticization effect induced by the presence of HBP. This effect is particularly interesting for epoxy thermosets, which are characterized by good mechanical properties, although they are brittle and fragile. By increasing the toughness properties of these photocured resins it may be possible to broad their applications. Copyright © 2005 Society of Chemical Industry     

Preparation and properties of TiO2‐filled poly(acrylonitrile‐butadiene‐styrene)/epoxy hybrid composites

Poly (acrylonitrile‐butadiene‐styrene) (ABS) was used to modify diglycidyl ether of bisphenol‐A type of epoxy resin, and the modified epoxy resin was used as the matrix for making TiO₂ reinforced nanocomposites and were cured with diaminodiphenyl sulfone for superior mechanical and thermal properties. The hybrid nanocomposites were characterized by using thermogravimetric analyzer (TGA), dynamic mechanical analyzer (DMA), universal testing machine (UTM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The bulk morphology was carefully analyzed by SEM and TEM and was supported by other techniques. DMA studies revealed that the DDS‐cured epoxy/ABS/TiO₂ hybrid composites systems have two T_gs corresponding to epoxy and ABS rich phases and have better load bearing capacity with the addition of TiO₂ particles. The addition of TiO₂ induces a significant increase in tensile properties, impact strength, and fracture toughness with respect to neat blend matrix. Tensile toughness reveals a twofold increase with the addition of 0.7 wt % TiO₂ filler in the blend matrix with respect to neat blend. SEM micrographs of fractured surfaces establish a synergetic effect of both ABS and TiO₂ components in the epoxy matrix. The phenomenon such us cavitation, crack path deflection, crack pinning, ductile tearing of the thermoplastic, and local plastic deformation of the matrix with some minor agglomerates of TiO₂ are observed. However, between these agglomerates, the particles are separated well and are distributed homogeneously within the polymer matrix. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Toughening of dicyandiamide-cured DGEBA-based epoxy resins by CTBN liquid rubber

Toughening of a diglycidyl ether of bisphenol-A (DGEBA)-based epoxy resin with liquid carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer has been investigated. For this purpose six blend samples were prepared by mixing DGEBA with different concentrations of CTBN from 0 to 25 phr with an increment of 5 phr. The samples were cured with dicyandiamide curing agent accelerated by Monuron. The reactions between oxirane groups of DGEBA and carboxyl groups of CTBN were followed by Fourier-transform infrared (FTIR) spectroscopy. Tensile, impact, fracture toughness and dynamic mechanical analysis of neat as well as the modified epoxies have been studied to observe the effect of CTBN modification. The tensile strength of the blend systems increased by 26 % when 5 phr CTBN was added, and it remained almost unchanged up to 15 phr of CTBN. The elongation-at-break and Izod notched impact strength increased significantly, whereas tensile modulus decreased gradually upon the addition of CTBN. The maximum toughness of the prepared samples was achieved at optimum concentration of 15 phr of CTBN, whereas the fracture toughness (K _IC) remained stable for all blend compositions of more than 10 phr of CTBN. The glass transition temperature (T _g) of the epoxy resin significantly increased (11.3 °C) upon the inclusion of 25 phr of CTBN. Fractured surfaces of tensile test samples have been studied by scanning electron microscopic analysis. This latter test showed a two-phase morphology where the rubber particles were distributed in the epoxy resin with a tendency towards co-continuous phase upon the inclusion of 25 phr of CTBN.     

Synthesis and characterization of siliconized epoxy-1,3-bis(maleimido)benzene intercrosslinked matrix materials

Intercrosslinked network of siliconized epoxy-1,3-bis(maleimido)benzene matrix systems have been developed. The siliconization of epoxy resin was carried out by using various percentages of (5-15%) hydroxyl-terminated polydimethylsiloxane (HTPDMS) with γ-aminopropyltriethoxysilane (γ-APS) as crosslinking agent and dibutyltindilaurate as catalyst. The siliconized epoxy systems were further modified with various percentages of (5-15%) 1,3-bis(maleimido)benzene (BMI) and cured by using diaminodiphenylmethane (DDM). The neat resin castings prepared were characterized for their mechanical properties. Mechanical studies indicate that the introduction of siloxane into epoxy resin improves the toughness of epoxy resin with reduction in the values of stress-strain properties whereas, incorporation of bismaleimide into epoxy resin improves stress-strain properties with lowering of toughness. However, the introduction of both siloxane and bismaleimide into epoxy resin influences the mechanical properties according to their percentage content. Differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and measurement of heat distortion temperature were also carried out to assess the thermal behavior of the matrix samples. DSC thermogram of the BMI modified epoxy systems show unimodel reaction exotherms. The glass transition temperature (T_g), thermal degradation temperature and heat distortion temperature of the cured BMI modified epoxy and siliconized epoxy systems increase with increasing BMI content and this may be due to the homopolymerization of BMI rather than Michael addition reaction. The morphology of the BMI modified epoxy and siliconized epoxy systems were also studied by scanning electron microscopy.     

Effects of epoxy content on dynamic mechanical behaviour of PEI‐toughened dicyanate–novolac epoxy blends

By varying the cyanate/epoxy ratio, three polyetherimide(PEI)‐modified bisphenol A dicyanate–novolac epoxy resin blends with different epoxy contents were prepared. The effects of epoxy content on the dynamic mechanical behaviour of those blends were investigated by dynamic mechanical thermal analysis. The results showed that the glass transition temperature of the cyanate–epoxy network (T_g1) in the modified blend decreases with epoxy content. When the epoxy content increases, both the width of the glass transition of the cyanate–epoxy network and its peak density are depressed substantially. Although the tangent delta peak value of PEI is basically independent of epoxy content, the T_g of PEI (T_g2) decreases with epoxy content. T_g1 is independent of the PEI loading. When T_g1 is lower than T_g2, however, the T_g1 in the blend with revised phase structure is substantially lower than other blends. Copyright © 2004 Society of Chemical Industry     

Synthesis and characterization of bis(4‐cyanato 3,5‐dimethylphenyl) naphthyl methane/epoxy/glass fiber composites

Bis(4‐cyanato 3,5‐dimethylphenyl) naphthylmethane was prepared by treating CNBr with bis(4‐hydroxy 3,5‐dimethylphenyl) naphthylmethane in the presence of triethylamine at −5 to 5°C. The dicyanate was characterized by FT‐IR and NMR techniques. The prepared dicyanate was blended with commercial epoxy resin in different ratios and cured at 120°C for 1 hr, 180°C for 1 hr, and post cured at 220°C for 1 hr using diamino diphenyl methane (DDM) as curing agent. Castings of neat resin and blends were prepared and characterized by FT‐IR technique. The morphology of the blends was evaluated by SEM analysis. The composite laminates were also fabricated from the same composition using glass fiber. The mechanical properties like tensile strength, flexural strength, and fracture toughness were measured as per ASTMD 3039, D 790, and D 5528, respectively. The tensile strength increased with increase in cyanate content (3, 6, and 9%) from 322 to 355 MPa. The fracture toughness values also increased from 0.7671 kJ/m² for neat epoxy resin to 0.8615 kJ/m² for 9% cyanate ester epoxy modified system. The thermal properties were also studied. The 10% weight loss temperature of pure epoxy is 358°C and it increased to 398°C with incorporation of cyanate ester resin. The incorporation of cyanate ester up to 9% loading level does not affect the T_g to a very great extent. POLYM. COMPOS., 2008. © 2008 Society of Plastics Engineers     

Preparation and properties of high performance phthalide‐containing bismaleimide modified epoxy matrices

A series of intercrosslinked networks formed by diglycidyl ether of bisphenol A epoxy resin (DGEBA) and novel bismaleimide containing phthalide cardo structure (BMIPP), with 4,4′‐diamino diphenyl sulfone (DDS) as hardener, have been investigated in detail. The curing behavior, thermal, mechanical and physical properties and compatibility of the blends were characterized using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), notched Izod impact test, scanning electron microscopy (SEM) and water absorption test. DSC investigations showed that the exothermic transition temperature (T_p) of the blend systems shifted slightly to the higher temperature with increasing BMIPP content and there appeared a shoulder on the high‐temperature side of the exothermic peak when BMIPP content was above 15 wt %. TGA and DMA results indicated that the introduction of BMIPP into epoxy resin improved the thermal stability and the storage modulus (G′) in the glassy region while glass transition temperature (T_g) decreased. Compared with the unmodified epoxy resin, there was a moderate increase in the fracture toughness for modified resins and the blend containing 5 wt % of BMIPP had the maximum of impact strength. SEM suggested the formation of homogeneous networks and rougher fracture surface with an increase in BMIPP content. In addition, the equilibrium water uptake of the modified resins was reduced as BMIPP content increased. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Phenolic Hyperbranched Polymers as Additives in Cationic Photopolymerization of Epoxy Systems

Summary: A phenolic group containing hyperbranched polyester (HBP) was synthesized and employed as chain transfer agent in cationic photopolymerization of a biscycloaliphatic epoxy monomer ( CE ). The epoxy group conversion increases by increasing the amount of HBP in the photocurable resin, due to a chain transfer reaction involving the phenolic‐OH groups. HBP acts as a plasticizer inducing decrease of the T_g values together with an increase of the toughness properties. Meanwhile gel content increases together with the E′ values. By increasing the amount of HBP in the photocurable resin an increase of the density is evident indicating a decrease of free volume. Therefore an improvement of the gas barrier properties might be expected; at the same time an increase of the thermal stability is evident.

     

The effect of brominated epoxy on epoxy/phenolic reactive blends

Innovative reactive blends containing epoxy and brominated epoxy (BE) incorporated with resole-type phenolic were studied with the aim to elucidate the curing kinetics and the final thermomechanical characteristics of this unique system. Curing kinetics was investigated by means of the activation energy determined using differential scanning calorimetry (DSC ) at various heating rates analyzed by the Arrhenius equation. Both DSC and Fourier transform infrared revealed that bromine elimination at elevated temperatures (above 220 °C) had lowered the activation energy in the case of BE containing phenolic blends. The thermomechanical properties showed that the addition of conventional epoxy to resole decreased its thermal properties and modulus compared to neat resole. Distinctively, BE/resole blends exhibited increased glass-transition temperature, compared to diglycidyl ether of bisphenol A/resole blends in combination with higher elongation and toughness compared to neat resole. It was concluded that BE/epoxy resin/phenolic reactive systems offer high T _g, mechanical properties and toughness and hence are applicable for structural adhesives and for matrices of polymer-fiber composites. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47172.     

Mechanical Behavior and Fracture Toughness of Poly(L‐lactic acid)‐Natural Fiber Composites Modified with Hyperbranched Polymers

Summary: The use of hyperbranched polymers (HBP) with hydroxy functionality as modifiers for poly(L ‐lactic acid) (PLLA)‐flax fiber composites is presented. HBP concentrations were varied from 0 to 50% v/v and the static and dynamic tensile properties were investigated along with interlaminar fracture toughness. Upon addition of HBP, the tensile modulus and dynamic storage modulus (E′) both diminished, although a greater decline was noticed in the static modulus. The elongation of the composites with HBP showed a pronounced increase as large as 314% at 50% v/v HBP. The loss factor (tan δ) indicated a lowering of the glass transition temperature (T_g) due to a change in crystal morphology from large, mixed perfection spherulites to finer, smaller spherulites. The change in T_g could have also resulted from some of the HBP being miscible in the amorphous phase, which caused a plasticizing effect of the PLLA. The interlaminar fracture toughness measured as the critical strain energy release rate (G_IC) was significantly influenced by HBP. At 10% v/v HBP, G_IC was at least double that of the unmodified composite and a rise as great as 250% was achieved with 50% v/v. The main factor contributing to high fracture toughness in this study was better wetting of the fibers by the matrix when the HBP was present. With improved ductility of the matrix, it caused ductile tearing along the fiber‐matrix interface during crack propagation.

ESEM photograph of propagation region of the interlaminar fracture toughness specimens with 30% v/v of HBP.     

Preparation of 4‐(4‐hydroxyphenoxy)phenol diglycidyl ether with improved toughness behavior

A high‐performance difunctional epoxy resin, 4‐(4‐hydroxyphenoxy)phenol diglycidyl ether (DHPOP), was synthesized by a two‐step method. The curing behavior of DHPOP was investigated by nonisothermal differential scanning calorimetry method and the curing kinetics results revealed that the introduction of ether linkage could improve the activity of epoxy groups, leading to a lower curing temperature and apparent activation energy compared with that of the commercial bisphenol‐A diglycidyl ether (DGEBA). A series of copolymers were then prepared by varying the mass ratio of DHPOP and DGEBA, which were cured with 4,4′‐diaminodiphenyl methane. The effect of DHPOP contents on thermal and mechanical properties and fracture morphology was studied. As expected, with the increase of DHPOP in the network, the impact strength and char yield were significantly enhanced, while the glass transition temperature (T_g) remained unchanged because of the increase of crosslink density. The excellent toughness endows the DHPOP with the promising potential for the application as high‐performance resin matrix. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46458.     

Preparation,characterization, and property testing of condensation‐type silicone/montmorillonite nanocomposites

Nanocomposites (NCs) of silicone rubber and organically modified montmorillonite (OMMT) nanoparticles were prepared and characterized. It was shown that OMMT loadings of 2 and 3.5 parts per hundred resin/filler per weight (phr) produced exfoliation or delamination hybrids, whereas at a concentration of 5 phr, the filler seemed to retain its original crystallographic morphology, and the system shifted to an ordinary reinforced elastomer. Fourier transform infrared analysis, differential scanning calorimetry, and thermogravimetric analysis testing were performed for characterization and showed no effect of the nanofiller on the structural parameters of the composites, with the exception of a reduction in the crystallinity. Dynamic mechanical analysis revealed an increase in the glass‐transition temperature (T_g) at OMMT concentrations of 2 and 3.5 phr, whereas at 5 phr, T_g dropped again. Finally, mechanical testing showed an improvement in the tensile strength and stiffness, whereas improved solvent resistance was recorded by swelling experiments in toluene. This experimental study allowed us to explore the range where the OMMT filler produced NCs with silicone elastomers and, furthermore, showed that the incorporation of OMMT into silicone rubber did not introduce any chemical changes but increased the density of crosslinks; this led to a loss of crystallinity, an increase in T_g, and a significant improvement in the tensile properties. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Investigation of readily processable thermoplastic‐toughened thermosets. V. Epoxy resin toughened with hyperbranched polyester

This article describes the use of hyperbranched polyester oligomers (HBPs) as modifiers for epoxy thermosets. The effect of HBP molar mass, end group, and loading on prepolymer viscosity, thermoset fracture toughness, T_g, and high‐temperature dynamic storage modulus (E′) were measured. The HBP molar mass was systematically increased from nominal values of ∼ 1750 g mol (Generation 2, or G2) up to ∼ 14,000 g mol (Generation 5, or G5), which corresponds from a low of two layers of monomer up to a maximum of five layers of monomer around the central core. Toughness increased only modestly with the molar mass of the HBP. At 7% loading in the epoxy thermoset, the G5 HBP increased toughness by ∼ 60% over the untoughened control. Toughness increased to 82% above the untoughened control at a loading of 19% G5 HBP, but the toughness decreased at 28% HBP loading. The T_g and E′ were influenced by the HBP modifier, but the effect was not systematic and may have been due to competing effects of HBP molar mass and end group. The effect of the architecture of the thermoplastic modifier was investigated by introducing a linear aliphatic polyester (∼ 5400 g mol) with a repeat unit structure, which was similar to that of the HBP. At the molecular weight range investigated, neither the prepolymer viscosity nor the thermoset toughness of the HBP–epoxy was significantly different from that of the linear polyester in epoxy. Preliminary results are presented showing the effect of thermoplastic molecular weight and architecture on morphology. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 72: 151–163, 1999