Handbook of Epoxy Blends

Epoxy resins are a class of thermosetting polymers widely used for structural application. However, as epoxy resins are inherently brittle because of their highly cross-linked structure, a great effort has been made to improve the fracture toughness. A widely used method for this purpose is the addition of second-phase polymeric particles, and over the past decades, great success has been achieved in this area. This chapter provides a comprehensive overview of the development in rubber-toughened epoxy. First, we review the history of rubber-toughened epoxy and different kinds of rubbers used for toughening epoxy. Then, we summarize the factors affecting the toughening effect and mechanisms accounting for rubber-toughened epoxy. Finally, we discuss some new trends in this field.

Since longtime, academic and industrial research has been focused on the development of epoxy networks with excellent mechanical properties such as their fracture toughness and flexural and tensile properties by using rubber particles or low molar mass liquid rubbers. In fact, different routes have been developed to increase their resistance to damage such as the synthesis of new copolymers and block copolymers, the incorporation of unmodified or modified rubber particles, as well as the formation of graft interpenetrating polymer networks (graft-IPN). Nevertheless, the number of publications combining the keywords epoxy and rubber is considerable. Thus, various types of rubber have been studied as modifiers of epoxy networks, including functionalized or not butadiene–acrylonitrile rubber (NBR), polybutadiene (PBD), acrylate-based rubbers, natural rubber (NR), styrene–butadiene rubber (SBR), polydimethylsiloxane (PDMS), etc. For these reasons, this chapter gives an overview of the different preparation of epoxy–rubber networks and their final properties.

The incorporation of functionalized liquid rubbers is one of the most successful methods to toughen epoxies. These rubbers are initially miscible with epoxy oligomers at a given temperature, however, the reaction-induced phase separation occurs because of the increment in the molecular weight of the epoxy matrix as the curing reaction proceeds, thereby resulting in the precipitation of rubber as a dispersed phase. The properties of the resultant epoxy/rubber blends depend largely on their final morphologies, which in turn are determined by the phase separation of the epoxy/rubber blends in the curing process. In this chapter, we first introduce the thermodynamics of phase separation based on the Flory-Huggins solution theory and then discuss the miscibility of epoxy oligomers with liquid rubbers and some factors affecting the phase separation of epoxy/rubber blends, such as the structural properties of rubber, curing agent, and curing procedure. We also discuss the techniques used to study phase separation. Finally, some critical comments and conclusions are given.

The investigations on the morphological features of epoxy/rubber blends are of great importance as the morphology controls the property and performance of these blends. The characterization techniques like optical microscopy (OM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM) are commonly used to evaluate the morphology and phase distribution of the dispersed rubber particles in the epoxy matrix. These characterization techniques are used to explore the morphological features in epoxy systems modified with different kinds of rubbers such as liquid rubbers, preformed core–shell rubber particles, in situ-formed rubber particles, etc. Moreover, several factors which affect the final two-phase morphology in the epoxy/rubber blends are also explored using these techniques. The fracture surface characteristics are also explored using morphological investigation of the fracture/fatigue surface of the epoxy/rubber blends to establish the toughening mechanism operating in them. While both OM and SEM are widely used to reveal the microstructure, AFM and TEM are used to trace out the nanostructure in such blends. The current chapter gives a detailed discussion on the use of such techniques to explore the morphology and the microscopic toughening phenomena operates in epoxy/rubber blends on the basis of published reports.

Epoxy resin is a kind of widely used thermoset resins known for its excellent properties. While its inherent brittleness largely limited its application, the incorporation of rubber phases into epoxy resins is a classical method and could effectively make up the shortcomings. In order to acquire a deeper understanding of the mechanism of these properties, it is necessary for epoxy/rubber blends to be structurally well characterized. This chapter focuses on the practical aspects of using the five spectroscopic methods most often applied, Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, Raman spectroscopy, optical microscopy (OM), and X-ray scattering spectroscopy, in the structural investigation and microstructures of epoxy/rubber blends to study the driving forces producing miscibility in the blends. The application of other spectroscopic methods is also discussed. It should be mentioned that the study of driving forces producing miscibility in epoxy/rubber blends is based on the application of at least a few different types of spectroscopic methods during the same study.

This chapter is an introduction to the rheology of epoxies and rubbers, their blends, classification, and types. Principles of rheology and polymer rheology are illustrated through conclusive plots and graphs. Rheology of rubbers, epoxies, and their blends are discussed with respect to their molecular structure, mobility, and conformational states. Rheology is explained with respect to the established viscoelasticity models for epoxies and their blends. A detailed account of rheometers and measurements is given. Miscibility, phase separation, and thermodynamics of epoxy-rubber systems form an integral part of this chapter. A correlation is drawn between rheology and shear behavior of these systems. Rheological insights on processing, manufacturing methods, curing reactions, and curing schedules are detailed for epoxy-rubber blend systems. Functional and product applications of rheology of epoxy/rubber blends are listed at the end.

During the past decades, the toughening of epoxy resins has received increasing attention because many different applications demand epoxy materials with improved mechanical properties. Different approaches have been employed to toughen the epoxy system: agents as liquid rubbers, block copolymers, core–shell particles, glass beads, epoxidized thermoplastics, hyperbranched organic, and hybrid compounds and combinations of them have been considered as toughening agents for epoxy systems. The morphology of epoxy resins and, consequently, their mechanical properties strongly depend on the cure kinetics, and in the case of soluble liquid rubbers, phase separation takes place as the polymerization proceeds. Subsequently, the evolution of size and distribution of the rubber particles in the epoxy during the curing reaction represents a critical point for the success of the effect of rubber systems in terms of mechanical improvement of neat resin system. Therefore, different methods have been studied and developed to control the cure kinetic parameters of epoxy resins, in order to develop models and control their final morphology and properties. Among them, some of the most used are those based on chemical changes such as differential scanning calorimetry (DSC) and infrared (IR) spectroscopy methods, as well as those centered on bulk property changes such as rheological and pressure–volume–temperature (PVT) methods. In this chapter, these methods are discussed and the results obtained on toughening various types of epoxy systems are compared. Moreover, the studies are extended to nanostructured systems, where the presence of the nanofiller plays a crucial role in the evolution of the reaction kinetics.

For rubber-modified epoxy blends, dynamic mechanical thermal analysis (DMTA) was mainly used to investigate the influence of rubber content on the relaxation of modified epoxies. The storage modulus and glass transition temperature generally decrease as the rubber content increases in epoxy matrix. It is therefore DMTA that enables us to study the variations of these parameters of rubber-toughened epoxy blends. The combination uses of microscopic observations and DMTA help us to identify the phase structures of rubber in epoxy matrix. DMTA also provides supporting evidence to phase structures of rubber inclusions in epoxy matrix and an important reference for thermal stability of rubber-modified epoxy blends.

The thermal properties of epoxy/rubber blends include glass transition, thermal conductivity, heat capacity, thermal expansion, and thermal stability and are systematically reviewed by a number of thermal analysis techniques including differential scanning calorimetry, thermogravimetric analysis, thermomechanical analysis, and dynamic mechanical analysis. The rubbers include usually used natural rubber, polybutadiene rubber, nitrile rubber, polyurethane rubber, silicon rubber, etc. Generally speaking, the addition of a rubber component to the epoxy resin will result in depression of glass transition temperature of it due to incomplete phase separation and incomplete curing reaction. It depends on the cross-linking density of the blends. The thermal conductivity of the blends is affected by the polar nature of rubber. The conductive rubber which is forming a continuous phase could enhance the thermal conductivity. Heat capacity of epoxy/rubber blends is affected not only by the polar nature of rubber but also the density of the formed blends. The thermal expansion behaviors are not only related to thermal and mechanical history but also depend on the network of modified epoxy resin. And further, the thermal stability of epoxy/rubber blends is mainly depending on the thermal stability of the rubber and the cross-linking density of the formed networks. The higher-heat-resistant rubber leads to higher thermal stability of epoxy/rubber blends.

Highly cross-linked epoxies that are susceptible to brittle failure can be effectively toughened by blending them with various types of rubber. Initially, a small amount of a miscible liquid rubber is incorporated into the matrix of the curing agent-incorporated epoxy resin, and then the whole mass is subjected to curing. The phase separation depends upon the formulation, processing, and curing conditions. The improvement in fracture toughness occurs due to the dissipation of mechanical energy by cavitation of the rubber particles, followed by shear yielding of the matrix. A few factors such as the size of the rubber particles, curing agent, cross-linking density, etc. play an important role in succeeding or failing to improve the toughness. This chapter provides an overview of the toughening mechanism of rubber-modified epoxies. The effects of a few major factors (i.e., the size of the rubber particles, curing agent, curing time and temperature, etc.) on the mechanical properties of rubber-modified blends were studied. The effect of the varieties of synthetic and natural liquid rubber on the impact, flexural, and tensile properties of the epoxy blend is compared and studied.

The water and solvent sorption behavior of epoxy/rubber polymer blends of various structures are discussed in this chapter. The sorption phenomena are explained in these blends with respect to the surrounding temperature, relative humidity, and moisture concentration profiles based on parameters like diffusion, part and laminate thickness, weight gain due to absorption over time, and saturation equilibration. A reasonable correlation is attempted through Fickian and non-Fickian absorption models for epoxies, rubbers, their blends, and the observed properties. A brief note is presented on the viscoelastic models of environmentally affected blends. The observed changes in the physical, chemical, mechanical, and electrical properties of the blends are documented. Here again, distinction is made between the sorption characteristics of solid rubber/epoxy and liquid rubber/epoxy blends. The hygrothermal performance of fiber composites fabricated with epoxy/rubber blends as the matrix materials is also discussed and presented along with their applications. Limitations, implications, and suggestions for future work have been included at the end of the chapter.

This chapter will be devoted to the study of ternary systems containing synthetic rubber or natural rubber in the epoxy system in the presence of nanoclay. Curing behavior, morphology, and properties of ternary systems containing epoxy, PMMA grafted natural rubber, and nanoclay were elaborated. Two types of nanoclay used in this coverage were natural montmorillonite (Cloisite Na) and organic chemically modified montmorillonite (Cloisite 30B). The amount of nanoclay used were 2,5, and 7 phr, while the amount of rubber was fixed at 5 phr. Poly(etheramine) was used as the curing agent, and the effect of the equivalent and higher molar ratio, 1.05, of amine-epoxy were also studied.The materials prepared were cured unmodified epoxy, cured toughened epoxy, cured unmodified epoxy/clay nanocomposites, and cured toughened epoxy/clay nanocomposites. Some experimental results related to transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-Transformed infrared analysis (FTIR), differential scanning calorimetry (DSC), thermal gravimetric analysis (TGA), and impact tests were discussed.

The Chapter highlights the importance of epoxy based particulate composites with special reference to rubber modified epoxy resins. A brief account on modeling of toughened epoxies is given. Thereafter, mechanical properties of particulate composites based on epoxies are discussed followed by mathematical modeling approaches towards them. The combined effects of particulate fillers with rubbery additives(both CTBN and other rubbery modifiers) on epoxies are then discussed. Finally, the effect of nanoparticles as fillers, in addition to rubbery additives are also reviewed, followed by present and potential applications of such materials.

Epoxy resins have been used in a very wide range of industrial applications for more than half a century. Paints, adhesives, coatings, and matrix material for many different kinds of composites are the main applications of epoxy resins. Excellent adhesion properties along with high mechanical strength and thermal stability are the major attractions of this family of engineering materials. However, epoxy resins suffer from the inherent brittleness which can potentially limit their applications. Among different approaches which have been employed to reduce this deficiency, rubber toughening has been practiced the most. Different types of rubber modifiers which are blended with epoxy resins for this purpose include reactive oligomers, preformed rubber particles, and di- or tri-block copolymers. This chapter tries to give a general overview of the whole concept of rubber-toughened epoxies to the reader. Delivering a more realistic sense of industrial applications of epoxy/rubber and epoxy/copolymer blends by means of practical examples is done in this chapter as well. The goal is that the reader can benefit from this chapter in expectations from epoxy blends in practice. Engineers interested in epoxy resins may find insights in this chapter for their developing industrial/research plans.

The incorporation of thermoplastics in epoxy matrices is considered to be a highly effective method to improve some mechanical properties of epoxy resins, especially the fracture toughness. In this chapter, a comprehensive review of the development in thermoplastic/epoxy blends is provided. Also the effects of the addition of different thermoplastics such as polysulfone (PSF), poly(ether sulfone) (PES), poly(ether imide) (PEI), and poly(phthalazinone ether)s on fracture toughness, modulus, and strength of the epoxy resins have been reviewed. Then, the reaction-induced phase separation and the influence factors of toughening effect such as content of additives, molecular weight and end groups, etc., are also summarized. In addition, the toughening mechanisms of thermoplastics/epoxy blends are described generally.

Epoxy/thermoplastic blends have received a high degree of attention owing to the fracture toughness improving without significantly compromising the thermal and mechanical properties. These materials are mostly prepared by mixing thermoplastic with epoxy monomers and curatives, and then undergoing a reaction-induced phase separation mechanism to form the final products. The mixing processes are very critical for forming the blends with the optimal properties and processabilities. A poor mixing can lead to localized property variation and deteriorate the morphological and mechanical properties of the final cured blends. Due to the importance of the mixing process, methods used for fabrication of epoxy/thermoplastic blends are described in detail in this chapter. Some novel techniques suitable for specific epoxy/thermoplastic blends are also discussed.

Reaction-induced phase separation in the epoxy/thermoplastic (TP) blends has received considerable interests since the early 1980s. Phase separation studies mostly focused on the miscibility, phase separation mechanism, and morphology formation in the various epoxy/TP systems and processing conditions. This is peculiarly important, since there are still some disagreements on the mechanisms when the traditional understandings of phase separation kinetics are used to explain the phase evolution and unique nodular structure in the present reactive systems. In the existing published work, sea-island, bicontinuous, and nodular structures were mostly reported. It seems that majority of the morphology formation analysis can reach some kind of agreement; however, questions have been raised again and again whenever consistent morphology and performance relationship cannot be obtained or reached. In this chapter, some of the previous reports were briefly reviewed and a more complete three-dimensional view of the phase structure evolution, including the combined in situ optical microscope and light scattering observation and comprehensive information on xy plane and z directions of the whole sample, was introduced. The significant difference between molecule mobility and dynamics of the epoxy oligomers and TP polymer chains was noticeable. Consequently, dynamically asymmetric phase separation was found useful for the discussion of phase separation kinetics of the epoxy/TP systems, which made the nodular structure formation and volume fraction of the TP-rich phase understandable. Layered structure and gradient morphology ubiquitously formed during the cross-linking reaction of many thermoset/thermoplastic systems, which was considered useful for interlamellar toughening or inter-parts adhesion applications.

The morphology of the blends has great impact on the thermal mechanical properties of multicomponent materials. For epoxy modified with thermoplastic resins, both thermodynamic and kinetic factors may affect the evolution and final phase structure of the blends. The chemical structure, end groups, side groups, molecular weight, and distribution of thermoplastics may influence the compatibility and interfacial conditions of blends, while the curing conditions and difference in the mobility of components have fundamental effects on the viscoelastic phase-separation process of these kinds of blends. Various techniques, such as optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscope (TEM), atomic force microscope (AFM), and light scattering (LS), have been employed in the morphology study of materials.

Light scattering is a popular in situ technique commonly used to study the morphology evolution in real time in heterogeneous systems. The immediate response and short sample preparation times are the distinct advantages of light scattering compared to electron microscopy to study the morphology evolution in polymer blends. The small-angle light scattering (SALS) technique is extensively used to study the reaction-induced phase separation in epoxy blends. Studies involving various epoxy/block copolymer and epoxy/thermoplastic blends were included in this chapter. During the epoxy curing process, the morphology change with respect to curing time and blend composition was followed via the change in scattering patterns using time-resolved light scattering technique (TRLS). The formation of secondary structures within the phase-separated primary domains and their mechanism of phase separation were also studied using light scattering technique. More importantly, in reactive blending, the light scattering technique will provide deeper understanding of the effect of polymer functionality and processing parameters on the phase-separated morphology. This chapter would give an overview of morphology evolution studied in various reactive epoxy blends using light scattering technique.

In this chapter, the main findings of spectroscopic analysis applied to epoxy/thermoplastic (TP) blends are presented. This includes vibrational spectroscopy (infrared and Raman), nuclear magnetic resonance spectroscopy, fluorescence spectroscopy, energy-dispersive X-ray spectroscopy, and dielectric relaxation spectroscopy. These techniques have been employed alone or combined to monitor polymerization processes, the glass transition, and structural relaxations, to study reaction-induced phase separation (RIPS), to obtain a complete chemical characterization of the different phases, or to analyze compositional gradients at the interphases. The study of the specific interactions between the epoxy components and the thermoplastic modifier allowed the interpretation of chain interdiffusion and miscibility data and helped to explain microstructure. Several examples are discussed.

Epoxy–thermosets hold value in many industrial applications, owing to their favorable characteristics including excellent adhesion, ease of processing, and thermal and chemical stability. Despite these benefits, the high cross-linking density of epoxy–thermosets also results in a brittleness that limits utility in many high-performance structures that are subject to high impact, such as aerospace and automotive applications. The inclusion of thermoplastics within the epoxy can improve toughness and impact resistance. Rheological analysis of epoxy–thermoplastic blends serves as a critical analytical technique for evaluating the viscoelastic properties during the pre-cure formulating stage and during the cure. Chemorheology is useful to characterize the distinct viscoelastic changes that occur during the cure of epoxy–thermoplastics, which involve phase separation, epoxy gelation, and vitrification. Through rheological modeling, the behavior during cure can be further evaluated to predict flow behavior and mechanisms of phase separation. Importantly, rheology meets the analytical demands in industrial applications, where details concerning preprocessing conditions, quality control, and cure cycle design are crucial.

Thermoplastic toughening of epoxy resins had aroused considerable interest in the past few decades. Functionalized as well as nonfunctionalized thermoplastics have been extensively used to toughen epoxy resins. The ultimate properties of the blends are strongly dependent on the cure conditions employed. Investigation of the cure kinetics is therefore very important in predicting the ultimate properties. Kinetic analysis gives information regarding the extent of cure, curing mechanism, activation energy, etc. Techniques such as differential scanning calorimetry, Fourier transform infrared spectroscopy, dielectric relaxation spectroscopy, etc. are useful tools for kinetic analysis. Various models have been developed for the evaluation of cure kinetics. This chapter summarizes the various aspects of the kinetics of epoxy resin/thermoplastic blends.

Epoxy/thermoplastic blends have been primarily developed to meet requirements in high-performance applications rendering toughness to the epoxy resins without sacrificing thermal resistance and mechanical performance. Engineering thermoplastics are the natural choice having high glass transition temperature, high thermal degradation temperatures, and impact resistance. The challenge is to understand and control phase separation in these systems and determine the correlation with mechanical properties. This chapter presents the application of dynamic mechanical thermal analysis (DMTA) as a suitable analytical tool to investigate the mechanical behavior and energy dissipation of epoxy resins modified with thermoplastics and aspects and consequences of miscibility, partial miscibility, or phase separation between thermoplastic domains and epoxy matrix. The type of thermoplastics utilized (amorphous vs. semicrystalline) and the influence of curing agents and curing temperature on the phase separation dynamics and nanophase of epoxy thermosets are discussed. Finally, application of DMTA to design and characterize the performance of a new generation of smart epoxy/thermoplastic blends with self-healing and shape memory properties is also discussed.

Incorporation of thermoplastics into the epoxy resin is a potential route for the development of toughened epoxy thermoset with augmented properties. The ultimate properties of the blend depend on the various transitions and transformations during epoxy polymerization such as the viscoelastic phase separation, secondary phase separation, gelation, and vitrification. In this chapter, it is aimed to give a detailed description on the various approaches for improving the fracture toughness of epoxy by blending with thermoplastics, the toughening mechanisms involved, and their effects on the thermal properties of the blends. All the systems have been analyzed with a special attention on their thermal properties. Thus, the thermoplastic-modified epoxy resins with improved fracture toughness along with the inherent properties of epoxy resins fall in the class of engineering materials, which is eligible for many high-end applications.

Thermoset (TS) epoxy resins have drawn a lot of attention over the last years in several fields including transportation, construction, and electronics owed to their excellent combination of mechanical properties, ease of processing, and low cost. However, their brittleness is a main disadvantage that limits their applications. Incorporation of thermoplastic (TP) polymers, i.e., poly(phenylene oxide) (PPO), polysulfone (PSF), or polyetherimide (PEI), has found to overcome this drawback. In this chapter, the phase separation and mechanical properties of selected TS/TP blends will be described, and the toughening mechanisms that have been proposed will be discussed. Further, the morphology and mechanical properties of TS/TP-based ternary nanocomposites, with special emphasis on those containing carbon-based nanofillers such as carbon nanotubes (CNTs), carbon black (CB), or graphene oxide (GO), will be addressed. These blends typically undergo reaction-induced phase separation upon curing, leading to different morphologies such as dispersed, co-continuous, dendritic, or phase inverted. Examples have been selected to demonstrate the importance of the asymmetric distribution of the nanofillers for developing new materials with enhanced properties, superior than those attained in the corresponding nanofiller-reinforced binary samples. These multifunctional materials are expected to have a wide range of applications, particularly in sensing devices, shape memory, and self-healing materials.

The present chapter deals with the applications and drawbacks of epoxy/thermoplastic blends. The fundamentals of epoxy/thermoplastic blends are discussed. The different toughening technologies applied to epoxy resins for advanced composites are thoroughly discussed and their commercial outcomes presented. Some applications like adhesives and self-healing are also introduced. For each application field, when appropriate, examples of commercial exploitation for the epoxy/thermoplastic blends are given to present the reader to the practical down to earth applications. In some cases, i.e., self healing, the future applications are presented.

During curing of epoxy resins shrinkage of the material appears. As long the resin is in the liquid or gel state, the material can easily relax and no stresses appear caused by shrinkage. After vitrification, the mobility of the polymer units is hampered and internal stresses develop. Thus, the shrinkage may cause problems during processing and in the final properties and application. One strategy to reduce the shrinkage is the addition of nonshrinking additives. Favorable is the addition of thermoplastics or rubbers, which in addition to the reduction of cure shrinkage often exhibit a toughening effect to the material. This chapter focuses on the influence of added thermoplastics, hyperbranched polymers, block-copolymers, or rubbers on the cure shrinkage and kinetics. The efficiency of the additives in reduction of shrinkage depends not only on their type and amount but also on their reactivity and the morphology of the blends which develops during the cure process. Beside the shrinkage, also other volumetric properties are discussed like changes in the thermal expansivity.

Formation of nanostructures inside epoxy thermosets by the inclusion of appropriate block copolymers (BCPs) has been emerged as a promising approach to optimize epoxy thermoset material properties for potential applications. For the last two decades, tremendous efforts have been made by researchers to create ordered or disordered nanostructures in epoxy thermosets by the incorporation of reactive or nonreactive BCPs in an attempt to develop toughened thermosets suitable for specific applications. This chapter briefly reviews the different mechanisms of phase separation in epoxy/BCP systems, such as self-assembly and reaction-induced microphase separation (RIMPS), and outlines some of the important features of nanostructured morphologies and their influence on fracture toughness of fabricated products.

Incorporating block copolymers into epoxy systems has emerged as a versatile and effective methodology not only to enhance their mechanical properties, but also as an intriguing strategy to design advanced materials with tailored properties. Knowledge of microphase separation mechanisms operating during the development of these materials is essential due to the straight relationship between block copolymer characteristics, epoxy system formulation, and curing conditions with the final nanodomain morphology. This chapter is focused on the thermodynamic and kinetic fundamentals describing microphase separation mechanisms by which the nanodomains are obtained. Moreover, key parameters affecting phase separation mechanisms and morphologies are discussed, explaining how different material properties can be tuned by controlling the nanostructure morphology.

The ability of block copolymers to generate nanostructures when mixed with epoxy thermosetting systems is deeply analyzed. Both amphiphilic and chemically modified di- or triblock copolymers have been used with this purpose. In both cases, one of the blocks is miscible with epoxy system (or even can react with it) before and after curing, while the other one is immiscible before or after curing, leading to morphology development by self-assembly (SA) or reaction-induced phase separation (RIPS), respectively. In some cases, nanostructure development can occur by a combination of both mechanisms or even some demixing of the miscible block can also happen, generating different morphologies. Depending on that and on other parameters like employed hardener, cure cycle, copolymer block ratio, and topology among others, different morphologies such as spherical or wormlike micelles, hexagonally packed cylinders, bilayer micelles, or mixtures of them can be obtained. The effect of nanofillers on morphologies of ternary systems based on epoxy systems nanostructured with block copolymers is also analyzed.

Epoxy/block copolymer blends exhibit unique nanostructure, morphologies, phase behavior, and physical properties, which are determined by the cross-linking reaction of the thermosetting resin, the self-assembly of the block copolymer, and the process of phase separation. Understanding of the influence of different types of ER-miscible blocks on the microdomain structure and dynamics, as well as the underlying molecular mechanism responsible for the structure formation and evolution in these blends is still lacking at a molecular level. In this chapter, a variety of advanced multiscale solid-state NMR techniques were used to characterize the heterogeneous dynamics, miscibility, microdomain, and interphase structure, as well as the cross-linked network in nanostructured epoxy/block copolymer (ER/BCP) blends, focusing on the role of ER-miscible blocks containing poly(ε-caprolactone) (PCL) or poly(ethylene oxide) (PEO) blocks having different intermolecular interactions with ER. 1H static and magic-angle spinning (MAS) experiments were used to detect the molecular mobility in these blends, and then detailed dynamic behavior and the miscibility of the BCPs with the cured-ER network were obtained by 1H dipolar filter experiments. Two-dimensional 13C-1H WISE experiment was used to gain information about the heterogeneous dynamics of individual components and to determine the extent of phase separation in the blends. 1H dipolar filter spin-diffusion experiments were used to quantitatively determine the evolution of interphase thickness. High-resolution 1H DQ filter and 1H-1H spin-exchange experiments under fast MAS were utilized to detect interphase composition and detailed intermolecular proximity between ER and BCPs in the interphase region. High-resolution 13C CPMAS experiments were employed to probe the driving force for the interphase formation and miscibility associated with the intermolecular interactions between ER and ER-miscible blocks. Finally, 13C T₁ spin–lattice relaxation experiments were used to quantitatively determine the amount of local destroyed network and dynamics of cross-linked network in all blends. On the basis of these NMR results, we proposed a model to describe the unique structure and dynamics of the interphase and cross-linked network, as well as the underlying mechanism responsible for the nanostructure formation in ER/BCP blends with different ER-miscible blocks.

The rheology of the epoxy/block copolymer blends is an important learning tool to understand the microphase separation of one of the blocks of block copolymer during the curing process. The knowledge about the viscoelastic changes during the thermosetting network formation allows to better understand the processes such as gelation, vitrification, and microphase separation. Different methods can be employed to determine the gelation point of the epoxy/block copolymer blends. The first method is related to the extrapolation of the complex viscosity (η*) to infinite, the second to the intersection of dynamic storage modulus (G′) and dynamic loss modulus (G″) curves, and the third one to the fact that tan δ is independent on frequency at gelation point. In this chapter, a few examples of rheology of epoxy/block copolymer blends will be presented with the main aim of showing the correlation between rheological behavior and final properties of thermosetting systems.

This chapter deals with the study of the cure kinetics of epoxy/block copolymer blends in order to give a comprehensive account about the effect of adding this kind of modifier on the reaction rate of the network formation. Non-isothermal runs at constant heating rates and isothermal runs at constant temperature were carried out in order to determine the total heats of reaction released during curing for the epoxy blends modified with different contents of block copolymers. It was found a clearly delay of cure kinetics with the increase of block copolymer content. In order to understand the parameters affecting epoxy curing kinetics, the influence of block copolymer blocks chemical structure, and the molar ratio between blocks on the curing rate was also analyzed. Fourier transform infrared spectroscopy was used for this purpose. The experimental curves of isothermal curing were fitted to a phenomenological autocatalytic model and also to mechanistic model. Kinetics parameters were calculated from the previous models. The increase observed in activation energy values with the increase of block copolymer content corroborated that the physical interactions between the block copolymer and the epoxy significantly affect the curing behavior, agreeing with the observed delay.

Dynamic mechanical thermal analysis (DMTA) is one of the most powerful techniques to investigate the phase separation dynamics of block-copolymer (BCP)-modified epoxy systems as it provides information such as phase separation, compatibility/miscibility between the BCP blocks and epoxy matrix, specific interactions among various phases, damping characteristics, and stiffness and toughness of the system. In this chapter, we focus on the dynamic mechanical properties of epoxy thermosets modified with diblock and triblock copolymers. The type of block copolymers and the effects of different curing agents, cure temperature, hardeners, etc. on the phase structure and phase separation dynamics of epoxy thermosets are discussed. An extremely fascinating feature of DMTA is that direct information about the phase structure could be derived from the tan δ peaks. Shift in the T _g , broadening of glass transition peaks, superimposition of two or more peaks, height and width of various transition peaks, etc. furnish ample evidence for the type and nature of the phase structure and an intuitive understanding of the related mechanism.

New ways to improve the thermal properties of epoxy systems have been interesting topic for polymer researchers for several years. The block copolymer-modified epoxy matrix has received a great deal of attention and is still being intensely studied. Differential scanning calorimetry (DSC) is the most commonly used technique to investigate the thermal properties of epoxy/block copolymer systems. It can generally provide information such as phase behavior, miscibility, glass transition temperature, melting temperature, etc. between the block copolymer blocks and the epoxy matrix. In this chapter, we have mainly focused on the changes in the glass transition properties of the thermosets modified with block copolymers. The influence of the type of block copolymers and curing agents used and the effects of cure time and temperature on the phase behavior and microphase separation of epoxy thermosets are also discussed.

In this chapter, we summarized the recent progress in the studies of mechanical properties of epoxy/block copolymer blends. It is recognized that nanostructures can be formed in epoxy/block copolymer blends via either self-assembly or reaction-induced microphase separation mechanism. The formation of nanostructures in the epoxy thermosets can more effectively improve the toughness of the epoxy thermosets, which has been called “toughening by nanostructures.” The toughening of nanostructured epoxy thermosets is quite dependent on type and shape of dispersed nanophases and the interactions between nanophases and epoxy matrix. In terms of the mechanism for energy dissipation, toughening mechanisms of the epoxy/block copolymer blends involve shear band, microcracking, crack pinning, and particle bridging. Depending on the inherent features of materials and operating conditions (e.g., applied loading), improvement of toughness can be achieved by the function of a single mechanism or through a complex combination of simultaneous and successive actions of different processes.

Epoxy polymers are well known for their superior barrier properties in various environmental conditions. Block copolymers and thermoplastics were added to these epoxy polymers to modify their hydrophilic/hydrophobic balance and control various macroscopic physical properties such as moisture absorption, cross-linking, and degradation. The hydrophilic/hydrophobic balance is an important property of any material that determines their fate in the real-world application. Here, in this chapter, water and solvent sorption in various epoxy blends were discussed. Specifically, the change in hydrophilic/hydrophobic balance in the presence of various amounts of block copolymers and thermoplastics were discussed. Contact angle and surface free energy measurements in these blends gave an indication of change in surface properties with respect to blend compositions. Additionally, the nano/micro channels produced by microphase separation of block copolymers and their bulk phase separated morphologies had a greater influence on the sorption behavior of the epoxy blends. This chapter would give an overview of water and solvent sorption behavior of various epoxy/block copolymer and epoxy/thermoplastic blends studied in recent years.