0 Introduction

Core-shell particles with a bilayer structural morphology have been attracting more and more attention because of their unique comprehensive performance and wide application potential as electronic, catalytic, sustainable, biomedical materials, and toughening agents of brittle polymers including epoxy resin and polylactide (PLA). The molecular design of the core-shell particles is considered as a key focus in order to obtain the core-shell particles with various structural features including high specific surface area, porous internal structure, rubbery elastic core but glassy rigid shell, resulting in great applicability in different fields of adsorption, catalysis, biomedicine, liquid chromatography, electronic printed circuit boards based on epoxy resin, and medical surgical suture based on PLA. As shown in Fig. 1, the publication of the academic articles regarding "core" and "shell" as simultaneous two keywords in the article title started in 1975 showed an exponential increasing trend from 1992 to 2019. There are totally 32199 articles including 343 review articles published in world leading journals such as Chemical Reviews ^[1-2], Chemical Society Reviews ^[3-7], and Progress in Materials Science ^[8] and 364 highly cited articles. This suggests that the scientific research and technology development on the core-shell materials are very hot worldwide.

The first review article published in 1992 summarized the preparation and challenge of core-shell composite latex particles via staged emulsion polymerization, particularly when the desired core polymer is more hydrophilic than the shell polymer^[9]. It is suggested that the optimization of several thermodynamic and kinetic variables including particle surface polarity, stage ratio, core particle size, the mode of monomer addition, and crosslinking degree could be used to control core-shell structure based on whole organic polymers. The second review article published in 1993 analyzed the fracture behavior and toughening mechanism of high-performance epoxies modified with core-shell rubber particles ^[10]. Synergistic toughening highly cross-linked epoxies could be realized by operating the matrix shear-yielding and the crack-deflection mechanisms upon fracture. The third important review article published in 1998 reviewed the synthesis and structural ordering of Core-shell polymer microspheres based on highly branched polymers by adjusting methyl methacrylate (MMA) monomer concentration and locking the cubic lattices of core-shell microspheres in a PMMA matrix ^[11]. It is indicated that these core-shell polymer microspheres would be useful in high technology including epoxy resin- and PLA-based optical and electronic devices, and surgical suture, respectively. The fourth historic review article published in 2000 introduced the discovery and preparation of nanocrystalline TiO₂-MoO₃ core-shell materials ^[12]. It is of interest that the nanocrystalline TiO₂-MoO₃ core-shell materials demonstrate significant photoabsorption energy that is correlated with both the TiO₂-MoO₃ core-shell nanoparticle size and the interaction between the TiO₂ core and the MoO₃ shell. The photoabsorption energy decreases from 2.88 to 2.60 eV with decreasing nanoparticle size from 8 to 4 nm. After 2006, the review articles regarding core-shell structured systems became more and reached the maximum 41 papers in 2020, as illustrated in Fig. 1. Representative historic review articles are listed as Refs. [1-8, 11, 13-44]. In this review, we start with various preparation methods of the core-shell nano-/microshperical particles, followed by a summary of their structural morphology, and finally by an outline of their important applications in many fields. Especially, the design and preparation of the wholly organic polymers-based core-shell particles are focused. We highlight the vital application and latest development progress of soft core-hard shell particles for effectively toughening epoxy resin and PLA. This review article would like to briefly summarize the current state of the core-shell nano-/microshperical particles based on polymers with potential applicability as toughening agents for the goal of achieving better future developments in these fields^[44-45].

1 Classification and Preparation of Core-shell Nanospheres

Core-shell nano- and micro-spheres can be categorized based on double or multiple materials into core-shell composite spheres, which are composed of two or more different components. The core-shell type nano- and micro-spheres can be defined as comprising an inner core substance and an outer layer shell material. There is close interaction between the core and shell layers in the nano- and micro-spheres of various combinations including inorganic core-inorganic shell^{[17, 46-77]}, inorganic core-organic shell^[78-83], organic core-inorganic shell^[84], and organic core-organic shell^[85-106] materials. The core layer could endow the core-shell spheres with additional functions, while the shell layer is closely related to their practical application. By the way, there are other core-shell nanowires, nanotubes, nanorods, nanoribbons, and nanoplates possessing outstanding functions and key applicabilities^[107-119].

The most common core-shell spheres are concentric core-shell spheres where a simple spherical core particle is totally covered by a shell of a different material. Different shaped core-shell nanoparticles have also given rise to immense research interest because of their various novel properties and functionalities. Generally, a nonspherical core would lead to the formation of core-shell nanoparticles having various shapes. The core-shell nanospheres can be simply classified into organic polymers, inorganic, and inorganic/organic hybrid. Thus, their preparation methods are quite different. As illustrated inFig. 2, 12 diverse methods have been invented for the successful preparation of core-shell nanospheres^{[17, 46-106]}.

Because the materials with various functionalities can be designed with more various materials through the arrangement and combination of core-shell structures, we can simply prepare novel materials with more unique properties, splendid functions, and more extensive and promising applicabilities. As summarized in Fig. 3, 21 types of applications of the core-shell nanospheres have been depicted so far^{[17, 46-106]}.

Li and Stöver summarized the common approaches to polymer-based core-shell particles in Scheme 1^[120-121]. It is suggested that precipitation polymerization is one of the best methods of synthesizing narrowly dispersed cross-linked core-shell polymer microspheres that could be useful as packing materials in high performance liquid chromatography shown in Fig. 4^[120]. This paper mainly focuses on the preparation of wholly organic polymer core-shell nanospheres including nanoparticles (i.e., core-shell rubber particles) for their important applications in epoxy resin and PLA toughening.

2 Research Backgrounds of Core-shell Rubber Particles as Key Toughening Components

Epoxy resin refers to the general name of curing compounds with cross-linked network structures that contain two or more epoxy groups in a molecular structure, taking aliphatic, alicyclic or aromatic organic compounds as the skeleton, and can react with curing agents under appropriate conditions ^[122]. Diglycidyl ether bisphenol A formed by the reaction of bisphenol A and epichlorohydrin, with the structural formula shown in Scheme 2, is the most widely used epoxy oligomer, accounting for about 70% of all epoxy oligomers uses ^[123-124].

Before curing, epoxy oligomer is a thermoplastic oligomer, which cannot be used directly in general. Some appropriate curing agents must be added for curing and cross-linking under certain conditions to generate a kind of network-like polymer that is infusible and insoluble. Reactive curing agents are mainly divided into amine curing agents and anhydride curing agents. Amine curing agents commonly like triethylenetetramine with high activity have a relatively simple curing process and can be cured rapidly even at room temperature. The curing process is mainly the crosslinking reaction among the active hydrogens on the nitrogen atoms and the epoxy groups, which resulting in curing into solid, but the volatility and toxicity of the amine curing agents are high, and especially the materials formed thus have high brittleness and low thermal deformation temperature. Therefore, the epoxy products cured by the amine curing agents are generally used as coatings and adhesives. Anhydride curing agents such as liquid methyl tetrahydrophthalic anhydride are basically non-irritating, low toxicity, low volatility, and long pot time. However, the curing time is long and needs to be heated for completely curing, so the anhydride curing agents cannot be used to prepare the products with high crosslinking degree. The curing mechanism is that the hydroxyl group on the resin causes the anhydride in the curing agents to open the ring structure to generate a monoester, and the carboxyl and epoxy groups in the monoester structure undergo esterification reaction to generate a diester structure. The hydroxyl group generated by esterification reaction can further open the ring of the anhydride in the curing agents. The curing rate is closely related to the concentration of hydroxyl group in the epoxy oligomers. Their cured products with epoxy oligomers have low shrinkage, creep resistance, good wear resistance, strong heat resistance, high stiffness, significant adhesion to many substrates, high mechanical strength, and excellent electrical properties. Therefore, their cured products are mainly used in electronic device packaging materials, copper clad laminate, circuit boards, high-voltage insulation components, structural adhesives, protective coatings, resin matrices for high-performance composites, and other fields.

As introduced above, the thermosetting resins that form a network structure after curing of epoxy oligomers have excellent mechanical properties (good dimensional stability, high strength, light weight) and strong resistance to acid, alkali, and solvent corrosion. However, most epoxy resin networks have huge difficulties in sliding, high internal stress, brittleness nature, poor weather resistance, low impact resistance, low resistance to the initiation and growth of cracks, and other problems, which are not sufficient to meet the stringent requirements of modern engineering technology, consequently leading to seriously limited scope of use^[125-126]. Efficient toughening of epoxy resin has always been recognized as a major problem in the world. In order to further expand the practical application of epoxy resins, it is necessary to toughen them.

There have been several review articles regarding the core-shell nanospheres or nanomaterials based on whole polymers^[41-44]. Toughening of epoxy resin systems using Core-shell rubber particles have also been reviewed just in 2021^[45]. Epoxy resin is one of the most important thermosetting resins that have been extensively applied as printed circuit board, electronic circuit board, copper clad laminate, insulation parts, and high and low voltage transformer accessories in aerospace, electrical and electronic, chemical, adhesives, and coatings industries because epoxy resin board has several key characteristics including very smooth and flat surface, dense structure and pure composition (i.e., without any bubbles or impurities), high dielectric properties, strong resistance to surface leakage and arc, powerful adhesion, good processing ability (basically no small molecule volatile, low-pressure molding, and soluble), very low curing molding shrinkage, excellent chemical stability, high dimensional stability and durability, and powerful antibacterial properties. Epoxy resin is formed by the reaction of its oligomer and curing agent. Generally, epoxy resin can be divided into six categories according to its chemical structure and the combination mode: glycidyl ethers, glycidyl esters, glycidyl amines, alicyclic epoxy resins, epoxide hydrocarbons, and others. As a result, it also has wide sources and low cost. Unfortunately, upon curing, the epoxy resin will have a large number of aromatic rings in the crosslinked structure, high chemical crosslinking density, low flexibility of molecular chain networks, and large internal stress, consequently leading to resin brittleness, poor impact resistance, and insufficient crack growth resistance, which largely limits its wider application in modern industrial field.

Therefore, the investigation on toughening epoxy resin has received great attention from academic researchers and industrial R & D developers.

At present, there are three main ways to toughen epoxy resins:

1) Compounding epoxy resins with nanoparticle elastomers^[127-130], thermoplastic nanofibers veils^[131-132], hyperbranched polymers^[133-134], and core-shell polymer for second phase toughening;

2) Continuously penetrating polyurethane into the three-dimensional cross-linked network of epoxy resin to form a semi-penetrating or interpenetrating network structure for efficient toughening^[135-136];

3) The introduction of flexible segments to reduce the crosslinking density or the introduction of micro phase separation structure for enhancing the deformation synergy of molecular segments^[137].

Core-shell rubber particles have a special double-layer or multi-layer structure. As a novel toughening agent of epoxy resin, it has a spherical soft core-hard shell bilayer structure. The shell should be mostly a glassy-state polymer, which can not only protect the core to keep its original shape, but also achieve good compatibility and dispersion between the core-shell polymer nanoparticles and the matrix resin through physical and chemical interactions. The optimal soft core polymer should be mostly in the rubbery state. As a stress concentration body, fracture energy would cause craze to absorb energy. Upon compounding core-shell rubber particles with epoxy oligomers, the presence of core-shell rubber particles can reduce the internal stress of the resulted epoxy system to some extent, and improve the adhesive strength and impact strength of the resulted composite, finally realizing a widely controllable range for strengthening the toughness. Moreover, the impact properties of the epoxy resins can also be greatly improved by optimizing the component and ratio of the core-shell particles, simultaneously remain the other properties including the thermal stability of the epoxy resin system. The materials design and preparation of core-shell rubber particles have made a great advance in the last 20 years, and have become one of the research and development focuses of toughening epoxy resins^[138-140].

Polylactide is commonly known as a biobased biodegradable renewable thermoplastic that could be classified as a sustainable material for replacing some fossil-based thermoplastics such as poly(ethylene terephthalate) or polystyrene (PSt). Regardless of the promising mechanical strength and modulus, like epoxy resins, unmodified PLA suffers from low ductility and high brittleness, which has largely limited the extensive use of PLA in various fields. Therefore, the enhancement of the toughness of PLA has been focused recently. It has been found that the elongation at break of PLA can be remarkably improved with the incorporation of plasticizers or other polymers including core-shell rubber particles^[141-143].

3 Core-Shell Rubber Particles

Core-shell rubber particles refer to composite particles formed by the emulsion polymerization of two or more monomers. The core-shell of particles are composed of different polymer components, and the core and shell also have different functions resulted from different components, showing a special double-layer structure or multi-layer structure^[144]. The core-shell rubber particles can be divided into soft core-hard shell and hard core-soft shell according to the physical hardness of core and shell polymers at ambient temperature. The hard core-soft shell type particles are very useful mainly as coatings and adhesives, and the soft core-hard shell type particles can mainly act as anti-impact modifiers or toughening agents or flexibilizer^[145-147]. Generally, the core-shell rubber particles have round and spherical shape with the minimum surface energy. The core-shell rubber particles with abnormal shapes like half-moon shape, strawberry shape, hollow shape, and berry shape, could be formed if micro phase separation due to different reaction conditions, generally including inversion, occurs^[148-149].

Since the 1990s, the core-shell rubber particles have innovatively been used to toughen thermosetting epoxy resins^[150-151], opening a new way of toughening modification. The core-shell polymer particles have reached stronger toughening effect than ordinary liquid rubbers, while the glassy transition temperature (T_g) and mechanical properties of the matrix materials would not decrease significantly^[152]. The biggest characteristic of the core-shell rubber particles as the toughening agents of epoxy resins is that the particle size is only controlled by the polymerization process, not by the blending processing conditions. The core is usually polybutadiene (PB) rubbers, poly(n-butyl acrylate) (PBA), poly(styrene-co-butadiene) rubber, polyurethane^[153], poly(ester-co-ether)^[154] or polysiloxane elastomers, and the hard shell is poly(methyl methacrylate) (PMMA)^[155] or polystyrene. In the process of blending the core-shell rubber particles with the matrix, on the one hand, the shell plays a role in protecting the core, making the core basically maintain its original shape and size before and after blending, and the resin can form a complete "island" phase separation structure after curing. On the other hand, the shell can also have active functional groups, so that the core-shell rubber particles can be grafted onto the epoxy matrix, remarkably improving the compatibility and adhesion of the system^[156], and achieving the purpose of powerful toughening. It can be found that this core-shell toughening agent has strong designability and controllability and thus suitable core-shell polymer particles can be artificially designed according to different unique needs. The typical core-shell structure is shown in Scheme 3.

However, the toughening effect of core-shell particles is not perfect, because the effect is sensitive to several factors such as core-shell ratio, particle size, additive amount, compatibility with the matrix, and interface interaction force between the shell layer and the matrix. If PMMA shell layer is too thick, the core rubber will not work efficiently, consequently leading to poor toughening effect, and thus minor improvement of mechanical properties of the system. When the rubber layer is relatively thick, i.e., corresponding shell layer is thin, the sticky rubbery core layer cannot be completely wrapped, badly leading to the agglomeration of the core-shell rubber particles. Note that it is difficult to exfoliate the aggregated core-shell particles again for efficient toughening.

The smaller the particle size of the core-shell particles, the larger the contact surface among the particles and the matrix is because of larger specific surface area if the dispersion of the particles in the matrix is uniform and adequate. When bearing external force, more energy is absorbed mainly through shear yielding, and more sites block the crack, which accomplishes good toughening effect. If the particles are too small, especially to nanometer size, due to strong surface interaction, the particles will tend to agglomerate, which would lead to poor dispersibility of the nanoparticles and finally weak toughening efficiency. The particles with larger sizes can toughen the matrix by crack anchoring, leading to weaker toughening effect than the shear yield caused by smaller particles. The number of large particles with the same mass will also be much less than smaller particles. Therefore, the particle size range from 100 nm to 1 μm is appropriate, but the optimal size is between 200 nm and 500 nm. Lin et al.^[157] prepared a series of core-shell particles with PBA as the core and the copolymer of PMMA and glycidyl methacrylate (GMA) as the shell with different particle sizes. It is found that the toughening effect on epoxy resin was the most obvious when the particle size of core-shell particles was ca 300 nm.

The toughening effect significantly depends on the core-shell particle content in the matrix. The toughening efficiency could be the highest when the addition amount is appropriate. With increasing the addition amount, more particles only play the role of filler, resulting in excessive concentration of local stress, and thus inconspicuous toughening effect and finally reduced mechanical properties of the matrix. In fact, the presence of too many core-shell particles will also cause many problems, such as low heat resistance and poor processibility resulting from high mixing viscosity^[158].

The interface interaction between the core-shell particulate toughener and the matrix has a great impact on the performance of the toughened system. Wang^[159] prepared PBA/PMMA core-shell structure particles to modify epoxy resin. It was found that the introduction of active acrylate GMA into the shell layer can significantly improve the toughness. Ning et al.^[160] proposed Hansen solubility parameter theory to provide an effective strategy for designing core-shell nanoparticles to efficiently toughen epoxy resin. The Hansen solubility parameter can be useful to well predict the dispersion of core-shell particles in the matrix and their interaction with the interface of the matrix.

4 Mechanism of Emulsion Polymerization for Synthesizing Core-shell Rubber Particles

At present, there are three main mechanisms for the formation of core-shell latex particles: grafting mechanism, interpenetrating network mechanism, and ionic bonding mechanism^[161].

4.1 Graft Polymerization Mechanism^[162]

For core-shell emulsion polymerization, if one of the core and shell monomers is vinyl compound and the other is acrylate monomer, a graft copolymer transition layer can be formed between the core and shell layers. The formation of core-shell latex particles is carried out according to the graft polymerization mechanism. Min et al.^[163] investigated the influence of various process parameters in PBA/PMMA system on the graft reaction of MMA on PBA core particles during seed emulsion polymerization. The results showed that with the increase of initiator concentration, the content of graft copolymer decreased. With enlarging surface area of seed particles, the amount of MMA grafted onto PBA seeds would rise, while the relative molecular weight of grafting copolymer would decline, indicating that the grafting polymerization reaction was a surface-controlled process. The initiation point could be in the aqueous phase or the emulsifier adsorption layer, and the graft polymerization belonged to the chain-transfer reaction mechanism resulting from dehydrogenation at position a shown in Scheme 4.

4.2 Mechanism of Interpenetrating Polymer Networks^[164]

Interpenetrating polymer network is a kind of network structure formed by the interpenetrating and cross-linking of two kinds of polymer chains. The cross-linking agent is added into the core-shell emulsion polymerization system to result in cross-linking reaction within the core layer or between core and shell layers, generating the interpenetrating network polymers. The formation of the network can obviously improve the miscibility between core and shell layers like the graft copolymer.

4.3 Ionic Bonding Mechanism^[165]

The mechanism of ionic bonding is that the core layer and shell layer polymers are combined together by ionic bonds to form core-shell latex particles having stable interaction between core and shell layers. In order to prepare the latex particles, it is necessary to introduce comonomers that can generate ionic bonds, such as sodium styrenesulfonate and trimethylaminoethyl methacrylate chloride. It is reported that the composite polymer emulsion made from copolymer monomers containing ionic bonds can be used to control the formation of heterogeneous structure due to the introduction of foreign ions on different macromolecular chains that can strongly inhibit phase separation^[166].

In particular, these polymerization mechanisms could all be useful to design and prepare high-performance toughening agents for meeting the needs of efficient toughening of intrinsically brittle polymers such as epoxy resin ad PLA. Among the mechanisms, it seems that the first graft polymerization mechanism has reached remarkable merits. Recent research reports on the high-performance toughening agents based on core-shell particles are mainly focused worldwide.

5 Synthesis Method of Core-shell Rubber Particles

There have been several typical preparation methods of core-shell rubbery particles, such as self-assembly method^[167] and template method, but the most common method is seed emulsion polymerization, i.e., known as core-shell emulsion polymerization. The seeds in the seed emulsion can be divided into self-forming and external types. For the common self-forming seed method, the core seed is first prepared by the emulsion polymerization to obtain the seed emulsion, then the second monomer is added into the seed emulsion for the obtainment of the shell layer onto the seeds by surface polymerization, and finally the core-shell polymer particles are prepared, as illustrated in Scheme 5.

Seed emulsion polymerization is simple in operation, and can result in the formation of core-shell polymer particles with tunable and narrow-distributed particle size, shapes, and properties by adjusting the formula of the core-shell monomers. According to the addition way of shell monomer, emulsion polymerization methods can be divided into the following 4 types:

1) Batch method^[168-171]: After adding emulsifier, initiator, and butyl acrylate monomer into ionic water in a four-neck distillation flask, the emulsion polymerization at a certain temperature for a period of time would result in the formation of poly(butyl acrylate) (PBA) particles. Then styrene (St) monomer was added for grafting polymerization on the PBA particles. As a result, a PBA core-polystyrene(PSt) shell composite particles are prepared by this two-stage method. The investigation on the morphology of PBA/PSt core-shell latex particles indicates that the percent grafting of the core-shell latex polymers was dependent on the addition method of the second-stage shell monomer, e.g., equilibrium swelling process, semi-batch process, and batch process.

Seeded dispersion polymerization at 70℃ in ethanol overnight has been used to prepare core-shell side-chain liquid-crystalline polyacrylate particles with amorphous non-crosslinked poly(phenyl methacrylate) as core and poly[4-methoxyphenyl 4-(6-(acryloyloxy)hexyl)oxy-benzoate] as liquid-crystalline shell if using 2-methyl-1, 4-phenylene bis(4-(6-(acryloyloxy)hexyl)-oxy-benzoate) and 4, 4'-bis(6-acryloyloxyhexyloxy) azobenzene, PVP, and AIBN as crosslinker, stabilizer, and initiator, respectively^[171]. Almost monodisperse core-shell spherical particles with an average diameter of 1690 nm, a coefficient of diameter variation of 3.8%, and the thickness of ca 100 nm for the cross-linked shell (Fig. 5(a)-(d)) have been obtained. Moreover, the photoswitched azobenzene moieties in the liquid-crystalline shells by alternative irradiation of 365 and 455 nm light might be useful for drug delivery and soft actuator applications. Similar seeded dispersion polymerization at 70℃ in ethanol overnight is also successful to prepare core-shell side-chain liquid-crystalline polyacrylate particles with poly[4-methoxyphenyl 4-(6-(acryloyloxy)hexyl)oxy-benzoate] as liquid-crystalline non-crosslinked core and poly[4-methoxyphenyl 4-(6-(acryloyloxy)hexyl)oxy-benzoate-co-4-(6-acryloxyhexyl-1-oxy)benzoic acid] as liquid-crystalline crosslinked shell when using 2-methyl-1, 4-phenylene bis(4-(6-(acryloyloxy)hexyl)-oxy-benzoate), PVP, and AIBN as crosslinker, stabilizer, and initiator, respectively^[192]. Smaller binary liquid-crystalline core-shell spheres with an average diameter of 1380 nm and a coefficient of the diameter variation of 8.1% (Fig. 5(e), (f)) were acquired.

Zhao and Urban^[172] reported PSt/poly(BA-co-GMA) and PSt(38.3%)/Poly(BA(7%)-co-methacrylic acid (MAA)(0.74%)) core-shell latex particles containing epoxies or acid functional groups that are very vital in many applications because these reactive groups can facilitate crosslinking reactions between core-shell particles and their toughening epoxy resin matrix, thus significantly enhancing the impact strength, flexibility, toughness, and chemical resistance of the resulted modified epoxy resins. TEM micrographs of PSt latex seeds and PSt/poly(BA-co-GMA) core-shell latex particles by a semibatch emulsion polymerization of respective n BA-GMA comonomer and n BA-MAA comonomer onto PSt seeds are shown in Fig. 6. The core-shell morphology was satisfactorily realized, exhibiting a PSt core as dark phase, whereas a poly(BA-co-GMA) shell as lighter circular appearance. It is revealed that the blend film of the 1∶1 wt mixture of PSt/poly(BA-co-GMA) and PSt/P(BA-co-MAA) core-shell latex particle dispersion (15 wt%) in pure water demonstrated two glass transition temperatures at 10 and 120 ℃ attributed to poly(BA-co-GMA) and poly(BA-co-MAA) phase and PSt phase, respectively.

2) Semi continuous method^[173-176]: Core monomer, initiator, and emulsifier are added into water to polymerize to a certain extent, and then the remaining shell monomer and initiator are dripped at a certain rate at a certain time interval. That is to say, core-shell rubbery particles were fabricated via a semi-continuous seed emulsion polymerization that involved successive two steps of the 1^st core preparation and the 2^nd shell formation. We can obtain core-shell rubbery particles with various core sizes by the following three steps; a) preparing the primary seed latex by polymerization of n BA monomer; b) preparing the secondary latex by adding the monomer to the primary latex and subsequent polymerization; and c) preparing tertiary latex by using secondary latex^[176]. Starved semicontinuous emulsion polymerization has also been used to prepare cationic core-shell PSt/PMMA spheres with a core-shell diameter ratio of 1∶7.5^[177]. As shown in Fig. 7(a)-(c), cross-linked PSt/PMMA(80/20) seeds of 70 nm in diameter gave 530 nm particles in three steps of PMMA growth. Growth of the 320 nm PSt seed latex (Fig. 7(d)) gave almost monodisperse 730 nm PSt/PMMA core-shell latex (Fig. 7(e)).

Klein et al^[178]. reported shape-tunable highly cross-linked PSt core-PMMA shell microspheres by seeded emulsion polymerization of styrene for the preparation of the cross-linked PSt cores and seeded dispersion polymerization of MMA for the formation of the PMMA shell onto the PSt cores, as shown in Fig. 8. This may be greatly interesting because no photobleaching occurs, such that the microspheres or microparticles could be tracked in real space over a long time.

On the contrary to PSt core-PMMA shell particles, the structures of composite core-shell (PMMA/PSt) latex particles have also been prepared via staged emulsion polymerization and can be controlled or optimized by altering the thermodynamic and kinetic variables including particle surface polarity, stage ratio, core particle size, the mode of monomer addition, and the degree of crosslinking. This gives a new design direction of the core-shell particles when the core polymer is expected to be more hydrophillic than the shell polymer^[179]. It is reported that the shell-layer stability in the core-shell particles could be controlled by adjusting initiators for the polymerization of styrene onto PMMA seeds^[180]. TEM observation in Fig. 9(a), (b) reveals that if using K₂S₂O₈ initiator, there is a diffuse and irregular interface between the PSt shell and the PMMA core^[180]. The shell is made up of small spherical PSt particles. If choosing tert-butyl hydroperoxide initiator, there is a smooth and sharp interface between the PSt shell and the PMMA core(Fig. 9(b)).

It has been confirmed that the PSt content on the shell surface was higher in the core-shell particles formed with tert-butyl hydroperoxide (85%) than in those prepared using K₂S₂O₈ (50%). Fig. 9(c) shows that the PSt shell layers of the original particles prepared using K₂S₂O₈ initiator are nearly homogeneous and compact PSt domains containing only a few occasional PMMA cores, seen as bright spots in the dark PSt areas, i.e., the PSt particles are partly engulfed in PMMA. In Fig. 9(d), there are two particles that exist as 1) dark and compact PSt disks something like part of PSt disks in Fig. 9(c) and 2) dark rings slightly like rings in Fig. 9(a), (b). The dark PSt disks represent the sections cut through inverted core-shell particles or the upper or lower cap of the particles. The dark rings should imply sectioned core-shell particles only if the entire section is taken somewhere from below the upper to above their lower shell caps^[180]. There may be a kinetic barrier to phase rearrangement in the core-shell particles prepared using tert-butyl hydroperoxide retained their core-shell structure.

3) Pre-swelling method^[181-183]: The shell monomer was added to the core polymer emulsion system for the shell monomer to pre-swell the surface of the core polymer latex particles for a period of time at room temperature first and then heated up to the set temperature at which initiator was introduced to initiate polymerization. Therefore, the pre-swelling method can be chosen to obtain core-shell particles with not only a perfect structure, but also a transition layer of interpenetrating network at the interface between the core and shell layers due to the proper crosslinking between the core and the shell layers, resulting in an improved adhesion of the two-layer interface. This is because in the pre-swelling polymerization process, after the addition of shell monomers, there is a quite long swelling time. Therefore, the second shell monomer can not only be fully dispersed on the surface of the seed latex particles, but also penetrate the interior of the slightly cross-linked core polymer.

As an example, the pre-swelling preparation route to PBA/PMMA core-shell latex particles is as follows^[183]:

a) For the synthesis of PBA seed emulsion, 180.0 g nBA, 360.0 g ammonia, 1.800 g ethylene glycol dimethacrylate (EDMA), and a certain amount of sodium dodecylsulfate (SDS) were added into a round bottom flask and stirred for 0.5 h at room temperature to obtain BA pre-emulsion. Add 1/3 of BA pre-emulsion into a four necked bottle equipped with condenser tube, and heat it to 75 ℃ in nitrogen. After adding 0.45 g K₂S₂O₈ and reacting for 0.5 h, the remaining BA pre-emulsion and 0.45 g K₂S₂O₈ were dropped at the same time within 1 h. After polymerization for 2 h, the pH value of the polymerization system was adjusted to about 9 with ammonia water for standby.

b) Add 80.0 g MMA, 160.0 g ammonia, 0.800 g EDMA, and 0.320 g SDS into the round bottom flask to prepare MMA pre-emulsion. After the MMA pre-emulsion was dripped into 360.0 g PBA emulsion in a four-neck flask equipped with a condenser tube, the mixture solution was stirred at room temperature for 2 h for pre-swelling process. After elevating the solution temperature to 75 ℃ and adding 0.400 g K₂S₂O₈, the polymerization reaction of MMA monomer occurs in nitrogen for 2 h^[183], accomplishing PBA core-PMMA shell particles.

4) Semi continuous pre-emulsification method^[184-187]: A mixture of core monomer, emulsifier, crosslinker, and water is fully stirred to form a pre-emulsion. After the initiator was added into 1/3 of the pre-emulsion, the solution obtained thus was heated to ca. 80℃. Upon the solution turned to blue color that implies the formation of the nanosized seed latex in the reaction system, and then the initiator and remained 2/3 pre-emulsion were dropped into the hot and blue solution at a constant rate within ca. 3 h. PBA seed emulsion has been prepared by the semi continuous pre-emulsification method. A detailed example is given below: The mixture of 100 g BA, 2 g nonylphenol polyoxyethylene ether, 1 g SDS, 2 g tripropylene glycol dihydrate acrylate and 58 g water was vigorously stirred for 30 min to obtain BA pre-emulsion. 1/3 of the BA pre-emulsion, (NH₄)₂S₂O₈ initiator (0.125 g) and 58 g water were added into a four-necked flask for polymerization at 82 ℃, and (NH₄)₂S₂O₈ (0.125 g) initiator was added into the flask. The polymerization lasts for 30 min after the system becomes blue. Then the residual 2/3 pre-emulsion was dropped into the blue reactive system within the dropping time of 2.5-3.5 h. After dropping, keep it warm for 1 h, and then heat it up to 90 ℃ for 1 h. After the reaction, let the emulsion cool to room temperature for standby. PBA core-PMMA shell latex particles by in-situ emulsion polymerization are prepared as follows: 53.5 g PBA seed emulsion obtained above, 32 g MMA, and 0.32 g tripropylene glycol dihydrate acrylate was mixed by high-speed stirring for 30 min to obtain pre-emulsion; 1/3 pre-emulsion, (NH₄)₂S₂O₈ initiator (0.08 g) and 73.58 g water was placed in a four necked flask and polymerized at 82 ℃. When the system emitted blue light, 30min later, another (NH₄)₂S₂O₈ initiator (0.08 g) was added into the flask. At the same time, the remaining 2/3 pre-emulsion was dripped by a constant pressure drop funnel for the dropping time of 2.5-3.5 h. After dropping, keep the temperature for 1 h, and then heat it to 90 ℃ for 1 h. Finally, the emulsion formed allows to cool down to room temperature, break the emulsion, separate, dry, and grind the solid white product to obtain powdered PBA/PMMA core-shell particles^[186].

6 Acrylates-based Core-shell Rubbery Particles

Acrylates-based CSR particles are a series of core-shell particles totally based on acrylates and their derivatives. The most representative is PBA core-PMMA shell acrylic copolymers through emulsion polymerization and post-treatment with BA, cross-linker, and MMA as starting reagents. PBA latex particles as the core is a rubbery state with lower T_g, which is equivalent to introducing flexible chain segments into the system during epoxy oligomer curing. The molecular chains of the cured product become soft or flexible, resulting in enhanced toughness. High T_g PMMA grafted on PBA core surface is conducive to the separation of core-shell particles from the resulted emulsion, improving their compatibility with epoxy resin and then enhancing the interfacial force, i.e., PBA core-PMMA shell particles are good impact modifiers for usually brittle epoxy resins^[188].

There are many reports on the synthesis of acrylates-based core-shell rubbery particles for significantly improved toughness of epoxy resins, PLA, and also other traditional polymers^[189-193]. Lee et al.^[189] reported two-stage soap-free emulsion polymerization without adding or only adding a small amount of emulsifier of BA and MMA monomers with potassium persulfate as initiator to prepare core-shell rubbery particles. The results show that the reaction rate and the number of core-shell rubbery particles will increase with the increase of initiator concentration, but the molecular weight of the polymers will decrease. It is of interest that the reaction rate and the average molecular weight of the second stage PMMA also increase with the increase of BA/MMA concentration ratio. Furthermore, uniform core-shell rubbery particles can be obtained by this method, but the stability of the emulsion system might be relatively low or the latex particles may agglomerate to some extent.

Zhu et al.^[190] have prepared acrylic core-shell rubbery particles by traditional three-steps emulsion polymerization of nBA, MMA, EDMA in the presence of K₂S₂O₈ initiator and sodium dodecyl-benzenesulfonate emulsifier in nitrogen. The 1^st step is batch emulsion polymerization of nBA at 75 ℃ for 2 h for synthesizing the PBA seed latex with the volume-average particle diameter of 86 nm by the dynamic light-scattering method and size polydispersity index of 1.09, the 2^nd step is semibatch seeded emulsion polymerization of nBA at 75℃ for 4.5 h for the obtainment of the volume-average particle diameter of 187 nm and size polydispersity index of 1.04, and the 3^rd step is seeded emulsion polymerization of MMA at 80℃ for 4.5 h for the preparation of the resulted core-shell particles with a core-shell ratio of 70∶30 wt/wt, the volume-average particle diameter of 217 nm, and size polydispersity index of 1.16.

Fu et al.^[191] synthesized core-shell rubbery particles consisting of a rubbery cross-linking PBA core and a rigid PMMA shell with a whole diameter of about 352 nm through seed emulsion polymerization in the NaHSO₃-K₂S₂O₈ redox initiator system. It is revealed that the polymerization had a very high instantaneous conversion of > 93% and overall conversion of 99%. The core-shell rubbery particles with core-shell weight ratio 80/20 are efficient modifier to toughen poly(butylene terephthalate) (PBT) by melt blending. It should be noticed that in this synthesis process, redox initiator will introduce electrolyte ions that might destroy the stability of emulsion, consequently leading to a small number of agglomerates in the final core-shell rubbery particles.

7 Core-shell Rubbery Particles as Powerful Toughener of Epoxy Resins and PLA

Some investigations have been reported on the toughening modification of epoxy resins and PLA by core-shell rubbery particles. It is believed that core-shell rubbery particles dispersed in epoxy resin matrix with rubbery PBA core as stress concentration body can induce many fine crazes and shear bands to dissipate system energy, and timely stop their excessive expansion without causing cracks. In addition, if the deformation of core-shell rubbery particles forming shear band near the material notch is too large, the core-shell particles will gradually form holes, which will significantly dissipate the stress concentration effect near the notch and induce shear yield, so as to achieve good toughening effect. In general, PBA core size, particle size and distribution, core-shell ratio, and shell structure^[192-193] are important factors affecting the toughening effect of core-shell rubbery particles on epoxy resin.

Zhang et al. ^[192] synthesized acrylic core-shell nanoparticles with PBA as the core and poly(MMA-co-GMA) copolymer as the shell by emulsion polymerization. TEM observation indicates that core-shell nanoparticles with BA/MMA/GMA(80g/20g/10g) ratio are nearly spherical, with a particle size of 50-100 nm. Particularly, the epoxy resins toughened with 10 wt% core-shell nanoparticles demonstrate simultaneously improved tensile and impact strengths of 13.5% and 59.7%, respectively, over the unmodified epoxy networks. Fortunately, apparent improvement in impact strength has not been accompanied with reduction of thermal resistance.

It has been further verified by PB core-PSt shell (grafted on PB core) rubber particles as impact modifiers to toughen PSt^[194]. Deng et al. discovered that both the elongation at break and impact strength of the melt injection molding specimens of the PB core-PSt shell rubber particles blended into PSt increased significantly first and then decline slightly with the increase of PB core size from 100 nm to 450 nm, exhibiting a maximal elongation at break of 44 % and maximal impact strength of 208 J/m at the PB core size of 300 nm shown in Fig. 10(a), (b), regardless of a slight decrease in tensile strength of high impact PSt/PB-g-PSt/PSt blends when enlarging rubber particle size. Simultaneously, a concurrent decrease of T_g and brittleness with increasing core size from 100 nm to 300 nm in Fig. 10(b) also confirms the highest blend toughness at the core size of 300 nm. So high toughening efficiency is apparently ascribed to the uniform dispersion of the core-shell particles on the fracture surfaces of the melt blends that have been observed in Fig. 10(c), (d).

Antonino et al. reported the epoxy resins based on bisphenol A diglycidyl ether toughened by PB/PMMA (core-shell) spherical particles with an average diameter of 100 nm^[195]. It was discovered that the elongation at break, shear strength, T-peel strength, and fracture energy were apparently enhanced to 6.37 %, 25.02 MPa, 9.14 N/mm, and 850 J/m² at the core-shell particle content of 30 wt% compared with 3.87 %, 20.53 MPa, 2.78 N/mm, and 156 J/m² respectively for corresponding pure epoxy resin. That is to say, the increase in toughness resulting from the 30 wt% addition of PB/PMMA (core-shell) rubber particles was up to 550%.

Su et al. ^[196] investigated toughening efficiency of hot-mix epoxy asphalt binders modified by 2 wt% core-shell rubbery spheres containing two core polymers i.e, PB core and poly(styrene-co-butadiene) core and the same PMMA shell with average diameters of 100-200 nm and 85-115 nm, respectively. It is found that the composite of the epoxy modified by PB core-PMMA shell spheres exhibits the highest tensile strength of 4.59 MPa, the 2^nd highest elongation at break of 442 %, and the highest toughness of 1190 MJ/m³. The composite of the epoxy modified by poly(styrene-co-butadiene) core-PMMA shell spheres exhibits the 2^nd highest tensile strength of 3.60 MPa, the highest elongation at break of 468 %, and the 2^nd highest toughness of 1020 MJ/m³. In other words, the tensile strength of the neat epoxy (3.0 MPa) increases by 53% and 20%, respectively after introducing PB/PMMA and poly(styrene-co-butadiene)/PMMA core-shell spheres, the elongation at break of the neat epoxy asphalt binders (309 %) increases by 42% and 51%, respectively, and the toughness of the neat epoxy asphalt binders (564 MJ/m³) increases by 110% and 82%, respectively. One earlier article depicted that the composite of the epoxy modified by 1 wt% PB core-PMMA shell spheres exhibits the even higher elongation at break of 545% that is 60% higher than the pure epoxy asphalt binders (341%)^[197].

Hao et al.^[141] discovered an efficient grafting/crosslinking inhibitor, i.e., tert-dodecyl mercaptan (TDDM), of PB core. As shown in Fig. 11(a), (b), the T_g and yield strain of the 60 wt% PB core (300 nm diameter)/40 wt% styrene-acrylonitrile-GMA(75∶25∶1) terpolymer shell decrease monotonically from -78.7℃ to -85.0℃ and 7.2 to 5.9% respectively with increasing TDDM content from 0 to 2.13 wt%, while the elongation at break increases monotonically from 90 to 296 %. Particularly, at the TDDM content of 1.18 wt%, the blend T5 of PLA(80 wt%) with the reactive core-shell particles (20 wt%) behaved the maximum elongation at break, tensile ductility, and impact toughness in Fig. 11(c), (d), (e), which was 30 times higher than the pristine PLA and 3 times higher than PLA/RCS-T0 blend, respectively. The greatly improved cavitation ability of RCS particles should be attributed to appropriate ability of inhibiting the grafting/crosslinking reactions from TDDM through chain transfer effect, achieving the high toughening ability of RCS particles and super-tough PLA blends^[141].

Liu et al.^[142] prepared a slightly cross-linked PB core (25 wt%) that was grafted with PMMA shell (75 wt %) with the CSR particle diameter of 150-180 nm. After adding PB core-PMMA shell particles into PLA, PLA/CSR(85/15 wt) blend obtained exhibits a high elongation at break (122%) that is higher than pure PLA (5%).

Lee et al.^[176] reported that crosslinked PBA(81%)-PMMA(19%) core-shell particles would enhance the elongation at break to 235% that is much higher than neat PLA (15%).

Petchwattana et al.^[143] reported very high toughening efficiency of a commercial white powder of poly(methyl methacrylate-co-ethyl acrylate) core-shell rubber particles with specific gravity of 0.48-0.56. It is discovered that PLA/tributyrin (as plasticizer)/poly(methyl methacrylate-co-ethyl acrylate) CSR particles(80/5/15 wt) demonstrates the maximum strain values at break of 425% that is much larger than neat PLA (8%).

Ning et al.^[198] prepared soft PBA core-rigid PMMA shell (50/50) spherical nanoparticles (100 nm diameter) crosslinked by EDMA via aqueous emulsion polymerization. It is suggested that the nanoparticles can keep a discrete dispersion state when they were transferred into epoxy oligomers by an organic phase(2-butone)-transfer method. They discovered that the addition of these nanoparticles of 10 wt % hardly ever influences the cure and glass transition temperature of the epoxy resin, but exhibits an impressive toughening effect on the epoxy matrix, including 3.5 times higher elongation at break(9.8%) and 9.5 times higher critical strain energy release rate (1283 J/m²) than neat epoxy resin regardless of slightly lower tensile strength and modulus. Clearly observed stress-induced whitening and cold drawing during tensile straining measurement should confirm the significant enhancement in fracture toughness resulted from the incorporation of the core-shell particles. Obviously, multilayer acrylic core-shell polymer particles with variable reactive-shell compositions made by a sequential emulsion polymerization are useful to successfully enhance the toughness of epoxy resins with fracture toughness of 890 J/m² for the toughened epoxy-matrix composite compared with 500 J/m² for the unmodified composite^[199].

Wang et al.^[200] synthesized and prepared a soft PBA core-rigid PMMA shell (40/60) spherical nanoparticle (110 nm diameter) and then added them to AG-80 epoxy oligomers to study the improvement of system toughness. The results exhibit an efficient toughening effect on the epoxy matrix, including 1.91 times higher impact strength (1.11 kJ/m²) than pure epoxy resin (0.58 kJ/m²), and other properties remained basically stable. The maximum impact strength of 1.13 kJ/m² was observed for 10 wt% core-shell rubbery particles modified diglycidyl ether of bisphenol A epoxy, revealing an increase by a factor of 11.4 above the unmodified epoxy (0.1 kJ/m²)^[201].

He et al.^[202] used commercial PBA core-PMMA shell particles (EM500A, LG chemical) with respective average diameters and densities of 400-600 nm and 1.07-1.08 g/cm³ to improve the mechanical properties of DGEBA epoxy resin (YD-128). The results show that the tensile strength, Young's modulus, and impact strength of epoxy resin composite containing 0.5 wt% core-shell particles achieve the maximum at 77 K and room temperature. Compared with the raw epoxy resin, these properties of the modified composite at 77K rise by about 17%, 18%, and 26%, respectively. Particularly, the epoxy resin composite containing 0.5 wt% core-shell particles has the highest dimensional stability or the lowest coefficient of thermal expansion of 6.14×10^-5 ℃^-1 below their glass-transition temperature of 75.6 ℃. Compared with the neat epoxy resin, the epoxy resin modified by 0.5 wt% core-shell particles exhibited the 12.9 % reduction of coefficient of thermal expansion.

Quan et al.^[203] used two types of particles including acrylic rubber core-PMMA shell particles with whole diameter of 203 nm and 16.9 nm shell thickness as well as butadiene acrylic copolymer nanoparticles with the diameter of 74.1 nm to toughen diglycidylether of bisphenol A epoxy (Epon828 from HEXION) with an epoxy equivalent molecular weight between 185 and 192 g/eq using dicyandiamide curing agent. It is depicted that the epoxy plate modified by 22 vol% core-shell particles and 16 vol% poly(butadiene-co-acrylate) nanoparticles has the largest elongation at break of 17.2%, while the 30 vol % core-shell rubbery particles-modified epoxy achieves the maximal fracture energy of 2671 J/m², due to the shear band yielding and plastic void growth as possible toughening mechanism.

Tsang et al.^[204] reported toughened epoxy resin with all-acrylic PBA core-shell rubber particles with the diameter of 300 nm to 50 μm (Paraloid EXL-2300G, Rohm and Haas, UK) as impact modifier, and found that the core-shell rubbery particle content of 10 wt% in epoxy resin results in the highest strain to failure and fracture energy of 3.5% and 108 J/m², respectively.

The PBA core-PMMA shell rubbery particles prepared by Wang et al.^[205] are basically spheric shapes with diameter of 200 nm and are well compatible with epoxy resin. The particles can be effectively dispersed in the epoxy matrix, accomplishing 1.48 times enhancement of impact strength at the PBA/PMMA core-shell rubbery particle content of 3 wt% compared with pure epoxy resin. The impact toughness of the epoxy system was obviously improved and the fracture morphology is ductile fracture. Recently, polysiloxane core-PMMA shell particles shown in Fig. 12 have been synthesized for toughening photocurable epoxy oligomers as advanced stereolithography resins with both high thermal stability and high transparency comparable to the resin without toughening agent^[206]. The elongation at break, impact strength, and fracture toughness of the epoxy-based UV-curable stereolithography resins containing 5 wt% polysiloxane core-PMMA shell nanoparticles decreased but their T_g increased with the increase of shell thickness. The resins toughened by the smallest core-shell particles exhibited the largest elongation at break of 12.84 % with the highest improvement of 797.9 %, the highest impact strength of 2.75 kJ/m² with the highest improvement of 65%, the highest fracture toughness of 5.28 MPa/m² with the greatest improvement of 218%, and the lowest T_g of 101.4℃ with the greatest reduction compared to the results of pure resin of 1.43%, 1.67 kJ/m², 1.66 MPa/m², and 108.2℃, respectively. It is suggested that the epoxy resins containing 5 wt% polysiloxane core-PMMA shell nanoparticles possessing the outstanding toughness can be used in 3D printing materials including self-tapping screw and micro-fluidic devices because the introduction of the core-shell particles has nearly no effects on the transparency, printing accuracy, and dimensional stability. By the way, it is reported that 4 wt% silicone core-glassy shell rubber particles with mean diameter of 180 nm have been added to toughen diglycidyl ether of bisphenol A epoxy, demonstrating higher elongation at break than original epoxy^[207].

Table 1 systematically summarized and compared the toughening efficiency of core-shell rubber particles to epoxy resin, polylactide, and polystyrene. It is found that a high core-shell particle content up to 15-30 wt % can afford the formation of the toughened PLA with a 3-50 times improvement of elongation at break and impact strength, unfortunately leading to a sharp decline of the corresponding tensile strength. If 0.5-30 wt% core-shell particles are added, the tensile strength of the modified epoxy resins would not decrease dramatically, but their elongation at break and impact strength thus would just rise slightly or maintain at absolutely low value. Particularly, their fracture toughness may increase significantly upon introducing the core-shell rubber particles into epoxy resins. Furthermore, it is reported that a reinforcing agent like carbon black nanoparticles could be required for basically maintain original tensile strength, realizing simultaneously toughening and reinforcing effects. In a word, the toughening efficiency of core-shell rubber particles varies significantly with changing the toughened matrix from epoxy resin to polylactide.

Very recently, a unique core-shell latex particle has been designed and prepared based on BA/MMA/St/DVB(24.9/16.6/8.3/0.06) as core monomers and BA/MMA/St/GMA/hydroxyethyl methacrylate/2-ethylhexyl acrylate/acrylic acid/DVB(18.3/10.0/8.0/5.0/5.0/2.0/1.7/0.14) as shell monomers in the presence of (NH₄)₂S₂O₈ initiator^[208]. Especially, when a special octafluoropentyl methacrylate(6.65 wt%) was added as the 9th monomer of the shell monomers, the core-shell latex particles-based coated film obtained thus exhibits simultaneously the strongest water resistance, the highest thermostability, the largest elongation at break(303.8%), the highest tensile strength (7.77 MPa), and the highest Young's modulus (155.73 MPa). It could be predicted that the toughening ability of the intrinsically tough core-shell elastic particles to brittle resins would be high.

8 Mechanism of Toughening Epoxy Resin from Core-shell Rubbery Particles

So far, several theories including rubber cavitation shear yield^[141], secondary transition temperature, direct energy absorption, crack core, multiple craze, shear yield, craze shear band, craze branching, and percolation^[209-211] have been proposed to describe the properties and toughening mechanism of core-shell rubbery particles in brittle polymers such as epoxy resins. However, the rubber cavitation shear yield^{[141, 212]} is the main mechanism for core-shell rubbery particles to efficiently enhance the energy absorption of epoxy resin under load.

When the composite system is subjected to an external force, the second phase particles are subject to both the hydrostatic tension and the three-dimensional static field. These two forces would cause the second phase to separate from the epoxy resin matrix, thus creating holes. Under the action of triaxial tensile stress, the growth of voids in core-shell rubbery particles in their toughening materials can be observed. Cavitation in the rubber particles will weaken and disperse the stress at the crack tip of the epoxy phase contacted. Secondly, the triaxial tensile stress state of thin ligaments in the interspace of epoxy resins changes to the uniaxial tensile stress state, and this new stress state induces the shear band. In fact, the role of core-shell rubbery particles is to generate internal cavities, reduce hydrostatic tension, and finally start the ductile shear yield mechanism ^[213]. The interaction between the stress field in front of the crack and core-shell rubbery particles results in the stress concentration in the surrounding matrix, so that a shear band or deformation zone appears in the shear yield mechanism. The enhancement of fracture toughness depends on the number of these particles: the larger the number of core-shell rubbery particles, the more the deformation areas before fracture. The micron sized core-shell rubbery particles can effectively improve the tensile shear yield of epoxy resins and cause plastic deformation ^[214]. The microscopic study shows that, compared with the undeformed core-shell rubbery spherical particles, the deformed core-shell rubbery spherical particles at the crack tip extend to the same size as the epoxy matrix, which also confirms the occurrence of plastic deformation^[215]. The internal cavitation within core-shell rubbery particles eliminates the plane strain constraint by reducing the modulus, and the generated concentrated deviatoric stress is sufficient to produce shear yield. Cavities formed by cavitation core-shell rubbery particles play a role in additional stress concentration^{[45, 216]}.

9 Conclusions

The artificial designability of core-shell particles like soft core-hard shell polymer particles and their good compatibility with matrix resins result in their wide applicability. Specially, their powerfully toughening effect to brittle polymer materials including epoxy resin and polylactide and even tough poly(butylene terephthalate) will make traditional materials become advanced materials with high performance, multifunctionality, and high added value. With the further development of new core-shell particles as highly efficient and multifunctional toughening agents, in-depth understanding of the toughening mechanism, and progress in the constantly improved material genome technology, wholly new toughening composite materials will be developed for more important and versatile applications as high-technological materials with many vital properties and functionalities including flexibility, strong adhesion, good processibility, low curing molding shrinkage, water resistance, chemical stability, dimensional stability, durability, powerful antibacterial property, heat conduction, electrical conductivity, high dielectric property, strong resistance to surface leakage and arc, wave absorption, electromagnetic shielding, damping, and shock absorption.

The problems existing in the study on toughening epoxy resin and PLA with core-shell rubber particles may be that a high core-shell particle content up to 15-30 wt % is necessary for a 3-50 times improvement of elongation at break and impact strength of modified PLA, unfortunately leading to a sharp decline of the corresponding tensile strength, as summarized in Table 1. If 0.5-30 wt% core-shell particles are added into epoxy resins, the tensile strength would not decrease dramatically, the elongation at break and impact strength would just rise slightly or maintain at absolutely low value, but the fracture toughness increases significantly. Thus, a reinforcing agent like carbon black or graphene^[217-218] could be required for basically maintain original tensile strength, realizing simultaneously toughening and reinforcing effects. Besides, it seems that most of the rubbery cores in current core-shell particles are too weak in a non-crosslinked state or slightly rigid something like glassy shell in a crosslinked state because it is not easy to obtain uniform and also appropriate crosslinked cores. Therefore, the authors suggest that thermoplastic poly(ester-co-ether) elastomers without chemical crosslinking sites are worth trying as hard elastic core materials for further research on toughening epoxy resin and PLA.