Characterization of the particle–matrix interface in rubber-modified epoxy by atomic force microscopy

Atomic force microscopy (AFM) was used to study the interphase regions in rubber-toughened epoxy polymers. The nature of the interphase region was varied by either the adducting of reactive oligomers or by crosslinking the shell polymer on core/shell latex particles. The adducted reactive oligomers were comprised of carboxyl-terminated, butadiene-actrylonitrile copolymers (CTBN) prereacted with either (1) a low molecular weight diglycidyl ether of bisphenol A-based epoxy, which results in an interphase with increased crosslink density, or (2) a high molecular weight epoxy based on the diglycidyl ether of propylene glycol (DEGP), which results in an interphase with decreased crosslink density. The second type of rubber particles is custom-made submicron core/shell latex particles of a poly(butadiene-co-styrene)[P(BS)] core with an acrylate shell. Two acrylate shells were (1) PMMA/AN shell containing 25% acrylonitile and (2) a similar PMMA/AN with 5% divinyl benzene. The toughness of these blends was characterized using linear elastic fracture mechanics. The features on the fracture surfaces were examined using both AFM and FESEM (field emission scanning electron microscopy). AFM was able to detect features not observed in SEM and also to quantify all of the fracture surface features. In particular, the height-to-width ratio of the rim surrounding cavitated particles provided a useful means for determining the ductility of the interphase region. Attempts were made to determine the size of the interphase region using the frictional mode and the tip-adhesion forces. Unfortunately, the results of both approaches are inconclusive at the present time; this is most likely due to the deformation surrounding the rubber particles detected in the fast fracture regions. © 1995 John Wiley & Sons, Inc.     

Structure and properties of epoxy resins modified with acrylic particles

The morphology and material properties of dicyandiamide (DICY)‐cured epoxy resins modified with acrylic particles consisting of a PBA (polybutyl acrylate) core and a PMMA (polymethyl methacrylate) shell and epoxy resins modified with acrylic rubber (PBA) particles alone were studied. It was found that the epoxy system modified with core/shell acrylic particles showed higher fracture toughness, indicating that the modification had a larger effect on improving the material properties of the epoxy resin. A characteristic shown by the core/shell acrylic particles is that they swell along with the epoxy resin under exposure to heat and gel before the latter cures. In this process, the epoxy resin penetrates the surface of the shell layer and a bond is formed between the epoxy matrix and the core/shell acrylic particles. This suggests that the epoxy matrix around the core/shell acrylic particles has the effect of increasing the level of energy absorption due to plastic deformation of the matrix. This is thought to explain why the epoxy resin modified with core/shell acrylic particles showed higher fracture toughness. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 74: 2955–2962, 1999     

Structure and adhesive properties of epoxy resins modified with acrylic particles

The morphology and physical properties of dicyandiamide (DICY)-cured epoxy resin modified with acrylic particles were studied. We used one homopolymer of methyl methacrylate (MMA) and three copolymers of MMA and glycidyl methacrylate (GMA) [P(MMA-GMA)] containing different amounts of GMA as the acrylic particles. When a mixture of the acrylic particles and the epoxy resin was heated, the particles were swollen with the epoxy resin and thus a soft gel with no fluidity was formed. Further heating to the reaction temperature of the DICY cured the soft gel. The structure of the acrylic particles strongly affected the physical properties of the soft gel and the cured epoxy resin. The cured system containing 5 mol% of GMA showed the best physical properties (impact strength and adhesive property), but there was a tendency for the physical properties to decline with a higher GMA content. We have determined that the GMA content of the acrylic particles affects the concentration of network chains in the system.     

耐水型环氧树脂增韧剂的制备及应用

用乳液聚合法合成了烷基化纳米二氧化硅/甲基丙烯酸甲酯/丙烯腈核壳型复合弹性粒子,并用于增韧环氧树脂。核壳粒子的形态由透射电镜观测,改性试样的断裂表面由扫描电镜观测,并对该增韧剂进行了实际应用的研究。结果表明:在复合粒子的添加量为10phr时,能大幅度提高环氧树脂的韧性及耐水性。     

ACR-g-GMA的制备及对PBT的增韧

要采用乳液聚合方法合成了以丙烯酸丁酯(BA)为橡胶相内核,甲基丙烯酸甲酯(MMA)为壳层,并在壳层接枝甲基丙烯酸环氧丙酯(GMA)的核壳结构聚合物(AcR-g-GMA)。用其增韧聚对苯二甲酸丁二醇酯(PBT),制备PBT/ACR-g-GMA合金。用傅立叶变换红外光谱考察接枝聚合物的环氧基团;用电子显微镜观察共混物中粒子分布的微观形态;测试了共混物的力学性能。结果表明:采用乳液聚舍方法能够将GMA接枝到ACR上,GMA可以增强两相间的界面结合力,ACR-g-GMA粒子能有效地增韧PBT。当ACR-g-GMA粒子中GMA的质量分数为3%,m(PBT)/m(ACR-g-GMA)为80/20时,共混物的缺口冲击强度可高达389 J/m。     

Effect of crosslink density on fracture behavior of model epoxies containing block copolymer nanoparticles

Model diglycidyl ether of bisphenol-A based epoxy resins containing well-dispersed 15 nm block copolymer (BCP) nanoparticles were prepared to study the effect of matrix crosslink density on their fracture behavior. The crosslink density of the model epoxies was varied via the controlled epoxy thermoset technology and estimated experimentally. As expected, it was found that the fracture toughness of the BCP-toughened epoxy is strongly influenced by the crosslink density of the epoxy matrix, with higher toughenability for lower crosslink density epoxies. Key operative toughening mechanisms of the above model BCP-toughened epoxies were found to be nanoparticle cavitation-induced matrix shear banding for the low crosslink density epoxies. The toughening effect from BCP nanoparticles was also compared with core-shell rubber-toughened epoxies having different levels of crosslink density. The usefulness of the present findings for designing toughened thermosetting materials with desirable properties is discussed.     

Core-shell particles designed for toughening the epoxy resins. II. Core-shell-particle-toughened epoxy resins

The diglycidyl ether of bisphenol A–m-phenylene diamine (DGEBA–MPDA) epoxy resin was toughened with various sizes and amounts of reactive core-shell particles (CSP) with butyl acrylate (BA) as a core and methyl methacrylate (MMA) copolymerized with various concentration of glycidyl methacrylate (GMA) as a shell. Ethylene glycol dimethacrylate (EGDMA) was used to crosslink either core or shell. Among the variables of incorporated CSP indicated above, the optimal design was to obtain the maximum plastic flow of epoxy matrix surrounding the cavitated CSP during the fracture test. It could be achieved by maximizing the content of GMA in a shell-crosslinked CSP, the particle size, and the content of CSP in the epoxy resin without causing the large-scale coagulations. The incorporation of reactive CSP could also accelerate the curing reaction of epoxy resins. Besides, it was able to increase the glass transition temperature of epoxy resins if the particle size ≤0.25 μm and the dispersion was globally uniform. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2313–2322, 1998     

Novel polymer composites based on a mixture of preformed nanosilica‐filled poly(methyl methacrylate) particles and a diepoxy/diamine thermoset system

In this work, a new material based on an epoxy thermoset modified with a thermoplastic filled with silica nanoparticles was investigated. When thermoplastic particles are filled with nanoparticles with unique properties such as high efficiency for absorbing ultraviolet light, electric or magnetic shielding, high electrical conductivity, and high dielectric constants, more than an enhancement of the mechanical properties is expected to be achieved for modified epoxy‐based thermosets. Particles of poly(methyl methacrylate) (PMMA) filled with silica nanoparticles were used to modify a thermoset based on a full reaction between diglycidyl ether of bisphenol A and 3‐(aminomethyl)benzylamine. When the preformed thermoplastic particles were mixed with the reactive constituents of the epoxy system under certain curing conditions in which total miscibility was avoided, uniform particle dispersions could be obtained. The relationships between the composition, morphology (nanoscale and microscale), glass‐transition temperature, mechanical properties, and fracture toughness were considered. Four main results were obtained for consideration of the potential of silica‐filled PMMA as an important modifier of brittle epoxy thermoset systems: (1) a good dispersion of the silica nanoparticles in the PMMA domains, (2) a good dispersion of the silica‐filled PMMA microparticles in the epoxy matrix, (3) the possibility of partial dissolution of the PMMA‐rich domains into the epoxy system, and (4) a slight increase in properties such as the hardness, indentation modulus, and fracture toughness. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Effects of particle size and rubber content on fracture toughness in rubber-modified epoxies

The effects of particle size of core-shell rubber on the fracture toughness of rubber-modified epoxies were investigated. Various sizes of core-shell rubber particles, from 0.16 to 1.2 μm in diameter, were synthesized by seeded emulsion polymerization. Particle size effects were clearly seen for lower crosslinked diglycidyl ether of bisphenol A (DGEBA)/piperidine resin. Fracture toughness increased as the particle size of core-shell rubber decreased from 1.2 to 0.4 μm. On the other hand, fracture toughness was constant in this range of particle sizes for higher crosslinked DGEBA/diaminodiphenylmethane (DDM) resin. Cavitation in the rubbery core and shear deformation in the matrix are the toughening mechanisms for DGEBA/piperidine resin, whereas cavitation is the only mechanism for DGEBA/DDM resin. Toughening effectiveness decreased with <0.2 μm core-shell rubber particles since they are difficult to cavitate. The effects of core-shell rubber content on fracture toughness of rubber-modified epoxies were also examined. The optimum rubber content for maximum toughness of rubber-modified epoxies decreased with decreased particle size of core-shell rubber in shear deformable DGEBA/piperidine resin. But the fracture toughness of rubber-modified DGEBA/DDM resins increased as the rubber content increased.     

Internal stress of epoxy resin modified with acrylic core-shell particles containing functional groups prepared by seeded emulsion polymerization

In order to reduce the internal stress in a cured epoxy resin, submicrometer-sized poly(butyl acrylate) (PBA)/poly(methyl methacrylate) (PMMA) core-shell particles having cross-links were dispersed in the resin prior to curing. For the introduction of cross-links, monoethylene glycol dimethacrylate or glycidyl methacrylate monomer was copolymerized. Cross-links in the PBA core reduced the shrinkage of the cured epoxy resin, and cross-links at the PMMA shell produced a strong interaction with the epoxy matrix. The internal stress was reduced effectively by the introduction of cross-links.     

Preparation of toughened PMMA through PEG-modified urethane acrylate/PMMA core–shell composite particles

Poly(urethane acrylate) (PUA)/poly(methylmethacrylate) (PMMA) core–shell composite particles were prepared by two-stage emulsion polymerization. The sizes of composite particles could be varied from 25 to 210 nm by introducing polyoxyethylene (POE) groups to the urethane acrylate molecular backbone. Core–shell morphology was identified by investigating the polarity of the surface of the core and shell polymer particles and by measuring the contact angle of the composite particles. A composite particle prepared with relatively small particles (about 20 nm) did not show the core/shell morphology, because the high polar surface of the core polymer particle and the low-stage ratio of the core to the shell cause the formation of a core/shell two-stage latex to be more thermodynamically unstable. The fracture toughness of rubber-toughened PMMA containing PUA/PMMA composite particles increased as the particle sizes decreased and the shell thickness of the composite particles increased. In particular, when the average size of the composite particle was about 43 nm and the stage ratio was 50/50, the fracture toughness of the rubber-toughened PMMA increased more than three times compared with that of pure PMMA. Furthermore, the transparency of toughened PMMA could be maintained up to 91% in the visible spectra range. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 2291–2302, 1998     

Effect of core-shell particles dispersed morphology on the toughening behavior of PBT/PC blends

Methyl methacrylate-co-styrene-co-glycidyl methacrylate grafted polybutadiene (PB-g-MSG) and styrene-co-glycidyl methacrylate grafted polybutadiene (PB-g-SG) core-shell particles were prepared to toughen poly (butylene terephthalate) (PBT) and polycarbonate (PC) blends. The compatibilization reaction between the epoxy groups of glycidyl methacrylate and the carboxyl groups of PBT induced the PB-g-SG particles dispersed in the PBT phase. On the other hand, the good miscibility between PMMA (the shell phase of PB-g-MSG) and PC induced the PB-g-MSG particles dispersed in the PC phase. The different phase morphology led to different toughening behavior. The PBT/PC/PB-g-MSG blends with the PC encapsulated morphology showed much lower brittle-ductile transition core-shell particles content (10-15 wt% or 15-20 wt%) compared with the PBT/PC/PB-g-SG blends (20-25 wt%). The difference between the toughening efficiency of the core-shell particles was due to the change of deformation mechanisms. In PBT/PC/PB-g-MSG blends, the cavitation of PB rubber phase led to the occurrence of shear yielding of the matrix. While in the PBT/PC/PB-g-SG blends, the debonding between PBT and PC interface induced the shear yielding of the matrix. The variation of the core-shell particles dispersed phase morphology also affected the crystallization properties and DMA results of the PBT/PC blends. Modification of the phase morphology provided an useful strategy to prepare PBT/PC blends with higher toughening efficiency.     

Interfacial studies in CTBN-modified epoxies

Rubber particle cavitation and concomitant shear deformation of the matrix is known to be a major source of toughening in rubber-modified epoxies. The role of the rubber-matrix interface in this toughening mechanism, however, is not well studied. It has been claimed by Chen and Jan [Polym. Eng. Sci., 31,577 (1991)] that introduction of a ductile interphase around the rubbery phase enhances plastic dilation of particles and thus contributes to fracture energy of modified blend. In spite of this promising development in rubber toughening, very few studies on the use of ductile interfaces to improve the fracture resistance of rubber-modified polymers have been initiated. The objective of this investigation is to examine the role of ductility of interface on the fracture toughness of rubber-modified epoxies. Both ductile and rigid interphases are incorporated around CTBN particles in a DGEBA epoxy matrix via end-capping of rubber with epoxy monomers different from that of the matrix. The results of this investigation suggest that introduction of a ductile interphase may indeed further improve the crack growth resistance of material under certain test conditions. In contrast, introduction of the rigid interphase, in the system studied, promoted interfacial debonding and plastic dilation but did not alter the mechanical performance of the rubber-modified blend. © 1995 John Wiley & Sons, Inc.     

以具有核壳结构的聚丙烯酸酯颗粒增韧环氧树脂胶粘剂

研制了PEA/PMMA~(…)核壳乳胶粒子,并用于改性环氧树脂。SEM照片指出PBA分散相的平均粒径尺寸。力学性能及Tg测试结果表明改性环氧树脂体系的力学性能大幅度提高,而Tg并未降低。PBA/PMMA核壳粒子为环氧树脂有效的增韧剂。     

环氧树脂的增韧

添加一种分相的橡胶粒子到环氧树脂中可增加其破坏韧性。已知的这些改性剂有 Ｃ Ｔ Ｂ Ｎ、微凝胶和核壳粒子等。这些橡胶增韧环氧树脂体系形成海岛结构,增韧的原因是橡胶粒子的撕裂并诱发母体的塑性变形。另一类替代反应性橡胶用于改性环氧的是多种强韧的热塑性塑料,环氧树脂变韧是形成了双连接相结构。综合讨论了改性剂和母体的性质对环氧树脂共混物韧性的影响以及增韧机理。     

Water-based radiation-curable latexes

Film-forming polystyrene/poly(n-butyl acrylate-co-glycidyl methacrylate) [PS/P(BA-co-GMA)] core–shell latex particles were prepared via a two-stage emulsion polymerization procedure using a polystyrene latex seed. Delayed addition of GMA was used in order to locate functional epoxy groups close to the particle surfaces. It was found that a temperature of 25°C at the second-stage polymerization, in combination with a redox initiator system, was essential for the formation of a uniform shell of BA–GMA copolymer around the PS core. The latex particle morphology was investigated by transmission electron microscopy (TEM). Reactive double bonds were introduced into the particle shells in order to produce a film-forming latex system that could be cured by ultraviolet (UV)-radiation without any need to use reactive multifunctional monomers or oligomers as crosslinkers. The surface-bound epoxy groups were used as grafting sites for amine or carboxyl functional unsaturated monomers, respectively. The grafting was demonstrated by Fourier transform infrared (FTIR) spectroscopy. Films prepared from modified and unmodified latexes were exposed to UV radiation in the presence of a photoinitiator. Crosslinking was tested by thermal mechanical analysis (TMA) and by determination of swelling and gel content of exposed films. It was demonstrated that films from the modified latexes after irradiation had significantly higher stiffness and gel content and showed lower swelling than unmodified ones. © 1996 John Wiley & Sons, Inc.     

Toughening mechanisms in epoxy-silica nanocomposites (ESNs)

Two types of nanosilica (NS) particles with different average particle sizes (20 nm and 80 nm in diameter, respectively) were used to fabricate epoxy-silica nanocomposites (ESNs) in this study. No significant differences in fracture behavior were observed between the epoxies filled with 20 nm NS particles and the epoxies filled with 80 nm NS particles. Interestingly, both types of NS particles were found to be more efficient in toughening epoxies than micron size glass spheres. As with micron size glass spheres, the fracture toughness of the ESNs were affected by the crosslink density of the epoxy matrix, i.e. a lower crosslinked matrix resulted in a tougher ESN. The increases in toughness in both types of ESNs were attributed to a zone shielding mechanism involving matrix plastic deformation. Moreover, the use of Irwin's formalized plastic zone model precisely described the relationship between the fracture toughness, yield strength and the corresponding plastic zone size of the various ESNs examined.     

Effect of blending sequence on the morphology and impact toughness of poly(ethylene terephthalate)/polycarbonate blends

The morphology of PET/PC/E‐GMA‐MA blends made by different mixing sequences was studied by transmission electron microscopy (TEM). The results suggest that migration of the E‐GMA‐MA copolymer from the PET phase to the PC phase occurred during the mixing of the (PET/E‐GMA‐MA) pre‐blend with the PC at 10% copolymer content. As a result of the migration, the E‐GMA‐MA particles are located in the PC phase rather than in the PET phase. This finding is not in agreement with the prediction made previously by others based on the possible reaction between the epoxy group of GMA and carboxyl group of PET. Core‐shell (PC/E‐GMA‐MA) particles formed in situ during blending and the size of the core‐shell particles was controlled by the blending sequence used. Mechanical properties of the ternary blends were tested at various temperatures. Although the blending sequence does not have a noticeable effect on the yield strength and modulus of the blends, it has a strong influence on the morphology formed, which determines the impact toughness. For blends made under optimum processing conditions, the brittle‐ductile transition occurred at a lower temperature and lower elastomer content. A study of the toughening mechanism suggested that the major toughening events were cavitation plus matrix shear yielding. It is postulated that the very high impact toughness found with the (PC/E‐GMA‐MA)/PET blend (at 10% E‐GMA‐MA) originated from the bimodal particle size distribution of the core‐shell particles formed in situ.     

Influence of spacer groups on grafting ability,curing ability,and film properties of water-based radiation curable latexes

Film-forming polystyrene/poly(n-butyl acrylate-co-glycidyl methacrylate) [PS/P(BA-co-GMA)] core–shell latex particles were prepared via a two-stage emulsion polymerization procedure using a PS latex seed. A delayed addition of GMA was used to locate the functional epoxy groups near the surface of the particles. The surface-bound epoxy groups were used as grafting sites for unsaturated carboxyl functional monomers having the unsaturated groups and the carboxylic group separated by 1, 5, or 10 oxyethylene units. Grafting and curing characteristics and film properties after irradiation were investigated as a function of the number of oxyethylene units. A BA-GMA [P(BA-co-GMA)] copolymer was used as a model system for the core–shell latex particles for quantification of the grafting reactions. The grafting was demonstrated by FTIR and ¹H-NMR spectroscopy. The effects of crosslinking was studied by thermal mechanical analysis and dynamical mechanical analysis. Differential photocalorimetry was also used for evaluation of the curing ability. It was demonstrated that the reagent having five oxyethylene units in the spacer group was grafted onto the polymer backbones to a larger extent than the other two reagents, and a more thoroughly cured film was obtained upon irradiation. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 897–906, 1998     

环氧官能化ABS增韧PBT及其共混物

合成了甲基丙烯酸环氧丙酯(GMA)接枝的丙烯腈-丁二烯-苯乙烯(ABS-g-GMA)核壳粒子增韧聚对苯二甲酸丁二醇酯(PBT),加入环氧树脂(Epoxy)为扩链剂进一步提高共混物的性能。红外光谱(FTIR)结果表明:GMA成功接枝到ABS粒子上;研究发现不同GMA含量的ABS-g-GMA粒子在PBT及PBT/Epoxy共混物中均匀分散;ABS-g-GMA对PBT增韧效果较好,Epoxy进一步提高了PBT/ABS-g-GMA共混物的冲击韧性及拉伸强度;ABS-g-GMA增韧PBT的机理是橡胶粒子的空洞化和PBT基体的剪切屈服。