橡胶增韧环氧树脂机理的研究

本文研究了固化剂种类、环氧基体平均网链长度和分散相与基体间键合情况对体系韧性等的影响.结果说明,基体平均网链长度是一个更为重要的影响因素.分散相与基体间的化学键合也是重要的.文中对橡胶增韧环氧树脂的机理提出了见解.在交联密度较低的材料中,在橡胶颗粒附近叠加的应力场诱发下发生纵深度较大的断裂过程.分散相与基体间的化学键合增大该应力场强度有利于加大断裂过程区.     

Toughening of epoxies via craze-like damage

The fracture behavior of a core-shell rubber (CSR) modified epoxy is investigated using both fracture mechanics and microscopy tools. The CSR-modified epoxy is found to be toughened via numerous line-array cavitations of the CSR particles, followed by plastic flow of the epoxy matrix. The toughening effect via the above craze-like damage process is found to be as effective as that of the well-known widespread rubber cavitation/matrix shear yielding mechanisms. The conditions for triggering the craze-like damage appear to be both stress state and rubber concentration dependent. The type of rubber tougheners utilized also plays a critical role in triggering this rather unusual craze-like damage in epoxy systems. © 1993 John Wiley & Sons, Inc.     

Influence of carboxyl‐terminated (butadiene‐co‐acrylonitrile) loading on the mechanical and thermal properties of cured epoxy blends

A mixture of epoxy with liquid nitrile rubber, carboxyl‐terminated (butadiene‐co‐acrylonitrile) (CTBN) was cured under various temperatures. The cured resin was a two‐phase system, where spherical rubber domains were dispersed in the matrix of epoxy. The morphology development during cure was investigated by scanning electron microscope (SEM). There was slight reduction in the glass transition temperature of the epoxy matrix (T_g) on the addition of CTBN. It was observed that, for a particular CTBN content, T_g was found to be unaffected by the cure temperature. Bimodal distribution of particles was noted by SEM analysis. The increase in the size of rubber domains with CTBN content is due probably to the coalescence of the rubber particles. The mechanical properties of the cured resin were thoroughly investigated. Although there was a slight reduction in tensile strength and young's modulus, appreciable improvements in impact strength, fracture energy, and fracture toughness were observed. Addition of nitrile rubber above 20 parts per hundred parts of resin (phr) made the epoxy network more flexible. The volume fraction of dispersed rubbery phase and interfacial area were increased with the addition of more CTBN. A two‐phase morphology was further established by dynamic mechanical analysis (DMA). © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2531–2544, 2004     

Structure and property relationships in model diglycidyl ether of bisphenol‐a and diglycidyl ether of tetramethyl bisphenol‐a epoxy systems. I. Mechanical property characterizations

Model epoxy networks, with variations in crosslink density and in epoxy monomer rigidity, were prepared to study how the network structure affects modulus, T_g, and toughness/toughenability of epoxy resins. Diglycidyl ether of bisphenol‐A and diglycidyl ether of tetramethyl‐bisphenol‐A, along with the corresponding chain extenders, were chosen to study how monomer backbone rigidity and crosslink density affect physical and mechanical properties of epoxies. The present study indicates that, as expected, the backbone rigidity of the epoxy network, not the crosslink density alone, will strongly influence modulus and T_g of epoxy resins. Upon rubber toughening, it is found that the rigidity of the epoxy backbone and/or the nature of the crosslinking agent utilized are most critical to the toughenability of the epoxy. That is, the well‐known correlation between toughenability and the average molecular weight between crosslinks (M_c) does not necessarily hold true when the nature of epoxy backbone molecular mobility is altered. The potential significance of the present findings for a better design of toughened thermosets for structural applications is discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2137–2149, 1999     

The effect of network architecture on the thermal and mechanical behavior of epoxy resins

The effects of crosslink functionality (f_c), molecular weight between crosslinks (M_c), and chain stiffness display on the thermal and mechanical behavior of epoxy networks are determined. Both f_c and M_c are controlled by blending different functionality amines with a difunctional epoxy resin. Chain stiffness is controlled by changing the chemical structure of the various amines. In agreement with rubber elasticity theory, the rubbery moduli are dependent on f_c and M_c, but independent of chain stiffness. The glassy moduli and secondary relaxations of these networks are relatively independent of f_c, M_c, and chain stiffness. However, the glass transition temperatures (T_g) of these networks are dependent on all three structural variables. This trend is consistent with free volume theory and entropic theories of T_g. f_c, M_c, and chain stiffness control the yield strength of these networks in a manner similar to that of T_g and is the result that both properties involve flow or relaxation processes. Fracture toughness, as measured by the critical stress intensity factor (K_Ic), revealed that f_c and M_c are both critical parameters. The fracture behavior is the result of the fracture toughness being controlled by the ability of the network to yield in front of the crack tip. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1371–1382, 1998     

Fracture mechanisms in rigid core-shell particle modified high performance epoxies

The fracture mechanisms of a high performance epoxy system modified with two types of preformed rigid core-shell particles (RCSP) were investigated. The use of the preformed RCSP anables the control of the dispersion of the toughener phase in the epoxy, which, in turn, allows the mechanical properties of the modified epoxy to be optimized. The toughening effect via the RCSP modification is found to be as good as that via the core-shell rubber modification. The moduli andT _g of these RCSP-modified epoxies are virtually unaltered via the RCSP modification, when compared with the neat epoxy resin equivalent. The toughening mechanisms in these toughened systems appear to be predominantly crack deflection, crack bifurcation, and microcracking. Approaches for effective toughening of high performance polymers via rigid polymers are discussed.     

The effects of monoepoxide chain termination and their derived soluble fractions on the structure‐property relationships of controlled epoxy networks

A new series of monoepoxide terminated controlled epoxy networks (CENs) and a corresponding soluble fraction polymer (SFP) were prepared to further investigate the effects of chain termination on epoxy thermoset structure‐property relationships. CENs having an initial molecular weight between crosslinks (M_c,i) of ～3000 g/mol using phenylglycidyl ether (PGE) as the chain terminator have thermal and mechanical properties consistent with previously studied monophenol terminated CENs. Glass transition temperature (T_g) decreases monotonically with PGE concentration (ε), whereas fracture toughness decreases sharply at a critical PGE concentration (ε_c). A PGE terminated SFP was prepared corresponding to the soluble fraction expected for the CEN composition at ε_c. The SFP behaves as a weak antiplasticizer in these epoxy thermosets; T_g is reduced and follows the inverse rule of mixtures, and fracture toughness is slightly reduced. By difference it is inferred that most of the deterioration of epoxy thermoset properties resulting from incorporation of chain terminators above ε_c is a result of the presence of nonelastically active pendant chains and by the increase in M_c. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 72–79, 2009     

Light scattering from epoxy-elastomer resins: A study of statistical methods in heterogeneity analysis

The solid-state light scattering from ABE (acrylonitrile butadiene epoxy) copolymers was studied as a function of rubber content by measuring the intensity distribution of scattered light under V_v polarization conditions. The range of heterogeneity has been quantified by the Debye–Bueche statistical treatment and by comparison with simulated scattering envelopes. The data indicate that below a critical rubber concentration of 20% the rubbery phase precipitates as spherical domains of nearly constant size. Phase inversion occurs at a rubber concentration of 20% and is accompanied by a maximum in polydispersity of the dispersed phase. Beyond this concentration, epoxidic domains are imbedded in a rubber and epoxy matrix. As a result of comparison of Debye–Bueche parameters with simulated values of these, it is concluded that the correlation distance is very sensitive to the presence of a small number of large particles.     

THE EFFECT OF MATRIX TOUGHNESS ON THE BRITTLE-DUCTILE TRANSITION OF HDPE/CaCO_3 BLENDS

The effects of HDPE matrix toughness on the brittle-ductile transition of HDPE/CaCO_3blends are investigated. Not all HDPE can be toughened by CaCO_3 particles. The ability of thematrix to yield plays a fundamental role in determing whether HDPE can be toughened or not.There exists a critical matrix toughness (I_(sc)≈45J/m) below which HDPE can not be toughenedobservably by CaCO_3 particle at given average size, and above which the critical matrix ligamentthickness (τ_?) is proportional to matrix impact strength.     

Rigidity percolation model of polymer fracture

A theory of the fracture of polymers with network microstructure was developed that was based on the vector, or rigidity percolation (RP) model of Kantor and Webman, in which the modulus, E, is related to the lattice bond fraction p, via E ～ [p ? p_c]^τ. The Hamiltonian for the lattice was replaced by the strain energy density function of the bulk polymer, U = σ²/2E, where σ is the applied stress and p was expressed in terms of the lattice perfection via the bond density ν, with the entanglement molecular weight, ν = ρ/M_e and appropriate measures of crosslink density for rubber, thermosets, and carbon nanotubes. The stored mechanical energy, U, was released by the random fracture of νD_o[p ? p_c] over stressed hot bonds of energy D_o ≈ 330 kJ/mol. The polymer fractured critically when p approached the percolation threshold p_c, and the net solution was obtained as σ = (2EνD_o [p ? p_c])^1/2 with a fracture energy, G_1c ～ [p ? p_c]. The fracture strength of amorphous and semicrystalline polymers in the bulk was well described by, σ = [ED_oρ/16 M_e]^1/2, or σ ≈ 4.6 GPa/M_e^1/2. Fracture by disentanglement was found to occur in a finite molecular weight range, M_c < M < M*, where M*/M_c ≈ 8, such that the critical draw ratio, λ_c = (M/M_c)^1/2, gave the molecular weight dependence of the fracture as G_1c ～ [(M/M_c)^1/2 ? 1]². The critical entanglement molecular weight, M_c, is related to the percolation threshold, p_c, via M_c = M_e/(1 ? p_c). Fracture by bond rupture was in accord with Flory's suggestion, G/G* = [1 ? M_c/M], where G* is the maximum fracture energy. Fracture of an ideal rubber with p = 1 was determined not to occur without strain hardening at λ > 4, such that the maximum stress, σ = E (λ ? 1/λ) = 3.75E. The fracture properties of rubber were found to behave as σ ～ ν, σ ～ E, and G_1c ～ ν. For highly crosslinked thermosets, it was predicted that σ ～ (Eν)^1/2, σ ～ (X ? X_c)^1/2, and G_1c ～ ν^?1/2, where X is the degree of reaction of the crosslinking groups and X_c defines the gelation point. When applied to carbon nanotubes (SWNT and MWNT) of diameter d and hexagonal bond density ν = j/b², the nominal stress as a function of diameter is σ(d) = [16 ED_o(p ? p_c) j/b]^1/2/d ≈ 211/d (GPa.nm) and the critical force, F_c(d) ≈ 166 d (nN/nm), in which j = 1.15, b = 0.142 nm, E ≈ 1 Tpa, and D_o = 518 kJ/mol. For polymer interfaces with Σ chains per unit area of length L and width X ～ L^1/2, G_1c is then ～ [p ? p_c], where p ～ ΣL/X. The results predicted by the RP fracture model were in good agreement with a considerable body of fracture data for linear polymers, rubbers, thermosets, and carbon nanotubes. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 168–183, 2005     

Synthesis of hydroxyl‐terminated poly(ether ether ketone) with pendent tert‐butyl groups and its use as a toughener for epoxy resins

Hydroxyl‐terminated poly(ether ether ketone) with pendent tert‐butyl groups (PEEKTOH) was synthesized by the nucleophilic substitution reaction of 4,4′‐difluorobenzophenone with tert‐butyl hydroquinone with potassium carbonate as a catalyst and N‐methyl‐2‐pyrrolidone as a solvent. Diglycidyl ether of bisphenol A epoxy resin was toughened with PEEKTOHs having different molecular weights. The melt‐mixed binary blends were homogeneous and showed a single composition‐dependent glass‐transition temperature (T_g). Kelley–Bueche and Gordon–Taylor equations gave good correlation with the experimental T_g. Scanning electron microscopy studies of the cured blends revealed a two‐phase morphology. A sea‐island morphology in which the thermoplastic was dispersed in a continuous matrix of epoxy resin was observed. Phase separation occurred by a nucleation and growth mechanism. The dynamic mechanical spectrum of the blends gave two peaks corresponding to epoxy‐rich and thermoplastic‐rich phases. The T_g of the epoxy‐rich phase was lower than that of the unmodified epoxy resin, indicating the presence of dissolved PEEKTOH in the epoxy matrix. There was an increase in the tensile strength with the addition of PEEKTOH. The fracture toughness increased by 135% with the addition of high‐molecular‐weight PEEKTOH. The improvement in the fracture toughness was dependent on the molecular weight and concentration of the oligomers present in the blend. Fracture mechanisms such as crack path deflection, ductile tearing of the thermoplastic, and local plastic deformation of the matrix occurred in the blends. The thermal stability of the blends was not affected by blending with PEEKTOH. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 541–556, 2006     

Structure of a typical amine-cured epoxy resin

A technique for estimating M_c, the molecular weight between crosslinks, of amine-cured epoxy resins is described. The technique is based upon the stoichiometry of the curing reaction and the amount of primary amino and epoxy groups remaining in the polymer at a given time. The M_c values so calculated are shown to be consistent with M_c results obtained from separate measurements of swelling and the polymer–solvent interaction parameter χ₁ for the range of polymer concentration in which both measurements could be obtained. A means of estimating the relative reaction rates of the primary and secondary amino groups with the epoxy groups is given. Under proper curing conditions the amine–epoxy reaction goes very nearly to completion. The presence or absence of an exotherm has no noticeable effect on the course of the reaction between bisphenol A-epichlorohydrin (Epon 828) and methylene dianiline.     

The effect of filler content on the lower yield stress of polymer composites

The effect of the concentration of the dispersed elastic filler on the lower yield stress of matrix composites based on plastic polymers is studied. As the matrix polymers, LDPE-HDPE and LDPE-(medium-density PE) are used. The elastic filler is rubber crumb prepared by roll grinding of worn tires or by deformation grinding of ethylene-propylene-diene rubber. Irrespective of the type of filler particles and their adhesion to the polymer matrix, the lower yield stress σ_d of the composite is described by the linear law σ_d = σ_dm(1 ? V _f ), where σ_dm is the lower yield stress of the polymer matrix and V _f is the volume content of the filler. Analysis of the published data shows that this relationship is quite general and describes the effect of rigid inorganic particles on the lower yield stress when adhesion between the filler particles and the matrix is poor.     

A new conception on the toughness of nylon 6/silica nanocomposite prepared via in situ polymerization

A novel method, in situ polymerization, was used for the preparation of nylon 6/silica nanocomposites, and the mechanical properties of the nanocomposites were examined. The results showed that the tensile strength, elongation at break, and impact strength of silica-modified nanocomposites exhibited a tendency of up and down with the silica content increasing, while those of silica-unmodified nanocomposites decreased gradually. It also exhibited that the mechanical properties of silica-modified nanocomposites have maximum values only when 5% silica particles were filled. Based on the relationship between impact strength of the nanocomposites and the matrix ligament thickness τ, a new criterion was proposed to explain the unique mechanical properties of nylon 6/silica nanocomposites. The nylon 6/silica nanocomposites can be toughened only when the matrix ligament thickness is less than τ_c and greater than τ_a, where τ_a is the matrix ligament thickness when silica particles begin to aggregate, and τ_c is the critical matrix ligament thickness when silica particles begin to toughen the nylon 6 matrix. The matrix ligament thickness, τ, is not independent, which related with the volume fraction of the inorganic component because the diameter of inorganic particles remains constant during processing. According to the observation of Electron Scanning Microscope (SEM), the process of dispersion to aggregation of silica particles in the nylon 6 matrix with increasing of the silica content was observed, and this result strongly supported our proposal. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 789–795, 1998     

Structured elastomeric submillimeter films displaying magneto and piezo resistivity

Structured elastomer films (100–150 µm) presenting piezo and magneto resistance are described. The films are composites of filler particles, which are both electrically conductive and magnetic, dispersed in an elastomeric matrix. The particles consist of magnetite (6 nm) grouped in silver‐coated aggregates (Fe₃O₄@Ag). The matrix is styrene–butadiene rubber (SBR) in which diethylene glycol (DEG) is added. The particles, SBR and DEG, are dispersed in toluene and then placed between two rare earth magnets. Formation of pseudo‐chains (needles) of inorganic material aligned in the direction of the magnetic field is obtained after solvent evaporation. The addition of DEG is substantial to obtain an electrically conductive material. The electrical conductivity is anisotropic and increases when applying normal stresses and/or magnetic fields in the direction of the needles. The elastomers, particles, and needless were characterized by XRD, SEM, EDS, FTIR, DSC, TGA, VSM, profilometry, and stress–strain analysis. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 574–586     

Graft interpenetrating polymer networks of polyurethane and epoxy. II. Toughening mechanism

Graft interpenetrating polymer networks (graft-IPNs) of polyurethane (PU) and the diglycidyl ether of bisphenol A (epoxy) were prepared by first grafting excess PU prepolymer to the epoxy and then simultaneously polymerizing the PU prepolymer and epoxy. The fracture properties, at high shear rate (e.g., impact) and low shear rate (e.g., pseudostatic tensile fracture energy measurement) of these graft-IPNs exhibit opposite behavior. Although dispersed rubber particles can enhance the Izod impact strength, toughening of the matrix of graft-IPNs was found to be the main contribution. In contrast, it was found that a heterogeneous morphology with suitably dispersed rubber domains of appropriate size as well as the toughness of the matrix are requirements for effectively increasing the fracture energy at low shear rate. A reinitiating crack in the plastic matrix is proposed as the main toughening mechanism and can be invoked to interpret the fracture behavior at high and low shear rates of the graft-IPNs.     

Nanostructured epoxy thermoset templated by an amphiphilic PCL‐b‐PES‐b‐PCL triblock copolymer

The PCL‐b‐PES‐b‐PCL triblock copolymer is used to incorporate into epoxy resin when the blends are cured with 4,4'‐diaminodiphenylsulfone (DDS) to afford the nanostructured epoxy thermosets. The differential scanning calorimetry (DSC) and Fourier transform‐infrared spectroscopy (FT‐IR) show that the nanostructured PCL‐b‐PES‐b‐PCL thermosets are accessed through the formation of the intermolecular hydrogen bonding interactions. The nanostructures are further evidenced by means of small‐angle X‐ray scattering (SAXS) and transmission electron microscopy (TEM). By considering the miscibility of the subchains of block copolymer with epoxy before and after the curing reaction, it is judged that the formation of the nanostructures follows the mechanism of reaction‐induced microphase separation. It is noted that the epoxy resin is significantly toughened in terms of the measurement of critical stress field intensity factor (K_IC). © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 424–432     

Compatibilization level effects on the structure and mechanical properties of rubber‐modified polyamide‐6/clay nanocomposites

Exfoliated polyamide‐6 (PA6)/organically modified montmorillonite clay (OMMT) nanocomposites (PNs) were modified with partially maleinized styrene–ethylene/butadiene–styrene triblock copolymers (SEBS) at three maleinization levels in an attempt to link in these materials high toughness with appropriate small‐strain and fracture tensile properties. OMMT stayed only in the PA6 matrix, and no preferential location in the matrix/rubber interphase was observed. The increased dispersed phase size upon the addition of OMMT was attributed to interactions between maleic anhydride (MA) functionalized SEBS and the surfactant of OMMT. The rubber particle size generally decreased when the MA content of SEBS increased, and this indicated compatibilization. The subsequent good adhesion led to tough nanocomposites across a wide range of both strain rates and fracture modes. As the critical interparticle distance (τ_c) decreased with the MA content, and the other parameters that could influence the surface‐to‐surface mean interparticle distance did not change, it is proposed that in these PNs higher adhesion leads to a smaller τ_c value. Finally, the presence in the matrix of a nanostructured clay makes the rubber content necessary for the toughness jump to increase and τ_c to decrease. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3611–3620, 2005     

Modification of epoxy resin using reactive liquid (ATBN) rubber

Epoxy resins are widely utilised as high performance thermosetting resins for many industrial applications but unfortunately some are characterised by a relatively low toughness. In this respect, many efforts have been made to improve the toughness of cured epoxy resins by the introduction of rigid particles, reactive rubbers, interpenetrating polymer networks and engineering thermoplastics within the matrix.In the present work liquid amine-terminated butadiene acrylonitrile (ATBN) copolymers containing 16% acrylonitrile is added at different contents to improve the toughness of diglycidyl ether of bisphenol A epoxy resin using polyaminoimidazoline as a curing agent. The chemical reactions suspected to take place during the modification of the epoxy resin were monitored and evidenced using a Fourier transform infrared. The glass transition temperature (T_g) was measured using a differential scanning calorimeter. The mechanical behaviour of the modified epoxy resin was evaluated in terms of Izod impact strength (IS), critical stress intensity factor, and tensile properties at different modifier contents. A scanning electron microscope (SEM) was used to elucidate the mechanisms of deformation and toughening in addition to other morphological features. Finally, the adhesive properties of the modified epoxy resin were measured in terms of tensile shear strength (TSS).When modifying epoxy resin with liquid rubber (ATBN), all reactivity characteristics (gel time and temperature, cure time and exotherm peak) decreased. The infrared analysis evidenced the occurrence of a chemical reaction between the two components. Addition of ATBN led to a decrease in either the glass transition temperature and stress at break accompanied with an increase in elongation at break and the appearance of some yielding. As expected, the tensile modulus decreased slightly from 1.85 to about 1.34 GPa with increasing ATBN content; whereas a 3-fold increase in Izod IS was obtained by just adding 12.5 phr ATBN compared to the unfilled resin. It is obvious that upon addition of ATBN, the Izod IS increased drastically from 0.85 to 2.86 kJ/m² and from 4.19 to 14.26 kJ/m² for notched and unnotched specimens respectively while K_IC varies from 0.91 to 1.49 MPa m^1/2 (1.5-fold increase). Concerning the adhesive properties, the TSS increased from 9.14 to 15.96 MPa just by adding 5 phr ATBN. Finally SEM analysis results suggest rubber particles cavitation and localised plastic shear yielding induced by the presence of the dispersed rubber particles within the epoxy matrix as the prevailing toughening mechanism.     

The effects of monophenol chain terminators on the structure–property relationships of controlled epoxy networks

Five families of new controlled epoxy thermosets (CENs) using three monophenol chain terminators were prepared to study systematic changes in the structure and amount of the monophenol and the initial molecular weight between crosslinks (M_c,i) on the properties of epoxy thermosets. Glass transition temperature (T_g) decreases with monophenol mole fraction (χ) in proportion to both the concentration and flexibility of the chain terminator. Distinct serial relations for T_g depression were observed for the three M_c,i families. Dynamic mechanical analysis (DMA) shows significant perturbations of the relaxation behavior with added terminator as evidenced by decrease in peak tan δ and in post T_g damping. The rubbery coefficients of thermal expansion (CTE) increases with monophenol concentration only at χ > 0.05 and shows distinct curvature versus temperature, but is largely invariant with monophenol flexibility. The thermal stability of terminated CENs decreases only slightly with χ and little difference was found with monophenol structure. Most surprisingly, fracture toughness decreases markedly and discontinuously with χ depending on M_c,i. The values of the critical monophenol concentration at which fracture toughness markedly decreases (χ_c) are inversely proportional to M_c,i but are independent of monophenol flexibility. No correlation of χ_c with any of the calculated network structure parameters was apparent. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1632–1640, 2008