Room-temperature curing epoxies and their elevated-temperature properties

Low-temperature curing epoxy formulations for elevated-temperature service were developed and characterized for adhesive repair and composite matrix. Balanced lap-shear and peel strengths for adhesive applications and flexural and interlaminar shear strengths for composites were obtained when a blend of tetra-functional and tri-functional epoxies was used. For low-temperature curing triethylenetetraamine (TETA) was proven to be effective combined with an amine terminated elastomer. The latter component served also as a toughener. Lower-functional epoxies and curing agents resulted in a peel strength increase and a simultaneous decrease in shear strength. Furthermore, the incorporation of the lower-functional constituents or silanes, resulted in development of a three-phase morphology compared to a conventional two-phase elastomer-epoxy morphology. Thermal analysis carried out by Differential Scanning Calorimetry (DSC) and Dielectric Loss Factor (DLF) indicated that the reduced temperature-curing formulations have relatively low activation energy and that the main cross-linking reaction takes place close to 50°C and is completed at 120°C with a maximum glass transition temperature at 180°C. Mechanical properties of the adhesive formulation were comparable or better than existing commercial products. Flexural and interlaminar shear strengths, for glass-epoxy composites incorporating a selected low-curing-temperature formulation, exhibited enhanced properties compared to commercially 120°C epoxy-glass prepreg composites.     

The concept of boundary interphase in composite mechanics

Summary The thermomechanical behaviour of a particle composite was evaluated under the assumption that a boundary interphase between the matrix and the filler particles develops, upon the thermomechanical properties of which the overall behaviour of the composite depends. This interphase exists in reality and it contains both areas of adsorption interaction in polymer surface layers onto filler particles as well as areas of mechanical imperfections. Under the assumption that the interphase is homogeneous and isotropic exhibiting perfect adhesion with both main phases a theory was developed providing quantitative means of assessing the adhesion efficiency between the phases and its effect on the thermomechanical behaviour of the composite. Experimental results are in good agreement with theoretical predictions.With 4 figures     

Dependence of the dielectric properties of suspensions on the volume fraction of suspended particles

It is shown that the fundamental expression for the complex permittivity epsilons* of a dilute suspension of monodispersed, spherical particles, epsilons*=epsilone*(1+3phid*), where epsilone* is the complex permittivity of the suspending medium and d* the dipolar coefficient, is strictly valid for any value of the volume fraction phi of particles in the suspension, provided that d* is interpreted as the ensemble average value of the dipolar coefficient of the particles and is defined in terms of the macroscopic electric field in the suspension.     

Mechanical properties of block copolymer vesicle and micelle modified epoxies

Block copolymers with and without reactive functionalities can improve fracture resistance in brittle epoxies even when added in relatively small amounts (<5 wt %). At certain compositions, amphiphilic block copolymers spontaneously self‐assemble into vesicles, spherical micelles, or wormlike micelles in thermoset resins, and these morphologies are retained with the full curing of the resins. The addition of such block copolymers leaves the glass‐transition temperature of these blends relatively unchanged, whereas the fracture resistance increases up to a factor of 3.5 for the vesicle‐modified blends. For epoxies modified with block copolymers self‐assembled into a spherical geometry (vesicles or spherical micelles), the fracture resistance scales with the ratio of the interparticle distance to the average vesicle (or spherical micelle) diameter (D_i/D_p) and increases as this quantity is reduced. Greater adhesion between the vesicle and epoxy resin improves the fracture resistance only at higher values of D_i/D_p, at which the materials are more brittle. Debonding and subsequent matrix plastic deformation are identified as the toughening mechanisms in these blends. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2444–2456, 2003     

Micellar structure and mechanical properties of block copolymer‐modified epoxies

Amphiphilic block copolymers provide a unique means for toughening epoxy resins because they can self‐assemble into different inclusion shapes before epoxy curing. The two examples reported here are spherical micelles and vesicles, which form in blends containing epoxy and symmetric or asymmetric poly(ethylene oxide)–poly(ethylene‐alt‐propylene) (PEO–PEP) block copolymer with PEO volume fractions of 0.5 and 0.26, respectively. The vesicles and spherical micelles were characterized by transmission electron microscopy and small‐angle X‐ray scattering (SAXS), respectively. SAXS data from the spherical micelles were fit to the Percus–Yevick model for a liquid‐like packing of spheres with hard‐core interactions. Mechanical properties of spherical‐micelle‐modified and vesicle‐modified epoxies in the dilute limit are compared. The glass‐transition temperature and Young's (storage) modulus were tested with dynamic mechanical spectroscopy, and compact‐tension experiments were performed to determine the critical plane‐strain energy release rate for fracture. Vesicles were most effective in improving the epoxy fracture resistance. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2996–3010, 2001     

The structure and properties of polymer-metallic coating interphase

The structure of the surface layer in polymers (LDPE and PET) decorated with a thin metal (gold and platinum) layer was studied after their deformation under different conditions. It was found that relatively thick coatings debonded from the polymer substrate during tensile drawing. Debonding was observed at low tensile strains (below 20–30%). During the further drawing of a polymer, a regular microrelief typical of deformable “rigid coating on a soft substrate” systems appeared on its surface. This phenomenon is explained by the fact that the debonding metal coating uncovers not the surface of the pure polymer but a certain modified layer, which has a higher elastic modulus than the pure polymer. The formation of this layer is associated with the inclusion of metal atoms into the polymer during the metal decoration by plasma immersion ion deposition. As a result of this inclusion, a modified layer, which has a higher glass transition temperature, a higher elastic modulus, and other mechanical properties, is formed between the coating and the polymer.     

The dynamic properties of plasticized epoxies over a wide frequency range

Summary The dynamic mechanical properties of a class of plasticized epoxies were investigated over a wide temperature range by means of a Rheovibron Viscoelastometer. Master curves for storage and loss moduli were produced by means of the frequency-temperature superposition principle, as well as for relaxation modulus by a conversion formula. The effect of plasticizer percentage on the said parameters, as well as the loss peaks, corresponding to and relaxations was investigated.     

Adhesion efficiency between phases in fibre-reinforced polymers by means of the concept of boundary interphase

Summary The role of the boundary interphase on the adhesion efficiency between fiber and matrix in the case of polymers reinforced with unidirectional fibers was investigated. A theoretical model was introduced considering that the composite material consists of three phases; that is, the fiber, the matrix and the interphase, material which is the part of the polymer matrix lying at the close vicinity of the fiber surface. The interphase material, having different physical properties from those of the bulk matrix, affects the overall behavior of the composite. Moreover, the quality of adhesion between the two main phases depends greatly on the nature of the interphase material. In this study we have considered that the interphase material is inhomogeneous in nature, with properties varying continuously from the fiber surface to the bulk matrix. The theory developed resulted in a criterion of the adhesion quality and in a prediction of the longitudinal modulus of elasticity of the fiber-composite.With 4 figures and 1 table     

Yielding and fracture in crosslinked epoxies

From a comparison of tensile properties of two epoxy networks crosslinked, respectively, with an aromatic diamine or an anhydride and from the available literature data on the fracture properties of thermosets, it has been tried to establish structure-property relationships in this field. Attention was focused on possible specific aspects of the crosslinked polymers. Surprisingly, the crosslink density does not seem to play a crucial direct role in fracture. In contrast, there is a relation between toughness and T_g: Aliphatic networks of low T_g are generally tougher than aromatic ones. The local mobility plays a very important role because it partly controls the value of Poisson's ratio (which determines the relative importance of dilatant stresses and shear stresses responsible, respectively, for brittle rupture and yielding) and, presumably, the activation volume of yielding (which partly determines the broadness of the temperature interval of ductility).     

Rheological properties of interphase adsorption layers of gossypin at a liquid-liquid phase separation boundary. III. Influence of chemical modification

The rheological properties of interphase absorption layers of gossypin derivatives at liquid phase separation boundaries and the stability of benzene emulsions have been investigated. It has been shown that the acylation of gossypin with acetic and succinic anhydrides lowers the strength properties of the adsorption layers of protein, leading to a change in the stability of benzene emulsions stabilized by gossypin.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, FAX (3712) 89-14-75. M. V. Lomonosov Moscow State University. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 528–533, July–August, 1994.     

Rheological properties of interphase adsorption layers of gossypin at a liquid-liquid phase separation boundary. III. Influence of chemical modification

The rheological properties of interphase absorption layers of gossypin derivatives at liquid phase separation boundaries and the stability of benzene emulsions have been investigated. It has been shown that the acylation of gossypin with acetic and succinic anhydrides lowers the strength properties of the adsorption layers of protein, leading to a change in the stability of benzene emulsions stabilized by gossypin. Institute of the Chemistry of Plant Substances, Academy of Sciences of the Republic of Uzbekistan, Tashkent, FAX (3712) 89-14-75. M. V. Lomonosov Moscow State University. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 528–533, July–August, 1994.     

Two-component interpenetrating polymer networks (IPNs) from polyurethanes and epoxies. II. Effect of different charge groups in polyurethanes and epoxies on properties and morphologies of two-component IPNs

Two-component interpenetrating polymer networks (IPNs) that contained charge groups in the backbones of the polyurethane and epoxy networks were studied. IPNs that contained opposite charge groups, similar charge groups, no charge groups, and corresponding pseudo-IPNs were prepared. A comparison of mechanical properties, water-resistance data, mechanical spectra, and electron microscopy showed that improved properties and morphologies resulted in IPNs that contained opposite charge groups. Presumably, interactions between the opposite charge groups in the constituent networks resulted in a forced compatibility between the two polymers which decreased the degree of phase separation.     

Estimation of interphase thickness and properties in PP/layered silicate nanocomposites

Nanocomposites were prepared from sodium montmorillonite (NaMMT) and organoclays (OMMT) with different particle sizes as a function of silicate content. Composite structure was characterized by various methods including X-ray diffraction (XRD), scanning electron microscopy (SEM) and rheology. Model calculations were carried out to estimate the thickness and yield stress of the interphase forming in the composites. The results proved the formation of an interphase, but the determination of interphase properties was hampered by several factors. First of all, the particle size of the filler changed quite considerably in PP/OMMT composites in spite of earlier observations and expectations. Particle characteristics changed even further when a relatively small amount (5 vol.%) of functionalized polymer (MAPP) was added to the composite. As a consequence, the estimation of the contact surface between the silicate and the polymer became extremely difficult. In spite of the uncertainties overall values of interphase properties were obtained using the results of all composites prepared. The prediction for the average thickness of the interphase is 0.23 μm and we obtained 51.2 MPa for interphase yield stress, but this estimate neglects the different interactions developing in composites containing the uncoated and the modified silicate, respectively.     

Attempt for the diffused interphase layer determination from the volume changes of microheterogeneous polymer mixtures

Summary For the experiments there have been prepared such microheterogeneous mixtures of isotactic polypropylene with polyethylene and of polyvinylchloride with polyethylene which contained maximum 60 volume per cent of dispersed phase component (iPP; PVC respectively) at 160°C for various diffusion times, whereat with sufficient exactness the tetrahedric lattice model could be applied. From the dependence of the mixture excess volume on dispersed phase volume fraction, calculated from additive and real densities, the volume fraction at the maximum dilatation ( _m ) was determined. At supposing theFickian course of the mutual diffusion, it was possible to calculate the thickness of the interphase layer.It was stated that for both polymer pairs the diffusion after 15 min had been practically stopped. This phenomenon, which could be explained by the kinetic hindrances during the interpenetration of the different nature polymer segments, is resulting in the deviation from theFickian course of diffusion.

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Zusammenfassung Zwecks Durchführung der Experimente wurden mikroheterogene Mischungen von isotaktischem Polypropylen mit Polyäthylen und von Polyvinylchlorid mit Polyäthylen vorbereitet, die höchstens 60 Vol.-% von dispergierter Phase (iPP oder PVC) bei 160°C für verschiedene Diffusionszeiten enthielten. Hierdurch konnte mit genügender Präzision mit dem Tetraeder-Gittermodell gearbeitet werden. Der Volumenbruch bei maximaler Dilatation ( _m ) wurde aus der Abhängigkeit des Exzessvolumens vom Volumenbruch der dispergierten Phase auf Grund von additiven und tatsächlichen Dichten berechnet. Unter Voraussetzung desFickschen Gesetzes für die Diffusion war es möglich, die Dicke der Zwischenphasenschicht auszurechnen.Man stellt fest, daß die Diffusion für beide Polymerpaare nach 15 min praktisch zum Stillstand kommt. Diese Erscheinung, die durch die kinetische Behinderung bei der Durchdringung von Polymersegmenten verschiedener Art erklärt werden könnte, ergibt Abweichungen vomFickschen Verlauf.

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With 2 figures and 1 table     

Phase behavior and interfacial properties of diblock copolymer-homopolymer ternary mixtures: Influence of volume fraction of copolymers and interaction energy

We use a Monte Carlo method to study the phase and interfacial behaviors of A-b-B diblocks in a blend of homopolymers, A and B, which are confined between two asymmetric hard and impenetrable walls. Our results show that, when the interaction strength is weak, the block copolymersare uniformly distributed in the ternary mixtures under considered concentrations. Under strong interaction strength, distribution region of the block copolymers changes from a single smooth interface to a curved interface or multi-layer interface in the ternary mixtures. Furthermore, our findings show that with increasing volume fraction of A-b-B diblock copolymer(фC), copolymer profiles broaden while фC≥ 0.4, a lamellar phase is formed and by further increasing фC, more thinner layers are observed. Moreover, the results show that, with the increase of фC, the phase interface first gradually transforms from plane to a curved surface rather than micelle or lamellar phase while with the increase of the interaction between A and B segments(ε_(AB)), the copolymer chains not only get stretched in the direction perpendicular to the interface, but also are oriented. The simulations also revealthat the difference between symmetric and asymmetric copolymers is negligible in statistics if the lengths of two blocksare comparable.     

Tensile modulus of polymer/CNT nanocomposites by effective volume fraction of nanoparticles as a function of CNT properties in the network

In this article, the effects of filler network and interphase between polymer matrix and nanoparticles on the tensile modulus of polymer/carbon nanotubes (CNT) nanocomposites are assumed by the effective volume fraction of nanoparticles. By this approach, the Takayanagi model is developed for polymer/CNT nanocomposites above percolation threshold. Also, the effective factors for filler network including the number (N ), aspect ratio (α ) and percolation threshold (? _p ) of CNT are correlated to three main parameters. The developed model is evaluated for some reported samples from previous papers, and the influences of main parameters on the modulus are examined. The acceptable predictability of the developed model for modulus of nanocomposites is illustrated by experimental results. The “α ” and “N ” parameters play positive roles in the modulus, while an inverse relation is observed between the modulus and the percolation threshold. The reasonable effects of these parameters on the tensile modulus of polymer/CNT nanocomposites are also discussed. Copyright © 2017 John Wiley & Sons, Ltd.     

Thermodynamic analysis of the phase separation in polyetherimide-modified epoxies

The miscibility of polyetherimides (PEIs) with epoxy monomers based on diglycidylether of bisphenol-A (DGEBA), and with reactive mixtures based on stoichiometric amounts of DGEBA and an aromatic diamine (DA) {either 4,4′-diaminodiphenylsulfone (DDS) or 4,4′-methylenebis[3-chloro 2,6-diethylaniline] (MCDEA)}; was experimentally studied. Cloud-point curves (temperature vs. composition) are reported for PEI-DGEBA and PEI-DGEBA-DA initial mixtures. Cloud-point conversions are reported for the reactive mixtures, for various PEI amounts and polycondensation temperatures. A thermodynamic model based on the Flory-Huggins-Staverman approach, taking polydispersity of both components into account, was used to analyze the experimental information. A single relationship between the interaction parameter and temperature, χ(T), could fit experimental results of mixtures of two commercial PEIs with DGEBA. The addition of DDS led to a decrease in miscibility whereas MCDEA improved the initial miscibility. In both cases, the interaction parameter decreased with conversion, meaning that PEI was more compatible with oligomeric species than with the mixture of starting monomers. The phase separation process in initially miscible rubber- or thermoplastic-modified thermosetting polymers is the result of two factors: increase in the average molar size of the thermosetting oligomer (main driving force favoring demixing), and variation of the interaction parameter with conversion, which may act to increase or decrease the cloud-point conversion determined by the first factor. © 1996 John Wiley & Sons, Inc.     

Thermoplastic composites based on flax fibres and polypropylene: Influence of fibre length and fibre volume fraction on mechanical properties

Natural-fibre-mat-reinforced thermoplastic (NMT) composites based on flax fibre mats and a Polypropylene (PP) matrix were manufactured using (i) a film-stacking method and (ii) a paper making process. The influence of fibre length and fibre content on stiffness and strength is reported and compared with data for glass-mat-reinforced thermoplastic (GMT) composites, including the influence of using maleic-anhydride grafted PP. The data is also compared with existing micromechanical models like Kelly-Tyson and Cox-Krenchel for strength and stiffness, respectively. A good agreement was found between theory and experiment in case of stiffness while in case of strength the experimental values fall well below the theoretical predictions. Results indicated that NMTs are of interest for low-cost engineering applications and can compete with commercial GMTs, especially when a high stiffness per unit weight is desirable. Results also indicated that the key area for future development lies not only in improved adhesion but mainly in improving the fibre strength.     

Chemical and structural characterisation of DGEBA-based epoxies by time of flight secondary ion mass spectrometry (ToF-SIMS) as a preliminary to polymer interphase characterisation

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) has become a powerful tool in the field of surface analysis since it provides information about the top few monolayers of a sample, i.e. on the chemical composition of the sample surface. Thus, the general question arises whether a surface-sensitive technique like ToF-SIMS would be appropriate to detect systematic chemical and/or structural changes in organic bulk polymers caused by varying a chemical content of the initial components or by tracking, e.g. curing processes in such materials. It is shown that careful sample preparation and the use of multivariate methods permit the quantitative acquisition of chemical and structural information about bulk polymers from the secondary ion signals. The hardener concentration and a cross-linking coefficient in diglycidyl ether of bisphenol A based epoxies were determined by ToF-SIMS measurements on samples with different resin to hardener ratio and varying curing time. In future work, we will use the developed method to investigate the local composition of adhesively bonded joints. In particular, the mapping of the chemical and structural properties in the so-called interphase will then be of interest.     

Monomer volume fraction profiles in pH responsive planar polyelectrolyte brushes

Spatial dependencies of monomer volume fraction profiles of pH responsive polyelectrolyte brushes were investigated using field theories and neutron reflectivity experiments. In particular, planar polyelectrolyte brushes in good solvent were studied and direct comparisons between predictions of the theories and experimental measurements are presented. The comparisons between the theories and the experimental data reveal that solvent entropy and ion‐pairs resulting from adsorption of counterions from the added salt play key roles in affecting the monomer distribution and must be taken into account in modeling polyelectrolyte brushes. Furthermore, the utility of this physics‐based approach based on these theories for the prediction and interpretation of neutron reflectivity profiles in the context of pH responsive planar polyelectrolyte brushes such as polybasic poly(2‐(dimethylamino)ethyl methacrylate) (PDMAEMA) and polyacidic poly(methacrylic acid) (PMAA) brushes is demonstrated. The approach provides a quantitative way of estimating molecular weights of the polymers polymerized using surface‐initiated atom transfer radical polymerization. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 956–964