Model samples of the interface of an adhesive joint containing small levels of aminopropyl triethoxysilane (APS) have been prepared in order to examine the interface formed with an aluminium substrate. X-ray photoelectron (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS) has been used to analyse and image the interface region in between the aluminium and an epoxy adhesive in order to ascertain the reactions by the organosilane, present as a minor component within the system. It was found that APS was present at the interface between the adhesive and the substrate and that it had reacted with the substrate forming a covalent bond and was also crosslinked within the adhesive. Evidence of near to full hydrolysis of APS is also present within the spectra.

Examination of the interface of a model adhesive joint by surface analysis: a study by XPS and ToF‐SIMS

We propose an alternative model of diffusion in polymers, which may explain formation of propagating sharp diffusion fronts to be observed in some experiments. Differently from typical Case II or Stefan problem models, plasticisation of the polymer matrix is not necessarily required. Instead, diffusing small molecules are assumed to be captured by some specific sites of the polymer matrix for certain retention times. For example, they may become adsorbed at the surface of the holes and microvoids that are considered to be present in glassy polymer regions according to the well-known dual-sorption model and are attributed to excess free volume frozen into the glassy matrix. On the other hand, the small molecules may become bound to polymer chains. In this adsorbed/bound state they have much smaller movability and thus are effectively excluded from the diffusion. This adsorption/binding is considered to be a reversible process of state changing, but with asymmetric probabilities for direct and reverse events. With this assumption, taking into account that the amount of space available for such immobilised molecules is limited (limited free volume, limited number of anchoring sites) the model naturally results in the formation of sharp diffusion fronts or strong concentration gradients. Such a diffusion front is a region of effective immobilisation (adsorption/binding, though reversible) of the free diffusing molecules. For the simplest prescribed boundary conditions the diffusion front propagates linearly with the square root of time. By means of imposing generalised boundary conditions, it is possible to model other regimes of propagation of the diffusion front, including constant velocity.

Sharp diffusion front in diffusion problem with change of state.

Results of an experimental study and numerical simulation of the curing kinetics of an epoxy binder under the conditions of violation of the stoichiometric equilibrium of its components are presented. The object of research is a two-component composition of an L epoxy resin and an EPH 161 hardener, certified as a binder for composite materials of aerospace purpose. As the basic model, we used a system of two ordinary differential equations describing the behavior of the resin and hardener during the curing process. The values of material parameters necessary for calculations were obtained by a rheological method, where the viscosity of binder was analyzed as a function of polymerization time and hardener concentration. The estimates of model parameters obtained by processing data of full-scale experiments are presented. The effective values of kinetic parameters at which the calculated time of yield loss is in good agreement with the experimental one are determined. The adequacy of the model was confirmed by direct measurements using the IR spectroscopy.

Modeling the Curing Kinetics of an Epoxy Binder with Disturbed Stoichiometry for a Composite Material of Aerospace Purpose

The character of the relationship between three parameters extremely important in terms of gas permeability of polymers, viz, selectivity of gas permeability F p , the fraction of the free volume ψ involved in the transfer of the gas molecules and the kinetic rigidity of the polymer matrix expressed through the glass transition point T g of the polymer, has been established. It is shown that the dependence of ψ on T g has a minimum on passing from highly elastic to glassy polymers and therefore to the same value of ψ may correspond two polymers in different physical states (highly elastic and glassy) and hence with different kinetic rigidity and value F p . It is concluded that the use of polymers with high T g as material for gas separating membrane is promising.

Selectivity of gas permeability of polymers

1.

Two principles of the physicochemical modification of epoxy polymers, leading to their strengthening, have been substantiated and experimentally confirmed: interstructural and structural modifications.




2.

The main laws, characteristics, and the mechanism of strengthening of epoxy polymers by small additions, antiplastification, dispersed filling, and vibroacoustic treatment of the oligomers have been established. These methods of modification are based on an increase in the energy of intermolecular interaction and the packing density, which compensate the defects, in the topological structure of the reticulated polymer.




2.

A correlation has been found to exist between the change in the supermolecular structure of epoxy polymers and their properties in the last three modification methods. It has been established that the associates of the original oligomers, having a thermal fluctuation character, represent the “precursors” of this structure, which during hardening and glass transition are converted to morphological elements of the reticulated polymer.

Principles of strengthening of epoxy binders

Electron microscopy, electron probe analysis, and a sorption-desorption method have been used to study the effects of surfactants on the structure of hardened epoxy resins (ER) and on the diffusion of water vapours through them. Two concentration ranges have been found with differing effects of the surfactants. Where their concentration was small (c ⩽ 0·5% w/w), the structural and property changes were not pronounced, but there was a steady change of the diffusionary and structural characteristics above 0·5% w/w surfactant content; this increased with concentration until phase separation of system ER-surfactant took place. The interpretation of the results is based on two properties of the surfactant, namely their plasticizing action and that of structural modification. Correlations linking the changes in diffusion of the ER with the density and the contents of the surfactants are based on the free volume theory.

The effect of surfactants on the structure and the diffusion properties of polyepoxides

The diffusion, sorption and phase state water in block copolymers of butadiene and isoprene with styrene and α-methylstyrene has been studied by sorptio, optical and gravimetric methods. When water interacts with the copolymers two types of kinetic sorption occur. At low temperatures and small partial water-vapour pressures (p/ps < 0·9) Fickian sorption occurs and an equilibrium concentration of the absorbed material is reached in the samples. At high temperatures and p/ps ⩾0·5 at a two stage type of vigorous water absorption takes place, without cessation over a long period of time, and the samples become turbid. It is shown that when the block copolymers absorb moisture the water aggregates at the polystyrene-rubber interfaces, because of the specific distribution of impurities in the polymer matrix. At high temperatures diffusion and sorption in such systems is complicated by the occurrence of chemical reactions.

Diffusion and sorption of water in block copolymers

A study was made of the effect of a filler on the supermolecular structure of crosslinked and plasticized epoxy resin. The investigation was based on electron microscopy, and on density measurements and determination of the phase state of the polymer. It is shown that changes in structural-morphological characteristics of the plasticized compositions are accounted for by thermodynamic parameters of the systems. Structural changes of an associative character both at micro- and macro-levels accompany the insertion of a filler into a plasticized epoxy matrix, while at the same time no morphological changes are observed, and the density of the crosslinked resin remains unaltered. It was found that the filler not only results in changes in matrix density, but also leads to the decomposition of the system hardened polymerplasticizer being localized in space.

Supermolecular structure of filled crosslinked polymers

The experimental results of diffusion and solubility studies of organic solvents and plasticizers into epoxy resins (ER) (polyhydroxylates) are given for a wide range of molecular weights (mol.wt.), temperatures, and compositions of the binary systems. The mol.wt. of the ER has been found to greatly affect the diffusion parameters of the binary system at all compositions. In dilute solutions of 0·9⩽ϕ1 (ϕ1-volume fraction of solvent), the dimensions and shape of the diffusing molecules mainly affect the auto-diffusion coefficients of the polymer, while those of the low mol.wt. compounds will be almost independent of the mol.wt. of the macromolecules. At medium composition (0·6⩽ϕ1⩽0·9) the thermodynamic parameters of the binary system determine the nature of the dependence of the autodiffusion coefficient on the mol.wt. of the ER. When ϕ1<0·5, the autodiffusion coefficients will be similar to the autodiffusion values of the solvents and any changes as a function of the mol.wt. of the ER will be related to the changes in internal mobility of the chains in the polymer matrix. The free volume theory is used to suggest a quantitative interpretation of the experimental results.

S.A. Nenakhov

Papers

The effect of surfactants on the structure and the diffusion properties of polyepoxides

Diffusion and sorption of water in block copolymers

Supermolecular structure of filled crosslinked polymers

The diffusion of low molecular weight substances into epoxy resins as a function of their molecular weight