Interphase stiffness

For analytical purposes, the fiber composites are conveniently modeled using axisymmetric three-phase (i.e. fiber-interlayer-matrix), four-phase (i.e. fiber-interlayer-matrix-composite medium) cylindrical composites, or in rare cases multi-layer composites (Zhang, 1993). These models are schematically presented in Fig. 7.9. The three-phase uniform interphase model is typified by the work of Nairn (1985) and Beneveniste et al. (1989), while Mitaka and Taya (1985a, b, 1986) were the pioneers in developing four-phase models with interlayer/interphase of varying stiffness and CTE values to characterize the stress fields due to thermo-mechanical loading. The four phase composite models contain another cylinder at the outermost surface as an equivalent composite (Christensen, 1979 Theocaris and Demakos, 1992 Lhotellier and Brinson, 1988). 

Decreased mobility of adsorbed chains has been observed and proved in many cases both in the melt and in the solid state [52-54] and changes in composite properties are very often explained by it [52,54]. Overall properties of the interphase, however, are not completely clear. Based on model calculations the formation of a soft interphase is claimed [51], while in most cases the increased stiffness of the composite is explained by the presence of a rigid interphase [55,56]. The contradiction obviously stems from two opposing effects. Imperfection of the crystallites and decreased crystallinity of the interphase should lead to lower modulus and strength and larger deformability. Adhesion and hindered mobility of adsorbed polymer chains, on the other hand, decrease deformability and increase the strength of the interlayer. 

The semi-Gaussian stiffness profile indicates diffusion processes occurring before the gelation of the reactive mixture of epoxy resin and curing agent. The diffusion processes are expected to modify the local structure of chemical crosslinks. Due to its finite width of 3/c=280 nm, the gradient zone has to be considered as a volume rather than as an area and, hence, it deserves to be called interphase. 

As compared to the interphases detected in the case of the Cu/epoxy systems, the interphase measured in the case of the C-fibre/PPS system exhibits a negative stiffness gradient, i.e. decreasing local stiffness of the thermoplastic PPS with increasing distance from the C-fibre surface. The mean width 3/c 107 nm of the stiffness profile is 2.6 times smaller than that of the stiffness profile measured on the Cu/epoxy replica sample. Taking into consideration the spatial constraints imposed on polymer chains due to the presence of the nearby hard wall represented by the surface of the C-fibres, the observed increase in local stiffness can be ascribed to the respective loss in chain flexibility and mobility. 

Polymer networks such as epoxies play an increasing role as adhesives in industry. Two properties are of special importance for their application (a) a strong adhesive bond is required between the solidified adhesive and the bonded object, which is often a metal (b) the mechanical stiffness of the adhesive has to be adapted to the desired level. As a consequence, the adhesive has to be selected according to its adhesion properties as well as its mechanical properties. Several studies have shown that both properties are linked as soon as the epoxy polymer layer is sufficiently thin the contact of the polymer with the substrate may induce in the polymer a broad interphase where the morphology is different from the bulk. Roche et al. indirectly deduced such interphases, for example from the dependence of the glass transition temperature on the thickness of the polymer bonded to a metal substrate [1]. Moreover, secondary-ion mass spectroscopy or Auger spectroscopy provided depth profiles of interphases in terms of chemical composition, which showed chemical variations at up to 1 pm distance from the substrate. 

In the present study an extended continuum mechanical model is derived which is able to predict either weak or stiff boundary layers in thin films. As a possible application, the formation of interphases in polymer films is investigated. In this case it was shown [7, 24, 37] that the local stiffness in the polymer depends on the combination of polymer and substrate. 

Finally, how do the mechanical properties and typical failure modes of a particular interphase microstructure influence stiffness, strength and durability under various conditions (Mechanics)... 

It was reported that the compatibilizer normally gets adsorbed on the surface of the clay platelets and alters the interphase [92]. The tensile strength and tensile modulus are always good for CPN compared with PP. The nano-level dispersion of clay in PP plays a vital role in such an improvement. The stiffness of the silicate layers contributes to the presence of immobilized (or) partially immobilized polymer phases [93]. The orientation of the silicate layer and molecular orientation also play a vital role in the improvement of the stiffness. 

The permeation of gases in such a complex structure is very difficult to model due to the lack of information on the phase structures and properties, as well as the complexity of such modelling. Qualitatively, the reduced mobility and the chain orientation in semi-ordered interphases due to the stiff and ordered crystallites would make the permeability smaller. For the Pebax grades with shorter polyether blocks and longer polyamide blocks, the tortuosity of the diffusion path will increase sharply when the polyether and amorphous PA phases become finely divided by the crystalhne phase. Nevertheless, we tried to use the PA phase crystallinity to simulate the CO2 and nitrogen permeabilities in Pebax films with the simple resistance model [35] to estimate the influence of the Pebax structure on the permeability. 

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