Interface interpenetrating

In developing criteria for the ranking of adhesive formulations or adherend surface treatments or primers, it is necessary to distinguish between two different situations. In one case (contact adhesion), a true interface is believed to exist across which intermolecular forces are engaged, while in the other, an interphase is formed by diffusive interpenetration or interdigitation between the adhesive and the adherend (diffusion interphase adhesion). Even in the case of contact adhesion, more often than not, an mi vphase of macroscopic thickness forms on... 

Intuitively the toughness of an interface would be expected to be related to the depth of interpenetration of the chains. Wool [32] argues that the fracture energy, r, for chain disentanglement at least, is proportional to the square of the interface thickness, which, via Eqs. 36 and 37, gives ... 

Fig. 3. XPS evidence of interpenetration at a silane/PVC interface (from Ref. [45]). Open circles primer dried at I75°C filled circles primer dried at 25°C.

Consequently, interpenetrating phase-separated D/A network composites, i.e. bulk heterojunction , would appear to be ideal photovoltaic materials [5]. By controlling the morphology of the phase separation into an interpenetrating network, one can achieve a high interfacial area within a bulk material. Since any point in the composite is within a few nanometers of a D/A interface, such a composite is a bulk D/A heterojunction material. If the network in a device is bicontinuous, as shown in Figure 15-26, the collection efficiency can be equally efficient. 

Topley and Hume [453], in a study of the dehydration of CaC03 6 H20, assumed the rapid initial formation of (on average) a single nucleus on the surface of each particle of reactant, represented as a sphere of radius a. In the absence of preferential surface development, the reaction interface penetrates the reactant at equal rates in all inward directions (kG = dr/df) and the volume of material reacted at time t is that volume of a sphere, having its centre at the site of surface nucleation and of radius kGt, which falls within the reactant. The fractional reaction, the zone of interpenetrating spheres, at time t is... 

The use of interpenetrating donor-acceptor heterojunctions, such as PPVs/C60 composites, polymer/CdS composites, and interpenetrating polymer networks, substantially improves photoconductivity, and thus the quantum efficiency, of polymer-based photo-voltaics. In these devices, an exciton is photogenerated in the active material, diffuses toward the donor-acceptor interface, and dissociates via charge transfer across the interface. The internal electric field set up by the difference between the electrode energy levels, along with the donor-acceptor morphology, controls the quantum efficiency of the PV cell (Fig. 51). 

Rikvold, PA Stell, G, D-Dimensional Interpenetrable-Sphere Models of Random Two-Phase Media Micro structure and an Application to Chromatography, Journal of Colloid and Interface Science 108, 158, 1985. 

It may be noted that the statement made above—that the surface potential in the electrolyte phase does not depend on the orientation of the crystal face—is necessarily an assumption, as is the neglect of S s1- It is another example of separation of metal and electrolyte contributions to a property of the interface, which can only be done theoretically. In fact, a recent article29 has discussed the influence of the atomic structure of the metal surface for solid metals on the water dipoles of the compact layer. Different crystal faces can allow different degrees of interpenetration of species of the electrolyte and the metal surface layer. Nonuniformities in the directions parallel to the surface may be reflected in the results of capacitance measurements, as well as optical measurements. 

As the concentration of the macromolecules in equilibrium with the interface increases, so that the coil population becomes crowded, a sudden reaching of a very gently upward sloping plateau is observed, but hardly ever any multilayer formation, since no proper outer boundary of the adsorbed layer of coils develops (see below). Instead, as more coils compete for the same interfacial area, adsorption of further molecules occurs as interpenetration is still resisted, at the expense of the site areas held per molecule, i.e., the adsorbed trains become shorter, the loops longer, and the area under the coils smaller the coils become sideways and upwards compressed, (15) (16), Fig. 3. Experimentally,... 

Relatively little is understood in the presence of non planar-non ideal interfaces, where electronic levels located in the band gap region act as recombination centers. Colloidal materials, low cost polycrystalline materials and films, interpenetrating networks of absorber and charge collecting phases (e.g., as in the DSSC cells), and the presence of redox active adsorbing species, all give rise to... 

Polymers interpenetration of polymer chains, phase separation, compatibility between polymers, interdiffusion of latex particles, interface thickness in blends of polymers, light-harvesting polymers, etc. 

The diffusion theory states that interpenetration and entanglement of polymer chains are additionally responsible for bioadhesion. The intimate contact of the two substrates is essential for diffusion to occur, that is, the driving force for the interdiffusion is the concentration gradient across the interface. The penetration of polymer chains into the mucus network, and vice versa, is dependent on concentration gradients and diffusion coefficients. It is believed that for an effective adhesion bond the interpenetration of the polymer chain should be in the range of 0.2-0.5 pm. It is possible to estimate the penetration depth (/) by Eq. (5),... 

Rossi et al. [30] evaluated rheologically mucins of different origin with polyacrylic acid and sodium carboxymethyl cellulose. The same group also reported a novel rheological approach based on a stationary viscoelastic test (creep test) to describe the interaction between mucoadhesive polymers and mucins [31,32]. Jabbari et al. [33] used attenuated total-reflection infrared spectroscopy to investigate the ehain interpenetration of polyaciylic acid in the mucin interface. 

Jabbari, E., Wisniewski, N., and Peppas, N.A., Evidence of mucoadhesion by chain interpenetration at a poly(acrylic acid)/mucin interface using ATR-FTIR spectroscopy, J. Control. Rel., 26 99-108 (1993). 

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