Interfacial stiffness

A basic concept is then the interfacial stiffness and the description in terms of the capillary wave Hamiltonian (Privman, 1992). To introduce these terms, we consider the one-dimensional interface z = h(x) of a two-dimensional system for simplicity. Noting that in lattice systems the interfacial energy jnt will depend on the angle 9 between the tangent to the interface and the x-axis, we write 9 = arctanfdft/dx)]... 

Figure 9. Interfacial Stiffness/Contact Pressure Calibration...

It has been also shown that when a thin polymer film is directly coated onto a substrate with a low modulus ( < 10 MPa), if the contact radius to layer thickness ratio is large (afh> 20), the surface layer will make a negligible contribution to the stiffness of the system and the layered solid system acts as a homogeneous half-space of substrate material while the surface and interfacial properties are governed by those of the layer [32,33]. The extension of the JKR theory to such layered bodies has two important implications. Firstly, hard and opaque materials can be coated on soft and clear substrates which deform more readily by small surface forces. Secondly, viscoelastic materials can be coated on soft elastic substrates, thereby reducing their time-dependent effects. 

Polypropylene block and graft copolymers are efficient blend compatibilizers. These materials allow the formation of alloys, for example, isotactic polypropylene with styrene-acrylonitrile polymer or polyamides, by enhancing the dispersion of incompatible polymers and improving their interfacial adhesion. Polyolefinic materials of such types afford property synergisms such as improved stiffness combined with greater toughness. 

Polycarbonates are manufactured via interfacial polymerization or through a melt esterification process. The properties of polycarbonate can differ greatly based on the method of polymerization. Specifically, the molecular weight distributions created by the two methods differ because of kinetic effects. Polycarbonates manufactured via interfacial polymerization tend to be less stable at high temperatures and less stiff than those produced via melt esterification, unless proper manufacturing precautions are taken. Therefore, when choosing a polycarbonate resin grade for a specific application, it is important to know the method by which it was produced. Either polymerization method can be performed as a continuous or batch process. 

In RIM processes, two or more reactive components are mixed together, starting the reaction between the components before the mixture is dispensed into the mold. This tends to increase the viscosity of the liquid that is dispensed due to an increase in molecular weight of the polymers or pre-polymers formed in the initial reaction. An increased viscosity can prohibit complete filling of the mold and permeation of the preform. This tends to decrease the adhesion between the matrix and the fibers. Poor interfacial adhesion between the reinforcement and matrix phase can cause a material to have less than desirable stiffness and strength. 

Step-growth polymerization, 22, 24-25, 23, 84-86, 86,90-92,114-115, 261 compared with chain-growth polymerization, 88-89, 88-89 interfacial polymerization, 91-92 laboratory activities on synthesis of nylon, 228-230 synthesis of polyesters in the melt, 231-233 synthesis of polyurethane foam, 234-237 molar mass and, 86, 86 polycondensation of poly ethylene terephthalate), 90-91 polymers produced by, 86 types of monomers for, 90 Stereochemistry, 28, 37-39,41-42, 70 tacticity, 103-105 Stereoisomers, 41 Stereoregularity, 70 Stiffness, 142, 261 Strain, 142-143, 261 Strength... 

Among the applications discussed in this chapter, the most prominent in recent years is CNT-reinforced polymer nanocomposites. The use of CNTs in polymers can provide superior mechanical properties (60). For instance, the addition of 1% CNTs might increase the stiffness of polymers by 10% and increase their resistance to fracture however, improvements in the properties of CNT-reinforced polymers largely depend on the dispersion of CNTs within the polymer matrix and the polymer-CNT interfacial properties. The following section highlights several studies regarding the processing of PLA-CNT nanocomposites. 

In conventional composites filled with carbon black or silica, the increase in stiffness is mainly associated with a change in the structure and dynamics of the polymer at the filler surface. On account of the enormous surface-to-volume ratio of the particles, the polymer in the interfacial region represents a significant fraction of the materials and its behavior significantly affects or even governs the properties of the composite. 

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