Variations in Properties and Composition

The relationship between structure and properties is often difficult to establish since the effects of atomic size, valencies, bond length, bond strength, stoichiometry, vacancies, and vacancy ordering are yet to be fully determined. A great deal of work is still needed. 

Variations in composition and the presence of defects and vacancies may considerably alter the properties and behavior of these materials. This is reflected by the high spread of values found in the literature. In order to be meaningful, a property should be associated with the composition of the carbide being tested this is often not the case and, although a great deal of information is available, the reported values are sometimes questionable. The data listed in the following chapters must be viewed with this in mind. 

A reason for the high spread is, in addition to the variation in composition and other Actors mentioned above, the paucity of large single crystals of high quality and uniformity. Measurements made on polycrystals and films have to contend with grain boundaries, grain growth, voids, and other defects which impart an additional measure of uncertainty in the results. 

Another reason is the effect of impurities, especially dissolved oxygen. Oxygen is difficult to remove altogether and may affect the physical properties of the material, particularly measurements of the lattice parameters.[1] 

---

TABLE 2.10 Variation in Properties and Composition of the Polyaniline Fiber Spun with and without a H3PO4 Dopant Exchange... 

Many combinations of diacids—diamines and amino acids are recognized as isomorphic pairs (184), for example, adipic acid and terephthalic acid or 6-aminohexanoic acid and 4-aminocyclohexylacetic acid. In the type AABB copolymers the effect is dependent on the structure of the other comonomer forming the polyamide that is, adipic and terephthalic acids form an isomorphic pair with any of the linear, aliphatic C-6—C-12 diamines but not with -xylylenediamine (185). It is also possible to form nonrandom combinations of two polymers, eg, physical mixtures or blends (Fig. 10), block copolymers, and strictly alternating (187—188) or sequentially ordered copolymers (189), which show a variation in properties with composition differing from those of the random copolymer. Such combinations require care in their preparation and processing to maintain their nonrandom structure, because transamidation introduces significant randomization in a short time above the melting point. 

Other elements of glass structures must be included in order to discuss more subtle variations in properties and various spectra. The field strength of both network-forming and network-modifying cations must be included in discussions of trends in properties with glass compositional variations. Since many modern studies include variations in the identity of the anions which link the structure, their field strengths and ionic radii are also important. The atomic radii of mobile cations, anions, and atoms and molecules must be considered in structural models seeking to explain transport properties. 

The most important stmctural variables are again polymer composition, density, and ceU size and shape. Stmctural foams have relatively high densities (typically >300 kg/m ) and ceU stmctures similar to those in Figure 2d which are primarily comprised of holes in contrast to a pentagonal dodecahedron type of ceU stmcture in low density plastic foams. Since stmctural foams are generally not uniform in ceU stmcture, they exhibit considerable variation in properties with particle geometry (103). 

Electrodeposition is more flexible than electroless deposition, in that it is not limited by the requirement of having a catalytically active surface. Electrodeposition allows a wider variation in the alloy composition and in the deposit properties than does electroless deposition. This flexibility has not been widely exploited, however, and most of the electrodeposited alloys have had compositions similar to those obtained by electroless deposition (i.e. CoP or CoNiP). 

Since 1985, a major effort has been devoted to incorporating heterocyclic units within the backbone of poly(arylene etherjs (PAE). Heterocyclic units within PAE generally improve certain properties such as strength, modulus and the glass transition temperature. Nucleophilic and electrophilic aromatic substitution have been successfully used to prepare a variety of PAE containing heteorcyclic units. Many different heterocyclic families have been incorporated within PAE The synthetic approaches and the chemistry, mechanical and physical properties of PAE containing different families of heterocyclic units are discussed. Emphasis is placed on the effect variations in chemical structure (composition) have upon polymer properties. 

In spite of the apparent sensitivity to the material properties, the direct assignment of the phase contrast to variation in the chemical composition or a specific property of the surface is hardly possible. Considerable difficulties for theoretical examination of the tapping mode result from several factors (i) the abrupt transition from an attractive force regime to strong repulsion which acts for a short moment of the oscillation period, (ii) localisation of the tip-sample interaction in a nanoscopic contact area, (iii) the non-linear variation of both attractive forces and mechanical compliance in the repulsive regime, and (iv) the interdependence of the material properties (viscoelasticity, adhesion, friction) and scanning parameters (amplitude, frequency, cantilever position). The interpretation of the phase and amplitude images becomes especially intricate for viscoelastic polymers. 

The gradient composition in FGMs not only results in a spatial variation in properties but will also generate residual stresses, which will affect the mechanical properties. One of the potential advantages of FG components is the positive influence of compressive residual surface stresses on the strength and wear resistance. A correct design of the gradient for an optimal distribution of the residual stresses is therefore important, as discussed in this chapter. 

---