Materials Terms Links

In this book, an attempt is made to provide an overview of the science of magnetic ceramics. Chemical aspects are covered in terms of synthetic methods and crystallography. Physics is introduced to provide a theoretical basis to magnetism, which is necessary to interpret the property measurements. Materials science links together physics and chemistry and, in addition, provides the framework for a scientific understanding of fabrication and testing, leading to applications. 

In this Chap. 5 of the book on Thermal Analysis of Materials, the link between microscopic and macroscopic descriptions of crystals is given in Sects. 5.1-3. This is followed by a thermodynamic analysis of melting of crystals and isotropization of mesophases in terms of entropy and enthalpy in Sects. 5.4 and 5.5. The final section deals with the properties of liquids and glasses (Sect. 5.6). 

In this last Chapter of Thermal Analysis of Polymeric Materials, the link between microscopic and macroscopic descriptions of multi-component macromolecules is discussed, based on the thermal analysis techniques which are described in the prior chapters. The key issue in polymeric multi-component systems is the evaluation of the active components in the system. The classical description of the term component was based on smaU-molecule thermodynamics and refers to the number of different molecules in the different phases of the system (see Sect. 2.2.5). If chemical reactions are possible within the system, the number of components may be less than the different types of molecules. It then represents the species of molecules that can be varied independendy. For example, the three independent species CaO, CO2, and CaCOj represent only two components because of the equation that links their concentrations ... 

Two types of thermoset polyimides are currently prepared commercially. They are based on low molecular weight bis imides such as bis maleimides or bis-5-norbomene-2,3-dicarboximides. Due to unsaturations, the materials cross-link by free-radical mechanism into tight networks. Michael type additions of primary and secondary amines to the bis maleimides are often used to chain-extend them before cross-linking. This reduces the cross-linking density and the brittleness [115]. The materials are designated by the term PMR, for polymerizable monomeric reactants. 

The term electrochromism was chosen in 1961 on the model of thermochromism and photochromism [58]. It qualifies the ability of a material to change its optical properties when an electrical potential is applied across it [59]. The optical properties of an elec-trochromic material are linked to its oxidation state and hence can be manipulated by the oxidation—reduction process, ie, gain or loss of electrons. The required voltage for an electrochromic colour change is very low — only a few volts are necessary. The colour of an electrochromic material remains even when the current has ceased to flow (so-called memory effect [60]), and its colour change is reversible when the inverted potential is applied. Colour change can occur from a colourless to a coloured state or from one colour to another, and even in the infrared or UV ranges. 

A thermoplastic adhesive should normally have a glass transition temperature above 40 °C if it is to resist creep. Flexible thermoplastic adhesives (Tg below 15 °C) should be used only where there is no long-term stress or risk of diffusion into porous materials. Cross-linked materials might be used in... 

Cross-linking polyethylene enhances its heat resistance (in terms of resistance to melt flow) since the network persists even about the crystalline melting point of the uncross-finked material. Cross-linked polyethylene thus finds application in the cable industry as a dielectric and as a sheathing material. Three main approaches used for cross-linking polyethylene are (1) radiation cross-linking, (2) peroxide cross-linking, and (3) vinyl silane cross-linking. 

Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state). 

We noted above that the presence of monomer with a functionality greater than 2 results in branched polymer chains. This in turn produces a three-dimensional network of polymer under certain circumstances. The solubility and mechanical behavior of such materials depend critically on whether the extent of polymerization is above or below the threshold for the formation of this network. The threshold is described as the gel point, since the reaction mixture sets up or gels at this point. We have previously introduced the term thermosetting to describe these cross-linked polymeric materials. Because their mechanical properties are largely unaffected by temperature variations-in contrast to thermoplastic materials which become more fluid on heating-step-growth polymers that exceed the gel point are widely used as engineering materials. 

Fibers. The principal type of phenoHc fiber is the novoloid fiber (98). The term novoloid designates a content of at least 85 wt % of a cross-linked novolak. Novoloid fibers are sold under the trademark Kynol, and Nippon Kynol and American Kynol are exclusive Hcensees. Novoloid fibers are made by acid-cataly2ed cross-linking of melt-spun novolak resin to form a fuUy cross-linked amorphous network. The fibers are infusible and insoluble, and possess physical and chemical properties that distinguish them from other fibers. AppHcations include a variety of flame- and chemical-resistant textiles and papers as weU as composites, gaskets, and friction materials. In addition, they are precursors for carbon fibers. 

The distinctions between these homopolymers arise from the different ways in which the monomer units are hooked together in polyacetal chains. Starch (qv), plant nutrient material, is composed of two polysaccharides a-amylose and amylopectin. cx-Amylose is linear because of exclusive a (1 — 4) linkages, whereas amylopectin is branched because of the presence of a (1 — 6) as well as a (1 — 4) links. The terms linear and branched refer only to primary stmcture. 

The way to think of them all is as Si04 tetrahedra (or, in polymer terms, monomers) linked to each other either directly or via a metal ion (M) link. When silica is combined with metal oxides like MgO, CaO or AI2O3 such that the ratio MO/SiOj is 2/1 or greater, then the resulting silicate is made up of separated Si04 monomers (Fig. 16.4a) linked by the MO molecules. (Olivene, the dominant material in the Earth s upper mantle, is a silicate of this type.)... 

Whilst the term thermosetting plastics arose out of the fact that early products of this type were cross-linked by subjecting the intermediate-stage materials to elevated temperature, the term is also widely used where cross-linking takes place at normal ambient temperatures. 

It is somewhat difficult conceptually to explain the recoverable high elasticity of these materials in terms of flexible polymer chains cross-linked into an open network structure as commonly envisaged for conventionally vulcanised rubbers. It is probably better to consider the deformation behaviour on a macro, rather than molecular, scale. One such model would envisage a three-dimensional mesh of polypropylene with elastomeric domains embedded within. On application of a stress both the open network of the hard phase and the elastomeric domains will be capable of deformation. On release of the stress, the cross-linked rubbery domains will try to recover their original shape and hence result in recovery from deformation of the blended object. 

The term ABS was originally used as a general term to describe various blends and copolymers containing acrylonitrile, butadiene and styrene. Prominent among the earliest materials were physical blends of acrylonitrile-styrene copolymers (SAN) (which are glassy) and acrylonitrile-butadiene copolymers (which are rubbery). Such materials are now obsolete but are referred to briefly below, as Type 1 materials, since they do illustrate some basic principles. Today the term ABS usually refers to a product consisting of discrete cross-linked polybutadiene rubber particles that are grafted with SAN and embedded in a SAN matrix. 

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