Amorphous hard materials

The melt temperature of a polyurethane is important for processibiUty. Melting should occur well below the decomposition temperature. Below the glass-transition temperature the molecular motion is frozen, and the material is only able to undergo small-scale elastic deformations. For amorphous polyurethane elastomers, the T of the soft segment is ca —50 to —60 " C, whereas for the amorphous hard segment, T is in the 20—100°C range. The T and T of the mote common macrodiols used in the manufacture of TPU are Hsted in Table 2. 

Block copolymers can contain crystalline or amorphous hard blocks. Examples of crystalline block copolymers are polyurethanes (e.g. B.F. Goodrich s Estane line), polyether esters (e.g. Dupont s Hytrel polymers), polyether amides (e.g. Atofina s Pebax grades). Polyurethanes have enjoyed limited utility due to their relatively low thermal stability use temperatures must be kept below 275°F, due to the reversibility of the urethane linkage. Recently, polyurethanes with stability at 350°F for nearly 100 h have been claimed [2]. Polyether esters and polyether amides have been explored for PSA applications where their heat and plasticizer resistance is a benefit [3]. However, the high price of these materials and their multiblock architecture have limited their use. All of these crystalline block copolymers consist of multiblocks with relatively short, amorphous, polyether or polyester mid-blocks. Consequently they can not be diluted as extensively with tackifiers and diluents as styrenic triblock copolymers. Thereby it is more difficult to obtain strong, yet soft adhesives — the primary goals of adding rubber to hot melts. 

Poly(methyl methacrylate) (PMMA), an amorphous plastic material, is extremely stable to aging and to weathering it is hard and glass clear. 

At very high pressures, above 12 GPa, and temperatures above 1000 K, a transparent,yellowish, ultra-hard material, believed to consist of the remnants of collapsed molecules, is formed. In several cases ultrasonic, scratch, and indentation studies have shown this material to have a bulk modulus and hardness far exceeding that of diamond [123,131,147], although these reports are by no means uncontested [124,148,149]. The material is extremely disordered and probably has a high fraction of sp2 coordinated bonds, but the structure is unknown. There are some similarities with amorphous carbon (ta-C),but differences in Raman spectra and mechanical properties show that the structures differ. The question of bond types is interesting, since sp2 bonds are known to be stronger than sp3 ones. The materials are semiconducting and have Debye temperatures near 1450 K, somewhat lower than that of diamond [150]. 

Michaels et al.50 53) reported the chemical and physical properties of the polyelectrolyte complex of NaSS-PVBMA. This complex is used as a membrane in various fields. Polyelectrolyte complexes prepared by casting homogeneous solutions of ternary-solvent systems mentioned above are transparent and amorphous. Dried polyelectrolyte complexes are hard materials whereas wetted ones are rubber-like or skin-like. The polyelectrolyte complex with an equimolar composition is neutral whereas that with a non-equimolar composition can display ion-exchange properties. The range of the water content depends on the excess of polycation or polyanion in the complex. 

A characteristic feature of the material under investigation is its very low microhardness - between 25 and 35 MPa depending on the crystalline modification present. These values are up to 5-6 times lower than those for semicrystalline homo-PBT regardless of the crystalline modification. Moreover, the obtained values for H of PEE (Table 6.3, Fig. 6.5) are about half the amorphous hardness, // , of PBT, being 54 MPa as reported by Giri ef a/. (1997). This means that there should be other factors responsible for the very low H values of the copolymer. 

Multipurpose Applications. Cake thickness is varied, dependent upon the application. Thin cakes from finer particles or more amorphous, compressible materials versus thick cakes for hard, easy filling crystals. 

Glass is only one example of an amorphous material. Another is paper, composed of randomly oriented cellulose molecules. Many familiar objects are made up of amorphous solid materials, all lacking long-range structure or order. They are aperiodic substances—substances that do not display periodicity. Consequently, it is hard to analyze the structure of amorphous materials as each sample is unique. 

The solid polymeric blomaterlals may be subdivided Into soft and/or rubbery materials, amorphous and hard materials, and seml-crystalllne materials. Figure 2 shows examples In each of these categories for a wide variety of blomaterlals applications. 

In order to complete this review, a brief overview of magnetic materials other than oxides is presented in this chapter. Soft and hard metallic alloys are discussed first instead of a detailed account of the numerous alloy systems, this overview focuses on the mechanisms for obtaining specific microstructures, which, in turn, lead to a precise control of anisotropy in soft materials, and to coercivity in hard materials. These discussions include examples of classic alloys, as well as the recently developed soft amorphous alloys and the impressive supermagnets with extremely high coercive fields. 

---