Amorphous brittleness

The common form of asphalt is a black, compact, amorphous, brittle mass of dull luster, which breaks showing a polished surface and fuses at 188-90°F (ca 87°), d 1.40-1.42 at 77°F (25°C) and Moh s hardness 1 to 2... 

Typical stress-strain curves for plastics are shown in Fig. 1.2. The figure shows qualitatively (a) the steep, but non-linear curve associated with amorphous, brittle thermoplastics such as unmodified polystyrene, (b) the equivalent graph for a similar brittle thermoplastics material to which rubber has been added to produce a high impact grade and (c) an intrinsically tough thermoplastics material, such as a nylon (polyamide). 

Fig. 43. Campositioii ranges for the formation of the amorphous phase, and the changes of and in (a) Al-Y-Ni and (b) Al-Ce-Ni systems. Double open circle, amorphous (ductile) open circle, amorphous (brittle) semi-open circle, amorphous plus crystalline solid circle, crystalline. The asterisk represents 7",.

The amorphous brittle and ductile homopolymers are illustrated in Section 1.2 and the amorphous copolymers in Section 1.3. The other polymers form separate groups and are illustrated in Chapter 3 (block copolymers). Chapter 4 (polymer blends). Chapter 5 (rubber-toughened polymers). Chapter 6 (particle-filled micro-and nanocomposites), and Chapter 7 (fiber-reinforced polymers). 

The characteristic mechanical property of the amorphous polymers is high strength and a brittle up to ductile deformation behavior. The reason for this behavior is the formation of localized deformation zones under load, such as crazes, deformation bands, or shear bands [12]. The typical type of deformation seen in the amorphous brittle, glassy polymers is the craze. Crazes are often visible with the naked eye in reflected light see Fig. 1.4. The word craze recalls a macroscopic crack-like appearance craze comes from an old English word. 

Crystalline tellurium has a silvery-white appearance, and when pure exhibits a metallic luster. It is brittle and easily pulverized. Amorphous tellurium is found by precipitating tellurium from a solution of telluric or tellurous acid. Whether this form is truly amorphous, or made of minute crystals, is open to question. Tellurium is a p-type semiconductor, and shows greater conductivity in certain directions, depending on alignment of the atoms. 

Figure 9.3. Stress-strain curves for (a) rigid amorphous plastics material showing brittle fracture and (b) rubbery polymer. The area under the curve gives a measure of the energy required to break the...

For reasons explained below, the effect of increasing the vinyl alcohol content in EVOH is quite different to that of increasing the vinyl acetate content in EVA. In the case of ethylene-vinyl acetate (EVA) copolymers, increasing the vinyl acetate content up to about 50% makes the materials less crystalline and progressively more flexible and then rubbery. In the range 40-70% vinyl acetate content the materials are amorphous and rubbery, whilst above 70% the copolymers become increasingly rigid and brittle. 

As with other rigid amorphous thermoplastic polymers such as PVC and polystyrene (see the next chapter) poly(methyl methacrylate) is somewhat brittle and, as with PVC and polystrene, efforts have been made to improve the toughness by molecular modification. Two main approaches have been used, both of which have achieved a measure of success. They are copolymerisation of methyl methacrylate with a second monomer and the blending of poly(methyl methacrylate) with a rubber. The latter approach may also involve some graft copolymerisation. 

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

In the crystalline region isotactic polystyrene molecules take a helical form with three monomer residues per turn and an identity period of 6.65 A. One hundred percent crystalline polymer has a density of 1.12 compared with 1.05 for amorphous polymer and is also translucent. The melting point of the polymer is as high as 230°C. Below the glass transition temperature of 97°C the polymer is rather brittle. 

The brittleness of isotactic polystyrenes has hindered their commercial development. Quoted Izod impact strengths are only 20% that of conventional amorphous polymer. Impact strength double that of the amorphous material has, however, been claimed when isotactic polymer is blended with a synthetic rubber or a polyolefin. 

The polymers are amorphous with brittle points (quite closely related to the Tg) of about -15°C and - 0°C respectively. 

X-ray evidence shows the material to be completely amorphous as might be expected from such a complex mixture. The specific gravity ranges from 1.05 to 1.10. It is slightly harder than gypsum and therefore just not possible to scratch with a fingernail. Yellow in colour, it is less brittle than other hard natural resins and may therefore be carved or machined with little difficulty. The refractive index is 1.54. 

When the temperamre is lowered, rubbers become stiff and brittle. All rubbers eventually stiffen to a rigid, amorphous glass at the glass transition temperature (Tg). This temperature also indicates the low-temperature service limit of the rubber. Tg values are dependent on the structure, degree of cross-linking (vulcanization) and isomeric composition of the rubber. 

Transition region or state in which an amorphous polymer changed from (or to) a viscous or rubbery condition to (or from) a hard and relatively brittle one. Transition occurs over a narrow temperature region similar to solidification of a glassy state. This transformation causes hardness, brittleness, thermal expansibility, specific heat and other properties to change dramatically. 

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