Amorphous elevated-temperature strength

Most cephalospoiin antibiotics are white, off-white, tan, oi pale yeUow sohds that are usually amorphous, but can sometimes be obtained crystalline. The cephalosporins do not usually have sharp melting points, but rather decompose upon heating at elevated temperatures. The acid strength, p of the... 

Penicillins have several properties that are characteristic of /i-lactam antibiotics. They are obtained in relatively pure form as off-white, tan, or yellow freeze-dried or spray-dried solids that are usually amorphous. Alternatively they are sometimes obtained as crystalline solids, often as hydrates. Penicillins do not usually have sharp melting points, but decompose upon heating to elevated temperatures. Most natural members have a free carboxyl group and commercial preparations are generally either supplied as salts, most frequently as sodium salts, or in zwitterionic form as hydrates, e.g.. amoxicillin trihydrate. The acid strength of the carboxyl group in aqueous solution varies from pAT = 2.73 for oxacillin to p= 3.06 for carbenicillin. 

The crystallinity of the pary lenes determines two of their most important practical characteristics mechanical strength at elevated temperatures and solvent resistance. The crystallinity of parylenes is confined to small-submicromctcr domains that arc randomly dispersed throughout an amorphous continuum. Because the crystalline domains are much more resistant to permeation than the amorphous phase, they retain their reinforcing structural role even in the presence of permeants in the amorphous phase, thus giving the parylenes dieir resistance to solvent attack. 

Amorphous thermoplastic introduced in 1982. The material is characterized by high strength and rigidity at elevated temperatures, long-term heat resistance, and highly stable dimensional and electrical properties. 

Amorphous or fused silica has long been recognized as a very valuable material of construction, especially for use at elevated temperatures, because it has great strength and a coefficient of expansion so low as to make it resistant to thermal shock. On the other hand, lack of practicable means of fabrication has impeded its widespread use even in view of its outstanding properties. 

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