8.3 Phase Transformations 285 Table 8.3.Glass transition temperatures of various materials Material Ts(C) Polyethylene (low-density) -105 Polypropylene(atactic) -20 Polypropylene(isotactic) 0 Polyvinyl chloride 81 Polystyrene 95 Chalcogenide AsGeSeTeb 245 Soda lime glassb 520-600 Fused quartzb 1,175 apolymer;bceramic Although the glass transition and melting are distinct phase transitions,both involve the movements of atoms,ions or molecules in the solid.These movements are more extensive during melting than during the glass transition.Therefore, a material that has a high melting temperature tends to have a high Tg. Table 8.3 lists the Tg values of various polymers and ceramics.The Tg values are higher for ceramics than for polymers.This is consistent with the higher melting temperatures of ceramics. Among the polymers,polyethylene has a very low Te because of an absence of functional groups that cause intermolecular interactions,and an absence of bulky side groups.The bulky side groups as well as the intermolecular interactions hinder the sliding of molecules relative to one another.Polyvinyl chloride has a higher Tg because of the chloride functional group in its structure,which promotes intermolecular interactions.Polystyrene has an even higher Tg because of its bulky benzeneside group.Isotactic polypropylene(with the-CH3 side groupsof different mers on the same side of the polymer molecular chain)has a higher Te than atactic polypropylene(with the-CH,side groups of different mers positioned randomly on both sides of the polymer molecular chain)because the former is associated with more order and hence better packing of the molecular chains relative to one another.The better packing hinders the sliding of molecules relative to one another,thus causing a higher Tg. Isotactic polypropylene is a commonly used thermoplastic polymer.The addi- tion of rubber to it results in a composite that is tough and flexible.Polypropylene- polyethylene copolymers(with two types of mer in the same molecule)are attrac- tive because the presence of the polyethylene component increases the low tem- perature impact.A shortcoming of polypropylene is its tendency to degrade upon exposure to ultraviolet(UV)radiation.In order to increase the UV resistance, carbon black can be added as a filler that absorbs the UV radiation. Among ceramics,fused quartz has a higher Tg than soda lime glass.This is consistent with the higher melting temperature of fused quartz. A chalcogenide is a compound with at least one chalcogen ion(sulfur,selenium or tellurium:all elements in Group IV of the periodic table)and atleast one element that is more electropositive.For example,AsGeSeTe is a chalcogenide that has Se8.3 Phase Transformations 285 Table 8.3. Glass transition temperaturesTg of various materials Material Tg (°C) Polyethylene (low-density)a −105 Polypropylene (atactic)a −20 Polypropylene (isotactic)a 0 Polyvinyl chloridea 81 Polystyrenea 95 Chalcogenide AsGeSeTeb 245 Soda lime glassb 520–600 Fused quartzb 1,175 aPolymer; b ceramic Although the glass transition and melting are distinct phase transitions, both involve the movements of atoms, ions or molecules in the solid. These movements are more extensive during melting than during the glass transition. Therefore, a material that has a high melting temperature tends to have a high Tg. Table 8.3 lists the Tg values of various polymers and ceramics. The Tg values are higher for ceramics than for polymers. This is consistent with the higher melting temperatures of ceramics. Among the polymers, polyethylene has a very low Tg because of an absence of functional groups that cause intermolecular interactions, and an absence of bulky sidegroups.Thebulky sidegroupsaswellastheintermolecular interactionshinder the sliding of molecules relative to one another. Polyvinyl chloride has a higher Tg because of the chloride functional group in its structure, which promotes intermolecular interactions. Polystyrene has an even higher Tg because of its bulky benzenesidegroup.Isotacticpolypropylene(withthe–CH3 sidegroupsofdifferent mers on the same side of the polymer molecular chain) has a higher Tg than atactic polypropylene (with the –CH3 side groups of different mers positioned randomly on both sides of the polymer molecular chain) because the former is associated with more order and hence better packing of the molecular chains relative to one another. The better packing hinders the sliding of molecules relative to one another, thus causing a higher Tg. Isotactic polypropylene is a commonly used thermoplastic polymer. The addi￾tion of rubber to it results in a composite that is tough and flexible. Polypropylene– polyethylene copolymers (with two types of mer in the same molecule) are attrac￾tive because the presence of the polyethylene component increases the low tem￾perature impact. A shortcoming of polypropylene is its tendency to degrade upon exposure to ultraviolet (UV) radiation. In order to increase the UV resistance, carbon black can be added as a filler that absorbs the UV radiation. Among ceramics, fused quartz has a higher Tg than soda lime glass. This is consistent with the higher melting temperature of fused quartz. A chalcogenide is a compound with at least one chalcogen ion (sulfur, selenium or tellurium: all elements in Group IV of the periodic table) and at least one element that is more electropositive. For example, AsGeSeTe is a chalcogenide that has Se