The Degree Of Crystallization: Rate Of Crystallization

The rate of crystallization is dependent on temperature, of which there are two significant points: ( The glass transition temperature. Below Tg, there is virtually no molecular motion on a local scale. Polymers have many of the properties associated with ordinary organic glasses including hardness and stiffness. @TmThe crystalline melting point. This is the temperature at which crystals melt and a crystalline polymer resembles an amorphous polymer, which has no short-range order. Tm generally increases as the degree of crystallinity increases.

The rate of crystallization(both nucleation and growth) depends on temperature as shown in Fig1.20. Above Tm, no crystallization exists and below Tg no further movement of molecules is possible to nucleate or grow crystals. The fastest rate of crystallization occurs midway between T and Tm. The longer a polymer remains between the two, the greater the amount of crystallization. For example: For PP, Tg is-10 to-20C and Tm is 150 to 175 C, PP will therefore continue to crystallize slowly at room temperature (21 C)until equilibrium is reached. For PTFE, Tg is-110'C and Tm 330C, PTFE crystallizes rapidly at room temperature(21C).