Rubber crystals, melting range

The view that the degree of imperfection depends on the amount of supercooling is borne out by the observations of Bekkedahl and Wood on rubber. They showed that the melting range (for fast melting) is lower the lower the temperature at which the rubber had been allowed to crystallize. Other crystalline polymers exhibit parallel behavior. 

The melting range of rubber crystals as a function of the crystallisation temperature. 

Fig. 2.6 Melting range of natural rubber as a function of the temperature of crystallization. (From Wood and Bekkedahl (8))...

In these polyester TPEs, the hard polyester segments can crystallize, giving the polymer some of the attributes of semicrystalhne thermoplastics, most particularly better solvent resistance than ordinary rubbers, and also better heat resistance. Above the melting temperature of the crystalhne regions, these TPEs can have low viscosity and can be molded easily in thin sections and complex stmctures. Properties of thermoplastic polyester elastomers can be fine-tuned over a range by altering the ratio of hard to soft segments. 

When the crystallization is conducted over the complete accessible temperature range, i.e. from below the melting temperature to above the glass temperature, a more complex temperature dependence evolves. A typical example is given in Fig. 9.6 for natural rubber.(19) Here the time taken for half of the crystallization to develop is plotted as a function of the crystallization temperature. Many other homopolymers behave in a similar manner. This type of temperature dependence is not restricted to long chain polymers. It is also observed during the crystallization... 

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