Self-seeding effect

The base-line drawing procedure is the main reason for the relative low repeatability of the Hf-value determination. The self-seeding effect, clearly influencing the Tc-value, makes calculation of an average Tc-value meaningless. The He-, Tm2-and Hf2-values are hardly influenced, thus ... 

It is interesting to note the magnitude of the undercooling necessary to recrystallize PHB in the absence of self-seeding—almost 120°C Similar effects can be seen with other polymers but the maximum undercoolings measured are usually only 60 or 70 C. The massive temperature range over which nucleation and crystallization can be observed in PHB is one of the major reasons for academic interest in the material as a model polymer for morphological and crystallization kinetics studies. 

A scale has been proposed by which the effectiveness of a nucleation catalyst can be assessed. In addition to comparing the crystallization temperature on cooling the nucleated polymer with that of the pure polymer, the crystallization temperature of the self-seeded polymer is also taken into account.(268,269) The self-seeded polymer (270) is taken to represent the ideal nucleated system. The efficiency of a nucleating agent, E, expressed as a percentage, is given as (268,269)... 

In addition to the nucleating agents discussed in Section 18.4, many other materials have been found to be effective. Whilst the nylons may be self-nucleating, partieularly if there is some unmelted crystal structure, seeding with higher melting point polymers can be effective. Thus nylon 66 and poly(ethylene terephthalate) are reported to be especially attractive for nylon 6. 

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