The Wikipedia article on nylon sounds competant to me:
http://en.wikipedia.org/wiki/Nylon#Bulk_properties
>From this, it appears that the molecular structure of nylon
"below Tm [the melting point]" is "dependent on details of
formation", which would seem to argue against the possibility
of adjusting it via boiling.
If you often boil nylon strings, you might (in accordance
with the above aritcle) try cooling them rapidly (eg, with
tongs and ice water), in an effort to increase their
flexibility.
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