in comparing the fractional crystallinity measured by
DSC it must be carried out at the same temperature. In
amorphous PET samples crystallisation occurs at 50 K
above the glass transition temperature, and it is reasonable
to consider that the fractional crystallinity
measured at 375 K does not change until this temperature
is exceeded. The original fractional crystallinity of
PET samples, crystallised at different temperatures and
times, as measured by the First Law method, density
and WAXD are compared in Table 1. There is sufficient
agreement between the various methods to suggest that
there is nothing unique about the crystallinity measured
by DSC.