4. Conclusion
Based on the DSC and XRD results, a mechanism for the isothermal
crystallization of PO blended with MP up to a concentration of 8%
could be proposed. This is schematically presented in Fig. 11, along
with a comparison with the crystallization mechanism of PO. At a
high degree of undercooling, PO showed a two-step crystallization
with the formation of α crystals in the first step. In the second step,
these crystals transformed into β′1 crystals and additional β′1 crystals
were formed from the melt. At a lower degree of undercooling, PO
directly crystallized into the β′1 polymorph. For the MP blends, an
earlier onset of crystallization was observed. It was suggested that
the high-melting MP initiated the crystallization process, possibly accompanied
by co-crystallization of some of the highest-melting components
of PO. The crystallization of MP triggered the crystallization
of the higher-melting point TAGs of PO in the β′1 polymorph. When
the degree of undercooling was high enough, the lower-melting
point TAGs of PO crystallized in the α polymorph, which subsequently
transformed to β′2 crystals. When the concentration of MP was
rather low, it is possible that α formation occurred both on the previously
crystallized MP/high-melting TAGs and in the melt. As indicated
in Fig. 11, the temperature above which PO formed no α crystals anymore,
i.e. the cut-off temperature, was higher when MP had been
added to PO. Furthermore, it has been shown that the addition of
MP to PO yielded a coarser crystal structure.