ABSTRACT
Appendix ........................................................................................................... 116 Keywords .......................................................................................................... 117 References ......................................................................................................... 117
6.1 CRYSTALLIZATION
Is crystallization from the melt controlled by entanglement? Chain molecules are entangled in the melt above the entanglement molecular mass Me. For poly(ethylene) this molecular mass amounts to Me = 3800 g/mol. One may estimate the relaxation time for disentanglement by estimating the time a chain needs diffusing across the distance corresponding to its end-to-end distance. The chain may have disentangled itself after traversing this distance. It is:
10 cm / s 10 s 100 nm
D O R
= =
The rate of crystallization for example of poly(ethylene oxide) (PEO) or poly(ε-caprolactone) (PCL) was found at undercooling ∆Tc = 30 K:
Rate of crystallization = O (10-3 s-1) Comparison shows:
Hence, entanglement does not constitute a barrier for crystallization. The unmixing of crystallizable and non-crystallizable parts of the chains proceeds without entanglement barrier. Experimental evidence for disentanglement during crystallization is given by:
• Fractionation of chains during crystallization in samples with broad molecular mass distribution
• Phase separation during crystallization in miscible amorphous/crystalline blends. In the melt, different species are entangled. After crystallization, we have neat crystallites of the respective constituent.
