Poster Presentation 1
EFFECT OF CRYSTALLINITY ON MELT MEMORY OF RANDOM ETHYLENE
1-ALKENE COPOLYMERS
Xuejian Chen1, Al Mamun1, Rufina G. Alamo1
1
Department of Chemical and Biomedical Engineering, FAMU-FSU College of Engineering, 2525 Pottsdamer St,
Tallahassee, Florida 32310-6046
Abstract
A strong memory effect of crystallization has been observed in melts of random ethylene copolymers even
above the equilibrium melting temperature. Melt memory is correlated with self-seeds that increase the
crystallization rate of ethylene copolymers. The seeds are associated with molten ethylene sequences from the
initial crystals that remain in close proximity and are unable to diffuse quickly to the randomized melt state.
Fast diffusion is restricted by topological chain constraints (loops, knots, and other entanglements) that build
in the intercrystalline region during crystallization. The effect of topological constraints on melt memory, or
on number of remaining seeds in the melt, was analyzed studying copolymers with different levels of
crystallinity. There is a threshold level of crystallinity below which copolymers do not display strong melt
memory. A faster development of the initial crystallinity by dynamic cooling traps more efficiently knots and
loops around the crystallites leading to a lower crystallinity threshold compared to slower isothermal
crystallization. Higher thresholds of crystallinity level is required for observing melt memory in copolymers
with lower molecular weight due to faster sequence diffusion in the melt. Besides, the threshold crystallinity
level is found to display Arrhenius type temperature dependence with the activation energy being independent
of molecular weight, further highlighting its correlation with diffusion. Finally, with increasing branch
content, the threshold crystallinity level decreases as a result of creating more entanglements by selecting a
larger number of sequences with shorter length to make the same amount of crystal.
Fig 1. Evolution of melt and subsequent crystalline structure from different initial crystallinity.
Reference:
1. X. Chen, A. Mamun, R. G. Alamo, Macromol. Chem. Phys. 2015, 216, 1220.
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