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Nucleation in medium density polyethylenes: comparisons between metallocene and chromium catalysts
Author(s) -
Janimak J J,
Stevens G C
Publication year - 2000
Publication title -
polymer international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.592
H-Index - 105
eISSN - 1097-0126
pISSN - 0959-8103
DOI - 10.1002/1097-0126(200011)49:11<1329::aid-pi487>3.0.co;2-5
Subject(s) - nucleation , materials science , lamellar structure , polyethylene , crystallinity , crystallization , low density polyethylene , crystallization of polymers , metallocene , chromium , high density polyethylene , polymer , supercooling , chemical engineering , thermodynamics , polymer chemistry , chemistry , composite material , polymerization , metallurgy , physics , engineering
Nucleation of metallocene‐catalysed medium density polyethylenes is reported and the results compared with a chromium‐catalysed polyethylene. Results from stepwise incremental temperature jump experiments in and around the melting temperature region show enhanced nucleation behaviour in chromium‐catalysed polyethylene that was absent in the metallocene‐catalysed polyethylenes. Conventional Avrami analysis has been applied to describe the overall crystallization kinetics. Avrami exponents of approximately two were observed for the primary lamellar domain growth process. While Avrami exponents fluctuated between lamellar and spherulitic growth, rate constants increased and crystallization half‐times decreased with decreasing crystallization temperature. The chromium catalysed polyethylene was found to crystallize faster and have higher crystallinity. This effect of enhanced nucleation was not seen in the other materials over the temperature range we explored in these experiments. In estimating the nucleation density at constant supercooling, the metallocene materials were at least several orders of magnitude greater than the chromium‐catalysed polyethylene. © 2000 Society of Chemical Industry