Macromolecules 2000, 33, 3392-3403
Influence of Structural and Topological Constraints on the Crystallization
and Melting Behavior of Polymers.
2. Poly(arylene ether ether ketone)
Herve´ Marand,* Azar Alizadeh, Robin Farmer, Ravi Desai, and Vesselin
Velikov
Departments of Chemistry and Materials Science and Engineering,
Virginia Polytechnic Institute and State University,
Blacksburg, Virginia 24061-0212
Received August 11, 1999; Revised Manuscript Received February 4, 2000
ABSTRACT:
The secondary crystallization and its influence on the glass transition
are studied as a function of crystallization temperature and time by differential
scanning calorimetry for PEEK. The multiple melting behavior resulting
from isothermal annealing from the glass or crystallization from the melt
is discussed in the context of models considering either a melting-recrystallization-remelting
process or a bimodal population of primary and secondary crystals. The
heating rate dependence of the multiple melting behavior indicates that
reorganization of primary crystals occurs during heating for samples annealed
from the glassy state but is insignificant for those crystallized from
the melt. For either mode of crystallization, the high- and low-temperature
endothermic regions are associated with the melting of primary and secondary
crystals, respectively. Investigations of the low endotherm transition
temperature and heat of fusion as a function of crystallization time and
temperature lead to the following conclusions: the melting temperature
of secondary crystals increases linearly with the logarithm of secondary
crystallization time at a rate, B(T), increasing linearly with decreasing
temperature; the Avrami exponent, which characterizes the initial stage
of secondary crystallization, is constant below ca. 310 °C (n = 1/2)
but increases gradually with temperature above 310 °C; the late stage
of secondary crystallization is characterized by a linear increase
in crystallinity with logarithm of time. Studies of the evolution of the
glass transition after secondary crystallization indicate that the calorimetric
Tg increases linearly with the logarithm of time at a rate, b(T), increasing
with decreasing temperature. Finally, a qualitative model of polymer crystallization
of semiflexible polymer chains is proposed. This model considers the effect
of structural constraints (chain stiffness) on the nature of the amorphous
phase after primary crystallization and the effect of topological constraints
(pinning of amorphous chains) on the secondary crystallization behavior.