The confinement of crystallizable blocks within AB or ABC microphase-separated block
copolymers in the nanoscopic scale can be tailored by adequate choice of composition, molecular weight,
and chemical structure. In this work we have examined the crystallization behavior of a series of AB and
ABC block copolymers incorporating one or two of the following crystallizable blocks: polyethylene, poly(ε-caprolactone), and poly(ethylene oxide). The density of confined microdomain structures (MD) within
block copolymers of specific compositions, in cases where the MD are dispersed as spheres, cylinders, or
any other isolated morphology, is much higher than the number of heterogeneities available in each
crystallizable block. Therefore, fractionated crystallization takes place with one or several crystallization
steps at decreasing temperatures. In specific cases, the clear observation of exclusive crystallization from
homogeneous nuclei was obtained. The results show that, regardless of the specific morphological features
of the MD, it is their vast number as compared to the number of heterogeneities present in the system
that determines the fractionated character of the crystallization or in extreme cases homogeneous
nucleation. The self-nucleation behavior was also found to depend on the composition of the copolymers.
When the crystallizable block is confined into spheres or cylinders and exhibits homogeneous nucleation,
the self-nucleation domain disappears. This is a direct consequence of the extremely high density of
microdomain structures that need to be self-seeded (on the order of 1015−1016/cm3). Therefore, to increase
the density of self-nuclei, the self-nucleation temperature has to be decreased to values so low that
extensive partial melting is achieved, and some of the unmelted crystal fragments can be annealed, in
some cases even before self-nucleation takes place.
4-Urazoylbenzoic acid groups are attached to the chain ends of
polyisobutylene. The
cooperative assembling process of these polar groups is studied by DSC
and dielectric and dynamic
mechanical spectroscopy. The melting of the ordered clusters
occurs in the temperature range 380−390
K. Distortions within the U4A clusters (Σ process) are monitored
below the “melting” temperature
T
m
with dielectric spectroscopy. On a larger length scale, these
distortions also lead to stress relaxation
which can be probed by dynamic mechanical measurements. Near
T
m, the relaxation of U4A
multiplets
(α* relaxation) is detected with dielectric spectroscopy. In
this temperature range, dynamic mechanical
measurements show a transition from elastic behavior to viscous flow.
The results are rationalized with
molecular modeling calculations.
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