2 research outputs found
Crystallization of Isotactic Polypropylene under the Spatial Confinement Templated by Block Copolymer Microdomains
We investigate the crystallization behavior of isotactic
polypropylene
(iPP) under the influence of nanoscale confinement templated by the
microphase-separated structure of an iPP-based diblock copolymer system,
isotactic polypropylene-<i>block</i>-atactic polystyrene
(iPP-<i>b</i>-aPS). Three types of iPP microdomains, i.e.,
lamellae, cylinder, and sphere, were generated by controlling the
composition of the diblock. The effect of microdomain morphology on
the nucleation mechanism, crystallization kinetics, self-nucleation
behavior, the population of the helical sequence of iPP block in the
melt state, and crystal orientation have been systematically studied.
It was found that the crystallization rate of iPP was predominantly
controlled by homogeneous nucleation when the crystallization process
was largely confined within the individual cylindrical and spherical
microdomains. Such a nucleation mechanism and the highly frustrated
crystal growth in the isolated microdomains led to the absence of
Domain II and atypical crystallization kinetics in Domain III in the
self-nucleation study. The population of the longer helical sequence
of iPP block revealed by infrared spectroscopy was found to be affected
by temperature but not by the spatial confinement, chain stretching,
and junction point constraint imposed by the microdomains. Finally,
the orientation of α-form iPP crystals in the lamellae-forming
iPP-<i>b</i>-aPS was identified over a broad range of crystallization
temperatures (<i>T</i><sub>c</sub>). Different from other
crystalline–amorphous diblocks, a lamellar branching of α-form
iPP was observed in the lamellar microdomains at <i>T</i><sub>c</sub> lying between 15 and 80 °C, where the daughter
lamellae developed from the perpendicularly orientated parent iPP
crystals with a specific angle of 80° or 100°. Once the
sample was crystallized at <i>T</i><sub>c</sub> ≤
10 °C, the iPP crystals became randomly oriented
Control of Nanostructural Dimension by Crystallization in a Double-Crystalline Syndiotactic Poly(4-methyl-1-pentene)-<i>block</i>-poly(l‑lactide) Block Copolymer
The control of nanostructural dimension
by crystallization-induced chain stretching was investigated in a
novel double-crystalline block copolymer, syndiotactic polyÂ(4-methyl-1-pentene)-<i>block</i>-polyÂ(l-lactide) (<i>s</i>PMP–PLLA),
featuring a lamellar phase. Because of the similar glass transition
temperatures of <i>s</i>PMP and PLLA, their blocks could
crystallize under soft confinement (i.e., a crystallization temperature
higher than the glass transition temperatures of the constituent blocks)
in <i>s</i>PMP–PLLA. With the strong segregation
of <i>s</i>PMP–PLLA, the first-crystallized <i>s</i>PMP block was templated by microphase separation to form
confined crystalline <i>s</i>PMP lamellae within the microphase-separated
lamellar texture. Most interestingly, the first-crystallized <i>s</i>PMP block may also induce significant stretching of the
PLLA chains from the lamellar interface, resulting in the increase
of microdomain thickness of the PLLA block. With the increase of crystallization
temperature, this chain stretching may become more significant, resulting
in a large increase (∼34%) of the lamellar long period. The
double-crystalline lamellar morphologies having homeotropic orientation
for both <i>s</i>PMP and PLLA crystals can be acquired in
the shear-aligned <i>s</i>PMP–PLLA as evidenced by
simultaneous 2D small-angle X-ray scattering and wide-angle X-ray
diffraction, giving uniform birefringence under polarized light microscope
with thermal reversibility. As a result, the switchable lamellar nanostructures
having significant dimensional change can be carried out by simply
controlling crystallization or melting of the crystallizable blocks