2 research outputs found
Syndiotactic Polyallyltrimethylsilane-Based Stereoregular Diblock Copolymers: Syntheses and Self-Assembled Nanostructures
Structurally well-defined stereoregular diblock copolymers
composed of syndiotactic polyallyltrimethylsilane (sPATMS) and polyÂ(methyl
methacrylate) (PMMA) were prepared by using an α-bromoester-terminated
sPATMS macroinitiator, which was chain extended by MMA using a cuprous
halide-based atom transfer radical polymerization (ATRP) system. The
α-bromoester-terminated sPATMS macroinitiator was prepared via
the esterfication of hydroxyl-capped sPATMS with α-bromoisobutyryl
bromide. The hydroxyl-capped sPATMS was generated via a selective
chain transfer reaction to triethylaluminum (TEA) during the syndiospecific
polymerization of allyltrimethylsilane (ATMS) conducted in the presence
of syndiospecific <i>ansa</i>-metallocene catalysts. The
proposed synthetic route not only offers the high-yield production
of stereoregular sPATMS-<i>b</i>-PMMA but also provides
the linking of the stereoregular block (sPATMS) with PMMA through
a controlled/living radical polymerization process. Moreover, the
proposed method offers effective control over the block chain length,
the molecular weight distribution (<i>M</i><sub>w</sub>/<i>M</i><sub>n</sub>) and the stereoregularity of sPATMS block.
Thus, the self-assembly of the resultant diblock copolymers produces
well-ordered nanostructures from microphase separation, as evidenced
by transmission electron microscopy and small-angle X-ray scattering
results
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