15 research outputs found
Effects of Nanoporous Anodic Alumina Oxide on the Crystallization and Melting Behavior of Poly(vinylidene fluoride)
Poly(vinylidene fluoride) (PVDF)
nanotubes were fabricated by melt-wetting
into porous anodic aluminum oxide (AAO) templates with two different
interfacial properties: one is pristine AAO, and the other is modified
by FOTS (AAO-F). Their crystallization and melting behaviors are compared
with those of a bulk sample. For the PVDF in AAO-F, the nonisothermal
crystallization temperature is slightly lower than that of bulk, and
the melting temperature is similar to that of bulk. For the PVDF in
pristine AAO, when the pore diameter is 200 nm, the crystallization
is induced by two kinds of nucleation: heterogeneous nucleation and
interface-induced nucleation. On the contrary, in the AAO template
with pore diameter smaller than 200 nm, only interface-induced nucleation
occurs. The melting temperature of PVDF crystals in the pristine AAO
is much higher than that of bulk which can be attributed to the presence
of an interfacial layer of PVDF on the template inner surface. The
interaction between PVDF and AAO template produces the interfacial
layer. Such an interfacial layer plays an important role in enhancing
the melting temperature of PVDF crystals. The higher melting peak
is always observed when the PVDF is nonisothermally crystallized in
the AAO template irrespective of the thermal erasing temperature suggesting
the interfacial layer is very stable on the AAO template surface.
If the PVDF nanostructures are released from AAO template, the higher
melting peak disappears with the enhancement of thermal erasing temperature
Polymorphism and Enzymatic Degradation of Poly(1,4-butylene adipate) and Its Binary Blends with Atactic Poly(3-hydroxybutyrate) and Poly(vinyl phenol)
The
influence of atactic poly(3-hydroxybutyrate) (aPHB) and poly(vinyl
phenol) (PVPh) on the crystallization, phase transition, and enzymatic
degradation behaviors of poly(1,4-butylene adipate) (PBA) was studied.
It was found that both aPHB and PVPh can lower the critical temperature
of neat α-PBA crystallization from 34 °C for neat PBA to
32 °C for the blends. Also the critical temperatures of neat
β-PBA crystallization decrease from 28 °C for neat PBA
to 26 and 24 °C for the PBA/aPHB and PBA/PVPh, respectively.
Moreover, the β-to-α phase transition can be accelerated
by incorporation of PVPh and aPHB. The β-to-α phase transition
completes at 55 °C during heating process for neat PBA, while
the temperatures for a complete β-to-α transition of PBA
in PBA/aPHB and PBA/PVPh are 50 and 45 °C, respectively. This
result should be attributed to the decreasing melting point of PBA
in its blends with aPHB or PVPh. Therefore, the melting of the original
β-PBA and accompanied recrystallization into α ones should
take place earlier and more quickly in the blends than that in neat
PBA. The analysis of enzymatic degradation demonstrates that the degradation
of PBA can be affected by crystalline morphology and the molecular
chain mobility of PBA in the amorphous region. The restricted mobility
of amorphous PBA imposed by aPHB and PVPh can slow down the degradation
rate of PBA in the blends. The higher <i>T</i><sub>g</sub> and stronger intermolecular interaction between PVPh and PBA result
in the slowest degradation of PBA in the PBA/PVPh blend. Furthermore,
in neat PBA, PBA/PVPh, or PBA/aPHB, the degradation rate of α-PBA
crystals obtained via annealing is slower than that of α-PBA
prepared by isothermal crystallization and even slower than that of
β-PBA
Effects of Composition and Melting Time on the Phase Separation of Poly(3-hydroxybutyrate)/Poly(propylene carbonate) Blend Thin Films
In this study, the effect of composition
and melting time on the
phase separation of poly(3-hydroxybutyrate)/poly(propylene carbonate)
(PHB/PPC) blend thin films was investigated. Optical microscopy under
phase contrast confirms the occurrence of phase separation of PHB/PPC
blends at 190 °C. Polarized optical and scanning electron microscopies
(POM and SEM) demonstrate that phase separation takes place along
both horizontal and vertical film planes, which should be attributed
to the two different interfaces and immiscible blends. A low PPC content
(e.g. 30 wt %) results in the formation of compact PHB spherulites
filling the whole space, whereas the noncrystallizable PPC spherical
microdomains scatter in the PHB region, and their size shows a remarkable melting-time dependence.
With the increasing PPC component and melting time, it is observed
from POM that the separated PHB domains scattered in the continuous
PPC phase, like the island structure. However, it can be revealed
by SEM micrographs that the PHB thick domains are actually connected
by its thin layer under the PPC layer. The real situation is, therefore,
a large amount of PPC aggregates to the surface to form a network
uplayer, whereas the PHB thick domains connected by its thin layer
form a continuous PHB region, leading to a superimposed bilayer structure.
There is also a small amount of PHB small domains scattered in the
PHB phase. The PHB thick domains crystallize cooperatively with the
PHB- or PHB-rich sublayer in a way just like the growth of pure PHB
spherulites. This superimposed bilayer by interplay between phase
separation and crystallization may provide availability to tailor
the final structure and properties of crystalline/amorphous polymer
blends
Effects of Composition and Melting Time on the Phase Separation of Poly(3-hydroxybutyrate)/Poly(propylene carbonate) Blend Thin Films
In this study, the effect of composition
and melting time on the
phase separation of poly(3-hydroxybutyrate)/poly(propylene carbonate)
(PHB/PPC) blend thin films was investigated. Optical microscopy under
phase contrast confirms the occurrence of phase separation of PHB/PPC
blends at 190 °C. Polarized optical and scanning electron microscopies
(POM and SEM) demonstrate that phase separation takes place along
both horizontal and vertical film planes, which should be attributed
to the two different interfaces and immiscible blends. A low PPC content
(e.g. 30 wt %) results in the formation of compact PHB spherulites
filling the whole space, whereas the noncrystallizable PPC spherical
microdomains scatter in the PHB region, and their size shows a remarkable melting-time dependence.
With the increasing PPC component and melting time, it is observed
from POM that the separated PHB domains scattered in the continuous
PPC phase, like the island structure. However, it can be revealed
by SEM micrographs that the PHB thick domains are actually connected
by its thin layer under the PPC layer. The real situation is, therefore,
a large amount of PPC aggregates to the surface to form a network
uplayer, whereas the PHB thick domains connected by its thin layer
form a continuous PHB region, leading to a superimposed bilayer structure.
There is also a small amount of PHB small domains scattered in the
PHB phase. The PHB thick domains crystallize cooperatively with the
PHB- or PHB-rich sublayer in a way just like the growth of pure PHB
spherulites. This superimposed bilayer by interplay between phase
separation and crystallization may provide availability to tailor
the final structure and properties of crystalline/amorphous polymer
blends
Effect of Anodic Alumina Oxide Pore Diameter on the Crystallization of Poly(butylene adipate)
Poly(butylene adipate) (PBA) was
infiltrated into the anodic alumina
oxide (AAO) templates with the pore diameter of around 30, 70, and
100 nm and PBA nanotubes with different diameters were prepared. The
crystallization and phase transition behavior of the obtained PBA
nanotubes capped in the nanopores have been explored by using X-ray
diffraction and differential scanning calorimetry. Only α-PBA
crystals form in the bulk sample during nonisothermal crystallization.
By contrast, predominant β-PBA crystals form in the AAO templates.
The β-PBA crystals formed in the nanopores with pore diameter
less than 70 nm prefer to adopt an orientation with their <i>b</i>-axis parallel to the long axis of the pore. During the
melt recrystallization, it was found that the critical temperature
(<i>T</i><sub>β</sub>), below which pure β-crystals
form, is 20 °C for bulk PBA. It drops down significantly with
the pore diameter for the PBA in the AAO template. Moreover, the β-crystals
in the porous template exhibit larger lattice parameters compared
with the bulk crystals. By monitoring the change of β-crystals
in the heating process, it was found that β-crystals in the
AAO template with the pore diameter of 30 nm (D30) melt directly while
the β-crystals transform to α-crystals in the template
with the pore diameter of 100 nm (D100). The intensity of (020) Bragg
peak of β-crystals decreases at a similar rate in both D30 and
D100 but disappears at a relatively lower temperature in D30. On the
other hand, the β(110) peak intensity of β-PBA crystals
formed in the D100 template decreases first at slower rate before
α crystals appear, and then at a faster rate once the β
to α phase transition takes place
Structure Evolution of Poly(3-hexylthiophene) on Si Wafer and Poly(vinylphenol) Sublayer
The
structure evolution of P3HT thin films on Si wafer and PVPh
covered Si wafer during heating, thermal annealing, and melt recrystallization
processes has been studied in detail using X-ray analysis techniques.
The effect of substrate on the crystallization behavior and interface
structure of P3HT films was explored. For the P3HT films deposited
on the Si substrate, it was found that the stability of P3HT crystals
is orientation dependent. The crystals oriented with <i>b</i>-axis normal to the substrate, that is, a face-on molecular orientation,
are less stable than those with the <i>a</i>-axis arranged
normal to the substrate (side-on molecular orientation). Thermal annealing
temperature plays an important role in the molecular structure of
P3HT including crystal structure, film thickness, and surface roughness.
After annealing at relatively high temperature, new crystals form
during the cooling process accompanied by the shrinking of <i>a</i>-axis. Moreover, the melt recrystallization favors the
formation of more stable P3HT crystals with side-on molecular orientation.
The PVPh substrate does not affect the crystallization behavior of
solution cast P3HT significantly but inhibits the formation of P3HT
crystal with face-on molecular orientation. However, the interfacial
morphology of P3HT and PVPh changes by annealing at elevated temperature.
The P3HT/PVPh interface changes from a sharply defined one into a
diffused one at around 160 °C. The PVPh sublayer inhibits the
melt recrystallization of P3HT to some extent, leading to a slight
expansion of the <i>a</i>-axis
Effects of Composition and Melting Time on the Phase Separation of Poly(3-hydroxybutyrate)/Poly(propylene carbonate) Blend Thin Films
In this study, the effect of composition
and melting time on the
phase separation of poly(3-hydroxybutyrate)/poly(propylene carbonate)
(PHB/PPC) blend thin films was investigated. Optical microscopy under
phase contrast confirms the occurrence of phase separation of PHB/PPC
blends at 190 °C. Polarized optical and scanning electron microscopies
(POM and SEM) demonstrate that phase separation takes place along
both horizontal and vertical film planes, which should be attributed
to the two different interfaces and immiscible blends. A low PPC content
(e.g. 30 wt %) results in the formation of compact PHB spherulites
filling the whole space, whereas the noncrystallizable PPC spherical
microdomains scatter in the PHB region, and their size shows a remarkable melting-time dependence.
With the increasing PPC component and melting time, it is observed
from POM that the separated PHB domains scattered in the continuous
PPC phase, like the island structure. However, it can be revealed
by SEM micrographs that the PHB thick domains are actually connected
by its thin layer under the PPC layer. The real situation is, therefore,
a large amount of PPC aggregates to the surface to form a network
uplayer, whereas the PHB thick domains connected by its thin layer
form a continuous PHB region, leading to a superimposed bilayer structure.
There is also a small amount of PHB small domains scattered in the
PHB phase. The PHB thick domains crystallize cooperatively with the
PHB- or PHB-rich sublayer in a way just like the growth of pure PHB
spherulites. This superimposed bilayer by interplay between phase
separation and crystallization may provide availability to tailor
the final structure and properties of crystalline/amorphous polymer
blends
Confinement Effects on the Crystallization of Poly(3-hydroxybutyrate)
Poly(3-hydroxybutyrate)
(PHB) is taken as an example to explore
(i) whether the confined crystallization occurring in anodized
aluminum oxide (AAO) nanopores is the same as that in ultrathin films,
and (ii) whether the interfacial effect of curve surface is the same
as flat surface. The crystallization behavior of PHB in AAO and thin
films (sandwiched between two plates) has been compared. A curvature-dependent
crystallization behavior of PHB is identified. Stable intermediate
structures of PHB confined in narrow pores (diameter <100 nm),
which have never been observed in bulk, are obtained for the first
time. In larger pores (pore diameter >100 nm), crystallization occurs both in the center
and interfacial regions in contact with AAO inner wall. This implies
that the strength of interfacial layer weakens with decreased curvature
and has been further proved by the crystallization behavior of its
sandwiched ultrathin film. It is found that the highly restricted
interface layer incapable of crystallization is about 30 nm for film,
which is much thinner than that for nanorods (approximately 100 nm).
We have also identified two different relaxations of PHB nanorods
corresponding to the interfacial effect and spatial confinement, respectively.
While the relaxation correlated to the interfacial effect with a slower
relaxation time strengthens, the relaxation corresponding to spatial
confinement with a faster relaxation time (bulklike α-relaxation)
weakens with decreased pore size. This is completely different from
that of sandwiched thin film, where only a thickness-independent α-relaxation
same as bulk is observed <sup>1</sup> (Macromolecules 2006, 39, 5967). Therefore,
the reduced crystallization kinetics of thin film is attributed to
the reduction of long-range chain mobility. By contrast, the inhibited
crystallization of PHB nanorods in AAO is attributed to the reduced
segmental dynamics and the reduced chain mobility of PHB layer at
interface
Efficient Thermally Activated Delayed Fluorescence Conjugated Polymeric Emitters with Tunable Nature of Excited States Regulated via Carbazole Derivatives for Solution-Processed OLEDs
For
main-chain-type thermally activated delayed fluorescence (TADF)
polymeric emitters, conjugated or nonconjugated monomers are generally
copolymerized as the spacers to suppress the inter- and intramolecular
exciton concentration quenching. In this work, carbazole derivatives
are introduced into the main chains of the conjugated polymers based
on the TADF unit, 2-(10<i>H</i>-phenothiazin-10-yl)dibenzothiophene-<i>S</i>,<i>S</i>-dioxide (DBTO2-PTZ). It is found that
the content of carbazole derivatives units could not only effectively
suppress exciton quenching and nonradiative transition but also manipulate
the distribution of molecular orbits and Δ<i>E</i><sub>ST</sub> values and even regulate the nature of excited states.
Therefore, the upconversion of triplet exciton from triplet to singlet
excited state could be regulated by introducing the different contents
of carbazole derivatives. Among three synthesized polymers, COP-10
displays a relatively higher <i>k</i><sub>RISC</sub> and
PLQY in the film state. The optimized OLED device without any TADF
assistant dopant could reach to an EQE<sub>max</sub> of 15.7% with
a lower turn-on voltage of 3.2 V
Crystal Morphology of Poly(3-hydroxybutyrate) on Amorphous Poly(vinylphenol) Substrate
The
crystalline morphology and orientation of poly(3-hydroxybutyrate)
(PHB) thin film on the poly(vinylphenol) (PVPh) sublayer with different
thickness was studied by atomic force microscopy, X-ray diffraction,
and infrared spectroscopy. PVPh sublayer influences the morphology
of PHB greatly. Although edge-on lamellae form on both Si and PVPh
surfaces at relatively lower crystallization temperature, the morphology
of them is quite different. It appears as sheaflike edge-on lamellar
morphology on PVPh sublayer. In addition, the edge-on lamellae prefer
to form on the PVPh sublayers at much higher crystallization temperature
compared with that on Si wafer. The PVPh layer thickness also influences
the crystalline morphology of PHB. On a 30 nm thick PVPh layer, sheaflike
edge-on lamellae form in a wide range of crystallization temperatures.
When the PVPh thickness increases to 65 nm, fingerlike morphology
is observed when the crystallization temperature is lower than 95
°C. The fingerlike morphology is caused by a diffusion-limited
aggregation process, and it requires an optimum condition. Thickness
ratio between PHB and PVPh sublayer and temperature are two key factors
for the formation of fingerlike morphology