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>_g 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>_β), 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 ¹ (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>_ST 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>_RISC and PLQY in the film state. The optimized OLED device without any TADF assistant dopant could reach to an EQE_max 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