Layered double hydroxide (LDH) platelets with nanosized and microsized level were synthesized and used as fillers in an isotactic polypropylene (PP) matrix. The nucleation and crystallization behavior of PP/LDH composites (denoted as 1-PPLx and 2-PPLx for composites containing nanosized and microsized LDH, respectively; x represents the mass percentage of LDH) was investigated by differential scanning calorimetry and polarized optical microscopy techniques. It is found that the crystallization temperature of PP/LDH composites is largely enhanced and the half crystallization time is reduced remarkably relative to pure PP, especially for 2-PPLx composite. The 2-PPLx composite exhibits stronger heterogeneous nucleating ability and faster crystallization rate than 1-PPLx samples with the same LDH loading. In addition, the crystallized PP/LDH composites possess significantly enhanced thermal stability, gas barrier, and flame-retardant properties relative to neat PP, which would show a broad application prospect in engineering plastics and packing industry.Topics: Phase transition; Thermal properties;

Poly(3-hexylthiophene) (P3HT) nanotubes were prepared using anodized aluminum oxide (AAO) templates with two different diameters of ca. 240 nm and 60 nm. The structure and optical properties of the P3HT nanotubes were studied by using X-ray diffraction and UV-vis spectrometry. The results show that annealing and melt-crystallization can enhance the crystallinity and content of the well-ordered crystalline P3HT aggregates capped in the wider AAO template (240 nm). While melt-recrystallization leads to the highest crystallinity of the P3HT nanotubes, annealing leads to the formation of P3HT aggregates with the highest content of ordered structure (55.1%) and the narrowest free exciton bandwidth (w = 6.9 meV). By contrast, both annealing and melt-recrystallization do not remarkably influence the crystallinity and structural order of the P3HT nanotubes capped in the narrower AAO template (60 nm). The annealing and melt-recrystallization treatments show even a negative effect on the UV absorption properties of P3HT nanotubes. After releasing the P3HT nanotubes from the AAO, the existence of free surface results in a tremendous enhancement of crystallinity and an increase of the lateral size of crystals independent of the diameter. With respect to the optical properties, a further improvement in the optical properties of the released P3HT nanotubes with larger diameters is achieved through annealing; but annealing has a negative effect on the optical properties of the released P3HT nanotubes with smaller diameters.

PBA nanotubes with different diameters have been prepared. The crystallization behavior and phase transition behavior have been explored by using X-ray diffraction and DSC. For isothermal melt crystallization, the temperature dependence of the crystal phase and the orientation of PBA crystals in the Anodic Alumina Oxide (AAO) templates are very different from that of the bulk. In the AAO templates, especially in the narrow nanopores, the b-axis of the α phase prefers to adopt the orientation parallel to the long axis of the pore. In addition, an in situ X-ray experiment indicates that some molecular chains cannot pack into the crystal lattice in the AAO template at high crystallization temperature, and they are able to crystallize only after cooling back to room temperature. The core–shell structure of PBA exists in the AAO template, which leads to the incomplete crystallization of the α form at higher crystallization temperature and the formation of β-crystals in the cooled sample. The phase transition behavior of β-crystals in the heating process is also affected by nanoporous confinement. The expansion of the β-crystal unit cell is depressed and the phase transition behavior of β to α is altered in the AAO template. At a slow heating rate, compared to the β-PBA in bulk, β-crystals transit to α-crystals at a slower rate in the templates. At a fast heating rate, less β-crystals transit to α-crystals and more β-crystals prefer to melt directly in the AAO templates.

The crystallization behavior and morphology of poly(3-hydroxybutyrate) (PHB) ultrathin films sandwiched between Si wafers and amorphous thin polymer layers were studied by using grazing incident X-ray diffraction (GIXD) technology. Two kinds of amorphous polymers, i.e., atactic polystyrene (aPS) and poly(vinyl phenol) (PVPh), were used to check the influence of interaction between the PHB and capping layer on the crystallization behavior of PHB. The results show that the crystallization behavior of the PHB thin film depends on the film thickness, crystallization temperature and the capping layers. The PHB ultrathin films of 16 nm in thickness cannot crystallize and an amorphous layer is always obtained within the experimental time scale irrespective of crystallization temperature and capping layer properties. When the PHB layer is 60 nm thick, flat-on lamellae of PHB form under the PS layer at any crystallization temperature but edge-on and flat-on lamellae coexist under the PVPh layer. This implies that the PS layer favors the formation of flat-on PHB lamellae in thin films, while the PVPh layer encourages the formation of edge-on lamellae. For PHB films of 110 nm, edge-on lamellae show up at a lower crystallization temperature under the PS layer. In contrast, no edge-on lamellae were identified under the PVPh layer at higher crystallization temperatures.

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.

The crystallization and melting behaviors of PBA thin films with different thicknesses placed on Si wafers and PVPh surfaces under varied conditions were studied by using grazing incident X-ray diffraction (GIXD) and infrared reflection-absorption spectroscopy (IR-RAS). The results show that crystallization of PBA during the solvent evaporation process on either Si wafer or PVPh surfaces produces always β-form crystals regardless of the film thickness. However, the melting behavior of the β-PBA crystals on the PVPh surface is quite different from those on a Si wafer. On the Si wafer, the melting of β-PBA crystals in films with thicknesses ranging from 39 to 293 nm is not altered obviously. The β-to-α phase transition always takes place during the heating process before melting. By contrast, on the PVPh sublayer, the melting of the β-PBA crystals depends on the thicknesses of both PBA and PVPh layers. A thinner PBA layer and a thicker PVPh layer favor a direct melting of the β-PBA crystals without the occurrence of β-to-α phase transition. The phase transition temperature of the PBA film with the same thickness reduces with the thickening of the PVPh layer. IR-RAS results indicate that the intermolecular hydrogen bonds increase with temperature and thickening of PVPh sublayers. The reduced phase transition temperature can be attributed to the increasing hydrogen bonds formed at the interface of PVPh and PBA. The surface property of PVPh films is investigated conveniently through monitoring the formation of hydrogen bonds between PVPh and PBA at the interface. The formation of hydrogen bonds at temperatures which are much lower than the glass transition temperature of PVPh suggests that the segmental mobility of PVPh molecular chains evolves in the glassy state. It is the higher segmental mobility of PVPh in the thicker film that alters the phase transition behavior of the PBA layer.

The cold crystallization and melting behavior of poly(3-hydroxybutyrate)(PHB) layer on amorphous Poly(vinyl phenol) (PVPh) and Si wafer substrate were studied by using Grazing incidence X-ray diffraction and infrared reflection-absorption spectroscopy. Compared to the PHB on Si wafer, the PVPh layer shows great influence on the crystallization and melting behavior of PHB layer. The depression extent of melting temperature increases with the increase of PVPh thickness when the thickness of PVPh is smaller than a critical value. Infrared reflection-absorption spectroscopy study is carried out to better understand the structure evolution of PHB and its interaction with PVPh in the heating process. By monitoring and decomposing the CO stretching bands, several points can be identified: (1) melting temperature decreases with PVPh thickening; (2) the fraction of intermolecular hydrogen bonds formed between the OH groups of PVPh and CO groups of PHB is very small below glass transition temperature of PVPh and it increases significantly above 100 °C. Moreover the fraction increases with PVPh thickness. The effect of PVPh thickness on the formation of hydrogen bonds is attributed to the roughness, molecular mobility and/or molecular orientation of PVPh.

In polymer processing operations, the molten polymer chains are frequently subjected to shear or/and elongation flow fields, which will produce molecular chain orientation of the melt. This leads to the orientation-induced crystallization has been an important subject in the field of polymer physics. Systematic studies indicated that the chain orientation influences the crystallization kinetics, the final morphology as well as the polymorphic behavior of the polymers. In this article, the effects of preorientation on the crystallization of isotactic polypropylene (iPP) concerning the above mentioned aspects have been reviewed. In particular, the formation mechanism of orientation-induced β-iPP crystallization has been discussed according to the recent experimental results. It is suggested that the local order of the macromolecular chain segments in the melt is most important for β-nucleation of iPP. The formation of β-iPP nuclei may be restricted in a certain chain orientation window of the iPP melts. Chain orientation outside of this window results in the formation of α-iPP.Figure optionsDownload full-size imageDownload as PowerPoint slide

The specular and off-specular X-ray reflectivities were efficiently employed to study the evolution of surface morphology as a function of temperature in a single layer of poly(3-hydroxybutyrate) (PHB) and a bilayer of PHB/poly(vinyl phenol) (PVPh) on Si substrates. The results indicate that the changes of thickness and surface roughness caused by pre-melting of PHB crystals are not obvious for the single layer, whereas the surface roughness of the PHB layer and the intensity of the off-specular X-ray reflectivity for the bilayer exhibit a remarkable non-monotonic change in the temperature range of 100–150°C; the roughness parameter evaluated by the specular X-ray reflectivity reaches its maximum at 120°C. The interaction at the interface between PVPh and PHB certainly contributes to the non-monotonic changes. Such interaction also affects the crystallization and melting behavior of PHB thin film greatly. The crystallization of PHB thin film is inhibited even on the glassy surface of PVPh sublayer. In the melting process, the PHB crystals on PVPh sublayer feature a three-section melting curve separated by a plateau region of 120–140°C.