PBT And Its Nanocomposites Crystallization And Melting Behavior Of Block Copolymer Self-assembly Behavior TMAFM

For semicrystalline polymers, the crystallization and melting behaviors are vital to the performance of material. Poly(butylene terephthalate) (PBT) is a semicrystalline thermoplastic engineering material with a high crystallization rate, which is well suited for injection-molding because of its short processing cycles and excellent dimensional stability. To optimize the properties of materials, the crystallization kinetics of linear PBT, branched PBT, PBT ionomers, glass fiber reinforced PBT, as well as PBT blends with other polymers have been investigated. However, up to now the crystallization and melting behaviors of PBT nanocomposites have not been well understood. Most studies on thermal behavior of PBT have been focused on the dual or even multiple melting endotherms, which are also found in many other semicrystalline polymers. Various interpretations have been proposed for the origin of the double melting endotherms. However, the present explanation details for the dual melting endotherms are not distinctly. Moreover, it is not clear that how the melting behavior of semicrystalline polymers, especially the dual melting endotherms will be affected with the addition of nanoparticles.In this work, PBT/organo-ATT nanocomposites were prepared by a simple melt-mixing method. Their crystallization and melting behaviors both were studied mainly by Differential scanning calorimetry (DSC). The main conclusions are given as following:1. The crystal structure and isothermal crystallization behaviors of PBT nanocomposites were studied by X-ray diffraction (XRD) and DSC. The XRD results indicated that the addition of ATT didn't alter the crystal structure of PBT and the crystallites in all the samples were triclinic α-crystals. Compared with neat PBT, PBT nanocomposites exhibited higher crystallization rates during the isothermal crystallization. At the same time, the regime II/III transitions were observed in PBT and nanocomposites based on Hoffman-Lauritzen theory, however the transition temperature increased with the addition of ATT. The fold surface free energy (σ_e) of polymer chains in the nanocomposites was lower thanthat in neat PBT. It should be reasonable to treat ATT as a good nucleating agent for the crystallization of PBT, which played a determinant effect on the reduction in σ_e during the isothermal crystallization of the nanocomposites, even if the existence of ATT could restrict the segmental motion of PBT. The crystallization behaviors of two kinds of PBT nanocomposites with commercially rod-like silicate ATT or layered silicate montmorillonite (MMT) were studied respectively. The crystallization rates of these two nanocomposites were increased compared with neat PBT; however, the variation trends in the half-time of crystallization for these two nanocomposites were different with increasing the loadings of nanoparticles.2. After dried in vacuum, the activated and grafted ATT were analyzed by Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA) to identify the surface groups and to determine the amounts of organic molecules grafted to the ATT nanorods. Scanning electron microscopy (SEM) was used to examine the morphology of PBT/ATT nanocomposites. The nonisothermal crystallization behaviors of melt-crystallized PBT composites were studied by DSC. The advanced integral isoconversional method suggested by Vyazovkin was firstly used to analyze the DSC data of nonisothermal crystallization for nanocomposites, and then the dependences of the effective activation energy (E_α) on the temperature were obtained. The results showed that the character of dependence of E_a on temperature for PBT nanocomposites were different from those for neat PBT.3. The melting behaviors of PBT and its nanocomposites were studied by DSC method. Most of DSC curves had double melting endotherms peaks, T_m1 and T_m 2, which should correspond to the melting processes for the unusual crystal and the usual crystal respectively. If annealed at higher temperature or for a longer time, the perfection of the unusual crystal would be improved. For the origin of the exotherm peak appearing before T_m2 , we had a different explanation from those suggested by other researchers. If the unusual crystal formed during the original annealing process, the exotherm should indicate the recrystallizaiton following an initial crystal melting; otherwise, the exotherm should correspond to the crystallization of amorphous chains. With the addition of the nanorods, the crystallization was accelerated and simultaneously the stability of the unusual crystal was improved, which induced a higher equilibriummelting temperature.Part II The self-assembly behaviors of block copolymer studied by tapping-mode atomic force microscopyThe self-assembly behaviors of block copolymers in the bulk or in the selective solvent have always received much attention by many researchers. Tapping mode atomic force microscopy (TMAFM), as a powerful tool for investigating surfaces of soft materials, has been frequently used to distinguish different components of block copolymers for its excellent lateral resolution. Although the tapping mode can result in lower lateral forces compared with conventional contact mode, the vertical force still lies in the certain range so that tip-indentation or tip-penetration phenomena couldn't be avoided in the imaging process, which will cause a "narrowing" effect on the topography of the sample. At the same time, the tip may bring the "broadening" effect on the sample due to its certain curvature diameter. These two kinds of effects will induce the different artifacts in height images. In addition, the origin of the phase shift in phase images has been discussed quite extensively in the recent years. It is generally accepted that the phase signal reflects the viscoelastic properties of the different components on the surfaces of heterogeneous samples. However, it has not been reported that how to distinguish the described "artifacts" in the height images together with the phase images, and thus to obtain the real information of topography. In this work, thin film of poly(styrene-b-isoprene-b-styrene) (SIS) and micelles of Poly(styrene-b-isoprene-b-methyl methacrylate) (SIM) in selective solvent were studied as following,1. Thin films of SIS were prepared by solution casting on the freshly cleaved mica surface or quartz glass surface. The films were then directly scanned using TMAFM. It was observed that the copolymer microphase separated into stripes mostly parallel to the film surface and some spiral structures were formed by circulated stripes. We believed that the occurrence of the spirals was due to the instability caused by the block copolymer microphase separation coupled with the hydrodynamic effect during the course of the film formation.2. The micelles solution was prepared by SIM in selective solvent and then spin-coated on the surface of the substrate. Two kinds of morphologies of micelles on the substrate were observed by TMAFM. Furthermore, two sets ofTMAFM images, each of which contained one artifact induced by the "narrowing" or "broadening" effects of the tip, were obtained under the varied set-point amplitude (A_sp) with constant free oscillating amplitude (A₀). Such suggestions were then verified by the self-consistent mean field theory (SCMFT) simulation and transmission electron microscopy (TEM) measurement. Finally we obtained the partial real information for the morphology and the structure of SIM micelle on the substrate through analyzing the height images combined with phase images.