| Combinatorial method is a high efficient way to investigate the condensed state of various crystalline polymers. In the thesis, we chose temperature gradient to prepare combinatorial sample libraries of various crystalline polymers including linear low density polyethylene (LLDPE),poly(vinylidene fluoride)(PVDF), poly(ethylene terephthalate)(PET) and branched low molecular weight polyethylene/ultrahigh molecular weight polyethylene (B-LMWPE/UHMWPE) blend. For these different polymers, temperature dependent condensed states such as crystalline transition, crystal orientation, crystallization and the relationship between crystallization and phase separation have been investigated systematically. The main results of these researchs are as follows.The crystalline morphology transition from banded to sheaf-like in micron-thick linear low-density polyethylene films was reported systematically.The observation by polarized optical microscope (POM) shows that the banded spherulites gradually transits to the sheaflike crystalline from lower crystallization temperature to higher one. Result of wide angle X-ray diffraction (WAXD) shows that the lamellae exhibt edge-on orientation in sheaf-like spherulites. Result of small angle X-ray scaterring (SAXS) shows that the long period of the multilamellar crystal increases with increasing the crystallization temperature (Tc). Therefore, the crystalline transition is confirmed to result from the thickening long period and the weakening surface stress of the multilamellar crystals as the crystallization temperature increased. The mechanism is different from the confinement effect of the ultrathin film. Moreover, based on the high-selectivity of combinatorial method, a series of unique spherulitic morphologies such as nested, multiple-nested and hollow spherulitic crystals were successfully constructed, which may provide a potential application in model substrate and surface modification studies.Temperature dependences of spherulites morphology and crystal orientation of PVDF thin films were firstly investigated using temperature gradient combinatorial methods. The combinatorial methods apply a temperature gradient with temperature ranging from137to169℃.The radius and amount of the spherulites observed by POM show the gradient change depending on the crystallization temperature (Tc). Meanwhile, the preferential orientation of the crystallites was found with increasing Tc by two-dimensional wide-angle X-ray diffraction (2D-WAXD) and Fourier transform infrared spectroscopy (FTIR). The crystal sizes were estimated based on Scherrer’s equation and correlation function from WAXD and SAXS profiles respectively. Results show that the crystallization at169℃gives the most developed crystalline state, in which the spherulite size is maximal, the crystallite size is also the longest and a-axis is almost parallel to the normal to the film surface. Namely, the b and c axes are in the film plane. Besides, the b-axis size of lamellae is very long, about200nm, indicating that the crystallization rate is maximal along the b direction. The thermal inducing perferential orientation at higher temperatures attributed to confinement of the film thickness. This confinement also induced that the spherulites grew in a two-demensional mode at higher temperatures while in a three-dimensional mode at lower temperatures, which was proved by the isothermal melt-crystallization experiments.The effect of annealing on the crystallization of amorpous PET film was investigated. The temperature gradient (T-gradient) ranges from50～250℃. The light transmittance of the film decreases as the annealing temperature increases. Multiple melting endotherms were observed in the differential scanning calorimetry (DSC) profiles of the samples annealed above120℃. The broaden of diffractions in the2D-WAXD pattern and1D-WAXD profile suggests the existing of many small crystallites, and the shift of the diffraction means the exsiting of paracrystalline. Results of FTIR show that the conformation of ethylene glycol moiety transits from gauche to trans when the temperature is above Tcc. The trans ratio keeps stable during the120～200℃, but increases distinctly after200℃. The variation trendency is similar with that of the crystallinity of PET. The measurement of WAXD-DSC combined machine suggests two key conformation transition inducing crystallization in the amorphous PET. The conformation trasition of ethylene glycol moiety is the first transtion starting at about120℃, it induces the crystallization. The conformation transition of the terephthaloyl moiety is the second one which strats at200℃, enhancing the crystallization. Besides, the WAXD profiles of WAXD-DSC were not changed when the heating temperature reached the low melting temperature, indicating that the low melting is not due to the small crystallites but attributed to the melt of the paracrystal in coexisting structure of crystallite and paracrystal.Crystallization and phase separation of polyethylene blend were firstly studied under the controlled T-gradient condition. B-LMWPE and UHMWPE blend films were prepared by casting the blend solutions on the T-gradient stage and then evaporating the solvent. The morphological evolution of blend films was investigated in details. The blend films (10/1) showed a continuous gradients surface. With increasing temperature, the degrees of crystallinity for both of the B-LMWPE and UHMWPE decreased in the blend films. The results indicated that phase separation behavior of B-LMWPE and UHMWPE blend with composition10/1or5/1occurred in both of solution and gel. It is suggested that phase separation and crystallization compete with each other in the later period of the formation of blend film. Another fact is that high viscosity of blend solution plays a more important role in phase separation rather than in crystallization, and it will be the main disincentive in inhibiting Marangoni phenomenon when the composition of UHMWPE increases in the blend.Based on the above results, temperature dependence of lamellae orientation of B-LMWPE/UHMWPE blend film was further studied by transmission electron microscopy (TEM),2D-WAXD and2D-SAXS. The film was prepared by solution casting under the controlled T-gradient. TEM and X-ray scattering observations demonstrate that no clear lamellar crystal exists in B-LMWPE-rich phase while a large amount of lamellae are found in UHMWPE-rich phase. And these lamellae show a significant temperature dependence of preferred orientation. Results of2D-WAXD and2D-SAXS indicate that the lamellae stack regularly and preferentially parallel to the film surface at lower temperatures. At the temperatures near to the transition region, namely near to the gelation temperature, a part of the lamellae become to orient in an irregular way. At higher temperatures, almost all of the lamellae preferentially orient along the normal to the film surface and they randomly distribute around that direction. It’s worth noting that the preferred orientation of the lamellae may influence the drawability of B-LMWPE/UHMWPE blend films. |
