Ya Cao

Influence of the stereo-defect distribution on the crystallization behavior of Ziegler-Natta isotactic polypropylene

The relationship between the stereo-defect distribution and the crystallization behavior of Ziegler-Natta isotactic polypropylene (ZN-iPP) is an important issue, which has not been clearly studied up to now. In this work, the crystallization behavior of a series of iPP samples with similar average isotacticity but different stereo-defect distribution, polymerized with the same Ziegler-Natta catalyst system, was studied by means of differential scanning calorimetry (DSC) and polarized optical microscopy (POM) observation. The results of isothermal crystallization kinetics indicated that, as the distribution of stereo-defects becomes more uniform, the overall crystallization rate decreases gradually. Meanwhile, the results of self-nucleation isothermal crystallization kinetics showed that, the crystal growth rate decreases gradually and the energy barrier of crystal growth increases. Moreover, the POM observation illustrated that not only the crystal growth rate, but also the nucleation rate decrease gradually as the stereo-defect distribution becomes more uniform. The results above indicated that for iPP polymerized with the same Ziegler-Natta catalyst system, stereo-defect distribution plays an important role in determining the nucleation kinetics, crystal growth kinetics and the overall crystallization kinetics of the resin.

Hydrophobic Modification of Poly(Ethylene Terephthalate) with Epoxy-Modified Polysiloxane by Reactive Extrusion

Hydrophobic poly(ethylene terephthalate) (PET) was prepared by reactive blending of PET with double epoxy groups modified polysiloxane (diepsi) or multiepoxy groups modified polysiloxane (multiepsi). The structure of the modified PET was characterized by 1H NMR and intrinsic viscosity. The grafted ratio of diepsi and multiepsi was 1.3 and 0.03 wt%, respectively. With the introduction of diepsi and multiepsi to PET, the water contact angle of modified PET increased from 73° to a maximum of 106°, showing the good hydrophobicity of the modified PET. The samples of modified PET were washed with different solvents and their water contact angles did not significantly decrease, indicating good durability of the hydrophobicity. Moreover, the films of modified PET prepared by solution casting (phenol and 1, 1, 2, 2-tetrachloroethane mixed solution) showed excellent hydrophobicity with the contact angle 151°. Scanning electron microscopy images showed the surface of the modified PET films to be rougher than those of unmodified PET because the introduction of polysiloxane to PET resulted in phase separation during the solvent evaporation process.

Effect of Temperature and Comonomer Content on Thermal Behavior and Crystallization Property of the Propylene–Ethylene Random Copolymers

The effects of ethylene units content and crystallization temperature on the conformations, and the thermal and crystallization behavior were investigated by a combination of Fourier transform infrared (FTIR) spectroscopy, wide angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC). The characterization of FTIR spectroscopy proves that the longer helical conformation sequences of the propylene–ethylene random (PER) samples decrease, whereas the shorter helical conformation sequences increase with the increase in ethylene units content. The increase of the shorter helical conformation sequences is favorable for the formation of the γ-phase in the crystals. A group of broad endothermic peaks can be seen clearly in the DSC curves of PER copolymers, which may be associated with the melting of mixtures of the α- and γ-forms in the crystals. The melting point, crystallization temperature, and crystallinity degree of the PER copolymers decrease with the increase in ethylene units contents. Three typical melting peaks of the PER copolymers crystallized isothermally between 80°C and 130°C were observed. The two higher melting peaks result from melting of the α- and γ-phase in the crystals, whereas the materials crystallized on quenching give the lowest peak. The WAXD results confirm that the PER copolymers crystallize from the melt, as mixtures of α and γ forms, in a wide temperature range. The critical number ζlim of the crystallizable units for the α-form increases with the increase in crystallization temperature for PER copolymers, which is favorable for the formation of the γ phases. The amount of γ-form increases with the increase in crystallization temperature at the expense of its α component, then reaches a maximum value at the crystallization temperature of 115°C, and finally decreases with further increase in the crystallization temperature.

Effects of Polymerization and Crosslinking Technologies on the Crystallization Behaviors and Gel Network of Crosslinked Polyethylene

In the present study, we investigated the influences of polymerization and crosslinking technologies on the branched-chain structures, crystallization behaviors, and gel networks of two types of low-density polyethylenes, LDPE1 and LDPE2, polymerized under different polymerization conditions in commercial large-capacity production lines. High temperature 13C NMR analysis suggests that the molar contents of both long-chain branches and short-chain branches of LDPE2 were markedly higher than those of LDPE1 due to a higher polymerization temperature for LDPE2. The high content of chain branches of LDPE2 leads to its relatively higher storage modulus, entanglement density, and gel content, as indicated by dynamic mechanical analysis. In addition, when the crosslinking temperature reached 180°C, the LDPE2 crosslinked with 2 wt% dicumylperoxide (XLPE2-D2) displayed a relatively wider processing time window and a faster crosslinking rate than XLPE1-D2. Successive self-nucleation and annealing fractionation indicated that increasing the concentration of dicumylperoxide can improve the crosslinking degree, but resulted in lower crystallinity and thinner lamellar crystals.