Synthesis, Characterization And Crystallization Behavior Of Isotactic Polystyrene And Ethylene-styrene Copolymer

Isotactic polystyrene (iPS) and styrene-ethylene copolymers (PES) were synthesized with spherical MgCl₂-supported Ziegler-Natta catalytic system consisted of titanium tetra- chloride, diisobutyl o-phthalate (DIBP) as internal electron donor, triisobutyl aluminium as cocatalyst and diphenyl dimethoxy silane as external electron donor. The effects of polymerization temperature, cocatalyst, external electron donor, monomer concentration and pressure of molecular hydrogen on the synthesis of iPS were investigated. The crystallization behavior of iPS films with various molecular weight distributions prepared from solution and isothermally crystallized from glassy state were revealed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). The pre-polymerization in the preparation of PES was studied by the particle morphology of propylene pre-polymerization and the PES were fractionated with octane and xylene as solvent by temperature gradient elution fractionation method (TGEF). The crystallization behavior of PES and its fractions were investigated by differential scanning calorimetry (DSC), polarized light microscopy (PLM) with hot-stage and wide angle X-ray diffraction (WAXD). The tensile and dynamic mechanical properties of PES were characterized to verify the conclusions on its structure and crystallization. The sulfonation of PES was carried out and the contact angle between the PES films and alcohol were obtained to characterize the level of the modification. The results are follows.1. The catalytic efficiency of isospecific polymerization of styrene is far above the values of the reported in references;particularly it can be 7700 gPS/gTi.h under the suitable conditions. The curves of iPS molecular weight distribution can be fitted with several Schulz-Flory most-probable distributions and the results show that the fraction of lower molecular weight in the product increases when triethyl aluminium is used as cocatalyst. On the contrary, the high molecular weight products can be obtained with triisobutyl aluminium as cocatalyst. The introduction of molecular hydrogen results in higher catalytic efficiency and broader molecular weight distribution.2. There appear two peaks in the DSC endotherm of iPS, called annealing peak and melting peak, respectively. The temperature of annealing peak increases with the rising temperature of isothermal crystallization. With the increasing of iPS molecular weight distributions, the nucleation density increases and the size of iPS spherulites decrease, at the same crystallization temperature and time. The lamellar thickness in the spherulites is about 25 ± lOnm, larger than the typical value of polymer lamellar thickness. The orientations of lamellae are all edge-on in the spherulites with narrow and broad molecular weight distributions, while both the edge-on and flat-on lamellae can be observed in the sample with medium molecular weight distribution.3. The uniform spherical particles of polypropylene pre-polymerized can be obtained under the conditions that Al/Ti molar ratio is 80-100, Si/Ti molar ratio is 5-10, and the temperature is 40-50 °C. With the pre-polymerization process for 10 ¹5min, the uniform large polyethylene particles are yielded with high catalytic efficiency. The catalytic efficiency can be enhanced and the molecular weight and molecular weight distribution can be controlled when the molecular hydrogen is introduced during the ethylene polymerization. The styrene-ethylene copolymers can be obtained with three polymerization technologies with different orders of the monomers introduction. The styrene content in PES is relatively higher with styrene pre-polymerization technology.4. The PES can be fractionated effectively with octane as the extraction solvent by temperature gradient elution fractionation method. The components in the copolymers are identified as three major types based on the thermal behaviors and FT-IR and 13C-NMR spectrum: high styrene content and short ethylene sequence copolymer, ethylene-predominant copolymer and highly molecular weight copolymer containing multi-ethylene segments.5. The spherulitic growth rates and melting points of PES decreased with the increase of styrene content, and the unit cell dimensions of the orthorhombic crystals of copolymers are larger than that of pure polyethylene. The peak in the WAXD curve corresponding to (200) reflections disappears gradually and the peak corresponding to (110) reflections is weakened with the increasing of styrene content.The crystallinites of PES, however, are larger than that of pure PE under the same crystallization conditions, which is correlated to the nucleation rate after the insertion of styrene unit in the polyethylene chains. Both isothermal and non-isothermal crystallization kinetics show the overall crystallization rate of PES with higher styrene content is larger than that of low styrene-content copolymer.6. The tensile strength and elongation at break of PES prepared with styrene pre-polymerization technology are larger than that of pure polyethylene. On the contrary, the tensile strength of PES prepared with propylene pre- polymerization is decreased with the increase of styrene content and the elongation at break is less than that of PE. There appears a new weak peak in the dynamic mechanical thermal analysis of PES, which corresponds to the irregular structure due to the insertion of styrene units. The contact angle between the PES films and alcohol is reduced for the sulfonation of copolymers.