In olefin polymerization with MgCl2‐supported Ziegler–Natta (Z–N) catalysts, the apparent propagation rate constant (kp)a calculated by Rp = (kp)a [C*] CMe (CMe is equilibrium monomer concentration in the reaction system) declines with reaction time for gradually developed monomer diffusion limitation in the polymer/catalyst particles. In this work, a simplified multi‐grain particle model was proposed to build correlation between (kp)a and other kinetic parameters that can be determined experimentally. Rate profiles of propylene polymerization and ethylene‐1‐hexene copolymerization by three MgCl2‐supported Z–N catalysts were determined, and the (kp)a data was calculated using [C*] determined by quench‐labelling the propagation chains with acyl chloride. Decline of (kp)a in each polymerization process was precisely fitted by the linear correlation between lg(kp)a and [(ρcatmp)/(ρpmcat) + 1]1/3 developed on the particle model. Real propagation rate constant (kp) was estimated by extrapolating the fitting line to the starting point of polymerization, where no concentration gradient exists. According to the particle model, the slope of the lg(kp)a versus [(ρcatmp)/(ρpmcat) + 1]1/3 line (lgd) represents the degree of monomer diffusion limitation. Variations of parameter d found in the studied reaction systems can be reasonably explained based on the knowledge of olefin diffusion in the polymer phase.
In this article, a series of diblock copolymer polyethylene-b-poly(ethylene glycol)s (PE-b-PEGs) with various molecular weight of polyethylene segment was blended with linear low-density PE. The PE/PE-b-PEG blend porous membranes with high porosity were obtained by thermally induced phase separation (TIPS) process. The isothermal crystallization kinetics of PE/LP/PE-b-PEG blends indicated that the introduction of PE-b-PEG could inhibit the growth rate of polyethylene crystals which could increase the pore size and porosity of the membranes. The PE/PE-b-PEG blend membranes with PE 1300 -b-PEG 2200 showed the largest pore size and porosity due to its crystallization behavior during TIPS. The surface of the membranes became smoother and the morphology of the membranes could be effectively tuned by introducing PE-b-PEG. Compared with the PE membrane, the PE/PE-b-PEG blend membranes exhibited higher hydrophilicity (the water contact angle decreased from 1128 to 848), water permeability (the permeation flux increased from 80 to 440 L/m 2 h under 0.1 MPa), rejection performance (completely reject carbon particles in the filtration of carbon ink solution), and fouling resistance (the value of protein adsorption dropped from 0.25 to 0.05 mg/cm 2 ). The hydrophilicity and fouling resistance of PE/PE-b-PEG blend membranes increased as the length of PE segment in PE-b-PEGs decreased.Among the methods for the preparation of PE porous membranes, thermally induced phase separation (TIPS) is the best method. [12][13][14][15][16][17][18][19] By this method, the pore structure could be easily controlled and also the modifying reagent can be introduced into the matrix. Therefore, the preparation and modification of PE membranes can be simultaneously achieved. Previously, a polyethylene-b-poly(ethylene glycol) (PE-b-PEG, 50 wt % of PEG, M n 5 1400) diblock copolymer was introduced into PE membrane by TIPS, which significantly enhanced membrane's hydrophilicity and water flux performance. 6,[20][21][22][23] In our recent work, a series of PE-b-PEG diblock co-polymers containing PE segment with different molecular weight was successfully synthesized. 24 In this work, the PE-b-PEGs were used as the modifying reagent by TIPS to prepare porous membranes from PE. Due to the good compatibility between PE matrix and PE-b-PEG diblock copolymer, PE-b-PEG can be dispersed more uniformly in PE matrix than other hydrophilic polymers. Therefore, the pore structure and pore size distribution of PE/PE-b-PEG blend membrane prepared by TIPS method can be more uniform. Interestingly, the influence of the molecular weight of V C 2018 Wiley Periodicals, Inc.
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