Multidimensional high‐temperature liquid chromatography: A new technique to characterize the chemical heterogeneity of Ziegler‐Natta‐based bimodal HDPE

High temperature two‐dimensional liquid chromatography (HT 2D‐LC) was recently introduced as a new technique to analyze the heterogeneities with regard to composition and molar mass present in model blends of polyolefins and various olefin copolymers. The method uses graphite as stationary phase and solvent gradients of 1‐decanol → 1,2,4‐trichlorobenzene as mobile phase for the compositional separation. With the aim to maximize the chromatographic resolution, the influence of the separation's temperature in the first dimension was evaluated: approaching the θ‐temperature of polyethylene (PE) in 1‐decanol selectively enhances the retention of higher molar mass PE standards while that of the lower molar mass ones is hardly affected. A bimodal ethylene/1‐butene copolymer and its temperature rising elution fraction (TREF) fractions were separated by HT 2D‐LC. For the first time, both axes of the contour plot were calibrated with regard to chemical composition and molar mass, respectively. Prefractionation of the bulk sample by TREF enhances the detectability of separated components of the 2D separation. The influence of the separation temperature, that is, working around the θ‐temperature of PE in 1‐decanol, can be used to enhance the chromatographic resolution of the 2D chromatography. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Particle morphology and morphogenesis of nascent polyethylene produced with a spherical MgCl2‐supported Ziegler–Natta catalyst in slurry process

Ethylene polymerizations were conducted in slurry process with a spherical MgCl₂‐supported Ziegler–Natta catalyst activated by triethylaluminum, and the morphology of nascent polymer particles was observed by scanning electron microscope. Three kinds of typical microscale morphologies: nodular and rope‐like structures on the external particle surface, and nodular structures in the bulk of the particle were observed. These structures are composed of polyethylene lamellae and amorphous phase filling the space between the lamellae. There is close relation between the polymerization activity and microporosity of the particles. When the activity was higher than 1500 g PE/g Cat, large number of tiny pores appeared in the particles, and rope‐like structures appeared on their surface. Formation of the morphologies can be reasonably explained by models based on multiscale solid structure of the catalyst particle and dynamic interactions among the growing polymer phases. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45679.     

Influence of metal‐oxide‐supported bentonites on the pyrolysis behavior of polypropylene and high‐density polyethylene

In this article, we report on the pyrolysis of polypropylene (PP) and high‐density polyethylene (HDPE) in the absence and presence of plain and metal‐oxide‐impregnated bentonite clays [BCs; acid‐washed bentonite clay (AWBC), Zn/AWBC, Ni/AWBC, Co/AWBC, Fe/AWBC, and Mn/AWBC] into useful products. Thermal and catalytic runs were performed at 300°C in the case of PP and at 350°C in the case of HDPE for a contact time of 30 min. The effects of different catalysts and their concentrations on the overall yields and the yields of liquid, gas, and residue were studied. The efficacy of each catalyst is reported on the basis of the highest liquid yields (in weight percentage). The derived liquid products were analyzed by Fourier transform infrared spectroscopy and gas chromatography–mass spectroscopy; this confirmed the presence of paraffins, olefins, and naphthenes. The results indicate the catalytic role of impregnated BCs compared to plain BC with the optimum efficiency shown by Co/AWBC in the case of PP and Zn/AWBC in the case of HDPE toward the formation of liquid products in a desirable C range with the enrichment of olefins and naphthenes in the case of PP and paraffins and olefins in the case of HDPE compared to the thermal run. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41221.     

Blending ultrahigh‐molecular‐weight polyethylene and poly(ethylene/10‐undecen‐1‐ol) in one nascent particle

Ultrahigh‐molecular‐weight polyethylene (UHMWPE)/polar polyethylene (PE) composites were blended in one nascent particle by in situ polymerization with a hybrid catalyst. Polystyrene‐coated SiO₂ particles were used to support the hybrid catalyst. Fe(acac)₃/2,6‐bis[1‐(2‐isopropylanilinoethyl)] was supported on SiO₂ for the synthesis of UHMWPE, whereas [PhN?C(CH₃)CH?C(Ph)O]VCl₂ was immobilized on a polystyrene layer to prepare a copolymer of ethylene and 10‐undecen‐1‐ol (polar PE). Importantly, the core part of the supports (the polystyrene layer) exhibited pronounced transfer resistance to 10‐undecen‐1‐ol; this provided an opportunity to keep the inside iron active sites away from the poisoning of 10‐undecen‐1‐ol. Therefore, UHMWPE was simultaneously synthesized with polar PE by in situ polymerization. Interestingly, the morphological results show that UHMWPE and the polar PE were successfully blended in one nascent polymer. This improved the miscibility of the composites, where most of the chains were difficult to crystallize because of the strong interactions between the PE chains and polar chains. The blends showed an extremely low crystallinity, that is, 9.9%. Finally, the hydrophilic properties of the polymer composites were examined. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46652.     

Effect of different additives and their mixtures on thermal stability of polyethylene synthesized using iron and titanocene hybrid catalysts

Traditionally, additives are introduced into a polymer matrix via extrusion process which consumes a high amount of energy. In this study, the use of different additives including antioxidants for the in‐reactor stabilization of polyethylene has been investigated in order to provide an energy saving system. Particular attention was dedicated to the efficiency of antioxidant influencing the catalysts activity and properties of polymers. The effect of the addition of Irganox 1330 and Irgafos 168 antioxidants and zinc stearate on the activities of metallocene, post‐metallocene, and their supported hybrid were studied. In addition, the effect of different additives on the thermal characteristics of the synthesized polymers and oxidative induction time (OIT) was evaluated. Our polymerization results exhibited that the factors such as chemical structure of antioxidant and its steric hindrance, type of catalysts, and their hybrid could affect the catalyst performance and OIT contents. The use of antioxidants mixture and hybrid of catalysts is a way that can increase oxidation resistance of polymers considerably. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45482.     

Characterization of ethylene‐propylene copolymers with high‐temperature gradient adsorption liquid chromatography and CRYSTAF

Blends of linear polyethylene (PE) and isotactic polypropylene (iPP) with different average molar masses and a series of ethylene‐propylene (EP) copolymers with different chemical composition as well as blends of PE, Ipp, and EP copolymers were separated using a carbon‐column packing (Hypercarb®) and gradients of 1‐decanol or 2‐ethyl‐1‐hexanol → 1,2,4‐trichlorobenzene (TCB). The separation is based on full adsorption of linear PE on the carbon sorbent at temperature 160°C. However, iPP is not adsorbed and elutes in size exclusion mode. The random EP copolymers have been adsorbed in the column packing and separated according to their average chemical composition after application of the gradient starting with alcohol and ending with pure TCB. The elution volumes of the copolymers depended linearly on the average concentration of ethylene in the copolymers. The HPLC elution profiles were correlated with the CRYSTAF elution profiles. In contrast to CRYSTAF, fully amorphous polyolefin samples were separated with the high‐temperature adsorption liquid chromatography. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Simultaneous measurement of the molecular weight distribution and 5‐ethylidene‐2‐norbornene content across the molecular weight distribution of ethylene–propylene–diene terpolymer via a new size exclusion chromatography–ultraviolet–refractive index method

A size exclusion chromatography (SEC)–UV–refractive index (RI) method was developed to measure the 5‐ethylidene‐2‐norbornene (ENB) content across the molecular weight distribution (MWD) in ethylene–propylene–diene terpolymer (EPDM) at room temperature. The ratio of the UV and RI signals at the same effective elution volume was converted to ENB content. The feasibility of using this method to measure the ENB content across the MWD in EPDM at high temperature was also demonstrated. Prior understanding was that ENB had insufficient UV absorbance relative to high‐temperature SEC solvents to allow for useful measurements. We demonstrated this by using high‐boiling‐point solvents, such as decalin, with a low UV absorbance in the UV wavelength range of interest for ENB. These solvents also gave rise to a high enough specific RI increment (dn/dc) for EPDM that a suitable RI detector response was obtained. Additionally, this methodology could be readily applied to other polymers soluble at high temperature as long as the polymers contained a UV chromophore. These include polymers containing vinyl, conjugated vinyl, aromatic ring, carbonyl, or halocarbon groups. This UV‐absorption‐based detection concept might also be extended to high‐temperature thermal‐gradient interactive chromatography‐UV, high‐temperature solvent‐gradient interactive chromatography‐UV (high‐temperature liquid chromatography‐UV), temperature‐rising elution fractionation‐UV, crystallization analysis fractionation‐UV, and crystallization elution fractionation‐UV. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43911.     

Effect of DMDBS (3 : 2, 4‐bis(3,4‐dimethyldibenzylidene) sorbitol) and NA11 (sodium 2,2′‐methylene‐bis(4,6‐di‐tertbutylphenyl)‐phosphate) on electret properties of polypropylene filaments

Polypropylene (PP) composite filaments containing two different nucleating agents—DMDBS (3 : 2, 4‐bis(3,4‐dimethyldibenzylidene) sorbitol) and NA11 (sodium 2,2′‐methylene‐bis(4,6‐di‐tertbutylphenyl)‐phosphate) were melt spun to modify polymer electrostatic charging characteristics. Sample filaments were charged with a corona instrument and their surface potentials were measured. Initial surface potential as well as potential stability was monitored through an accelerated decay procedure. NA11 was found to be more efficient as an electret additive leading to a 50% increase in charge stability. Filaments with DMDBS exhibited a faster decay. Charging at elevated temperatures resulted in enhanced charge density and stability for both additives. The fiber microstructure was examined by Wide Angle X‐ray Diffraction and Differential Scanning Calorimetry. Rather than reducing the crystal sizes, X‐Ray diffractograms suggest that the crystal size increases with the addition of nucleating agents, while the degree of crystallinity appears to remain unaltered. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2068–2075, 2013     

Comonomer effects in copolymerization of ethylene and 1‐hexene with MgCl2‐supported Ziegler‐Natta catalysts: New evidences from active center concentration and molecular weight distribution

In this article, comonomer effects in copolymerization of ethylene and 1‐hexene with four MgCl₂‐supported Ziegler‐Natta catalysts using either ethylene or 1‐hexene as the main monomer were investigated. It was found that no matter which monomer was used as the main monomer, the polymerization activity was significantly enhanced by introducing small amount of comonomer. In copolymerization with ethylene as the main monomer, the strength of comonomer effects was much stronger in active centers producing low‐molecular‐weight polymer than those producing high‐molecular‐weight polymer. In copolymerization with 1‐hexene as the main monomer, the number of active centers ([C*]/[Ti]) was determined, and the propagation rate constants (k_p) were calculated. Deconvolution of the polymer molecular weight distribution into Flory components were made to study the active center distribution. Introduction of small amount of ethylene caused marked increase in the number of active centers and decrease in average chain propagation rate constant. Introducing internal electron donor in the catalyst enhanced not only the number of active centers but also the chain propagation rate constant. In copolymerization of 1‐hexene with small amount of ethylene, the internal donor weakened the comonomer effects to some extent and changed the distribution of comonomer effects among different types of active centers. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41264.     

Processability‐enhanced bimodal high‐density polyethylene with comb‐branched high‐density polyethylene

Two solution reactors in series were utilized to synthesize comb‐branched high‐density polyethylene (HDPE), cbHDPE, where the first reactor prepares vinyl‐terminated HDPE macromers catalyzed by an organometallic catalyst favoring beta hydride transfer and the second reactor copolymerizes HDPE macromers with ethylene using a different organometallic catalyst capable of incorporating macromers. A bimodal HDPE, biHDPE with bimodalities in molecular weight, and hexene content of the desired composition distribution was also prepared in a gas phase reactor using silica supported dual organometallic catalysts. By blending 3% solution‐made cbHDPE into the gas‐phase biHDPE, the resulting trimodal HDPE preserves the excellent stiffness and toughness of the bimodal HDPE while having exceptional melt strength and processability. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45755.     

Influences of the chain structure of PE‐b‐PEG on the properties of PE/PE‐b‐PEG blend membranes prepared by TIPS

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 PE₁₃₀₀‐b‐PEG₂₂₀₀ 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 112° to 84°), water permeability (the permeation flux increased from 80 to 440 L/m² 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/cm²). The hydrophilicity and fouling resistance of PE/PE‐b‐PEG blend membranes increased as the length of PE segment in PE‐b‐PEGs decreased. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46499.     

Kinetics of the propylene polymerization with prepolymerization at high temperature using Ziegler‐Natta catalyst

The bulk polymerization of propylene in liquid monomers with Ziegler‐Natta catalyst at 95°C is studied, using alkyl aluminum as the cocatalyst and dicyclopentyldimethoxysilane as the external donor. The highest catalyst activity is shown at the cocatalyst/Ti molar ratio of 300, which keeps relatively constant with the molar ratio increasing from 300 to 800. Besides, the catalyst activity is up to 65 kgPP/(gCat*h) in the range of cocatalyst/donor molar ratio from 12 to 16. The polymerization reaction rate curves with and without catalyst precontacting are similar, while the activity with catalyst precontacting are higher than that without precontacting. Furthermore, the kinetics of polymerization with and without prepolymerization are investigated in the range of the polymerization temperature from 70 to 95°C. It shows that at the high temperature, the polymerization rate increases with prepolymerization. Finally, the influence of prepolymerization at 95°C on the polymerization kinetics and particle properties is also described. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41816.     

Propylene polymerization over supported Ziegler–Natta catalysts: Effect of internal and external donors on distribution of active sites according to stereospecificity

Data on the content of fractions with different microtacticities for polypropylene (PP) samples produced over three catalysts [the “donor‐free” titanium–magnesium catalyst and catalysts with dibutyl phthalate and 1,3‐diether(fluorene) used as internal donors] upon polymerization in the absence/presence of an external donor (ED) have been obtained by preparative temperature rising elution fractionation method. The effect of internal and EDs on the distribution of PP fractions with different microtacticity is discussed. Data on molecular weight and thermophysical characteristics were obtained for individual fractions with different microtacticities. Correlations were found between microtacticity, molecular weight, and the melting points of these fractions. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46291.     

Development of stable and active PVA‐PSSA/SA‐PVA catalytic composite membrane for esterification enhancement

An active and stable catalytic composite membrane (CCM), poly(vinyl alcohol)–poly(styrene sulfonic acid)/sodium alginate–poly(vinyl alcohol) (PVA‐PSSA/SA‐PVA), was prepared to enhance the esterification of ethanol and propionic acid. The morphologies and crystal structures of the CCMs were investigated by Fourier transform infrared spectroscopy, scanning electron microscopy, and X‐ray diffraction. The effects of catalytic layer thickness, mass ratio of PVA to PSSA, concentration of catalytic layer solution, ratio of reaction volume to membrane area, and molar ratio of propionic acid to ethanol were discussed. The pervaporation results showed that the flux of CCM increased from 118 to 320 g m^?2 h^?1 compared with the SA‐PVA membrane because of the close affinity and low resistance of PSSA to water. After crosslinking with 3‐aminopropylmethyldiethoxysilane, the CCMs had good catalytic activities. The acid conversion reached 92.8% at 75 °C in 12 h, and the stabilization of the CCM was greatly improved. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46514.     

Kinetics of short‐duration ethylene polymerization with MgCl2‐supported Ziegler–Natta catalyst: Two‐stage initiation evidenced by changes in active center concentration

The kinetics of ethylene polymerization with a TiCl₄/MgCl₂‐type Ziegler–Natta catalyst was studied. Changes in polymerization activity and concentration of active centers ([C*]) in the first 5 min were determined. Initiation of the active centers was found to proceed in two stages. In the first stage, [C*]/[Ti] quickly rose to about 1% in less than 30 s and then remained stable in the subsequent 60 s. Then the [C*]/[Ti] value started to increase again, forming the second buildup stage. The polymerization activity was found to change roughly in parallel with the change in [C*]/[Ti]. Changes in the polymer/catalyst particle morphology and polymer molecular weight distribution with polymerization time were studied. A mechanistic model was proposed to explain the two‐stage kinetics: initiation of active sites on the outer surface of catalyst particles takes place in the first stage, and initiation of active sites buried inside the particles takes place in the second stage. These buried sites are released when the catalyst particles are fragmented by the expanding polymer phase. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45187.     

Kinetics of short‐duration ethylene–propylene copolymerization with MgCl2‐supported Ziegler–Natta catalyst: Differentiation of active centers on the external and internal surfaces of the catalyst particles

Ethylene–propylene copolymerization with a TiCl₄/MgCl₂ type ZN catalyst was conducted for different durations from 30 to 600 s, and changes of polymerization rate, concentration of active centers ([C^*]) and copolymer chain structure with time were traced. The copolymerization rate decayed with time, but [C^*]/[Ti] increased in the same period. This was attributed to release of more active sites through disintegration of catalyst particles by the growing polymer phase. Ethylene content of the copolymer quickly decreased in the period of 30–90 s, meaning that the active centers activated in the reaction process have stronger ability of incorporating propylene than those activated at the very beginning. The copolymer samples were fractionated into two parts, namely n‐heptane soluble fraction (random copolymer) and insoluble fraction (segmented copolymer with high ethylene content). With continuation of the copolymerization, active centers producing the random copolymer chains increased much faster than active centers producing the segmented copolymer chains, and became the dominant centers after 120 s. Consequently, proportion of the soluble fraction sharply increased with time. All these results indicate that the active centers located on the external surface of catalyst particles are highly different from those buried inside the particles. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46030.     

Highly effective organometallic‐mediated radical polymerization of vinyl acetate using alumina‐supported Co(acac)2 catalyst: A case study of adsorption and polymerization

An alumina support system for cobalt^(II) acetylacetonate (Co(acac)₂) catalyst was studied for the cobalt‐mediated radical polymerization (CMRP) of vinyl acetate (VAc). We report a simple but efficient technique to produce this supported catalysts through the adsorption of Co(acac)₂ on the surface of alumina particles. Moreover, kinetic and thermodynamic study of Co(acac)₂ adsorption on the alumina support were conducted and the influence of effective parameters were investigated. It was found that using alumina‐supported Co(acac)₂ for radical polymerization of VAc yields polymers with controlled molecular weight, narrow molecular weight distribution, and high purity. For the alumina‐supported CMRP, changing the polymerization mechanism and domination of termination pathway compared to degenerate transfer pathway resulted in a 2.5 times increase in polymerization rate (k_ap) and a drop in induction time while maintaining a good control of the VAc polymerization. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46057.     

Crystallization and fracture behaviors of high‐density polyethylene/linear low‐density polyethylene blends: The influence of short‐chain branching

Two commercial polyethylene samples, linear high‐density polyethylene (HDPE) and branched linear low‐density polyethylene with almost the same molecular weight distribution but different contents of short‐chain branching (SCB) were melt blended based on the consideration of practical application. Dynamic rheology analysis indicated good compatibility of all the blends with different compositions. Common differential scanning calorimeter (DSC) tests and successive self‐nucleation and annealing (SSA) treatment showed several interesting phenomena. First, without consideration of the effect of molecular weight and molecular weight distribution impact, co‐crystallization occurred at all ratios even the two components had a considerable difference in SCB distribution. Second, in SSA curves the area of the first two melting peaks, i.e., the amount of the thick lamellas of the two components showed an obvious positive deviation with the increase of HDPE content owing to the crystal perfection improved by the co‐crystallization. Essential Work of Fracture tests proved the co‐crystallization effects had a positive effect on the improvement of the resistance to crack propagation. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Butyl chloride as promoter in ethylene polymerization by magnesium ethoxide‐supported catalyst

The effects of butyl chloride as a promoter in the ethylene polymerization were studied using a Mg(OEt)₂/TiCl₄/triethyl aluminum (TEA) Ziegler–Natta catalyst system, where Mg(OEt)₂, TiCl₄, TEA were used as support, catalyst, and activator, respectively. The influence of BC on the catalyst performance, polymerization rate, and polymer properties were investigated. This study strongly indicates that BC could act as a promoter with high performance in the ethylene polymerization. There was a remarkable increase in the catalyst yield and polymerization rate, in particularly, in the presence of hydrogen which was used for controlling the molecular weight. A reduction in the polymer molecular weight was observed in the presence of BC and hydrogen. The morphology of the polymers was evaluated through scanning electron microscopy and particle size distribution. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40189.     

Rheological,thermal, and morphological properties of low‐density polyethylene/ultra‐high‐molecular‐weight polyethylene and linear low‐density polyethylene/ultra‐high‐molecular‐weight polyethylene blends

The dynamic rheological behavior of low‐density polyethylene (LDPE)/ultra‐high‐molecular‐weight polyethylene (UHMWPE) blends and linear low‐density polyethylene (LLDPE)/UHMWPE blends was measured in a parallel‐plate rheometer at 180, 190, and 200°C. Analysis of the log–additivity rule, Cole–Cole plots, Han curves, and Van Gurp curves of the LDPE/UHMWPE blends indicated that the blends were miscible in the melt. In contrast, the rheological properties of LLDPE/UHMWPE showed that the miscibility of the blends was decided by the composition of LLDPE. The differential scanning calorimetry results and scanning electron microscopy photos of the LLDPE/UHMWPE blends were consistent with the rheological properties, whereas with regard to the thermal and morphological properties of LDPE/UHMWPE blends, the results reveal three endothermic peaks and phase separation, which indicated a liquid–solid phase separation in the LDPE/UHMWPE blends. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013