Mathematical Modeling of Temperature Rising Elution Fractionation (TREF) of Polyethylene and Ethylene/1‐Olefin Copolymers

Temperature rising elution fractionation (TREF) is a characterization technique widely used to estimate chemical composition distribution (CCD) of semicrystalline copolymers. Although several mathematical models have been previously proposed to elucidate the TREF fractionation mechanism, all previous TREF models assume equilibrium fractionation; thus, they cannot describe important kinetics effects observed in TREF experiments. In this work, a new TREF model is developed incorporating crystallization and dissolution kinetic models during the fractionation process. The proposed model describes the effects of molecular weight, comonomer content, cooling rate, heating rate, and solvent flow rate on experimental TREF profiles for both polyethylene and ethylene/1‐olefin copolymers very well.     

Effect of Operating Conditions on Dynamic Crystallization of Ethylene/1‐Octene Copolymers

Dynamic crystallization (DC) is a new characterization technique for measuring the chemical composition distribution (CCD) of semicrystalline copolymers. This technique fractionates polymers based on chain crystallizabilities under a constant cooling rate; a solvent is also fed through the column at a constant flow rate during the crystallization to enhance the physical separation of the polymer fractions. In this work, a DC model for ethylene/1‐olefin copolymers on the basis of population balance, crystallization kinetics, and axial dispersion is proposed. This model is found to describe the experimental DC profiles of an ethylene/1‐octene copolymer at various operation conditions very well.

     

Cocrystallization of Blends of Ethylene/1‐Olefin Copolymers: An Investigation with Crystallization Analysis Fractionation (Crystaf)

Summary: Crystallization analysis fractionation (Crystaf) is a technique for estimating the chemical composition distribution (CCD) of semi‐crystalline copolymers. Cocrystallization may happen during Crystaf analysis, affecting Crystaf profiles and leading to misinterpretation of the CCD. This study investigates this phenomenon and determines the main factors leading to cocrystallization by analyzing a series of ethylene/1‐olefin copolymers. We considered three factors affecting cocrystallization: comonomer type, cooling rate and chain crystallizability. The results showed that cooling rate and similarity in chain crystallizability are the key factors regulating cocrystallization during Crystaf analysis.

Crystaf analysis of blends of copolymers.     

Characterization of the Chemical Composition Distribution of Ethylene/1‐Alkene Copolymers with HPLC and CRYSTAF—Comparison of Results

A series of copolymers of ethylene with propene, 1‐hexene, 1‐octene, and 1‐octadecene is characterized by size‐exclusion chromatography (SEC), nuclear magnetic resonance spectroscopy (NMR), crystallization analysis fractionation (CRYSTAF), and high‐temperature interactive liquid chromatography. Four different solvent pairs are used as the mobile phase, while porous graphite is used as the column packing. The elution volumes of the polymer samples do not correlate with their average molar mass (SEC); however, they correlate with the average chemical composition (NMR). High performance liquid chromatography (HPLC) enables separation of the copolymers over the full range of their composition and independent of their crystallinity. Dependence between the elution volume and the average chemical composition distribution (CCD) of the copolymer is linear and it is a function of the length of branches as well as the type of the mobile phase. The CCDs of copolymers derived from HPLC profiles are similar to, yet broader than the CCDs obtained with CRYSTAF.

     

Characterization of Ethylene/α‐Olefin Copolymers Using High‐Temperature Thermal Gradient Interaction Chromatography

Several crystallization‐based techniques are used to measure the chemical‐composition distribution of polyolefins, but they are limited to semicrystalline polyolefins. Recently, high‐temperature thermal gradient interaction chromatography (HT‐TGIC) has been developed to quantify the chemical‐composition distribution of semicrystalline and amorphous polyolefins, thus broadening the range of techniques available for the analysis of polyolefin chemical‐composition distribution. In HT‐TGIC, the fractionation mechanism relies on the interaction of polyolefin chains with a graphite surface upon temperature change in an isocratic solvent. In the present investigation, a series of ethylene/1‐octene copolymers having approximately the same molecular weight average and different comonomer fractions (up to 25% of 1‐octene) is synthesized using a metallocene catalyst to investigate the fractionation mechanism of HT‐TGIC. Three copolymer samples and their blends are also studied to determine which operation parameters influence the HT‐TGIC peak shape and position. The cooling rate has no significant effect on the desorption temperature and the broadness of the HT‐TGIC chromatograms. On the other hand, the heating rate and the elution flow rate substantially influence the peak temperature and breadth.

     

FTIR Microanalysis and Phase Behaviour of Ethylene/1‐Hexene Random Copolymers

Ethylene/1‐hexene random copolymers with 1‐hexene content in the range of 1–5 mol‐%, synthesised in the presence of new heterogeneous catalyst systems based on bis‐carboxylato and ‐bis‐chloro‐carboxylato titanium chelate complexes, have been characterised by FTIR microspectroscopy (FTIR‐M), DSC calorimetry and X‐ray scattering. The co‐monomer content and sequence distribution in the various samples were determined by means of both FTIR‐M and ¹³C NMR spectroscopy. The deformation bands of methyl groups in the region of 1 400–1 330 cm^?1 were used for the structural analysis of these copolymers. The effect of composition on the crystallinity and phase transitions of copolymers was analysed both in 1 500–1 300 and 760–690 cm^?1 frequency ranges as a function of the annealing temperature. A neat variation of the absorbance ratio of methyl band at 1 378 cm^?1 was recorded between 110 and 130 °C corresponding to the melting range of the copolymer crystals. The crystallisation behaviour of the copolymers was examined by DSC in dynamic and isothermal conditions; the isothermal kinetics were analysed according to the Avrami model. A marked decrease in the bulk crystallisation rate, accompanied by changes in the nucleation and growth of crystals, was found with an increase in the co‐monomer content. The melting behaviour of isothermally crystallised samples was also investigated and the melting temperatures of the copolymers at equilibrium conditions were related to the composition; the experimental data were consistent with the Flory exclusion model of side branches from the crystalline phase. The lowering of crystal growth rate in the copolymers has been accounted for by an increase in the free energy of formation of critical size nuclei due to the effect of the side branches.

     

Simultaneous Deconvolution of the Bivariate Molecular Weight and Chemical Composition Distribution of Ethylene/1‐Hexene Copolymers

The bivariate molecular weight and chemical composition distribution (MWD×CCD) of ethylene/1‐hexene copolymers can be measured using TREF×GPC cross fractionation characterization (CFC). In this work, the experimental MWD×CCD of ethylene/1‐hexene copolymers made with a Ziegler–Natta catalyst under different polymerization conditions are measured by CFC and deconvoluted to identify the minimum number of site types present in the catalyst. Blends of ethylene/1‐hexene copolymers produced with a metallocene catalyst with known MWD×CCD are used to validate the proposed technique. This is a powerful methodology to better understand the nature of active sites on multisite catalysts, and can be beneficial for the development of copolymers with precisely controlled microstructures.     

Chemical Composition Separation of EP Copolymers by CEF and HT‐SGIC: Crystallization versus Adsorption

Preparative temperature rising elution fractionation (prep TREF) suffers from co‐crystallization effects and, therefore, cannot provide reliable chemical composition distribution (CCD) information. This limitation can be overcome when prep TREF is combined with further fractionation methods such as crystallization elution fractionation (CEF) or high‐temperature solvent‐gradient‐interaction chromatography (HT‐SGIC) as a new approach. By CEF, significant amounts of (co‐crystallizing) amorphous ethylene‐propylene (EP) copolymer are identified in semicrystalline TREF fractions of a heterophasic ethylene/propylene copolymer (HEPC). Complete compositional fractionation with no influence of crystallization effects is accomplished by HT‐SGIC. Prep TREF–HT‐SGIC is found to be the most selective and suitable method for the fast and complete CCD analysis of such complex EP copolymers with CEF providing complementary information.

     

Metallocenic Isotactic Poly(propylene) and its Copolymers with 1‐Hexene and Ethylene

A comprehensive study of the structure and properties has been performed for copolymers of propylene‐1‐hexene, CiPH, and propylene‐ethylene, CiPE, synthesized by an isotactic metallocene catalyst system. The comonomer content constitutes the most important factor affecting the structure and properties of these CiPH and CiPE copolymers, although the length of the comonomer is also very important. Thus, a considerable decrease in crystallinity is observed in the two kinds of copolymers as the comonomer content increases. The structure in the CiPH copolymers evolves, however, from the typical, monoclinic crystal lattice to mesomorphic‐like, ordered entities for the highest 1‐hexene molar fraction, whereas in the CiPE copolymers the structural evolution with molar fraction goes from a monoclinic lattice to an almost amorphous material. All of these variations in crystal structure significantly influence the viscoelastic and mechanical behavior of these CiPH and CiPE copolymers. Consequently, the location and intensity of the different relaxation mechanisms, as well as the rigidity parameters (storage and Young's moduli and microhardness) and deformation mechanism are strongly dependent upon composition.

     

Thermal Properties and SSA Fractionation of Metallocene Ethylene‐Oct‐1‐ene Copolymers with High Comonomer Content Cross‐linked by Dicumyl Peroxide or β‐Radiation

Two metallocene ethylene‐oct‐1‐ene copolymers, differing in comonomer content and in molecular weight, were cross‐linked either by dicumyl peroxide or β‐radiation. The effect of high comonomer content on the crystalline morphology, once the materials were cross‐linked, was analyzed by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). The gel content was determined in boiling xylene, and the cross‐linking process was monitored by FT‐IR spectroscopy. Two endotherms were distinguished: the first one was associated to the primary crystallization, and the second one to the shorter sequences that are excluded from the primary crystallization. The successive self‐nucleation annealing (SSA) technique has revealed that as the comonomer content increases the crystal size distribution is more homogeneous, and therefore, the melting and crystallization behaviour is reversible, because of its fringed‐micellar morphology. In spite of their crystalline morphology with very low crystallites, DCP cross‐linked samples displayed a considerable decrease in crystallinity and in crystal size, whereas β‐irradiated samples showed no significant decline in crystal size. Slight changes in crystallinity were detected and attributed to the heat generation that every irradiation process involves and affects smaller crystallites preferentially. DMA analysis has confirmed DSC results on crystalline size and crystallinity variations induced by both cross‐linking processes. By means of FT‐IR spectroscopy, it was detected that a high comonomer content induces oxidation during cross‐linking. Moreover, β‐irradiation samples exhibited a lower degree of oxidation than DCP cross‐linked samples.

The heating scan of DCP cross‐linked EG8411 after being submitted to successive self‐nucleation annealing.     

Ethylene/10‐Undecenoic Acid Copolymers Prepared with Different Metallocene Catalysts

The synthesis and characterisation of several functionalised polyethylenes, obtained by direct copolymerisation of ethylene with 10‐undecenoic acid (UA), have been carried out. Two metallocene complexes of C_s symmetry with different bridges were used: a dimethylsilyl bridge complex Me₂Si(Cp)(Flu)ZrCl₂ and a phenylidene bridge complex Ph₂C(Cp)(Flu)ZrCl₂. For comparison, results obtained with the Cp₂ZrCl₂/MAO system are also reported. The copolymerisation activity depends strongly on the metallocene complex and on the added amount of the UA comonomer. High incorporation levels of up to 7.3 mol‐% are achieved when using the phenylidene bridged catalyst presenting a rigid structure and a wide bite angle. The effect of comonomer content and catalyst geometry on the crystalline structure and mechanical behaviour of the different copolymers are comprehensively discussed.

     

Joint Effect of Poly(ethyhlene‐co‐1‐octene) Chain Length and 1‐Octene Fraction on High‐Temperature Thermal Gradient Interaction Chromatography

High‐temperature thermal gradient interaction chromatography (HT‐TGIC) fractionates polyolefins based on an adsorption–desorption mechanism. Several factors influence the shape and position of HT‐TGIC chromatograms, notably polymer microstructure, analytical conditions, and, to a lesser extent, solvent type. This article investigates the joint influence of chain length and comonomer content of a series of polyethylene and ethylene/1‐octene copolymers having similar 1‐octene fractions (0–13 mol%) and a wide range of molecular weights on HT‐TGIC fractionation. For each series of copolymers having similar 1‐octene fraction, the elution peak temperature decreases exponentially and the profiles become increasingly broader below a critical number average chain length value. The authors use Monte Carlo simulation and Stockmayer distribution to explain the observed behavior, finding that no simple correlation exists between ethylene sequences in the copolymers and peak elution temperature, but that there is strong evidence that axial dispersion is responsible for symmetrical broadening of the HT‐TGIC profiles. The authors also study the HT‐TGIC of binary blends, finding that components with similar 1‐octene contents and dissimilar chain lengths tend to increase co‐adsorption/co‐desorption effects.

     

Positron Annihilation in Norbornene‐Ethylene Copolymers

Norbornene‐ethylene amorphous copolymers with high content in norbornene are obtained with metallocene catalysts and present a high glass transition temperature. The size and concentration of the free‐volume holes of these copolymers, which affect to their gas barrier properties, has been studied by positron annihilation lifetime spectroscopy. The results of positron annihilation studies have shown that the microstructure of all samples is similar and they can be related to the dynamic mechanical parameters (temperatures and activation energies of the relaxations) as well as to the stress‐strain characteristics and microhardness of the copolymers. Moreover, it has been shown that in the case of the norbornene‐ethylene copolymers investigated the influence of the molecular rigidity on the glass transition temperature prevails over that of the pore dimensions.

Apparent activation energies and reciprocal values of tan δ for the γ relaxation as functions of volumes of the holes determined at T_g ?50 °C.     

Amorphous‐Crystalline PSnPEOn Heteroarm Star Copolymers: Crystallinity and Crystallization Kinetics

Summary: The crystallization behavior and kinetics of poly(ethylene oxide) in polystyrene/poly(ethylene oxide) heteroarm star copolymers were studied by differential scanning calorimetry and optical microscopy. A comparison between star and linear amorphous‐crystalline block copolymers showed that the macromolecular architecture is an important factor affecting crystallinity. The following points were observed: the equilibrium melting point is higher in the star copolymers, the crystallinity reduces as the number of arms increases, leading to smaller and ill‐defined spherulites, and crystallization proceeds faster in linear copolymers at low supercooling.

Half crystallization times, t_1/2, calculated from the Avrami analysis of the latent heats, obtained during the isothermal crystallization experiments as a function of supercooling, ΔT, for all copolymers.     

Effect of Level of Crystallinity on Melt Memory Above the Equilibrium Melting Temperature in a Random Ethylene 1‐Butene Copolymer

A strong memory effect of crystallization has been observed in melts of random ethylene copolymers even above the equilibrium melting temperature. Melt memory is correlated with self‐seeds that increase the crystallization rate of ethylene copolymers. The seeds are associated with molten ethylene sequences from the initial crystals that remain in close proximity and are unable to diffuse quickly to the randomized melt state. Fast diffusion is restricted by topological chain constraints (loops, knots, and other entanglements) that build in the intercrystalline region during crystallization. The effect of topological constraints on melt memory, or on number of remaining self‐seeds in the melt, is analyzed studying the melting and subsequent crystallization of a model ethylene 1‐butene copolymer with 2.2 mol% ethyl branches prepared with different levels of crystallinity. There is a critical threshold level of crystallinity of 6–12% to observe the effect of melt memory on the subsequent crystallization rate of this copolymer. A faster development of the initial crystallinity may more efficiently trap knots and loops around the crystallites, leading to a lower crystallinity threshold than for slow or isothermally crystallized copolymers.

     

Ethylene/1‐Hexene Copolymers Produced with MAO/(nBuCp)2ZrCl2 Supported on SBA‐15 Materials with Different Pore Sizes

The influence of the support architecture (structure and pore size) on the activity of MAO/(nBuCp)₂ZrCl₂ heterogeneous catalysts in the polymerization of ethylene and 1‐hexene is studied through the characterization of the polymers' microstructure. SBA‐15 silica‐based mesostructured materials are synthesized with different pore sizes and compared with commercial silica. All SBA‐15 supported catalysts lead to higher catalytic activities than amorphous silica and give rise to ethylene/1‐hexene copolymers with bimodal CCD, which suggests that the carrier structure plays an important role not only in the global rate of the process but also in the copolymer composition. The structures and pore sizes of the support show a marked influence on the polymer CCD shape.

     

Chain Features and Their Influence on the Thermal Stability of Poly(propylene‐co‐1‐nonene) Copolymers

The thermal stability of several isotactic polypropylenes and propylene‐co‐1‐nonene copolymers is assessed under nitrogen by means of thermogravimetric analysis (TGA). The samples involve wide ranges of molecular weight, isotactic average length, and 1‐nonene content, in order to perform a comprehensive analysis of the effect that chain features exert on the apparent activation energy (E_a), in the initial stages of the molten state degradation. The degradation process correlates with chain mobility and, accordingly, with chain features that are linked to. Thus, microstructure and chain size are found to play a key role. In fact, isotactic average length of propylene sequences and molecular weight are driving factors in the E_a required for main chain thermal scission.     

Mathematical Modeling and Optimization of Run Parameters of Crystallization Analysis Fractionation (CRYSTAF)

Blends of a random poly(propylene) copolymer with different types of polyethylene were used to develop a sample independent statistic mathematical model which describes the quality of phase separation of polymer blends obtained by CRYSTAF. By coupling the abstract model with experimental data, process parameters influencing the non‐equilibrium CRYSTAF separation process can be determined. It could be shown that the stirring speed applied during the fractionation process strongly influences the resolution of the derived CRYSTAF profile and thus the quality of fractionation. Nonlinear optimization of the models' response function leads to optimized run parameters for the CRYSTAF process which results in CRYSTAF profiles of high resolution and thus to a high quality in fractionation.

     

Effect of Solvent Type on High‐Temperature Thermal Gradient Interaction Chromatography of Polyethylene and Ethylene–1‐Octene Copolymers

The effects of the solvent type and operation conditions on the high‐temperature thermal gradient interaction chromatography (HT‐TGIC) of ethylene homopolymers, ethylene–1‐octene copolymers, and their blends are investigated. While the HT‐TGIC profiles of single polymers measured with 1,2,4‐trichlorobenzene (TCB) and chloronaphthalene (CN) are similar, they are always narrower when o‐dichlorobenzene (ODCB) is used, particularly for samples with lower 1‐octene fractions. Significant differences between the experimental and the calculated profiles of binary blends are observed with all three solvents, but better peak separation is seen when the ODCB is used. Having higher fractions of a 1‐octene‐poor component in the blend causes a more significant distortion of the shape expected for the peak from the component with the higher 1‐octene fraction. The effect of the molecular weight on HT‐TGIC profiles is also studied using samples with the same comonomer content and different molecular weights. Samples with low molecular weight have broader distributions and significant lower‐temperature tails, particularly when TCB is used. Chain crystallization after adsorption effects may also play a minor role for low‐comonomer samples. Finally, HT‐TGIC profiles are compared with their equivalent crystallization elution fractionation (CEF) profiles. The HT‐TGIC curves are broader than the equivalent CEF profiles, but these differences decrease as the comonomer content increases.

     

Synthesis of Propylene/1‐Butene Copolymers with Ziegler‐Natta Catalyst in Gas‐Phase Copolymerizations, 1

Summary: The appropriate choice of comonomers can be used to create a wide range of polymer properties, leading to considerable improvement of product performance. Experimental runs were performed to evaluate the effect of 1‐butene on the crystallinity, the melt temperature and the molecular weight distribution (MWD) of the final propylene/1‐butene copolymer resins. According to the results obtained, the melt temperature of the copolymer material can be reduced significantly compared to that of the polypropylene homopolymer. The incorporation of 1‐butene into the copolymer chain leads to a decrease of the sealing initiation temperatures of propylene polymer resins. GPC analyses of copolymer samples showed that 1‐butene concentration does not seem to significantly influence either the shape of the MWD or the polydispersity indexes for a given set of reaction conditions. Therefore, a family of propylene/1‐butene random copolymers grades can be successfully developed for gas phase processes for packaging and film applications.