The slug flow behavior of polyethylene particles polymerized by Ziegler-Natta and metallocene catalysts

We investigated the flow behavior of polyethylene particles polymerized by Ziegler-Natta catalyst and Metallocene catalyst. The employed polymer particles were linear low-density polyethylene (LLDPE), and average particle size was 600 Μm. Different flow behavior of polymer particles in a fluidized bed was observed with different polymerization catalysts in the bubbling and slugging flow regimes. The flow behavior determined from the pressure drop fluctuation in the lower and upper section of the bed was analyzed with statistical methods. Presented at the Int’/Sym. on Int’l Symp. on Chem. Eng. (Cheju, Feb. 8–10, 2001), dedicated to Prof. H. S. Chun on the occasion of his retirement from Korea University.     

Effect of cocatalyst on the chemical composition distribution and microstructure of ethylene-hexene copolymer produced by a metallocene/Ziegler-Natta hybrid catalyst

A silica-magnesium bisupport (SMB) was prepared by a sol-gel method for use as a support for metallocene/Ziegler-Natta hybrid catalyst. The SMB was treated with methylaluminoxane (MAO) prior to the immobilization of TiCl₄ and rac-Et(Ind)₂ZrCl₂. The prepared rac-Et(Ind)₂ZrCl₂/TiCl₄/MAO/SMB catalyst was applied to the ethylenehexene copolymerization with a variation of cocatalyst species (polymerization run 1: triisobutylaluminum (TIBAL) and methylaluminoxane (MAO), polymerization run 2: triethylaluminum (TEA) and methylaluminoxane (MAO)). The effect of cocatalysts on the chemical composition distributions (CCDs) and microstructures of ethylene-hexene copolymers was examined. It was found that the catalytic activity in polymerization run 1 was a little higher than that in polymerization run 2, because of the enhanced catalytic activity at the initial stage in polymerization run 1. The chemical composition distributions (CCDs) in the two copolymers showed six peaks and exhibited a similar trend. However, the lamellas in the ethylene-hexene copolymer produced in polymerization run 1 were distributed over smaller sizes than those in the copolymer produced in polymerization run 2. It was also revealed that the rac-Et(Ind)₂ZrCl₂/TiCl₄/MAO/SMB catalyst preferably produced the ethylene-hexene copolymer with non-blocky sequence when TEA and MAO were used as cocatalysts.     

A different approach to morphological diversity and surface nucleation in linear polyethylene

The classical Kossel-Stranski crystal model is adapted to a critical re-examination of surface nucleation in polyethylene. Several aspects of the diverse morphologies displayed by this polymer are well accounted for. The suggestion is made that curved {200} faces in lenticular crystals arise as a consequence of normal growth rather than nucleated growth on these faces. Processes generally involved in surface nucleation and subsequent layer spreading are discussed with emphasis upon entropic considerations. This leads to the view that, although nucleation kinetics are followed, molecular conformations can become more tightly clumped than is commonly thought to follow inevitably from layer spreading.     

Thermoreversible gelation of mixed triblock and diblock copolymers in n-octane

The thermoreversible gelation of blends of polystyrene-block-poly(ethylene/butylene)-block-polystyrene (SEBS) and polystyrene-block-poly(ethylene/propylene) (SEP) copolymers in n-octane was studied. The solvent is selective for the polyolefine blocks of the copolymers. The influence of the composition of the hybrid gels on the sol-gel transition and on the mechanical properties of the gels was analyzed. The sol-gel transition temperature increased with the concentration of both type of copolymers and did not depend on the hybrid gel composition for SEBS2 proportions higher than 50% at a total copolymer concentration higher than 6 wt%. The mechanical properties of the different gels were examined through oscillatory shear and compressive stress relaxation measurements. The elastic storage modulus increased with the triblock copolymer concentration but kept almost constant with the diblock copolymer concentration for SEBS concentrations higher than 5.0%. The stress relaxation rate was not dependent on the concentration of triblock and diblock copolymers, but the hybrid gels show lower stress relaxation rates than the pure SEBS2 gels. In the hybrid SEBS/SEP gels the SEP chains impart stability to the micelles or nodes of the network whereas the SEBS chains are responsible for the bridges that keep the gel as one-phase system.     

Crystallization kinetics of some new olefinic block copolymers

Blocky ethylene-octene copolymers synthesized by chain-shuttling polymerization differ from statistical copolymers in their rapid rate of crystallization and in the formation of space-filling spherulites even when the crystallinity is as low as 7%. The bulk crystallization rate, measured with DSC, was rapid even in copolymers with a relatively large fraction of non-crystallizable soft block and only slowed somewhat as the amount of crystallizable hard block decreased from 100 to 18 wt%. As measured with the polarized optical microscope, the linear spherulite growth rate exhibited the same dependence on soft block content as the bulk crystallization rate. The fold surface energy was extracted from an analysis of the growth rate according to the Lauritzen-Hoffman theory. A gradual increase in the fold surface energy with soft block content reflected some increasing disorder of the fold surface. In contrast, even a small amount of statistically distributed comonomer was very effective in disrupting the fold surface regularity as demonstrated by the high fold surface energy.     

Thermoreversible micellization and gelation of a blend of pluronic polymers

Aqueous solutions of a blend consisting of a PEO-PPO-PEO triblock copolymer, Pluronic F127 (EO₁₀₀-PO₆₅-EO₁₀₀) and a PPO-PEO-PPO triblock copolymer, Pluronic-R 25R4 (PO₁₉-EO₃₃-PO₁₉) were studied. Thermoreversible micellization and gelation properties of the blend were examined as a function of temperature and molar ratio of 25R4 to F127 by means of micro DSC and rheology. The completely thermoreversible behaviors of micellization and gelation were observed for all the blend solutions with two F127 concentrations (10 and 15 wt%) and various 25R4/F127 molar ratios (0-4) even though the pure 25R4 solution itself was not thermoreversible. At a given concentration of F127, three effects of 25R4 on F127 were found as follows. (a) The micellization temperature of F127 shifts to a lower temperature with increasing 25R4 content, implying a “salt-out like” effect of 25R4. (b) Beyond the primary peak for micellization a secondary peak appears due to the effect of 25R4. (c) At the molar ratio of 25R4/F127 = 3:1, the gelation of the 15 wt% F127 solution occurs twice at low and high temperatures, respectively. When the ratio > 3:1, the gelation occurs only at high temperatures. The possible mechanisms involved in these unique behaviors of micellization and gelation have been proposed and discussed. The effect of 25R4 on F127 was compared with another Pluronic polymer F108 (EO₁₃₃-PO₅₀-EO₁₃₃).     

Real-time dynamic light scattering on gelation and vitrification

The time-resolved dynamic light scattering has been employed to investigate gelation of divinylbenzene (DVB; branched chains) and vitrification of styrene (St; linear chains) during radical polymerization in bulk. The intensity correlation function (ICF) was obtained every 30 s during polymerization process. The DVB system exhibited a power law behavior in ICF characteristic of gelation threshold, which was followed by vitrification. In the case of St, two relaxation modes appeared before vitrification, which were assigned to be the cooperative and the slow relaxation modes.     

Effect of comonomer type on the crystallization kinetics and crystalline structure of random isotactic propylene 1-alkene copolymers

Isothermal crystallization kinetics and properties related to the crystalline structure of four series of random propylene 1-alkene copolymers have been comparatively studied in this work. Comonomers studied include ethylene, 1-butene, 1-hexene and 1-octene in a concentration range up to 21 mol%. All copolymers were synthesized with the same metallocene catalyst to provide an equivalent random distribution and a similar content of stereo and regio defects within the series. This has ensured that differences in crystallization kinetics and in crystalline properties of copolymers with matched compositions reflect the affinity of the comonomer type for co-crystallization with the propene units, and the effect of content and type of co-unit in the development of the crystalline structure. In the nucleation-driven crystallization range, that is for T_cs > T_{c max}, the values of the rate follow the sequence PB > PE > PH = PO for comonomer contents <13 mol%, and PB > PE > PH > PO for >13 mol% comonomer. These trends in overall crystallization are guided by differences in undercooling due to a similar progression of the degree of participation of the comonomer in the crystalline lattice. The variation of the rates at T_cs < T_{c max} follows the melt segmental dynamics driven by differences in T_g, especially at the highest co-unit contents, resulting in a reverse rate sequence for PHs and POs >15 mol%, i.e., PB > PE ∼ PO > PH. In addition to crystallization kinetics, a comparative polymorphic analysis and unit cell expansion, crystalline morphology, and melting behavior have been instrumental in resolving the partitioning of the four types of co-units between crystalline and non-crystalline regions. 1-Butene units participate at the highest level followed by the ethylene units, as demonstrated by solid-state NMR. However, both units are defects that hinder crystallization, as given by the decreasing rates, decreased levels of crystallinity and lowered melting temperatures with increasing co-unit content. All crystalline properties of PHs and POs conform to a rejection model of the 1-octene units from the crystals in the whole compositional range, and rejection of the 1-hexene units for PH <13 mol%, a conclusion also supported by NMR. The ability of PH >13 mol% to pack comonomer-rich sequences into a stable trigonal lattice leads at T_cs > T_{c max} to an increased number of crystallizable sequences, and to faster crystallization rates than for matched PO copolymers.     

Diffusion and annealing in crystallizing polymers

Ultraviolet and fluorescent microscopy of additives in crystallizing polypropylene can be used to study the development of crystallinity within growing spherulites. As the spherulite forms, impurity species including fluorescent additives are initially rejected then subsequently diffuse back in. Simultaneously the interior of the spherulite is increasing in crystallinity and tending to push the impurities back towards the outside.The effect of these processes on the final distribution of impurities within the spherulite is calculated. These results suggest that non-uniformity in additive concentrations will have little impact on the stabilization of the polymer against photo-oxidation.     

Polymerization of 4-n-alkylstyrenes with typical Ziegler-Natta and metallocene catalysts

Polymerization of 4-n-alkylstyrenes (alkyl side group; methyl, ethyl, propyl and buthyl) was carried out with the η-C₅(CH₃)₅TiCl₃-methylaluminoxane (MAO) and TiCl₃-triethylaluminum (TEA) catalyst systems. When the η-C₅(CH₃)₅TiCl₃-MAO catalyst was used, the stereoregularity of resulting polymers markedly depended on the length of substituted alkyl groups, i.e., the catalyst gave highly syndiotactic poly(4-methylstyrene), but produced atactic polymers for the monomers with ethyl, propyl and buthyl substituents. On the other hand, all the poly(4-n-alkylstyrene)s obtained with the TiCl₃-TEA catalyst were highly isotactic. As a result, a large difference was observed in the thermal properties of polymers obtained between the two catalyst systems. Received: 6 September 1996/Accepted: 16 October 1996     

Structural formation and gelation behavior of cold-crystallized poly(trimethylene terephthalate)

Experimentally observed a +-type H_v and a circular-type V_v scattering patterns from small-angle light scattering (SALS) measurement indicated a random assembly of sheaf-like or rod-like superstructure for poly(trimethylene terephthalate) (PTT) under cold-crystallization. The long period and crystallite thickness of cold-crystallized PTT have been determined by small-angle X-ray scattering (SAXS). The crystallite thickness is very small, ca 2 nm, which is close to the c-axis (chain axis) of unit cell in PTT crystal, 18.64 Å. The intermolecular crystallization may be viewed as a behavior of physical gelation. The liquid-solid transition in PTT crystallization from glassy state through the gel point has been investigated by dynamic mechanical experiments. The power law relaxation modulus, E(t)=St^−Δ identifies the gel point, and small frequency window is sufficient to determine the relaxation exponent, Δ. The gel point occurs at the very early stage of crystallization, in which the relative degree of crystallinity is found to be about 2-3%. The Δ value is ca 0.52, which is independent of crystallization temperature.     

Dispersion and gelation behavior of alumina suspensions with Isobam

As a copolymer of isobutylene and maleic anhydride, Isobam, a newly-developed gelling agent, plays a significant role in ceramic forming process. However, dispersion and gelation behavior of Isobam has not been discussed in detail yet. To investigate the influence of other dispersants, pH and electrolytes on dispersion and gelation behavior of Isobam, zeta potential, viscosity and modulus curves during gelation were measured from 50?vol% alumina suspensions with 0.1–0.7?wt% Isobam. A two-step bridging mechanism is proposed to explain the gelation behavior of Isobam, providing guidance on usage of Isobam in combination of other additives to achieve suspensions with high solids loading and adjustable gelation characteristic.     

Nonisothermal crystallization of metallocene propylene–decene‐1 copolymers

The nonisothermal crystallization behavior of one metallocene‐based isotactic polypropylene and three propylene–decene‐1 copolymers was studied. The effects of comonomer content and cooling rate were investigated. It was found that comonomer units enchained systematically reduce the crystallization temperature (T_c), melting temperature (T_m), fusion enthalpy (ΔH_f), and crystallinity (X_c). Such an effect becomes more evident at a faster cooling rate. With increasing comonomer content, the supercooling required for crystallization increases and the overall crystallization rate is reduced. The Avrami equation is applicable to describe the nonisothermal crystallization kinetics of propylene–decene‐1 copolymer. It was shown that, although the reduced crystallization rate constant Z_c increases with comonomer content, the Avrami exponent decreases with comonomer content and cooling rate, leading to the smaller overall crystallization rate and larger crystallization half‐time of the copolymer with higher comonomer content. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1724–1730, 2004     

Isothermal crystallization kinetics of nylon 6, blends and copolymers using simultaneous small and wide-angle X-ray measurements

Crystallization behavior in a number of blends and copolymers of nylons (polyamides) was investigated using time-resolved X-ray scattering data obtained simultaneously in the small- and wide-angle regimes. The following samples studied were: nylon 6 homopolymers (N6) of different molecular weights, copolymers of N6 and nylon 6,6 (N6/66), and blends of these with an amorphous nylon (N6I/T), which is a 70:30 random copolymer of poly(hexamethylene isophthalamide) and poly(hexamethylene terephthalamide). Addition of comonomers and blending with the N6I/T reduces the crystallinity of N6. Isothermal crystallization data obtained at several temperatures showed the expected faster crystallization kinetics at higher degrees of supercooling. Comonomer units in the N6 backbone reduce the rate of crystallization. N6I/T affects the crystallization (lamellar growth) behavior of N6/N66: the rate is higher at temperatures above the T_g of N6I/T (120 °C) where the crystallization is nucleation-driven, and lower below the T_g of N6I/T where it is growth-driven. Lamellar spacing decreases with an increase in the degree of supercooling, and this decrease is smaller in the blends than in the homopolymer. Larger lamellar spacing in N6I/T blends is due not to the insertion of N6I/T segments into the interlamellar regions but to an increase in the lamellar thickness. Blending seems to change the morphology by affecting the crystallization behavior rather than by thermodynamic phase separation. Residual monomers, which act as plasticizers, dramatically reduce the crystallization rate, whereas shear or similar mechanical history of the resin considerably accelerate the crystallization rate.     

Influence of molecular composition on the development of microstructure from sheared polyethylene melts: Molecular and lamellar templating

A combination of in situ and ex situ X-ray scattering techniques and transmission electron microscopy has been used to study the crystallization behaviour of polyethylene, following the imposition of melt shear. In the case of a branched material, the imposition of shear flow up to a rate of 30 s⁻¹ was found to induce no anisotropy. Although shearing the linear material only ever induced a very small degree of anisotropy in the melt, for shear rates >0.15 s⁻¹, subsequent crystallization resulted in increasing anisotropy. Blends of the above two polyethylenes were produced, in which the linear material constituted the minority fraction (∼10%). Isothermal crystallization at temperatures where extensive crystallization of the branched material does not occur demonstrated that the behaviour of the linear component of the sheared blend mirrored that of the linear polyethylene alone. However, in addition, it was found that when crystallized in the presence of an oriented morphology, the branched polymer also formed anisotropic structures. We have termed the process templating, in which the crystallization behaviour of the bulk of the system (∼90% branched material) is completely altered (spherulitic to oriented lamellar) by mapping it onto a pre-existing minority structure (∼10% linear polymer).     

Performance control of asymmetric poly(phthalazinone ether sulfone ketone) ultrafiltration membrane using gelation

We studied the influence of the gelation conditions on the formation kinetics of the polyphthalazine ether sulfone ketone (PPESK) membrane via wet phase inversion process experimentally and theoretically. Membrane formation and its morphology were first observed with an online optical microscope - CCD camera system. The resulting membranes prepared under various gelation conditions were then characterized by the gelation parameter, optical microscope, and SEM. Lastly, the relationship between the final membrane structure/permeation properties and the gelation parameter was discussed extensively. The results showed that both the gelation rate and the membrane flux increased dramatically as the gelation temperature increased. Moreover, the membrane structures became loose, and the porosity of membrane increased. Different non-solvent could change the solubility parameter between the polymer and the non-solvent, and thus the gelation rate greatly. With the increasing number of carbons in non-solvent, the gelation rate became slow, and the membrane gradually changed from a finger structure into a sponge structure. Adding NMP into the non-solvent changed the difference in the chemical potential and the solubility parameter between the polymer solution and the non-solvent, which in turn changed the gelation rate of polymer solution greatly. With the increasing concentration of NMP in non-solvent, the gelation rate became very slow and sponge structures formed with the non-solvent system of 80% NMP. A novel conclusion could be made that we could control the flux and reject of membrane just by changing the mean diffusion coefficient of skin, D, and the diffusion coefficient of skin, D1, in the process of membrane formation. This work was presented at 13 ^th YABEC symposium held at Seoul, Korea, October 20–22, 2007.     

Isothermal crystallization of intercalated and exfoliated polyethylene/montmorillonite nanocomposites prepared by in situ polymerization

Polyethylene/montmorillonite (PE/MMT) nanocomposites with different dispersion states of MMT were prepared by in situ polymerization. Isothermal crystallization of the intercalated nanocomposite, in which the PE chains were confined in the MMT layers, was studied and was compared with that of the exfoliated nanocomposite. It is observed that the intercalated sample has longer induction period, longer crystallization half time and larger crystallization activation energy than the exfoliated sample, showing that crystallization of PE is retarded due to confinement of the MMT layers. Analysis of crystallization kinetics shows that Avrami exponent (n) increases gradually with crystallization temperature. However, the maximal value of n is 2.0 for the intercalated sample, but it can reach 3.0 for the exfoliated sample. It is inferred that the stems of the PE crystals confined in the MMT layers are parallel to the MMT layers. The Hoffman-Weeks extrapolation method cannot be applied in the intercalated sample because of the small lateral surface of the PE crystals. Based on the depression of the melting temperature, the specific free energy of the PE/MMT interface was estimated, which is about 1.0 mJ/cm², much smaller than the free energy of the lateral surface of PE crystals. This is attributed to the origin of the strong nucleation effect of MMT.     

Investigation of gelation mechanism and the effects of APS on AlN slurries with Isobam

A novel and simple gelling system based on a water-soluble copolymer of isobutylene and maleic anhydride (commercial name, Isobam) was adopted to manufacture various ceramics via spontaneous gelation. The gelation mechanism of AlN slurry was investigated using infrared spectroscopy. Results show that Isobam was hydrolyzed to produce the –COOH group, and the hydrogen bonding was formed between the –COOH and –CONH₂ groups of Isobam during spontaneous gelation. Meanwhile, the gelling process of the Isobam system could be accelerated via enhancing hydrogen bonding through APS addition. With 1 wt% addition of APS, the gelation time reduced to 23 h, the viscosity of 50 vol% AlN slurry decreased to 0.21 Pa·s, and the flexural strength of green body improved by 48%. Therefore, the presence of hydrogen bonding in the Isobam gelation mechanism was confirmed, and it could be enhanced by introducing APS to accelerate gelation.     

Rheological study of the gelation of cross-linking polyhedral oligomeric silsesquioxanes (POSS)/PU composites

The gelation process of N-Phenylaminomethyl-POSS/PU (polyurethane) nanocomposites during curing and at the stable state after curing was investigated by rheology. An increase in dynamic shear moduli, G′ and G″, was observed during the dynamic temperature ramps of the sample. In time-resolved mechanical spectroscopy (TRMS), G′ and G″ increased with curing time at constant curing temperatures over a wide of frequencies. The gelation time of the composites decreased with the rise of curing temperature or with the increase of POSS concentration. The relaxation exponent n at the critical gel was around 0.73. The gel stiffness S decreased as curing temperature increased or as POSS concentration increased. After the completion of the curing reaction, the critical concentration of POSS beyond which the gelation of POSS/PU composites would happen was found around 2.5 wt%. The viscoelastic properties of crosslinking POSS/PU fitted time–temperature-superposition well which implied the incorporation of multifunctional POSS into PU increased the homogeneity of crosslinking POSS/PU composites. Surprisingly, the modulus of the fully cured materials between 2.7 wt% and 6 wt% could also be supposed onto a master curve at high temperature, which implied self-similarity of network near the critical gel. The similar microstructure of POSS/PU at stable state was also confirmed by TEM. The network formation mechanism and the fine structure of the crosslinking POSS/PU were firstly investigated which would provide technical and theoretical basis for other hybrid crosslinking systems.     

Molecular weight dependence of melt crystallization behavior and crystal morphology of low molecular weight linear polyethylene fractions

Melt crystallization behavior and corresponding crystal morphology of five low molecular weight (3,900 ≤ MW ≤ 20,800) linear polyethylene (PE) fractions have been investigated. The overall crystallization data indicate that the lower molecular weight (MW) fraction possesses a higher crystallization rate at the same undercooling (ΔT). On the contrary, at the same crystallization temperature (T_c) the rate increases with MW. The Avrami exponent (n) varies from ca. 3 to 4 with decreasing ΔT for the fractions studied, which implies the nucleation process changes from athermal type to thermal type as T_c increases. For the low MW PE’s, the different crystal growth regimes (regime I and II) have been first time identified via linear crystal growth rate (G) measurements. The regime I/II transition temperatures are close to previously reported data, which were obtained through a different method. As reported for intermediate MW PE’s, the transitions occur at an almost constant ΔT of 17.5±1 °C for each fraction studied. Morphological study shows that single crystals could be formed isothermally at low ΔT’s. Typical banded spherulites and axialites, which are MW and ΔT dependent, are also observed. Orthorhombic structure is ascertained to be the dominant crystal structure that exists irrespective of MW and crystal growth regime.