Effect of crystallization conditions on the morphological structure of ethylene-1-butene copolymers obtained by different ziegler-natta catalyst systems

Four ethylene- 1 -butene copolymers of about the same comonomer content but obtained with different supported Ziegler-Natta catalyst systems have been studied. The effects of the catalyst and the crystallization conditions on the morphological structure have been analyzed. These two factors'clearly affect the melting endotherms and the most probable crystallite thickness of the copolymers, although no important differences were found in the crystalline contents. The catalyst system influences the melting pattern due to changes in the chemical composition distribution, i.e., variations in the comonomer content between chains of different molecular weight.     

Triple crystallization behavior of fractionated ethylene/α-olefin copolymers of different catalyst type

Non-isothermal crystallization processes in fractions of Ziegler-Natta (ZN) and single site (SS) based ethylene/1-butene and ethylene/1-hexene copolymers have been studied by differential scanning calorimetry (DSC). Fractionation of used copolymers was done according to molar mass (MM) and composition (comonomer content). It was observed in DSC scans that for fractions with high MM (larger than 10 kg/mol) in addition to the main high-temperature crystallization peak (HTCP), a very-low temperature crystallization peak (VLTCP) is present at temperatures in between 60–75 °C. Such peak is absent for the first fractions having very-low MM. The partial crystallinity and peak temperatures, obtained from VLTCP, increase with MM and level off at MM around 60–100 kg/mol. It was found that the crystallinity as related to the area of the VLTCP is catalyst type dependent, and is higher for the SS catalyst compared to the ZN. Peak temperature of VLTCP linearly decreases with increasing comonomer content at fixed MM while the partial crystallinity practically does not change with comonomer content.     

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.     

Blends of propylene-ethylene and propylene-1-butene random copolymers: I. Morphology and structure

Samples of propylene-ethylene (EP) and propylene-(1-butene) (BP) random copolymers with various comonomer content (2-3.1 wt% ethylene, 9.9 wt% 1-butene), were melt-mixed in Brabender internal mixer at various compositions (25/75, 50/50, 75/25). Films of copolymers and blends, as well as of a homopolymer sample (iPP), obtained by compression moulding and with different thermal history were characterized by optical and scanning electron microscopy (OM, SEM), small-angle light scattering (SALS), small- and wide angle X-ray scattering (SAXS, WAXS) and differential scanning calorimetry (DSC). It was found that all copolymers and blends studied crystallized exclusively in monoclinic α-modification forming spherulitic structure in a very broad undercooling range. The average size of spherulites is smaller in the copolymer containing 1-butene as compared to those containing ethylene or to iPP homopolymer, due to enhanced heterogeneous nucleation in BP copolymer. SEM microscopic observations demonstrated that EP and BP copolymers were miscible at all examined compositions and form homogeneous blends. Structural and morphological analysis indicated that the comonomer units are incorporated into growing crystallites in both EP and BP copolymers, while the non-crystallizing material is rejected out of the crystallites. For small concentrations of comonomer some of non-crystallizing species are pushed ahead of the front of growing spherulite into interspherulitic regions. For higher comonomer concentration these species are mostly trapped in intraspherulitic regions. Melting behavior of copolymers reflects the incorporation of comonomer into crystalline phase: melting temperature and crystallinity degree decrease in copolymers and blends as compared to plain iPP.     

Lamellar morphology of random metallocene propylene copolymers studied by atomic force microscopy

Four sets of propylene based random copolymers with co-units of ethylene, 1-butene, 1-hexene and 1-octene, and a total defect content up to ∼9 mol% (including co-unit and other defects), were studied after rapid and isothermal crystallization. Etched film surfaces and ultramicrotomed plaques were imaged so as to enhance contrast and minimize catalyst and co-catalyst residues. While increasing concentration of structural irregularities breaks down spherulitic habits, the formation of the gamma polymorph has a profound effect on the lamellar morphology. Lamellae grown in the radial axis of the spherulite and branches hereon are replaced in γ-rich copolymers with a dense array of short lamellae transverse or tilted to the main structural growth axis. This is the expected orientation for γ iPP branching from α seeds. Spherulites are formed in copolymers with non-crystallizable units (1-hexene and 1-octene) up to ∼3 mol% total defect content and were observed up to ∼6 mol% in those with partially crystallizable comonomers (ethylene and 1-butene). However, lamellae were observed in all the copolymers analyzed, even in the most defective ones, highlighting the important role of the gamma polymorph in propagating lamellar crystallites in poly(propylenes) with a high concentration of defects. Long periods measured from AFM and SAXS are comparatively analyzed.     

Morphological and kinetic partitioning of comonomer in random propylene 1-butene copolymers

The partitioning of the 1-butene co-unit between crystalline and non-crystalline regions of random, homogeneous propylene 1-butene copolymers (PB) has been studied by WAXD, ¹³C NMR, and FTIR in a series of copolymers with a concentration of 1-butene ranging from 2 to ～ 20 mol%. A partial inclusion of the 1-butene co-unit in the crystallites is identified by the expansion of the unit cell, and quantified by extracting ¹³C NMR spectra of the crystalline regions. For slowly cooled copolymers, about 30% of the chain’s 1-butene co-units are incorporated into the crystallites. Analyses of FTIR absorbances associated with crystalline 1-butene provide additional quantitative information on the morphological partitioning of the co-unit and give evidence to support that the incorporation of the comonomer into the crystalline regions is controlled by crystallization kinetics. The presence of the comonomer in the crystalline region affects the observed vibration of the most sensitive iPP 3/1 regularity bands associated with the evolution of crystallites, i.e. 841 cm^?1 (12 isotactic units). The frequency of this band shifts toward higher values with increasing comonomer and with increasing undercooling, in support of an increasing concentration of entrapped crystalline 1-butene. The frequency shift is absent in copolymers with co-units that are excluded from the crystalline regions, such as the 1-octene comonomer.     

Effect of multifunctional comonomers on the properties of poly(ethylene terephthalate) copolymers

The properties of poly(ethylene terephthalate) (PET) and its copolymers containing 0.04–0.15 mol% of pentaerythritol and trimethylolethane (TME) have been investigated. The molecular weight of the copolymers increased with comonomer content, and this effect was observed significantly with pentaerythritol copolymers, resulting in broad molecular weight distribution. The comonomer effect on the mechanical properties was small. The shear viscosity of the copolymers showed shear thinning within the experimental shear rate range. The crystallization rate and birefringence of the fibres containing 0.103 mol% pentaerythritol increased with the spin draw ratio, whereas they decreased with comonomer content. © 2002 Society of Chemical Industry     

Nonisothermal melting and crystallization studies of homogeneous ethylene/α-olefin random copolymers

A study of nonequilibrium melting, nonisothermal, and isothermal crystallization behavior of ethylene/1-octene (EO) random copolymers, produced using metallocene catalysts has carried out. As branch (or defect) content increases, the nonisothermal and isothermal crystallization rates, melting temperatures, and heats of fusion decrease. There is also a branch length effect on melting temperature depression, the melting temperature depression of EO random copolymers with hexyl branches were significantly larger than those of ethylene/1-butene (EB) and ethylene/1-propene (EP) copolymers having ethyl and methyl branches, respectively. The melting temperatures of homogeneous random copolymers have been found to be always lower than those of fractions of heterogeneous copolymers, having approximately the same branch content and molecular weight. Hence, defect distribution in copolymer systems is at least as important a parameter as the defect content. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 1893–1905, 1998     

Isothermal crystallization of metallocene‐based propylene/α‐olefin copolymers

Isothermal crystallization and subsequent melting behavior of two propylene/hexene‐1 copolymers and two propylene/octene‐1 copolymers prepared with metallocene catalyst were investigated. It is found that γ‐modification is predominant in all copolymers. The Avrami exponent shows a weak dependency on comonomer content and comonomer type. At higher crystallization temperatures (T_c) the crystallization rate constant changes more rapidly with T_c and the crystallization half‐time substantially increases. Double melting peaks were also observed at high T_c, which is attributed to the inhomogeneous distribution of comonomer units along the polymer chains and the existence of crystals with different lamellar thicknesses. The equilibrium melting temperatures (T) of the copolymers were obtained by Hoffman–Weeks extrapolation. It was found that the T decreases with increasing comonomer content, but are independent of comonomer type, implying that comonomer units are excluded from the crystal lattice. Dilation of the crystal lattice was also observed, which depends on crystallization, comonomer content, and comonomer type. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 240–247, 2005     

Effects of shearing and comonomer content on the crystallization behavior of poly(butylene succinate‐co‐butylene 2‐ethyl‐2‐methyl succinate)

Poly(butylene succinate‐co‐butylene 2‐ethyl‐2‐methyl succinate) (PBSEMS) random copolymers were prepared with different comonomer compositions. The effects of shearing and comonomer content on the crystallization behavior of these copolymers were investigated at 80 °C. The thermal and morphological properties of the resulting samples were also discussed. The copolymers showed a longer induction time and a slower crystallization rate with increasing comonomer content. The promoting effect of shear on the overall crystallization behavior was more notable for those copolymers containing more 2‐ethyl‐2‐methyl succinic acid (EMSA) units. The melting temperature of ‘as‐prepared’ poly(butylene succinate) (PBS) was ca. 115 °C, while that of the copolymers varied from 112 to 102 °C. Higher comonomer contents in the copolymers gave rise to lower melting temperatures and broader melting peaks. In addition, the isothermally crystallized samples showed multiple melting endothermic behavior, the extent of which depended on the comonomer content. The copolymers showed different wide‐angle X‐ray diffraction (WAXD) patterns from that of neat PBS, depending on the comonomer content and shear applied during crystallization. With increasing comonomer content, the copolymers crystallized without shearing, showing the shifting of a diffraction peak to a higher angle, while those crystallized under shear did not show any peak shift. Copyright © 2004 Society of Chemical Industry     

Crystallization of the γ form in random propylene‐ethylene copolymers

Wide‐angle X‐ray scattering and differential scanning calorimetry measurements have been conducted on seven random copolymers of propylene with ethylene in order to study the γ phase formation as a function of the comonomer content. The lamellar morphology of the samples was also investigated by small‐angle X‐ray scattering. The content of the γ phase was found to go through a maximum with crystallization temperature and to increase with comonomer concentration, up to a point (ethylene ≥6.5 wt%) where the latter parameter became less influential. The multiple melting endotherms behaviour of the samples was studied by DSC and temperature‐controlled diffractometric techniques. The attribution of the DSC peaks to the different isotactic polypropylene polymorphs that form in these conditions was confirmed. The results obtained permitted us to ascertain that, in the experimental conditions chosen, some further formation of crystallites takes place during the quenching to room temperature after the crystallization isotherm. In this phase, the chains organize themselves in stacks with thin lamellae, forming a distinct population with respect to those formed on isothermal crystallization. The melting of the thinner lamellae determines a convergence of the two populations into just one, still retaining an organization in stacks, that gradually disappears until complete melting of the material. Copyright © 2004 Society of Chemical Industry     

乙烯／1—丁烯气相共聚反应及共聚产物的结构与性能研究

研究了Ti-Mg催化剂催化乙烯/1-丁烯的气相共聚合反应。常压下聚合动力学曲线为衰减型,催化效率10.2～11kg PE/gTi,产物密度0.905～0.922g/cm~3,Φ-100流化床共聚合(1.2MPa)催化效率137～173kgPE/gTi,产物特性粘度[η]、MI_(2.16)MFR_(10.0/2.16)分别为1.09～2.22、1.0～6.9、7.8～10.4,共聚单体中1-丁烯含量为17％～38％(体积百分数)时,共聚物熔点为117.0～121.8,结晶度22.9％～34.9％,密度0.914～0.939g/cm~3,支化度16.7～22.7。结果表明,共聚物熔点、结晶度、密度随共聚单体1-丁烯含量增加而降低,而支化度则相反。     

Side chain liquid crystalline polyether macromonomers and their polymerization

Summary Copolymers of ethylene and 1-octene, 1-tetradecene and 1-octadecene were synthetized in order to study the effect of the comonomer chain length and amount on density, melting temperature and heat of fusion. They were also compared with the properties of the copolymers obtained with the heterogeneous titanium catalyst.It could be seen that the density/melting-area of the copolymers obtained with the zirconocene catalyst was much broader than with the titanium catalyst. It could also be seen that with the same comonomer content 1-tetradecene gave lower density of the copolymers than 1-octene. However, no difference were seen between the densities of the copolymers obtained with 1-tetradecene and 1-octadecene.The melting temperature of the copolymers was seen to be the lower the longer the comonomer was. In the area of the high branching amount the linear correlation was not in work. It was also shown that the differences in the heat of fusions could only be seen at high amount of branches (>15 branches/1000C): the longer the comonomer the lower amount of heat was needed to melt the copolymer.     

Rheological,thermal and crystallisation properties of ethylene,propylene and α-olefin copolymers. I Rheological properties

Abstract

A series of random ethylene, propylene/1-butene, 1-pentene, 1-hexene, 1-heptene, 1-octene copolymers, ethylene and propylene homopolymers were prepared and investigated. The rheological properties (steady state and dynamic shear viscosity, creep compliance and plateau modulus), of copolymer samples with different co-unit content and molecular masses were determined and compared with the properties of homopolymers. The effects of the length of counit and the comonomer content were investigated. The copolymers exhibited similar rheological properties to the homopolymer but they have a lower shear viscosity, normal viscosity, higher steady state creep compliance and smaller plateau modulus values. The effect of comonomer content was evaluated on the bases of free volume theory.     

Analysis of the chemical composition distribution of ethylene/α-olefin copolymers by solution differential scanning calorimetry: an alternative technique to Crystaf

A series of ethylene/1-hexene copolymers synthesized with a metallocene catalyst with varying comonomer contents but constant molecular weights were analyzed with crystallization analysis fractionation (Crystaf), solid state differential scanning calorimetry (solid state DSC) and solution differential scanning calorimetry (solution DSC). Experimental solution DSC exotherms were compared to Crystaf profiles obtained under similar crystallization conditions. At the same cooling rates (0.1 °C/min), considerable differences were found for samples with low levels of short chain branching although the discrepancy became less significant with increasing branching. Very good agreement was found between the Crystaf profiles of metallocene copolymers obtained at a cooling rate of 0.1 °C/min and solution DSC exotherms of samples crystallized at the rate of 0.01 °C/min. Good agreement was also observed between the Crystaf and solution DSC profiles of a Ziegler-Natta linear low-density polyethylene (LLDPE) sample when crystallized at the same cooling rate of 0.2 °C/min. In conclusion, solution DSC is a useful technique for simulating profiles obtained by Crystaf analysis, although very slow analysis times must be used for samples with less than 4 mol% comonomer.     

Analysis of polyolefins and olefin copolymers using Crystaf technique: Resolution of Crystaf curves

An experimental technique, crystallization analysis fractionation (Crystaf), is used to analyze compositional uniformity of ethylene/α‐olefin copolymers and isotactic polypropylene. A computerized method for quantifying Crystaf data is developed based on resolution of Crystaf curves into their elemental components, with each component representing a fraction of the polymer with the same degree of chain imperfection. This analysis of Crystaf curves gives three parameters characterizing crystallizable polymer material: (a) the number of compositionally uniform components, (b) properties of each compositionally uniform component (in the case of ethylene/α‐olefin copolymers, the comonomer content), and (c) the quantity of each component. Crystaf analysis of several ethylene/1‐hexene copolymers produced with supported Ti‐based Ziegler‐Natta catalysts shows the existence of two groups of copolymer components. The first group includes components with low comonomer content, in the Crystaf analysis they precipitate at high temperatures as several relatively sharp peaks. The second group includes components with high comonomer contents; they precipitate at much lower temperatures, as a broad overlapping group of peaks. The peak resolution technique was applied to analysis of ethylene/α‐olefin copolymers prepared with a supported catalyst at different temperatures, a copolymer produced with a pseudo‐homogenous Ziegler‐Natta catalyst, and to isotactic polypropylene. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Structures and cocrystallization behavior of copolyesters based on poly(octamethylene terephthalate) and poly(octamethylene 2,6-naphthalate)

We have synthesized poly(octamethylene terephthalate) (POT), poly(octamethylene 2,6-naphthalate) (PON), and poly(octamethylene terephthalate-co-octamethylene 2,6-naphthalate)s [P(OT-co-ON)s] with various comonomer composition by melt-polycondensation reaction and investigated their chain structures, crystalline structures, melting and cocrystallization behavior by using ¹H NMR spectroscopy, wide angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC), respectively. It was observed that P(OT-co-ON)s exhibit clear melting and crystallization peaks in DSC thermograms and sharp diffraction peaks in WAXD patterns throughout the copolymer composition, resulting from the cocrystallization behavior of OT and ON units in copolymers. When the melting and crystallization temperatures of P(OT-co-ON)s are compared as a function of the copolymer composition, there exists an eutectic point at around 23 mol% ON, where the crystal transformation from POT-type to PON-type occurs. It was confirmed from WAXD patterns of the melt-crystallized samples that the crystal transformation from POT-type to PON α-type to PON β-type occurs with the increment of the comonomer ON content in copolymers, i.e., POT-type crystals for POT and P(OT-co-ON) with 11 mol% ON, PON α-type crystals for P(OT-co-ON)s with 23-48 mol% ON, and PON β-type crystals for PON and P(OT-co-ON)s with 62-87 mol% ON. Both DSC and WAXD results demonstrate the isodimorphic cocrystallization of P(OT-co-ON)s. Based on the Wendling-Suter model for cocrystallization thermodynamics, it was found that the average defect free energy for the inclusion of OT units into PON β-type crystals is much lower than the value of the incorporation of ON units into POT-type crystals.     

Crystallization of a miscible propylene/ethylene copolymer blend

The crystallization behavior and morphological patterns of a miscible blend of two propylene/ethylene (P/E) copolymers that differed in ethylene content were studied. Metallocene-catalyzed P/E copolymers containing 3.1 and 11.0 mol% ethylene were chosen for blending. The difference in ethylene content was small enough to ensure miscibility of the pair in the melt, and the ethylene content was low enough to ensure that both were crystallizable. The blends were characterized by differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), optical microscopy (OM) and atomic force microscopy (AFM). The complex melting endotherm of the blends consisted of a broad low temperature peak at T_m1, a high temperature peak at T_m2, and an intermediate peak at which was not characteristic of either constituent and depended on blend composition. The multiple melting peaks arose from distinct crystal populations. All the blends exhibited a mixed morphological texture of α-radial lamellae with short, densely packed γ-overgrowths, interspersed with areas of α-crosshatch. The high temperature peak at T_m2 was assigned to the melting of the α-radial lamellae which formed from chains of the lower comonomer constituent. The broad low temperature peak at T_m1 was attributed to the melting of γ-crystal overgrowths on the radial lamellae. The new peak at was thought to arise from the melting of the α-crosshatch lamellae. The lamellar thickness, and hence , correlated with the crystallization temperature, which decreased as the blend was made richer in the higher comonomer constituent.     

The cocrystallization behavior of binary blends of isotactic polypropylene and propylene-ethylene random copolymers

Rapidly crystallized blends of metallocene isotactic polypropylene and propylene-ethylene random copolymers with an ethylene content varying from 0.76 to 7 mol% were found to cocrystallize to different degrees depending on the composition of the comonomer and content of copolymer in the blend. The degree of molecular mixing was studied using differential scanning calorimetry and solvent extraction techniques. A high extent of cocrystallization is obtained in the whole composition range of blends with a copolymer having up to ～ 2 mol% of ethylene. The degree of cocrystallization decreases gradually with increasing ethylene content or with increasing copolymer content in the blend. It is found that for ethylene contents as high as 5–7 mol% the copolymer rich blends show partial separate crystallization of the propylene ethylene copolymer. Thus, these crystals were selectively extracted at temperatures just above the dissolution temperature of the pure copolymer. In these blends, the fractional content of segments from the copolymer molecules incorporated in the cocrystal is low, yet it prevents extraction of these molecules at temperatures above the dissolution temperature of the copolymer. The degree of cocrystallization is explained by differences in crystallization kinetics of the pure components. The percentage of extracted material was found to be directly related to the dissolution temperature of the cocrystal which was also found to be a linear function of the inverse of the crystallite thickness. The high extent of cocrystallization observed for these polypropylene blends contrasts with comparable blends of polyethylenes. The blends of linear PE with a copolymer of 4 mol% branch units and higher, form separate crystals even after rapid crystallization.     

Synthesis, crystallization and tensile properties of poly(ethylene terephthalate-co-2,6-naphthalate)s with low naphthalate units content

A series of poly(ethylene terephthalate-co-naphthalate)s (PETN copolymers) with low naphthalate units content was synthesized. A melting point depression was observed, while the glass transition temperatures were slightly higher than that of Polyethylene terephthalate (PET). Crystallization rates of the copolymers decreased with increasing comonomer content. WAXD patterns showed that only PET crystals were formed. Co-crystallization behaviour was evaluated on the basis of the Wendling–Suter model. The tensile properties of the copolymers PETN 97/3 and PETN 94/6, Young's modulus yield stress and elongation at break was significantly improved compared to PET. WAXD showed that some crystalline precursor was generated during drawing of the specimens. DSC traces of the drawn specimens showed enhanced crystallization rates compared to that of the original amorphous specimens.