Quantifying short chain branching microstructures in ethylene 1-olefin copolymers using size exclusion chromatography and Fourier transform infrared spectroscopy (SEC-FTIR)

A rapid method for the SEC-FTIR analysis of short chain branching distribution (SCBD) across the molecular weight distribution (MWD) is described and its application demonstrated using ethylene 1-olefins copolymers. Chromatograms are generated using the root mean square absorbance over the 3000-2700 cm⁻¹ spectral region (i.e. FTIR serves as a concentration detector). Spectra from individual time slices of the chromatogram are subsequently analyzed for comonomer branch levels using chemometric techniques. Furthermore, we are able to estimate error in the reported SCB content of each slice. Using the appropriate training sets, chemometric models can be constructed which provide SCB versus MWD profiles with sufficient precision to detect trends resulting from catalyst and process changes in LDPE and/or HDPE samples. We demonstrated the method by showing the results of a model that has enabled us to accurately quantify branching levels in polyolefins within ±0.5/1000 total carbons (i.e. ca. 0.1 mol%) in samples with relatively low levels of SCB (i.e. <10 SCB/total carbons) and mixed branch types.     

Dynamic mechanical and rheological properties of metallocene-catalyzed long-chain-branched ethylene/propylene copolymers

The dynamic mechanical and rheological properties of five long-chain branched (LCB) and three linear ethylene/propylene (EP) copolymers were investigated and compared using a dynamic mechanical analyzer (DMA) and an oscillatory rheometer. The novel series of LCB EP copolymers were synthesized with a constrained geometry catalyst (CGC), [C₅Me₄(SiMe₂N^tBu)]TiMe₂, and had various propylene molar fractions of 0.01-0.11 and long-chain branch frequencies (LCBF) of 0.05-0.22. The linear EP copolymers were synthesized with an ansa-zirconocene catalyst, rac-Et(Ind)₂ZrCl₂ (EBI), and contained similar levels of propylene incorporation as the CGC copolymers, but no LCB. In dynamic mechanical analysis, the dynamic storage moduli (G′) and loss moduli (G″) of the copolymers decreased with an increase of propylene molar fraction. The α- and β-transitions of the CGC copolymers were overlaid with each other. High damping (tan δ) values were found with the CGC copolymers at temperatures below 0 °C. In oscillatory rheological analysis, compared to the linear EBI counterparts, the LCB CGC copolymer melts showed higher zero shear activation energies, broader plateaus of δ and larger elastic contributions, which are essential characteristics of LCB polymers. It was found that the long chain branching was the determining factor in controlling rheological properties of the polymer melts while the short chain branching from propylene incorporation played a decisive role in affecting dynamic mechanical properties. This work represents the first rheological evidence of LCB in EP copolymers synthesized with CGC.     

A new turbidimetric approach to measuring polyethylene short chain branching distributions

A new instrument to analyze the short chain branching distribution of polyethylenes has been described. Turbidity analysis of ethylene/α-olefin copolymers by turbidity fractionation analysis can provide short chain branching distribution information that is similar to CRYSTAF and TREF. In these experiments, the turbidity of a polymer solution is monitored while changing its temperature at a controlled rate. The turbidimetric response is to the precipitation or dissolution of the crystallized polymer at a given temperature. With an approach similar to CRYSTAF, the differential of the turbidity profile provides valuable SCBD information for polymers with broad and narrow compositions such as Ziegler-Natta LLDPE and homogeneous polymers catalyzed by single-site catalysts.     

Does the length of the short chain branch affect the mechanical properties of linear low density polyethylenes? An investigation based on films of copolymers of ethylene/1-butene, ethylene/1-hexene and ethylene/1-octene synthesized by a single site metallocene catalyst

Three nearly identical linear low density polyethylene resins based on copolymers of ethylene with 1-butene (B), 1-hexene (H) and 1-octene (O) were utilized to investigate the effect of short chain branch length on the mechanical properties of blown and compression molded (quenched and slow cooled) films. The content of short chain comononer in the three copolymers was ca. 2.5-2.9 mol% that corresponded to a density of 0.917-0.918 g/cm³. Within a given series, the tensile properties of these films do not show any significant difference at slow deformation rates (up to 510 mm/min), even though the DSC and TREF profiles of ‘H’ and ‘O’ differed slightly in comparison to ‘B’. However, at higher deformation rates (ca. 1 m/s), the breaking strength of these films was found to increase with increasing short chain branch length. In addition, the Spencer impact and Elmendorf tear strength of the blown films were also observed to increase with increasing short chain branch length. Further, dart impact strength and high-speed puncture resistance (5.1 m/s) of 1-octene and 1-hexene based samples was also observed to be higher than that based on 1-butene. The blown films displayed low and comparable levels of equivalent in-plane birefringence and crystalline orientation by wide angle X-ray scattering. This confirms that the differences in mechanical properties in the blown film series are not attributable to differences in molecular orientation. The deformation behavior of both the compression molded and blown films were also investigated in a well-defined controlled regime by analyzing their essential work of fracture. It was found that the essential work of fracture of films based on 1-hexene and 1-octene was higher than that of films based on 1-butene. While the origin of these differences in mechanical properties with increasing short chain branch length is not fully understood, the present investigation confirms this effect to be pronounced at high deformation rates for both the blown and compression molded quenched films.     

Comparison of methods for characterizing comonomer composition in ethylene 1-olefin copolymers: 3D-TREF vs. SEC-FTIR

In this paper, two methodologies for determining comonomer composition in ethylene 1-olefin copolymers, namely three detectors coupled to a temperature rising elution fractionation unit (3D-TREF) and size exclusion chromatography coupled to a Fourier transfer infrared detector (SEC-FTIR), are examined and compared. Because the two methods are based on different separation mechanisms, insight into the resin's molecular architecture is gained from two entirely different, yet complementary perspectives. The choice of which method to use will be determined by the specific structure vs. property issue under study. Comparative results from the analysis of copolymers produced by Ziegler-Natta, chromium and metallocene catalysts show that both the methods are useful for characterizing LLDPE resins. However, the 3D-TREF method may offer more insight into the heterogeneity of resin blends, particularly when the blend components have similar molecular weights. Although some MW-dependency information of the temperature fractions can be ascertained via viscometer and light scattering detectors, SEC-FTIR is the more appropriate method to detect compositional heterogeneity in resin blends that are composed of two or more resins with the same copolymer compositions, but with different molecular weights.     

Copolymerization behavior of Cp2ZrCl2/MAO and [(CO)5WC(Me)OZrCp2Cl]/MAO: a comparative study on ethylene/1-pentene copolymers

Ethylene/1-pentene copolymers were synthesized using Cp₂ZrCl₂(1)/MAO and [(CO)₅WC(Me)OZr(Cp)₂Cl](2)/MAO catalyst systems. The copolymers were characterized by SEC, DSC, FTIR and ¹³C NMR spectroscopy. The copolymers synthesized with [(CO)₅WC(Me)OZr(Cp)₂Cl](2)/MAO had higher average molecular weights and broader polydispersities compared to those produced with Cp₂ZrCl₂(1)/MAO. The chemical heterogeneity was investigated by SEC-FTIR and fractionation techniques. All copolymers showed a higher incorporation of the 1-pentene in the low molecular weight fraction as revealed by SEC-FTIR. Crystallization analysis fractionation (CRYSTAF) showed a broad chemical composition distribution (CCD) for all the copolymers synthesized with these two catalyst systems. Selected copolymers were also analyzed using an automated preparative molecular weight fractionation.     

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.     

The polydispersity of ethylene sequence in metallocene ethylene/α-olefin copolymers III: crystallization and melting behavior of short ethylene sequence

Crystallization and melting behavior of short ethylene sequence of metallocene ethylene/α-olefin copolymer with high comonomer content have been studied by standard DSC and modulated-temperature differential scanning calorimetry (M-TDSC) technique. In addition to high temperature endotherm around 120°C, a low temperature endotherm is observed at lower temperatures (40-80°C), depending on time and temperature of isothermal crystallization. The peak position of the low temperature endotherm T_m^low varies linearly with the logarithm of crystallization time and the slope, D, decreases with increasing crystallization temperature T_c. The T_m^low also depends on the thermal history before the crystallization at T_c, and an extrapolation of T_m^low (30.6°C) to a few seconds has been obtained after two step isothermal crystallization before the crystallization at 30°C. The T_m^low is nearly equal to T_c, and it indicates that the initial crystallization at low temperature is nearly reversible. Direct evidence of conformational entropy change of secondary crystallization has been obtained by using M-TDSC technique. Both the M-TDSC result and the activation energy analysis of temperature dependence suggest that crystal perfection process and conformational entropy decreasing in residual amorphous co-exist during secondary crystallization.     

Separation of propene/1-alkene and ethylene/1-alkene copolymers by high-temperature adsorption liquid chromatography

A high performance liquid chromatography column (HPLC) Hypercarb^® packed with porous graphite has proven to discriminate polyolefin molecules due to differences in their adsorption and desorption behaviour. While linear polyethylene (PE) and syndiotactic polypropylene (sPP) are adsorbed on the graphite packing, isotactic polypropylene (iPP) is not adsorbed. The column operates at 160 °C with 1-decanol as sample solvent and mobile phase. We have now tested this HPLC system for separations of random propene/1-alkene and ethylene/1-hexene copolymers: While copolymers of propene with 1-butene, 1-hexene and 1-octene copolymers eluted in size exclusion mode without adsorption, propene/1-octadecene and ethylene/1-hexene copolymers are strongly retained and eluted only after application of a linear gradient starting from 1-decanol and ending with pure 1,2,4-trichlorobenzene. The retention of propene/1-alkene (>11 carbons in the side chain) copolymers increases with the concentration of comonomer, making this HPLC system suitable to separate these copolymers according to their chemical composition. In contrast, the retention of ethylene/1-hexene samples decreases with increasing 1-hexene content. Branching in this case shortens the length of continuous methylene sequences of the polymer backbone, which are expected to adsorb in a planar conformation to the graphite layers. This is the first report on the separation of short chain branched polyolefins by high-temperature adsorption liquid chromatography.     

Effect of short chain‐branching distribution on crystallinity and modulus of metallocene‐based ethylene–butene copolymers

In this article, the short chain‐branching distribution (SCBD) of some metallocene‐based ethylene–butene copolymers was evaluated by DSC, and some conventional ethylene copolymers were also studied for the purpose of comparison. It is found that metallocene‐based ethylene copolymers have a relative narrower SCBD. These copolymers were crystallized under different modes, and the crystallinity and initial modulus of them were examined. The metallocene‐based ethylene copolymers contain less interfacial regions, and the melting temperatures of them decrease more rapidly with the decrease of density than those of conventional ethylene copolymers. Moreover, the metallocene‐based and conventional ethylene copolymers of similar density have close initial modulus when they are quenched or annealed at 100°C, but conventional ethylene copolymers show higher initial modulus when stepwise crystallized from 120°C. These differences in crystallinity and initial modulus were explained based on their differences in short‐chain branching distributions. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1709–1715, 2000     

Miscibility of binary blends of ethylene/norbornene copolymers: Comparison to a lattice cluster theory

The mutual miscibility of five binary ethylene/norbornene copolymers of the N_xE_1−x/N_yE_1−y type where x and y represent the norbornene content in the first and second copolymers, respectively (with x/y of 0.36/0.52, 0.36/0.56, 0.36/0.62, 0.46/0.60 and 0.46/0.66), in five different compositions was studied by rheology and differential scanning calorimetry. Most of the blends do not obey the principle of time-temperature superposition, i.e., they can be considered as thermorheologically complex, and hence immiscible. The experimental data from rheology were compared to a recent lattice cluster theory. The theoretical miscibility diagram correctly predicts the experimental cases for most blend compositions except for the very asymmetric compositions.     

DSC analysis of ethylene/1-butene copolymers obtained with different ziegler–natta catalysts

The crystallization and melting behaviour of two sets of ethylene/1-butene copolymers have been analysed by DSC. The samples, with comonomer content in the range from 0 to 21.5 mol%, were obtained by industrial processes using both Mg/Ti-based catalyst systems. The composition dependences of melting and crystallization temperatures were found to be strictly affected by the catalyst type. Moreover, logarithmic plots of the melting and crystallization enthalpy as a function of the ethylene content (mol%) in the copolymers fitted linear relationships whose slopes have been related to the critical sequence length of crystallizable ethylene units, depending on the catalytic system. These results are compared with those reported in the literature for ethylene/1-butene copolymers synthesized by other catalysts and are accounted for by a different distribution of the comonomer units in the macromolecules of the two sets of samples.     

Linear low-density polyethylenes by co-polymerization of ethylene with 1-hexene in the presence of titanium precursors and organoaluminium co-catalysts

The co-polymerization of ethylene with 1-hexene has been performed with titanium precursors based on carboxylato ligands in the presence of organoaluminium compounds as activators to afford linear low-density polyethylenes (LLDPEs). The influence of the polymerization parameters was studied with particular reference to the type and amount of catalyst components, solvent, temperature, ethylene pressure, 1-hexene concentration. The chloro-substituted bis-carboxylato titanium complex resulted the most active precursor in the co-polymerization, allowing to obtain copolymers with modulable composition in the 1-5 mol% range of 1-hexene units. The obtained copolymers were characterized by thermal analysis, X-ray diffraction spectroscopy, FTIR and NMR techniques.     

Nuclear magnetic resonance in solid ethylene/α-olefin copolymers: Relaxation,spin-diffusion and lamellar widths

¹H and ¹³C-n.m.r. spectra and spin-lattice relaxation behaviour (laboratory frame, T₁ (¹³C and ¹H), and rotating frame, T_1ρ (¹H)) are reported for a range of solid ethylene copolymers with α-olefins having different types and concentrations of branches. The spin-diffusion model for relaxation in semicrystalline polymers is summarised and some new theoretical results given. The ¹³C high-resolution n.m.r. spectra obtained using both cross-polarisation (c.p.) and single pulse excitation (s.p.e.) methods associate side chain resonances mainly with the mobile, short T_1ρ protons and, hence, the more disordered region of the solids, consistent with the short ¹³C T₁ components. ¹H laboratory-frame spin-lattice relaxation is single component whilst those for ¹³C and the ¹H on-resonance rotating-frame relaxation are both multiexponential processes, requiring a minimum of three components to describe them. Annealing of samples, which is known to increase the lamellar thicknesses of the crystalline region, causes large increases in the longer relaxation time components. The ¹H and ¹³C spin-lattice relaxation data for a set of annealed and quenched ethyl-branched materials having different branch contents are compared in detail with the predictions of the spin-diffusion relaxation model. The results are internally and semi-quantitatively consistent with this theory and it is concluded that for the ¹H spin-lattice relaxation the model is clearly appropriate and for ¹³C it is consistent with observations. Questions concerning the relevance of spin-diffusion for the magnetically dilute ¹³C nucleus at natural abundance are mentioned and possible alternative explanations for the relationships observed are referred to briefly.     

Influences of three different ethylene copolymers on the toughness and other properties of polylactide

The aim of this study was to investigate influences of three different ethylene copolymers on the toughness and other properties of very brittle biopolymer PLA (polylactide). For this aim, PLA was melt blended by twin-screw extruder with various amounts of ethylene vinyl acetate (EVA), ethylene-methyl-acrylate (EMA) and ethylene-n-butyl acrylate-glycidyl-methacrylate (EBA-GMA). SEM and DSC analyses indicated that these ethylene copolymers were thermodynamically immiscible with phase separation in the form of 1–5?µm sized round domains in the PLA matrix. Rubber toughening mechanisms of EVA, EMA and EBA-GMA were very effective to improve ductility and toughness of PLA significantly. Depending on the type and content of the ethylene copolymers, the highest increases in % elongation at break, Charpy impact toughness and G_IC fracture toughness values of PLA were as much as 160, 320 and 158%, respectively. Although there were no detrimental effects of using EVA, EMA and EBA-GMA on the thermal properties of PLA, they resulted in certain level of reductions in stiffness, strength and hardness values.     

Effect of composition and comonomer type on the rheology, morphology and properties of ethylene-α-olefin copolymer/polypropylene blends

The rheology, morphology, thermal and mechanical properties of blends containing low molecular weight polypropylene (PP) and metallocene-based ethylene-α-olefin copolymers (ECs) have been investigated. Evaluation of the thermo-rheological properties of the blends showed that they are immiscible, both in the solid and melt state, over the whole range of compositions. Rheological properties were correlated to blend morphology, by using the Palierne emulsion model. The resulting low values of interfacial tension confirmed excellent compatibility between the phases.Addition of ECs in PP resulted in significant ductility improvement. The transition from plastic to elastomeric properties coincided with phase inversion. The butene-based EC was found to be the most beneficial in terms of impact properties.Fine morphologies, with sub-micron sizes of the dispersed phase, were obtained upon post-extrusion shearing of the blends.     

Binary blends of linear ethylene copolymers over a wide crystallinity range: Rheology, crystallization, melting and structure properties

This study deals with the physical properties of melt-compounded blends of three linear ethylene copolymers covering a large crystallinity range, namely 77% - 46% - 16% for the high density - linear low density - ultra low density copolymers, respectively. The melt behavior assessed from the zero-shear viscosity (η_o) reveals immiscibility of the three binary systems over the whole composition range. However, the change from positive to negative deviation of η_o with respect to the log-additivity mixing law as a function of composition suggests a structural transition from partial miscibility at the interface of the phase-separated domains to incompatibility. Crystallization and melting behaviors of the blends corroborate the occurrence of phase separation in the three systems. For most blends, the temperature shift of the crystallization (T_c) and melting (T_m) peaks as compared to the ones of the pure copolymers yet indicates partial miscibility in the crystalline and/or in the amorphous regions. It is pointed out that miscibility in the amorphous phase resulting from partial miscibility in the melt may, on its own, entail T_m depression of the crystals via surface free energy effect without necessarily implying cocrystallization and crystal thickness reduction. In several cases, the presence of intermediate endotherm and exotherm between the two main peaks of the melting and crystallization traces, respectively, discloses hybrid crystals assigned to a composition gradient at the interface of the phase-separated domains. A marked positive deviation of the upper T_c from the linear mixing rule is observed for the three systems. A nucleating effect from the interface of the phase-separated domains is suggested to promote early crystallization in the upper T_c phase. The SAXS data reveal electron density fluctuations at a much larger scale than that of the semi-crystalline structure demonstrating the occurrence of micro-phase separation in the melt prior to crystallization. Solubility of low T_m chain species in the amorphous layers of the high T_m phase is also evidenced. AFM and DMTA support micro-phase separation in the three systems and provide complementary information on the crystalline habits in the phase-separated domains of the blends.     

Synthesis of 1-hexene/1,7-octadiene copolymers using coordination polymerization and postfunctionalization with triethoxysilane

Homo- and copolymerization of 1-hexene (H) and 1,7-octadiene (O) were done using two different catalysts 1,4-bis(2,6-diisopropylphenyl)acenaphthenediiminedibromo nickel (II) and rac-ethylenebis(indenyl)zirconium dichloride [rac-Et(Ind)₂ZrCl₂]. The metallocene catalyst showed higher activity than the nickel α-diimine catalyst in homo- and copolymerization. The ¹H NMR studies confirmed the formation of copolymers containing 8–47% of 1,7-octadiene. In the copolymerization of hexene and diene, as the amount of incorporated diene in the copolymers increased, their T _g increased. TGA results showed that thermal stability of the polymer increases with the increase of 1-hexene incorporation in the polymer chain. Finally 1-hexene/1,7-octadiene copolymers were functionalized by triethoxysilane in the presence of hexachloroplatinic acid. The ¹H NMR spectrum of the functionalized samples showed that the double bonds in the copolymer structure were completely eliminated. The DSC analysis showed higher T _gs for the functionalized copolymer. © 2020 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48934.     

Synthesis, characterization and properties of functional star and dendritic block copolymers of ethylene oxide and glycidol with oligoglycidol branching units

Well-defined, four-arm star block copolymers of ethylene oxide and glycidol were prepared via controlled anionic polymerization using protected glycidol. The length of the poly(ethylene oxide) block was varied from DP = 10 to 50, while the length of the short polyglycidol block remained nearly constant, at DP = 4-6. Star block copolymers with hydroxyl groups at the ends of the arms after conversion to the corresponding alkoxides were used as multifunctional macroinitiators for the sequential polymerization of ethylene oxide and protected glycidol. After deprotection, the branched block copolymers of ethylene oxide and glycidol had narrow molar mass distributions and multiple hydroxyl groups (up to 200) at the peripheries. The structure and functionality were determined using size exclusion chromatography with a light scattering detector and nuclear magnetic resonance spectroscopy. The thermal properties of the synthesized copolymers were also investigated, as well as the hydrophilic dye uptake to the hydrophobic phase containing copolymers.     

Self‐induced short chain branching in homopolymers and 1‐hexene copolymers of ethylene produced with amido functionalized ansa half‐sandwich complexes as catalysts

Amido ansa 3‐substituted indenyl complex precursors can be activated with methylaluminoxane and used for prepolymerization with ethylene to give a heterogeneous catalyst for olefin polymerization. Homo polymerization of ethylene with 1‐(3‐pent‐4‐enylindenylidene) dimethylsilyl'butylamidotitaniumdichloride (1), 1‐(3‐hex‐5‐enylindenylidene)dimethylsilyl'butylamidotitanium‐dichloride (2), and 1‐(3‐pent‐4‐enylindenylidene) (oct‐7‐enyl)methylsilyl'butylamidotitaniumdichloride (3) produces polyethylenes that contain ethyl branches. The ethyl branching in the polymers made with complexes 1 and 2 is barely above the ¹³C NMR detection limit, but the level observed in the polymer made with complex 3 is 17 times greater. Copolymerization of ethylene and 1‐hexene using prepolymerized 3 yields copolymers containing both ethyl and butyl branches. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 734–739, 2006