Molecular heterogeneity of ethylene‐propylene rubbers: New insights through advanced crystallization‐based and chromatographic techniques

For a long time ethylene‐propylene rubber (EPR) copolymers with high comonomer contents were believed to be amorphous materials with a random copolymer composition. This is not completely correct as has been shown by temperature rising elution fractionation (TREF) combined with differential scanning calorimetry (DSC), crystallization analysis fractionation (CRYSTAF), and high temperature–high‐performance liquid chromatography (HT‐HPLC). When using only conventional crystallization‐based fractionation methods, the comprehensive compositional analysis of EPR copolymers was impossible due to the fact that large fractions of these copolymers do not crystallize under CRYSTAF conditions. In the present work, HT‐HPLC was used for the separation of the EPR copolymers according to their ethylene and propylene distributions along the polymer chains. These investigations showed the existence of long ethylene sequences in the bulk samples which was further confirmed by DSC. The results on the bulk samples prompted us to conduct preparative fractionations of EPR copolymers having varying ethylene contents using TREF. Surprisingly, significant amounts of crystallizing materials were obtained that were analyzed using a multistep protocol. CRYSTAF and DSC analyses of the TREF fractions revealed the presence of components with large crystallizable sequences that had not been detected by the bulk samples analyses. HT‐HPLC provided a comprehensive separation and characterization of both the amorphous and the crystalline TREF fractions. The TREF fractions eluting at higher temperatures showed the presence of ethylene‐rich copolymers and PE homopolymer. In order to obtain additional structural information on the separated fractions, HT‐HPLC was coupled to Fourier transform‐infrared (FT‐IR) spectroscopy. The FT‐IR data confirmed that the TREF fractions were separated according to the ethylene contents of the eluted samples. Preparative TREF analysis together with a combination of various analytical methods proved to be useful tools in understanding the complex molecular composition of these rubber samples. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2015 , 53, 863–874     

Ethylene/1‐pentene copolymers synthesized using the metaloxycarbene complex catalyst system [(CO)5W=C(Me)OZr(Cp)2Cl]/MAO

Ethylene/1‐pentene copolymers were prepared using a [(CO)₅W= C(Me)OZr(Cp)₂Cl] (1)/MAO catalyst system. 1‐Pentene incorporation in the copolymer was monitored using ¹³C‐NMR spectroscopic methods. The weight average molecular weights (M_w) of the copolymers were between 142,000 and 629,000 g/mol, with polydispersity indexes (PDIs) ranging from ≈ 2 to 90, as analyzed by size exclusion chromatography (SEC). Melting and crystallization temperatures, determined using differential scanning calorimetry (DSC) and crystallization analysis fractionation (CRYSTAF), decreased linearly as the amount of 1‐pentene in the copolymer increased. SEC‐FTIR revealed that the 1‐pentene is predominantly incorporated in the low molecular weight fraction. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5121–5133, 2004     

Synthesis and characterization of model diblock copolymers of poly(dimethylsiloxane) with poly(1,4‐butadiene) or poly(ethylene)

Model diblock copolymers of poly(1,4‐butadiene) (PB) and poly(dimethylsiloxane) (PDMS), PB‐b‐PDMS, were synthesized by the sequential anionic polymerization (high vacuum techniques) of butadiene and hexamethylciclotrisiloxane (D₃) in the presence of sec‐BuLi. By homogeneous hydrogenation of PB‐b‐PDMS, the corresponding poly(ethylene) and poly(dimethylsiloxane) block copolymers, PE‐b‐PDMS, were obtained. The synthesized block copolymers were characterized by nuclear magnetic resonance (¹H and ¹³C NMR), size‐exclusion chromatography (SEC), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and rheology. SEC combined with ¹H NMR analysis indicates that the polydispersity index of the samples (M_w/M_n) is low, and that the chemical composition of the copolymers varies from low to medium PDMS content. According to DSC and TGA experiments, the thermal stability of these block copolymers depends on the PDMS content, whereas TEM analysis reveals ordered arrangements of the microphases. The morphologies observed vary from spherical and cylindrical to lamellar domains. This ordered state (even at high temperatures) was further confirmed by small‐amplitude oscillatory shear flow tests. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1579–1590, 2006     

Investigation by combined solid‐state NMR and SAXS methods of the morphology and domain size in polystyrene‐b‐polyethylene oxide‐b‐polystyrene triblock copolymers

The microphase structure of a series of polystyrene‐b‐polyethylene oxide‐b‐polystyrene (SEOS) triblock copolymers with different compositions and molecular weights has been studied by solid‐state NMR, DSC, wide and small angle X‐ray scattering (WAXS and SAXS). WAXS and DSC measurements were used to detect the presence of crystalline domains of polyethylene‐oxide (PEO) blocks at room temperature as a function of the copolymer chemical composition. Furthermore, DSC experiments allowed the determination of the melting temperatures of the crystalline part of the PEO blocks. SAXS measurements, performed above and below the melting temperature of the PEO blocks, revealed the formation of periodic structures, but the absence or the weakness of high order reflections peaks did not allow a clear assessment of the morphological structure of the copolymers. This information was inferred by combining the results obtained by SAXS and ¹H NMR spin diffusion experiments, which also provided an estimation of the size of the dispersed phases of the nanostructured copolymers. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 55–64, 2010     

Polyolefin analysis by single‐step crystallization fractionation

Temperature rising elution fractionation (TREF) fractionates polymer chains with respect to their crystallizability, independently of molecular weight effects. In order to achieve a good fractionation, TREF requires a time‐consuming polymer deposition step over an inert support before the elution step. A single‐step crystallization fractionation method has been developed recently,^1,2 Crystallization Analysis Fractionation (CRYSTAF), in which the chemical composition (or short chain branching) distribution of olefin copolymers can be measured by monitoring on‐line polymer concentration in solution at decreasing temperatures. For the present experimental investigation, a CRYSTAF‐prototype has been assembled and used to fractionate several linear low‐density polyethylene (LLDPE) samples. These results were compared to the ones measured by the commercial CRYSTAF apparatus from Polymer ChAR. Additionally, CRYSTAF results from Polymer ChAR were compared to analytical TREF results. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 539–552, 1999     

Synthesis and characterization of model polybutadiene‐1,4‐b‐polydimethylsiloxane‐b‐polybutadiene‐1,4 copolymers

Model copolymers of poly(butadiene) (PB) and poly(dimethylsiloxane) (PDMS), PB‐b‐PDMS‐b‐PB, were synthesized by sequential anionic polymerization (high vacuum techniques) of 1,3‐butadiene and hexamethylciclotrisiloxane (D₃) on sec‐BuLi followed by chlorosilane‐coupling chemistry. The synthesized copolymers were characterized by nuclear magnetic resonance (¹H NMR), size‐exclusion chromatography (SEC), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). SEC and ¹H NMR results showed low polydispersity indexes (M_w/M_n) and variable siloxane compositions, whereas DSC and TGA experiments indicated that the thermal stability of the triblock copolymers depends on the PDMS composition. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2726–2733, 2007     

Ethylene/1‐hexene copolymers synthesized with a single‐site catalyst: Crystallization analysis fractionation,modeling, and reactivity ratio estimation

A series of poly(ethylene‐co‐1‐hexene) samples made with rac‐ethylene bis(indenyl)zirconium dichloride/methylaluminoxane were analyzed by crystallization analysis fractionation (CRYSTAF). The nine samples had comonomer contents of 0–4.2 mol % 1‐hexene with a narrow range of molecular weights (34,000–39,000 g/mol). Because all the copolymer samples had narrow, unimodal chemical composition distributions, they were ideal as calibration standards for CRYSTAF. A linear calibration curve was constructed relating the peak crystallization temperature from CRYSTAF operated at a cooling rate of 0.1 °C/min and the comonomer content as determined by ¹³C NMR. Reactivity ratios for ethylene and 1‐hexene were estimated by the fitting of reactant liquid‐phase compositional data to the Mayo–Lewis equation. It was found that a value of the 1‐hexene reactivity ratio could not be unequivocally determined from the set of samples analyzed because the range of comonomer incorporation was too narrow. Stockmayer's bivariate distribution was used to model the fractionation process in CRYSTAF, and although a good fit to experimental CRYSTAF profiles was attained, the model did not fully describe the underlying crystallization phenomena. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 2595–2611, 2002     

Characterization of Ethylene Copolymers with Liquid Chromatography and Melt Rheology Methods

Summary: Melt rheology and polymer chromatography methods were applied to characterize molecular heterogeneities in products of free radical copolymerization of ethylene with methyl acrylate and vinyl acetate comonomers performed in continuously stirred tank and tubular reactors. We found that the ethylene–vinyl acetate copolymers made in both reactors had similar linear viscoelastic properties typical to branched products of the high pressure process. But the ethylene–methyl acrylate copolymers obtained in the tubular reactor had unusually high melt viscosity at low shear rate and much lower onset of shear thinning despite the narrower molecular weight distribution and the lower overall amount of long-chain branches compare to their autoclave counterparts with similar average molecular weight and chemical composition. Using interaction polymer chromatography method called gradient elution at critical point of adsorption we found that ethylene-acrylate copolymers from the tubular reactor had very broad chemical composition distribution, which was consistent with a significant difference in reactivity ratios between ethylene and acrylate comonomers. Such chemical composition heterogeneity can be a reason for the observed unusual rheological properties of these copolymers.     

Copolymerization of ethylene and 1-hexene with supported metallocene catalysts: Effect of support treatment

The effect of different catalyst support treatments in the 1-hexene/ethylene copolymerization with supported metallocene catalysts was investigated through the analysis of molecular weight and chemical composition distributions by means of high temperature gel permeation chromatography (GPC) and crystallization analysis fractionation (CRYSTAF). The molecular weight distributions of all copolymers are narrow, indicating a uniform catalyst site. However, with respect to chemical composition distribution, some supported catalysts show broad and sometimes bimodal distributions, which indicates the presence of two or more active site types.     

Synthesis and characterization of poly(diethylsiloxane) and its copolymers with different diorganosiloxane units

Poly(diethylsiloxane) and its copolymers with various kinds of R₁R₂SiO (R₁ = R₂ = methyl or phenyl, or R₁ = methyl and R₂ = phenyl) units have been prepared by the equilibrium polymerization of cyclosiloxanes. All the polymers have been characterized by ¹H and ²⁹Si NMR, gel permeation chromatography, differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) measurements. The results indicate that a random distribution of different units has been obtained in the structures of copolymers containing 50 mol % diethylsiloxane units content. DSC and DMA show that the presence of 2.5 mol % diphenylsiloxane units or 5.0 mol % methylphenylsiloxane units in the copolymer can disrupt the crystallinity and lead to noncrystalline copolymers with low glass‐transition temperatures (ranging from ?133 to ?137 °C according to DSC). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2722–2730, 2003     

Separation and Characterization of Ethylene-Propylene Copolymers by High-Temperature Gradient HPLC Coupled to FTIR Spectroscopy

Summary: The chromatographic separation of ethylene-propylene (EP) copolymers with regard to chemical composition was accomplished by a new technique - high- temperature gradient HPLC. Using a mobile phase of ethylene glycol monobutylether (EGMBE) and 1,2,4–trichlorobenzene (TCB), and silica gel as the stationary phase, copolymers with different ethylene contents were separated according to their chemical compositions. Using a sample solvent of n-decanol and a column temperature of 140 °C, chromatographic conditions were established that correspond to separation in a precipitation-redissolution mechanism. With the aim to obtain further information on the separation process, the HPLC system was coupled to FTIR spectroscopy through a LC-Transform interface. The FTIR data confirmed that the copolymers were separated according to the ethylene content of the eluted samples.     

A novel and versatile potassium‐based catalyst for the ring opening polymerization of cyclic esters

Poly(lactide) (PLA), poly(ε‐caprolactone) (PCL) and poly(trimethylene carbonate) (PTMC) homopolymers of high molecular weight were prepared using potassium‐based catalyst. Polymerizations were carried out in toluene at room temperature. The chemical structure of the polymers was investigated by ¹H and ¹³C NMR. The physical properties investigated by GPC and DSC for the polymers obtained are similar to those prepared using tin octanoate based catalyst. Using a sequential polymerization procedure, PLA‐b‐PCL, PLA‐b‐PTMC, and PCL‐b‐PTMC diblock copolymers were synthesized and characterized in terms of their composition and physical properties. The formation of diblock copolymers was confirmed by NMR and DSC measurements. In vitro cytotoxicity tests have been carried out using MTS assay and the results show the biocompatibility of these polymers in the presence of the fibroblast cells. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 5348–5362, 2008     

Synthesis and characterization of biodegradable poly(ester anhydride) based on ε‐caprolactone and adipic anhydride initiated by potassium poly(ethylene glycol)ate

Novel biodegradable poly(ester anhydride) block copolymers based on ε‐caprolactone (ε‐CL) and adipic anhydride (AA) were prepared by sequential polymerization. ε‐CL was first initiated by potassium poly(ethylene glycol)ate and polymerized into active chains (PCL‐O^?K⁺), which were then used to initiate the ring‐opening polymerization of AA. The effects of the AA feed ratio, solvent polarity, monomer concentration, and temperature on sequential polymerization were investigated. The copolymers, obtained under different conditions, were characterized by Fourier transform infrared, ¹H NMR, gel permeation chromatography (GPC), and differential scanning calorimetry (DSC). The GPC results showed that the weight‐average molecular weights of the block copolymers were approximately 6.0 × 10⁴. The DSC results indicated the immiscibility of the two components. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1511–1520, 2003     

HPLC of synthetic polymers

Summary Statistical copolymers of styrene and acrylonitrile have been separated by high performance precipitation chromatography according to the increase in acrylonitrile content. The separation mechanism was strongly depending on the combination of stationary phase and eluent composition. By applying a typical normal phase gradient from n-heptane (precipitation) to dichloromethane the polymers were adsorbed, after dissolution of the polar groups of the stationary phases. The elution curves became broader with increasing acrylonitrile content of the polymer. By the addition of 20% methanol to the dichloromethane the adsorption could be minimized and copolymer elution was dependent solely on solubility and independent of stationary phase polarity. Separation according to chemical composition was also possible without precipitation with a large pore silica and a normal phase gradient from dichloromethane to the same eluent containing 2.75% methanol.Part I see reference 19     

Linear low density polyethylene: Microstructure and sealing properties correlation

The microstructures of four commercial linear low density polyethylenes (LLDPE) were evaluated and correlated with their sealing properties. Atomic Force Microscopy (AFM), Temperature Rising Elution Fractionation (TREF), Differential Scanning Calorimetry (DSC) and Crystallization Analysis Fractionation (CRYSTAF) experiments revealed that the comonomer distribution is one of the main factors that influence the sealing properties. The superior sealing performance showed by metallocene LLDPE samples in comparison to Ziegler-Natta LLDPE samples can be attributed to their well balanced chemical composition distribution.     

Advanced Characterization of Propylene-Based Copolymers and Olefin Block Copolymers

Summary: The newly developed interactive separation of polyolefins by high temperature liquid chromatography (HTLC) provides new information about the chemical composition distribution of polyolefin elastomers. The technique has the advantage of being quantitative in its separation, and has high resolution for the separation of polyolefins by their chemical composition without influence by cocrystallization. Chemical composition distributions can be determined for individual polymers and blends which contain the full range of comonomer typically present in polyethylene and poylypropylene homopolymers, semi-crystalline copolymers, and amorphous copolymers. HTLC analysis in combination with the other fractionation techniques, such as DSC, TREF, NMR, and xylene fractionation, can be used to estimate the amount of olefin block copolymer present in a block composite produced by chain shuttling catalysis.     

Crystallization Elution Fractionation and Thermal Gradient Interaction Chromatography. Techniques Comparison

Summary: The chemical composition distribution has been shown to be the most critical and discriminating parameter in understanding the performance of industrial polyolefins with non homogeneous comonomer incorporation. The chemical composition distribution is being analyzed by well known techniques such as temperature rising elution fractionation, TREF, crystallization analysis fractionation, CRYSTAF and crystallization elution fractionation, CEF. These techniques separate according to crystallizability and provide a powerful and predictable separation of components based on the presence of branches, irregularities or tacticity differences, independently of the molar mass. TREF, CRYSTAF and CEF can not be used, however, for the separation of more amorphous resins, and may not always provide the best solution for complex multi-component resins due to the existence of some co-crystallization. The application of high temperature interactive HPLC to polyolefins opened a new route to characterize these types of polymers. The use of solvent gradient HPLC for separation of polyethylene and polypropylene and the developments in HPLC on carbon based columns extended further the application of high temperature HPLC in polyolefins. A new approach has been developed recently using the carbon based column but replacing solvent gradient by a thermal gradient which facilitates the analysis of polyethylene copolymers and provides a powerful tool for the analysis of elastomers. Thermal gradient interaction chromatography (TGIC) is being compared with TREF and CEF with the analysis of model samples. The advantages/disadvantages of each technique are being investigated and discussed. The combination of TGIC and TREF/CEF provides an extended range of separation of polyolefins.     

Synthesis and characterization of in-chain silyl-hydride functional SBR and self-crosslinking elastomer

Functional in-chain silyl-hydride(Si-H) SBR copolymers of 4-vinyiphenyldimethylsilanol(VPDMS) and butadiene were synthesized by living anionic polymerization,in which active group Si-H was not lost and its content was controllable. Corresponding self-crosslinking elastomers were obtained by hydrosilation of Si-H group with vinyl bonds in chain.The copolymers and elastomers were characterized by ~1H NMR,size exclusion chromatography(SEC),Fourier transform infrared (FTIR) spectroscopy,differential scanning calorimetry(DSC),and thermogravimetry analysis(TGA) techniques.     

Biodegradable poly(sebacic anhydride-co-caprolactone) multi-block copolymers: Synthesis, characterization, crystallinity and crystalline morphology

Biodegradable poly(sebacic anhydride-co-caprolactone) (PSA-co-PCL) multi-block copolymers were prepared by condensation of acylated PSA and PCL prepolymers with different weight ratios. The homopolymer and copolymers were characterized by ¹H-NMR, gel permeation chromatography (GPC), differential scanning calorimeter (DSC) and atom force microscope (AFM). ¹H-NMR and GPC has indicated the formation of PSA-co-PCL multi-block copolymers, in which PSA and PCL segments are randomly distributed. The incorporation of PCL segments into the molecule chains even at a content of 20 wt% could significantly decrease the molecular weight distribution of the copolymer and increase its weight average molecular weight, as compared with PSA homopolymer. DSC has revealed that the melting temperature and degree of crystallinity for both SA and CL components are strongly composition dependent, implying the hindrance effect of the two components on crystallinity of each other. AFM observation has shown the difference in crystalline structures between PSA and PCL phases in the copolymers. In-vitro degradation tests performed at 37 °C in PBS buffer solution (pH 7.4, 0.1 M) have demonstrated the acceleration of degradation rate of the sample with increasing SA content in the copolymer.     

长软链段对苯二甲酸乙二酯-环氧乙烷多嵌段共聚物的组成不均一性研究

合成了不同软链段长度和不同硬链段含量的系列对苯二甲酸乙二酯－环氧乙烷(PET-PEO)多嵌段共聚物,用NMR质子港测定了硬链段含量,对部分溶于氯仿的PET－PEO多嵌段共聚物进行了分离,并分别测定其氯仿可溶物和不溶物的硬链段含量、熔融热谱和热结晶谱.揭示了PET－PEO多嵌段共聚物的组成不均一性及其对软镇段长度和硬链段含量的依赖性,进而用DSC热谱证明了软链段和硬链段的结晶能力与PET－PEO多嵌段共聚物组成不均一性密切相关.