Preparative Temperature Rising Elution Fractionation Research Articles

Preparative temperature rising elution fractionation of polyolefin elastomer and its chain microstructure

AbstractPolyolefin elastomer (POE) has very weak crystalline ability, consequently, applying the conventional preparative temperature‐rising elution fractionation (P‐TREF) to separate is challenging. Here, a unique, home‐built P‐TREF apparatus with an extensive range of temperatures from −80°C to 150°C is applied to first fractionate POE depending on its crystallizability. The main fractions are eluted at 0°C, 8°C, 15°C, 20°C, 25°C, 30°C, and 35°C. The corresponding weight percentages of fractions are 8.31, 13.38, 15.59, 12.05, 13.39, 17.30, and 10.53 wt%, respectively. The chain structures of fractions are further analyzed by high‐temperature gel permeation chromatography (HT‐GPC), 13C‐nuclear magnetic resonance spectroscopy (13C‐NMR), differential scanning calorimetry (DSC), and successive self‐nucleation and annealing (SSA). The crystallinity of the fraction grows continually as the elution temperature rises. The 1‐octene comonomer concentrations within the fractions decreases from 13.8 to 7.9 mol% when the elution temperature rises from −10°C to 40°C. These findings enable for the detailed recognition of the chain microstructure of POE resin and the extension of the TREF approach to POE resins. This lays the groundwork for fundamental studies and practical uses in industry.

Comparison of chain structures between special polyethylene resins for natural gas pipes by preparative temperature rising elution fractionation and cross-fractionation

Comparison of chain structures between special polyethylene resins for natural gas pipes by preparative temperature rising elution fractionation and cross-fractionation

Study of Heterogeneity of Ethylene/1-Octene Copolymers Synthesized with Zirconium and Titanium Complexes Bearing Diamine-bis(phenolate) Ligands.

A series of ethylene/1-octene copolymers synthesized with diamine-bis(phenolate) complexes activated with Al(iBu)3/[Ph3C][B(C6F5)4] were subjected to preparative temperature rising elution fractionation (TREF). The complexes used differed in the type of metallic center (Zr or Ti) or the amine donor in the pendant arm of the ligand (NMe2 or NiPr2). The obtained fractions were then characterized via FTIR, DSC and GPC methods. It was found that all the copolymers had very broad chemical composition distributions, and the most heterogeneous was the copolymer produced by the titanium complex bearing a ligand with the NiPr2 donor group. The difference in the comonomer incorporation into the macromolecules of the fractions was as high as 8.3 mol%. The melting temperature and molecular weight of the fractions changed nearly linearly with the increased elution temperature. Copolymers produced by zirconium catalysts were also fractionated by molecular weight using the solvent/non-solvent technique with subsequent analysis of the fractions. It was shown that the fractions have a similar composition, low molecular weight distribution and very broad comonomer distribution. Therefore, the comonomer content in the fractions was not a function of the molecular weight as was observed for the copolymers synthesized with the Ziegler-Natta catalysts.

Comparison of chain microstructure between two propylene−ethylene copolymer resins with bimodal melting temperature distribution

Polypropylene (PP) has been commonly used as packaging materials due to the favorable mechanical and optical properties related to its diverse and complex chain microstructure which can be better understood through effective fractionation. In this work, two commercial PP resins (A and B) with similar bimodal melting temperature distribution in the range of 100–160 °C but different ethylene content (9.1–12.3 mol%) are fractionated by preparative temperature rising elution fractionation (P-TREF) and relevant fractions are characterized by a variety of techniques such as high temperature-gel permeation chromatography (HT-GPC), differential scanning calorimetry (DSC), 13C nuclear magnetic resonance (NMR) spectroscopy, Fourier transform-infrared (FT-IR) spectroscopy, and wide-angle X-ray diffraction (WAXD). The chain microstructures of the two resins used as food packaging film exhibit heterogeneity and some differences, which are mainly caused by broad and different distribution of ethylene comonomers. Resin B contains more ethylene units than resin A, and the ethylene units are incorporated into the PP chains in the forms of individual unit or very short segments, resulting in large amount of fractions with relatively low isotacticity and crystallinity that eluted at the temperature range of 35–90 °C. These fractions account for 84.07 wt% of resin B, whereas 59.66 wt% of resin A. The propylene sequences of different isotacticity and crystallinity generated by the insertion of ethylene comonomers in resin B are mainly responsible for its biomodal melting behavior. However, ethylene comonomers tend to insert into the PP chains of resin A in the forms of long polyethylene (PE) blocks and generate propylene−ethylene block copolymers with high crystallizability which can be eluted at high temperatures of 120–140 °C. These fractions constitute 3.16 wt% of resin A, but barely exist in resin B. Moreover, 35.83 wt% of resin A were eluted at 110 °C, whereas only 13.22 wt% of resin B can be eluted at this temperature. These fractions with high isotacticity and molecular weight eluted at high temperatures contribute to the higher flexural modulus of resin A. Both long ethylene blocks and propylene sequences of different lengths contribute to the biomodal melting peaks on the thermogram of resin A.

The chain microstructure and condensed structure of polyethylene resin used for Biaxially stretched film

AbstractTo explore the desirable chain structure and condensed structure of polyethylene used for biaxial stretching, the blends of two polyethylenes (PE1 and PE2) with different molecular weight and branch distribution were investigated. Their condensed structure including relaxation and crystallization behavior was studied by rheology, dynamic mechanical analysis (DMA), and non‐isothermal crystallization kinetics. And the chain microstructure was characterized by preparative temperature rising elution fractionation (P‐TREF), and successive self‐nucleation and annealing (SSA). It was demonstrated that the structural variables were not changing monotonically with the increment of the PE1 content. The longest relaxation time, lowest and less varied crystallization rate always appeared when the blend contained 15 wt% of PE1. This excellent structure was owing to the formation of more branches on the short chains, resulting in the uniform distribution of lamellae size and thus showing the widest biaxial stretchable temperature range (i.e., 8°C).

Chain microstructure of two highly impact polypropylene resins with good balance between stiffness and toughness

For highly impact polypropylene (HIPP), a good balance between stiffness and toughness has attracted much attention from both industrial and academic research. Herein, two HIPP resins (A and B) with different ethylene contents and impact resistances especially at low temperatures are investigated. Their chain microstructure is studied by combining preparative temperature rising elution fractionation (P-TREF) with multiple characterization techniques such as high-temperature gel permeation chromatography (HT-GPC), Fourier transform infrared (FTIR) spectroscopy, 13C nuclear magnetic resonance (13C NMR), wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), rheometer, and thermal fractionation. Both samples are mainly composed of four portions: amorphous ethylene−propylene random copolymer (EPR), ethylene−propylene (EP) segmented and blocky copolymer, and isotactic polypropylene homopolymer. Sample A contains ~30 wt% EPR fractions eluted at 35 °C, ~9 wt% EP segmented and blocky copolymers eluted at 80–100 °C, and ~19 wt% highly isotactic polypropylene fractions eluted at 125–140 °C, which are higher than the contents of corresponding fractions in sample B. Moreover, most fractions of sample A have higher molecular weights than the corresponding fractions of sample B. The contribution of different components to stiffness and toughness is also discussed. The excellent impact resistance of sample A is demonstrated from the perspective of chain microstructure.

Temperature rising elution fractionation and fraction characterization of Trans-1, 4-poly(isoprene-co-butadiene)

In order to analyze the chain structure and sequence distribution of trans-1,4-poly(isoprene-co-butadiene) copolymers (TBIR) with different butadiene content, a preparative-temperature rising elution fractionation (P-TREF) technique was used to fractionate the TBIR into 10 to 12 fractions based on the elution temperature ranged from −40 °C to 70 °C. NMR, DSC and GPC were used to characterize the microstructure, butadiene incorporation, [BI], [II] and [BB] dyad sequence concentration, melting point, molecular weight and molecular weight distribution of the fractions. Two or multi-model GPC curves are observed for the lower temperature elution fractions and the molecular weight of the fractions shows an increased tendency with an increase in the elution temperature. The fractions obtained at −40 °C elution temperature are copolymers with short trans-1, 4-polyisoprene (TPI) blocks, high butadiene incorporation and lower melting temperature. The fractions obtained at 50 °C elution temperature are copolymers containing medium-length TPI blocks and short trans-1, 4-butadiene (TPB) blocks. The fractions obtained at the other elution temperatures are mainly copolymers with medium-length TPI blocks. The chain structure models of the TBIR fractions are proposed and possible formation mechanism is discussed.

Preparative Temperature Rising Elution Fractionation of One Poly(1-butene) Copolymer and Its Chain Microstructure Characterization

Generally, commercial P-TREF (preparative temperature rising elution fractionation) equipment carries out an experiment for polyolefin samples between room temperature (or the lowest temperature at 20 °C) and 150 °C. Although we have applied the traditional TREF method in polyethylene and propylene resin in our previous studies, it failed to fractionate poly(1-butene) with a polymorph structure. Up to now, it has been a challenge to realize an effective P-TREF separation procedure for poly(1-butene) resins. Based on the analysis and experiments, it was found that a low temperature was important for poly(1-butene) resin. Herein, a novel P-TREF instrument with a broad temperature range from −80 to 150 °C is designed and fabricated. By applying this equipment, one poly(1-butene) copolymer, i.e., 1-butene/ethylene random copolymer, is first effectively fractionated according to its crystallizability by the TREF principle. The main fractions are eluted at 30, 42, and 46 °C, corresponding to the weight percent ...

Influence on properties and phase structure of single gas-phase reactor made impact polypropylene copolymers

Impact polypropylene copolymers (IPC) are materials important for many commercial applications. These materials are usually synthetized by different methods involving two consecutive reactions: in liquid, gas or liquid-gas phase. This work presents a method of synthesis in a laboratory scale based on two sequential steps in only one reactor in gas phase. Variables such as H2 addition, reaction time, and composition during the second step were studied, and their influence on the formation of propylene-ethylene copolymer materials with different properties was analyzed. The IPC materials so obtained were characterized by analytical temperature rising elution fractionation (TREF), calorimetric methods (DSC), 13C nuclear magnetic resonance (13C NMR), gel permeation chromatography with an infrared detector (GPC-IR5), Charpy impact, and scanning electron microscopy (SEM).The isotactic polypropylene matrix (iPP) obtained in the first step of the gas-phase process displays spherical morphology and lower mean particle size than those obtained in the liquid-phase process, even though the polymer grains experimented a certain tendency to agglomerate.In particular, hydrogen addition caused a significant decline in the catalyst productivity and dramatically shortened the length of the propylene homopolymer chains. Similarly, the presence of hydrogen on the synthesis of ethylene-propylene copolymers was demonstrated to lead to materials with very low molecular weight, low ethylene incorporation, and rubbery phases irregularly distributed along the iPP matrix and therefore with poor impact properties.On the other hand, ethylene-propylene copolymers synthesized without hydrogen yielded a suitable combination of molecular weights and molecular weight distribution that can contribute to good polymer processing and were proven to incorporate adequate amounts of ethylene mainly randomly distributed into the iPP matrix. SEM measurements revealed that the amorphous rubbery phase was homogeneously dispersed in the iPP matrix, and the Charpy test allowed to rank these materials as IPC.Finally, fractionation of IPC materials by preparative TREF provided information about the microstructure formation. Subsequent studying of molecular weights and composition of the fractions by GPC-IR, analytical TREF, and DSC measurements based on the ethylene-propylene composition and ethylene distribution in the molar mass molecules was shown to enable the design of IPC materials. Additionally, the optimal IPC material was compared with the best IPC using liquid-phase polymerization, and the results showed that the range of ethylene-propylene composition, as well as the ethylene distribution in IPC molar mass molecules, correlated with IPC mechanical behavior.

Comprehensive branching analysis of polyethylene by combined fractionation and thermal analysis

AbstractLinear low density polyethylene (LLDPE) and low density polyethylene (LDPE) differ significantly in their branching types and branching distributions. For a comprehensive analysis, preparative temperature rising elution fractionation and/or preparative molar mass fractionation are used to fractionate typical LLDPE and LDPE bulk resins into narrowly distributed fractions. The chain structures of the bulk resins and their fractions are further analysed using SEC, crystallization analysis fractionation, DSC and high‐temperature HPLC to provide detailed information on short chain branching in LLDPE and long chain branching in LDPE. For LDPE it is shown that the multiple fractionation approach is a powerful source of sample libraries that may have similar molar masses and different branching structures or alternatively similar branching but different molar masses. The analysis of these library samples by thermal analysis provides a much deeper insight into the molecular heterogeneity of the samples compared to bulk sample analysis. © 2018 Society of Chemical Industry

Propylene polymerization over supported Ziegler–Natta catalysts: Effect of internal and external donors on distribution of active sites according to stereospecificity

ABSTRACTData on the content of fractions with different microtacticities for polypropylene (PP) samples produced over three catalysts [the “donor‐free” titanium–magnesium catalyst and catalysts with dibutyl phthalate and 1,3‐diether(fluorene) used as internal donors] upon polymerization in the absence/presence of an external donor (ED) have been obtained by preparative temperature rising elution fractionation method. The effect of internal and EDs on the distribution of PP fractions with different microtacticity is discussed. Data on molecular weight and thermophysical characteristics were obtained for individual fractions with different microtacticities. Correlations were found between microtacticity, molecular weight, and the melting points of these fractions. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46291.

Chemical composition separation of a propylene–ethylene random copolymer by high temperature solvent gradient interaction chromatography

A propylene–ethylene random copolymer was fractionated by preparative temperature rising elution fractionation (TREF). The structural heterogeneity of the bulk sample and its TREF fractions was studied by high temperature liquid chromatography with a solvent gradient elution from 1-decanol to 1,2,4-trichlorobenzene. HPLC alone cannot resolve those propylene-ethylene copolymers with high ethylene content in the bulk sample, due to their low weight fractions in the bulk sample and a small response factor of these components in the ELSD detector, as well as their broad chemical composition distribution. These components can only be detected after being separated and enriched by TREF followed by HPLC analysis. Chemical composition separations were achieved for TREF fractions with average ethylene contents between 2.1 and 22.0mol%, showing that copolymers with higher ethylene contents were adsorbed stronger in the Hypercarb column and eluted later. All TREF fractions, except the 40°C fraction, were relatively homogeneous in both molar mass and chemical composition. The 40°C fraction was rather broad in both molar mass and chemical composition distributions. 2D HPLC showed that the molar masses of the components containing more ethylene units were getting lower for the 40°C fraction. HPLC revealed and confirmed that co-crystallization influences the separation in TREF of the studied propylene-ethylene copolymer.

Effect of Microstructure of High Density Polyethylene on Catalytic Degradation: A Comparison Between Nano Clay and FCC

The effect of microstructure of high density polyethylene (HDPE) on catalytic degradation over fluid catalytic cracking (FCC) catalyst and Nano clay is investigated. An ethylene/1-hexene copolymer (HDPE) was fractionated based on 1-hexene content to different fractions using preparative temperature rising elution fractionation (P-TREF) method. The short chain branch (SCB) content of each fraction calculated based on 1H NMR results. The homogenous SCB content fractions were subsequently analyzed using TGA and thermal degradation behavior of samples were compared with catalytic degradation. The results showed that FCC, in contrast to nano clay, is more sensitive to microstructure of HDPE during catalytic degradation. Therefore, linear chains can start degradation sooner on FCC than the branched chains. In comparison of nano clay and FCC it was found that the nano clay, reduces temperature at maximum degradation rate (Tmax), but FCC reduces the T5% and Tmax significantly.

Exploring the Compositional Heterogeneity of Vis‐Broken Impact Poly(propylene) Copolymers by Advanced Fractionation Methods

Vis‐breaking or rheology control is an important technical process to improve the processability of impact poly(propylene) copolymers (IPCs). In the vis‐breaking process the molar mass and its dispersity are reduced and the crystallinity changes and polar carbonyl functionalities are introduced due to the reaction of the polymer with peroxide. Although the fundamental principles of vis‐breaking are well understood, not much research has been devoted to the investigation of the molecular changes brought about by the vis‐breaking process. In the present study, the effect of vis‐breaking on the molecular parameters of IPCs is elucidated using advanced fractionation methods. After the analysis of the bulk sample properties, the samples are fractionated using preparative temperature rising elution fractionation and the fractions are analyzed by Fourier transform infrared spectroscopy, differential scanning calorimetry, crystallization analysis fractionation, and high temperature high performance liquid chromatography techniques. For the first time, an effective multidimensional analytical approach is established to study compositional heterogeneities in vis‐broken IPC materials. image

Comparison of chain structures between high-speed extrusion coating polyethylene resins by preparative temperature rising elution fractionation and cross-fractionation

Two polyethylene (PE) resins (samples A and B) are synthesized as high-speed extrusion coatings with similar minimum coating thickness and neck-in performance but different maximum coating speeds. Both samples are separated into seven fractions using preparative temperature rising elution fractionation. The microstructures of the original samples and their fractions are studied by high-temperature gel permeation chromatography, Fourier transform infrared spectroscopy, 13C nuclear magnetic resonance spectroscopy, differential scanning calorimetry, and successive self-nucleation/annealing thermal fractionation. Compared with sample B, sample A has a broader MWD, more LCB contents, and less SCB contents. Moreover, sample A contains slightly more 30 °C and 50 °C fractions with lower molecular weights, and more fractions at 75 °C and 85 °C with higher molecular weight. The chain structure and its distribution in the two PE resins are studied in detail, and the relationship between the chain structure and resin properties is also discussed.

Molecular chain heterogeneity of a branched polyethylene resin using cross-fractionation techniques

The typical standard long chain branching (LCB) polyethylene (PE) SRM1476 is issued by the American National Bureau of Standards, which is widely used as a standard reference material in chromatographic experiments. Although some works have been reported in literatures, they are not consistent, such as the data about its molecular weight and molecular weight distribution. Therefore, it is necessary to further investigate the microstructure features of SRM1476 in order to better serve as a reference object in comparison with other unknown resins. In this study, the molecular chain heterogeneity of SRM1476 is investigated extensively and compared with the linear PE SRM1475. Based on the characterization of the original sample, SRM1476 actually has both LCB and short chain branching (SCB) structures, as well as the SCB content is more than LCB content in 13C-NMR results. After successive self-nucleation and annealing (SSA) thermal fractionation, SRM1476 shows a broad-range endotherm with multiple melting peaks (more than eight peaks), which proves the molecular chain heterogeneity in SRM1476. SRM1476 is fractionated into nine fractions by preparative temperature rising elution fractionation (TREF). At the high elution temperature region, the amounts of fractions are more than 90 %, and their molecular weights are higher. At the low elution temperature region below 50 °C, molecular weights of fractions are lower and their amounts are less than 5 %. Differential scanning calorimetry (DSC) melting curves of TREF fractions with higher elution temperatures shift toward higher temperature with sharper melting peaks. In the results of TREF-SSA cross-fractionation, each fraction shows a broad-range endotherm with multiple melting peaks that shift toward the high elution temperature region with the elution temperature. The arithmetic mean methylene sequence lengths of TREF fractions gradually increase from 33 to 105 as elution temperature increases. Results from TREF-DSC and TREF-SSA cross-fractionations indicate that SRM1476 and its TREF fractions have both intramolecular and intermolecular heterogeneity.

On the multimodality of preparative TREF fractionation as detected by advanced analytical methods.

Preparative temperature rising elution fractionation (prepTREF) is the standard technique for the preparative fractionation of polyolefins according to crystallisability. For olefin copolymers such as linear low-density polyethylene (LLDPE), it was believed that the TREF elution temperature correlates directly with the copolymer composition. For copolymers having different bulk comonomer contents, the prepTREF fractions of different samples collected at a given temperature were assumed to have the same chemical composition. It was acknowledged quite early that co-crystallisation effects may disturb TREF fractionation and fractions are obtained that are not completely homogeneous. This, however, has not been investigated quantitatively so far. The fundamental statement of prepTREF is challenged for the first time quantitatively using advanced analytical techniques including high-temperature high-performance liquid chromatography (HPLC). Ethylene-1-octene copolymers having bulk comonomer contents ranging from 0.3 to 6.4mol% were fractionated by prepTREF, and the fractions were analysed by high-temperature size exclusion chromatography, crystallisation analysis fractionation, differential scanning calorimetry and high-temperature HPLC. All analytical results prove that the TREF fractions collected from different samples at the same elution temperature have different chemical compositions. The chemical compositions of the fractions correlate with the compositions of the bulk samples in that the comonomer contents of similar TREF fractions increase with an increase of the comonomer content of the bulk samples. These results are in clear contrast to the previous assumption that the TREF fraction composition is independent of the bulk copolymer composition for a given copolymer type. Graphical abstract Advanced analysis of LLDPE by combination of HT-HPLC, DSC and CRYSTAF.

Calibration curve establishment and fractionation temperature selection of polyethylene for preparative temperature rising elution fractionation

A series of copolymers of ethylene with 1-hexene synthesized using a metallocene catalyst are selected and mixed. The blend is fractionated via preparative temperature rising elution fractionation (P-TREF). All fractions are characterized via high-temperature gel permeation chromatography (GPC), C-13 nuclear magnetic resonance spectroscopy (C-13-NMR), and differential scanning calorimetry (DSC). The changes in the DSC melting peak temperatures of the fractions from P-TREF as a function of elution temperature are almost linear, thereby providing a reference through which the elution temperature of TREF experiments could be selected. Moreover, the standard calibration curve (ethylene/1-hexene) of P-TREF is established, which relates to the degree of short-chain branching of the fractions. The standard calibration curve of P-TREF is beneficial to study on the complicated branching structure of polyethylene. A convenient method for selecting the fractionation temperature for TREF experiments is elaborated. The polyethylene sample is fractionated via successive self-nucleation and annealing (SSA) thermal fractionation. A multiple-melting endotherm is obtained through the final DSC heating scan for the sample after SSA thermal fractionation. A series of fractionation temperatures are then selected through the relationship between the DSC melting peak temperature and TREF elution temperature.

The ablative behavior of high linear chains and its role in the thermal stability of polyethylene: a combined P‐TREF−TGA study

AbstractEthylene copolymers contain short chain branches (SCBs) which have great influence on their properties. An ethylene/1‐hexene copolymer distinguished in terms of the number of butyl SCBs was precisely separated based on differences in crystallizability using the preparative temperature‐rising elution fractionation (P‐TREF) method and was then studied via TGA and DSC. A comparison between the results obtained and the literature suggested a short chain branch distribution (SCBD) functionality for a included in the general linear form Tm (°C) = −a(SCBD) × (SCB) + b. P‐TREF−TGA results showed that the highly linear chains acted as ablative layers which could increase the thermal stability and durability of polyethylene in the absence of any mineral additive. Furthermore, the P‐TREF−TGA data displayed an interesting interrelationship between temperature at maximum rate of degradation (Tmax) and the number of butyl SCBs over all the heating rates (10, 25, 50 and 100 °C min‐1). The role of the number of butyl SCBs in thermal degradation was exhausted by higher heating rates, whereas the ablation capability was enhanced. Kinetic studies demonstrated that the activation energy dropped on increase in butyl branch content within the backbone. © 2015 Society of Chemical Industry

The interrelationships between microstructure and melting, crystallization and thermal degradation behaviors of fractionated ethylene/1-butene copolymer

The interrelationships between microstructure and melting, crystallization and thermal degradation behaviors of a commercial Ziegler–Natta (Z–N)-based ethylene/1-butene copolymer were investigated. The copolymer was fractionated based on short chain branch (SCB) content by preparative temperature-rising elution fractionation (P-TREF) method. A broad multipeak chemical composition distribution was obtained for both molecular weight distribution and short chain branch distribution. A difference of about 48 °C in melting temperature (Tm) has been observed for fractions with 50 branches of different SCB contents. A logarithmic relationship was obtained between the methylene sequence length calculated based on proton-1 nuclear magnetic resonance results and the P-TREF elution temperature (ET). A relationship between the SCB content and the inverse of lamellae thickness (Lc) was established. Two linear functionalities were found for Tm versus ET and crystallization temperature (Tc) versus the SCB content. To the best of our knowledge, as a first effort, surprisingly a linear relationship between the temperature at the maximum rate of degradation (Tmax) and the SCB content was acquired, which showed less sensitivity to the SCB content than Tm. Also, it was found that the degradation initiation temperature (T5%) and activation energy (Ea) are increased by the decreasing SCB content. Moreover, it was demonstrated that Hosoda equation is not applicable for different ethylene/α-olefin copolymers based on different catalyst types with dissimilar fractionations and experimental conditions.