Correlation between GPC,DSC, and Analytical TREF for Linear Polyethylene

Analytical temperature rising elution fractionation (TREF) of linear polyethylene (PE) samples with different densities was done in 1-chloronaphthalene using a gel permeation chromatograph (GPC) coupled with a gas chromatograph. The corrected peak elution temperatures completed the previously obtained data in trichlorobenzene, xylene, and dibutoxymethane. A mathematical correlation was found for diluted linear PE samples between the α parameter of the Mark-Houwink-Sakurada equation governing the retention time in GPC, the bulk melting temperature measured by differential scanning calorimetry (DSC), and the TREF peak elution temperature. The extrapolation to the melting temperature measured by DSC gives α = 0.5, thus confirming the hypothesis that polymer conformations in the melt are similar to those in a theta solvent.     

Elution Temperatures of Polyethylene Copolymers in Temperature Rising Elution Fractionation (TREF): Comparison in Xylene and Trichlorobenzene Diluents

Temperature rising elution fractionation (TREF) became the preferred technique to characterize the short chain branching distribution of polyethylene copolymers. Due to technical limitations, preparative TREF (PTREF) is usually done in xylene, while trichlorobenzene is used in analytical TREF (ATREF). Attempts to correlate the TREF elution temperatures based on data published by different authors erroneously showed higher elution temperatures for xylene than for trichlorobenzene. Our study rectifies this error. The experiments were done in both solvents on the same analytical TREF instrument. For the analyzed polyethylene copolymers, we found that the average elution temperature in xylene is 3.7° ± 1°C lower than in trichlorobenzene.     

Characterization of polypropylenes prepared with bis‐indenyl type metallocene and MgCl2‐supported TiCl4 catalyst systems

The homo‐polypropylene: m‐PP, prepared with rac‐Me₂Si[2‐n‐Pr‐4‐(9‐Phenanthryl)‐Ind]₂ZrCl₂, showed 99.6% of [mmmm] and 162.8°C of melting temperature (Tm). This polymer was compared by TREF analysis with the homo‐polypropylene: Ti‐PP, which was produced by our latest MgCl₂‐supported TiCl₄ catalyst system and showed 99.0% of [mmmm] and 165.7°C of Tm. It was indicated that m‐PP has narrower stereoregurality distribution than Ti‐PP and Tm of the fraction eluted from m‐PP at the highest temperature range is 163.6°C, while that from Ti‐PP reaches to 167.3°C. The characters and advantages of each polymer are discussed on the basis of these results. In addition, an advantage of this metallocene was made clear by characterization of polypropylenes copolymerized with ethylene.     

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), 13 C nuclear magnetic resonance spectroscopy(13C-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.     

Changes in the melting behavior with the radial distance in isotactic polypropylene spherulites

The melting of highly tactic i‐polypropylene occurs in two stages even for crystallization at 145 °C, a temperature at which reorganization during scanning is negligible. A comparison of two such polypropylenes, one nucleated and the other not nucleated, together with fractions from the latter, has been made with electron microscopy following permanganic etching, in addition to differential scanning calorimetry. This has allowed the two melting stages to be assigned to two components of the lamellar morphology, with progressive changes in both occurring with increasing radial distance within a spherulite. The highest melting temperature is for dominant radial lamellae far from a spherulite center. The lowest melting regions are the evenly crosshatched spherulite centers and a narrow peripheral band. Lower melting is attributed to the suppression of isothermal lamellar thickening paralleling recent direct demonstrations in polyethylene. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2342–2354, 2003     

Combination of TREF, high-temperature HPLC, FTIR and HPer DSC for the comprehensive analysis of complex polypropylene copolymers

A novel, powerful analytical technique, preparative temperature rising elution fractionation (prep TREF)/high-temperature (HT)-HPLC/Fourier transform infrared spectroscopy (FTIR)/high-performance differential scanning calorimetry (HPer DSC)), has been introduced to study the correlation between the polymer chain microstructure and the thermal behaviour of various components in a complex impact polypropylene copolymer (IPC). For the comprehensive analysis of this complex material, in a first step, prep TREF is used to produce less complex but still heterogeneous fractions. These chemically heterogeneous fractions are completely separated by using a highly selective chromatographic separation method—high-temperature solvent gradient HPLC. The detailed structural and thermal analysis of the HPLC fractions was conducted by offline coupling of HT-HPLC with FTIR spectroscopy and a novel DSC method—HPer DSC. Three chemically different components were identified in the mid-elution temperature TREF fractions. For the first component, identified as isotactic polypropylene homopolymer by FTIR, the macromolecular chain length is found to be an important factor affecting the melting and crystallisation behaviour. The second component relates to ethylene–propylene copolymer molecules with varying ethylene monomer distributions and propylene tacticity distributions. For the polyethylene component (last eluting component in all semi-crystalline TREF fractions), it was found that branching produced defects in the long crystallisable ethylene sequences that affected the thermal properties. The different species exhibit distinctively different melting and crystallisation behaviour, as documented by HPer DSC. Using this novel approach of hyphenated techniques, the chain structure and melting and crystallisation behaviour of different components in a complex copolymer were investigated systematically.

Fractionation of HDPE on quench cooling

In a previous paper we discussed co-crystallization in a LDPE/HDPE blend using TREF and DSC. As part of that study it was observed that pure HDPE showed an unexpected fractionation behavior when quench crystallized in TREF. The overall peak broadened and two peaks appeared instead of the previously observed single peak for slow cooled HDPE.The development of two peaks was observed for all commercial HDPEs investigated, independent of their melting indices and densities. TREF and GPC were used in an attempt to evaluate the origin of the two HDPE components.The authors appreciate support from CAPES-BRAZIL (C.A.F.). Additional thanks go to Dr. G. W. Knight in Dow Chemical Company for kindly providing the polymer samples and performing the GPC analysis.     

Differential thermal analysis of polyethylene under high pressure

The design of a differential thermal analysis apparatus for use at elevated pressure is described. Experiments on melting and crystallization of folded-chain crystals of polyethylene and poly(ethylene–butene-1) copolymer, and melting of extended-chain polyethylene crystals have been conducted at pressures up to 4200 bars. The precision in transition temperature measurement was ±1°C. The Clausius-Clapeyron equation predicts the melting point increase with pressure at atmospheric pressure to be 32.0°C/kb. The melting point depression due to copolymerization remained constant over the complete pressure range analyzed on the poly(ethylene–butene-1) used in this study. Crystallization of polyethylene is retarded at elevated pressures, and a 50% larger degree of supercooling is necessary at 5000 bars to give a crystallization rate equal to that observed at atmospheric pressure. The difference in melting point between folded-chain and extended-chain polyethylene increases from 8.4°C at 1 bar to 25.6°C at 3000 bars.     

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.     

Polyolefin characterization in dibutoxymethane by high temperature gel permeation chromatography with a new evaporative light scattering detector

A new evaporative light scattering detector (ELSD) for the analysis of polyolefins by high temperature gel permeation chromatography (GPC) was recently introduced by Agilent Technologies. For the first time, we investigated the possibility to use this detector to measure the molecular weight distributions (MWD) of different types of polyolefins (polypropylene, linear and low-density polyethylene) in dibutoxymethane (DBM, butylal). These samples were previously characterized by GPC in trichlorobenzene (TCB) with a differential refractive index (DRI) detector in an interlaboratory study conducted by the International Union of Pure and Applied Chemistry (IUPAC) [1], and in a recent publication by GPC with the new ELSD in TCB [2]. The signal to noise of ELSD using DBM is about 10 times lower than that for TCB. However, the ELSD signal power exponent for DBM was measured as 1.35, which is much closer to unity than the value of 1.61 for TCB. After applying the required corrections to linearize the response of the ELSD signal as a function of concentration, similar average molecular weights to those measured in the interlaboratory study using DRI, were obtained for the analyzed resins.     

Comportement Thermique du Polypropylene au Cours du Vieillissement

Several samples of polypropylene were studied by thermal analysis. The photo-oxidation and the aging of polypropylene films showed a mass loss more than 7% in heating from 20 to 220°C (5°C min-1), cooling to 20°C and reheating to 220°C. The authors observed also a decrease of the melting and crystallization temperatures. The non aged samples or these ones with preservatives are thermo-oxidised and presented an exothermic peak at about 200°C in DTA heating. The DTA-TG simultaneous apparatus is very useful in the study of polypropylene oxidation by making comparative trials according to a well definite procedure. This revised version was published online in July 2006 with corrections to the Cover Date.     

Synthesis and block‐specific complexation of poly(ethylene oxide)–poly(tetrahydrofuran)–poly(ethylene oxide) triblock copolymers

Well‐defined ABA triblock copolymers in which A stands for poly(ethylene oxide) (PEO) and B for poly(tetrahydrofuran) (PTHF) were synthesized by end‐capping bifunctionally living PTHF with different polyethylene glycol–monomethylethers. Differential scanning calorimetry analysis of these copolymers showed two melting points: one around 55 °C due to the PEO blocks, and one around 30 °C due to the PTHF segments, demonstrating that these block copolymers show extensive phase separation. Upon addition of sodium thiocyanate, crystalline complexes with PEO were formed and as a consequence, the melting points of the PEO segments had shifted to approximately 170 °C, whereas the melting points of the PTHF segments decreased slightly. The obtained materials behave as thermoplastic elastomers up to 160–175 °C. The influence of the relative lengths of the PEO and the PTHF segments on the thermal and mechanical properties of the materials have been investigated. Copyright © 1999 John Wiley & Sons, Ltd.     

Synthesis and characterization of thermoplastic polyesters and polyamides from sebacyl bisketene

Sebacyl bisketene was generated in solution at ?78°C. Copolymerization in solution at 0°C with the secondary diamines, piperazine and N,N′dimethyl-1,6-hexamethylenediamine, yielded the polyamides poly(1,4-piperazylsebacyl) and poly[(methylimino)hexamethylene(methylimino)sebacyl], respectively. The polyamides were obtained in yields of 50–90%. The former had a glass transition temperature (T_g) at 30°C and a melting temperature at 165°C, whereas the latter had only a T_g at ?15°C. The polymers were insoluble in the usual polyamide solvents. Copolymerization with the diol bisphenol A yielded poly(oxy-1,4-phenyleneisopropylidene-1,4-phenyleneoxysebacyl). The polyester was obtained in yields up to 99%. Gel permeation chromatography (GPC) determinations showed molecular weights up to 50,000 when acetone was the reaction solvent but only 12,000 when tetrahydrofuran (THF) was the reaction solvent; the T_g for the polyester varied with the molecular weight with a maximum at 15°C. Tensile properties were obtained for the polyesters with molecular weights greater than 35,000.     

L-丙交酯/ε-己内酯共聚物分子链结构非均匀性的研究

利用升温淋洗分级(TREF)和连续自成核退火热分级(SSA)等方法研究了L-丙交酯(LLA)和ε-己内酯(ε-CL)共聚物(PLC)分子链结构的非均匀性.合成了2种不同摩尔比的PLC共聚物,用TREF的方法,将同一个共聚物分级出3个级份.用核磁共振氢谱(1H-NMR)和凝胶渗透色谱(GPC)等方法对每个级份的分子结构进行了表征.结果表明,随着淋洗温度的升高,共聚物级份中的ε-CL单元的含量明显降低,而且级份的数均分子量增大,分子量分布指数降低.在SSA热分级研究中,发现PLC各级份间存在明显的分子链间和分子链内结构上的差异,这种链结构差异使得各个级份表现出不同的熔融行为.     

Correlation of the elution behavior in temperature rising elution fractionation and melting in the solid‐state and in the presence of a diluent of polyethylene copolymers

Compositionally homogeneous poly(ethylene‐α‐olefin) random copolymers with 1‐butene and 1‐hexene comonomers have been studied. The melting of solution‐crystallized specimens of these copolymers in the presence of trichlorobenzene as a diluent with differential scanning calorimetry (DSC) is well correlated with analytical temperature rising elution fractionation (A‐TREF) elution temperature profiles. This indicates that the A‐TREF experiment is essentially a diluent melting experiment. Furthermore, the correction of the corresponding solid‐state melting endotherms of these copolymers with Flory's diluent melting equation yields curves that also correlate very well with the DSC diluent melting curves and the A‐TREF elution temperature profiles. Values of χ, the Flory–Huggins interaction parameter, are determined for these copolymers in trichlorobenzene. χ decreases as short‐chain branching increases. The A‐TREF elution temperature profiles of one of these copolymers are the same, within experimental error, for dilute‐solution crystallizations of the copolymer performed over an extremely broad time schedule (10 s to 3 days). This indicates the profound effect of the branches, as limiting points of the ethylene sequences, in controlling the crystal thickness distribution, which in turn controls the melting point in the presence of the diluent, or the elution temperature from the A‐TREF. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2819–2832, 2001     

OXIDATION OF POLYPROPYLENE HOMOPOLYMER IN THE MOLTEN STATE IN PRESENCE OF AIR

The oxidation of polypropylene (PP) homopolymer in air was performed using dodecanol-1 as an accelerator. The experiments were conducted under atmospheric pressure at 180-220°C. Spectroscopic data indicated the formation of polar groups such as ketones, esters, alcohols, anhydrides etc. as determined by FTIR and ESCA. The scanning electron microscopy (SEM) showed the variations of morphology of the oxidation products. The fusion temperatures were determined by differential scanning calorimetry (DSC). The variations of solubility of PPO as compared with the original PP were investigated in solvents such as MEK, THF, and toluene.

Surface tension and molecular weights were determined by tensiometry and gel premeation chromatography (GPC). The melt flow index (MFI) of different samples were determined.     

Analysis of microstructure of ethylene–1-hexene copolymer prepared over thermally pretreated MgCl2/THF/TiCl4bimetallic catalyst

Thermally pretreated catalysts were prepared by heating MgCl₂/THF/TiCl₄ (TT-0) at 80°C for 5 min (TT-1) and 60 min (TT-2), and at 108°C for 5 min (TT-3) and 60 min (TT-4). Ethylene–1-hexene copolymers were prepared with these catalysts. The TT-1 catalyst produced more blocky and higher 1-hexene content polymer than TT-0, 2, 3, and 4. Temperature rising elution fractionation (TREF) analysis was used to investigate the chemical composition distribution of the ethylene–1-hexene copolymer, exhibiting bimodal distribution for TT-0 and trimodal for TT-1, 2, 3, and 4. A portion of higher hexene content of the copolymer markedly increased when the copolymerization was performed with TT-1, indicating that copolymerization active sites were newly generated. Portion of homopolyethylene increased drastically when the copolymerization was performed with TT-4, indicating that ethylene homopolymerization active sites were increased. Gel permeation chromatography (GPC) also revealed that three kinds of active sites existed on the catalyst. ¹³C-NMR spectrum of each fraction after TREF analysis suggested that the isospecific active site could polymerize 1-hexene well, resulting in random and alternating copolymers. A scheme for generation of the active site and change of its nature during thermal treatment of bimetallic complex catalyst is proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 291–300, 1998     

Deformation of disentangled polypropylene crystalline grains into nanofibers

It was shown that a solid‐state deformation of polypropylene (PP) being in the form of partially disentangled powder is possible by blending with another molten polymer. During mixing of disentangled polypropylene powder with polystyrene at the temperature below melting of polypropylene crystals the shear forces deform powder grains into nanofibers. All disentangled powder particles larger than 0.7 µm underwent deformation into nanofibers having the mean thickness between 100 and 200 nm. Polypropylene nanofibers got entangled during blending and form a network within polystyrene matrix, reinforcing it. Network of entangled nanofibers can be further deformed with pronounced strain hardening and strength reaching 70 MPa at 135 °C. Blending resulted in generation of PP nanofibers and formation of PP nanofibers entangled network, thus formation of “all‐polymer nanocomposites” in one step compounding. The crucial feature for ultra‐deformation of PP grains by shearing during mixing is disentanglement of macromolecules. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1983–1994     

Rheological properties of UHMWPE/iPP blends

The effects of isotactic polypropylene (PP) on the rheological properties of ultra high molecular weight polyethylene (UHMWPE) have been measured in a broad range of composition (0, 5, 15, 30 wt% PP) at various temperatures (110, 130, and 150°C) and a specific gel concentration of 6 wt%. The result showed that the viscosity of the UHMWPE significantly decreased with the addition and increasing amount of PP. Regardless of temperature, the viscosity function followed the power‐law behavior. Copyright © 2009 John Wiley & Sons, Ltd.     

New understanding on the memory effect of crystallized iPP

In this study, recovery processes of isotactic polypropylene(iPP) melted spherulites at 135 °C after melting at higher temperatures(170 °C–176 °C) were investigated with polarized optical microscopy and Fourier transform infrared spectroscopy. The recovery temperature was fixed to exclude the interference from heterogeneous nuclei. After melting at temperatures between 170 °C and 174 °C, the melted spherulite could recover back to the origin spherulite at low temperatures. Interestingly, a distinct infrared spectrum from iPP melt and crystal was observed in the early stage of recovery process after melting at low temperatures, where only IR bands resulting from short helices with 12 monomers or less can be seen, which indicates that the presence of crystal residues is not the necessary condition for the polymer memory effect. Avrami analysis further indicated that crystallization mainly took place in melted lamellae. After melting at higher temperatures, melted spherulite cannot recover. Based on above findings, it is proposed that the memory effect can be mainly ascribed to melted lamellae, during which crystalline order is lost but conformational order still exists. These conformational ordered segments formed aggregates, which can play as nucleation precursors at low temperatures.