Use of FT-IR Spectrometry for On-Line Detection in Temperature Rising Elution Fractionation

Summary: Temperature rising elution fractionation (TREF) has become a popular analytical technique that is able to determine the chemical composition distribution (CCD) of an ethylene/α-olefin copolymer. An infrared (IR) detector is commonly used in TREF detection to measure the concentration of the polymer solution exiting the column as a function of elution temperature. The chemical composition of the eluting polymer at a given elution temperature can be predicted from the relationship between comonomer content and TREF elution temperature pre-established through ¹³C nuclear magnetic resonance (NMR) analysis of TREF fractions. In this article, a Fourier transform infrared (FT-IR) spectrometer has been coupled with a TREF instrument to provide a more powerful tool for characterizing complex olefin copolymers. The Partial Least Squares (PLS) technique is used when analyzing the FT-IR spectra of the eluting polymer solutions. The power of on-line FT-IR detection in TREF is demonstrated using a few complex copolymer systems, such as ethylene-octene copolymer, polystyrene grafted ethylene-vinyl acetate copolymer and ethylene-methyl acrylate copolymer.     

Modeling the Fractionation Process in TREF Systems. III. Model Validation With Low Molecular Weight Homopolymers

Low molecular weight semicrystalline homopolymers are used as a model system for temperature rising elution fractionation (TREF) analysis. An already proposed thermodynamic model for TREF analysis is used to characterize TREF fractions from low molecular weight polyethylenes M?_n = 500 to 3000 and some of their mixtures. In this molecular weight range it is possible, under appropriate crystallization, conditions, to form extended-chain crystals, and therefore lamellar thicknesses become comparable to extended-chain lengths. Lamellar thicknesses calculated from TREF spectra permit calculations of the molecular weights of the fractions, up to a limit of about 142 CH₂, where partially folded-chain crystallites appear under these operating conditions. Also homopolymers blends are fractionated and the TREF spectra analyzed to test model predictions. It is shown that appearance of chain folding may set a resolution limit to the analysis of commercial copolymers by TREF. © 1996 John Wiley & Sons, Inc.     

Simultaneous Deconvolution of the Molecular Weight and Chemical Composition Distribution of Polyolefins Made with Ziegler-Natta Catalysts

Heterogeneous Ziegler-Natta catalysts produce polyolefins that have broad distributions of molecular weight (MWD) and chemical composition (CCD). For such broad distributions, mathematical models are useful to quantify the information provided by polyolefin analytical techniques such as high-temperature gel permeation chromatography (GPC), temperature rising elution fractionation (TREF), and crystallization analysis fractionation (CRYSTAF). In this paper, we developed a mathematical model to deconvolute the MWD and CCD of polyolefins simultaneously, using Flory's most probable distribution and the cumulative CCD component of Stockmayer's distribution. We have applied this procedure to “model” polyolefin resins and to one industrial linear low-density polyethylene (LLDPE) resin. The proposed methodology is able to deconvolute theoretical distributions even when random noise is added to the MWDs and CCDs, and it can be used to calculate the minimum number of active site types on heterogeneous Ziegler-Natta catalysts.     

Modeling the fractionation process in TREF systems. II. Numerical analysis

Temperature raising elution fractionation (TREF) allows qualitative short chain branching (SCB) analysis in copolymers. In order to make the analysis quantitative, information on how such fractionation occurs must be incorporated into the interpretation of TREF spectra. In a previous work a model of the fractionation was proposed and some preliminary results given. In this article a rigorous mathematical analysis of the solution of the fractionation model is presented by defining two problems. The direct problem, of little practical use, helps to understand the fractionation process. The inverse problem consists in obtaining the distribution of crystallizable lengths (DCL) (directly related to the SCB distribution) from the TREF spectrum. This last problem (i.e., the main interest of this work) is explained in detail. TREF experiments are simulated solving the so-called direct problem using DCLs of different shapes. The synthetic TREF spectra are then processed using the inverse algorithm. The synthetic experiments demonstrate the adequacy of the proposed algorithm as a tool of analysis. A linear ethylene polymer was used to test, experimentally, the numerical procedure. The results obtained are in agreement with those obtained in earlier studies on the same sample by Raman spectroscopy. © 1996 John Wiley & Sons, Inc.     

Characterization of polypropylene�Cpolyethylene blends by temperature rising elution and crystallization analysis fractionation

The introduction of single-site catalysts in the polyolefins industry opens new routes to design resins with improved performance through multicatalyst-multireactor processes. Physical combination of various polyolefin types in a secondary extrusion process is also a common practice to achieve new products with improved properties. The new resins have complex structures, especially in terms of composition distribution, and their characterization is not always an easy task. Techniques like temperature rising elution fractionation (TREF) or crystallization analysis fractionation (CRYSTAF) are currently used to characterize the composition distribution of these resins. It has been shown that certain combinations of polyolefins may result in equivocal results if only TREF or CRYSTAF is used separately for their characterization.     

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     

Fractionation process in TREF systems: Validation of thermodynamic model and calculation procedure by Raman LAM studies

In this paper, possible sources for the unexpected distributions of crystalline sequence lengths calculated from temperature rising elution fractionation (TREF) calibration experiments, as reported in a previous work, are investigated. With this aim, chain folding and cocrystalization phenomena were explored in the conditions of crystallization as used for TREF or crystallization analysis fractionation (CRYSTAF). Slow crystallizations were performed from xylene solutions of model low molecular weight ethylene homopolymers with narrow molecular weight distributions. The same experiments were performed with homopolymers having narrow molecular weight distributions and with blends having wide molecular weight distributions. The resulting distributions of the lengths of crystalline methylene sequences were directly studied by Raman in the so‐called longitudinal acoustic mode (LAM) and by DSC. For ethylene homopolymers with molecular weights below 2000 g/mol, the results from Raman LAM indicate that slow crystallization in TREF or CRYSTAF systems occurs in the extended‐chain mode. For higher molecular weights, evidence of chain folding was found. In the case of blends, independent crystallization was observed for each molecular weight when the molecular weight ranges used for the blends are relatively narrow. Cocrystallization was observed when this range was increased. Overall, these results strongly support the inverse technique calculation procedure developed by our group for the calculation of distributions of lengths of crystallizable sequences from TREF spectra. In this context, the results confirm that the unexpected crystallizable sequence lengths found in our previous work really exist and can be associated to chain folding or cocrystallization phenomena. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3083–3092, 2005     

负载型催化剂制备的聚丙烯等规度分布

负载型Ziesler－Natta催化剂中存在许多活性中心［'3，为了解其本质，需对其各自产生的聚合物进行分离，以往采用的溶剂抽提法［'－'j只能将聚合物大致分级．最近，升温淋洗分级法（TREF）已被运用于聚丙烯的分级卜，'，其原理是根据聚合物的结晶度分级D'，影响聚丙烯结晶性的主要因素是等规度，而分子量到达一定程度后其影响较小，故通过TREF分级可得到聚丙烯的等规度分布．TREF法的淋洗温度可控，故分级效果较好．该法在分级前需对样品进行等温结晶处理，以消除抽提法由于样品未必充分结晶而带来的误差．本文用TREF法对不同催化…     

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.     

Crystallization Elution Fractionation of LLDPEs Made with Metallocene Catalysts

Summary: Temperature rising elution fractionation (TREF) and crystallization analysis fractionation (CRYSTAF) fractionate semicrystalline polymers according to their crystallizabilities from dilute solution and have been widely used to measure the CCD of LLDPE. A new fractionation technique, known as crystallization elution fractionation (CEF), has been developed recently. The main difference between CEF and TREF and CRYSTAF is that the crystallization cycle in CEF is performed dynamically under solvent flow in a long column that contains an inert support material. In this paper, several metallocene-LLDPE resins have been analyzed by CEF to investigate the effect of cooling cycle parameters, comonomer fraction, polymer molecular weight, and blend cocrystallization on the fractionation. This new technique can be used to obtain CCDs with better resolution and in shorter times than TREF and CRYSTAF.     

Application of Fractionation Techniques to the Study of Olefin Polymerization Kinetics and Polymer Degradation

Summary: Temperature rising elution fractionation (TREF) has been regarded as a powerful technique for study of semicrystalline polymers. In this paper, two examples of unique applications of TREF were introduced. One was the study on the influence of extraction of internal donor on the variation of isospecific active sites of a MgCl₂- supported Ziegler catalyst, and the other was the estimation of the relationship between polymer micro-tacticity and degradation rate of isotactic polypropylene (iPP). The former example revealed the conversion from high to low isospecific site by the extraction of internal donors, whereas the latter showed a negative correlation between the level of isotacticity and the degradation rate. These results demonstrated that TREF was useful in these research applications.     

结晶分级技术在支化聚乙烯研究中的应用

介绍了近年发展起来的几种结晶分级技术及其在支化聚乙烯结构表征及性能研究方面的应用。利用升温淋洗分级技术(TREF),可根据结晶特性的不同将高分子分离成多个分布较窄的级份,通过分别表征各级份的链结构,从而可获得高分子链结构方面较为准确的信息。基于差示扫描量热技术(DSC)发展起来的两类热分级技术,主要包括逐步结晶热分级(SC)和连续自成核退火分级(SSA)技术,虽然不能从物理上对高分子进行分级,但通过选择适当的操作参数,也能得到一系列与升温淋洗分级实验类似的链结构信息,并且具有设备简单、操作方便、样品用量少、耗时短等优点。本文结合我们自己的工作,对各种分级技术的原理、实验操作及应用进行了系统综述,并展望了结晶分级技术发展的某些可能趋势。     

Updated “Onion Skin” Model for Temperature Rising Elution Fractionation

Recent developments on the temperature rising elution fractionation (TREF) technique, understanding the impact strength of polyethylene blends based on their chemical structure, as well as ongoing discussions on REACH legislation regarding the oligomer fraction of polymers, are all reasons for better comprehension of the separation mechanism in TREF. To achieve this goal, two carefully chosen blends of linear metallocene polyethylene were analyzed by TREF over a large domain of crystallization rates. The results allowed updating the “onion skin” model for the crystallization kinetics during the cooling step of TREF. The advantages and limitations of the TREF technique for different applications are discussed.     

茂金属聚乙烯的短链支化结构不均匀性研究进展

茂金属支化聚乙烯技术是聚烯烃工业发展史上最重要的技术进展之一,该类产品具有优异的抗穿刺、抗撕裂、抗冲击性质,在薄膜、重包装等领域具有广泛的用途,其力学性能与共聚单体的种类、含量及其在分子链上的分布有密切关系。本文从催化剂活性中心特点和聚合反应工艺条件两方面对聚合物结构的影响出发,阐述了茂金属聚乙烯短链支化结构不均匀性产生的原因,通过核磁共振、升温淋洗分级和热分级等表征支化不均匀性的研究方法,介绍了茂金属支化聚乙烯分子主链上共聚单体的序列结构组成及分布,以及共聚物的结晶性能等方面的差异。     

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     

Study of the compositional heterogeneity of ethylene–hexene-1 copolymers by thermal fractionation technique by means of differential scanning calorimetry

The successive self-nucleation/annealing technique (SSA) by differential scanning calorimetry has been applied to study the heterogeneity of ethylene–hexene-1 copolymers produced with supported catalytic systems of different compositions: highly active supported Ziegler–Natta (Z–N) catalysts—a titanium–magnesium catalyst TiCl₄/MgCl₂ (TMC) and a vanadium–magnesium catalyst VCl₄/MgCl₂ (VMC), a supported zirconocene catalyst Me₂Si(Ind)₂ZrCl₂/SiO₂ (MAO), and a chromium-oxide catalyst CrO₃/SiO₂. Comparative data by SSA technique with the same temperature program were obtained for copolymers differed by MWD from narrow to very broad (Mw/Mn = 2.4–54) and short chain branching distribution from narrow (zirconocene catalyst) to very broad (TMC and chromium oxide catalysts). It is demonstrated that copolymers produced with the zirconocene catalyst have the narrowest melting range and do not contain thick lamellae. The widest lamella thickness distribution has been found for a copolymer produced with the chromium-oxide catalyst. Copolymers produced with the supported Z–N catalysts are ranked in the middle with a more narrow lamella thickness distribution for copolymer prepared with VMC as compared with the one produced with TMC. The SSA results are compared with the data on copolymer fractionation by TREF. It is shown that these methods give a good correlation for copolymers with narrow short-chain branching distribution produced with the supported zirconocene catalyst. In the case of copolymers produced with TMC, TREF yields a higher content of the high-branched fractions.     

Active Sites and Mechanisms for Direct Oxidation of Benzene to Phenol over Carbon Catalysts

The direct oxidation of benzene to phenol with H₂O₂ as the oxidizer, which is regarded as an environmentally friendly process, can be efficiently catalyzed by carbon catalysts. However, the detailed roles of carbon catalysts, especially what is the active site, are still a topic of debate controversy. Herein, we present a fundamental consideration of possible mechanisms for this oxidation reaction by using small molecular model catalysts, Raman spectra, static secondary ion mass spectroscopy (SIMS), DFT calculations, quasi in situ ATR‐IR and UV spectra. Our study indicates that the defects, being favorable for the formation of active oxygen species, are the active sites for this oxidation reaction. Furthermore, one type of active defect, namely the armchair configuration defect was successfully identified.     

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     

Site‐Selective Acylations with Tailor‐Made Catalysts

The acylation of alcohols catalyzed by N,N‐dimethylamino pyridine (DMAP) is, despite its widespread use, sometimes confronted with substrate‐specific problems: For example, target compounds with multiple hydroxy groups may show insufficient selectivity for one hydroxyl, and the resulting product mixtures are hardly separable. Here we describe a concept that aims at tailor‐made catalysts for the site‐specific acylation. To this end, we introduce a catalyst library where each entry is constructed by connecting a variable and readily tuned peptide scaffold with a catalytically active unit based on DMAP. For selected examples, we demonstrate how library screening leads to the identification of optimized catalysts, and the substrates of interest can be converted with a markedly enhanced site‐selectivity compared with only DMAP. Furthermore, substrate‐optimized catalysts of this type can be used to selectively convert “their” substrate in the presence of structurally similar compounds, an important requisite for reactions with mixtures of substances.     

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.