Temperature Rising Elution Fractionation Technique Research Articles

Determination of the Composition of LDPE/LLDPE Blends via 13C NMR

AbstractPolyethylene film properties can be manipulated by blending linear low‐density polyethylene (LLDPE) with low density polyethylene (LDPE) resin. The accurate determination of LDPE content in those blends, however, is challenging. Differential scanning calorimetry analysis has been traditionally used to determine the LDPE content, but the result can have significant error due to several limitations. Due to co‐crystallization, significant error can also result using temperature rising elution fractionation technique. In principle, the LDPE content would be determined by 13C NMR analysis if the short‐chain branch (SCB) content of the LDPE could be determined accurately. But unambiguous assignment of all peaks in the 13C NMR spectrum of a LDPE is difficult, if possible. Consequently, the SCB content of LDPE cannot be determined accurately. Therefore, a new approach is required to analyze the LDPE/LLDPE blend composition by 13C NMR on a routine basis. In this paper, a robust 13C NMR method to determine the LDPE content in LDPE/LLDPE blends is presented. By identifying unique LDPE peaks in the 13C NMR spectrum of a LDPE/LLDPE blend compared to that of LLDPE, the blend composition can be determined. The accuracy and precision of this method will also be discussed.

Chain structure comparison of two low density polyethylene resins fractionated by temperature rising elution fractionation and thermal fractionation

Two low-density polyethylene resins were fractionated into 11 fractions using the temperature rising elution fractionation technique. The resins were of injection molding grade with similar densities but different melt indexes and mechanical properties. High-temperature gel permeation chromatography (HT-GPC), 13C-nuclear magnetic resonance (13C-NMR) spectroscopy, differential scanning calorimetry (DSC), and successive self-nucleation/annealing (SSA) thermal fractionation were employed to characterize the original resins and their fractions. Resin A had a higher molecular weight and broader molecular weight distribution than resin B did. The highest contents were found for the 80 °C and 75 °C fractions of resins A and B, respectively. In addition, the highest molecular weights for resins A and B at 2.14 × 105 and 1.42 × 105 g/mol, respectively, were attained for the fractions eluted at 80 °C. According to the results of 13C-NMR, the total branching contents of resins A and B, including short- and long-chain branching, were 3.63 mol% and 4.89 mol%, respectively. DSC analysis revealed that the crystallinity of fractions 3–10 of resin A was lower than that of resin B. We noted the presence of approximately eight multiple melting peaks in the original sample and fractions from the results of SSA. These peaks indicated the inhomogeneity of the molecular chain. Lw and Ln of resin A were longer than those of resin B. Lastly, the relationship between chain structure and property was also discussed.

Influence of isotacticity and its distribution on degradation behavior of polypropylene

The influence of isotacticity and its distribution on the degradation behavior of polypropylene (PP) was examined on a series of PP fractions prepared with the aid of temperature rising elution fractionation technique. It was found that the length of the isotactic sequence predominantly decided the stability of PP fractions, where a faster propagation of oxidation occurred for PP chains with a longer isotactic sequence. Two PP fractions with different tacticity were mixed as the simplest model of polymer to examine the degradation behavior when the primary structure is distributed. It was found that even if two samples have the same average mesopentad, the details of the constituent differentiate their stability, which can be explained as a consequence of infectious degradation spreading.

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.

Structural evaluation of copolymers of ethylene and 1‐octadecene by using the temperature rising elution fractionation technique

Structural evaluation of copolymers of ethylene and 1‐octadecene by using the temperature rising elution fractionation technique

Structural evaluation of copolymers of ethylene and 1-octadecene by using the temperature rising elution fractionation technique

Three samples of ethylene-octadecene copolymers having different quantitative composition were analyzed structurally and in terms of their thermal behavior. The samples were fractionated by temperature rising elution fractionation, presenting different chemical composition distributions (CCD) that are essentially the result of the proportion of incorporated octadecene. The CCD profiles were relatively wide for samples generated by metallocene catalysts. The analyses of the fractions showed that the melting and crystallization temperatures decrease with increasing comonomer incorporation, but this relation is affected by the average molecular weight of the chains. The melting thermograms of those fractions having higher proportions of octadecene may be divided into two characteristic regions: the first one, at a higher temperature, originates from the melting of the least modified chains, which crystallize more perfectly. The second one is formed by the melting of chains having a high degree of comonomer incorporation, which melt in a diffuse manner over a wide range of temperatures. It is probable that the morphology of the crystals formed in this region does not follow the folded chain model, and are better represented by a model involving the alignment of chain segments (bundling). © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 221–227, 2001

Characterizing polyethylene‐based blends with temperature rising elution fractionation (TREF) techniques

AbstractThe temperature rising elution fractionation (TREF) technique makes use of the differential crystallizabilities, introduced by short chain branches (SCB), to fractionate polyethylenes (PE) on the basis of the SCB level of any one chain. This allows one to probe the SCB distribution of a polymer. The TREF technique has been applied to blends of PE by recognizing that the two commercial types of low density polyethylene, high pressure (HP‐LDPE) and linear low density (LLDPE), have different short chain branching distributions. Other common copolymers of ethylene, like ethylene vinyl acetate (EVA) or terpolymers like ethylene/propylene/hexadiene (EPDM), also have different SCB types, levels, and distributions and therefore have different TREF behavior. We have constructed a TREF apparatus that can be used in a number of different types of blend studies. The first is an Analytical‐TREF, which gives a profile of the amount of polymer with a given SCB level and can be used to quantify HP‐LDPE/LLDPE blends. We have also modified the technique to construct a stop‐flow TREF apparatus, which enables one to separate fractions on the basis of the branching levels. The fractions can then be analyzed by size exclusion chromatography, IR, 13C NMR, or DSC. The use of these TREF techniques to characterize blends of HP‐LDPE/LLDPE, LLDPE/EVA, PE/EPDM, and PE/polyisobutylene is discussed.

Determination of branching distributions in polyethylene and ethylene copolymers

AbstractA description is given of an analytical temperature‐rising elution fractionation (TREF) system for the purpose of determining short‐chain branching (SCB) or copolymer distributions in poly‐ethylenes and ethylene copolymers. The system achieves fractionation on the basis of crystallizability and is shown to be very little influenced by molecular weight in the normal high polymer range. Sample preparation by slow cooling from relatively dilute solution followed by continuous elution with a simultaneous and fairly rapid rate of temperature rise proves to be an efficient fractionation process. An on‐line detector and data system allows application of a calibration curve to give realistic SCB distribution data in a convenient manner. The potential value of the TREF technique for providing structural information is illustrated by examples which include low‐density high‐pressure resins made by both tubular and autoclave reactors, high‐ and low‐density resins made by low‐pressure processes, and copolymers of ethylene with vinyl acetate and ethyl acrylate.