Temperature Rising Elution Fractionation Research Articles

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.

A novel approach for accurate determination of polyethylene and polypropylene content in polyolefin blends and recyclates by cross-fractionation chromatography

The accurate characterization of post-consumer recycled (PCR) polyethylene (PE) and polypropylene (PP) blends is essential for repurposing them into higher-demand applications. This study improves the accuracy of the Temperature Rising Elution Fractionation coupled with Gel Permeation Chromatography (TREFxGPC) approach for both virgin and post-consumer recycled PP/PE blends. Model blends and virgin polymers were used to estimate polymer-specific IR-detector response factors. Moreover, a correction for the molecular weight-dependent amount of PP eluting in the TREF temperature range characteristic of PE was found. Accounting for these factors improved the results of the method, reducing the error in composition determination from 8 % to 2 %. Therefore, our approach provides better quantitative assessment capabilities for recycled blends. Furthermore, the refined TREFxGPC not only reliably determines the PCR PP/PE blend composition but also reveals their bivariate molecular weight and chemical composition distribution (MWD-CCD). Understanding the full microstructure of the blend can lead to superior recycled products, enabling the use of PCR in more demanding applications.

20 Years of polyolefin HPLC: Accomplishments and challenges

The first publication on the interaction of polyolefins with stationary phases for high performance liquid chromatography at high temperatures (HT-HPLC) appeared in 2003. After the successful application of temperature rising elution fractionation and the introduction of crystallization fractionation for the analysis of polyolefins with regard to chemical composition this was a step towards methods for chemical composition analysis that are not based on crystallizability of the polyolefin. Polyolefins as semicrystalline polymers comprise crystallizable and amorphous components, and HT-HPLC opened the way to separate all sample components irrespective of their crystallizability. The first application of this new approach was the separation of ethylene–methyl methacrylate block copolymers, presented in 2005. The development of the first commercial HT-HPLC instrument and the discovery of graphitic materials as efficient stationary phases for HT-HPLC were further important milestones in the development of advanced separation techniques for complex polyolefins. The ultimate goal of comprehensively analyzing complex polyolefins by composition and molar mass was achieved by high temperature two-dimensional chromatography, which is based on the online coupling of HT-HPLC and HT-SEC. Further important developments include HT-HPLC using temperature gradients and the combination of solvent and temperature gradients. In this review, the most important developments in the field over the last 20 years are discussed, and different techniques, experimental protocols and some major applications are presented. In conclusion, challenges for future developments are outlined.

Influence of Cooling Rate on the Temperature Rising Elution Fractionation of One Poly(1‐butene‐co‐ethylene) Resin and Microstructure of Its Fractions

AbstractTREF (temperature rising elution fractionation) is a method to separate resins based on crystallizability. The influence of TREF cooling rate (V0) on the fractionation process is still unclear. Herein, a poly(1‐butene‐co‐ethylene) resin is fractionated by the TREF method at three different cooling rates (1, 2, and 4 °C h−1), and then the fractions are analyzed by high‐temperature gel permeation chromatography (HT‐GPC), differential scanning calorimetry (DSC), and 13C‐nuclear magnetic resonance spectroscopy (13C‐NMR). The cooling rate V0 affects the crystallization process of the PB‐1 resin, resulting in different fraction contents at the same elution temperature. Furthermore, the cooling rate also affects the chain microstructure of fractions obtained from the same elution temperatures, i.e., the content of 1‐butene and ethylene monomer units, average sequence length and isotacticity. The lower cooling rate is beneficial to the effective separation of the fractions with weak crystallizability. These results have important guidance for optimizing fractionation conditions and obtaining different narrow distribution fractions of poly(1‐butene) (PB‐1) resin.

Crystallization, structure and properties of PP-b-PE block copolymer

In recent years, a commercialized polypropylene (PP) and polyethylene (PE) block copolymer has been synthesized using chain-shuttling polymerization, but its molecular structure and properties are still limitedly understood. In this work, we analyzed the chain composition, mechanical properties, and structural transformations occurring during the deformation of commercial PP-PE block copolymer obtained via chain-shuttle technology. The chain composition and crystallization properties of PP-PE block copolymers obtained by chain shuttling technique were analyzed using nuclear magnetic resonance (NMR), temperature-rising elution fractionation (TREF), and differential scanning calorimetry (DSC) techniques. The sample has a mass ratio of 49:51 of PE chains to PP chains, but the crystallinity of the two components differs significantly. The PP displays limited crystallinity, indicating short PP chain segments. Some PE sequences are incorporated into the PP matrix as random copolymers, resulting in elastomeric behavior. We investigated the In-situ structural changes of crystals during stretching using synchrotron radiation 2D X-ray diffraction/scattering. In the elastic deformation region, the strain has a significant influence on the orientation of PE crystals but has little effect on the orientation of PP crystals. After the yield point, the strain-induced PE crystal failure resulting in a decrease in stress. In the strain hardening region, the orientation of PE induces recrystallization, forming a layered structure with tensile orientation. At the same time, the PP crystals are disoriented under the action of strain until they break. The results from this work provide significant insight into the design of PP-PE block copolymer products to meet the performance requirements needed for engineering applications.

Separation of amorphous and semicrystalline parts of LLDPE with high‐temperature interactive liquid chromatography

AbstractLinear low‐density polyethylene (LLDPE) materials may contain components, which are soluble at room temperature. These may influence physical properties or raise concerns for end‐users and regulatory bodies because LLDPE is widely used for food packaging. Due to the nonpolar and amorphous nature of the room temperature soluble part of LLDPE, its molecular characterization with common methods for polyolefin analysis, such as Temperature Rising Elution Fractionation (TREF), Crystallization Fractionation (CRYSTAF), or Differential Scanning Calorimetry (DSC), has proven to be very challenging. Separation of the amorphous part of LLDPE, as well as its semicrystalline part, is, however, realizable with high‐temperature liquid chromatography (HT‐LC), which is demonstrated for the first time in this work. This technique enables to evaluate the chemical composition distribution of the room‐temperature solubles and non‐solubles in LLDPE samples. It was found that LLDPE synthesized with metallocene catalysts contained only a very small amount of amorphous material. In contrast, LLDPE prepared with Ziegler‐Natta catalysts contained up to 14 wt.% of the amorphous copolymer. The latter was soluble at room temperature, had a weight‐average molar mass ranging from 60 to 200 kg/mol, and contained about 10 mol% more 1‐olefin than the non‐soluble part of LLDPE.

Chain microstructure comparison of two transparent polypropylene resins fractionated by temperature rising elution fractionation

Chain microstructure comparison of two transparent polypropylene resins fractionated by temperature rising elution fractionation

Enhanced characterization of the chemical structure of high-density polyethylene by size exclusion/thermal fractionation on poly(styrene-co-divinylbenzene) columns

Following the recent trends in polymer analysis, a novel fractionation method was developed using xylene as an alternative to trichlorobenzene (TCB) and offering a more exhaustive characterization of the chemical structure of high-density polyethylene (HDPE). The method is based on a linearized evaporative light scattering detector (LinELSD), which has a much better signal-to-noise for the solutions of HDPE in xylene, as compared with the traditional differential refractive index (DRI) and infrared (IR) detectors. The low viscosity of xylene opens the possibility to use the poly(styrene-co-divinylbenzene) (PS-DVB) columns not only for gel permeation chromatography (GPC) but also for temperature-rising elution fractionation (TREF). An immediate application is the simultaneous measurement of the wax fraction’s content and molecular weight (MW) in HDPE. Furthermore, there are strong indications that the thermogram profile corresponding to the isothermal extraction at 70 °C provides a fingerprint of the HDPE synthesis catalyst.

Thermal Fractionation of Polyolefins: Brief History, New Developments and Future Perspective

For semi-crystalline polymer materials, the difference in chain structure often leads to different physical properties; therefore, in-depth analysis of the chain structure is of great significance. With the continuous development of advanced instruments, many research means have emerged to characterize the structure of molecular chains. Among them, fractionation techniques provide effectively structural information on inter- and intra-molecular comonomer distribution, branching degree, and sequence length, etc. This work briefly presents the history of developments of various classical fractionation means such as temperature-rising elution fractionation, stepwise crystallization and successive self-nucleation and annealing, while focusing on the present and future of their applications.

Effect of the pore architecture of Ziegler-Natta catalyst on its behavior in propylene/1-hexene copolymerization

The role of the pore architecture of the Ziegler-Natta (ZN) catalysts in the propylene polymerizations is unveiled, towards the catalyst activity, H2– and comonomer-response, and final properties of the synthesized polymer. In this regard, two commercially available ZN catalysts (Cat-A and Cat-B belong to the 4th generation, containing diisobutyl phthalate as internal donor) with a similar composition but with different pore structures were employed in propylene/1-hexene copolymerizations in the presence of H2 as a chain transfer agent. Although pore diameter does not alter H2-response noticeably, it significantly affects the comonomer incorporation as well as the distribution of the isotactic sequence length in the copolymer backbone. Cat-A with a lower surface area, a larger pore diameter, and a wider inlet showed slightly higher activities in homopolymerization and copolymerization experiments. According to the self-nucleation and annealing (SSA) results, the copolymer produced from Cat-A demonstrated lower isotactic sequence length and lamellar thickness contents owing to its larger pore diameter with wider inlets. Moreover, Temperature Rising Elution Fractionation (TREF) profiles asserted SSA data in which the copolymer from Cat-A represented a higher content of low elution temperatures fractions, due to its higher comonomer content. These findings will lead to catalysts for different polypropylene grades.

Cocrystallization of Polyethylene Blends during TREF–GPC Cross‐Fractionation Characterization

AbstractCross‐fractionation characterization (CFC) between temperature rising elution fractionation (TREF) and gel permeation chromatography (GPC) is a technique developed to determine the bivariate molecular weight and chemical composition distribution (MWDxCCD), a unique and detailed microstructural fingerprint of polyolefin. Like other crystallization‐based characterization techniques, CFC may suffer from cocrystallization effects during analysis, which can mislead the interpretation of polymer microstructures. However, cocrystallization phenomena during CFC analysis are not investigated, especially for industrial polymers. In this study, the cocrystallization effect during CFC analysis is systematically investigated using two series of binary blends based on four individual polyethylene samples. These individual samples have distinct microstructural characteristics, representing polyethylene produced using different polymerization processes and catalytic systems. The effects of microstructural characteristics and blending ratio on cocrystallization level are examined. When the CFC profiles of individual components do not overlap, a high level of cocrystallization can be observed. Furthermore, the behavior of how the blending ratio affects cocrystallization depends on the characteristics of CFC profiles of individual component and their interactions. The level of cocrystallization in binary blends increases with the weight fraction of the component having microstructures eluted in the cocrystallization temperature range.

Stretchability and Properties of Biaxially Oriented Polypropylene Film

Abstract The double bubble tubular film (DBTF) process is a more economical way of producing biaxially oriented film, when compared to the tenter biaxially oriented film process. This film manufacturing technique has been widely used to produce biaxially oriented films because of good shrinkability and high physical properties. Recently, shrinkage film for this usage is required to have good stretchability and shrinkage strength. But the biaxial stretchability changes of polypropylenes and the reasons for such have not been reported upon systematically. In this report, the relationship between stretchability and material design of various polypropylenes (PP) for biaxially oriented film, which are most popular polymers, was investigated. Several different ethylene contents and melt flow indices (MI) of PP were examined. There was a correlation between MI, ethylene content and the stretchability of the DBTF. A random copolymer with MI 2 g/10 min and ethylene content 4.0 wt.% gave good stretchability. The most suitable composition distribution for stretching and the properties of biaxially oriented films were studied by using the temperature rising elution fractionation.

Fast successive self‐nucleation and annealing (SSA) thermal fractionation protocol for the characterization of polyolefin blends from mechanical recycling

AbstractThe sorting stage of mechanical recycling of post‐consumer polyolefins has severe challenges. Polypropylene (PP) is often contaminated with polyethylene (PE) and vice versa. To meet quality requirements, characterization of the recycled pellets is needed. To address this problem, fast characterization generating a statistical assessment of the content of the various batches from recycling is required. This investigation shows that the use of fast scanning rates (in a conventional Differential Scanning Calorimeter) in the successive self‐nucleation and annealing (SSA) protocol can reduce the thermal fractionation time, without losing resolution power, as long as the increase in heating/cooling rate is compensated by reducing sample mass. Using a “coupled SSA protocol” for polypropylene and polyethylene fractions at a rate of 10 °C/min, the measurement time is approximately 420 min. Implementing mass compensation, faster heating rates (i.e., 30 °C/min) and using a single‐fraction protocol, sufficient to determine the content of PP and high‐density PE, reduced the time of the measurement to 75 min. Examples of fractionations of commercial post‐consumer and post‐industrial recycled polyolefin blends conducted at a faster rate are provided. The derived polyolefin content is compared with the standard temperature rising elution fractionation analysis to assess the validity of the proposed method.

Development of a robust calibration method for improving the long-term precision of polyethylene comonomer content distribution using crystallization elution fractionation

Comonomer content distribution (CCD), also commonly known as chemical composition distribution (CCD) and short chain branching distribution (SCBD), describes the variation of short chain branching composition between individual polymer chains in polyolefin materials. It is of particular importance for controlling polyolefin performance. Crystallization-based separation methods have evolved over the past four decades aiming at resolution, speed, precision, and accuracy. Two of the commonly used techniques are Crystallization Elution Fractionation (CEF) and Temperature Rising Elution Fractionation (TREF), where polymer chains are physically separated along the column or on the surface of the support based on their crystallinity, respectively. CEF analysis takes much less time than TREF. There is a critical need for precise temperature calibrations for data repeatability. This report demonstrates a novel CEF methodology using a two-point technique to consistently and conveniently calibrate the comonomer composition and column temperature. This column temperature calibration methodology was adopted in a study by tracking the reproducibility over a period of 8 years, using multiple instruments located in different laboratories and in different geographies. The results exhibited superior repeatability, with less than 0.3% of the relative error calculated from 3000 data points of the eluting peak temperature, thus demonstrating this as a robust method for industrial labs that require good quality controls.

A new GPC/TREF/LinELSD instrument to determine the molecular weight distribution and chemical composition: application to recycled polymers

The linearized evaporative light scattering detector (LinELSD) incorporates an inverse power function facilitating the characterization of polymers in solution. To show the benefits of the LinELSD, a versatile liquid chromatograph was built by combining a high temperature gel permeation chromatograph (HT-GPC), a fast cooling/heating oven of a gas chromatograph (GC), and the LinELSD. Apart from the main GPC functionality to measure the molecular weight distribution (MWD) of polymers, the injected samples can also be eluted as a function of polymer type by applying a temperature rising elution fractionation (TREF) program on the column placed in the GC oven. The hybrid GPC/TREF/LinELSD instrument removes the limitations of the commercial GPC and TREF systems manufactured with differential refractive index (DRI) or infrared (IR) detectors, which require a sensible difference between the refractive indices of polymer and solvent, or the absence of CH2 and CH3 groups in the solvent. Thus, the new instrument qualifies to characterize the chemical structure of recycled polymers, demanding more flexible analytical conditions than those previously developed for the virgin resin analysis. Moreover, the hybrid instrument allows to investigate polyolefin solutions in xylene and toluene, which are the preferred solvents for the selective extraction of high-performance polymers from mixed plastic waste, thus qualifying as a unique analytical solution to assess the emergent solvent-based purification industrial processes.

Engineered PP impact copolymers in a single reactor as efficient method for determining their structure and properties

Impact polypropylene (PP) copolymers (IPCs) are important materials for many commercial applications. These materials are usually synthetized through different methods involving two consecutive reactions in the same phase or in different phases. Here, a laboratory-scale synthesis method based on a sequential liquid- and gas-phase two-step process in a single reactor is developed. Propylene homopolymers and IPCs were synthesized with varying amounts of comonomers and hydrogen. The IPC materials obtained were fully characterized via analytical temperature rising elution fractionation (TREF), differential scanning calorimetry (DSC), 13C nuclear magnetic resonance (13C NMR), gel permeation chromatography with an infrared detector (GPC-IR5), Charpy impact, scanning electron microscopy (SEM), and cross-fractionation chromatography (CFC). The addition of only ethylene to the second step in the absence of hydrogen led to the creation of an ethylene-propylene (EP) copolymer with similar impact strength to that of a propylene homopolymer. The addition of hydrogen to the first step dramatically shortened the length of the PP chains and inhibited catalytic active centers that led to EP copolymer synthesis. This material exhibited very low molecular weight, low ethylene incorporation, and rubbery phases irregularly distributed along the isotactic polypropylene (iPP) matrix, resulting in the formation of an EP copolymer material with poor impact properties. IPCs synthesized without hydrogen and with a 50/50 (v/v) mixture of propylene/ethylene monomers in the second step enhance ethylene incorporation, facilitating adequate homogeneous and heterogeneous ethylene distribution and resulting in a high increment of amorphous ethylene-propylene-rubber (EPR) domains, which remarkably improves impact properties. Additionally, a criterion based on the ratio between EEE and EPE + PEP triads ranging between 1 and 2 was also established to predict the impact resistance of any heterophasic PP. Fractionation of the optimal sample provided a detailed understanding about the microstructure of this copolymer through the study of the molecular weight and composition of the fractions via GPC-IR, analytical TREF, and DSC measurements. Finally, the liquid–gas-phase two-step IPC material was compared, by means of SEM and CFC measurements, with synthesized IPC using liquid–liquid-phase two-step polymerization, and the results showed that the range of EP composition as well as ethylene distribution in the molar mass molecules of the IPCs was correlated to their mechanical behavior. This proves that crystalline families composed of high-molecular-weight EP copolymers in the liquid–gas-phase process can act as a compatibilizing agent between the iPP matrix and the elastomeric rubbery phase, allowing one to improve the impact resistance of the IPC, more so than that of IPCs obtained in the gas–gas and liquid–liquid phases. The results indicate that the synthesis of IPC resins in a single reactor is an efficient experimental method for fundamental research on IPCs.

Quantification of PP contamination in recycled PE by TREF analysis for improved the quality and circularity of plastics

Polyethylene constitutes more than one-third of the world's plastic and is one of the main components in municipal waste streams. Unfortunately, varying amounts of polypropylene (PP) are usually found in recycled high-density polyethylene (rHDPE), which erodes their mechanical properties and reduces the quality of these recyclates and their possible reuse in the industry. Differential Scanning Calorimetry (DSC) is the technique commonly used to detect and determine the presence of PP in these blends, but intrinsic limitations of this method lead to inaccurate results. Thus, alternative techniques must be employed to solve these drawbacks and help to estimate more accurately the levels of the PP impurities in rHDPE. In this work, crystallization techniques, such as Temperature Rising Elution Fractionation (TREF) is used to fractionate PE and PP blends into their components to obtain quantitative information. Municipal waste samples of rHDPE contaminated with different amounts of PP have been characterized by DSC and TREF. Comparative results of both techniques show an underestimation of PP values when DSC is used as analytical technique. Results prove that TREF analysis irrespective of the crystallization rate used is valid for the identification and quantification of PP impurities in rHDPE samples, even at low PP percentages in the blend lower than 2 wt. %, which could help to control the quality of rHDPE and achieve the priority of reusing and recycling adopted by the European Union for a Circular Economy of Plastics.

Crystallization kinetics, crystalline structures and properties of PB/PP blends regulated by poly(butene-block-propylene) copolymers

Blending isotactic polybutene-1 (PB) with isotactic polypropylene (PP) is a facial strategy to improve the mechanical properties and promote polymorphous transformation of PB. As the immiscible polymer pairs with strong crystallizability, the interactions between PP and PB in the melt state influence the subsequent processes like phase separation and crystallization, and then determine the crystalline structures and properties of the blends. Herein, two kinds of poly(butene-block-propylene) (PB-b-PP) copolymers with different sequence structures fractionated from PB/PP in-reactor alloy using temperature rising elution fractionation were utilized to compatibilize the PB/PP (80/20) blend. The phase morphologies, crystallization kinetics of PP and PB phases, and crystalline structures in PB/PP/PB-b-PP blends were characterized by phase contrast optical microscopy (PCOM), differential scanning calorimeter (DSC), and small-angle X-ray scattering (SAXS). The PB-b-PP copolymer with longer PP blocks (A) showed more satisfied compatibilization for PB/PP blends and PB/PP/PB-b-PP-2.5 blend with 2.5 wt% copolymer as compatibilizer exhibited the most stable phase morphology with the smallest PP domains in melt state at 200 °C. During isothermal crystallization processes of the PB/PP/PB-b-PP blends at 135 °C for PP crystallization and then 90 °C for PB crystallization, the crystallization rate including nucleation rate and nucleation density, and crystallinity of PP and PB (form II) increased, and achieved the largest when the copolymer concentration was 2.5 wt% in PB/PP/PB-b-PP blend. The copolymer A significantly accelerated crystallization rate of PP compared with that of copolymer B. The PB/PP/A-2.5 blend with the largest lamellae thickness and highest crystallinity of PB form I, combined with the smallest spherulites, exhibited the highest modulus, tensile strength and toughness. The compatibilizing mechanism and roles of PB-b-PP copolymers for PB/PP blends were discussed to understand the physical and mechanical behaviors of the blends.

Influence of isothermal crystallization temperature on the temperature rising elution fractionation for a poly(1-butene-co-ethylene) resin

Temperature Rising Elution Fractionation (TREF) is effective for separating polyolefin resins according to crystallizability. A relatively low isothermal crystallization temperature is necessary to fractionate poly(1-butene) (PB-1) resin because of its slow crystallization kinetics. This study aims to investigate the influence of isothermal crystallization temperature of TREF on the fractionation and chain microstructure of eluted fractions for one poly(1-butene-co-ethylene) with 5.8 mol% ethylene. The resin is fractionated using P-TREF under four different isothermal crystallization temperatures, i.e. 0 °C, −5 °C, −10 °C, and −30 °C, and then the fractions are characterized by high-temperature gel permeation chromatography (HT-GPC), 13C nuclear magnetic resonance spectroscopy (13C NMR), and differential scanning calorimetry (DSC) to acquire their chain microstructure information. The isothermal crystallization temperature significantly affects the nucleation and growth of the crystals, which consequently results in differences in the relative content of eluted fractions. With the decrease of isothermal crystallization temperature from 0 °C to −30 °C, the copolymer is fractionated more effectively since fractions are more evenly distributed among elution temperatures rather than concentrated at lower elution temperatures. Furthermore, the isothermal crystallization temperature influences the chain microstructure of fractions obtained at the same elution temperature, which are different in ethylene content, isotacticity, and average sequence length. The distribution of ethylene content and isotacticity becomes broader with the decrease in isothermal crystallization temperature. Lower isothermal crystallization temperature is conducive to effective separation of the fractions with weak crystallizability. These results are of great significance for the selection of fractionation conditions and chain microstructure analysis of poly(1-butene-co-ethylene) resins.

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.