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

Abstract

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 13 C-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 broadrange 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.

Study on the water‐tree ageing characteristics of polyethylene/organic montmorillonite and crosslinked polyethylene/organic montmorillonite nanocomposites

“…Each SSA melting peak represents a set of methylene sequences of equal or similar length [31]. It can be seen from Figure 4 and Table 5 that the crystallisation behaviour, lamellar thickness, and distribution in PE and its nanocomposite before and after crosslinking are quite different.…”

Deformulation of commercial linear low‐density polyethylene resins by advanced fractionation and analysis

“…Accordingly, this technique is a selective fractionation technique for both crystalline and non‐crystalline materials . A more comprehensive approach for evaluating molecular heterogeneity in terms of CCD and MMD in complex polymer systems is to perform preparative fractionations as in preparative temperature rising elution fractionation (pTREF), solvent gradient fractionation and preparative molar mass fractionation (pMMF) . Fractions in milligram or gram quantities are obtained that differ in chemical composition (pTREF) and molar mass (pMMF).…”

Comprehensive branching analysis of polyethylene by combined fractionation and thermal analysis

“…A more comprehensive approach to evaluating molecular heterogeneity in terms of branching distribution, CCD and MMD in heterogeneous systems is to perform fractionations on a preparative scale as in preparative temperature rising elution fractionation (pTREF), solvent gradient fractionation and preparative molar mass fractionation (pMMF) . These techniques produce fractions in milligram or gram quantities that differ in branching (pTREF) and molar mass (pMMF) which can then be subjected to further analysis by NMR, FTIR or thermal analysis to provide in‐depth information on branching and molar mass …”