Effect of chain microstructure on mechanical properties of metallocene LLDPE resins by using TREF×HT-GPC and TREF×SSA methods

Abstract

Analyzing commodity metallocene linear low-density polyethylene (mLLDPE) for its chain structure and mechanical performance is crucial for developing high-value polyethylene products. This paper concentrates on the cross-fractionation analysis of two mLLDPE samples (A and B) with similar comonomer incorporation, density, melting index, and molecular weight but different mechanical properties. The mLLDPE products were fractionated by preparative temperature rising elution fractionation (P-TREF), and the P-TREF fractions were further analyzed by successive self-nucleation and annealing (SSA) as well as the high temperature gel permeation chromatography (HT-GPC). The TREF × SSA and TREF × HT-GPC cross-fractionation techniques provided detailed information on molecular structure, especially the methylene sequence distribution and short-chain branch distribution. This elucidated the impact of internal chain structures on their mechanical properties. After TREF classification, sample B exhibited a much broader quality distribution than that of sample A. The TREF × HT-GPC results showed that the preference of the 1-hexene comonomer incorporation into long chains causes more entanglements in the inter-lamellar region and thus benefits sample B's elasticity and toughness. The TREF × SSA test revealed that the presence of more thick lamellae in sample A explained the larger bending modulus and higher stiffness. Three-dimensional maps of molecular weight, elution temperature, and relative concentration of all fractions were established using TREF data together with HT-GPC and SSA data, which directly visualized the subtle differences in the molecular microstructure between the two samples.