Molecular Weight and Branching Distribution of a High Performance Metallocene Ethylene 1‐Hexene Copolymer Film‐Grade Resin

Abstract

The bivariate, or cross branching distribution of a gas-phase produced, film-grade ethylene 1-hexene copolymer with enhanced Elmendorf tear in machine direction, MD, and in transverse direction, TD, (> 400 g/mil) and high dart impact has been characterized through the analysis of fractions obtained by molecular weight and 1-hexene composition. The molecular weight fractions, obtained by a solventnon-solvent fractionation technique, are each mixtures of molecules with at least two different 1-hexene compositions, one component with a constant relatively high density ($1 mol% hexene) and a second of a lower density broadly distributed along the molecular weight fractions. The content of the low density component increases with increasing molecular weight of the fraction while the level of 1-hexene decreases. The mixed compositional character of these fractions is easily inferred by their high crystallization rates and both high melting and crystallization temperatures compared to the values of model random ethylene copolymers. The set of compositional fractions obtained by TREF display an increasing 1-hexene concentration with increasing molecular weight, and except for the highest molecular weight components (Mw > 150,000 g/mol) their melting and crystallization behavior followed the random pattern. Higher than expected melting temperatures and a constancy of the high melting temperature peak with increasing crystallization temperature, suggests that the intra-chain 1-hexene distribution of the highly branched, high molecular weight fraction deviates strongly from the random behavior. These structural features and the bimodal character of the composition distribution of this resin, that contains high molecular weight chains with both low and high 1-hexene contents, are correlated with the enhanced key film properties.

Effect of Level of Crystallinity on Melt Memory Above the Equilibrium Melting Temperature in a Random Ethylene 1-Butene Copolymer

“…In this path, fast crystallization is important to ensure stability of the material during and after processing, and external nucleants are often added to decrease the free energy barrier for nucleation and increase crystallization rate. It is known that the crystallization of ethylene 1‐alkyl copolymers (LLDPE) depends strongly on the content and distribution of the comonomer, both inter and intramolecularly . Furthermore, we have shown recently that the state of the melt also affects greatly the crystallization rate of random copolymers, even at very high temperatures, such as those well above the equilibrium melting point of the copolymers .…”

Recent Advances in High-Temperature Fractionation of Polyolefins

“…The proposed approach was validated successfully with model ethylene/1-butene and ethylene/ 1-octene copolymers. Alamo and co-workers [41] studied the effects of molar mass and branching distribution on mechanical properties of ethylene/1-hexene copolymer film grade resins produced by a metallocene catalyst. Molar mass fractions were obtained by solvent/non-solvent techniques while P-TREF was used for fractionation according to the 1-hexene content.…”

Effect of Heterogeneous Short Chain Branching Distribution on Acceleration or Retardation of the Rate of Crystallization from Melts of Ethylene Copolymers Synthesized with Ziegler‐Natta Catalysts

“…In fact, enhanced performance in tear and impact is usually reached by LLDPEs with complex and broad bivariate distributions. [7,8] A recent work compared the effect of melt memory on crystallization of narrow and broadly distributed commercial LLDPEs, [6] and found a remarkable behavior of the latter. The narrow LLDPEs displayed a strong memory of crystallization and enhanced crystallization rates from temperatures (T melt ) above the equilibrium melting point akin to the behavior of model ethylene 1-butene random copolymers.…”

“…[6,7] It is known from early work that blends of ethylene copolymers with a difference in comonomer content > 8 mol % phase separate in the melt. [9,[14][15][16][17] The phase diagram shows an upper critical solution temperature (UCST) at $ 150 C which is the temperature between regions B and C in Figure 6.…”