Perfectly Controlled Lamella Thickness and Thickness Distribution: A Morphological Study on ADMET Polyolefins

Abstract

Lamella thickness distribution (LTD) plays a critical role in determining the mechanical properties of polyethylene. LTD is predominantly governed by the intermolecular chemical composition distribution, but intrachain heterogeneity also results in a broadened LTD. Polyethylene synthesized by acyclic diene metathesis (ADMET) contains pristine microstructures free from inter and intrachain heterogeneity and therefore represent ideal models to investigate these phenomena. The crystalline structures of ADMET polyethylene with ethyl or n-hexyl branches every 21 st backbone carbon (EB21and EO21, respectively) were characterized by transmission electron microscopy (TEM), small X-ray scattering and wide angle X-ray diffraction (SAXS and WAXD), and differential scanning calorimetry (DSC). The samples were crystallized for various periods at temperatures near the DSC crystallization peak temperatures: 10 8C for EB21 and 0 8C for EO21. TEM observation exhibited that EB21 displays straight lamellar crystals with axialitic organization and an average thickness of about 55 Å. This corresponds to twice the ethylene sequence length between branches, suggesting that one lamellar stem spans three branches and includes one ethyl branch within the lamella. The lamella thickness distribution was very narrow compared with that of the cross-fraction of ethylene/1-butene copolymer prepared via Ziegler-Natta polymerization. Similarly it was found from the same characterization methods that EO21 also displays a narrow lamella thickness distribution albeit with thinner lamellae, averaging 25-26Å thick. Judging from this lamella thickness, EO21 is considered to have a lamella stem composed of a single ethylene sequence between two braches, suggesting that the n-hexyl branch is entirely excluded from a crystalline phase.

(nBuCp)2ZrCl2‐catalyzed ethylene‐4M1P copolymerization: Copolymer backbone structure, melt behavior, and crystallization

“…The [ECC] and [CCC] values (where C 5 4M1P) of Table 2 as well as the literature support this conclusion. 31,[88][89][90][91] Note that the formation of such amorphous and crystalline phases influences the copolymer mechanical and various end-use properties. In parallel, Figure 6 summarizes how Gibbs-Thompson (GT) equation successfully modeled the combined influence of 4M1P-induced interchain and intrachain copolymer composition distributions on the resulting polyethylene bulk crystallinity.…”

Systematic Studies of Morphological Changes of Precision Polyethylene

“…Precision polyethylene, bearing butyl groups on every 39th carbon ( Bu‐39 ), displays a narrow lamellar thickness distribution relative to LLDPE (thicknesses between 38 and 54 Å for Bu‐39 and 46 and 94 Å for a polymer with random butyl branches, Bu‐random ) . Likewise, Et‐21 and Hex‐21 also have narrow lamellar thickness distributions . As mentioned above, Et‐21 forms lamellae with average thickness of about twice the distance between ethyl groups …”

Morphology control in precision polyolefins

“…From this work, we can conclude that melting point is determined by branch-to-branch distance-up to 75 carbons (Fig. 5) [27]. We believe chains fold to form lamellae devoid of large defects.…”

“…The crystalline structures of ADMET polyethylene with ethyl or n-hexyl branches were studied using TEM [27]. These measurements showed that the ethyl-branched sample displayed straight lamellar crystals with an average thickness of 55 Å .…”