By Zhe Zhou
Ethylene-hexene linear low-density polyethylene is widely used in packaging industry due to its outstanding mechanical properties. Some of them contain long chain branching (LCB) which improves viscoelastic properties. Therefore, it is important to detect and quantify LCB content to enable production of improved LLDPEs for targeted applications. 13C NMR is an intrinsically quantitative method and can measure LCB directly. [1] However, 13C NMR LCB measurement in ethylene-hexene LLDPE has challenges, such as ethylene-hexene-ethylene branch (EHE Br) peak overlap with the LCB methine peak in conventional solvents for dissolving ethylene-hexene LLDPE. We explored using 1-choloronaphthelane/para-dichlorobenzene-d4 (PDCB-d4) (9:1, w:w) as solvent [2] to separate the LCB methine peak from the EHE Br peak. Although excellent separation of the LCB methine peak from the EHE Br peak can be achieved on 600 and 700 MHz 10 mm NMR cryoprobes, the LCB methine peak overlaps with EHE Br peak’s downfield 13C satellite. Therefore, a new anti-incredible natural abundance double quantum transfer experiment (anti-INADEQUATE) inverse gated NMR pulse sequence, ainadigsp1d.2, was developed to remove 13C satellite peaks and quantify LCB content in ethylene-hexene LLDPE. [3] To improve sensitivity, we further developed a much more sensitive method “Refocused insensitive nuclei enhanced by polarization transfer-anti-incredible natural abundance double quantum transfer experiment (RINEPT-anti-INADEQUATE)” for LCB measurement. [4] This new method provides a 4.5 times sensitivity increase compared with the anti-INADEQUATE method. This means that a single day NMR measurement duration with the current RINEPT-anti-INADEQUATE method can achieve similar signal to noise ratio as 20 days NMR measurement time with the previous anti-INADEQUATE method. Combining either crossing point approach [5] or our simple scaling method, [6] quantitative LCB measurement can be achieved.
[1] W. DeGroot, D. Gillespie, R. Cong, Z. Zhou, R. Paradkar, “Molecular Structure Characterization of Polyethylene”, Handbook of Industrial Polyethylene and Technology, Scrivener Publishing LLC, 2017, 139.
[2] Z. Zhou, D. Baugh, P. P. Fontaine, Y. He, Z. Shi, S. Mukhopadhyay, R. Cong, B. Winniford, M. Miller, “Long-chain branch measurement in substantially linear ethylene polymers by 13C NMR with halogenated naphthalenes as solvents”, Macromolecules, 2017, 50, 7959.
[3] Z. Zhou, C. Anklin, R. Cong, X. Qiu, R. Kuemmerle, “Long-chain branch detection and quantification in ethylene-hexene LLDPE with 13C NMR”, Macromolecule, 2021, 54, 757.
[4] Z. Zhou, C. Anklin, R. Kuemmerle, R. Cong, X. Qiu, J. DeCesare, M. Kapur, R. Patel, “Very sensitive 13C NMR method for the detection and quantification of long-chain branches in ethylene-hexene LLDPE”, Macromolecule, 2021, 54, 5985.
[5] J. Hou, Y. He, X. Qiu, “Speedy, robust and quantitative analysis of polyolefins using sensitivity-enhanced 13C NMR spectroscopy”, Macromolecules, 2017, 50, 2407.
[6] Z. Zhou, S. Pesek, J. Klosin, M. Rosen, S. Mukhopadhyay, R. Cong, D. Baugh, B. Winniford, H. Brown, K. Xu, “Long chain branching detection and quantification in LDPE with special solvents, polarization transfer techniques, and inverse gated 13C NMR spectroscopy”, Macromolecules, 2018, 51, 8443.
Session #7: The tough stuff- measuring difficult things in soft materials