Long-Chain Branch Detection and Quantification in Ethylene–Hexene LLDPE with 13C NMR

Ethylene–hexene linear low-density polyethylene (LLDPE) is widely used in the packaging industry due to its outstanding mechanical properties. Some LLDPEs 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. However, 13C NMR LCB measurement in ethylene–hexene LLDPE has challenges, such as the overlap of the ethylene–hexene–ethylene branch (EHE Br) peak with the LCB methine peak in conventional solvents used for dissolving ethylene–hexene LLDPE. Recently a 1-choloronaphthelane/para-dichlorobenzene-d4 (PDCB-d4) (9:1, w/w) solvent was developed to separate the LCB methine peak from the ethylene–octene–ethylene branch (EOE Br) peak in an ethylene–octene copolymer. Here, we explore using this solvent for 13C NMR LCB measurement in ethylene–hexene LLDPE. Based on 13C NMR simulation, a high level of LCB in low hexene content LLDPE might be detected and measured (with some errors due to incomplete separation from the EHE Br peak) with 400 and 500 MHz 10 mm NMR cryoprobes. However, low sensitivity and LCB peak overlap with the EHE Br peak pose even more difficulties for detecting the low level LCB in high hexene content LLDPE. Although 600 and 700 MHz 10 mm NMR cryoprobes provide much better sensitivity and excellent separation of the LCB methine peak from the EHE Br peak, the LCB methine peak overlaps with the 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. The LCB results obtained with this new NMR pulse sequence are in very good agreement with expected values.