No abstract

Recent global demand of polyethylene was 187 billion pounds with 23% being low density polyethylene (LDPE). LDPE is used in a variety of applications including liners, consumer bags, heavy duty sacks, caps, closures, toys, lamination, and agricultural films. A key feature of LDPE is the presence of long chain branching (LCB) which has a significant impact on physical and rheological properties, including melt strength. Previously it was reported that there are no C6 branches in LDPE, and therefore the common practice has been to use the resonance around 32.2 ppm as a measure of LCB content. Herein, the existence of C6 branches in LDPE is reported for the first time, enabled by the use of 1-chloronaphthalene as the NMR solvent. It is shown that the C6 branches have a contribution to the resonance around 32.2 ppm. This finding suggests that C6 branches should be measured in LDPE samples and be excluded from LCB quantification because they do not affect LDPE rheological properties as LCB does. The data obtained strongly suggest that C7 branches are present in much lower concentrations in the LDPE samples studied. The quantification of these resonances traditionally requires long acquisition time, even with the use of a cryoprobe. In this work, different polarization transfer techniques, refocused insensitive nuclei enhanced by polarization transfer (RINEPT), and distortionless enhancement by polarization transfer (DEPT) were compared in terms of their ability to enhance CH2 sensitivity, thus leading to a greater ability to observe the CH2 resonances belonging to the C6 branches. It was found that RINEPT with hard 180° 13C pulses is most suitable for this purpose. A much faster method is proposed for measuring LCB in LDPE that employs a combination of a conventional quantitative 13C NMR technique and a polarization transfer technique.

Substantially linear ethylene polymers (SLEPs) are important commercial products which are used in various applications such as packaging, electrical insulation, toys, pipes, footwear, roofing, automotive, fabrics, and much more. SLEPs can be produced using molecular catalysts which can lead to long chain branching (LCB). The amount of LCB has an influence on viscoelastic properties which affect film production and processing as well as mechanical and optical properties. Thus, it is important to accurately measure LCB content. 13C NMR is one of the methods that have been used to characterize LCB. It is quite challenging to measure LCB with 13C NMR in the presence of short chain branches (SCBs) longer than four carbons due to the overlap of LCB signals with SCB signals. In this paper, we describe the use of halogenated naphthalenes as suitable solvents to separate 13C signals related to LCB from SCB. The new method presented here allows for better quantification of LCB in polymer samples with a diverse array of branching types.