Macromolecules, Vol.51, No.21, 8443-8454, 2018
Long Chain Branching Detection and Quantification in LDPE with Special Solvents, Polarization Transfer Techniques, and Inverse Gated C-13 NMR Spectroscopy
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 C-6 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 C-6 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 C-6 branches have a contribution to the resonance around 32.2 ppm. This finding suggests that C-6 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 C-7 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 C-6 branches. It was found that RINEPT with hard 180 degrees C-13 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 C-13 NMR technique and a polarization transfer technique.