(96d) Fast and Non-Invasive Measurement of Cross-Link Density in Polymers by Td- NMR

Mechanical and thermal properties of the cured polymer products are strong functions of their cross-link density (CLD). A continuous monitoring of the curing process is done through measurement of CLD during production. Swelling and rheological measurements are the two most commonly used routine CLD measurement methods in the polymer industry. The swelling experiment is time consuming, poorly repeatable, involves handling of organic solvents, and has ample opportunity to introduce human error. The rheological measurements, although mechanized, have poor repeatability and not fast. A fast method of measuring CLD in polymers would enable better process monitoring and quality control in cross-linked/cured polymer products. A fast, non-invasive, and automated TD-NMR (time-domain nuclear magnetic resonance) measurement of polymer CLD can be done using a small bench-top instrument without any sample preparation. In TD-NMR, the signal is obtained from the hydrogen atoms present in the polymer molecules and it varies with their mobility. When cross-linking is introduced in a polymer, TD-NMR looks directly at the molecular level and measures the mobility of the polymer strands, which in turn provides a measure of the CLD. The basics of TD-NMR, as applies to quantitative study of polymers, will be elaborated in the presentation. Through quantitative analyses of the NMR signals from various polymers, it is shown to correlate very well with the CLD of the sample. Experimental data from the TD-NMR measurement of PE-foam samples will be shown as an example. This easy TD-NMR measurement of CLD seems to have promise in process monitoring and quality control of cross-linking in polymer industry.