Time-domain nuclear magnetic resonance to assess microscale solubility in highly viscous and/or disperse polymer systems

Journal article, 2026

Determination of the physical state of a polymer, whether it is undissolved (solid), as in the case of a microplastic, swollen or fully dissolved, is essential when considering the properties and behaviour of polymeric formulations, but remains challenging within the complex formulations found in many fast moving consumer goods (FMCG). This information is also critical when considering the regulatory controls associated with the application of polymeric formulations. It is, therefore, important to be able to determine and quantify the presence and relative proportion of undissolved polymers within a formulation. Such quantification needs to be simple, rapid and robust for regulatory purposes, but has proved challenging to implement in complex formulations. In this paper, we address this challenge by introducing a novel measure of the degree of solid and undissolved polymer, based on time domain (TD) nuclear magnetic resonance (NMR). Using ¹H solid-echo (SE) Carr-Purcell-Meiboom-Gill (CPMG) transverse NMR relaxation curves, we calculate a restricted mobility index, which provides a measure of the proportion of immobile hydrogen atoms present in a sample, arising from solid and undissolved polymeric components within the formulation. Restricted mobility indices are determined for polyvinyl pyrrolidone (PVP), polyquaternium-37 and cellulose polymer formulations as a function or temperature, pH, concentration and processing regime. This method is rapid, easy to implement and does not suffer from the same level of subjectivity as multicomponent fitting or inverse Laplace transforms. The robustness and repeatability of this method is demonstrated across a variety of different TD NMR instruments.