Quantification of Large Long Periods in Rigid Polymer Systems by 1H Spin Diffusion in HetCor NMR with Heavy Peak Overlap

Domain sizes in complex polymer materials on the 2- to 400-nm scale can be probed by ¹H spin diffusion NMR with ¹³C detection, which may be competitive with microscopy. In glassy systems, two-dimensional ¹H–¹³C heteronuclear correlation (HetCor) NMR with ¹H spin diffusion is the method of choice. Limits to its applicability have been overcome here by improved data analysis. Single-spectrum referencing eliminates the need for asymptotic equilibration and expands the range of accessible domain sizes to long periods of ~ 400 nm and makes time-consuming measurements with series of mixing times unnecessary. Systematic ¹H peak overlap correction in two-domain systems after local equilibration within 3 ms greatly expands the applicability of quantitative long-period determination from HetCor NMR with ¹H spin diffusion. It usually works even if the ¹H spectra of the two components are fully overlapped, as long as their fractional intensity contributions to at least one ¹H peak are distinctly different. This is documented for microphase-separated diblock copolymers of polystyrene and PMMA (alkyl slices) and of polystyrene and poly(4-vinyl pyridine), a polystyrene analogue. Based on extensive spin diffusion simulations utilizing coarse graining to reduce simulation times, convenient graphs are presented that enable conversion of a measured equilibration percentage to a tight range of minimum and maximum long period, as a robust, model-independent result.