Effect of the diamagnetic susceptibility in NMR measurements in diblock copolymers

Symmetric diblock copolymers in the lamellar phase are studied by NMR. ²H NMR spectra obtained in macroscopically oriented monodomains show an absolute frequency shift of a few ppm, which depends of the orientation of the sample with respect to the magnetic field. This is interpreted as due to the anisotropy of the diamagnetic susceptibility, related to the lamellar structure itself rather than to an orientational order of polymer chain segments. The spectra obtained in both 2D ¹³C and ¹H rotor-synchronized MAS experiments reflect the same effect. Specifically, we show that ¹H rotor-synchronized MAS experiments allow us to discriminate unambiguously the effect of diamagnetic susceptibility anisotropy and the effect of molecular orientation.