NMR Study of the Dissolution of Core-Crystalline Micelles

Short fragments of the core-crystalline micelles formed by a sample of poly(ferrocenyldimethylsilane)-block-poly(isoprene) (PFS-b-PI) block copolymer (BCP) underwent self-seeding in decane when heated above its dissolution temperature. Variable temperature (VT) ¹H NMR and diffusion-ordered pulsed-gradient spin–echo (DOSY) NMR were used to monitor the behavior of micelles that dissolved as a function of increasing temperature. We examined a sample of micelle fragments of PFS₆₅-b-PI₆₃₇ characterized by Lₙ = 39 nm and Lw/Lₙ = 1.13. The PI corona had high mobility and gave a ¹H NMR signal in both micellar and unimer forms. In contrast, the PFS component could only be detected for the dissolved unimer. We found from ¹H NMR that essentially all the BCP molecules were incorporated into the micelles at temperatures up to and including 50 °C, at the limit of NMR detection. Both PFS and PI resonances could be detected between 70 and 100 °C, and the integration ratio of the PFS-to-PI peaks increased with temperature. DOSY NMR measured the self-diffusion coefficients (Dₛ) of the micelle fragments and unimer at these temperatures. The hydrodynamic radii (Rₕ) for these species were calculated from Dₛ using the Stokes–Einstein equation. The PFS signals gave Rₕ values in the range of 5–6 nm at temperatures between 80 and 100 °C, consistent with unimer diffusion. PI signals were fitted by an exponential decay at 25 °C with Rₕ = 38 nm characteristic of the micelle fragments and at 90, 95, and 100 °C with Rₕ ≈ 6 nm, corresponding to unimer. At intermediate temperatures (70–85 °C), PI signals were fitted to a sum of two exponential terms, consistent with a fast diffusing species and a slow diffusing species. Interestingly, we noticed that the size of the micelle fragments at elevated temperatures (80 and 85 °C) was sensitive to sample history; samples heated directly to the elevated temperatures were found to be shorter than those heated stepwise.