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 L_n = 39 nm and L_w/L_n = 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 (D_s) of the micelle fragments and unimer at these temperatures. The hydrodynamic radii (R_h) for these species were calculated from D_s using the Stokes-Einstein equation. The PFS signals gave R_h 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 R_h = 38 nm characteristic of the micelle fragments and at 90, 95, and 100 °C with R_h ≈ 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.