Polyferrocenylsilane Crystals in Nanoconfinement: Fragmentation, Dissolution, and Regrowth of Cylindrical Block Copolymer Micelles with a Crystalline Core

Two samples of rod-like micelles in decane were prepared by seeded growth from a sample of a poly(isoprene-b-ferrocenyldimethylsilane) diblock copolymer (PI1000-PFS50, where the subscripts indicate the degree of polymerization). These micelles were uniform in length with a mass/length of 1.9 molecules/nm. The longer micelles (L-1250) had a number-average length L-n = 1243 nm, whereas the shorter micelles (L-250) had L-n = 256 nm. We used transmission electron microscopy (TEM) to examine the behavior of these micelles when dilute solutions of L-1250 or L-250 or their mixtures were heated at temperatures ranging from 40 to 75 degrees C and then cooled to room temperature. At 55 degrees C, the L-1250 sample underwent kinetically controlled fragmentation to give a broad distribution of micelle lengths. At this temperature, fragmentation was much less prominent in the L-250 sample. At higher temperatures, micelles with narrow distributions of lengths were obtained in each case (L-w/L-n approximate to 1.01). This process operates under thermodynamic control, and L-n values increased strongly with an increase in temperature. These results indicate that the micelles fragment, and polymer molecules dissolve, as the samples were heated. The fraction of surviving fragments decreased significantly at elevated temperatures, presumably reflecting a distribution of crystallinity in the cores of the micelle precursor. When the solutions were cooled, the surviving fragments served as seeds for the epitaxial growth of the micelles as the polymer solubility decreased. The most striking result of these experiments was the finding that fragments formed from the L-1250 micelles had a distribution of dissolution temperatures shifted by about 5 degrees C to higher temperature than the shorter L-250 micelles.