Explosive dissolution and trapping of block copolymer seed crystallites

The study of the dissolution of polymer crystals is a challenging task. Here the authors use crystallization-driven self-assembly of coil-crystalline block copolymers as a trapping technique to track the change in length of 1D seed crystallites during annealing

NMR Study of the Dissolution of Core-Crystalline Micelles

Short fragments of the core-crystalline micelles formed by a sample of poly­(ferrocenyl­dimethylsilane)-<i>block</i>-poly­(isoprene) (PFS-<i>b</i>-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-<i>b</i>-PI₆₃₇ characterized by <i>L</i>_n = 39 nm and <i>L</i>_w/<i>L</i>_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 (<i>D</i>_s) of the micelle fragments and unimer at these temperatures. The hydrodynamic radii (<i>R</i>_h) for these species were calculated from <i>D</i>_s using the Stokes–Einstein equation. The PFS signals gave <i>R</i>_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 <i>R</i>_h = 38 nm characteristic of the micelle fragments and at 90, 95, and 100 °C with <i>R</i>_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

PFS-b-PNIPAM:A first step towards polymeric nanofibrillar hydrogels based on uniform fiber-like micelles

Amphiphilic crystalline-coil diblock copolymers polyferrocenyldimethylsilane-<i>block</i>-poly­(<i>N</i>-isopropylacrylamide) of two different block ratios (PFS₅₆-<i>b</i>-PNIPAM₁₉₀ and PFS₂₆-<i>b</i>-PNIPAM₅₂₀) were synthesized by a copper-catalyzed azide–alkyne coupling reaction. They exhibited pronounced differences in self-assembly in alcohol solvents. While PFS₅₆-<i>b</i>-PNIPAM₁₉₀ formed mixtures of spherical and rod-like micelles in ethanol and 2-propanol, PFS₂₆-<i>b</i>-PNIPAM₅₂₀ formed long fibers of uniform width in these solvents. We used a seeded growth protocol to grow rod-like PFS₂₆-<i>b</i>-PNIPAM₅₂₀ micelles of uniform lengths. There were two surprising features of this experiment: First, micelle growth was unusually slow and required a long aging time (40 days) for them to reach their final length. Second, the micelles were characterized by a low number of polymer chains per unit length as determined by multiangle light scattering. This result suggests a loose packing of PFS chains in the micelle core. In an attempt to prepare thermoresponsive nanofibrillar hydrogels from these micelles, we explored approaches to transfer them from 2-propanol to water. These attempts were accompanied by extensive fragmentation of the micelles. We believe the fragility of these micelles is related to the loosely packed nature of the PFS chains in the micelle core. Fragmentation may also be affected by the cononsolvency effect of 2-propanol-water mixtures on the PNIPAM corona of the micelles. We could show, however, that the micelle fragments in water retained their anticipated thermoresponsive behavior

How a Small Modification of the Corona-Forming Block Redirects the Self-Assembly of Crystalline-Coil Block Copolymers in Solution

In this study, we examine how the self-assembly of crystalline-coil block copolymers in solution can be influenced by small changes in the chemical structure of the corona-forming block. Three samples of poly­(ferrocenyldimethylsilane)-<i>block</i>-poly­(2-vinylpyridine) that form long fiber-like micelles uniform in width in 2-propanol, were treated with methyl iodide to convert a small fraction (0.1% to 6%) of the pyridines to methylpyridinium groups. When these partially quaternized samples (PFS-<i>b</i>-P2VP^Q) were subjected to the same self-assembly protocol, very different structures were obtained. For PFS₃₆-<i>b</i>-P2VP₅₀₂^Q, the presence of positive charges led to the formation of much shorter rod-like micelles. In contrast, for PFS₁₇-<i>b</i>-P2VP₁₇₀^Q and PFS₃₀-<i>b</i>-P2VP₃₀₀^Q, complex platelet structures were obtained. We explain the complexity of these structures in terms of a distribution of compositions, in which the polymer chains with the highest extent of methylation are the least soluble in 2-PrOH and the first to associate, leading to two-dimensional aggregates. The less quaternized polymer chains remaining in solution have a stronger tendency to form elongated fiber-like micelles that grow from the ends of the initially formed planar structures. In this way, we show that small extents of chemical modification of the corona forming chains can modify the self-assembly process and that simple one-pot protocols can lead to diverse hierarchical structures

Probing the Growth Kinetics for the Formation of Uniform 1D Block Copolymer Nanoparticles by Living Crystallization-Driven Self-Assembly

Living crystallization-driven self-assembly (CDSA) is a seeded growth method for crystallizable block copolymers (BCPs) and related amphiphiles in solution and has recently emerged as a highly promising and versatile route to uniform core–shell nanoparticles (micelles) with control of dimensions and architecture. However, the factors that influence the rate of nanoparticle growth have not been systematically studied. Using transmission electron microscopy, small- and wide-angle X-ray scattering, and super-resolution fluorescence microscopy techniques, we have investigated the kinetics of the seeded growth of poly­(ferrocenyldimethylsilane)-<i>b</i>-(polydimethylsiloxane) (PFS-<i>b</i>-PDMS), as a model living CDSA system for those employing, for example, crystallizable emissive and biocompatible polymers. By altering various self-assembly parameters including concentration, temperature, solvent, and BCP composition our results have established that the time taken to prepare fiber-like micelles <i>via</i> the living CDSA method can be reduced by decreasing temperature, by employing solvents that are poorer for the crystallizable PFS core-forming block, and by increasing the length of the PFS core-forming block. These results are of general importance for the future optimization of a wide variety of living CDSA systems. Our studies also demonstrate that the growth kinetics for living CDSA do not exhibit the first-order dependence of growth rate on unimer concentration anticipated by analogy with living covalent polymerizations of molecular monomers. This difference may be caused by the combined influence of chain conformational effects of the BCP on addition to the seed termini and chain length dispersity