Explosive dissolution and trapping of block copolymer seed crystallites

Abstract

Enhanced control over crystallization-driven self-assembly (CDSA) of coil-crystalline block copolymers has led to the formation of intricate structures with well-defined morphology and dimensions. While approaches to build those sophisticated structures may strongly differ from each other, they all share a key cornerstone: a polymer crystallite. Here we report a trapping technique that enables tracking of the change in length of one-dimensional (1D) polymer crystallites as they are annealed in solution at different temperatures. Using the similarities between 1D polymeric micelles and bottle-brush polymers, we developed a model explaining how the dissolving crystallites reach a critical size independent of the annealing temperature, and then explode in a cooperative process involving the remaining polymer chains of the crystallites. This model also allows us to demonstrate the role of the distribution in seed core crystallinity on the dissolution of the crystallites.

Fig. 1

Investigation of crystallite dissolution by seed trapping. Schematic diagram describing a the control and b the seed trapping experiments performed to study the dissolution of PFS₅₃-b-PI₆₃₇ crystallites heated for 30 min in decane at different temperatures. In this scheme we use a color code to represent different chemical species: red represents polyferrocenyldimethylsilane (PFS) (either as the PFS block of a unimer or the crystalline core of a micelle), blue represents polyisoprene (PI), gray represents polydimethylsiloxane (PDMS), while the black spheres represent the platinum nanoparticles from Karstedt’s catalyst used to stain PI. c and d TEM micrographs of PFS₅₃-b-PI₆₃₇ micelles obtained by heating PFS₅₃-b-PI₆₃₇ seeds in decane for 30 minutes at c 63 °C, and d 73 °C, and cooling them to room temperature. e Length distribution histograms of the micelles shown in (c) (63 °C, green columns), and (d) (73 °C, purple columns). TEM micrographs of triblock comicelles obtained by heating PFS₅₃-b-PI₆₃₇ seeds 25 minutes at f 63 °C and g 73 °C then adding PFS₆₀-b-PDMS₆₆₀ unimer, and after five more minutes, letting the solution cool to room temperature. h Length distribution histograms of the surviving PFS₅₃-b-PI₆₃₇ seeds after the sample was annealed at 63 °C (green columns) and 73 °C (purple columns). Samples f and g were trapped with excess of PFS₆₀-PDMS₆₆₀ unimer, and stained with Karstedt’s catalyst to highlight the PI rich regions. Scale bars, 500 nm

Fig. 2

Effect of the dissolution temperature on the length and number of surviving seeds. a Evolution of the number average length of the PFS₅₃-b-PI₆₃₇ seed crystallites regrown at 23 °C as a function of annealing temperature (control experiment). b Evolution of the apparent mass percentage of surviving seeds as a function of annealing temperature. c Evolution of the length of the surviving seeds as a function of annealing temperature. The horizontal dashed line corresponds to the average length of the trapped seeds (averaged over the whole annealing temperature range). Error bars correspond to the s.e.m. of the length distributions determined by tracing more than 200 stained trapped seeds for each annealing temperature (see Supplementary Methods). d Length distributions of the surviving seeds at each annealing temperature. The surviving seeds were trapped by the addition of PFS₆₀-b-PDMS₆₆₀ prior to cooling, and stained with Karstedt’s catalyst

Fig. 3

Seed fragmentation. a schematic representation of the cross-section of a seed crystallite. The core, a central crystalline layer sandwiched by two amorphous layers, is surrounded by the corona chains that form superblobs of diameter D. Schematic representations of the fragmentation of seeds of core length, L, due to corona chain repulsion in the case of b D < L < 2D, c L ≈ D, and d D/2 ≤ L < D. Length distribution histograms of the PFS₅₃-b-PI₆₃₇ seeds e before annealing, and f after annealing at 50 °C

Fig. 4

Fitting of the length of micelles obtained by self-seeding. a Fit (eq. 6, dashed line) of the number average length of the PFS₅₃-b-PI₆₃₇ seed crystallites regrown at 23 °C (filled squares) as a function of annealing temperature (control experiment). b Fit (dashed line) of the apparent mass percentage of surviving seeds as a function of annealing temperature (filled squares)