Phase Control of Colloid-like Block Copolymer Micelles by Tuning Size Distribution via Thermal Processing

Abstract

The formation of a Frank–Kasper (FK) phase in a one-component block copolymer (bcp) has been attributed to the sufficiently large conformational asymmetry parameter that leads to the deformation of the micellar core into the polyhedral shape templated by the Voronoi cells for relieving the packing frustration of the coronal blocks. Here we present the insight into the evolution of body-centered cubic (BCC) and Laves C14 phases from the corresponding metastable liquidlike packing (LLP) phases in a conformationally symmetric poly(2-vinylpyridine)-block-poly(dimethylsiloxane) (P2VP-b-PDMS) forming the colloid-like micelle with a hard P2VP core and a soft PDMS corona at the temperature of micelle ordering (T g corona < T a < T g core). Different thermal processing conditions were applied to produce four distinct types LLP phase characterized by different average micelle size and size dispersity. The LLP phase was found to transform to BCC, C14 and a reorganized LLP phase with increasing polydispersity of particle size, showing the existence of a proper range of size dispersity for the formation of the Laves phase. The real-space analysis of the local environments of the particles revealed that the C14 phase can accommodate the particles with a broader range of interparticle distance and a less uniform local environment, such that the PDMS coronal blocks of the micelles with greater size dispersity suffered a lower degree of packing frustration when they organized into a C14 lattice. In view of the colloid-like nature of the micelle, a size fractionation process found in the crystallization of polydisperse colloidal particles was proposed as the mechanism for directing the effective development of C14 phase from the LLP phase. Due to the metastable nature of the ordered phases, their order–disorder transition temperatures were found to depend strongly on the structures of the LLP phases from which they developed and a strong memory effect underlying the phase transitions was identified.

Tuning the Complex Spherical Phase of Sugar-Based Block Co-Oligomer via Single-Monomer-Mediated Composition Variation

“…The reduction of the ΔF s by packing into Laves phase is typically unable to outweigh the correspondingly high ΔF el ; therefore, Laves phase has been considered as a metastable structure in neat BCP. ,, Experimentally, Laves C14 and C15 phases have been observed after the BCP was subjected to special thermal processing that yielded the micelles with a relatively broad size distribution and large volume asymmetry in the LLP phase ,, Provided that such a size dispersity cannot be adjusted effectively to reach the equilibrium size distribution during micelle ordering, the broadly distributed micelles inherited from the LLP phase would organize into Laves phase, in that the unit cell of the Laves phase provides a heterogeneous environment effectively accommodating the micelles with large size asymmetry. , Laves phase can become a thermodynamically stable structure in the binary blends of BCPs with different chain lengths or in the blends of a BCP with a homopolymer. − In these cases, the distribution of copolymer or homopolymer molecules within or among the micelles alleviates the ΔF el resulting from the significant volume asymmetry, thus favoring the stability of the Laves phase.…”