Growth
and termination of cylindrical micelles with cholesteric
liquid crystal (LC) cores (seeds) were achieved experimentally, following
the simulation studies of liquid-crystallization-driven self-assembly
(LCDSA) of block copolymers. The fluidity of LC cores was proven to
be crucial for this success. To a seed solution, the added block copolymer
unimers had a high tendency to form new aggregates. The formed aggregates
had a less-ordered cholesteric LC structure as compared with those
in the seed and subsequently fused with the seeds. After the fusion,
the fluidity allowed a rearrangement of the rod blocks within the
elongated segments to match the LC structure in the seeds. The fusion
and rearrangement were repeated in cycles, which completely consumed
the newly formed aggregates and led to a seeded-growth behavior. Under
a condition that the interactions between LC blocks are stronger,
the newly formed aggregates had smectic LC cores, which fused with
the seeds terminating the growth. The termination is attributed to
the higher energy barrier for the transition from the smectic LC structure
to cholesteric active ends. This work created a theoretical basis
for further exploration of living assembly using LC block copolymers,
which are building blocks for a wide range of functional materials.
Supramolecular polymerization of
nanoscale particles has been considered
as an effective route to prepare hierarchical nanostructures with
controlled geometry and functions. However, so far, less is known
about its mechanism, especially the polymerization kinetics which
is fundamentally important for the controllable synthesis of hierarchical
structures. In the present work, we discovered a temperature-induced
supramolecular step-growth polymerization which can provide a simple
and robust route for preparing one-dimensional hierarchical nanowires.
The polymerization units are spindlelike micelles self-assembled from
amphiphilic poly(γ-benzyl-l-glutamate)-graft-poly(ethylene glycol) (PBLG-g-PEG) graft copolymers.
Because of the imperfect coverage of PEG grafts on PBLG cores at both
ends of the micelles, structural defects appear. At low temperatures,
PBLG tends to be more hydrophobic, and resultantly, these defects
become reactive and induce polymerization. Kinetic studies revealed
that at low temperatures and high micelle concentrations, the polymerization
rate becomes relatively faster. The present work demonstrates a new
kind of supramolecular polymerization and its reaction kinetic mechanism.
The obtained results could provide a guidance for the controllable
construction of hierarchical structures.
Supramolecular polymerization has been a fascinating frontier of supramolecular chemistry in fabricating well-defined hierarchical nanostructures. However, the reverse process, that is, supramolecular depolymerization in which superstructures disassemble into subunits, is far less explored. In particular, the mechanism and kinetics of supramolecular depolymerization have not yet been reported. In this work, we discovered a thermal-induced supramolecular depolymerization of nanowires formed by supramolecular step-growth polymerization of preassembled micelles. With increasing temperature, the intermicelle interaction between the micellar subunits is broken, resulting in depolymerization of the nanowires into micellar subunits. Accompanying the supramolecular depolymerization, chain transfer between the subunits occurs. A theoretical model was proposed to reveal the mechanism and kinetics of the supramolecular depolymerization. It was found that the depolymerization behavior obeys the rules of random depolymerization. The present work could provide useful information for understanding the underlying principles of supramolecular degradation. In addition, the temperature-induced supramolecular depolymerization of hierarchical nanostructures may find potential applications in biomedical fields.
Branched nanostructures with tunable arm numbers are prepared through the assembly of silica rods mediated by coalescence of catalyst droplets on the end of the rods.
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