Effect of Molecular Architecture and Symmetry on Self-Assembly: A Quantitative Revisit Using Discrete ABA Triblock Copolymers

Stabilizing hexagonally close-packed phase in single-component block copolymers through rational symmetry breaking

Despite being predicted to be a thermodynamically equilibrium structure, the absence of direct experimental evidence of hexagonally close-packed spherical phase in single-component block copolymers raises uncomfortable concerns regarding the existing fundamental phase principles. This work presents a robust approach to regulate the phase behavior of linear block copolymers by deliberately breaking molecular symmetry, and the hexagonally close-packed lattice is captured in a rigorous single-component system. A collection of discrete A1BA2 triblock copolymers is designed and prepared through an iterative growth method. The precise chemical composition and uniform chain length eliminates inherent size distribution and other molecular defects. Simply by tuning the relative chain length of two end A blocks, a rich array of ordered nanostructures, including Frank-Kasper A15 and σ phases, are fabricated without changing the overall chemistry or composition. More interestingly, hexagonally close-packed spherical phase becomes thermodynamically stable and experimentally accessible attributed to the synergistic contribution of the two end blocks. The shorter A blocks are pulled out from the core domain into the matrix to release packing frustration, while the longer ones stabilize the ordered spherical phase against composition fluctuation that tends to disrupt the lattice. This study adds a missing puzzle piece to the block copolymer phase diagram and provides a robust approach for rational structural engineering.

Low Tg, strongly segregated, ABA triblock copolymers: a rheological and structural study

ABA triblock copolymers can form microphase separated structures where the B blocks form bridges between A domains, leading to reversible networks interesting for a variety of applications such as pressure sensitive adhesives or thermoplastic elastomers. However, a major drawback of these systems is their rapid loss of mechanical properties upon temperature increase. A potential way to circumvent this limitation would be to design ABA triblock copolymers that keep their microphase separation at high temperatures. In this paper, we report on all-soft ABA triblock copolymers having a poly(n-butyl acrylate) (PnBA) central block and poly(heptafluorobutyl acrylate) (PHFBA) outer blocks. By introducing fluorinated units, the incompatibility between the blocks is largely increased, allowing strong segregation between the block domains, which preserve the microphase separation up to high temperatures despite the low glass transition temperature of the blocks, as shown by temperature dependent SAXS measurements. We study the properties of different copolymers, with similar PHFBA volume fractions but different block lengths. Linear shear rheology measurements revealed the presence of a second, low frequency, plateau whose onset and length depend on the PnBA and PHFBA length, respectively. This plateau also persists up to higher temperatures for longer PHFBA blocks.

Discrete Miktoarm Star Block Copolymers with Tailored Molecular Architecture

Molecular architecture is a critical factor in regulating phase behaviors of the block copolymer and prompting the formation of unconventional nanostructures. This work meticulously designed a library of isomeric miktoarm star polymers with an architectural evolution from the linear-branched block copolymer to the miktoarm star block copolymer and to the star-like block copolymer (i.e., 3AB → 3(AB1)B2 → 3(AB)). All of the polymers have precise chemical composition and uniform chain length, eliminating inherent molecular uncertainties such as chain length distribution or architectural defects. The self-assembly behaviors were systematically studied and compared. Gradually increasing the relative length of the branched B1 block regulates the ratio between the bridge and loop configuration and effectively releases packing frustration in the formation of the spherical or cylindrical structures, leading to a substantial deflection of phase boundaries. Complex structures, such as Frank-Kasper phases, were captured at a surprisingly higher volume fraction. Rationally regulating the molecular architecture offers rich possibilities to tune the packing symmetry of block copolymers.

Unraveling Dynamic Helicity Inversion and Chirality Transfer through the Synthesis of Discrete Azobenzene Oligomers by an Iterative Exponential Growth Strategy

Unraveling the chirality transfer mechanism of polymer assemblies and controlling their handedness is beneficial for exploring the origin of hierarchical chirality and developing smart materials with desired chiroptical activities. However, polydisperse polymers often lead to an ambiguous or statistical evaluation of the structure-property relationship, and it remains unclear how the iterative number of repeating units function in the helicity inversion of polymer assemblies. Herein, we report the macroscopic helicity and dynamic manipulation of the chiroptical activity of supramolecular assemblies from discrete azobenzene-containing oligomers (azooligomers), together with the helicity inversion and morphological transition achieved solely by changing the iterative chain lengths. The corresponding assemblies also differ from their polydisperse counterparts in terms of thermodynamic properties, chiroptical activities, and morphological control.

Effect of chain architecture on the structure, dynamics, and rheology of thermoresponsive poloxamer hydrogels and associated blends

Poloxamers, ABA triblock polymers composed of a poly(propylene oxide) (PPO) midblock (B) and poly(ethylene oxide) (PEO) endblocks (A), are widely studied for biomedical applications. Aqueous poloxamer 407 (P407; also referred to as F127) undergoes a solution-to-gel transition with increasing temperature, driven by the formation and ordering of micelles onto periodic lattices; however, the gel temperature and resulting modulus has limited tunability. Here, reverse P407 (RP407), a BAB polymer of the same composition and molar mass but the inverted architecture, is synthesized via anionic polymerization. The micellization and gelation temperatures of RP407 are higher than that of P407 and the PPO endblocks allow for intermicelle bridging; however, both single-component solutions favor body-centered cubic (BCC) packings. Further, aqueous RP407 displays a "soft gel" region with interesting rheological behavior, including viscoelastic aging and thermal hysteresis. Combining P407 and RP407 yields solutions with intermediate transition temperatures and alters the size and micelle packing. While the single-component solutions produce BCC packings, the blends form close-packed structures and larger micelles of higher aggregation numbers. Blends of P407 with an analogous AB diblock (E111P32) display similar behavior, whereas RP407/diblock blends form intermediate-sized BCC-packed micelles. These differences in packing and aggregation alter the local environments within the gels, which could have implications for applications such as drug delivery and protein stabilization.

Local Chain Feature Mandated Self-Assembly of Block Copolymers

This work demonstrates an effective and robust approach to regulate phase behaviors of a block copolymer by programming local features into otherwise homogeneous linear chains. A library of sequence-defined, isomeric block copolymers with globally the same composition but locally different side chain patterns were elaborately designed and prepared through an iterative convergent growth method. The precise chemical structure and uniform chain length rule out all inherent molecular defects associated with statistical distribution. The local features are found to exert surprisingly pronounced impacts on the self-assembly process, which have yet to be well recognized. While other molecular parameters remain essentially the same, simply rearranging a few methylene units among the alkyl side chains leads to strikingly different phase behaviors, bringing about (i) a rich diversity of nanostructures across hexagonally packed cylinders, Frank-Kasper A15 phase, Frank-Kasper σ phase, dodecagonal quasicrystals, and disordered state; (ii) a significant change of lattice dimension; and (iii) a substantial shift of order-to-disorder transition temperature (up to 40 °C). Different from the commonly observed enthalpy-dominated cases, the frustration due to the divergence between the native molecular geometry originating from side chain distribution and the local packing environment mandated by lattice symmetry is believed to play a pivotal role. Engineering the local chain feature introduces another level of structural complexity, opening up a new and effective pathway for modulating phase transition without changing the chemistry or composition.

Regioisomeric Giant Triblock Molecules: Role of the Linker

In this study, polyhedral oligomeric silsesquioxane (POSS) based giant triblock molecules with precisely defined regio-configuration are modularly prepared through highly efficient coupling reactions. The length of the linker connecting neighboring nanoparticles is elaborately designed to regulate the geometric constraints. The triblock molecules adopt a folded packing during phase separation, and the regio-configuration imparts direct influence on the self-assembly behaviors. The ortho-isomers form periodic structures with a larger domain size, larger interfacial curvature, and enhanced phase stability. The regio-effect is closely related to the length and symmetry of the linker. As the linker extends, the neighboring particles gradually decouple, and the regio-effect diminishes. The symmetry of the linker shows an even more profound impact. This work quantitatively scrutinized the role of the linker, opening an avenue for engineering the assembled structures with molecular precision.

Phase Diagrams of Polymerization-Induced Self-Assembly Are Largely Determined by Polymer Recombination

In the current work, atom transfer radical polymerization-induced self-assembly (ATRP PISA) phase diagrams were obtained by the means of dissipative particle dynamics simulations. A fast algorithm for determining the equilibrium morphology of block copolymer aggregates was developed. Our goal was to assess how the chemical nature of ATRP affects the self-assembly of diblock copolymers in the course of PISA. We discovered that the chain growth termination via recombination played a key role in determining the ATRP PISA phase diagrams. In particular, ATRP with turned off recombination yielded a PISA phase diagram very similar to that obtained for a simple ideal living polymerization process. However, an increase in the recombination probability led to a significant change of the phase diagram: the transition between cylindrical micelles and vesicles was strongly shifted, and a dependence of the aggregate morphology on the concentration was observed. We speculate that this effect occurred due to the simultaneous action of two factors: the triblock copolymer architecture of the terminated chains and the dispersity of the solvophobic blocks. We showed that these two factors affected the phase diagram weakly if they acted separately; however, their combination, which naturally occurs during ATRP, affected the ATRP PISA phase diagram strongly. We suggest that the recombination reaction is a key factor leading to the complexity of experimental PISA phase diagrams.