Molecular Nanoparticles Are Unique Elements for Macromolecular Science: From “Nanoatoms” to Giant Molecules

Transition Kinetics of Self-Assembled Supramolecular Dodecagonal Quasicrystal and Frank-Kasper σ Phases in ABn Dendron-Like Giant Molecules

A series of noncrystalline AB_n dendron-like giant molecules DPOSS-MPOSS_n (n = 2-6, DPOSS: hydrophilic polyhedral oligomeric silsesquioxane (POSS) cage; MPOSS: hydrophobic POSS cage) were synthesized. These samples present a thermodynamically stable phase formation sequence from the hexagonal cylinder phase (plane group of P6mm), to the Frank-Kasper (F-K) A15 phase (space group of Pm3̅n), and further to the F-K σ phase (space group of P4₂/mnm), with increasing the number of MPOSS in a single molecule (n, from 2 to 6). Moreover, for DPOSS-MPOSS₅ and DPOSS-MPOSS₆, an intriguing dodecagonal quasicrystal (DQC) structure has been identified and revealed as a kinetic favorable metastable phase at lower temperatures, while the thermodynamically stable phase is the σ phase. The detailed investigation of the transition kinetics between the DQC and σ phase in these samples makes it possible to identify how the self-assembly directs the phase transition in terms of molecular and supramolecular aspects.

5 nm Ordered Structures Self-Assembled by C2 -Symmetric Hybrids with Polyhedral Oligomeric Silsesquioxane and Hexa-peri-Hexabenzocoronene

Hybrids consisting of polyhedral oligomeric silsesquioxane (POSS) and hexa-peri-hexabenzocoronene (HBC) with a dumbbell topology and C₂ symmetry were designed and synthesized. They self-assemble into 5 nm ordered structures. In particular, the increased steric effect with increasing POSS units stabilizes a square columnar phase (Col_squ ) which is important in nanotemplating. These hybrids containing discotic liquid crystal HBC and POSS units have an excellent etching contrast and present an approach to obtain 5 nm nanopatterns.

Multiscale Self-Assembly of Mobile-Ligand Molecular Nanoparticles for Hierarchical Nanocomposites

Multiscale hierarchical morphologies are greatly desired for fabricating nanocomposites with tunable macroscopic properties, but challenges remain in precisely manipulating the spatial arrangement of nanoparticles in polymer matrices across multiple length scales. Here, we demonstrate a class of mobile-ligand nanoparticle system built upon 1 nm anionic polyoxometalate molecular nanoparticles and cationic terminated polymer chains by electrostatic interaction. The highly rearrangeable polymer chains can serve as mobile ligands to direct the polyoxometalates to align into sub-10 nm anisotropic superlattice-like nanoarrays in the bulk state. Moreover, these nanoarrays can further serve as structural units to assemble into hierarchically ordered morphologies in polymer matrices, e.g., percolated networks over hundreds of micrometers which are comprised of cylindrically packed polyoxometalate superlattices down to sub-10 nm scale. These hierarchical morphologies enable the nanocomposites with reinforced mechanical performance. The presented mobile-ligand approach can provide a paradigm to design functional polymer nanocomposites with improved properties such as mechanical reinforcement and collective optical and electronic functions.

Competition and Cooperation among Different Attractive Forces in Solutions of Inorganic-Organic Hybrids Containing Macroionic Clusters

Hybrids composed of nanoscale inorganic clusters and organic ligands are ideal models for understanding the different attractive forces during the self-assembly processes of complex macromolecules in solution. The counterion-mediated attraction induced by electrostatic interaction from the large, hydrophilic macroionic clusters can compete or cooperate with other types of attractive forces such as hydrophobic interactions, hydrogen bonding, π-π stacking, and cation-π interactions from the organic ligands, consequently determining the solution behaviors of the hybrid molecules including their self-assembly process and the final supramolecular structures. The incorporation of organic ligands also leads to interesting responsive behaviors to external stimuli. Through the manipulation of the hybrid composition, architecture, topology, and solution conditions (e.g., solvent polarity, pH, and temperature), versatile self-assembled morphologies can be achieved, providing new scientific opportunities and potential applications.

Bottlebrush-Colloid Janus Nanoparticles

A bottlebrush-colloid Janus nanoparticle (JNP) with a ball-and-stick structure is reported. A single poly(4-vinyl benzyl chloride) (PVBC) polymer chain was grafted onto the amine-capped Fe₃O₄@NH₂ nanoparticle. pH-responsive 2-diethylaminoethyl methacrylate (DEAEMA) and water-soluble oligo(ethylene glycol) methacrylate (OEGMA) were sequentially grown from the PVBC backbone by ATRP, forming a core-shell bottlebrush. The synthesized PVBC₂₀₈-g-(PDEAEMA₁₃-b-POEGMA₈)-Fe₃O₄@NH₂ JNPs are dispersible in water and can be manipulated by a magnet. The Fe₃O₄ NPs with exposed -NH₂ groups facilitate accumulation at acidic sites. Hydrophobic dyes can be loaded within the PDEAEMA at pH ≥ 7.5, while they are released at pH values below 6.8. The composite JNPs are promising as a guided pH-responsive delivery vector toward acidic solid tumors.

Synthesis, Self-Assembly and Characterization of Tandem Triblock BPOSS-PDI-X Shape Amphiphiles

In this article, we report the facile synthesis, self-assembly, and characterization of shape amphiphiles (BPOSS-PDI-X) based on isobutyl-functionalized polyhedral oligomeric silsesquioxane (BPOSS), perylene tetracarboxylic diimide (PDI), and (60)fullerene (C60) moieties. Firstly, an asymmetrically functionalized diblock shape amphiphile precursor (BPOSS-PDI-OH) was obtained through the one-pot reaction between perylene-3,4,9,10-tetracarboxylic dianhydride and two different amines, namely BPOSS-NH2 and 3-amino-1-propanol. It was further conjugated with C60-COOH to give a tri-block shape amphiphile (BPOSS-PDI-C60). Their chemical structures were thoroughly characterized by NMR, IR and MALDI-TOF MS spectrometry. In order to gain insights on the structure-property relationship, their self-assembly in gas phase, in solution, and in solid state were characterized using traveling wave ion mobility mass spectrometry (TWIM-MS), UV/Vis absorption, fluorescence emission spectrophotometer, and transmission electron microscopy, respectively. It was found that BPOSS-PDI-OH formed more complicated dimers than BPOSS-PDI-C60. Both samples showed unique aggregation behaviors in solution with increasing concentration, which could be attributed neither to H- nor to J-type and might be related to the discrete dimers. While BPOSS-PDI-C60 could hardly crystalize into ordered structures, BPOSS-PDI-OH could form nanobelt-shaped single crystals, which may hold potential applications in microelectronics.

Controllable Self-Assembly of Amphiphilic Tadpole-Shaped Polymer Single-Chain Nanoparticles Prepared through Intrachain Photo-cross-linking

We report the use of intramolecular cross-linking chemistry as a tool to control the self-assembly of amphiphilic diblock copolymers (di-BCPs). Two amphiphilic di-BCPs of poly( N, N'-dimethylacrylamide)- block-polystyrene (PDMA- b-PS) with photo-cross-linkable cinnamoyl groups in either hydrophobic or hydrophilic blocks were prepared using reversible addition-fragmentation chain transfer polymerization. Intramolecular photo-cross-linking of cinnamoyl groups led to the formation of tadpole-shaped polymer single-chain nanoparticles (SCNPs) consisting of a self-collapsed block as the "head" and an un-cross-linked block as the "tail". When intramolecular photo-cross-linking was carried out in hydrophobic PS blocks, a clear morphological transition from branched cylindrical micelles (for the linear di-BCP) to completely spherical micelles at a dimerization degree of ∼63% was observed. A pattern of morphological transitions from cylindrical micelles to spherical micelles is observed through stepwise downsizing the length of cylindrical micelles when increasing the self-collapse degree of PS blocks, whereas, in case of photo-cross-linking carried out in hydrophilic PDMA blocks, the size of micelles showed a dramatic increase due to the shift of hydrophobic-to-hydrophilic balance. When the cross-linking degree of PDMA blocks reached >60%, tadpole-shaped SCNPs assembled into nonconventional aggregates with a nonsmooth surface. Our results illustrate the impact of chain topologies on the self-assembly outcomes of amphiphilic di-BCPs, which likely opens a door to control the micellar morphologies from just one parent linear di-BCP, rather than resynthesizing BPCs with different volume fractions of the two blocks.

Versatile Construction of Single-Tailed Giant Surfactants with Hydrophobic Poly(ε-caprolactone) Tail and Hydrophilic POSS Head

Giant surfactants refer to a new kind of amphiphile by incorporating functional molecular nanoparticles with polymer tails. As a size-amplified counterpart of small-molecule surfactants, they serve to bridge the gap between small-molecule surfactants and amphiphilic block copolymers. This work reports the design and synthesis of single-tailed giant surfactants carrying a hydrophobic poly(ε-caprolactone) (PCL) as the tail and a hydrophilic cage-like polyhedral oligomeric silsesquioxane (POSS) nanoparticle as the head. The modular synthetic strategy features an efficient "growing-from" and "click-modification" approach. Starting from a monohydroxyl and heptavinyl substituted POSS (VPOSS-OH), a PCL chain with controlled molecular weight and narrow polydispersity was first grown by the ring-opening polymerization (ROP) of ε-CL under the catalysis of stannous octoate, leading to a PCL chain end-capped with heptavinyl substituted POSS (VPOSS-PCL). To endow the POSS head with adjustable polarity and functionality, three kinds of hydrophilic groups, including hydroxyl groups, carboxylic acids, and amine groups, were installed to the periphery of POSS molecule by a high-efficiency thiol-ene "click" reaction. The compounds were fully characterized by NMR, gel permeation chromatography (GPC), MALDI-TOF mass spectrometry, and TGA analysis. In addition, the preliminary self-assembly study of these giant surfactants was also investigated by TEM and dynamic laser light scattering (DLS), which indicated that they can form spherical nanoparticles with different diameters in aqueous solution. This work affords a straightforward and versatile way for synthesizing single-tailed giant surfactants with diverse head surface functionalities.

Polymers on nanoparticles: structure & dynamics

Grafting polymers to nanoparticle surfaces influences properties from the conformation of the polymer chains to the dispersion and assembly of nanoparticles within a polymeric material. Recently, a small body of work has begun to address the question of how grafting polymers to a nanoparticle surface impacts chain dynamics, and the resulting physical properties of a material. This Review discusses recent work that characterizes the structure and dynamics of polymers that are grafted to nanoparticles and opportunities for future research. Starting from the case of a single polymer chain attached to a nanoparticle core, this Review follows the structure of the chains as grafting density increases, and how this structure slows relaxation of polymer chains and affects macroscopic material properties.

Effect of Chain Architecture on Phase Behavior of Giant Surfactant Constructed from Nanoparticle Monotethered by Single Diblock Copolymer Chain

The phase behaviors of the giant surfactant constructed from a nanoparticle (NP) monotethered by a single AB diblock copolymer chain were investigated by combining self-consistent field theory and density functional theory. Three types of giant surfactants with different chain architectures were constructed via changing the location of NP on the diblock copolymer chain. The simulation results show that the introduction of the NP can induce phase separation of the originally disordered AB diblock copolymers, and phase diagrams as a function of the chain length ratio of A block and the attraction between A block and NP were constructed for the three giant surfactant systems. Via changing the location of NP from the end of B block to the AB-junction point and to the end of A block, the conformational entropies of the systems gradually decrease, leading to a significant difference in phase behaviors. When the NP is tethered to the end of B block, the giant surfactant system has the smallest phase-separation region in the phase diagram, and the resulting ordered structures have the smallest feature sizes. However, when the NP is tethered to the end of A block, the giant surfactant system has the largest phase-separation region, as well as the largest feature sizes of ordered structures. Moreover, the distributions of the NPs within microphase-separated domain can be well tailored by changing the chain length ratio of A block or the attraction between A block and NP in all of the three giant surfactant systems. These findings provide the guideline for the preparation of polymer-nanoparticle composites with controllable morphologies, desirable feature sizes, and precise NP distributions in experiments.

The effect of steric repulsion between highly branched hydrophilic blocks on inverse cubic mesophase formation in block copolymers

The solution-based self-assembly of block copolymers (BCPs) into nanoparticulate or microparticulate inverse cubic mesophases, or polymer cubosomes, is of growing interest. This phenomenon could yield new polymeric mesoporous materials with three-dimensionally organized mazes of large water channels. In addition to the ratio of its hydrophilic and hydrophobic blocks, the architecture of a BCP critically influences self-assembly. BCP bilayers having triply periodic minimal surfaces with cubic lattice structures are formed. Here we report the synthesis and self-assembly of BCPs with T8 polyhedral oligomeric silsesquioxane (POSS), an inorganic cage molecule. POSS can be asymmetrically functionalized to host structural modules as branching units of the hydrophilic block that contain various spatial and chemical environments. BCPs, each containing seven poly(ethylene glycol) chains and the hydrophobic polystyrene block PEG350₇-POSS-b-PS, were prepared to investigate the effect of the highly branched hydrophilic block architecture on self-assembly. We found that the BCPs self-assembled into polymer cubosomes. Regardless of the block ratio, only the primitive cubic phase (Schwarz P surface, Im3̄m space group) was observed in the cubosomes. This is in contrast to the self-assembly of conventional BCPs into inverse mesophases, in which the internal lattice preferentially transforms into double-diamond lattices to yield a Schwarz D surface as a consequence of block ratio changes. Our results suggest that BCP architecture may be related to the symmetry of the lattice formed via self-assembly in solution.

The solution-based self-assembly of block copolymers (BCPs) into nanoparticulate or microparticulate inverse cubic mesophases, or polymer cubosomes, is of growing interest.

Hierarchically ordered nanostructures of a supramolecular rod-coil block copolymer with a hydrogen-bonded discotic mesogen

Supramolecular liquid crystalline block copolymers prepared via hydrogen bonding exhibit hierarchical structures that can be tuned by varying the molar ratio of the discotic hydrogen-bonding acceptor to the block copolymer donor.

Symmetry-guided, divergent assembly of regio-isomeric molecular Janus particles

Varying a single regio-configuration diverges the assembly outcome into distinct morphologies as mandated by their molecular symmetries.

Sequence isomeric giant surfactants with distinct self-assembly behaviors in solution

Two sequence isomeric giant surfactants exhibit distinguished self-assembly behaviors, which is caused by the different molecular packing conformations induced by their distinct molecular sequences.

Bioassemblies Fabricated by Coassembly of Protein Molecules and Monotethered Single-Chain Polymeric Nanoparticles

Molecular nanoparticles have been used as building blocks in the synthesis of functional materials. The grand challenges in the synthesis of the functional materials are precise control of the structures and functionalities of the materials by using nanoparticles with different architectures and properties. Monotethered single-chain polymeric nanoparticles (SCPN) are a type of nanosized asymmetric particles formed by intramolecular cross-linking of linear diblock copolymer chains. Monotethered SCPNs can be used as elemental building blocks for the fabrication of well-defined advanced structures. In this research, synthesis of biohybrid materials based on coassembly of bovine serum albumin (BSA) molecules and monotethered SCPNs is investigated. Due to the asymmetric structure of the SCPNs, positively charged SCPNs and negatively charged protein molecules coassemble into biohybrid vesicles with SCPNs on the layers and protein molecules in the walls. The self-assembled structures were analyzed by using dynamic light scattering, transmission electron microscopy, cryo-transmission electron microscopy, and atomic force microscopy. The average size of the biohybrid vesicles can be controlled by the molar ratio of SCPNs to BSA. The protein molecules in the biohybrid vesicles maintain most of the activities. This research paves a new way for the synthesis of functional biohybrid structures, and the materials can be used as protein carriers.

Regioisomeric Tandem Triblock Shape Amphiphiles Based on Polyhedral Oligomeric Silsesquioxanes

We report the design, synthesis and self-assembly of a series of tandem triblock shape amphiphile regioisomers composed of one rod-like fluorinated polyhedral oligomeric silsesquioxane (FPOSS) cage tethered with two spherical tert-butyl-POSS (tBPOSS) cages in para-, meta-, and ortho-configurations. Precision syntheses are achieved through sequential "click" reactions and the compounds are thoroughly characterized by combined techniques of NMR, FT-IR and MALDI-TOF MS spectroscopy. Only the ortho-isomer forms an ordered lamellae structure induced by the crystallization of perfluoroalkyl-chains and the other two remain amorphous due to frustrated packing by unfavorable regio-configuration. The distinct assembly behaviors of these three regioisomers reflect the delicate influence of a minute structural difference on precision macromolecular assembly.

Hierarchically ordered structures of disk-cube triads containing hexa-peri-hexabenzocoronene and polyhedral oligomeric silsesquioxane

Obtaining nanoscale-ordered structures is important for the development of nanotechnology. We designed and synthesized a series of disk-cube triads containing one hexa-peri-hexabenzocoronene (HBC) and two polyhedral oligomeric silsesquioxane (POSS) moieties, HBC-2POSS. The two POSS units were linked via ester or amide bonds. With the amide linkage used, the hydrogen bonding that was introduced affected the balance between the π-π interaction of HBC cores and crystallization interaction of POSS units. Hierarchically ordered structures were obtained from HBC-2POSS triads owing to the synergistic effect of multiple secondary interactions: π-π interaction, hydrogen bonding, and crystallization interaction. As organic-inorganic hybrid materials, these HBC-2POSS triads are promising candidates for templates <10 nm.

Multilevel Manipulation of Supramolecular Structures of Giant Molecules via Macromolecular Composition and Sequence

We have successfully synthesized a series of monodispersed chain-like giant molecules with precisely controlled macromolecular composition and sequence based on polyhedral oligomeric silsesquioxane (POSS) nanoparticles using an orthogonal "click" strategy. Their nonspherical supramolecular structures, such as lamellae, double gyroids, and hexagonal packed cylinders, are mainly determined by the composition (namely, the number of incorporated amphiphilic nanoparticles). In addition, by precisely alternating the sequence of arranged nanoparticles in the giant molecules with identical chemical compositions, the domain sizes of their supramolecular structures could be fine-tuned. This is attributed to the macromolecular conformational differences caused by collective hydrogen bonding interactions in each set of sequence isomeric giant molecules. This work has demonstrated multilevel manipulation of supramolecular structures of giant molecules: coarse tuning by composition and fine-tuning by sequence.

Anisotropic convergence of dendritic macromolecules facilitated by a heteroleptic metal-organic polyhedron scaffold

Anisotropic dendrimers with bipolar shapes were systematically obtained using a heteroleptic metal-organic polyhedron (MOP) as a robust core scaffold. The structure of one of these polyhedral shapes was unambiguously determined by single-crystal X-ray analysis, which revealed that the bulky dendrons converge to both axial positions of the heteroleptic MOP core.

POSS solid solutions exhibiting orientationally disordered phase transitions

Examples of solid solution behavior for molecular crystals are limited, due to difficulty in designing a crystalline mixture of different molecules. Here, we found that a mixture of two different monofunctionalized heptaisobutyl-substituted octasilsesquioxanes formed solid solutions showing a single phase-transition temperature, which exhibited orientationally disordered (OD) phases.

Highly Asymmetric Phase Behaviors of Polyhedral Oligomeric Silsesquioxane-Based Multiheaded Giant Surfactants

This work reports the molecular design, synthesis, and systematic study on the bulk self-assembly behaviors of three series of polyhedral oligomeric silsesquioxane (POSS)-based multiheaded giant surfactants XDPOSS-PS_n (X = 2, 3, and 4), which are composed of two, three, or four hydrophilic hydroxyl-group-functionalized DPOSS cages attached via one junction point to a hydrophobic polystyrene (PS) chain. These series of hybrid polymeric amphiphiles with precisely defined chemical structure and controllable molecular architecture are synthesized by the sequential usage of "click" reactions. By tuning molecular weights of the PS tail, we established full phase diagrams of XDPOSS-PS_n as a function of the volume fractions of PS chains (V_f^PS). We found that the self-assembled structures were greatly influenced by the molecular architecture. Strikingly, our results showed that the lamellar morphology, which usually existed at relatively symmetric compositions in common diblock copolymers, became the thermodynamically stable phase in the 3DPOSS-PS_n and 4DPOSS-PS_n samples even at an asymmetric composition up to V_f^PS = 0.842, with the ratio between the thicknesses of PS and DPOSS lamellae up to 5.32. This unusual phenomenon induced by molecular architectural variation could be explained by the large cross-sectional area of DPOSS cages at the nanophase-separated domain interface and high elastic deformation energy of clustered DPOSS cages which have relatively rigid conformation. The unique bulk self-assembly behaviors in our POSS-based multiheaded giant surfactants provide insights in developing hybrid nanomaterials toward unconventional nanostructures.

Plasmid-Templated Control of DNA-Cyclodextrin Nanoparticle Morphology through Molecular Vector Design for Effective Gene Delivery

Engineering self-assembled superstructures through complexation of plasmid DNA (pDNA) and single-isomer nanometric size macromolecules (molecular nanoparticles) is a promising strategy for gene delivery. Notably, the functionality and overall architecture of the vector can be precisely molded at the atomic level by chemical tailoring, thereby enabling unprecedented opportunities for structure/self-assembling/pDNA delivery relationship studies. Beyond this notion, by judiciously preorganizing the functional elements in cyclodextrin (CD)-based molecular nanoparticles through covalent dimerization, here we demonstrate that the morphology of the resulting nanocomplexes (CDplexes) can be tuned, from spherical to ellipsoidal, rod-type, or worm-like nanoparticles, which makes it possible to gain understanding of their shape-dependent transfection properties. The experimental findings are in agreement with a shift from chelate to cross-linking interactions on going from primary-face- to secondary-face-linked CD dimers, the pDNA partner acting as an active payload and as a template. Most interestingly, the transfection efficiency in different cells was shown to be differently impacted by modifications of the CDplex morphology, which has led to the identification of an optimal prototype for tissue-selective DNA delivery to the spleen in vivo.

Cylindrical micelles of a POSS amphiphilic dendrimer as nano-reactors for polymerization

A low generation amphiphilic dendrimer, POSS-AD, which has a POSS core and eight amphiphilic arms, was synthesized and used as a nano-reactor to produce well-defined polymer nano-cylinders. Confirmed by small-angle X-ray scattering (SAXS), Raman and NMR spectrometry, monodispersed cylindrical micelles that contain a hydrophilic cavity with a diameter of 2.09 nm and a length of 4.26 nm were produced via co-assembling POSS-AD with hydrophilic liquids, such as H₂O and HEMA in hydrophobic solvents. Taking the HEMA/POSS-AD cylindrical micelles as nano-reactors, polymerization of HEMA within the micelles results in polymer nano-cylinders (POSS-ADNPs) with a diameter of 2.24 nm and a length of 5.02 nm. The study confirmed that despite the inability to maintain specific shape in solution, low generation dendrimers form well-defined nano-containers or nano-reactors, which relies on co-assembling with hydrophilic guest molecules. These nano-reactors are robust enough to maintain their shape during the polymerization of the guest molecules. Polymer nano-cylinders with dimensions less than 10 nm can thus be produced from the HEMA/POSS-AD micelles. Since the chemical structure of low-generation dendrimers and the contents of the co-assembled nano-reactors can be easily adjusted, the concept holds the potential for the further developments of low-generation amphiphilic dendrimers.