Crystallization-driven Self-assembly Research Articles

Asymmetric Polymerization-Induced Crystallization-Driven Self-Assembly of Helical, Rod-Coil Poly(aryl isocyanide) Block Copolymers

Polymerization-induced crystallization-driven self-assembly (PI-CDSA) is combined, for the first time, with helical, rod-coil block copolymer (BCP) self-assembly to enable scalable and controllable in situ synthesis of chiral nanostructures of variable shape, size, and dimensionality. Herein, we report newly developed asymmetric PI-CDSA (A-PI-CDSA) methodologies in the synthesis and in situ self-assembly of chiral, rod-coil BCPs composed of poly(aryl isocyanide) (PAIC) rigid-rod and poly(ethylene glycol) (PEG) random-coil components. Using PEG-based nickel(II) macroinitiators, the construction of PAIC-BCP nanostructures with variable chiral morphologies is accomplished at solids contents ranging 5.0-10 wt %. At low core-to-corona ratios for PAIC-BCPs, we demonstrate the scalable formation of chiral one-dimensional (1D) nanofibers via "living" A-PI-CDSA whose contour lengths can be tuned through alterations to unimer-to-1D seed particle ratio. At high core-to-corona ratios, A-PI-CDSA was implemented for the rapid fabrication of molecularly thin, uniform hexagonal nanosheets via spontaneous nucleation and growth aided by vortex agitation. Investigations into 2D seeded, living A-PI-CDSA revealed a brand-new paradigm in the context of CDSA where hierarchically chiral, M helical spirangle morphologies (i.e., hexagonal helicoids) are size-tuned in three dimensions (i.e., heights and areas) via alterations to unimer-to-seed ratio. These unique nanostructures are formed in situ at scalable solids contents up to 10 wt % via rapid crystallization about screw dislocation defect sites in an enantioselective fashion. The liquid crystalline nature of PAIC blocks dictates the hierarchical assembly of these BCPs, with chirality translated across length scales and in multiple dimensions affording large amplifications in chiroptical activity with g-factors reaching -0.030 for spirangle nanostructures.

Chemical shield effect of metal complexation on seeded growth of poly(ε-caprolactone) core-forming blends

Seeded growth approach is an ambient temperature, living crystallization-driven self-assembly (CDSA) process of crystallizable block copolymers (BCPs) in selective solvents, which enables to the formation of micelle particles in a controllable and designable way. However, the seeded growth of crystallizable BCPs under spatial restriction exerted by external interactions is rarely explored. In present work, the seeded growth of polymer blend involving homopolymer and BCP is extensively studied with the unique complexation interaction between metal ions and poly(4-vinylpyridine) (P4VP) corona segments. Incorporation of certain amount of metal ions with 1D PCL-b-P4VP (PCL = poly(ε-caprolactone)) seeds would result in the inactivity of crystalline seeds. This chemical shield effect is largely related to the binding strength and molar ratio of metal ion to 4VP groups. The metal complexation between Cu2+ and PCL/PCL-b-P4VP blend unimer that is for seeded growth would also lead to the failure of successful epitaxial growth, which is caused by the cross-linking effect of P4VP segments in the presence of metal ions. Nevertheless, this cross-linking effect could be efficiently removed by adding a ligand that has a much stronger binding strength with Cu2+. Our finding provides a deep understanding of metal complexation on the seeded growth of crystallizable BCPs, and also provides useful information about the chemical shield effect exerted by corona conformation that will guide future studies of living CDSA.

Supramolecular Networks Obtained by Block Copolymer Self-Assembly in a Polymer Matrix: Crystallization Behavior and Its Effect on the Mechanical Response

In recent years, there has been growing interest in the study of supramolecular networks obtained by self-assembly of amphiphilic molecules due to their responsive behavior to different external stimuli. The possibility of embedding supramolecular networks into polymer matrices opens access to a new generation of functional polymers with great potential for various applications. However, very little is known about how the dynamics of the supramolecular network is affected by diffusional and topological limitations imposed by the polymer matrix. In this work, we investigate the behavior of supramolecular networks embedded into a rubbery polymer. Crystallization-driven self-assembly of a poly(ethylene-block-ethylene oxide) (PE-b-PEO) diblock copolymer was used to generate supramolecular networks in dimethacrylate monomers, which were then photopolymerized at room temperature. PE-b-PEO self-assembles into nanoribbons with a semicrystalline PE core bordered by coronal chains of PEO, and the nanoribbons, in turn, bundle into lamellar aggregates with an average stacking period of around 45 nm. The nanoribbons are interconnected through crystalline nodes in a 3D network structure. Small-angle X-ray scattering experiments show that the polymer matrix preserves the structure of the supramolecular network and avoids its disintegration when the material is heated above the melting temperature of PE cores. Successive self-nucleation and annealing studies reveal that the polymer matrix does not influence the crystallization–melting processes of PE, which take place through the interconnected cores of the supramolecular network. In contrast, the matrix imposes strong effects of topological confinement on the crystallization of PEO, limiting the dimensions of the crystalline lamellae that can be formed. Mechanical tests show that the deformation capacity of these materials can be precisely tuned by programming the temperature within the melting range of the supramolecular network. This behavior was also characterized by shape memory cyclic tests.

Uniform Two-Dimensional Crystalline Platelets with Tailored Compositions for pH Stimulus-Responsive Drug Release.

Two-dimensional, size-tunable, water-dispersible particle micelles with spatially defined chemistries can be obtained by using "living" crystallization-driven self-assembly (CDSA) approach. Nevertheless, a major obstacle of crystalline particles in drug delivery application is the difficulty in accessing to cargo within crystalline cores. In the present work, we design four different types of biocompatible two-dimensional platelets with a crystalline poly(ε-caprolactone) (PCL) core, a hydrophobic poly(4-vinylprydine) (P4VP) segment, and a water dispersible poly(N,N-dimethyl acrylamide) (PDMA) block in ethanol by seeded growth method. Transferring those uniform platelets with tailored compositions to an aqueous solution in the presence of a hydrophobic drug leads to efficient encapsulation of the cargo in the P4VP segments via hydrophobic interactions. These drug-loaded platelets exhibit pH-responsive release behavior in aqueous media due to the protonated-deprotonated process of P4VP blocks in acidic and neutral solutions. This work provides initial insight into biocompatible PCL platelets with low dispersity and precise chemistry control in stimulus-responsive drug delivery fields.

Heterobimetallic block copolymers with a combined main-chain/side-chain topology.

We report for the first time the synthesis of heterobimetallic block copolymers with a combined main-chain/side-chain topology. The photophysical, electrochemical, and thermal properties and ring-opening metathesis polymerization-induced crystallization-driven self-assembly (ROMPI-CDSA) behavior of these polymers can be significantly modulated by switching the metal identity.

Semi-conductive micellar networks of all-conjugated diblock and triblock copolymer blends.

The crystallization-driven self-assembly of the blends of the all-conjugated block copolymers of poly(3-hexylthiophene) (P3HT) and poly(3-ethylhexylthiophene) (P3EHT) results in the cross-linking of the one-dimensional nanowires of P3HT-b-P3EHT, which is achieved by intercalating P3HT-b-P3EHT-b-P3HT into the nanowire cores. The micellar networks constitute flexible and porous materials that conduct electricity upon doping.

Synthesis and living crystallization-driven self-assembly of backbone asymmetric and symmetric π-conjugated oligo(p-phenylene ethynylene)-based block copolymers

The backbone symmetry of π-conjugated segments is one of the most important structural factors that affect their crystalline behaviors.

Uniform and Length-Tunable, Paramagnetic Self-Assembled Nitroxide-Based Nanofibers for Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) is a widely used non-invasive medical imaging tool. Nanoparticle-based MRI contrast agents have received considerable attention due to their high loading capability for magnetic species, enhanced accumulation in lesions, and versatile surface functionalization. Anisotropic nanoparticles such as nanofibers can exhibit significant advantages over their well-explored spherical counterparts in terms of their pharmacokinetic and biodistribution profiles. Herein, we report the retrosynthetic design, synthesis, and characterization of uniform and length-tunable paramagnetic core-shell nanofibers for MRI through the use of the seeded-growth “living” crystallization-driven self-assembly (CDSA) approach. Triblock copolymer (TriBCP) precursors with a crystallizable polycarbonate core-forming segment, a nitroxide-bearing central region, and a hydrophilic poly(ethylene glycol) (PEG) terminal corona-forming segment were prepared via sequential living organocatalytic ring-opening polymerization (ROP). Low dispersity nanofibers of length ca. 80 nm relevant for biomedical applications were prepared for detailed studies by living CDSA and these possessed an average number of nitroxides per nanofiber of >8000. Subsequent evaluation of the water-proton relaxivities demonstrated that tuning the hydrophilicity of the central segment in the TriBCP allowed access to nanofibers with impressive performance compared to most existing polymer-based nitroxide-based contrast agents. As a result of their 1D morphology, the synthetic nanofibers therefore represent promising organic radical contrast agents (ORCAs) for MRI applications.

Living Crystallization-Driven Self-Assembly of Oligo(p-phenylene vinylene)-Containing Block Copolymers: Impact of Branched Structure of Alkyl Side Chain of π-Conjugated Segment

Living Crystallization-Driven Self-Assembly of Oligo(p-phenylene vinylene)-Containing Block Copolymers: Impact of Branched Structure of Alkyl Side Chain of π-Conjugated Segment

Precise and Controllable Assembly of Block Copolymers†

Comprehensive SummaryWith the development of nanotechnology, the precise synthesis of nanoparticles with nicely‐defined dimensions and structures has been well‐developed, and the functionalization and subsequent applications of the resultant nanostructures are becoming increasingly important. Comparing to inorganic nanoparticles, the nanostructures based on soft matters, especially block copolymer assemblies, are much lower in cost, easier to fabricate and richer in morphology. However, the dimensional control over the block copolymer assemblies is not as easy. Only in recent decade, with the discovery of living Crystallization‐Driven Self‐Assembly (CDSA) by Manners and Winnik, researchers become able to precisely tune the sizes of block copolymer assemblies in a relatively wide range. This discovery has inspired tremendous research effort in the self‐assembly field, and considerable progress has been made recently. This review summarizes the main progress in the precise and controllable self‐assembly field in the past five years, and is mostly focused on four aspects, including in‐depth understanding of the assembly methods, extension of this method to two‐dimensional nanostructures, utilization of this method to fabricate hierarchical structures, and the potential applications of these well‐defined nanostructures. We hope not only to make a periodic systematic summary of previous studies, but also to provide some useful thinking for the future development of this field.

Multicompartment Nanoparticles by Crystallization-Driven Self-Assembly of Star Polymers: Combining High Stability and Loading Capacity.

Herein novel multicompartment nanoparticles (MCNs) that combine high stability and cargo loading capacity are developed. The MCNs are fabricated by crystallization-driven self-assembly (CDSA) of a tailor-made 21 arm star polymer, poly(L-lactide)[poly(tert-butyl acrylate)-block-poly(ethylene glycol)]20 [PLLA(PtBA-b-PEG)20 ]. Platelet-like or spherical MCNs containing a crystalline PLLA core and hydrophobic PtBA subdomains are formed and stabilized by PEG. Hydrophobic cargos, such as Nile Red and chemotherapeutic drug doxorubicin, can be successfully encapsulated into the collapsed PtBA subdomains with loading capacity two orders of magnitude higher than traditional CDSA nanoparticles. Depolarized fluorescence measurements of the Nile Red loaded MCNs suggest that the free volume of the hydrophobic chains in the nanoparticles may be the key for regulating their drug loading capacity. In vitro study of the MCNs suggests excellent cytocompatibility of the blank nanoparticles as well as a dose-dependent cellular uptake and cytotoxicity of the drug-loaded MCNs.

Nanocomposites of epoxy with one-dimensional fibrous poly(ε-caprolactam) nanocrystals via crystallization-driven self-assembly

In this contribution, we reported the preparation of nanocomposites of epoxy with one-dimensional (1D) poly(ε-caprolactam) (PA6) nanocrystals via crystallization-driven self-assembly. First, a novel diblock copolymer composed of poly(ethylene oxide) and poly(ε-caprolactam) subchains (PEO-b-PA6) was synthesized via the anionic ring-opening polymerization (ROP) of ε-caprolactam. It was found that the PEO-b-PA6 diblocks displayed crystallization-driven self-assembly (CDSA) behavior in the selective solvents (e.g., water); the 1D fibrous nanocrystals of PA6 were obtained via CDSA approach. Such a CDSA behavior was utilized to generate 1D fibrous PA6 nanocrystals into epoxy thermosets. In this case, the epoxy precursors were used as the solvent selective for PEO subchain of the diblock. Notably, the 1D fibrous PA6 nanocrystals were generated in the epoxy precursors via the CDSA approach. Upon curing, the nanocomposites (i.e., the nanostructured thermosets containing PA6 nanocrystals) were successfully obtained. It was found that epoxy thermoset was significantly nanoreinforced by the PA6 nanocrystals. In the meantime, the fracture toughness of the materials was significantly improved with the incorporation of 1D fibrous PA6 nanocrystals.

Soft nanobrush-directed multifunctional MOF nanoarrays

Controlled growth of well-oriented metal-organic framework nanoarrays on requisite surfaces is of prominent significance for a broad range of applications such as catalysis, sensing, optics and electronics. Herein, we develop a highly flexible soft nanobrush-directed synthesis approach for precise in situ fabrication of MOF nanoarrays on diverse substrates. The soft nanobrushes are constructed via surface-initiated living crystallization-driven self-assembly and their active poly(2-vinylpyridine) corona captures abundant metal cations through coordination interactions. This allows the rapid heterogeneous growth of MOF nanoparticles and the subsequent formation of MIL-100 (Fe), HKUST-1 and CUT-8 (Cu) nanoarrays with tailored heights of 220~1100 nm on silicon wafer, Ni foam and ceramic tube. Auxiliary functional components including metal oxygen clusters and precious metal nanoparticles can be readily incorporated to finely fabricate hybrid structures with synergistic features. Remarkably, the MIL-100 (Fe) nanoarrays doped with Keggin H3PMo10V2O40 dramatically boost formaldehyde selectivity up to 92.8% in catalytic oxidation of methanol. Moreover, the HKUST-1 nanoarrays decorated with Pt nanoparticles show exceptional sensitivity to H2S with a ppb-level detection limit.

Amphiphilic Block Copolymers: Their Structures, and Self-Assembly to Polymeric Micelles and Polymersomes as Drug Delivery Vehicles.

Self-assembly of amphiphilic block copolymers display a multiplicity of nanoscale periodic patterns proposed as a dominant tool for the 'bottom-up' fabrication of nanomaterials with different levels of ordering. The present review article focuses on the recent updates to the self-association of amphiphilic block copolymers in aqueous media into varied core-shell morphologies. We briefly describe the block copolymers, their types, microdomain formation in bulk and micellization in selective solvents. We also discuss the characteristic features of block copolymers nanoaggregates viz., polymer micelles (PMs) and polymersomes. Amphiphilic block copolymers (with a variety of hydrophobic blocks and hydrophilic blocks; often polyethylene oxide) self-assemble in water to micelles/niosomes similar to conventional nonionic surfactants with high drug loading capacity. Double hydrophilic block copolymers (DHBCs) made of neutral block-neutral block or neutral block-charged block can transform one block to become hydrophobic under the influence of a stimulus (physical/chemical/biological), and thus induced amphiphilicity and display self-assembly are discussed. Different kinds of polymer micelles (viz. shell and core-cross-linked, core-shell-corona, schizophrenic, crew cut, Janus) are presented in detail. Updates on polymerization-induced self-assembly (PISA) and crystallization-driven self-assembly (CDSA) are also provided. Polyion complexes (PICs) and polyion complex micelles (PICMs) are discussed. Applications of these block copolymeric micelles and polymersomes as nanocarriers in drug delivery systems are described.

Scalable and Uniform Length-Tunable Biodegradable Block Copolymer Nanofibers with a Polycarbonate Core via Living Polymerization-Induced Crystallization-Driven Self-assembly.

Uniform 1D block copolymer (BCP) nanofibers prepared by the seeded-growth approach termed living crystallization-driven self-assembly (CDSA) offer promising potential for various applications due to their anisotropy, length tunability, and variable core and coronal chemistries. However, this procedure consists of a multi-step process involving independent BCP synthesis and self-assembly steps, where the latter is performed at low solution concentrations (<1 wt %), hindering scale-up. Here, we demonstrate the use of a one-pot BCP synthesis and self-assembly process, polymerization-induced CDSA (PI-CDSA), to access length-disperse nanofibers with a biodegradable crystalline poly(fluorenetrimethylenecarbonate) (PFTMC) core and a hydrophilic poly(ethylene glycol) (PEG) corona derived from PEG-b-PFTMC at concentrations up to 20 wt %, 400 times higher than those previously reported. Furthermore, living PI-CDSA could be used to access scalable, low dispersity, and length-tunable 1D PEG-b-PFTMC nanofibers at concentrations of up to 10 wt %. This provides the first example of living PI-CDSA involving an all-organic and biodegradable BCP that utilizes a conveniently implemented BCP synthesis protocol and does not involve living anionic polymerization. Significantly, samples of low-dispersity nanofibers of controlled lengths from 100 to 660 nm (Lw/Ln = 1.08-1.20) were prepared, allowing for upscaled access to well-defined biodegradable nanofibers at useful length-scales for applications in nanomedicine. Interestingly, detailed studies revealed a key role for PFTMC homopolymer impurities in the BCP prepared in situ in the formation of nanofibers under the reaction conditions used.

Role of Competitive Crystallization Kinetics in the Formation of 2D Platelets with Distinct Coronal Surface Patterns via Seeded Growth.

Low dispersity 2D platelet micelles with controllable surface patterns were prepared by seeded-growth/living crystallization-driven self-assembly (CDSA) of block copolymer/homopolymer (BCP/HP) blends of poly(ferrocenyldimethylsilane)-b-poly(2-vinyl pyridine) (PFS-b-P2VP) and PFS. The precise morphology was found to be dependent on the proportion of the P2VP corona block, which can be efficiently controlled by changing the molar concentration ratio of PFS-b-P2VP/PFS, (cB/cH)t, as well as their relative rates of crystallization, (GB/GH)t. In the case where their molar concentration ratio was comparable to their crystallization rate ratio, platelets with a uniform distribution of P2VP coronal chains were formed. In other cases, as the concentration ratio increased (or decreased) during the living CDSA process, hierarchical structures were formed, including chain-like assemblies consisting of end-to-end linked rectangular platelets and fusiform (tapered) micelles. (GB/GH)t was adjusted by tuning the degree of polymerization of the crystallizable PFS core-forming block and the BCP block ratio and by varying the terminus of the HP or changing the solvent used. Furthermore, the open edge of the platelets remained active for further growth, which permitted control of the morphology and dimensions of the platelets. Interestingly, in cases where the molar concentration ratio was lower than the crystallization rate ratio, growth rings were observed after two or more living CDSA steps. This study on the formation of platelet micelles by living CDSA of BCP/HP blends under kinetic control offers a considerable scope for the design of 2D polymer nanomaterials with controlled shape and surface patterns.

Generation of One-Dimensional Fibrous Polyethylene Nanocrystals in Epoxy Thermosets.

The one-dimensional (1D) polyethylene (PE) nanocrystals were generated in epoxy thermosets via crystallization-driven self-assembly. Toward this end, an ABA triblock copolymer composed of PE midblock and poly(ε-caprolactone) (PCL) endblocks was synthesized via the ring opening metathesis polymerization followed by hydrogenation approach. The nanostructured thermosets were obtained via a two-step curing approach, i.e., the samples were cured first at 80 °C and then at 150 °C. Under this condition, the one-dimensional (1D) fibrous PE microdomains with the lengths up to a couple of micrometers were created in epoxy thermosets. In contrast, only the spherical PE microdomains were generated while the thermosets were cured via a one-step curing at 150 °C. By the use of the triblock copolymer, the generation of 1D fibrous PE nanocrystals is attributable to crystallization-driven self-assembly mechanism whereas that of the spherical PE microdomains follows traditional self-assembly mechanism. Compared to the thermosets containing the spherical PE microdomains, the thermosets containing the 1D fibrous PE nanocrystals displayed quite different thermal and mechanical properties. More importantly, the nanostructured thermosets containing the 1D fibrous PE nanocrystals displayed the fracture toughness much higher than those only containing the spherical PE nanocrystals; the KIC value was even three times as that of control epoxy.

RAFT polymerization-induced self-assembly of semifluorinated liquid-crystalline block copolymers

Abstract It is a major challenge to prepare commercially viable scale semifluorinated liquid-crystalline block copolymers (SEFL-BCPs) using solution processing techniques. The technology of selectively solvating one block in a suitable solvent to realize self-assembly provides a promising route for the preparation of core-corona block polymer materials with extensive potential applications. However, considerable limitations have been discovered after much practice. BCP assemblies often require a separate synthesis step and are performed at high dilution. Herein, a one-pot approach combining polymerization-induced and crystallization-driven self-assembly (PISA-CDSA) was used to obtain well-defined, precise compositions of SEFL-BCPs. It is first synthesized via reversible addition-fragmentation chain transfer ethanol dispersion polymerization between 1H,1H,2H,2H-heptadecafluorodecyl acrylate and poly(N,N-dimethylacrylamide) at a concentration up to 20% v/v. Various morphologies, including 1D fiber-like micelles, 2D lamellar structures, and fusion structures, were first observed via transmission electron microscopy. This scalable PISA-CDSA strategy is greatly expanding the morphology scope and applicability of the polymer liquid crystal materials science field.

AIE-Active, Stimuli-Responsive Fluorescent 2D Block Copolymer Nanoplatelets Based on Corona Chain Compression.

Aggregation-induced emission (AIE) represents a powerful tool in nanoscience as a result of enhanced luminescence in the condensed state. Although AIEgenic materials have been utilized in a wide range of applications, well-defined self-assembled nanoparticles with tailorable and uniform dimensions and morphology remain challenging to access. Herein, we use the seeded growth, living crystallization-driven self-assembly (CDSA) method to prepare size-tunable and uniform AIE-active 2D nanoplatelets from amphiphilic block copolymer (BCP) precursors with a crystallizable core-forming block and a corona-forming block to which tetraphenylethene (TPE) groups were covalently grafted as AIE-active pendants. The nanoplatelets were formed as a result of a solvophobicity-induced 1D to 2D morphology preference change, which accompanied the seeded growth of a BCP with a quaternized corona-forming block bearing the TPE luminogen. The 2D nanoplatelets exhibited a solvent-responsive fluorescent emission, and examples with coronas containing homogeneously distributed AIE-active TPE groups and Hg(II)-capturing thymine units exhibited excellent performance as proof-of-concept "turn-on" sensors for Hg(II) detection with a rapid response, high selectivity, and a low detection limit (5-125 × 10-9 M, i.e., 1-25 ppb). The fluorescence intensity was found to be nonlinear with respect to analyte concentration and to increase with the area of the nanoplatelet. This behavior is consistent with a cooperative mechanism based on changes in the steric compression of the corona chains, which gives rise to a restriction of the intramolecular motion (RIM) effect.

Precise Control of Two-Dimensional Platelet Micelles from Biodegradable Poly(p-dioxanone) Block Copolymers by Crystallization-Driven Self-Assembly

Two-dimensional (2D) core–shell nanoparticles have been attracting increasing interest due to their wide applications in materials science. Living crystallization-driven self-assembly (CDSA) is an ambient temperature, seeded growth method of crystallizable block copolymers (BCPs) in selective solvents, which has been demonstrated to be a powerful tool for the creation of one-dimensional (1D)/2D nanomaterials with precise control over size and compositions. Nevertheless, the development of an efficient living CDSA approach is a challenge for the case of semicrystalline poly(p-dioxanone) (PPDO) as a core-forming block, where the dimensional control is poor. Herein, we demonstrate that the insufficient size control of 2D PPDO platelets could be overcome through modulation of solvent compositions or elevating the crystallization temperatures for PPDO. The possible mechanism involves an improved unimer solubility that avoids fast unimer aggregation. As a result, uniform 2D platelet micelles with a controlled area over a substantial size range are created via epitaxial growth of the unimer. It is noteworthy that the shape control of 2D platelet micelles from quasi-square to elongated hexagon to lozenge shapes can be accessible by regulating the crystallization conditions such as adding different amounts of cosolvents or crystallization at an elevated temperature. Meanwhile, spatially defined block comicelles can be achieved via the seeded growth approach from PPDO core-forming BCPs with different corona functionalities at elevated temperatures. The excellent water stability and biocompatibility properties of 2D PPDO platelet micelles further enable them to have a potential application as cargo vehicles in the drug delivery field.