Pore‐Functionalized Nanoporous Materials Derived from Block Copolymers

This review deals with nanoporous materials made from the self‐assembly of block copolymers with a special interest in the chemical functions covering the surface of their nanopores. A detailed overview of the existing methods and strategies to generate well‐defined organic functional groups covering the surface of the pore walls is provided. This further enables to finely tune the affinity of the pore walls and to perform well‐defined chemical reactions onto them, which is essential for further dedicated applications.

     

Self‐Assembled Micelles Based on PEG‐Polypeptide Hybrid Copolymers for Drug Delivery

A series of amphiphilic poly(L ‐leucine)‐block‐poly(ethylene glycol)‐block‐poly(L ‐leucine) (PLL‐PEG‐PLL) hybrid triblock copolymers have been synthesized. All the blocks in this system have good biocompatibility and low toxicity. The PLL‐PEG‐PLL copolymers could self‐assemble into micelles with PLL blocks as the hydrophobic core and PEG blocks as the hydrophilic shell, which were characterized by FT‐IR, ¹H NMR, and transmission electron microscopy analysis. The critical micellar concentration of the copolymer was 95.0 mg · L⁻¹. The circular dichroism spectrum shows that the PLL segments adopt a unique α‐helical conformation, which is found to play an important role in controlling the drug release rate. The drug release could be effectively sustained by encapsulation in the micelles. The copolymers may have potential applications in drug delivery.

     

Orthogonally Self‐Assembled Multifunctional Block Copolymers

We report the synthesis of telechelic poly(norbornene) and poly(cyclooctene) homopolymers by ring‐opening metathesis polymerization (ROMP) and their subsequent functionalization and block copolymer formation based on noncovalent interactions. Whereas all the poly(norbornene)s contain either a metal complex or a hydrogen‐bonding moiety along the polymer side‐chains, together with a single hydrogen‐bonding‐based molecular recognition moiety at one terminal end of the polymer chain. These homopolymers allow for the formation of side‐chain‐functionalized AB and ABA block copolymers through self‐assembly. The orthogonal natures of all side‐ and main‐chain self‐assembly events were demonstrated by ¹H NMR spectroscopy and isothermal titration calorimetry. The resulting fully functionalized block copolymers are the first copolymers combining both side‐ and main‐chain self‐assembly, thereby providing a high degree of control over copolymer functionalization and architecture and bringing synthetic materials one step closer to the dynamic self‐assembly structures found in nature.     

Gold‐Decorated Chiral Macroporous Films by the Self‐Assembly of Functionalised Block Copolymers

We describe a new and very versatile method to place chosen chemical functionalities at the edge of the pores of macroporous materials. The method is based on the synthesis and self‐assembly of inorganic block copolymers (BCPs) having chiral rigid segments bearing controllable quantities of randomly distributed functional groups. The synthesis of a series of optically active block copolyphosphazenes (PP) with the general formula [N?P(R‐O₂C₂₀H₁₂)_0.9(FG)_0.2]_n‐b‐[N?PMePh]_m (FG=‐OC₅H₄N ( 6 ), ‐NC₄H₈S ( 7 ), and ‐NC₄H₈O ( 8 )), was accomplished by the sequential living cationic polycondensation of N‐silylphosphoranimines, using the mono‐end‐capped initiator [Ph₃P?N?PCl₃][Cl] ( 3 ). The self‐assembly of the phosphazene BCPs 6 – 8 led to chiral porous films. The functionality present on those polymers affected their self‐assembly behaviour resulting in the formation of pores of different diameters (D_n=111 ( 6 ), 53 ( 7 ) and 77 nm ( 8 )). The specific functionalisation of the pores was proven by decorating the films with gold nanoparticles (AuNPs). Thus, the BCPs 6 and 7 , having pyridine and thiomorpholine groups, respectively, were treated with HAuCl₄, followed by reduction with NaBH₄, yielding a new type of block copolyphosphazenes, which self‐assembled into chiral porous films specifically decorated with AuNPs at the edge of the pores.     

ABC Triblock Terpolymer Self‐Assembled Core–Shell–Corona Nanotubes with High Aspect Ratios

Nanotubes have attracted considerable attention due to their unique 1D hollow structure; however, the fabrication of pure nanotubes via block copolymer self‐assembly remains a challenge. In this work, the successful preparation of core–shell–corona (CSC) nanotubular micelles with uniform diameter and high aspect ratio is reported, which is achieved via self‐assembly of a poly (styrene‐b‐4‐vinyl pyridine‐b‐ethylene oxide) triblock terpolymer in binary organic solvents with assistance of solution thermal annealing. Via direct visualization of trapped intermediates, the nanotube is believed to be formed via large sphere—large solid cylinderical aggregates—nanotube transformations, wherein the unique solid to hollow transition accompanied with the unidirectional growth is distinct from conventional pathway. In addition, by virtue of the CSC structure, gold nanoparticles are able to be selectively incorporated into different micellar domains of the nanotubes, which may have potential applications in nanoscience and nanotechnology.

     

Hierarchical Self‐Assembly of PDMA‐b‐PS Chains into Granular Nanoparticles: Genesis and Fate

The hierarchical self‐assembly of an amphiphilic block copolymer, poly(N,N‐dimethylacrylamide)‐block‐polystyrene with a very short hydrophilic block (PDMA₁₀‐b‐PS₆₂), in large granular nanoparticles is reported. While these nanoparticles are stable in water, their disaggregation can be induced either mechanically (i.e., by applying a force via the tip of the cantilever of an atomic force microscope (AFM)) or by partial hydrolysis of the acrylamide groups. AFM force spectroscopy images show the rupture of the particle as a combination of collapse and flow, while scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of partly hydrolyzed nanoparticles provide a clear picture of the granular structure.

     

Tuning the Chirality of Block Copolymers: From Twisted Morphologies to Nanospheres by Self‐Assembly

New advances into the chirality effect in the self‐assembly of block copolymers (BCPs) have been achieved by tuning the helicity of the chiral‐core‐forming blocks. The chiral BCPs {[N?P(R)‐O₂C₂₀H₁₂]_200?x[N?P(OC₅H₄N)₂]_x}‐b‐ [N?PMePh]₅₀ ((R)‐O₂C₂₀H₁₂=(R)‐1,1′‐binaphthyl‐2,2′‐dioxy, OC₅H₄N=4‐pyridinoxy (OPy); x=10, 30, 60, 100 for 3 a – d , respectively), in which the [N?P(OPy)₂] units are randomly distributed within the chiral block, have been synthesised. The chiroptical properties of the BCPs ([α]_D vs. T and CD) demonstrated that the helicity of the BCP chains may be simply controlled by the relative proportion of the chiral and achiral (i.e., [N?P(R)‐O₂C₂₀H₁₂] and [N?P(OPy)₂], respectively) units. Thus, although 3 a only contained only 5 % [N?P(OPy)₂] units and exhibited a preferential helical sense, 3 d with 50 % of this unit adopted non‐preferred helical conformations. This gradual variation of the helicity allowed us to examine the chirality effect on the self‐assembly of chiral and helical BCPs (i.e., 3 a – c ) and chiral but non‐helical BCPs (i.e., 3 d ). The very significant influence of the helicity on the self‐assembly of these materials resulted in a variety of morphologies that extend from helical nanostructures to pearl‐necklace aggregates and nanospheres (i.e., 3 b and 3 d , respectively). We also demonstrate that the presence of pyridine moieties in BCPs 3 a – d allows specific decoration with gold nanoparticles.     

Helical Packing of Nanoparticles Confined in Cylindrical Domains of a Self‐Assembled Block Copolymer Structure

Theoretical models predict that a variety of self‐assembled structures of closely packed spherical particles may result when they are confined in a cylindrical domain. In the present work we demonstrate for the first time that the polymer‐coated nanoparticles confined in the self‐assembled cylindrical domains of a block copolymer pack in helical morphology, where we can isolate individual fibers filled with helically arranged nanoparticles. This finding provides unique possibilities for fundamental as well as application‐oriented research in similar directions.     

Nanoporous Films with Sub‐10 nm in Pore Size from Acid‐Cleavable Block Copolymers

Nanoporous thin films with pore size of sub‐10 nm are fabricated using an acid‐cleavable block copolymer (BCP), a benzoic imine junction between poly(ethylene oxide) (PEO) and poly(methacrylate) (PMAAz) bearing an azobenzene side chain (denoted as PEO‐bei‐PMAAz) as the precursor. After a thermal annealing, the block copolymers are self‐assembled to form highly ordered PEO cylinders within a PMAAz matrix normal to the film, even in the case of low BCP molecular weight due to the existing of the liquid crystalline (LC) azobenzene rigid segment. Thus, PMAAz thin films with pore size of ≈7 nm and density of ≈10¹² cm⁻² are obtained after removal of the PEO minor phase by breaking the benzoic imine junction under mild acidic conditions. This work enriches the nanoporous polymer films from BCP precursors and introduces the LC property as a functionality which can further enhance the mechanical properties of the films and broaden their applications.

     

Supramolecular Chemistry in the Formation of Self‐Assembled Nanostructures from a High‐Molecular‐Weight Rod–Coil Block Copolymer

The self‐assembled nanostructures of a high‐molecular‐weight rod–coil block copolymer, poly(styrene‐block‐(2,5‐bis[4‐methoxyphenyl]oxycarbonyl)styrene) (PS‐b‐PMPCS), in p‐xylene are studied. The cylindrical micelles, long segmental cylindrical micelle associates, spherical micelles, and spherical micelle associates are observed with increased copolymer concentration. The high molecular weight of PS leads to the entanglement between PS chains from different micelles, which is the force for supramolecular interactions. Short cylindrical micelles are connected end‐to‐end via this supramolecular chemistry to form long segmental cylindrical micelle associates, analogue to the condensation polymerization process, with direction and saturation. On the other hand, spherical micelles assemble via supramolecular chemistry to form spherical micelle associates, yet without any direction due to their isotropic properties.

     

Self‐Assembled Asymmetric Block Copolymer Membranes: Bridging the Gap from Ultra‐ to Nanofiltration

The self‐assembly of block copolymers is an emerging strategy to produce isoporous ultrafiltration membranes. However, thus far, it has not been possible to bridge the gap from ultra‐ to nanofiltration and decrease the pore size of self‐assembled block copolymer membranes to below 5 nm without post‐treatment. It is now reported that the self‐assembly of blends of two chemically interacting copolymers can lead to highly porous membranes with pore diameters as small as 1.5 nm. The membrane containing an ultraporous, 60 nm thin separation layer can fully reject solutes with molecular weights of 600 g mol^?1 in aqueous solutions with a water flux that is more than one order of magnitude higher than the permeance of commercial nanofiltration membranes. Simulations of the membrane formation process by dissipative particle dynamics (DPD) were used to explain the dramatic observed pore size reduction combined with an increase in water flux.     

Self‐Assembly of Diblock Copolymers into Novel Snowflake‐Shaped Aggregates Driven by Quadruple Hydrogen Bonding

Two well‐defined diblock copolymers with quadruple hydrogen‐bonding groups on one block, denoted PSUEA‐1 and PSUEA‐2 , have been synthesized, and novel snowflake‐shaped nanometer‐scale aggregates, self‐assembled by such diblock copolymers in non‐polar solvents, have been observed. The micellar dimensions were investigated by DLLS and SLLS. Their morphologies were studied by TEM. Since the degrees of polymerization of the Upy‐containing blocks of PSUEA‐1 and PSUEA‐2 are quite similar and the polystyrene block of the PSUEA‐1 is longer than that of the PSUEA‐2 , a subtle but identifiable difference between the sizes and structures of the PSUEA‐1 and PSUEA‐2 aggregates was noticed and characterized.

     

Aqueous Self‐Assembly of Purely Hydrophilic Block Copolymers into Giant Vesicles

Self‐assembly of macromolecules is fundamental to life itself, and historically, these systems have been primitively mimicked by the development of amphiphilic systems, driven by the hydrophobic effect. Herein, we demonstrate that self‐assembly of purely hydrophilic systems can be readily achieved with similar ease and success. We have synthesized double hydrophilic block copolymers from polysaccharides and poly(ethylene oxide) or poly(sarcosine) to yield high molar mass diblock copolymers through oxime chemistry. These hydrophilic materials can easily assemble into nanosized (<500 nm) and microsized (>5 μm) polymeric vesicles depending on concentration and diblock composition. Because of the solely hydrophilic nature of these materials, we expect them to be extraordinarily water permeable systems that would be well suited for use as cellular mimics.     

Hybrid Compound Block Copolymer Micelles Encapsulating Gold Nanoparticles

We report here on the formation of hybrid compound block copolymer micelles encapsulating gold nanoparticles, utilizing a direct and general preparation method. The giant hybrid compound micelles are structured with micelles of PS‐b‐P2VP with gold nanoparticles in their P2VP core and PI‐b‐PS chains as the outer part of the compound micelles. The gold nanoparticles were produced using gold ion‐loaded PS‐b‐P2VP micelles as a nanoreactor, in a PS selective solvent (toluene), by the subsequent reduction of gold ions. The synthesis of the gold nanoparticles was monitored by UV‐vis spectroscopy. The gold containing micelles were then encapsulated in larger micelles of PI‐b‐PS copolymer, by successive utilization of toluene and heptane with the intermediate evaporation of toluene. The nanoassembly of the compound materials comprised a PI corona and a PS compound core, with P2VP/Au⁰ domains, and was characterized using UV‐vis spectroscopy, dynamic light scattering and transmission electron microscopy.

     

Synthesis of Continuous Mesoporous Alumina Films with Large‐Sized Cage‐Type Mesopores by Using Diblock Copolymers

Mesoporous alumina films with large‐sized cage‐type mesopores were prepared by using commercially available diblock copolymer (PS‐b‐PEO) and economic inorganic salt (AlCl₃) as aluminum source. The obtained mesopore sizes drastically expand from 35 nm to 80 nm when the amount of ethanol in the precursor solutions were controlled. More interestingly, under an optimized amount of ethanol as co‐solvent, there was no significant change of micelle morphology on the substrate, even though the relative amount of PS‐b‐PEO to alumina source was dramatically varied. When the amount of alumina precursor was decreased, the pore walls gradually became thinner, thereby improving pore connectivity. The ordered mesoporous alumina films obtained in this study exhibit high thermal stability up to 1000 °C, and their frameworks are successfully crystallized to γ‐alumina phase. This technique could also be applicable for creating other metal oxide thin films with large mesopores.     

Ultrathin and Large‐area Macroporous Polymeric Membranes Formed Through Self‐assembly of Sub‐10 nm Elastic Nanoparticles

A variety of sub‐10 nm nanoparticles are successfully prepared by crosslinking of polystyrene‐b‐poly(1,3‐butadiene) (PS‐b‐PB) and polystyrene‐b‐poly(4‐vinyl pyridine) (PS‐b‐P4VP) block copolymer micelles and inverse micelles. Among them, the core‐crosslinked PS‐b‐PB micelles can self‐assemble into ultrathin (< 10 nm) macroporous (pore size <1 µm) membranes in a facile way, i.e., by simply drop‐coating the particle solution onto a mica surface. No continuous/porous membranes are produced from shell‐crosslinked PS‐b‐PB micelles and both forms of PS‐b‐P4VP micelles. This suggests that the unique structure of the block copolymer precursor, including the very flexible core‐forming block and the glassy corona‐forming block and the specific block length ratio, directly determines the formation of the macroporous membrane.

     

Self‐Assembled Isoporous Block Copolymer Membranes with Tuned Pore Sizes

The combination of nonsolvent‐induced phase separation and the self‐assembly of block copolymers can lead to asymmetric membranes with a thin highly ordered isoporous skin layer. The effective pore size of such membranes is usually larger than 15 nm. We reduced the pore size of these membranes by electroless gold deposition. We demonstrate that the pore sizes can be controlled precisely between 3 and 20 nm leading to a tunable sharp size discrimination in filtration processes. Besides fractionation of nanoparticles and biomaterials, controlled drug delivery is an attractive potential application.