Functional Nanoporous Materials From Boronate-Containing Stimuli-Responsive Diblock Copolymers

Functional nanoporous polymeric materials have been prepared from novel polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymer precursors containing a reversible boronate ester junction between both blocks. To this purpose, homopolymers presenting either...

Clip Chemistry: Diverse (Bio)(macro)molecular and Material Function through Breaking Covalent Bonds

In the two decades since the introduction of the "click chemistry" concept, the toolbox of "click reactions" has continually expanded, enabling chemists, materials scientists, and biologists to rapidly and selectively build complexity for their applications of interest. Similarly, selective and efficient covalent bond breaking reactions have provided and will continue to provide transformative advances. Here, we review key examples and applications of efficient, selective covalent bond cleavage reactions, which we refer to herein as "clip reactions." The strategic application of clip reactions offers opportunities to tailor the compositions and structures of complex (bio)(macro)molecular systems with exquisite control. Working in concert, click chemistry and clip chemistry offer scientists and engineers powerful methods to address next-generation challenges across the chemical sciences.

Nanoporous Thin Films Formed from Photocleavable Diblock Copolymers on Gold Substrates Modified with Thiolate Self-Assembled Monolayers

Nanoporous thin films formed on electrodes are considered functional elements of electrochemical sensing systems, thus motivating methods for their development. We report a preparative strategy detailing the effects of surface modification of gold substrates with thiolate self-assembled monolayers (SAMs) on the properties of nanoporous thin films derived from polystyrene-block-poly(ethylene oxide) having a photocleavable o-nitrobenzyl ester junction (PS-hν-PEO). Two PS-hν-PEO having similar PEO volume fractions (≈0.2) but different molecular weights (10 and 23 kg/mol) were used to prepare films (30-100 nm thick) spin-cast on gold substrates unmodified and modified with cysteamine, thioctic acid, and 6-hydroxy-1-hexanethiol SAMs. Solvent vapor annealing followed by PEO removal led to the formation of nanopores with average diameters of 12 and 19 nm from the smaller and larger PS-hν-PEO, respectively. Cyclic voltammograms of 1,1'-ferrocenedimethanol showed that nanoporous films on cysteamine SAMs afforded nanopores reaching the underlying substrates at higher density than those on the other substrates. This result was attributed to balanced affinity of the cysteamine SAM surface with PS and PEO, which enhanced the vertical orientation of PEO microdomains. The generation of carboxyl groups associated with the photocleavage reaction was revealed by pH-dependent changes in the voltammogram of Fe(CN)₆^3- that reflected electrostatic effects regulated by the protonation state of the carboxyl groups. The SAMs underneath the nanoporous films could be replaced by treatment with a thiol solution, as verified by voltammograms of l-ascorbic acid. These results suggest that thiolate SAM modification provides a simple means to control the interfacial orientation of PEO microdomains in thin PS-hν-PEO films.

Porous Ultra-Thin Films from Photocleavable Block Copolymers: In-Situ Degradation Kinetics Study of Pore Material

On the basis of the major application for block copolymers to use them as separation membranes, lithographic mask, and as templates, the preparation of highly oriented nanoporous thin films requires the selective removal of the minor phase from the pores. In the scope of this study, thin film of polystyrene-block-poly(ethylene oxide) block copolymer with a photocleavable junction groups based on ortho-nitrobenzylester (ONB) (PS-hν-PEO) was papered via the spin coating technique followed by solvent annealing to obtain highly-ordered cylindrical domains. The polymer blocks are cleaved by means of a mild UV exposure and then the pore material is washed out of the polymer film by ultra-pure water resulting in arrays of nanoporous thin films to remove one block. The removal of the PEO materials from the pores was proven using the grazing-incidence small-angle X-ray scattering (GISAXS) technique. The treatment of the polymer film during the washing process was observed in real time after two different UV exposure time (1 and 4 h) in order to draw conclusions regarding the dynamics of the removal process. In-situ X-ray reflectivity measurements provide statistically significant information about the change in the layer thickness as well as the roughness and electron density of the polymer film during pore formation. 4 H UV exposure was found to be more efficient for PEO cleavage. By in-situ SFM measurements, the structure of the ultra-thin block copolymer films was also analysed and, thus, the kinetics of the washing process was elaborated. The results from both measurements confirmed that the washing procedure induces irreversible change in morphology to the surface of the thin film.

Photoinduced Modulation of Polymeric Interfacial Behavior Controlling Thin-Film Block Copolymer Wetting

The tunable surface-wetting properties of photosensitive random copolymer mats were used to spatially control the orientations of thin-film block copolymer (BCP) structures. A photosensitive mat was produced via thermal treatment on spin-coated random copolymers of poly(styrene-ran-2-nitrobenzyl methacrylate-ran-glycidyl methacrylate), synthesized via reversible-deactivation radical polymerization. The degree of UV-induced deprotection of the nitrobenzyl esters in the mat was precisely controlled through the amount of UV-irradiation energy imparted to the mat. The resulting polarity switching of the constituents collectively altered the interfacial wetting properties of the mat, and the tunability allowed lamellar or cylinder-forming poly(styrene-b-methyl methacrylate) BCP thin films, applied over the mat, to change the domain orientation from perpendicular to parallel at proper UV exposures. UV irradiation passing through a photomask was capable of generating defined regions of BCP domains with targeted orientations.

Visible light mediated BODIPY/Azo/cyclodextrin based supramolecular polymer assemblies in different water content solutions

A novel visible light responsive supramolecular polymer based on oligo(ethylene glycol) modified BODIPY (BDP), tetramethoxyazobenzene (Azo) and dimeric β-cyclodextrin (β-CD-C) was reported.

Fabrication techniques enabling ultrathin nanostructured membranes for separations

The fabrication of nanostructured materials is an area of continuous improvement and innovative techniques that fulfill the demand of many fields of research and development. The continuously decreasing size of the smallest patternable feature has expanded the catalog of methods enabling the fabrication of nanostructured materials. Several of these nanofabrication techniques have sprouted from applications requiring nanoporous membranes such as molecular separations, cell culture, and plasmonics. This review summarizes methods that successfully produce through-pores in ultrathin films exhibiting an approximate pore size to thickness ratio of one, which has been shown to be beneficial due to high permeability and improved separation potential. The material reviewed includes large-area, parallel, and affordable approaches such as self-organizing polymers, nanosphere lithography, anodization, nanoimprint lithography as well as others such as solid phase crystallization and nanosphere lens lithography. The aim of this review is to provide a set of inexpensive fabrication techniques to produce nanostructured materials exhibiting pores ranging from 10 to 350 nm and a pore size to thickness ratio close to one. The fabrication methods described in this work have reported the successful manufacture of nanoporous membranes exhibiting the ideal characteristics to improve selectivity and permeability when applied as separation media in ultrafiltration.

A photocleavable stabilizer for the preparation of PHEMA nanogels by dispersion polymerization in supercritical carbon dioxide

Synthesis of PHEMA nanogels stable in water by a scCO₂ process.

Photo-responsive polymers: synthesis and applications

Photo-responsive polymers are able to change their structure, conformation and properties upon light irradiation.

Photopatterning of Block Copolymer Thin Films

Block copolymers are potentially useful materials for large-area 2-D patterning applications due to their spontaneous self-assembly into sub-50 nm domains. However, most thin film engineering applications require patterns of prescribed size, shape, and organization. Photopatterning is a logical choice for manipulating block copolymer features since advanced lithography tools can pattern areas as small as a single block copolymer domain. By exposing either the block copolymer or a responsive interfacial surface to patterned radiation, precise control over placement, orientation, alignment, and selective development of block copolymer domains can be achieved. This Viewpoint highlights some of the recent research in photopatterning block copolymer thin films and identifies areas of future opportunity.

Asymmetric block copolymer membranes with ultrahigh porosity and hierarchical pore structure by plain solvent evaporation

Membranes with a hierarchical porous structure and an isoporous skin could be manufactured from a block copolymer blend by pure solvent evaporation (drying induced phase separation).

Distortion of Ultrathin Photocleavable Block Copolymer Films during Photocleavage and Nanopore Formation

Highly ordered block copolymer thin films have been studied extensively during the last years because they afford versatile self-assembled morphologies via a bottom-up approach. They promise to be used in applications such as polymeric membranes or templates for nanostructured materials. Among the block copolymer structures, perpendicular cylinders have received strong attention due to their ability to fabricate highly ordered nanopores and nanowires. Nanopores can be created from a thin block copolymer film upon the removal of one block by selective etching or by dissolution of one polymer block. Here we demonstrate the utilization of polystyrene-block-poly(ethylene oxide) diblock copolymer (PS-hν-PEO) with an ortho-nitrobenzyl ester (ONB) as the photocleavable block-linker to create highly ordered thin films. Removal of the PEO block by choosing an appropriate solvent upon photocleavage is expected to yield arrays of nanopores decorated with functional groups, thus lending itself to adsorption or filtration uses. While the feasibility of this approach has been demonstrated, it is crucial to understand the influence of removal conditions (i.e., efficiency of photocleavage as well as best washing solvent) and to evaluate changes in the surface topology and inner structure upon photocleavage. To this end, the time dependence evolution of the surface morphology of block copolymer thin films was studied using grazing-incidence small-angle X-ray scattering (GISAXS) technique in combination with scanning probe microscopy.

Tailored pore sizes in integral asymmetric membranes formed by blends of block copolymers

A simple way to generate isoporous membranes with tailored pore sizes is shown. Block copolymers of different compositions are blended in solution, and membranes are obtained by solution casting followed by nonsolvent-induced phase separation. This enables the preparation of integral asymmetric membranes with a defined pore size for given sets of block copolymers just by choosing the right blend composition.

Isoporous block copolymer membranes

The developments in membranes based on tailored block copolymers are reported with an emphasis on isoporous membranes. These membranes can be prepared in different geometries, namely flat sheets and hollow fibers. They display narrow pore size distributions due to their formation by self-assembly. The preparation of these membranes and possibilities to further functionalize such membranes will be discussed. Different ways to control the pore size will be addressed, and the potential of block copolymer blends to fabricate membranes with tailored pore sizes will be shown.

Self-assembly of an interacting binary blend of diblock copolymers in thin films: a potential route to porous materials with reactive nanochannel chemistry

Self-assembly of a binary mixture of poly(styrene)336-block-poly(4-vinyl pyridine)25 (PS336-b-P4VP25) and poly(ethylene glycol)113-block-poly(4-hydroxy styrene)25 (PEG113-b-P4HS25) is shown to give rise to a cylindrical morphology in thin films through pyridine/phenol-based hetero-complementary hydrogen bonding interactions between the P4VP and P4HS copolymer segments. Removal of the cylindrical phase (PEG-b-P4HS) allowed access to porous materials having a pore surface decorated with P4VP polymer blocks. These segments could be transformed into cationic polyelectrolytes through quaternization of the pyridine nitrogen atom. The resulting positively charged nanopore surface could recognize negatively charged gold nanoparticles through electrostatic interactions. This work, therefore, outlines the utility of the supramolecular AB/CD type of block copolymer towards preparation of ordered porous thin films carrying a chemically defined channel surface with a large number of reactive sites.

Directed self-assembly of block copolymers: a tutorial review of strategies for enabling nanotechnology with soft matter

Self-assembly of soft materials is broadly considered an attractive means of generating nanoscale structures and patterns over large areas. However, the spontaneous formation of equilibrium nanostructures in response to temperature and concentration changes, for example, must be guided to yield the long-range order and orientation required for utility in a given scenario. In this review we examine directed self-assembly (DSA) of block copolymers (BCPs) as canonical examples of nanostructured soft matter systems which are additionally compelling for creating functional materials and devices. We survey well established and newly emerging DSA methods from a tutorial perspective. Special emphasis is given to exploring underlying physical phenomena, identifying prototypical BCPs that are compatible with different DSA techniques, describing experimental methods and highlighting the attractive functional properties of block copolymers overall. Finally we offer a brief perspective on some unresolved issues and future opportunities in this field.