Photopatterning of Cross‐Linkable Epoxide‐Functionalized Block Copolymers and Dual‐Tone Nanostructure Development for Fabrication Across the Nano‐ and Microscales

The self‐assembly of block copolymers in thin films provides an attractive approach to patterning 5–100 nm structures. Cross‐linking and photopatterning of the self‐assembled block copolymer morphologies provide further opportunities to structure such materials for lithographic applications, and to also enhance the thermal, chemical, or mechanical stability of such nanostructures to achieve robust templates for subsequent fabrication processes. Here, model lamellar‐forming diblock copolymers of polystyrene and poly(methyl methacrylate) with an epoxide functionality are synthesized by atom transfer radical polymerization. We demonstrate that self‐assembly and cross‐linking of the reactive block copolymer materials in thin films can be decoupled into distinct, controlled process steps using solvent annealing and thermal treatment/ultraviolet exposure, respectively. Conventional optical lithography approaches can also be applied to the cross‐linkable block copolymer materials in thin films and enable simultaneous structure formation across scales—micrometer scale patterns achieved by photolithography and nanostructures via self‐assembly of the block copolymer. Such materials and processes are thus shown to be capable of self‐assembling distinct block copolymers (e.g., lamellae of significantly different periodicity) in adjacent regions of a continuous thin film.     

Fabrication of Covalently Crosslinked and Amine-Reactive Microcapsules by Reactive Layer-by-Layer Assembly of Azlactone-Containing Polymer Multilayers on Sacrificial Microparticle Templates

We report on the fabrication of covalently crosslinked and amine-reactive hollow microcapsules using 'reactive' layer-by-layer assembly to deposit thin polymer films on sacrificial microparticle templates. Our approach is based on the alternating deposition of layers of a synthetic polyamine and a polymer containing reactive azlactone functionality. Multilayered films composed of branched poly(ethylene imine) (BPEI) and poly(2-vinyl-4,4-dimethylazlactone) (PVDMA) were fabricated layer-by-layer on the surfaces of calcium carbonate and glass microparticle templates. After fabrication, these films contained residual azlactone functionality that was accessible for reaction with amine-containing molecules. Dissolution of the calcium carbonate or glass cores using aqueous ethylenediamine tetraacetic acid (EDTA) or hydrofluoric acid (HF), respectively, led to the formation of hollow polymer microcapsules. These microcapsules were robust enough to encapsulate and retain a model macromolecule (FITC-dextran) and were stable for at least 22 hours in high ionic strength environments, in low and high pH solutions, and in several common organic solvents. Significant differences in the behaviors of capsules fabricated on CaCO(3) and glass cores were observed and characterized using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Whereas capsules fabricated on CaCO(3) templates collapsed upon drying, capsules fabricated on glass templates remained rigid and spherical. Characterization using EDS suggested that this latter behavior results, at least in part, from the presence of insoluble metal fluoride salts that are trapped or precipitate within the walls of capsules after etching of the glass cores using HF. Our results demonstrate that the assembly of BPEI/PVDMA films on sacrificial templates can be used to fabricate reactive microcapsules of potential use in a wide range of fields, including catalysis, drug and gene delivery, imaging, and biomedical research.     

Nanostructured Thin Films via Self‐Assembly of Block Copolymers

Thin films of incompatible block copolymers self‐assemble into highly regular supramolecular structures with characteristic dimensions in the 10–100 nm regime. There is increasing interest in controlling the resulting structures and utilizing them, for instance in the area of nanotechnology. So far, research has concentrated mainly on exploiting the melt structure of diblock copolymers. Recent work on block copolymer solutions and more complex co‐ and terpolymer architectures has revealed a rich variety of novel thin‐film structures, some of which exhibit high complexity and order. In addition, by use of mean field dynamic density functional theory along with well‐controlled experiments, the fundamentals of thin film structure formation have been elucidated. We highlight some aspects of these studies and point to future directions in this lively field of materials science.     

Negative‐Tone Block Copolymer Lithography by In Situ Surface Chemical Modification

Negative‐tone block copolymer (BCP) lithography based on in situ surface chemical modification is introduced as a highly efficient, versatile self‐assembled nanopatterning. BCP blends films consisting of end‐functionalized low molecular weight poly(styrene‐ran‐methyl methacrylate) and polystyrene‐block‐Poly(methyl methacylate) can produce surface vertical BCP nanodomains on various substrates without prior surface chemical treatment. Simple oxygen plasma treatment is employed to activate surface functional group formation at various substrates, where the end‐functionalized polymers can be covalently bonded during the thermal annealing of BCP thin films. The covalently bonded brush layer mediates neutral interfacial condition for vertical BCP nanodomain alignment. This straightforward approach for high aspect ratio, vertical self‐assembled nanodomain formation facilitates single step, site‐specific BCP nanopatterning widely useful for various substrates. Moreover, this approach is compatible with directed self‐assembly approaches to produce device oriented laterally ordered nanopatterns.     

Directed Self‐Assembly of Star‐Block Copolymers by Topographic Nanopatterns through Nucleation and Growth Mechanism

Exploring the ordering mechanism and dynamics of self‐assembled block copolymer (BCP) thin films under confined conditions are highly essential in the application of BCP lithography. In this study, it is aimed to examine the self‐assembling mechanism and kinetics of silicon‐containing 3‐arm star‐block copolymer composed of polystyrene (PS) and poly(dimethylsiloxane) blocks as nanostructured thin films with perpendicular cylinders and controlled lateral ordering by directed self‐assembly using topographically patterned substrates. The ordering process of the star‐block copolymer within fabricated topographic patterns with PS‐functionalized sidewall can be carried out through the type of secondary (i.e., heterogeneous) nucleation for microphase separation initiated from the edge and/or corner of the topographic patterns, and directed to grow as well‐ordered hexagonally packed perpendicular cylinders. The growth rate for the confined microphase separation is highly dependent upon the dimension and also the geometric texture of the preformed pattern. Fast self‐assembly for ordering of BCP thin film can be achieved by lowering the confinement dimension and also increasing the concern number of the preformed pattern, providing a new strategy for the design of BCP lithography from the integration of top‐down and bottom‐up approaches.     

Automated Buildup of Biomimetic Films in Cell Culture Microplates for High‐Throughput Screening of Cellular Behaviors

An automatic method is established for layer‐by‐layer (LbL) assembly of biomimetic coatings in cell culture microplates using a commercial liquid‐handling robot. Highly homogeneous thin films are formed at the bottom of each microwell. The LbL film‐coated microplates are compatible with common cellular assays, using microplate readers and automated microscopes. Cellular adhesion is screened on crosslinked and peptide‐functionalized LbL films and stem cell differentiation in response to increasing doses of bone morphogenetic proteins (2, 4, 7, 9). This method paves the way for future applications of LbL films in cell‐based assays for regenerative medicine and high‐throughput drug screening.     

Influence of Chain Architecture on Nanopore Accessibility in Polyelectrolyte Block‐Co‐Oligomer Functionalized Mesopores

Functionalized ordered mesoporous silica materials are commonly investigated for applications such as drug release, sensing, and separation processes. Although, various homopolymer functionalized responsive mesopores are reported, little focus has been put on copolymers in mesopores. Mesoporous silica films are functionalized with responsive and orthogonally charged block‐co‐oligomers. Responsive 2‐dimethylamino)ethyl methacrylate)‐block‐2‐(methacryloyloxy)ethyl phosphate (DMAEMA‐b‐MEP) block‐co‐oligomers are introduced into mesoporous films using controlled photoiniferter initiated polymerization. This approach allows a very flexible charge composition design. The obtained block‐co‐oligomer functionalized mesopores show a complex gating behavior indicating a strong interplay between the different blocks emphasizing the strong influence of charge distribution inside mesopores on ionic pore accessibility. For example, in contrast to mesopores functionalized with zwitterionic polymers, DMAEMA‐b‐MEP block‐co‐oligomer functionalized mesopores, containing two oppositely charged blocks, do not show bipolar ion exclusion, demonstrating the influence of the chain architecture on mesopore accessibility. Furthermore, ligand binding–based selective gating is strongly influenced by this chain architecture as demonstrated by an expansion of pore accessibility states for block‐co‐oligomer functionalized mesopores as compared to the individual polyelectrolyte functionalization for calcium induced gating.     

Initiated Chemical Vapor Deposition: A Versatile Tool for Various Device Applications

Advances in device technology have been accompanied by the development of new types of materials and device fabrication methods. Considering device design, initiated chemical vapor deposition (iCVD) inspires innovation as a platform technology that extends the application range of a material or device. iCVD serves as a versatile tool for surface modification using functional thin film. The building of polymeric thin films from vapor phase monomers is highly desirable for the surface modification of thermally sensitive substrates. The precise control of thin film thicknesses can be achieved using iCVD, creating a conformal coating on nano‐, and micro‐structured substrates such as membranes and microfluidics. iCVD allows for the deposition of polymer thin films of high chemical functionality, and thus, substrate surfaces can be functionalized directly from the iCVD polymer film or can selectively gain functionality through chemical reactions between functional groups on the substrate and other reactive molecules. These beneficial aspects of iCVD can spur breakthroughs in device fabrication based on the deposition of robust and functional polymer thin films. This review describes significant implications of and recent progress made in iCVD‐based technologies in three fields: electronic devices, surface engineering, and biomedical applications.

     

Temperature‐Enhanced Solvent Vapor Annealing of a C3 Symmetric Hexa‐peri‐Hexabenzocoronene: Controlling the Self‐Assembly from Nano‐ to Macroscale

Temperature‐enhanced solvent vapor annealing (TESVA) is used to self‐assemble functionalized polycyclic aromatic hydrocarbon molecules into ordered macroscopic layers and crystals on solid surfaces. A novel C₃ symmetric hexa‐peri‐hexabenzocoronene functionalized with alternating hydrophilic and hydrophobic side chains is used as a model system since its multivalent character can be expected to offer unique self‐assembly properties and behavior in different solvents. TESVA promotes the molecule's long‐range mobility, as proven by their diffusion on a Si/SiO_x surface on a scale of hundreds of micrometers. This leads to self‐assembly into large, ordered crystals featuring an edge‐on columnar type of arrangement, which differs from the morphologies obtained using conventional solution‐processing methods such as spin‐coating or drop‐casting. The temperature modulation in the TESVA makes it possible to achieve an additional control over the role of hydrodynamic forces in the self‐assembly at surfaces, leading to a macroscopic self‐healing within the adsorbed film notably improved as compared to conventional solvent vapor annealing. This surface re‐organization can be monitored in real time by optical and atomic force microscopy.     

可光交联苯并噻二唑与芴共聚物的制备及性能

采用Suzuki偶合法将苯并噻二唑结构单元引入聚芴主链,同时在芴的C-9位上引入烯丙基,合成了可交联发绿光的苯并噻二唑与芴共聚物。通过对共聚物溶液的紫外光谱和荧光光谱研究发现,引入可交联基团烯丙基以后,聚合物的光学性能并未发生明显变化。可交联的共聚物在加入光引发剂后,在紫外光作用下可以进行光交联,并且在照射9 min左右聚合物完全交联。通过对交联聚合物荧光光谱的研究发现,聚合物薄膜在交联前后的荧光光谱基本不变,表明交联对聚合物的发光性能并无影响。交联共聚物薄膜在紫外线的照射下发出明显的绿色荧光。     

Layer‐By‐Layer Films as a Biomimetic Reservoir for rhBMP‐2 Delivery: Controlled Differentiation of Myoblasts to Osteoblasts

Efficient delivery of growth or survival factors to cells is one of the most important long‐term challenges of current cell‐based tissue engineering strategies. The extracellular matrix acts as a reservoir for a number of growth factors through interactions with its components. In the matrix, growth factors are protected against circulating proteases and locally concentrated. Thus, the localized and long‐lasting delivery of a matrix‐bound recombinant human bone morphogenetic protein 2 (rhBMP‐2) from a biomaterial surface would mimic in vivo conditions and increase BMP‐2 efficiency by limiting its degradation. Herein, it is shown that crosslinked poly(L ‐lysine)/hyaluronan (HA) layer‐by‐layer films can serve as a reservoir for rhBMP‐2 delivery to myoblasts and induce their differentiation into osteoblasts in a dose‐dependent manner. The amount of rhBMP‐2 loaded in the films is controlled by varying the deposition conditions and the film thickness. Its local concentration in the film is increased up to ≈500‐fold when compared to its initial solution concentration. Its adsorption on the films, as well as its diffusion within the films, is evidenced by microfluorimetry and confocal microscopy observations. A direct interaction of rhBMP‐2 with HA is demonstrated by size‐exclusion chromatography, which could be at the origin of the rhBMP‐2 “trapping” in the film and of its low release from the films. The bioactivity of rhBMP‐2‐loaded films is due neither to film degradation nor to rhBMP‐2 release. The rhBMP‐2‐containing films are extremely resistant and could sustain three successive culture sequences while remaining bioactive, thus confirming the important and protective effect of rhBMP‐2 immobilization. These films may find applications in the local delivery of immobilized growth factors for tissue‐engineered constructs and for metallic biomaterial surfaces, as they can be deposited on a wide range of substrates with different shapes, sizes, and composition.     

Cover Picture: Crosslinked Nanoparticle Stripes and Hexagonal Networks Obtained via Selective Patterning of Block Copolymer Thin Films (Adv. Mater. 18/2005)

The construction and fixation of ordered nanoparticle arrays allow not only the exploitation of the array's collective physical properties but also the possibility of manipulating discrete aggregates within a larger‐scale assembly scheme. A simple approach utilizing block copolymer thin films as templates and coordination chemistry as a mild crosslinking mechanism is reported by Shenhar, Rotello, and co‐workers on p. 2206. The cover image (by Nicholas Fischer) shows terpyridine‐functionalized gold nanoparticles organized in stripes on top of a microphase‐separated thin film of polystyrene‐block‐poly(methyl methacrylate) and crosslinked thorough the formation of iron–bisterpyridine complexes.     

Microcapsules for cell entrapment by template polymerization of a synthetic hydrogel coating around calcium alginate gel: preliminary development

Stable, water-soluble copolymers of glycidyl methacrylate with n-vinyl pyrrolidinone (I) were prepared by free-radical polymerization. Water-soluble copolymers of 2-hydroxyethyl methacrylate with methacrylic acid (II) were examined as co-reactants with (I) to form hydrogel matrices. Upon mixing of (I) and (II) in solution, a covalently crosslinked hydrogel was formed, presumably by etherification of epoxy functionality on (I) with hydroxyls on (II). Synthetic hydrogel-coated gel beads were prepared from an aqueous mixture of sodium alginate and (II) by treatment with solution containing (I) and calcium ion. An elastic, defect free and indefinitely stable covalently crosslinked hydrogel coating was formed around the calcium alginate by surface reaction of (I) and (II). Encapsulated guinea-pig red blood cells suffered minimal lysis over a 4 day period due to isolation and protection from the reacting species by the interior alginate gel matrix.     

UV written waveguides using crosslinkable PMMA-based copolymers

Crosslinkable copolymers poly(methylmethacrylate/2-methacryloylethylmethacrylate) (P(MMA/MAOEMA)) were developed for waveguide applications. P(MMA/MAOEMA) can crosslink under either UV exposure or heating. The UV-induced refractive index change in unreacted P(MMA/MAOEMA) is found to depend on the fluence. UV exposure of thermally crosslinked P(MMA/MAOEMA) can induce further structure change and thus index change, and therefore, was found to be useful for creating the core layers in optical waveguides. The photosensitivity of the thermally crosslinked polymers is sufficient for the fabrication of low loss (<1 dB/cm) channel waveguides in the thermally crosslinked copolymer system.     

Oriented mesoporous organosilicate thin films

Coassemblies of block copolymers and inorganic precursors offer a path to ordered inorganic nanostructures. In thin films, these materials combined with domain alignment provide highly robust nanoscopic templates. We report a simple path to control the morphology, scaling, and orientation of ordered mesopores in organosilicate thin films through the coassembly of a diblock copolymer, poly(styrene-b-ethylene oxide) (PS-b-PEO), and an oligomeric organosilicate precursor that is selectively miscible with PEO. Continuous films containing cylindrical or spherical pores are generated by varying the mixing composition of symmetric PS-b-PEO and an organosilicate precursor. Tuning interfacial energy at both air/film and film/substrate interfaces allows the control of cylindrical pore orientation normal to the supported film surfaces. Our method provides well-ordered mesoporous structures within organosilicate thin films that find broad applications as highly stable nanotemplates.     

Three‐Dimensional Nanofabrication by Block Copolymer Self‐Assembly

Thin films of block copolymers are widely seen as enablers for nanoscale fabrication of semiconductor devices, membranes, and other structures, taking advantage of microphase separation to produce well‐organized nanostructures with periods of a few nm and above. However, the inherently three‐dimensional structure of block copolymer microdomains could enable them to make 3D devices and structures directly, which could lead to efficient fabrication of complex heterogeneous structures. This article reviews recent progress in developing 3D nanofabrication processes based on block copolymers.     

MMA/MPEOMA/VSA copolymer as a novel blood-compatible material: ex vivo platelet adhesion study

MMA/MPEOMA/VSA copolymers with both pendant polyethylene oxide (PEO) side chains and negatively chargeable side groups were synthesized by random copolymerization of methyl methacrylate (MMA), methoxy PEO monomethacrylate (MPEOMA; PEO mol. wt, 1000), and vinyl sulfonic acid sodium salt (VSA) monomers with different monomer composition to evaluate their blood compatibility. MMA/MPEOMA copolymer (with PEO side chains) and MMA/VSA copolymer (with negatively chargeable side groups) were also synthesized for the comparison purpose. The synthesized copolymers were coated onto polyurethane (PU) tubes (inner diameter, 4.6 mm) by a spin coating. The platelet adhesion of the MMA/MPEOMA/VSA copolymer-coated tube surfaces was compared with that of tube surface coated with MMA/MPEOMA or MMA/VSA copolymer with similar MPEOMA or VSA composition, using an ex vivo canine arterio-artery shunt method. The platelet adhesion was evaluated by radioactivity counting of technetium (99mTc)-labeled platelets adhered on the surfaces after 30 and 120 min of blood circulation. The MMA/MPEOMA/VSA copolymer (monomer molar ratio 9/0.5/0.5 or 8/1/1) was better in preventing platelet adhesion on the surface than the MMA/MPEOMA or MMA/VSA copolymer with similar MPEOMA or VSA composition, probably owing to the combined effects of highly mobile, hydrophilic PEO side chains and negatively charged VSA side groups.     

A Hydrogel‐Film Casting to Fabricate Platelet‐Reinforced Polymer Composite Films Exhibiting Superior Mechanical Properties

The fabrication of mechanically superior polymer composite films with controllable shapes on various scales is difficult. Despite recent research on polymer composites consisting of organic matrices and inorganic materials with layered structures, these films suffer from complex preparations and limited mechanical properties that do not have even integration of high strength, stiffness, and toughness. Herein, a hydrogel‐film casting approach to achieve fabrication of simultaneously strong, stiff, and tough polymer composite films with well‐defined microstructure, inspired from a layer‐by‐layer structure of nacre is reported. Ca²⁺‐crosslinked alginate hydrogels incorporated with platelet‐like alumina particles are dried to form composite films composed of horizontally aligned alumina platelets and alginate matrix with uniformly layered microstructure. Alumina platelets are evenly distributed parallel without precipitations and contribute to synergistic enhancements of strength, stiffness and toughness in the resultant film. Consequentially, Ca²⁺‐crosslinked alginate/alumina (Ca²⁺‐Alg/Alu) films show exceptional tensile strength (267 MPa), modulus (17.9 GPa), and toughness (3.60 MJ m⁻³). Furthermore, the hydrogel‐film casting allows facile preparation of polymer composite films with controllable shapes and various scales. The results suggest an alternative approach to design and prepare polymer composites with the layer‐by‐layer structure for superior mechanical properties.     

Preparation of Macroscopic Block‐Copolymer‐Based Gyroidal Mesoscale Single Crystals by Solvent Evaporation

Properties arising from ordered periodic mesostructures are often obscured by small, randomly oriented domains and grain boundaries. Bulk macroscopic single crystals with mesoscale periodicity are needed to establish fundamental structure–property correlations for materials ordered at this length scale (10–100 nm). A solvent‐evaporation‐induced crystallization method providing access to large (millimeter to centimeter) single‐crystal mesostructures, specifically bicontinuous gyroids, in thick films (>100 µm) derived from block copolymers is reported. After in‐depth crystallographic characterization of single‐crystal block copolymer–preceramic nanocomposite films, the structures are converted into mesoporous ceramic monoliths, with retention of mesoscale crystallinity. When fractured, these monoliths display single‐crystal‐like cleavage along mesoscale facets. The method can prepare macroscopic bulk single crystals with other block copolymer systems, suggesting that the method is broadly applicable to block copolymer materials assembled by solvent evaporation. It is expected that such bulk single crystals will enable fundamental understanding and control of emergent mesostructure‐based properties in block‐copolymer‐directed metal, semiconductor, and superconductor materials.     

Templated Solid‐State Dewetting of Thin Silicon Films

Thin film dewetting can be efficiently exploited for the implementation of functionalized surfaces over very large scales. Although the formation of sub‐micrometer sized crystals via solid‐state dewetting represents a viable method for the fabrication of quantum dots and optical meta‐surfaces, there are several limitations related to the intrinsic features of dewetting in a crystalline medium. Disordered spatial organization, size, and shape fluctuations are relevant issues not properly addressed so far. This study reports on the deterministic nucleation and precise positioning of Si‐ and SiGe‐based nanocrystals by templated solid‐state dewetting of thin silicon films. The dewetting dynamics is guided by pattern size and shape taking full control over number, size, shape, and relative position of the particles (islands dimensions and relative distances are in the hundreds nm range and fluctuate ≈11% for the volumes and ≈5% for the positioning).