Surface-initiated, ring-opening metathesis polymerization: formation of diblock copolymer brushes and solvent-dependent morphological changes

In this article, we report the formation of diblock copolymer brushes on a gold surface by surface-initiated, ring-opening metathesis polymerization (SI-ROMP) with the newly developed ruthenium catalyst [(H2IMes)(3-Br-py)2(Cl)2Ru=CHPh]. Taking advantage of the highly improved activity of the ruthenium catalyst and the rapid initiation step of ROMP, we successfully formed thin films of well-defined block copolymers with 5-norbornene-2-endo,3-endo-dimethanol and norbornene carboxylic acid methyl esters (44:56 endo/exo). The catalyst was found to be active enough to polymerize endo isomers of norbonene derivatives from the surface as well as to form diblock copolymer brushes. SI-ROMP of diblock copolymers from the surface was confirmed by ellipsometry, infrared spectroscopy, and X-ray photoelectron spectroscopy. After the formation, the polymer-grafted substrates were immersed in various solvents, and the selective swelling characteristics of polymer brushes were investigated by atomic force microscopy.     

BSA adsorption on bimodal PEO brushes

BSA adsorption onto bimodal PEO brushes at a solid surface was measured using optical reflectometry. Bimodal brushes consist of long (N=770) and short (N=48) PEO chains and were prepared on PS surfaces, applying mixtures of PS(29)-PEO(48) and PS(37)-PEO(770) block copolymers and using the Langmuir-Blodgett technique. Pi-A isotherms of (mixtures of) the block copolymers were measured to establish the brush regime. The isotherms of PS(29)-PEO(48) show hysteresis between compression and expansion cycles, indicating aggregation of the PS(29)-PEO(48) upon compression. Mixtures of PS(29)-PEO(48) and PS(37)-PEO(770) demonstrate a similar hysteresis effect, which eventually vanishes when the ratio of PS(37)-PEO(770) to PS(29)-PEO(48) is increased. The adsorption of BSA was determined at brushes for which the grafting density of the long PEO chains was varied, while the total grafting density was kept constant. BSA adsorption onto monomodal PEO(48) and PEO(770) brushes was determined for comparison. The BSA adsorption behavior of the bimodal brushes is similar to the adsorption of BSA at PEO(770) monomodal brushes. The maximum of BSA adsorption at low grafting density of PEO(770) can be explained by ternary adsorption, implying an attraction between BSA and PEO. The contribution of primary adsorption to the total adsorbed amount is negligible.     

Amphipolar core-shell cylindrical brushes

Methacryloyl-endfunctionalized block copolymers consisting of styrene and 2-vinylpyridine were polymerized to polyblockcomacromonomers with a much higher main chain than side chain degree of polymerization. Like homopolymacromonomers these molecules exhibit the structure of cylindrical brushes. Since the vinylpyridine block is coupled to the polymerizable group, the resulting cylindrical macromolecules exhibit a core of vinylpyridine and a shell of polystyrene, thus resembling an amphipolar unimolecular micelle of cylindrical shape. The micellar character of the structure is demonstrated by loading the cylindrical brushes with tetrachloroauric acid in toluene. Subsequent reduction leads to the formation of colloidal gold, most probably within the brush in analogy to similar work on block copolymer micelles.     

Interaction forces between polyethylene oxide-polypropylene oxide ABA copolymers adsorbed to hydrophobic surfaces

Block and graft copolymers are frequently used as stabilizing agents in colloidal dispersions. One common material is the range of polymers known as "Pluronics," which is a BASF trade name for ABA block copolymers composed of a propylene oxide anchoring block (B block) and two ethylene oxide buoy or stabilizing blocks (A block); the equivalent ICI (Uneqima) trade name is Synperonic. In the work presented here the interactions between adsorbed layers of these materials immersed in 10(-2) M sodium sulfate solutions are presented. The block copolymers investigated had an approximately fixed molecular weight of around 3250 Da for the anchoring B block, whilst the molecular weight of the stabilizing polyethylene oxide chains varies around 800-6500 Da. Hydrophobic glass surfaces were used as the test substrate. It was found that in the absence of polymer a long ranged attractive interaction is observed, typical for the interaction between hydrophobic surfaces in aqueous media, but that in the presence of the polymers a repulsion was observed. The repulsion became longer ranged as the molecular weight of the ethylene oxide chain increased. On separation of the surfaces, the interaction was slightly longer ranged, suggesting that the two polymer layers intertwine and stretch each other on separation. This effect was more noticeable for the higher molecular weight polymers. The compression data were well described using a scaling analysis for the interaction between polymer brushes.     

Theoretical and experimental studies on the surface structures of conjugated rod-coil block copolymer brushes

A combined theoretical and experimental investigation of conjugated rod-coil block copolymer brushes is reported. The theoretical study for the surface structures of rod-coil block copolymer brushes was established based on the simulation method of dissipative particle dynamics. The effects of solvent stimuli, grafting density, and rod-coil block ratio of the polymer brushes on the surface structures were examined. The rod blocks of polymer brushes were found to be well-dispersed on the surface in their good solvents. On the other hand, aggregative domains of the rod blocks were formed in their poor solvents with the conformations of isolated islands or worm-like structures depending on the grafting density of the polymer brushes. The aggregative domains tend to stay on top of the coil blocks for small rod-to-coil block ratio. However, the submergence of the aggregative domains into the coil blocks is thermodynamically preferred for large enough rod-to-coil block ratio. New multifunctional amphiphilic rod-coil block copolymers, poly-[2,7-(9,9-di-n-hexylfluorene)]-block-poly-[poly(ethylene glycol) methyl ether methacrylate]-block-poly-[3(tripropoxysilyl)propyl methacrylate] (PF-b-PPEGMA-b-PPOPS), with two different block ratios were synthesized and used to prepare the corresponding polymer brushes via the grafting- method. The effects of stimuli factors on the surface structures characterized by the atomic force microscopy images were consistent with the theoretical results. Furthermore, the photophysical properties of PF-b-PPEGMA-b-PPOPS brushes were significantly varied by the solvent stimuli. The emission peaks originated from the aggregation and/or excimer formation of PF blocks were observed after methanol treatment. The photoluminescence intensity and its efficiency were well correlated to the surface structure and the methanol content in mixed solvents. Our study demonstrates how the surface structures and photophysical properties of rod-coil block copolymer brushes response to environmental stimuli.     

From self-assembled monolayers to chemically patterned brushes: Controlling the orientation of block copolymer domains in films by substrate modification

Block copolymer lithography is emerging as one of the leading technologies for patterning nanoscale dense features. In almost all potential applications of this technology, control over the orientation of cylindrical and lamellar domains is required for pattern transfer from the block copolymer film. This review highlights the state-of-art development of brushes to modify the substrates to control the assembly behaviors of block copolymers in films. Selected important contributions to the development of self-assembled monolayers, polymer brushes and mats, and chemically patterned brushes are discussed.     

Preparation and characterization of V-shaped PS-b-PEO brushes anchored on planar gold substrate through the trithiocarbonate junction group

Trithiocarbonate group was introduced into the polystyrene-b-poly(ethylene oxide) (PS-b-PEO) block copolymers as the junction of the blocks through RAFT polymerization. Mixed PS and PEO brushes with a V-shape were prepared by anchoring the trithiocarbonate group on the planar gold substrate. The morphology of the V-shaped brushes was characterized by atomic force microscopy (AFM) and the surface composition responsive to solvent treatment was detected by X-ray photoelectron spectroscopy (XPS). Different morphologies were observed for the V-shaped PS-b-PEO brushes, depending on the chain structure and solvent treatment. The highly selective solvent for PEO, ethanol, can intensify or induce microphase separation of the V-shaped brushes, leading to vertical microphase separation. When the V-shaped brushes are treated with the co-solvent, THF, miscible morphology, lateral microphase separation, and vertical microphase separation are observed as the PS block length increases. After treatment with the non-selective poor solvent, cyclohexane, the V-shaped PS(106)-b-PEO(113) brush, exhibits a laterally microphase-separated morphology, but the V-shaped PS(52)-b-PEO(113) and PS(253)-b-PEO(113) brushes are vertically microphase-separated.     

Behavior of polymers at surfaces and interfaces as observed in simulated systems

Cooperative motion algorithm (CMA) is used to simulate polymer chains in three types of dense systems reflecting special cases of polymer behavior at surfaces and interfaces: polymer brushes with variable grafting density in the range 0–1, both in a neutral solvent and in a polymer melt, layers of end-functionalized polymers between parallel end-adsorbing walls, and copolymers of various distributions of comonomer units (random, block and gradient copolymers) at interfaces with noncompatible polymers.     

Kumada chain-growth polycondensation as a universal method for synthesis of well-defined conjugated polymers

Kumada chain-growth polycondensation (KCGP) is a novel method for the synthesis of well-defined conjugated polymers. Because the Ni-catalyst can transfer in an intramolecular process to the propagating chain end, the polymerization follows chain-growth mechanism. With this newly developed method, various conjugated polymers, such as polythiophenes, poly(p-phenylene) (PPP), polypyrrole (PPy), and polyfluorene with controlled molecular weights and relatively narrow polydispersities (PDIs), have been prepared. Especially, the polymerizations for poly(3-alkylthiophene)s (P3ATs), PPP, and PPy exhibited quasi-living characteristics, which allows preparing polymer brushes, fully-conjugated block copolymers, and macroinitiators and macro-reactants for the synthesis of rod-coil block copolymers. In the current review, the progress in this new area is summarized.     

Dielectric studies on the dynamics of polymer chains in various environments

Stockmayer's type-A chain such as cis-polyisoprene (PI) having dipole moments aligned parallel to the chain contour exhibits slow dielectric response, reflecting fluctuation of the end-to-end vector of the chain ( dielectric normal modes ). This article intends to review our own work on dielectric spectroscopy using PI as a probe for studying dynamics of large scale motion of the probe chain in binary blends with polybutadiene (PB) and in block copolymers with PB or with polystyrene. Through such studies we were able to see the features of polymer chain dynamics in various environments: Rouse-Zimm-like dynamics in overlapping-but-unentangled regimes, reptation with and without accompanying constraint release, behaviour of a subchain incorporated into block copolymers with miscible segments, and dynamics of polymer brushes in strongly segregating block copolymers in bulk and in solutions.     

甲壳型液晶高分子的构筑、自组装及其功能化

甲壳型液晶高分子的发展很大程度上依赖于聚合物自组装的发展,而各种可设计、可预测、可调控的自组装策略的涌现,将甲壳型液晶高分子研究推向前所未有的高度,同时也极大地丰富了高分子化学与物理的内容,提升了研究水准.研究表明,侧链"甲壳效应"在调控甲壳型液晶高分子有序结构等方面有着重要作用.本综述从甲壳型液晶高分子设计合成、液晶相态调控、嵌段共聚物自组装和功能化应用等方面,总结和评述了近年来该领域国内的最新研究进展.最后,本综述总结了甲壳型液晶高分子在发展中所面临的主要问题,并对其发展趋势进行了展望.     

Synthesis,thermo-responsive micellization and caffeine drug release of novel PBMA-<Emphasis Type="Italic">b</Emphasis>-PNIPAAm block polymer brushes

Brush-like block copolymers with poly(t-butyl methacrylate) (PBMA) and poly(N-isopropylacrylamide) (PNIPAAm) as side arms, PBMA-b-PNIPAAm, were designed and synthesized via a simple free radical polymerization route. The chemical structure and molecular weight of these polymer brushes were characterized and determined by nuclear magnetic resonance (¹H NMR), Fourier transform infrared spectrometry (FTIR) and gel permeation chromatography (GPC). The micellar formation by these polymer brushes in aqueous solutions were detected by a surface tension technique, and the critical micelle concentration (CMC) ranged from 1.53 to 8.06 mg L⁻¹. The morphology and geometry of polymer micelles were investigated by transmission electron microscope (TEM) and dynamic light scattering (DLS). The polymer micelles assume the regularly-spherical core-shell structure with well-dispersed individual nanoparticles, and the particle size was in the range from 36 to 93 nm. The PNIPAAm segments exhibited a thermoreversible phase transition, so the resulting block polymer brushes were temperature-sensitive and the low critical solution temperature (LCST) was determined by UV-vis spectrometer at about 28.82–29.40°C. The characteristic parameters of the polymer micelles such as CMC, micellar size and LCST values were affected by their compositional ratios and the length of hydrophilic or hydrophobic chains. The evaluation for caffeine drug release behavior of the block polymer micelles demonstrated that the self-assembled micelles exhibited thermal-triggered properties in controlled drug release.     

Strongly stretched polymer brushes

Polymer “brushes” are formed when long-chain molecules are somehow attached by one end at an interface with a relatively small area per chain. Such adsorbed brushes in the presence of solvent may be used to modify surface properties, stabilize colloidal particles, etc. Strongly segregated block copolymer phases, or interfacial layers of such “polymeric surfactants” may also be modeled in terms of “melt brushes,” (i.e., brushes without solvent). In both cases, when chain attachments are crowded on the interface, the chains stretch out to avoid neighboring chains. The resulting physical state has properties markedly different from polymer solutions, gels, or weakly adsorbed polymer layers. When the chains are strongly stretched, their statistical mechanics become simpler, as fluctuations around the set of most probable conformations are suppressed. This makes possible many pencil-and-paper calculations of brush properties, including bending and compressional moduli, and detailed knowledge of the chain conformations. As a recent example, I will describe calculations of phase diagrams of strongly segregated block copolymers including bicontinuous double-diamond phases. © 1994 John Wiley & Sons, Inc.     

Motion of integrated CdS nanoparticles by phase separation of block copolymer brushes

A new method of reversibly moving CdS nanoparticles in the perpendicular direction was developed on the basis of the phase separation of block copolymer brushes. Polystyrene-b-(poly(methyl methacrylate)-co-poly(cadmium dimethacrylate)) (PS-b-(PMMA-co-PCdMA)) brushes were grafted from the silicon wafer by surface-initiated atom transfer radical polymerization (ATRP). By exposing the polymer brushes to H2S gas, PS-b-(PMMA-co-PCdMA) brushes were converted to polystyrene-b-(poly(methyl methacrylate)-co-poly(methacrylic acid)(CdS)) (PS-b-(PMMA-co-PMAA(CdS))) brushes, in which CdS nanoparticles were chemically bonded by the carboxylic groups of PMAA segment. Alternating treatment of the PS-b-(PMMA-co-PMAA(CdS)) brushes by selective solvents for the outer block (a mixed solvent of acetone and ethanol) and the inner PS block (toluene) induced perpendicular phase separation of polymer brushes, which resulted in the reversible lifting and lowering of CdS nanoparticles in the perpendicular direction. The extent of movement can be adjusted by the relative thickness of two blocks of the polymer brushes.     

Ordering mechanism of asymmetric diblock copolymers confined between polymer-grafted surfaces

Using self-consistent-field calculation, we study the equilibrium morphology of asymmetric block copolymer melts between two brush-covered surfaces. We discuss systematically the morphologies formed as a function of the grafting density of brushes and show a series of reentrant structural transformations between the lamellar and hexagonal phases. Such a selection of ordered microdomains has been attributed to the interplay among entropy effects of stretched brushes, the preferential interaction of the blocks with the grafted substrate, and the bulk microphase-separated behavior of asymmetric copolymers. Interestingly for the affinitive brush-copolymer interfaces, the bulk hexagonal phase may still be remained due to entropy elasticity of brushes, and the frustration can be relieved by deforming the shape of "soft" polymer-brush surface, in contrast to the hard-wall cases. The result demonstrates a simple way to realize molecular self-assembly for confined copolymer films with well-controlled thickness and interfacial environment.     

Stimuli-responsive polyelectrolyte block copolymer brushes synthesized from the Si wafer via atom-transfer radical polymerization

Surface-tethered oppositely charged weak polyelectrolyte block copolymer brushes composed of poly(2-vinyl pyridine) (P2VP) and poly(acrylic acid) (PAA) were grown from the Si wafer by atom-transfer radical polymerization. The P2VP-b-PAA brushes were prepared through hydrolysis of the second PtBA block to the corresponding acrylic acid. The P2VP-b-PAA brushes with different PAA block length were obtained. The P2VP-b-PAA brushes revealed a unique reversible wetting behavior with pH. The difference between the solubility parameters for P2VP and PAA, the changes of surface chemical composition and surface roughness, and the reversible wetting behavior illustrated that the surface rearrangement occurred during treatment of the P2VP-b-PAA brushes by aqueous solution with different pH value. The reversible properties of the P2VP-b-PAA brushes can be used to regulate the adsorption of the sulfonated PS nanoparticles.     

Effects of chain architectures on the surface structures of conjugated rod-coil block copolymer brushes

Rod-coil block copolymers are of unique and interesting characteristics since their physical properties can be reversibly tuned in response to the external stimuli, such as change in solvent quality. In this study, dissipative particle dynamics is used to investigate the surface structures of rod-coil polymer brushes tethered onto a surface. When immersed in the selective solvent for the coil blocks, rod blocks tend to form aggregates. Our results show that linear and Y-shaped polymer brushes exhibit similar aggregative behavior. However, some of the surface structures can be acquired within experimentally attainable surface grafting density only for Y-shaped polymer brushes. On the other hand, comblike polymer brushes are found to possess more diverse aggregative manners than linear brushes. Surface structures with aggregates taking the forms of cones, cylinders, or layers of spheres are found. By controlling the aggregative structures, it is possible for us to adjust the physical properties, such as optical function, of the material.     

Recent advances in the solution self‐assembly of amphiphilic “rod‐coil” copolymers

Solution self‐assembly of amphiphilic “rod‐coil” copolymers, especially linear block copolymers and graft copolymers (also referred to as polymer brushes), has attracted considerable interest, as replacing one of the blocks of a coil‐coil copolymer with a rigid segment results in distinct self‐assembly features compared with those of the coil‐coil copolymer. The unique interplay between microphase separation of the rod and coil blocks with great geometric disparities can lead to the formation of unusual morphologies that are distinctly different from those known for coil‐coil copolymers. This review presents the recent achievements in the controlled self‐assembly of rod‐coil linear block copolymers and graft copolymers in solution, focusing on copolymer systems containing conjugated polymers, liquid crystalline polymers, polypeptides, and polyisocyanates as the rod segments. The discussions concentrate on the principle of controlling over the morphology of rod‐coil copolymer assemblies, as well as their distinctive optical and optoelectronic properties or biocompatibility and stimuli‐responsiveness, which afford the assemblies great potential as functional materials particularly for optical, optoelectronic and biological applications. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 1459–1477     

Supramolecular Self‐Assembly of Nonlinear Amphiphilic and Double Hydrophilic Block Copolymers in Aqueous Solutions

Supramolecular self‐assembly of block copolymers in aqueous solution has received ever‐increasing interest over the past few decades due to diverse biological and technological applications in drug delivery, imaging, sensing and catalysis. In addition to relative block lengths, molecular weights and solution conditions, chain architectures of block copolymers can also dramatically affect their self‐assembling properties in selective solvents. This feature article mainly focuses on recent developments in the field of supramolecular self‐assembly of amphiphilic and double hydrophilic block copolymers (DHBCs) possessing nonlinear chain topologies, including miktoarm star polymers, dendritic–linear block copolymers, cyclic block copolymers and comb‐shaped copolymer brushes.

     

Facile Fabrication of Concentrated Polymer Brushes with Complex Patterning by Photocontrolled Organocatalyzed Living Radical Polymerization

Photocontrolled surface‐initiated reversible complexation mediated polymerization (photo‐SI‐RCMP) was successfully applied to fabricate concentrated polymer brushes with complex patterning structures. Positive‐type patterned polymer brushes were obtained by photo‐SI‐RCMP under visible light (550(±50) nm) using photomasks. A particularly interesting finding was that negative‐type patterned polymer brushes were also obtainable in a facile manner. A nonspecial UV light (250–385 nm) enabled the preparation of pre‐patterned initiator surfaces in a remarkably short time (1 min), leading to negative‐type patterned polymer brushes. Based on this unique selectivity between visible and UV light, the combination of two patterning techniques enabled the preparation of complex patterned brushes, including diblock copolymers, binary polymers, and functional binary polymers, without multistep immobilization of one or more initiators on the surfaces.