Organic/inorganic nanoobjects with controlled shapes from gelable triblock copolymers

A functional gelable triblock copolymer, poly(2-vinylpyridine)-block-poly(3-(triethoxysilyl)propyl methacrylate)-block-polystyrene (P2VP-b-PTEPM-b-PS), was prepared by the combination of reversible addition-fragmentation chain transfer (RAFT) mediated radical polymerization and copper catalyzed click chemistry. Bulk microphase separation of P2VP₃₁₀-b-PTEPM₅₈-b-PS₃₂₂ under different conditions was studied in order to prepare organic/inorganic nanoobjects by a procedure of crosslinking PTEPM phases and dispersing in a solvent. The conditions included using different annealing solvents and adding stearic acids to form supramolecular complexes with P2VP blocks respectively. Then the packed cylinders with P2VP cores and PTEPM shells dispersed in the PS matrix, lamella with alternating PS, PTEPM and P2VP layers, and the inverse cylindrical morphology with PS cores and PTEPM shells dispersed in the matrix of P2VP/stearic acid complex were obtained respectively just from the same triblock copolymer sample. After crosslinking PTEPM microdomains by sol-gel process and dispersing in solvents, a series of organic/inorganic polymeric nanoobjects, including two types of nanofibers with inverse internal structure and one novel kind of nanoplates, were produced. Further modification of the fibers with P2VP cores has been studied.     

Studies on phase separation in polyblends of block copolymers

The morphologies of polyblends of block copolymers of styrene and butadiene with similar composition but different molecular weights have been examined with an electron microscope. The micrographs show that some supramolecular features, with different morphologies to those in the matrix, are randomly dispersed. These results, we believe, provide evidence of the incompatibility of the block copolymers in spite of their chemical identity.     

Photoresponsive liquid crystalline block copolymers: From photonics to nanotechnology

Being one of the most fascinating multi-functional materials, photoresponsive liquid crystalline block copolymers (PLCBCs) have attracted much attention because of their light controllable properties of supramolecularly self-assembled structures. These originate from their unique features combining the advanced function of photoresponsive liquid crystalline polymers (PLCPs) with the inherent property of microphase separation of block copolymers (BCs). Benefiting from recent progresses in materials chemistry, diverse PLCBCs have been designed and synthesized by controlled polymerization using different synthetic routes and strategies. Generally, PLCBCs show different performance depending on their self-organization and molecular composition, with the PLCP blocks in the minority phase or in the majority phase. One of the most important properties of PLCBCs is supramolecular cooperative motion, resulted from the interactions between liquid crystalline elastic deformation and microphase separation, which enables them to self-assemble into regularly ordered nanostructures in bulk films with high reliability. These nanostructures contribute to improving the optical performance of polymer films by eliminating the scattering of visible light, in favor of their photonic applications. With the help of liquid crystal alignment techniques, both parallel and perpendicular patterning of nanostructures has been fabricated in macroscopic scale with excellent reproducibility and mass production, which provides nanotemplates and nanofabrication processes for preparing varieties of nanomaterials. Recent findings about PLCBCs including their synthesis, diagram of microphase separation, structure-property relationship, precise control of nanostructure as well as their applications in photonics to nanotechnology are reviewed.     

Theory of chromatography of ring-shaped block copolymers

A theory is developed for describing liquid chromatography of ring diblock and multiblock copolymers. The chromatographic behavior of ring block copolymers at different adsorption interactions is analyzed theoretically and compared with that of linear block copolymers; typical chromatograms are simulated by using the theory. In particular, it is shown that under the critical interaction condition for one block the chromatographic retention of a ring diblock copolymer is dependent of the length of the ‘critical’ block; this behavior differs qualitatively from that of a linear diblock copolymer. Ring copolymers are always more retained than linear ones, therefore such copolymers can be separated. Especially good separation of heterogeneous ring and linear block copolymers is predicted at near-critical interaction conditions. According to the theory, ring diblocks and multiblocks can be separated as well, if one component of a copolymer is adsorbing, while the other one is not adsorbing.     

Self-organization of stack-up block copolymers into polymeric supramolecules

Polyethylene oxide –b– polypropylene oxide -b- polyethylene oxide (EO₁₀₆PO₇₀EO₁₀₆) block copolymer self-organizes into polymeric supramolecules, characterized by NMR as phase transition from the isotropic stack-up block structure to the ordered cubic polymeric supramolecular structure. Its dependence on both temperature and copolymer concentration is clearly shown by the changes in line shape and chemical shift of the PO₇₀ block β, γ resonances.     

Orientational phase transitions in the hexagonal cylinder phase and kinetic pathways of lamellar phase to hexagonal phase transition of asymmetric diblock copolymers under steady shear flow

For asymmetric diblock copolymers under steady shear flow, the orientational phase transitions in the hexagonal cylinder phase and the kinetics of lamellar to hexagonal phase transition were studied based on the time-dependent Ginzburg-Landau approach. As to orientational phase transitions in the hexagonal cylinder phase, the simulation results show that the parallel orientation is stable at low shear rate and the perpendicular orientation is stable at high shear rate. In addition, different kinetic pathways of lamellar phase to hexagonal phase transition are observed after a sudden temperature jump from one phase to other. When the temperature jump is deep into the hexagonal phase from the shear-orientated lamellar phase under steady shear flow, the lamellae are transformed into hexagonally ordered cylinders directly via the short so-called modulated instability of the lamellar layers. However, if the temperature jump is only slight into the hexagonal phase with or without steady shear flow, lamellae are transformed into hexagonally ordered cylinders going through a distinct modulated and perforated lamellar phase stage. Moreover, without steady shear flow the perforated lamellar phase stage can survive for longer time, indicating that the steady shear flow cannot lead to stabilization of perforated lamellar phase. The simulation results also indicate that the perforated lamellar phase has abab…stacking.     

(Polystyrene-g-polyisoprene)-b-polystyrene comb-coil block copolymer in selective solvent

Comb-coil block copolymers consisting of two components polystyrene (PS) and polyisoprene (PI) were synthesized through combining TEMPO living free radical polymerization (LFRP) and anionic polymerization using “grafting-onto” strategy. Two typical samples with the same backbone but grafted with different numbers and lengths of branches, forming lamellar and cylinder phases, respectively, were investigated. As the selective solvent was added into these block polymers, the micro-structures transformed from disordered or weakly ordered structure into well-organized lamellar structure in the intermediate polymer concentration. The power law behavior of the lamellae space with respect to polymer concentration indicates that the two samples form the well-organized structure through different paths. The sample with longer branch length thus less branch points forms lamellae phase with smaller lamellae space. This difference at lamellae spacing is attributed to the two different ways that the chain assembles.     

Detection of the glass relaxation process of the PS-phase in block copolymers

The relaxation behavior of the glass process of the polystyrene (PS) phase in three different types of block copolymers has been investigated. Four triblock copolymers are used, namely, two styrene-butadiene-styrene (S-B-S), styrene-styrene butadiene-styrene (S-SB-S) and styrene-ethylene butylene-styrene (S-EB-S). The materials were crosslinked by means of dicumylperoxide (DCUP) with concentrations up to 10 phr, i.e. 10 g of DCUP per 100 g of the polymer. Dynamic mechanical measurements have been done for the complex shear compliance in wide temperature and frequency windows, 30-150 °C and 10⁻³-10 Hz, respectively. Complete master curves have been constructed for all samples and analyzed using Cole-Cole function. It is observed that crosslinking reduces the relaxation strength of the low frequency process much more than that of the high frequency process. Therefore, the glass relaxation process of the PS-phase can be easily detected in the crosslinked samples. The relaxation frequency of the maximum of the glass relaxation process of the PS-phase is found to be independent of the crosslinking concentration. The relaxation behavior of block copolymers with respect to crosslinking is compared to styrene butadiene rubber (SBR) statistical copolymer. A method to detect the glass transition temperature (T_g) of the investigated block copolymers is suggested.     

Crystallization of block copolymers in restricted cylindrical geometries

Using Tapping Mode atomic force microscopy, we studied crystallization of polymers in confined cylindrical geometries of micro-phase separated asymmetric diblock copolymers, deposited in thin films onto solid substrates. We observed that in most cases crystallization occurs separately and independently in each cylinder of the mesophase pattern. We followed the kinetics of the crystallization process with time and found an influence of the lateral size of the cylindrical morphology on the rate of crystallization. The present data are compared to previous results on spherical mesophase patterns. The growth rate within one single cylinder was found to be not necessarily constant. Depending on the amount of crystallized material and the molecular weight, structural relaxations of the crystals can have very strong influence on the amorphous matrix and can even lead to the destruction of the mesophase pattern, allowing for break-out crystallization. As a consequence of break-out crystallization, crystallization kinetics changed considerably since the crystal growth was not limited anymore to the confinement imposed by the cylinders.     

A simplified apparatus for the preparation of block copolymers with low polydispersity ratios

A simplified apparatus for the preparation of block copolymers by anionic polymerization is described. Low polydispersity can be obtained without the need for a high degree of manipulative skill.     

Self-assembly of POSS-containing block copolymers: Fixing the hierarchical structure in networks

A series of P(MMA-co-GMA)-b-PMAPOSS block copolymers (BCPs) were synthesized by ATRP. The BCPs contain POSS (polyhedral oligomeric silsesquioxane) as a pendant unit on a particular polymer block chain. In selective solvents, the BCPs self-assemble to form ordered micellar-like structures. Spherical, cylindrical or vesicle-like morphologies were produced by tuning the BCP and mixed solvent compositions. Crosslinking of the BCP by the reaction of the glycidyl groups in the P(MMA-co-GMA) block of the micelle shell (GMA = glycidyl methacrylate) with a diamine results in a long-range structure ordering. The hexagonally packed cylindrical arrangement of the BCP networks was revealed by SAXS and TEM. The hierarchical, ordered structure of the POSS-containing hybrids was fixed by crosslinking. In addition, transient physical gels were prepared from triblock copolymers with outer MAPOSS blocks, and their rheological behavior was investigated.     

Nanostructured silica templated by double hydrophilic block copolymers with a comb-like architecture

An original way to synthesize nanostructured materials is to use new structuring agents constituted of double hydrophilic block copolymers (DHBC). The originality of these structuring agents is multiple: in water, the hydrosoluble DHBC copolymers can become amphiphilic and form micelles in specific conditions, i.e. after addition of other molecules or after a change of a physicochemical parameter (pH), which selectively makes one of the blocks insoluble in water. The addition of a silica precursor to a micelle suspension can lead to the formation of hybrid mesostructured materials, precursors for mesoporous silica. The micellization process may be reversible and the micelles can then be removed from the silica materials in an aqueous solution at room temperature after application of a dissociation stimulus, leading to the mesoporous materials. A new original DHBC is used here for silica structuring: instead of a classical linear diblock copolymer, it is a diblock copolymer with a linear polyacid block (PAA) and a poly(ethylene oxide) based neutral block (PAMPEO) with a comb-type architecture. It is synthesized by controlled radical polymerization (RAFT method) which permits a control of the block lengths. It is shown here that these new DHBC polymers can form polyion complex micelles by complexation with a natural polyamine and that the micellization is reversible as a function of the pH. It is also shown that the new pH sensitive micelles can act as structuring agents in the preparation of mesoporous silica materials.     

Linear-dendritic block copolymers: The state of the art and exciting perspectives

Concurrent with the rapid development of both dendrimers and hyperbranched polymers, a novel class of block copolymer architectures has emerged from the combination of these dendritic architectures with linear chains, the “linear-dendritic block copolymers” (LDBCs). This review gives a comprehensive summary of the state of the art in this rapidly developing field from pioneering early work to promising recent approaches.The different strategies leading to these hybrid architectures with either perfect dendrimer/dendron building blocks or imperfect, yet more conveniently accessible hyperbranched segments, are reviewed and compared. The consequences of the unusual polymer topology for supramolecular structures both in solution and in the solid state are summarized, and important differences in comparison with classical linear block copolymer structures are highlighted. Current challenges in the area of block copolymers, nanotechnology and potential applications of linear-dendritic block copolymers are also considered.     

Hexagonally ordered arrays of metallic nanodots from thin films of functional block copolymers

We demonstrate a new and simple route to fabricate highly dense arrays of hexagonally close packed inorganic nanodots using functional diblock copolymer (PS-b-P4VP) thin films. The deposition of pre-synthesized inorganic nanoparticles selectively into the P4VP domains of PS-b-P4VP thin films, followed by removal of the polymer, led to highly ordered metallic patterns identical to the order of the starting thin film. Examples of Au, Pt and Pd nanodot arrays are presented. The affinity of the different metal nanoparticles towards P4VP chains is also understood by extending this approach to PS-b-P4VP micellar thin films. The procedure used here is simple, eco-friendly, and compatible with the existing silicon-based technology. Also the method could be applied to various other block copolymer morphologies for generating 1-dimensional (1D) and 2-dimensional (2D) structures.     

Synthesis of amphiphilic block copolymers based on poly(dimethylsiloxane) via fragmentation chain transfer (RAFT) polymerization

Dihydroxy terminated poly(dimethyl siloxane) (PDMS) was modified to form a di(trithiocarbonate) functional molecule capable of forming tri-block copolymers via the reversible-addition-fragmentation chain transfer (RAFT) process. Two statistical copolymer blocks were grown from the central PDMS block, comprising units of N,N-dimethyl acrylamide (DMA) and 2-(N-butyl perfluorooctanefluorosulfonamido) ethyl acrylate (BFA), to form A-B-A triblock macromolecules. The molecular weight of these block copolymers were found to increase with conversion while the polydispersity of the molecular weight distribution remains under 1.25. An unusual and interesting kinetic phenomenon was observed in that the copolymerization behaviour of DMA and BFA was influenced by the initial PDMS block. We surmise that this might be a direct observation of a ‘bootstrap’ effect.     

Domain nucleation dictates overall nanostructure in composites of block copolymers and model nanorods

The detailed nanostructure of composites formed from block copolymers and nanoparticles is known to depend sensitively on the preferred morphology of the block copolymer, on the shapes of the particles, and on interactions between the two components. But it can also depend on the kinetics of self-assembly in the polymer, and there are circumstances under which the kinetics of morphologically selective domain nucleation and growth determine the overall nanostructure of the composite. To study the mechanism of morphological seeding in block-copolymer nanocomposites, we have combined cylinder phases of polystyrene-block-polyisoprene diblock (as a solution in dibutylphthalate) and poly(styrene-block-isoprene-block-styrene) triblock (as a blend with homopolystyrene) copolymers with gold nanorods of different diameters and surface treatments. Polarized optical microscopy and transmission electron microscopy on these composites demonstrate that the nanorods selectively nucleate coaxial domains of copolymer cylinders (i.e., domains of cylinders aligned along the same axis as the nanorod). These single nucleation events occur regardless of nanorod diameter and surface character, and determine the order of most of the surrounding polymer. Mesoscale modeling of the nucleation process, performed with nanorods of different diameters and with different polymer-surface interactions, illustrates the mechanism by which copolymer-dispersed nanorods with different sizes and surface chemistry can template the organization of cylindrical copolymer domains.     

Microphase separation and molecular conformation of AB2 miktoarm star copolymers by dissipative particle dynamics

We simulate the microphase separation behavior and analyze the molecular conformation of AB₂ miktoarm star copolymers via dissipative particle dynamics (DPD). The phase diagram is constructed by varying the composition and interaction parameter. Through a mapping of the interaction parameter for a finite chain length, we find that the phase diagram via DPD is in near quantitative agreement with that predicted by the self-consistent mean-field (SCMF) theory. However, when the B composition is small, AB₂ is not able to form the ordered microstructure as easily as SCMF has predicted. Instead, only a tube-like phase is formed. This aggregated micelle-like phase via DPD, which is ignored in the SCMF study, has been frequently observed in experiments. In the analysis of the radius of gyration (R_g), when the interaction parameter increases, the R_g values of each A and B arm remain relatively unchanged; while the overall radius of gyration of AB₂ significantly increases. Furthermore, the angle between A and B arms shows an increasing trend while the angle between B and B arms shows a decreasing behavior with the interaction parameter. These results reveal that in order to reduce the contacts between A and B, the A and B arms tend to separate from each other, and the two B arms are squeezed onto the same side.     

Chain extension and block copolymer synthesis using silane radical atom abstraction coupled with nitroxide mediated polymerization

Silane radicals were used to abstract bromine termini from monobrominated polystyrene (PStBr) in the presence of excess monomer and 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), generating polymer radicals that underwent chain extension. Typically, 70-85% of the PStBr precursors were activated by silane radical atom abstraction (SRAA) and were elongated by nitroxide mediated polymerization (NMP), shifting to higher number-average molecular weight (M_n) values as observed by gel permeation chromatography (GPC). Chain extension did not occur until the temperature was elevated to 130 °C, with no increase in M_n values observed when the reaction was held at 80 °C, which is the temperature of the SRAA phase. The NMP phase of the reaction showed a linear correlation between M_n values and monomer consumption, along with first order kinetics with respect to styrene. SRAA/NMP was then applied to the synthesis of polystyrene-b-poly(n-butyl acrylate) and polystyrene-b-poly(p-methylstyrene), with analysis by GPC indicating the formation of block copolymers with a similar amount of unreacted PStBr remaining. Quantitative activation and elongation of the polymer precursors were prevented due to the ability of both the t-butoxy radicals and tris(trimethylsilyl)silane radicals to add across monomeric double bonds, competing with atom abstraction. Reactions were thus performed in which the monomer was added only after the transition to the higher temperature, which resulted in improved activation of the PStBr.     

Structures and rheological properties of reactive solutions of block copolymers. Part I. Diblock copolymers in a liquid epoxy monomer

P(S-b-MMA) and P(B-b-MMA) diblock copolymers (BCP) have been solubilized in a liquid epoxy. The obtained solutions have been characterized by rheology and small angle X-ray scattering (SAXS). As in the solid state, BCP can self-organize in solution to form well-ordered micellar structures. The two blocks respective roles have been clearly identified: at room temperature the PS or the PB block microsegregates while the PMMA block for which epoxy constitutes a good solvent, acts as a stabilizer of the microphase separation. The geometries and thermal stabilities of the ordered structures depend strongly on the molar masses and the chemical nature of the BCP blocks. For instance, the total molar mass of the BCP has to be high enough to obtain a periodic structure. On the contrary, if this molar mass is too high, too long relaxation times prevent the system from reaching its equilibrium. For the P(S-b-MMA) copolymer solution, a transition temperature from an order to a disorder state (T_ODT) is observed. The origin of this transition has been attributed to a solubilization of the PS domains around T_ODT. Macroscopically, this transition can be defined as a solid-like to a liquid-like transition. In the case of the P(B-b-MMA) copolymer solution, no order-disorder transition has been observed: it can be explained by the fact that the PB blocks are not soluble in epoxy at any temperature, up to T=200 °C.     

Temperature and salt-induced micellization of some block copolymers in aqueous solution

Micellization of three commercial samples of ethylene oxide (EO)-propylene oxide (PO) (PEO-PPO-PEO) triblock copolymers (pluronics) P65 (EO₂₅PO₃₀EO₂₅, mw PPO=1750, %PEO=50), P85 (EO₂₅PO₄₀EO₂₅, mw PPO=2250, %PEO=50), and F88 (EO₁₀₂PO₄₀EO₁₀₂, mw PPO=2250, %PEO=80) in aqueous sodium chloride solution (0–3 M) was examined by cloud point, surface tension, dye spectral change, fluorescence, and viscosity measurements over a temperature range of 25–50°C. Salt-induced micellization and micelle growth were observed. The presence of sodium chloride enhanced hydrophobicity in the PPO moiety and reduced hydrophilicity of PEO moieties, favoring micellization at relatively lower concentrations than in water at ambient temperature. The effect of salt on micellization is similar to that of increasing temperature. The critical micelle concentrations (CMC) and critical micelle temperatures showed a marked decrease in the presence of added salt. CMC obtained by different methods were in good agreement.