Order‐Order Transition of C → sdG → sL → S in ABC Triblock Copolymer Thin Film Induced by Solvent Vapor

The morphology transition of polystyrene‐block‐poly(butadiene)‐block‐poly(2‐vinylpyridine) (SBV) triblock thin film induced in benzene vapor showing weak selectivity for PS is investigated. The order‐order transitions (OOT) in the sequence of core‐shell cylinders (C), sphere in ‘diblock gyroid’ (sdG), sphere in lamella (sL) and sphere (S) are observed. The projection along (111) direction in Gyroid phase (sdG(111)) is found to epitaxially grow from C(001) in the film. Instead of sdG(111), sdG(110)_0.1875 develops to the phase of sL. Consequently, the film experiences the transition sequence of sdG(111) → sdG(211) → sdG(110)_0.25 → sdG(110)_0.1875 between C and sL. The mechanism is analyzed from the total surface area of the blocks.

     

Conformational Study on Thin Films of Symmetric AnB2nAn Triblock Copolymer

Summary: The behavior of symmetric A_nB_2nA_n triblock copolymer films confined between two hard neutral walls was explored by Monte Carlo simulation. The thicknesses of the films were between ≈1R_g0 and ≈7R_g0, where R_g0 is the unperturbed radius of gyration in the bulk. The confinement leads to a lamellar structure normal to the wall and the order‐disorder transition (ODT) temperature was found to be a function of film thickness. When the film thickness (D) was less than a critical value, D_C, which is between 3R_g0 and 4R_g0, the ODT temperature (T*_ODT) reduced by chain length N (T*_ODT/N) decreased with decreasing film thickness. However, T*_ODT/N was nearly independent of the film thickness when it was greater than D_C. In the case of strong confinement (D < D_C), the B block shrinks along the direction perpendicular to the wall and stretches along the direction parallel to the wall with decreasing film thickness, and the volume occupied by the B block shrinks. Under weak confinement conditions (D > D_C), the volume of the B block is nearly independent of film thickness. The conformations of the B block in the disordered state are quite different from those in the lamellae. If the film is thick enough, the volume of the B block approaches its value in the unperturbed state, regardless of the morphology. When temperature decreases, the B block stretches in the direction perpendicular to the A/B interface and shrinks in the other two directions. In addition, decreasing the temperature leads to the chains adopting two main extreme conformations, coiling or stretching as much as they can. The scaling behavior of the fraction of bridge chains vs. the temperature obtained in the weak segregation limit was different from that predicted in the strong segregation limit.

Schematic diagram of the X, Y and Z axis definition.     

Monte Carlo Simulations of the Morphologies and Conformations of Triblock Copolymer Thin Films

Summary: The morphologies and conformations of triblock copolymer (ABA and ABC) thin films confined between two identical walls were investigated by Monte Carlo simulation using bond length fluctuation and cavity diffusion algorithm on cubic lattice. Effects of the wall‐block interactions, copolymer chain composition and film thickness on morphologies, as well as on the fraction of chain “bridge” conformation f_bridge are presented in detail. In ABA thin film, column, parallel, perforated and perpendicular lamellas were discriminated, furthermore, the transition of morphology and the variation of f_bridge of ABA film along with the increase of thickness were revealed. In ABC thin film, lamella especially perpendicular lamella morphologies are predominant in varying the wall‐block interactions and the thickness. The results are consistent with some theoretical predictions such as DDFT and simulations reported in literature.

Isodensity profile of A₅B₅A₅ thin film.     

Order‐order transitions of a triblock copolymer with a homopolymer (ABC/A) blend film induced by saturated solvent vapor annealing

The morphology transition of binary mixtures of polystyrene‐block‐poly(butadiene)‐block‐poly(2‐vinylpyridine)(SBV) triblock and polystyrene (PS) homopolymer thin films was investigated as a function of the volume fraction of added homopolymer and the annealing time in benzene vapor. It was found that the weight ratio of PS in the blends influenced the transition process. When PS content was >5%, the order‐order transition (OOT) of core‐shell cylinders (C) →sphere in “diblock Gyroid” (sdG) → sphere in lamella (sL) → sphere (S) was observed, which was similar to ABC triblock copolymer except for the increased surface area of the PS phase. When PS content reached to 10–30%, the OOT in the sequence of C → sL → S was observed. The disappearance of the Gyroid phase is due to the change of the effective volume fraction. Further increasing the PS content, C phase also disappeared and sL → S was expected to take place. © 2014 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2014 , 52, 1030–1036     

Formation of Honeycomb Structures and Superhydrophobic Surfaces by Casting a Block Copolymer from Selective Solvent Mixtures

Poly(vinyl phenol)‐block‐polystyrene (PVPh‐b‐PS) diblock copolymers are synthesized by sequential anionic polymerization with sec‐butyl lithium as the initiator. The PVPh‐b‐PS diblock copolymer is cast (on a substrate) from several solvent mixtures that contain tetrahydrofuran/toluene ratios of 1:0.1, 1:1, and 1:2. After solvent evaporation the resulting films are characterized by SEM, TEM, and contact angle measurements. A honeycomb structure is fabricated from the vesicle structure at relatively low toluene contents. On the contrary, at relatively higher toluene contents, a micelle structure with porous microspheres is formed, which possesses higher surface roughness and results in film surface superhydrophobicity. The simple method described here that uses common/selective mixed solvents may be easily extended to prepare honeycomb structures and superhydrophobic surfaces simultaneously from a wide variety of block copolymers by carefully controlling the weight composition of the block copolymer and the selective solvent content.

     

Microdomain Morphology of Symmetrical Diblock Copolymer Thin Films Confined in a Slit

The microphase separation and morphology of symmetric diblock copolymer thin films confined in a slit with neutral or attractive surfaces were studied by the cell dynamic system method (CDS) and Monte Carlo simulation. The size effect, especially in CDS, was carefully investigated indicating that excessively small sizes in the X‐ and Y‐directions will give incorrect results although periodic boundary conditions are imposed. When the walls are neutral, parallel ordered lamella structure only exists over a short range, while irregular microdomain morphology occurs over the whole region. When directional quenching is applied, or the walls are attractive to one of the blocks, a periodical lamellar structure of alternating A‐rich and B‐rich layers occurs over the whole region of the film. Changing the slit width and the strength of interaction will influence the period and arrangement of lamellae. Agreement between the results from CDS and those from simulation is satisfactory indicating the reliability of the CDS method. Comparisons with corresponding experimental results are also discussed.     

溶剂尺寸对ABA两亲性三嵌段共聚物在选择性溶剂中自组装行为影响的Monte Carlo模拟

采用Monte Carlo模拟方法研究了溶剂尺寸对ABA两亲性三嵌段共聚物在选择性溶剂中自组装行为的影响。模拟结果表明,溶剂尺寸是决定共聚物聚集形态的重要因素之一。随着溶剂尺寸的增大,嵌段共聚物自组装所形成的胶束可以发生从球状到棒状再到囊泡状的转变。通过对各组分的相互作用对数随溶剂尺寸变化曲线的分析发现,增大溶剂尺寸会使溶剂的溶解性变差,由此引发体系中的一系列形态转变。此外,通过对体系自组装形貌结构相图的分析发现,增大溶剂尺寸或增加疏水作用同减小亲水作用对于自组装形态的改变具有同等效果。     

Structure and viscoelasticity of matched asymmetric diblock and triblock copolymers in the cylinder and sphere microstructures

A polystyrene–polyisoprene (PS–PI) diblock copolymer (10,000–50,000 g/mol) and a matched PS&ndashPI–PS triblock (10,000–100,000–10,000 g/mol) were employed to study the effect of chain architecture on the rheological response of ordered block copolymer melts. Both samples adopt hexagonal microstructures with PS cylinders embedded in a PI matrix; on further heating, an order–order transition (OOT) into a cubic array of spheres takes place prior to the order–disorder transition. Each morphology was verified by SAXS and TEM. Interestingly, at the OOT the low-frequency elastic modulus of the diblock increased abruptly, whereas that of the triblock decreased. In contrast, the modulus of the cubic phase was roughly independent of chain architecture. Chain relaxation parallel and perpendicular to the cylinders was probed by measuring the elastic modulus of a macroscopically aligned sample in directions parallel G and perpendicular (G) to the cylinder orientation. For both materials G < G < G where G is the elastic modulus of a randomly oriented sample. This result is attributed to the ability of the unentangled PS blocks to move along the direction of the cylinder axis, and thus relax the stress in the PI matrix in the parallel alignment. In each of the three cylindrical orientations the triblock had a larger modulus than the diblock, which is attributed to the presence of bridging PI blocks that connect distinct PS domains. About 20° below the OOT G showed a distinct change in its temperature dependence, which, coupled with SAXS measurements, is indicative of the onset of an undulation in the cylinder diameter that presages the pinching off of cylinders into spheres, as recently predicted by theory. The use of oriented samples also permitted SAXS confirmation of an approximate epitaxial relationship between the cylinder and the sphere unit cells, although a distinct change in the location of the structure factor maximum, q^*, is noted at the OOT. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2811–2823, 1997     

Control of Induced Chirality in Optically Active Poly(N‐propargylcarbamate) Films by Solvent Vapor

Chiral polyethyne derivatives with lyotropic liquid‐crystalline properties are found to be able to self‐assemble, forming two chiral organizations with opposite handedness in solid thin films by selection of the casting solvent and its concentration. After the film preparation, chiral organization could also be induced by simple exposure to an appropriate organic solvent's vapor for several minutes without thermal treatment. Furthermore, irreversible inversion of the handedness of the chiral organization in the film could be achieved by exposure to solvent vapor.

     

Structural Evolution of Perpendicular Lamellae in Diblock Copolymer Thin Films during Solvent Vapor Treatment Investigated by Grazing‐Incidence Small‐Angle X‐Ray Scattering

The structural evolution in poly(styrene‐b‐butadiene) (P(S‐b‐B)) diblock copolymer thin films during solvent vapor treatment is investigated in situ using time‐resolved grazing‐incidence small‐angle X‐ray scattering (GISAXS). Using incident angles above and below the polymer critical angle, structural changes near the film surface and in the entire film are distinguished. The swelling of the film is one‐dimensional along the normal of the substrate. During swelling, the initially perpendicular lamellae tilt within the film to be able to shrink. In contrast, at the film surface, the lamellae stay perpendicular, and eventually vanish at the expense of a thin PB wetting layer. During the subsequent drying, the perpendicular lamellae reappear at the surface, and finally, PS blocks protrude. By modeling, the time‐dependent height of the protrusions can be quantitatively extracted.

     

Photoreversible Gelation of a Triblock Copolymer in an Ionic Liquid

The reversible micellization and sol–gel transition of block copolymer solutions in an ionic liquid (IL) triggered by a photostimulus is described. The ABA triblock copolymer employed, denoted P(AzoMA‐r‐NIPAm)‐b‐PEO‐b‐P(AzoMA‐r‐NIPAm)), has a B block composed of an IL‐soluble poly(ethylene oxide) (PEO). The A block consists of a random copolymer including thermosensitive N‐isopropylacrylamide (NIPAm) units and a methacrylate with an azobenzene chromophore in the side chain (AzoMA). A phototriggered reversible unimer‐to‐micelle transition of a dilute ABA triblock copolymer (1 wt %) was observed in an IL, 1‐butyl‐3‐methylimidazolium hexafluorophosphate ([C₄mim]PF₆), at an intermediate “bistable” temperature (50 °C). The system underwent a reversible sol–gel transition cycle at the bistable temperature (53 °C), with reversible association/fragmentation of the polymer network resulting from the phototriggered self‐assembly of the ABA triblock copolymer (20 wt %) in [C₄mim]PF₆.     

Improved Conductivity Induced by Photodesorption in SnO2 Thin Films Grown by a Sol-Gel Dip Coating Technique

Thin films of undoped and Sb-doped SnO2 have been prepared by a sol-gel dip-coating technique. For the high doping level (2–3 mol% Sb) n-type degenerate conduction is expected, however, measurements of resistance as a function of temperature show that doped samples exhibit strong electron trapping, with capture levels at 39 and 81 meV. Heating in a vacuum and irradiation with UV monochromatic light (305 nm) improves the electrical characteristics, decreasing the carrier capture at low temperature. This suggests an oxygen related level, which can be eliminated by a photodesorption process. Absorption spectral dependence indicates an indirect bandgap transition with Eg 3.5 eV. Current-voltage characteristics indicate a thermionic emission mechanism through interfacial states.     

Selective Formation of Spinel Iron Oxide in Thin Films by Complexing Agent-Assisted Sol-Gel Processing

The structure of iron oxide was controlled by regulating the hydrolytic polymerization of aquo iron complexes with organic polydentate ligands such as diols. Iron oxides were prepared by calcining the precursor polymers obtained from iron nitrate nonahydrate and diols. When the diols were 1,2-pentanediol, 1,2-hexanediol and 1,2-octanediol, α-Fe₂O₃ with corundum structure appeared exclusively or as the main crystalline phase, in spite of the amount of diol used and the calcination temperature. In the case of 1,2-decanediol and 1,2-dodecanediol, when five moles of the diols were used to one mole of iron nitrate and the calcination temperatures were below 400°C, ψ-Fe₂O₃ with spinel structure appeared as the main phase and, when less than five moles of the diols were used, α-Fe₂O₃ appeared exclusively or as the main phase, irrespective of the calcination temperature. This tendency was also observed in thin films. Thus, a transparent magnetic film composed of γ-Fe₂O₃ could be prepared by applying a benzene solution of the iron polymer, obtained with 5 equivalents of 1,2-decanediol, on quartz and calcining the gel film at 350°C.     

Complex Macrophase‐Separated Nanostructure Induced by Microphase Separation in Binary Blends of Lamellar Diblock Copolymer Thin Films

The nanostructures of thin films spin‐coated from binary blends of compositionally symmetric polystyrene‐b‐polybutadiene (PS‐b‐PB) diblock copolymer having different molar masses are investigated by means of atomic force microscopy (AFM) and grazing‐incidence small‐angle X‐ray scattering (GISAXS) after spin‐coating and after subsequent solvent vapor annealing (SVA). In thin films of the pure diblock copolymers having high or low molar mass, the lamellae are perpendicular or parallel to the substrate, respectively. The as‐prepared binary blend thin films feature mainly perpendicular lamellae in a one‐phase state, indicating that the higher molar mass diblock copolymer dominates the lamellar orientation. The lamellar thickness decreases linearly with increasing volume fraction of the low molar mass diblock copolymer. After SVA, well‐defined macrophase‐separated nanostructures appear, which feature parallel lamellae near the film surface and perpendicular ones in the bulk.

     

Tuning the Stability of CaCO3 Crystals with Magnesium Ions for the Formation of Aragonite Thin Films on Organic Polymer Templates

Thin‐film growth of aragonite CaCO₃ on annealed poly(vinyl alcohol) (PVA) matrices is induced by adding Mg²⁺ into a supersaturated solution of CaCO₃. Both the growth rate and surface morphology of the aragonite thin films depend upon the concentration of Mg²⁺ in the mineralization solution. In the absence of PVA matrices, no thin films are formed, despite the presence of Mg²⁺. Molecular dynamics simulation of the CaCO₃ precursor suggests that the transition of amorphous calcium carbonate to crystals is suppressed in the presence of Mg²⁺. The role for ionic additives in the crystallization of CaCO₃ on organic templates obtained in this study may provide useful information for the development of functional hybrid materials.     

Highly Luminescent Thin Films of the Dense Framework 3∞[EuIm2] with Switchable Transparency Formed by Scanning Femtosecond‐Pulse Laser Deposition

Highly luminescent switchable thin films of the dense framework ³_∞[EuIm₂] were deposited by a scanning laser ablation technique using a femtosecond laser. The films can be controlled in terms of film thickness and amount of material deposited such that the material properties of the bulk material are retained on the nanometer scale. Polycrystalline films are formed that can be switched between transparent at visible light and nontransparent at UV light due to the intrinsic luminescence of the hybrid material, expanding the concept of smart films. The new femtosecond pulsed laser deposition method also provides a novel approach for coatings with framework compounds and coordination polymers.     

Sphere‐to‐Rod Transition of Micelles formed by the Semicrystalline Polybutadiene‐block‐Poly(ethylene oxide) Block Copolymer in a Selective Solvent

We present a morphological study of the micellization of an asymmetric semicrystalline block copolymer, poly(butadiene)‐block‐poly(ethylene oxide), in the selective solvent n‐heptane. The molecular weights of the poly(butadiene) (PB) and poly(ethylene oxide) (PEO) blocks are 26 and 3.5 kg · mol⁻¹, respectively. In this solvent, micellization into a liquid PEO‐core and a corona of PB‐chains takes place at room temperature. Through a thermally controlled crystallization of the PEO core at −30 °C, spherical micelles with a crystalline PEO core and a PB corona are obtained. However, crystallization at much lower temperatures (−196 °C; liquid nitrogen) leads to the transition from spherical to rod‐like micelles. With time these rod‐like micelles aggregate and form long needles. Concomitantly, the degree of crystallinity of the PEO‐cores of the rod‐like micelles increases. The transition from a spherical to a rod‐like morphology can be explained by a decrease of solvent power of the solvent n‐heptane for the PB‐corona chains: n‐Heptane becomes a poor solvent at very low temperatures leading to a shrinking of the coronar chains. This favors the transition from spheres to a morphology with a smaller mean curvature, that is, to a cylindrical morphology.

     

Induced changes of polymer organization in a gel of poly(vinyl alcohol) crosslinked by Congo red

Different gel microstructures are induced at variable poly(vinyl alcohol) (PVA) and Congo red concentrations, as revealed by ultrarapid freezing and a replica technique for transmission electron microscopy. The polymer microstructures observed include random coils, rigid polymer rods, and long fibers. The development of the different polymer conformations is proposed to be dependent on the degree of intramolecular and intermolecular crosslinking and on the electrostatic interactions of the Congo red ions. The rigid‐rod conformation appears to be the most energetically stable form; it is disrupted by electrostatic effects around the polymer overlap concentration (C*_PVA). We propose that the gel microstructure influences the physical properties of the gel. Gels possessing the rigid‐rod microstructure have increased Young's storage modulus values. Two possible mechanisms of gelation are suggested. The first describes a one‐stage reaction when the polymer concentration approximates C*_PVA, where polymers in an extended random‐coil conformation undergo intermolecular crosslinking without any microstructural changes. The second describes a two‐stage reaction when the polymer concentration is less than or greater than C*, where a disorder–order transition results in the formation of rigid polymer rods and fibers followed by the formation of a macromolecular network. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1471–1483, 2001     

Ordering poly(trimethylsilyl styrene‐block‐D,L‐lactide) block copolymers in thin films by solvent annealing using a mixture of domain‐selective solvents

Controlling the morphology, domain orientation, and domain size of block copolymer (BCP) thin films is desirable for many applications in nanotechnology. These properties can be tuned during solvent annealing by varying the solvent choice and degree of swelling which affect the effective miscibility and volume fraction of the BCP domains. In this work, we demonstrate with a bulk lamellae‐forming BCP, poly(4‐trimethylsilylstyrene‐block‐D ,L ‐lactide) (PTMSS‐b‐PLA), that varying the composition of a mixture of solvent vapors containing cyclohexane (PTMSS‐selective) and acetone (PLA‐selective), enables formation of perpendicularly oriented lamellae with sub‐20‐nm pitch lines. The BCP domain periodicity was also observed to increase by 30%, compared to bulk, following solvent annealing. Furthermore, solvent annealing alone is shown to induce a transition from a disordered to an ordered BCP. We rationalize our observations by hypothesizing that the use of a combination of domain selective solvent mixtures serves to increase the effective repulsion between the blocks of the copolymer. We furnish results from self‐consistent field theory calculations to support the proposed mechanism. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2014 , 52, 36–45