Phase behavior in thin films of cylinder-forming ABA block copolymers: experiments

We experimentally establish a phase diagram of thin films of concentrated solutions of a cylinder forming polystyrene-block-polybutadiene-block-polystyrene triblock copolymer in chloroform. During annealing the film forms islands and holes with energetically favored values of film thickness. The thin film structure depends on the local thickness of the film and the polymer concentration. Typically, at a thickness close to a favored film thickness parallel orientation of cylinders is observed, while perpendicular orientation is formed at an intermediate film thickness. At high polymer concentration the cylindrical microdomains reconstruct to a perforated lamella structure. Deviations from the bulk structure, such as the perforated lamella and a wetting layer are stabilized in films thinner than approximately 1.5 domain spacings.     

Phase separation in thin films of end‐grafted block copolymers

An atomic force microscopy investigation was carried out on various thick (30–120 nm) polymethyl methacrylate‐b‐polystyrene and poly(2‐(dimethyl amino)ethyl methacrylate)‐b‐polystyrene films prepared via a grafting‐from method. The structure of the films was examined with both topographic and phase imaging. Several different morphologies were observed including a perforated lamellar phase with irregular perforations. In addition, complementary small‐angle X‐ray scattering and reflectometry results measurements on a non‐grafted polymer are presented. Copyright © 2009 John Wiley & Sons, Ltd.     

Nanoparticle directed domain orientation in thin films of asymmetric block copolymers

We investigated the thin film morphology of two different asymmetric block copolymers (BCP), polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and poly(n-pentyl methacrylate)-block-poly(methyl methacrylate) (PPMA-b-PMMA), loaded with pre-synthesized iron oxide nanoparticles (NP). The chemical composition of the BCP constituents determines the strength of the interaction between polymer chains and nanoparticles. In the case of NP/PS-b-P4VP system, the nanoparticles interact preferentially with the P4VP block and hence localize selectively in the P4VP cylindrical microdomains. However, for the NP/PPMA-b-PMMA system, the nanoparticles have no significant preference for the copolymer blocks and segregate at the polymer/substrate interface. Interestingly, this changes the effective substrate surface energy and hence leads to a remarkable change in domain orientation from parallel to perpendicular with respect to the substrate. These results clearly demonstrate the importance of both enthalpic and entropic factors which determine spatial distribution of NP in BCP films and influence domain orientation.     

Pattern formation and ordering in thin films of crystallisable block copolymers

We show that tapping-mode atomic force microscopy provides real-space and time-resolved observations of morphology and pattern formation resulting from crystallisation of annealed thin films of polyethelenoxide or microphase-separated low-molecular-weight hydrogenated poly(butadiene-b-ethyleneoxide) diblock copolymers. Differences in viscoelastic properties allow distinguishing crystalline and molten (amorphous) areas with a nanometer resolution.     

In-plane and out-of-plane defectivity in thin films of lamellar block copolymers

We investigate the ordering of poly(styrene-b-methyl methacrylate) (PS-PMMA) lamellar copolymers (periodicity L₀ = 46 nm) confined between a free surface and brushed poly(styrene-r-methyl methacrylate) silicon substrate. The processing temperature was selected to eliminate wetting layers at the top and bottom interfaces, producing approximately neutral boundaries that stabilize perpendicular domain orientations. The PS-PMMA film thickness (t = 0.5L₀ − 2.5L₀) and brush grafting density (Σ = 0.2–0.6 nm⁻²) were systematically varied to examine their impacts on in-plane and out-of-plane ordering. Samples were characterized with a combination of high-resolution microscopy, X-ray reflectivity, and grazing-incidence small-angle X-ray scattering. In-plane order at the top of the film (quantified through calculation of orientational correlation lengths) improved with tⁿ, where the exponent n increased from 0.75 to 1 as Σ decreased from 0.6 to 0.2 nm⁻². Out-of-plane defects such as tilted domains were detected in all films, and the distribution of domain tilt angles was nearly independent of t and Σ. These studies demonstrate that defectivity in perpendicular lamellar phases is three-dimensional, comprised of in-plane topological defects and out-of-plane domain tilt, with little or no correlation between these two types of disorder. Strong interactions between the block copolymer and underlying substrate may trap both kinds of thermally generated defects. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 339–352     

Monte Carlo simulations of cylinder-forming ABC triblock terpolymer thin films

We systematically study the cylinder-forming ABC triblock terpolymer thin films using canonical ensemble Monte Carlo simulations. The simulated annealing procedure is applied to the self-assembling process. By judicious choice of the system dimensions, we elaborately investigate the effect of film thickness on the orientation of the cylinders. This confined triblock terpolymer system exhibits different phase behavior under the weak and strong surface fields. In addition, we also investigate the ensemble-averaged chain orientations and relative density profiles.     

Phase behavior in thin films of confined colloid-polymer mixtures

Using self-consistent-field and density-functional theories, we first investigate colloidal self-assembly of colloid-polymer films confined between two soft surfaces grafted by polymers. With increasing colloidal concentrations, the film undergoes a series of transitions from disordered liquid --> sparse square --> hexagonal (or mixed square-hexagonal) --> dense square --> cylindrical structures in a plane, which results from the competition between the entropic elasticity of polymer brushes and the steric packing effect of colloidal particles. A phase diagram displays the stable regions of different in-layer ordering structures as the colloidal concentration is varied and layering transitions as the polymer-grafted density is decreased. Our results show a new control mechanism to stabilize the ordering of structures within the films.     

Hybrid nanocapsules: interactions of ABA block copolymers with liposomes

Amphiphilic ABA triblock copolymers, such as poly(2-methyloxazoline)-block-poly(dimethylsiloxan)-block-poly(2-methyloxazoline) (PMOXA-PDMS-PMOXA), form vesicular structures. Here, the interaction of these ABA molecules with lipids is investigated by electron microscopy, fluorescence spectroscopy, light scattering, and differential scanning calorimetry. Our observations suggest the formation of homogeneous mixed polymer-lipid composites, independent of preparation method, i.e. film hydration, dispersion, or detergent removal. When ABA polymersomes and liposomes are mixed, we observed monomer exchanges on a time scale of minutes. The possibility of forming mixed structures and the exchanges between preformed structures allow the combination of the properties of lipids and polymers such as stability and loading encapsulation capacity.     

Statistical thermodynamics of ABA block copolymers. II

A microstructural model for the phase-separated states of ABA triblock copolymers is proposed. It postulates the simultaneous existence of three phases: pure A, pure B, and a mixed region. Incorporation of the mixed region distinguishes this treatment from all other theories and is responsible for considerable flexibility in the model without increased numbers of parameters. Calculations of free energy changes required to establish specific microstructures permit the prediction of a favored one from among five possibilities: discrete spheres of A (or B), discrete cylinders of A (or B), and lamellae.     

Synthesis and properties of ABA propylene-ethylene block copolymers

Experimental data, which includes catalyst lifetimes, thermal analyses, fractionation by urea complexation, x-ray diffraction, and ¹³C-NMR spectroscopy, are presented to confirm the successful synthesis of ABA propylene-ethylene block copolymers. A dry catalyst system of DEAC-TiCl₃(AA) and a gas-phase polymerization technique was used to prepare the copolymers. PRP-and ERE-type copolymers (P-isotactic polypropylene, E-polyethylene, and R-random propylene-ethylene copolymer block) were prepared. Some preliminary physical property data are given which indicate that PRP-type copolymers can behave as elastomeric fibers. The stress-strain behavior also indicates block copolymer formation.     

Synthesis, post-modification and self-assembled thin films of pentafluorostyrene containing block copolymers

Block copolymers consisting of a pentafluorostyrene (PFS) block and a hydrophilic block were synthesized by RAFT polymerisation. The hydrophilic blocks consist of methacrylate derivatives, 4-hydroxystyrene or 4-vinylpyridine monomers. The block copolymers were obtained with narrow molecular weight distributions and the molecular weights were in good agreement with the theoretical values. In addition, a model thiol was reacted with the PFS moieties of the block copolymers. This polymer–analogous reaction was performed under ambient conditions in high yields resulting quantitatively in para-substitution of the pentafluorophenyl rings. Finally, thin films consisting of block copolymers that showed strong phase-segregation behaviour and ordered nanostructured surfaces consisting of both blocks were obtained.     

Grazing-incidence transmission small angle X-ray scattering from thin films of block copolymers

Thin films of lamellar and cylindrical block copolymers are popular systems for low-cost nanolithography. To be useful as nanoscale templates, the lamellae or cylinders must be oriented perpendicular to the substrate. Domain orientations are usually characterized by microscopy measurements of the film surface, but these techniques cannot detect tilted, bent, or tortuous domains in the film interior. We report a simple method to quantify out-of-plane disorder in thin films of block copolymers based on a variant of grazing-incidence small angle X-ray scattering (GI-SAXS). A typical GI-SAXS experiment illuminates the center of a substrate-supported film at a grazing angle of incidence (near the film/substrate critical angle), and the strong reflected signal is interpreted with the distorted-wave Born approximation. In a new approach, the beam footprint is moved to the far edge of the sample, allowing the acquisition of a transmission pattern. The grazing-incidence transmission data are interpreted with the simple Born approximation, and out-of-plane defects are quantified through analysis of crystal truncation rods and partial Debye-Scherrer rings. Significantly, this study demonstrates that grazing-incidence transmission small angle X-ray scattering can detect and quantify the buried defect structure in thin films of block copolymers. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

Robust control of microdomain orientation in thin films of block copolymers by zone casting

Block copolymers with chemically immiscible segments exhibit a variety of microphase-separated nanostructures on the scale of 10-100 nm. Controlling the orientation of these microphase separated nanostructures is vital in many applications such as lithography, membranes, data storage, and so forth. Typical strategies involve the use of external fields or patterned substrates. Here, we report a robust zone casting technique to achieve highly ordered thin films of block copolymers on centimeter-scale substrates. The robustness of this technique is its powerful control on diverse morphologies and exceptional tolerance on versatility of block copolymer chemistry as well as allowance of a wide spectrum of substrates. We demonstrate that perpendicular orientations with respect to the surface are achieved for block copolymers with both lamellar and cylindrical morphologies by controlling solution casting rate, temperatures, and block copolymer chemical structures. Thin films of both noncrystalline and crystalline block copolymers exhibit excellent orientational order and lateral order. However, the lateral order in the thin films of crystalline block copolymers shows dependence on casting temperature and melting temperature of the crystalline segment. Remarkably, all the ordering is independent of the substrates on which the block copolymer films are cast.     

Self‐assembly of metallo‐supramolecular block copolymers in thin films

The self‐assembly of a metallo‐supramolecular PS‐[Ru]‐PEO block copolymer, where ‐[Ru]‐ is a bis‐2,2′:6′,2″‐terpyridine‐ruthenium(II) complex, in thin films was investigated. Metallo‐supramolecular copolymers exhibit a different behavior as compared to their covalent counterparts. The presence of the charged complex at the junction of the two blocks has a strong impact on the self‐assembly, effecting the orientation of the cylinders and ordering process. Poly(ethylene oxide) cylinders oriented normal to the film surface are obtained directly regardless of the experimental conditions over a wide range of thicknesses. Exposure to polar solvent vapors can be used to improve the lateral ordering of the cylindrical microdomains. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4719–4724, 2008     

Crazing in thin films of styrene–butadiene–styrene block copolymers

The mechanism of craze initiation and growth and its relationship to mechanical properties has been studied in thin films of styrene–butadiene–styrene (SBS) block copolymers. Optical microscopy and transmission electron microscopy were used to examine three copolymers which has a spherical rubber domain morphology but varied in rubber content from 20 to 50%. With increasing rubber content, the crazes became longer and less numerous. Widening of the crazes was at least partially responsible for the higher strains achieved in the copolymers, especially for the composition with the highest rubber content where the crazes widened to form micronecks. Transmission electron microscopy revealed that craze initiation and growth at the craze tip occurred by cavitation in the polystyrene phase. Cavitation of the continuous phase rather than the rubber domains was attributed to the concentration of chain-end flaws in the polystyrene. Crazes in the block copolymers followed a meandering pathway and the boundaries between crazed and uncrazed material were indistinct. Incorporation of fibrillated rubber particles into the craze fibrils strengthened the craze. At higher rubber content, the craze widened in the stress direction by voiding and fibrillation, which produced a cellular morphology.     

Controlling the size of nanostructures in thin films via blending of block copolymers and homopolymers

The effects of molecular weight and concentration of poly (methyl methacrylate) (PMMA) homopolymer or symmetric short polystyrene-block-poly (methyl methacrylate) (PS-b-PMMA) diblock copolymer on the size of the nanostructures of its blends with symmetric long PS-b-PMMA diblock copolymer have been investigated by atomic force microscopy. By careful controlling of the film thickness, solvent selectivity, and annealing time, PMMA cylindrical microdomains oriented normal to the film surface were obtained in all thin films. With the addition of both low- and high-molecular-weight PMMA homopolymers, the cylindrical domain sizes increased although it was less obvious for the lower molecular weight homopolymer. In contrast to the homopolymer, adding the short chain diblock copolymer resulted in a decrease in the cylindrical domain size, which was ascribed to the reduction of the interfacial tension and increase in the stretching energy.     

Ordering in thin films of asymmetric diblock copolymers

The time evolution of the free surface of asymmetric diblock copolymers of polystyrene and poly(methyl methacrylate) on a strongly interacting surface was studied with atomic force microscopy. The surface morphology underwent morphological transitions to satisfy commensurability conditions. These transformations were consistent with recent self‐consistent field arguments predicting the phase transitions of copolymers as a function of thickness (see M. J. Fasolka, P. Banerjee, A. M. Mayes, G. Pickett, & A. C. Balazs, Macromolecules 2000, 33, 5702). © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 663–668, 2001     

Synthesis and properties of high strength high modulus ABA block copolymers

A series of novel ABA block copolymers were synthesized containing a rigid-rod (B) block for reinforcement and a flexible coil (A) block as the matrix. Poly[(benzo[1, 2d: 4, 5d′]bisthiazole-2,6-diyl)-1,4-phenylene] (PBT) was the rigid-rod (B) block utilized in this study and was polymerized in such a way as to provide carboxylic acid end-groups. The carboxy-terminated PBT was copolymerized with the AB monomers, 3,4-diaminobenzoic acid and 4-amino-3-mercaptobenzoic acid, which generates a benzimidazole or benzthiazole (A) block, as well as grafts the blocks together. Composition of the blocks could be varied by the weight of AB monomer used in the copolymerizations. Solution behavior of the copolymers in methanesulfonic acid was determined, and fibers were obtained by wet spinning techniques. The block copolymers exhibited typical tenacities of 200 ksi, 16 Msi modulus, and an elongation to break of 1.4%. Critical concentration values for fabrication increased approximately 3% over mechanical mixtures of the same heterocyclic components.     

Phase behavior of mixed Langmuir monolayers from amphiphilic block copolymers and an antimicrobial peptide

The behavior of binary monolayers from PMOXA-PDMS-PMOXA triblock copolymers and alamethicin, an antimicrobial peptide, was investigated in the context of formation of novel biocomposite nanostructured materials. The properties of mixed monolayers were studied by surface pressure-area isotherms and Brewster angle imaging. As reported previously, functionality of alamethicin relies on its aggregation properties in lipid mono- and bilayers. This is also the case in polymer matrixes, however, here the mixing properties differ from lipid-peptide systems due to the polymers' structural specificity. The peptide influence on the polymer films is provided in detail for the first time, and supported by the compressibility data to asses the elastic properties of such composite membranes.