Solvent-induced microphase separation in diblock copolymer thin films with reversibly switchable morphology

We have studied the surface morphology of symmetric poly(styrene)-block-poly(methyl methacrylate) diblock copolymer thin films after solvent vapor treatment selective for poly(methyl methacrylate). Highly ordered nanoscale depressions or striped morphologies are obtained by varying the solvent annealing time. The resulting nanostructured films turn out to be sensitive to the surrounding medium, that is, their morphologies and surface properties can be reversibly switchable upon exposure to different block-selective solvents.     

On the microphase separation kinetics of symmetric diblock copolymers

Time-resolved small-angle x-ray scattering studies were performed on symmetric diblock copolymers of polystyrene and poly(methyl methacrylate), P(S-b-MMA). Freeze-dried powders of P(S-b-MMA) having a molecular weight of 8.4×10⁴ were rapidly heated to temperatures above the glass transition temperature to initiate the microphase separation. The microphase separation process was found to consist of a rapid, local microphase separation followed by a long-term coarsening process. The period characterizing the lamellar microphase separated structure was found to increase initiallv and then saturate at longer times. These results are discussed in light of recent theoretical developments.In celebration of his 65th birthday, this article is dedicated to Prof. E. W. Fischer whose methodic and thorough approach to research has been and continues to be a model for us to follow. Es freut mich, daß ich mit Herrn Fischer gearbeitet habe. Ich habe vieles von ihm gelernt. Ich hoffe, daß auch ich so fleißig sein werde, wenn ich so jung bin wie er.     

Selective solvent annealing induced phase separation and dewetting in PMMA/SAN blend ultrathin films

Phase behaviors induced by solvent annealing in poly(methyl methacrylate) (PMMA) and poly(styrene‐ran‐acrylonitrile) (SAN) blend ultrathin films have been investigated by atomic force microscopy and grazing incidence small‐angle X‐ray scattering. Our results indicate that both the phase separation within the blend and the dewetting of the film induced by composition fluctuation take place upon the selective solvent annealing, producing complex structures containing upper droplets (of one phase) and mimic‐films (of the other rich‐phase). The use of acetic acid (the selective solvent for PMMA) generates PMMA mimic‐film and SAN droplets, while the introduction of DMF (exhibiting better solubility for SAN) vapor results in the formation of SAN mimic‐film and PMMA droplets. Essentially, the interaction at polymer/substrate interface, resultant wettability of selected component, solubility of PMMA and SAN in adopted solvent dominate not only the phase separation and the dewetting of the whole film but also the synergism of them. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1243–1251     

Self-recovery of chiral microphase separation in an achiral diblock copolymer system

Macroscopic regulation of chiral supramolecular nanostructures in liquid-crystalline block copolymers is of great significance in photonics and nanotechnology. Although fabricating helical phase structures via chiral doping and microphase separation has been widely reported, the chiral memory and self-recovery capacity of asymmetric phase structures are the major challenge and still deeply rely on the presence of chiral additives. Herein, we demonstrate the first controllable chiral microphase separation in an achiral amphiphilic block copolymer consisting of poly(ethylene oxide) and azobenzene (Azo) groups. Chirality can be transferred to the fabricated helical nanostructures by doping with chiral additives (tartaric acid, TA). After the removal of the chiral additives and then performing cross-linking, the formed helical nanostructures will completely dispense with the chiral source. The supramolecular chirality and the micron-scale phase structure can be maintained under UV irradiation and heating-cooling treatment, enabling a reversible “on–off” chiroptical switch feature. This work is expected to avoid the tedious synthesis and expensive raw materials and shows a great application prospect in chiral separation and so on.

A chirality-storing copolymer MPS structure will overcome the external chiral source dependence and avoid tedious synthesis and expensive raw materials.     

Viscoelastic properties of ultrathin polycarbonate films by liquid dewetting

A liquid dewetting method for the determination of the viscoelastic properties of ultrathin polymer films has been extended to study thickness effects on the properties of ultrathin polycarbonate (PC) films. PC films with film thicknesses ranging from 4 to 299 nm were placed on glycerol at temperatures from below the macroscopic glass transition temperature (T_g) to above it with the dewetting responses being monitored. It is found that the isothermal creep results for films of the same thickness, but dewetted at different temperatures can be superposed into one master curve, which is consistent with the fact of PC being a thermorheologically simple material. Furthermore, the results show that the T_g of PC thin films is thickness dependent, but the dependence is weaker than the results for freely standing films and similar to literature data for PC films supported on rigid substrates. It was also found that the rubbery plateau region for the PC films stiffens dramatically, but still less than what has been observed for freely standing polycarbonate films. The rubbery stiffening is discussed in terms of a recently reported model that relates macroscopic segmental dynamics with the stiffening. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1559–1566     

Apparent dewetting of ultrathin multilayered polyelectrolyte films incubated in aqueous environments

We have investigated and characterized changes in film morphology and surface structure that occur when ultrathin multilayered polyelectrolyte films fabricated from linear poly(ethylene imine) (LPEI), sodium poly(styrene sulfonate) (SPS), and two hydrolytically degradable polyamines (polymers 1 and 2) are incubated in physiologically relevant environments. Characterization of the physical erosion profiles of films having the structure (LPEI/SPS)10(1/SPS)4(2/SPS)4 (approximately 80 nm thick) by atomic force microscopy (AFM), reflective optical microscopy, and scanning electron microscopy (SEM) demonstrated that these materials undergo large-scale changes in surface structure and morphology upon incubation in phosphate-buffered saline (PBS) at 37 degrees C. The patterns and structures generated during this transformation (e.g., nucleation and growth of holes, coalescence of holes, formation of cell-type structures, and the subsequent breakup of these features into droplets) are similar in many ways to those observed for the dewetting of thin films of conventional polymers, such as polystyrene, on nonwetting surfaces. The processes reported here are sufficiently slow (they occur over approximately 100 h) and occur under sufficiently mild conditions (e.g., incubation in PBS at 37 degrees C) to permit characterization and quantification of the structures and features that arise during the course of these transformations. The apparent dewetting of these ultrathin films upon exposure to aqueous environments creates future opportunities to investigate and characterize processes of mass transport in this class of ionically cross-linked assemblies.     

The relation between the microphase separation transition and the glass transition in diblock copolymers

The microphase separation transition (MST) has been studied for short chain diblock copolymers poly(styrene-b-isoprene) and poly(styrene-b-mma). A detailed analysis of small-angle x-ray scattering (SAXS) profiles in the homogeneous phase allows determination of the interaction parameter and the spinodal temperature T_s of the MST. T_s for the PS/PI diblocks is found to be lower than the glass transition temperature of their hard blocks. This results in a coupling of the MST and the glass transition. Using both structural (SAXS) and thermal differential scanning calorimetry (DSC) methods it is shown that an endothermal peak found in the DSC diagrams is related to the combined effect of the MST and the glass transition. © 1992 John Wiley & Sons, Inc.     

Synthesis of all-conjugated diblock copolymers by quasi-living polymerization and observation of their microphase separation

We designed and synthesized the all-conjugated diblock copolymers poly(3-hexylthiophene-block-3-(2-ethylhexyl)thiophene)s (P(3HT-b-3EHT)s) via a modified Grignard metathesis (GRIM), a type of quasi-living polymerization, and studied their microphase-separated structures. The P(3HT-b-3EHT)s synthesized had well-controlled molecular weights and very narrow polydispersity indices (PDIs), which demonstrates the usefulness of GRIM polymerization for the synthesis of semiconducting block copolymers. P(3HT-b-3EHT)s self-organized to form clear microphase-separated patterns upon thermal treatment, as observed by AFM. Interestingly, the enhancement of the interchain interaction of the P3HT segments compared with the P3HT homopolymer was clearly observed from the UV-vis spectra, despite the fact that the amount of crystalline P3HT fraction was reduced to 83% of the total polymer amount in P(3HT-b-3EHT). It is suggested that the relatively unconstrained, amorphous segments of P3EHT can enhance the crystallization of P3HT segments to form an ordered self-organized nanostructure.     

Instability and dewetting of ultrathin solid viscoelastic films on homogeneous and heterogeneous substrates

Instability and dewetting engendered by the van der Waals force in soft thin (<100 nm) linear viscoelastic solid (e.g., elastomeric gel) films on uniform and patterned surfaces are explored. Linear stability analysis shows that, although the elasticity of the film controls the onset of instability and the corresponding critical wavelength, the dominant length-scale remains invariant with the elastic modulus of the film. The unstable modes are found to be long-wave, for which a nonlinear long-wave analysis and simulations are performed to uncover the dynamics and morphology of dewetting. The stored elastic energy slows down the temporal growth of instability significantly. The simulations also show that a thermodynamically stable film with zero-frequency elasticity can be made unstable in the presence of physico-chemical defects on the substrate and can follow an entirely different pathway with far fewer holes as compared to the viscous films. Further, the elastic restoring force can retard the growth of a depression adjacent to the hole-rim and thus suppress the formation of satellite holes bordering the primary holes. These findings are in contrast to the dewetting of viscoelastic liquid films where nonzero frequency elasticity accelerates the film rupture and promotes the secondary instabilities. Thus, the zero-frequency elasticity can play a major role in imposing a better-defined long-range order to the dewetted structures by arresting the secondary instabilities.     

A criterion for dewetting initiation from surface disturbances on ultrathin polymer films

Nanoindentation-induced defects on ultrathin (h = 17 nm) polystyrene (PS) films that are spin cast on silicon (Si) substrates, with residual depths of penetration lower than the film thickness (<17 nm), can either grow to initiate dewetting or level, which results in a flat polymer surface, upon heating above the glass-transition temperature (T(g)). The excess surface energy (DeltaF(gamma)) of the system, which is added to the initially flat coating with the formation of an indent, provides a critical value, DeltaF(gamma,crit) = 6.1 x 10(-16) J, which determines indent evolution upon annealing. An indent grows when DeltaF(gamma) > DeltaF(gamma,crit) and levels when DeltaF(gamma) < DeltaF(gamma,crit). This conclusion is in agreement with previous reports, which used DeltaF(gamma) to distinguish the two (dewetting/leveling) opposing processes (1) in the case of indents deeper than the film thickness and (2) in the case of built-in ordered surface disturbances by capillary force lithography.     

Effects of interaction range and compressibility on the microphase separation of diblock copolymers: Mean-field analysis

Using the random-phase approximation and self-consistent field calculations, we have investigated the effects of finite interaction range and compressibility on the order-disorder transition (ODT) and the lamellar structure of symmetric diblock copolymers. While the compressibility does not affect the ODT, both the values of chiN and bulk lamellar period at the ODT increase with increasing interaction range. On the other hand, both the free-energy density and bulk period of the lamellae increase with either increasing interaction range or decreasing compressibility. Even with a finite compressibility, the mean-field ODT is still a second-order phase transition. The scaling exponent of bulk lamellar period with chiN, however, decreases with increasing compressibility. Our mean-field analysis provides a well understood reference for the study of fluctuation effects in diblock copolymers with finite interaction range and compressibility.     

Dissipative particle dynamics simulation of microphase separation behaviors in graft-diblock copolymer films

Dependence of the microphase separation behaviors of graft-diblock copolymers (A _x )g(B _y ) in thin films on composition fraction, thickness of film and A–B repulsing strength is investigated preliminarily by dissipative particle dynamics. Several kinds of ordered mesostructures have been observed and the simulated phase diagrams show evident asymmetries, besides, the center of lamellas region shifts away from f _A = 0.5. Some of the mesostructures in the film can correspond to those in bulk. Decreasing the thickness of film as well as strengthening the A–B repulsion help the mesostructures enhance the degree of order.     

Dispersion and phase separation of carbon nanotubes in ultrathin polymer films

The inner structure and nanoscale distribution of the stiffness was studied for polymer-single-wall carbon nanotube composites. Dispersion of nanotubes in a polystyrene and polyurethane polymer matrix was achieved by a proper choice of the organic solvent (NMP) and sonification of polymer/SWNT solutions. Ultrathin nanocomposite films were prepared through a dip-coating procedure and possessed a noticeable degree of nanotube orientation in the direction of the applied shear force. Peculiarities of the phase separation in the films were studied by atomic force microscopy (with application of force modulation mode to map the nanotube distribution within the polymer matrix) and Raman spectroscopy.     

Microphase separation transition and ordering kinetics in thin films of diblock copolymers

Using x-ray reflectivity measurements, we have investigated the structure of films of a symmetric diblock copolymer of polystyrene-b-polyisoprene (M _w =15700). The film thickness is in the range of 1 m. In equilibrium the films consist of lamellae with a thickness of 15.3 nm. They are nearly completely oriented parallel to the substrate. The evolution of oriented structure is studied by time-dependent experiments. The time constants of the structure formation depend strongly on the annealing temperature. An enhancement of the diffuse intensity in the range of Yoneda scattering is evidence for an additional surface structure.     

Phase behavior and dewetting for polymer blend films studied by in situ AFM and XPS: from thin to ultrathin films

In this work, the film thickness (l0) effect on the phase and dewetting behaviors of the blend film of poly(methyl methacrylate)/poly(styrene-ran-acrylonitrile) (PMMA/SAN) has been studied by in situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The thinner film shows the more compatibility of the blend, and the phase separation of the film occurs at l0>5Rg (radius of gyration). An initially time-independent q*, the characteristic wavenumber of the phase image, which is in good agreement of Cahn's linearized theory for the early stage of spinodal decomposition, has been obtained in real space and discussed in detail. For 5Rg>l0>3Rg, a "pseudo-dewetting/(phase separation+wetting)" behavior occurs, where the pseudo-wetting is driven by the concentration fluctuation mechanism. For l0<3Rg, a "real dewetting/(phase separation+wetting)" behavior occurs.     

Nanostructured ultrathin films obtained by the spreading of diblock copolymers on a surface

We propose a theory of nanopattern formation in ultrathin films obtained by the spreading of A-B diblock copolymers on a surface. A blocks are strongly adsorbed (spread) on the substrate, and the strength of the adsorption can be varied. B blocks are incompatible with the substrate, but they can spread atop the layer of the A blocks. We predict disk-shaped micelles ordered with hexagonal symmetry and parallel stripe-shaped micelles and bilayers to be stable. The type of resulting structure and its geometrical parameters depend on the composition of the copolymer and interaction parameters. We interrelate these results with those obtained for the case of the spreading of both blocks on the substrate, and we construct a unified phase diagram.     

Morphology and microphase separation of star copolymers

Star copolymers have attracted significant interest due to their different characteristics compared with diblock copolymers, including higher critical micelle concentration, lower viscosity, unique spatial shape, or morphologies. Development of synthetic skills such as anionic polymerization and controlled radical polymerization have made it possible to make diverse architectures of polymers. Depending on the molecular architecture of the copolymer, numerous morphologies are possible, for instance, Archimedean tiling patterns and cylindrical microdomains at symmetric volume fraction for miktoarm star copolymers as well as asymmetric lamellar microdomains for star‐shaped copolymers, which have not been reported for linear block copolymers. In this review, we focus on morphologies and microphase separations of miktoarm (A_mB_n and ABC miktoarm) star copolymers and star‐shaped [(A‐b‐B)_n] copolymers with nonlinear architecture. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 1–21     

Simulations of microphase separation in crosslinked polymer blends

We investigate using bond‐fluctuation simulations the behavior of crosslinked symmetric binary A‐B polymer blends. The variation of the A‐B‐interaction parameter leads to microphase separation. We study it by focussing on the structure factor which we determine for different crosslink densities and for different values of the A‐B‐interaction parameter. The structure factor peaks at smaller values of the scattering vector q than predicted by de Gennes. This finding is in line with recent experiments, which, however, could not clarify its origin. We relate the finding to the topological disorder inherent in the network structure, which allows for large deformations during the separation process.     

Far-infrared studies of microphase separation in sulfonated ionomers

Far-infrared spectra of a series of un-neutralized and neutralized lightly sulfonated polystyrenes with varying sulfonation levels have been investigated to seek spectroscopic evidence for microphase separation known to control the physical properties of these polymers. Broad, strong absorbance bands, not found in the spectrum of unmodified polystyrene, are observed in the spectra of the sulfonated analogs. The effects on the far-infrared spectra both of sulfonation level and of the mass and charge of the neutralizing cation are discussed in terms of cation motion and the formation of ion-rich domains.     

Vertical phase separation and liquid-liquid dewetting of thin PS/PCL blend films during spin coating

Thin films of an amorphous polymer, polystyrene (PS), and a crystalline polymer, poly(ε-caprolactone) (PCL), blend were prepared by spin coating a toluene solution. Surface chemical compositions of the blend films were measured by X-ray photoelectron spectroscopy (XPS), and the surface and interface topographical changes were followed by atomic force microscopy (AFM). By changing the PS concentration and keeping the PCL concentration of the solution at 1 wt %, a great variety of morphologies were constructed. The results show that the morphology of the blend films can be divided into three regions with increasing PS concentration. In region I, PS island domains are embedded in PCL crystals when the PS concentration is lower than 0.3 wt % and the size of the PS island increases with increasing PS concentration. In region II, holes with different sizes surrounded by a low rim are obtained when the concentration of PS is between 0.35 and 0.5 wt %. After selectively washing the PS domains, we studied the interface morphology of PS/PCL and found that the upper PS-rich layer extended into the bottom PCL layer, forming a trench surrounding the holes. In region III, an enriched two-layer structure with the PS-rich layer on top of the blend films and the PCL-rich crystal layer underneath is obtained when the concentration of PS is higher than 0.5 wt %. Last, the formation mechanism of the different surface and interface morphologies is further discussed in terms of the vertical phase separation to a layered structure, followed by liquid-liquid dewetting and crystallization processes during spin coating.