Crystallization, melting, and morphology of a poly(ethylene oxide) diblock copolymer containing a tablet-like block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene}

A poly(ethylene oxide) diblock copolymer containing a short block of poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PEO-b-PMPCS) has been successfully synthesized via atom transfer radical polymerization (ATRP) method. The number average molecular weights (M_n) of the PEO and PMPCS blocks are 5300 and 2100 g/mol, respectively. Combining the techniques of differential scanning calorimetry (DSC), optical microscopy (OM), wide angle X-ray diffraction (WAXD), and small angle X-ray scattering (SAXS), we have found that the PMPCS blocks, which are tablet-like, can significantly affect the crystallization and melting of the diblock copolymer. The sample studied can form the crystals with a monoclinic crystal structure identical to that of the homo-PEO. The melting temperature (T_m) of the diblock copolymer increases monotonically with crystallization temperature (T_c), which is remarkably similar to the behavior of long period. On the basis of Gibbs-Thomson relationship, the equilibrium T_m of the diblock copolymer is estimated to be 65.4 °C. In a wide undercooling (ΔT) range (14 °C<ΔT<30 °C), the isothermal crystallization leads to square-shaped crystals. The PEO-b-PMPCS crystallization exhibits a regime I→II transition at ΔT of 19 °C. The PEO blocks are non-integral folded (NIF) in the crystals, and the PMPCS blocks rejected to lamellar fold surfaces prevent the NIF PEO crystals from transforming to integral folded (IF) ones. Furthermore, the PMPCS tablets may adjust their neighboring positions up or down with respect to the lamellar surface normal, forming more than one PMPCS layer to accompany the increase in the PEO fold length with increasing T_c.     

Ordering of block copolymer/nanoparticle composite thin films

The ordering behavior of polymer nanocomposites composed of gold nanoparticles confined in the polystyrene (PS) domains of PS based block copolymers was investigated. The results reveal that the self‐assembly of nanoparticles in the PS domains improved the ordering of microdomains. This is attributed to the presence of nanoparticles that reduced the degree of segregation of the system, causing slow phase separation. This facilitates the packing of the cylindrical microdomains, leading to a well‐ordered structure of the composite. When particles were incorporated into the major domains of cylindrically ordered block copolymer, the connectivity of the domains allowed particles to move to the top of the film to gain additional entropy of the system. In contrast, when particles were organized in parallel cylinders in the block copolymer, they were confined in the cylinders which prevented them from diffusing in the depth direction. The aggregation of nanoparticles was amplified when the composite was annealed in air. We believe that the results from this study will enable more understanding of the effects of nanoparticles on the ordering of block copolymer/nanoparticle composite thin films and will provide a tool in the fabrication of composite thin films. Copyright © 2012 Society of Chemical Industry     

Silicon and carbon substrates induced arrangement changes in poly(styrene-b-isoprene-b-styrene) block copolymer thin films

Poly(styrene-b-isoprene-b-styrene) (SIS) block copolymer ordering in thin films was studied using two selective substrates as carbon and silicon. Atomic force microscopy (AFM) and contact angle measurements were employed to examine the affinities between domains and surrounding interfaces. The surface morphology was examined by AFM using different amplitude ratios. Results showed polyisoprene (PI) domain layer formation in the outermost film layer. On the other hand, the layer close to substrate adopted different arrangements on silicon and carbon substrates. Topographical and phase images revealed that in both substrates with the thickest films, the interactions between substrate and block domains were not enough to induce surface ordering being the morphology independent of employed substrate. However, decreasing film thickness, SIS thin films displayed a variety of arrangements such as perforated lamellae and cylindrical morphologies. Depending on substrate, these morphologies were achieved in different film thicknesses. Finally, the thinnest film did not adjust to characteristic domain spacing commensurability and terraces formation was observed. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Study of the time evolution of the surface morphology of thin asymmetric diblock copolymer films under solvent vapor

The time development of the surface morphology of asymmetric polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) thin films ‘annealing’ in methanol vapor, a selective solvent for minority P4VP block, was investigated by atomic force microscopy(AFM). For PS-b-P4VP with cylindrical structure in bulk, as annealing time progressed, the surface morphology underwent structural transitions from featureless topography to hybrid morphology of cylindrical and spherical pits, to cylinders, to nanoscale depressions, back to cylinders again. The different film thickness made the number of the transitions observed, at any given annealing time, different. The thicker the film is the more transitions at a given annealing time can be observed. If the film was not thick enough, depressions appeared. For PS-b-P4VP with spherical structure in bulk, it displayed nanoscale depressions with the annealing time increasing. A possible mechanism of the transition of morphologies during solvent annealing was proposed.     

Theoretical study on morphology of ABCD 4-miktoarm star block copolymer

A real-space implementation of the self-consistent field theory (SCFT) has been used to study the morphologies of ABCD 4-miktoarm star block copolymers. For the sake of numerical tractability, the morphologies and the phase diagrams of ABCD 4-miktoarm star block copolymers are investigated in two dimensions (2D) by varying the volume fractions of the blocks and the interaction parameters. Many interesting and complex morphologies occur and compared with ABCD linear block copolymers; ABCD 4-miktoarm star block copolymers have more regular disciplines. We found that systems with similar components have similar morphologies and at the weaker segregation, the minority components always cannot separate from the other blocks and they easily dissolve to form one phase with other block(s), but with the increase of the segregation degree (large ), the ordered phases can be well separated. With the help of our computational prediction, experimental researchers can work more purposefully and efficiently.     

Morphology change of asymmetric diblock copolymer micellar films during solvent annealing

The morphology change of an asymmetric polystyrene-block-poly(2-vinyl pyridine) (PS-b-PVP) diblock copolymer micellar film was investigated during solvent vapor annealing in chloroform. Initially, smaller islands in nanometer-length scale form at the film surface. Further annealing results in the growth of the islands composed of the PS-b-PVP cylinders above the bottom brush layer. For comparison, a film of the block copolymer prepared from THF solution (without micellar structure) was also studied. The surface morphology of the film from THF evolves via spinodal dewetting mechanism during solvent vapor annealing. At a long time solvent vapor annealing, the two kinds of the films display the same surface morphologies, which are determined by the interplay between the surface field and autodewetting.     

Cylindrical phase of diblock copolymers confined in thin films. A real-space self-consistent field theory study

We have used real-space self-consistent field theory to search possible morphology of an asymmetric AB diblock copolymer thin film confined between two homogeneous hard walls. The volume fraction of the A block is fixed to be f=0.3, as expected, a cylindrical phase is stable without confinement (in the bulk). Our simulation reveals that under confinement, in addition to parallel and perpendicular cylinders, other phases, such as flat lamellae, perforated lamellae, undulated cylinders and undulated lamellae, are also stable due to the block-substrate interactions. Three new structures, i.e. undulated lamellae, undulated cylinders and parallel cylinders with non-integer period, are observed to be stable with suitable film thickness and block-substrate interaction. By systematically varying the film thickness and the interaction parameters between the two blocks, phase diagrams are constructed for typical block-substrate interactions. We compare the phase diagrams for weak and strong substrate preference and discuss the effects of confinement and substrate preference on the stability of various structures.     

Mechanisms of ordering in block copolymer sub-monolayer films upon selective solvent annealing

The solvent annealing induced two-dimensional ordering in poly(styrene–ethylene/butylenes–styrene) (SEBS) triblock copolymer sub-monolayer films with a thickness of 16 nm equaling half of the bulk domain thickness l₀ was investigated by time-resolved ex-situ atomic force microscopy (AFM). Cyclohexane, a selective solvent for majority poly(ethylene/butylenes) block was used. The detailed pathway information on the ordering of highly regular hexagonal spheres was obtained by repeatedly taking images of the same marked area on the sample surface after ex-situ annealing treatments. Two different ordering mechanisms were observed under two different solvent annealing conditions: 1) under a well-sealed environment with a slow cyclohexane evaporation rate, poorly-ordered short cylinders first break into spheres, then the evolution of spherical phase takes place to gradually improve the orderliness of spheres, and finally well-ordered hexagonal spheres are formed; 2) under a poorly-sealed environment with a fast cyclohexane evaporation rate, “holes” and “islands” are instantaneously formed on the sample surface first, and then the surface gradually get even annihilating hole and island areas. Within the hole areas, the transition from short cylinders to hexagonal spheres takes place via a not well-defined microphase separation structure, while in the island areas, the transitions between cylinders and spheres take place to improve the orderliness of cylinders, and finally the enough-ordered cylinders transform into hex-spheres.     

Phase behaviors of bidisperse nanoparticle/block copolymer mixtures in dilute solutions

The complex microstructures of bidisperse nanoparticles/diblock copolymer mixtures in dilute solutions have been investigated by a theoretical approach which combines the self-consistent field theory (SCFT) and the density functional theory (DFT). Special attention is payed to the role played by the block ratio and the interaction parameters between each component in the mixture. It is shown that the conformational entropy of the polymer chains, the block ratio of the diblock copolymer, the chemical difference between two kinds of particles and the steric packing effect of the particles play important roles in determining the morphologies of the systems. It is found that with the increase of the block ratio, the mixture undergoes a morphological transition from compound micelles to spherelike micelles. The increase of chemical difference between the two kinds of particles can promote the formation of “a jujube set in a cake”. When the selectivity of the particles is changed, another type of micelle emerges. Specifically, in the case where the particles are nonselective to the A- and B-blocks, ordered structures from the phase separation between the two types of particles emerge inside the micelles formed by the amphiphilic diblock copolymers in solutions.     

A study of the versatile micropatterns of diblock copolymer micelles: the effect of copolymer concentration, substrate, film thickness and micelles morphology

We report the novel use of polystyrene-block-poly(acrylic acid) (PS-b-PAA) diblock copolymer micelles as the nano-building blocks in fabricating orderly aligned three-dimensional micropatterns with high regularity through a one-step evaporation-induced cracking process. Crack patterns of square, rectangular, stripe-like and mesh-like structures in micron scale were obtained. The effect of the concentration of diblock copolymer, the properties of the substrates, the thickness of the drying layer, and the morphology of the micelles on the regularity of the crack patterns was studied. By regulating the above factors, we achieved micropatterns of various structures. We further developed a cheap, fast, and simple method for fabricating micromolded structures using the crack patterns as templates.     

Ordering and microdomain orientation in block copolymer films by thermal deprotection

Ordering and microdomain orientation for the films of symmetric polystyrene-b-poly(tert-butyl methacrylate)s (PS-b-PtBMAs) was investigated by in-situ grazing incidence small-angle X-ray scattering (GISAXS) and the electron microscopy. During thermal deprotection at higher temperature (200 °C), functional tert-butyl ester units in the PtBMA block component are integrated into inter- or intra-molecular anhydride linkages. It was observed that this process causes an increase in the Flory-Huggins interaction parameter (χ) between the two block components for disordered PS-b-PtBMA film, leading to a modulated nonequilibrium structure. Interestingly, for lamella-forming PS-b-PtBMA film, a significant chain stretching in lateral direction during thermal deprotection resulted in a characteristic strain-induced perpendicular orientation in the middle of the film confined between two parallel orientations of lamellar microdomains.     

Nanoporous hard etch masks using silicon-containing block copolymer thin films

Nanoporous hard etch masks with various pore sizes were fabricated using a new type of silicon-containing block copolymers, polystyrene-block-poly(4-(tert-butyldimethylsilyl)oxystyrene) with different molecular weights. Since organic-inorganic block copolymers have a large difference in etch resistance between the organic and inorganic blocks, a hard etch mask of silicon oxide can be directly produced upon oxygen plasma treatment. Orientation and hexagonal arrays of cylindrical nanodomains were manipulated simply by adjusting the relative composition of selective and non-selective solvents in the annealing solvent. When the cylindrical nanostructures aligned perpendicular to the substrate surface were exposed to an oxygen plasma, hexagonally arranged nanopore arrays of silicon oxide with controlled pore sizes were fabricated. These nanoporous hard etch masks can be applied to the nanopatterning processes that require high aspect ratio structures.     

Solvent vapor induced dewetting in diblock copolymer thin films

The dewetting pattern development of thin film of poly(styrene)-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer has been studied after ‘annealing’ in the PMMA block selective solvent vapor. Initially, typical circular dewetted holes are observed. Further annealing, however, results in the formation of fractal-like holes. The heterogeneous stress induced by the residual solvent remaining in the film after spin-coating induces the anisotropy of the polymer mobility during the annealing process, which triggers the formation of the intriguing surface patterns.     

Morphology of semicrystalline oxyethylene/oxybutylene block copolymer thin films on mica

The morphology of as-cast and annealed thin films of four symmetric semicrystalline block copolymers on mica was investigated by tapping mode atomic force microscopy (AFM) and grazing incidence X-ray diffraction (XRD). It is found that the morphology of the thin films is dependent on chain length of oxyethylene/oxybutylene block copolymers. The as-cast thin films of the shorter E_mB_n block copolymers on mica exhibit a multi-layered lamellar structure parallel to the surface, in which the stems of the E crystals in the first half polymer layer contacting mica are parallel to the mica surface and perpendicular to the mica surface in the upper polymer layers. In contrast, the as-cast thin film of longer E₂₂₄B₁₁₄ exhibits a structure with mixed orientations of lamellar microdomains on a half polymer layer parallel to the surface. After annealing, the multi-layered structure on mica is transformed into a half-layered, densely branched structure, which is formed following a diffusion-limited aggregation mechanism, opposed to the featureless half-layered structure on silicon. Upon annealing, the upper polymer layers gradually retreat and the remaining area becomes thicker, but in contrast the first half polymer layer contacting mica becomes thinner due to wetting and the parallel orientation of the E crystal stems. The densely branched structure and the different chain orientations of the E crystal stems in the first half polymer layer contacting mica are attributed to the strong interaction between the E block and mica, as revealed by our previous work. The width of branches was employed to analyze the kinetics of secondary crystallization. It is also found that the width of the branches and the velocity of crystal front decrease as the chain length increases.     

Deformation behaviour of styrene/butadiene star block copolymer/hPS blends: influence of morphology

The influence of morphology on micromechanical deformation behaviour of blends consisting of a lamellar forming styrene/butadiene star block copolymer and polystyrene homopolymer (hPS) was studied by transmission electron microscopy (TEM). The pure star block copolymer and the microphase separated blends revealing lamellar structure with polystyrene (PS) lamella thickness in the range of about 20 nm showed homogeneous plastic deformation of the PS lamellae. The macrophase separated blends with PS particles in lamellar matrix exhibited debonding at the particle–matrix interface associated with extensive plastic deformation of the surrounding matrix. The blends containing PS matrix deformed via crazing.     

Effects of additives on the morphologies of thin titania films from self-assembly of a block copolymer

The effects of additives of poly(methyl methacrylate) (PMMA) and HAuCl₄ on the morphologies of hybrid titania films formed via co-assembly of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymers, titania sol-gel precursor in a selective solvent were investigated. The results show that addition of PMMA or HAuCl₄ has an important influence on the morphologies of hybrid titania films. Addition of PMMA or HAuCl₄ can induce the morphology transition of the PS-b-PEO/titania sol-gel mixture from spherical micelles to vesicles. Therefore, the morphologies of the hybrid films formed on silicon substrate surfaces by spin-coating can be controlled by the addition of homopolymer (PMMA) or inorganic precursor (HAuCl₄) into the PS-b-PEO/titania sol-gel mixtures, allowing access to nanoparticles or nanoporous films. After removing the polymer matrix, nanoparticle aggregates or nanobowl-like structures are left behind on the substrate surfaces.     

TEM characterization of diblock copolymer templated iron oxide nanoparticles: Bulk solution and thin film surface doping approach

The morphology of a novel diblock copolymer, poly(norbornene methanol)-b-poly(norbornene dicarboxylic acid), was investigated before and after metal oxide doping by transmission electron microscopy (TEM) using a novel iodine vapor staining method to image the undoped polymer. A lamellar morphology was observed by TEM after staining the undoped diblock copolymer with iodine vapor. Thin film surface doping resulted in a confinement of the iron oxide nanoparticles within the lamellar domains. Spherical nanoparticle aggregates were observed through a bulk solution doping method. It was observed that the particles were templated by the underlying lamellar structure of the copolymer when the thin film surface doping method was used.     

Novel approach to synthesize morphology variant tungsten oxide thin films for efficient chemical sensing

A new approach has been designed to synthesize and modify the morphology of high quality nanocrystalline tungsten oxide (WO₃) thin films using hot filament chemical vapor deposition (HFCVD) technique. The thin films were successfully synthesized having completely distinct morphologies using different processing conditions in very short duration of time 4–6 min. Two types of morphologies resembling a broccoli decorated with WO₃ nanograins (BWN) and another having faceted, pyramidal growth of WO₃ nanograins (PWN) were obtained. Both as-prepared films displayed excellent uniformity and exhibited the formation of standard monoclinic structure without undergoing any additional calcination/sintering. The morphology variant WO₃ thin films were applied as sensing electrodes to detect toxic chemical such as diethylamine (DEA) chemical. Due to smaller particle size, higher surface area and higher oxygen contents, the BWN thin film offered promising performance in DEA sensing with good sensitivity of ～3.5 μAμM^?1cm^?2 and fast response ～10 s.     

Effect of nanoparticle surface functionality on microdomain orientation in block copolymer thin films under electric field

The electric field induced microdomain orientations has been an interesting research topic. In this article, the effect of nanoparticle surface functionality on microdomain alignments in block copolymer/nanoparticle hybrid thin films was investigated with transmission electron microscopy experiments. The presence of gold nanoparticles influenced the microdomain orientation behaviors of block copolymer/nanoparticle thin films. The possibility for complete alignment normal to the substrate was illustrated by controlling electric field strength, concentration, and surface ligands of nanoparticles. This work provides basic and essential data to understand the properties and behaviors of emerging block copolymer/nanoparticle hybrid thin films.     

Homopolymer influence on micellar solutions of block copolymer

The influence of polystyrene on the formation of poly(styrene-b-ethylene/propylene) micelles in 5-methyl-2-hexanone was investigated. This solvent is selective for the polystyrene blocks. Static and dynamic light scattering and viscosity measurements were carried out to determine whether polystyrene chains in the micellar solutions cause any change in the thermodynamic functions of micellization or in the structural parameters of the micelles. Two homopolystyrenes of different molar mass were used at a concentration of 10⁻²g cm⁻³. Homopolystyrene seems to slightly favour micelle formation, the lower mass polystyrene having a larger effect. The polystyrene chains do not significantly affect the hydrodynamic size and molar mass of the poly(styrene-b-ethylene/propylene) micelles.