Morphology of polystyrene/polystyrene-block-poly(methyl methacrylate)/poly(methyl methacrylate) composite particles

Polystyrene/polystyrene-block-poly(methyl methacrylate)/poly(methyl methacrylate) (PS/PS-b-PMMA/PMMA) composite particles were prepared by releasing toluene from PS/PS-b-PMMA/PMMA/toluene droplets dispersed in a sodium dodecyl sulfate aqueous solution. The morphology of the composite particles was affected by release rate of toluene, the molecular weight of PS-b-PMMA, droplet size, and polymer composition. ‘Onion-like’ multilayered composite particles were prepared from toluene droplets of PS-b-PMMA and of PS/PS-b-PMMA/PMMA, in which the weights of PS and PMMA were the same. The layer thicknesses of the latter multilayered composite particles increased with an increase in the amount of the homopolymers. PS-b-PMMA/PS composite particles had a sea-islands structure, in which PMMA domains were dispersed in a PS matrix. On the other hand, PS-b-PMMA/PMMA composite particles had a cylinder-like structure consisting of a PMMA matrix and PS domains.     

Synthesis and analysis of phase segregation of polystyrene-block-poly(methyl methacrylate) copolymer obtained by Steglich esterification from semitelechelic blocks of polystyrene and poly(methyl methacrylate)

Here, an alternative route to successfully synthesize polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) is reported. Steglich esterification was used as an effective, metal free approach for coupling carboxylic terminated PS and the hydroxyl end-functionalized PMMA chains obtained by nitroxide-mediated polymerization and atom transfer radical polymerization, respectively. α-Functionalization was obtained using 4,4′-azobis(4-cyanovaleric acid) and 2,2,2-tribromoethanol as initiators. The synthesis of PS-b-PMMA was confirmed by gel permeation chromatography and nuclear magnetic resonance (NMR), while the dependence of the diffusion coefficients of the polymers (PS, PMMA, PS/PMMA blend, and PS-b-PMMA) with their corresponding molecular weights was discussed based on the results of atomic force microscopy-based infrared spectroscopy, differential scanning calorimetry, and spectra of diffusion-ordered NMR spectroscopy. Differently from PS-b-PMMA, a partial segregation was observed for the PS/PMMA blend, affecting its thermal behavior and diffusion coefficient. The study here presented provides an easier and efficient strategy for the synthesis of PS-b-PMMA and new insights into the diffusion of polymers.     

Perpendicular lamellae induced at the interface of neutral self-assembled monolayers in thin diblock copolymer films

We obtained perpendicular lamellar orientations in thin films of symmetric polystyrene-block-poly(methyl methacrylate), PS-b-PMMA, on self-assembled monolayers (SAMs) of 3-(p-methoxyphenyl)propyltrichlorosilane (MPTS) prepared on silicon wafers. In contrast to completely parallel lamellae on silicon wafers having a native oxide layer, perpendicular lamellae at the MPTS interface with parallel lamellae at the air interface were directly observed by transmission electron microscopy (TEM) in cross-sectional view. The perpendicular lamellae at the MPTS interface were attributed to the non-preferential (neutral) MPTS-covered substrate to both PS and PMMA blocks. The neutrality of the SAMs of MPTS was confirmed by the similar interfacial tension values of the SAMs of MPTS with PS and PMMA, estimated by contact angle measurements.     

A new approach in the study of tethered diblock copolymer surface morphology and its tethering density dependence

A poly(l-lactic acid)-block-polystyrene-block-poly(methyl methacrylate) (PLLA-b-PS-b-PMMA) triblock copolymer was synthesized with a crystalline PLLA end block. Single crystals of this triblock copolymer grown in dilute solution could generate uniformly tethered diblock copolymer brushes, PS-b-PMMA, on the PLLA single crystal substrate. The diblock copolymer brushes exhibited responsive, characteristic surface structures after solvent treatment depending upon the quality of the solvent in relation to each block. The chemical compositions of these surface structures were detected via the surface enhanced Raman scattering technique. Using atomic force microscopy, the physical morphologies of these surface structures were identified as micelles in cyclohexane and “onion”-like morphologies in 2-methoxyethanol, especially when the PS-b-PMMA tethered chains were at low tethering density.     

Property of diblock copolymer having extremely narrow molecular weight distribution

Molecular weight distribution effect on the morphological behavior of polystyrene-block-polyisoprene (PS-b-PI) diblock copolymers was investigated. PS-b-PI samples were prepared by anionic polymerization and further fractionated by HPLC to obtain the fractions of similar average molecular weight and composition but of narrower distributions in both molecular weight and composition. The strategy is to use reversed-phase LC to fractionate the PI block and normal phase LC to fractionate the PS block with a minimal effect on the other blocks. The interfacial thickness, grain size and the phase transition behavior of the unfractionated and fractionated PS-b-PI were compared by X-ray reflectivity, small angle X-ray scattering, transmission electron microscopy and rheological measurements. The fractionated PS-b-PI with more homogeneous molecular weight and composition exhibits a narrower interface, larger grain size and a sharper morphological transition compared to the unfractionated PS-b-PI.     

Effects of substrate roughness on the orientation of cylindrical domains in thin films of a polystyrene-poly(methylmethacrylate) diblock copolymer studied using atomic force microscopy and cyclic voltammetry

This paper describes the orientation of cylindrical domains in thin films of a polystyrene-poly(methylmethacrylate) diblock copolymer (PS-b-PMMA; 0.3 as the PMMA volume fraction) on gold and oxide-coated Si substrates having different surface roughness. Atomic force microscopy images of PS-b-PMMA films having thickness similar to the domain periodicity permitted us to study the effects of substrate roughness and block affinity on domain orientation. PS-b-PMMA films on gold substrates showed metastable vertical domain orientation that was attained more slowly on rougher substrates. In contrast, the domains were horizontally oriented on oxide-coated Si regardless of surface roughness and the annealing conditions examined. In addition, cyclic voltammetry data for PS-b-PMMA films on gold substrates whose PMMA domains were etched suggested that the metastable vertically oriented domains reached the underlying substrates. These results indicate that PS-b-PMMA films containing vertically oriented cylindrical domains can be obtained by using rough gold substrates upon annealing under controlled conditions.     

Reaction-induced microphase separation in thermosetting blends of epoxy resin with poly(methyl methacrylate)-block-polystyrene block copolymers: Effect of topologies of block copolymers on morphological structures

Polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) block copolymers with linear and tetra-armed star-shaped topological structures were synthesized via sequential atomic transfer radical polymerization (ATRP). With pentaerythritol tetrakis(2-bromoisobutyrate) as the initiator, the star-shaped block copolymers with two sequential structures (i.e., s-PMMA-b-PS and s-PS-b-PMMA) were prepared and the arm lengths and composition of the star-shaped block copolymers were controlled to be comparable with those of the linear PS-b-PMMA (denoted as l-PS-b-PMMA). The block copolymers were incorporated into epoxy resin to access the nanostructures in epoxy thermosets, by knowing that PMMA is miscible with epoxy after and before curing reaction whereas the reaction-induced phase separation occurred in the thermosetting blends of epoxy resin with PS. Considering the difference in miscibility of epoxy with PMMA and/or PS, it is judged that the reaction-induced microphase separation occurred in the systems. The design of these block copolymers allows one to investigate the effect of topological structures of block copolymers on the morphological structures of the thermosets. By means of atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS), the morphology of the thermosets was examined. It is found that the nanostructures were formed in the thermosets containing l-PMMA-b-PS and s-PS-b-PMMA block copolymers. It is noted that the long-range order of the nanostructures in the epoxy thermosets containing l-PMMA-b-PS is obviously higher than that in the system containing s-PS-b-PMMA. However, the macroscopic phase separation occurred in the thermosetting blends of epoxy resin with s-PMMA-b-PS block copolymer.     

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.     

Compatibilisation of PPE/SAN blends by triblock terpolymers: Correlation between block terpolymer composition, morphology and properties

Immiscible blends of poly(2,6-dimethyl-1,4-phenylene ether) (PPE) and poly(styrene-co-acrylonitrile) (SAN) with a weight composition of 60/40 were compatibilised by polystyrene-block-polybutadiene-block-poly(methyl methacrylate) triblock terpolymers (SBM) using a two-stage melt-processing approach. In order to investigate the influence of the SBM composition on the compatibilisation efficiency, the block lengths of the triblock terpolymers were systematically varied. The resulting morphological features of the blend systems as function of SBM composition and processing parameters are correlated with the resulting thermal and thermo-mechanical properties. In the ideal case, SBM should be located at the interface as PS is miscible with PPE while PMMA is miscible with SAN. The elastomeric middle block as an immiscible component should remain at the interface. This particular morphological arrangement is known as the ‘raspberry morphology’. A detailed TEM analysis of the blend morphologies following initial extrusion-compounding revealed a high compatibilisation efficiency of the SBM types with equal lengths of the end blocks and, furthermore, the desired raspberry morphology was achieved. In contrast, high PS contents in comparison to the other blocks led to a pronounced micelle formation in the PPE phase. Further evaluation of the blend structures following injection-moulding indicated that the morphologies remain relatively stable during this second melt-processing step. A detailed thermal analysis of all blend systems supports the interpretation of the observed morphological features. The fundamental correlation between SBM composition and blend morphology established in this study opens the door for the controlled development of interfacial properties of such compatibilised PPE/SAN blends during melt-processing.     

Molecular weight effect of PMMA on its miscibility with PS-b-PEO at the air/water interface

The mixed monolayer behavior of poly(methyl methacrylate) (PMMA) with different molecular weights and polystyrene-block-polyethylene oxide (PS-b-PEO) was investigated from the measurements of surface pressure–area per molecule (π-A) isotherms at three different temperatures (10°C, 25°C, and 40°C). The miscibility and nonideality of the mixed monolayers were examined by calculating the excess surface area as a function of composition. The molecular weight of PMMA was demonstrated to have a major effect on its miscibility with PS-b-PEO. Negative or close to zero deviations from ideality of surface areas were observed for PMMA with the lowest molecular weight (12,000 g/mole). Mostly positive deviations were detected in mixed PMMA/PS-b-PEO monolayers with higher molecular weights of PMMA. Because of probable mushroom-to-brush transition effect, the miscibility between PMMA and PS-b-PEO was found to be the best at 25°C among the experimental temperatures. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Suspension polymerization stabilized by triblock copolymer with CdS nanoparticles

In this paper, a new method to prepare polymer colloid particles stabilized by triblock copolymer with CdS nanoparticles was described. Poly(ethylene glycol-block-styrene-block-2-(dimethylamino) ethyl methacrylate) (PEG-b-PS-b-PDMAEMA) triblock copolymer was synthesized by sequential ATRP method. Micelles with CdS nanoparticles in the corona were prepared by “in situ” reaction of hydrogen sulfide with cadmium ion clusters in the corona of the micelles. The size of the CdS nanoparticles is affected by molar ratio of DMAEMA to cadmium ions and polymer concentration in the solution. When introduced into o/w emulsion the micelles reassemble on the surface of styrene oil droplets. PS colloid particles stabilized by triblock copolymer with CdS nanoparticles were achieved by suspension polymerization. TEM image indicates that CdS nanoparticles locate at the surface of the PS colloid particles.     

Synthesis of poly(ethylene oxide)-block-poly(methyl methacrylate)-block-polystyrene triblock copolymers by two-step atom transfer radical polymerization

The synthesis of ABC triblock copolymer poly(ethylene oxide)-block-poly(methyl methacrylate)-block-polystyrene (PEO-b-PMMA-b-PS) via atom transfer radical polymerization (ATRP) is reported. First, a PEO-Br macroinitiator was synthesized by esterification of PEO with 2-bromoisobutyryl bromide, which was subsequently used in the preparation of halo-terminated poly(ethylene oxide)-block-poly(methyl methacrylate) (PEO-b-PMMA) diblock copolymers under ATRP conditions. Then PEO-b-PMMA-b-PS triblock copolymer was synthesized by ATRP of styrene using PEO-b-PMMA as a macroinitiator. The structures and molecular characteristics of the PEO-b-PMMA-b-PS triblock copolymers were studied by FT-IR, GPC and ¹H NMR.     

Tuned phase behavior of weakly interacting polystyrene-block-poly(n-pentyl methacrylate) by selective solvent

We investigated, via small angle X-ray scattering, depolarized light scattering, rheometry, and transmission electron microscopy, the phase behavior of the mixture of a symmetric polystyrene-block-poly(n-pentyl methacrylate) copolymer (PS-b-PnPMA) showing the closed-loop phase behavior and excellent baroplasticity, and dodecanol, a PnPMA-selective solvent. We found that the addition of a selective solvent is simple, but very effective to obtain various microdomains including hexagonally packed cylinders and gyroids. Also, with increasing temperature, the mixtures showed multiple ordered-to-ordered transitions (OOTs) in addition to upper ordered-to-disordered transition (UODT). The first observation of gyroid microdomains in PS-b-PnPMA is very important, although they have been widely reported in many block copolymers, for instance, PS-block-polyisoprene copolymer (PS-b-PI) and PS-block-poly(d,l-lactide) copolymer (PS-b-PLA). Since the gyroid microdomains of PS-b-PnPMA show excellent baroplasticity, external pressure instead of temperature could easily change the microdomains.     

Dispersion of polymer-grafted magnetic nanoparticles in homopolymers and block copolymers

The dispersion of magnetic nanoparticles (NPs) in homopolymer poly(methyl methacrylate) (PMMA) and block copolymer poly(styrene-b-methyl methacrylate) (PS-b-PMMA) films is investigated by TEM and AFM. The magnetite (Fe₃O₄) NPs are grafted with PMMA brushes with molecular weights from M = 2.7 to 35.7 kg/mol. Whereas a uniform dispersion of NPs with the longest brush is obtained in a PMMA matrix (P = 37 and 77 kg/mol), NPs with shorter brushes are found to aggregate. This behavior is attributed to wet and dry brush theory, respectively. Upon mixing NPs with the shortest brush in PS-b-PMMA, as-cast and annealed films show a uniform dispersion at 1 wt%. However, at 10 wt%, PS-b-PMMA remains disordered upon annealing and the NPs aggregate into 22 nm domains, which is greater than the domain size of the PMMA lamellae, 18 nm. For the longest brush length, the NPs aggregate into domains that are much larger than the lamellae and are encapsulated by PS-b-PMMA which form an onion-ring morphology. Using a multi-component Flory-Huggins theory, the concentrations at which the NPs are expected to phase separate in solution are calculated and found to be in good agreement with experimental observations of aggregation.     

Temperature-dependent interaction parameters of poly(methyl methacrylate)/poly(2-vinyl pyridine) and poly(methyl methacrylate)/poly(4-vinyl pyridine) pairs

Temperature-dependent interaction parameters (α) of poly(methyl methacrylate)/poly(2-vinyl pyridine) (PMMA/P2VP) pair and PMMA/poly(4-vinyl pyridine) (PMMA/P4VP) pair were obtained from the SAXS profiles at various temperatures, and curve fitting to the random phase approximation theory. For this purpose, symmetric P2VP-block-PMMA and P4VP-block-PMMA copolymers were synthesized anionically. The molecular weights of both block copolymers were controlled to exhibit the disordered state over the entire experimental temperatures. We found that the value of α for PMMA/P4VP was larger than PMMA/P2VP, similar to polystyrene (PS)/poly(vinyl pyridine) pairs. However, the difference between in α between PMMA/P2VP and PMMA/P4VP was much smaller than that between PS/P2VP and PS/P4VP. This might be attributed to the hydrophilic PMMA block compared with hydrophobic PS block. Finally, the order-to-disorder transition temperature for symmetric P2VP-block-PMMA copolymers was determined by small angle X-ray scattering and birefringence methods.     

Synthesis and characterization of polystyrene-block-polyisoprene nanofibers with different crosslinking densities

Polystyrene-block-polyisoprene or PS-b-PI with 74 styrene and 230 isoprene units was prepared by anionic polymerization. The diblock formed cylindrical micelles with PI core and PS corona in a PS-selective solvent N,N-dimethylacetamide. Nanofibers with different crosslinking densities were obtained after PI core crosslinking with differing amounts of sulfur monochloride. The nanofibers were characterized by elemental analysis, FTIR, transmission electron microscopy, and light scattering.     

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.     

Vesicle formation of polystyrene-block-poly (ethylene oxide) block copolymers induced by supercritical CO2 treatment

A novel route for a preparation of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) block copolymer vesicles induced by supercritical carbon dioxide (scCO₂) is demonstrated. When PS-b-PEO block copolymer solutions in tetrahydrofuran (THF) are treated with scCO₂ at 70 °C for different times, PS-b-PEO copolymers first assemble into aggregated spheres; then aggregated spheres change into large compound micelles and finally evolve into vesicles. The possible formation mechanism of the vesicles is discussed.     

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.