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     

Atomic force microscopy of dense and asymmetric cellulose-based membranes

The surface structures of dense and integrally skinned cellulose acetate (CA) and cellulose acetate butyrate (CAB) membranes, prepared by phase inversion under different casting conditions, are investigated by tapping mode atomic force microscopy (TM AFM). The results obtained show that: (i) The top and bottom surfaces of the dense CA membrane were quite uniform in comparison with the corresponding faces of asymmetric CA and CAB membranes. Despite the casting conditions the active and support layers of the asymmetric membranes display large differences on the roughness parameters. (ii) The asymmetric membranes prepared with an organic system as a non-solvent pore-former (method IV) display smaller nodule aggregates and lower values of the roughness parameters than the ones prepared using an inorganic system as swelling agent (method I). This is more pronounced for the CA membranes than for the CAB membranes. (iii) In the active layer of asymmetric CA membranes casted at longer evaporation times, the measured values of surface roughness parameters tend to decrease. Also, for these CA membranes, as the evaporation time increases the average size of the depression areas observed on the surface decreases.

The laboratory-made CA and CAB membranes display a wide range of nanofiltration and reverse osmosis permeation characteristics. These characteristics are correlated to surface roughness parameters of the active layers.     

Atomic force microscopy of soil and stream fulvic acids

Atomic force microscopy (AFM) was used to image fulvic acid (FA) deposited from aqueous solution on to the basal-plane surfaces of freshly cleaved muscovite, and allowed to air dry. Two fulvic acid samples were used: a soil fulvic acid (SFA) prepared by NaOH extraction from a muck soil underlying a freshwater fen in the New Jersey Pinelands and the IHSS standard Suwannee River fulvic acid (SRFA). The use of tapping-mode AFM (TMAFM), a relatively new technique which reduces the lateral frictional forces generally associated with contact-mode AFM, allowed excellent images of delicate FA structures to be obtained with minimal sample disturbance. Four main structures were observed on SFA. At low concentrations, sponge-like structures consisting of rings ( 15 nm in diameter) appeared, along with small spheres (10–50 nm). At higher concentrations, aggregates of spheres formed branches and chain-like assemblies. At very high surface coverage, perforated sheets were observed. On some samples, all of these structures were apparent, perhaps owing to concentration gradients on drying. SRFA samples were only imagined at higher concentrations. Spheres, aggregated branches, and perforated sheets were apparent. The results agree with previous work by Stevenson and Schnitzer [Soil Sci., 133(1992) 179], who applied TEM to soil FAs freeze-dried on muscovite. However, the TEM images did not detect the smaller spheres and sponge-like structures observed by AFM at low concentrations. The relevance of imaging dried samples remains questionable; hence, it is hoped in the future to use new in situ TMAFM to image FAs sorbed to surfaces in solution. Although TMAFM provided excellent images, a variety of artifacts and potential problems were encountered, as discussed.     

Structural evolution of cylindrical‐phase diblock copolymer thin films

We have measured the time evolution of the self‐assembly process in perpendicular‐oriented cylindrical‐phase diblock copolymer thin films using statistical analysis of high‐resolution scanning electron microscope (SEM) images. Within minutes of annealing above the polymer glass‐transition temperature, microphase separation between polymer blocks results in formation of uniform nanometer‐scale domains whose relative position is initially largely uncorrelated. On further annealing, the cylindrical polymer domains organize into a two‐dimensional hexagonal lattice whose characteristic grain size increases slowly with time (～t^1/4). © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1970–1975, 2004     

Atomic force microscopy studies on the dewetting of perfluorinated ionomer thin films

The surface structure and dewetting process of thin films of complex perfluorinated ion‐containing polymers have been studied with atomic force microscopy. These polymers, or ionomers, consist of hydrophilic, hydrophobic, and ionic groups, which are noncompatible with one another, and this results in the association of the polymers into supramolecular structures. These types of polymers have a broad range of technological uses, ranging from thin selective coatings to fuel cells in the form of polymer electrolyte membranes. As the technology calls for thinner films, the interfacial structure and dynamics (wetting/dewetting) of the films become critical in controlling the overall behavior of the polymers. The ionomer under consideration forms structured films consisting of bundles of micelles. These ultrathin films do not dewet above the glass‐transition temperatures of the polymers, contrary to what has been observed in thin diblock copolymers. Perturbing the system with a high‐ionic‐strength solution, however, results in a breakup of the primary aggregate and enhances the adhesion of the films and their stability. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 149–158, 2003     

PDMS-MDI-PEG多嵌段共聚物涂层表面微相分离行为研究

采用两步溶液聚合方法合成了一系列聚二甲基硅氧烷(PDMS)-4,4′-二苯基甲烷二异氰酸酯(MDI)-聚乙二醇(PEG)多嵌段共聚物.利用轻敲模式原子力显微镜(AFM)观察了嵌段共聚物的表明形貌,研究了退火、共聚物组成以及PEG分子量和不同的官能团对涂层表面微相分离行为的影响,同时对微相分离行为的形成机理也作了相应的探讨.研究表明,该嵌段共聚物即使在PDMS含量大于50wt%时,涂层表面仍呈现出规整有序的纳米级相分离结构,其中疏水相和亲水相分别由PDMS链段和MDI-PEG组分构成.     

Solvent-induced novel morphologies in diblock copolymer blend thin films

We report the morphology and phase behaviors of blend thin films containing two polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymers with different blending compositions induced by a selective solvent for the PMMA block, which were studied by transmission electron microscopy (TEM). The neat asymmetric PS-b-PMMA diblock copolymers employed in this study, respectively coded as a1 and a2, have similar molecular weights but different volume fractions of PS block (fPS=0.273 and 0.722). Another symmetric PS-b-PMMA diblock copolymer, coded as s, which has a PS block length similar to that of a1, was also used. For the asymmetric a1/a2 blend thin films, circular multilayered structures were formed. For the asymmetric a1/symmetric s blend thin films, inverted phases with PMMA as the dispersed domains were observed, when the weight fraction of s was less than 50%. The origins of the morphology formation in the blend thin films via solvent treatment are discussed. Combined with the theoretical prediction by Birshtein et al. (Polymer 1992, 33, 2750), we interpret the formation of these special microstructures as due to the packing frustration induced by the difference in block lengths and the preferential interactions between the solvent and PMMA block. Results obtained here suggest that diblock copolymer blend thin films treated with a selective solvent offer an alternative and attractive approach to control the self-organization of polymers.     

Microphase separation at the surface of block copolymers, as studied with atomic force microscopy

Atomic force microscopy (AFM) is used to study the phase separation process occurring in block copolymers in the solid state. The simultaneous measurement of the amplitude and the phase of the oscillating cantilever in the tapping mode operation provides the surface topography along with the cartography of the microdomains of different mechanical properties. This technique thus allows to characterize the size and shape of those microdomains and their organization at the surface (e.g. cubic lattice spheres, hexagonal lattice of cylinders, or lamellae). In this study, a series of symmetric triblock copolymers made of a inner elastomeric sequence (poly(butadiene) or poly(alkylacrylate)) and two outer thermoplastic sequences (poly(methylmethacrylate)) is analyzed by AFM in the tapping mode. The microphase separation and their morphology are essential factors for the potential of these materials as a new class of thermoplastic elastomers. Special attention is paid to the control of the surface morphology, as observed by AFM, by the molecular structure of the copolymers (volume ratio of the sequences, molecular weight, length of the alkyl side group) and the experimental conditions used for the sample preparation. The molecular structure of the chains is completely controlled by the synthesis, which relies on the sequential living anionic polymerization of the comonomers. The copolymers are analyzed as solvent-cast films, whose characteristics depend on the solvent used and the annealing conditions. The surface arrangement of the phase-separated elastomeric and thermoplastic microdomains observed on the AFM phase images is discussed on the basis of quantitative information provided by the statistical analysis by Fourier transform and grain size distribution calculations.     

Surface-induced morphologies in thin films of a rod-coil diblock copolymer

A block copolymer containing a rodlike block is studied for its adsorption and formation of nanostructured thin films on the substrate surface. The block copolymer is poly(styrene-b-3-triethoxysilylpropylisocyanate) (PS-b-PIC) of which the PIC chain consists of repeating amide units with triethoxysilyl side groups. As the copolymer chains are adsorbed onto silica surfaces, the PIC blocks pack laterally on the plane in a smectic manner, and the PS chains segregate along the ordered PIC chains, resulting in stripe patterns. The width of the stripes formed on the silica surface appeared to be much larger that on the carbon surface. This was accounted for by the bilayered smectic packing of the rod blocks that is induced by rod-surface attractive interaction. The periodicity of the stripe pattern on the carbon surface indicates that interdigitated packing is preferred by the copolymers on the hydrophobic surface in a manner similar to those in the bulk state of rod-coils. Excess rod-coils on the bilayered smectic layer resulted in a terraced morphology due to large difference in the periodicity between the bilayered smectic layer at the substrate surface and the interdigitated smectic layer in the bulk.     

Achieving structural control with thin polystyrene-b-polydimethylsiloxane block copolymer films: The complex relationship of interface chemistry,annealing methodology and process conditions

The structure of thin microphase-separated polystyrene-block-polydimethylsiloxane (PS–PDMS) films has been studied using state-of-the-art top-down and cross-sectional electron microscopy. This is the first time that the profile of PS–PDMS films has been measured in situ and these measurements allowed us to image the shape of the PDMS domains within the film as well as examine the wetting behavior of the block copolymer film on a variety of substrates. It was found that for each polymer, substrate chemistry and annealing method combination examined, there was a small range of film thicknesses whereby the films exhibited the optimal characteristics of high levels of ordering without dewetting or multilayering. Specifically, the optimum thickness for films treated by thermal annealing was greater than that for the equivalent solvent annealed film; a change that was correlated with morphology variations related to solvent swelling of the solvent annealed films. The surface chemistry also induced changes in the optimum film thickness. Selective surfaces were shown to control whether a PDMS wetting layer was formed or not, leading to either thicker or thinner wetting optimum film thicknesses; while undulating morphologies were observed for less selective surfaces. Concomitant changes in the periodicity were then hypothesized to occur as a result of confinement effects and the selectivity of the surface.     

Simulated annealing study of asymmetric diblock copolymer thin films

We report a simulated annealing study of the morphology of asymmetric diblock copolymer thin films confined between two homogeneous and identical surfaces. We have focused on copolymers that form a gyroidal morphology in the bulk. The morphological dependence of the confined films on the film thickness and the surface-polymer interaction has been systematically investigated. From the simulations it is found that much richer morphologies can form for the gyroid-forming asymmetric diblock copolymer thin films, in contrast to the lamella-forming symmetric and cylinder-forming asymmetric diblock copolymer films. Multiple morphological transitions induced by changing the film thickness and polymer-surface interactions are observed.     

Conductivity of diblock copolymer system filled with conducting nano-particles: Effect of copolymer morphology

We explore the effect of temperature-induced morphological changes in insulating diblock copolymer system (DBC) filled with conductive fillers on the conductivity of this composite. By making use of the developed method that relies on the consistent phase-field model of DBC, Monte-Carlo simulations of the filler distribution in DBC, and resistor network model, we quantitatively relate the morphology of filled DBC and its conductivity. In particular, we demonstrate that the order–disorder transition between the random and ordered microphases of DBC causes the conductor-insulator transition in the network of conductive fillers immersed in this system. The order–order transition between the ordered lamellae and cylindrical microphases of DBC is found to co-occur with a jump in the composite conductivity caused by restructuring of the conductive filler network.     

Getting to the bottom morphology of block copolymer thin films

We demonstrate a general approach for attaining the bottom morphology of block copolymer(BCP) thin films. In our former measurements on PS-b-PMMA films, surface morphology maps of the BCP films revealed distinct ordering regimes where the cylinders orient predominantly perpendicular or parallel to the interface and an ‘intermediate' regime where these morphologies coexist. However, this earlier work did not explore the bottom morphology of BCP thin films. In this study, we investigated the block copolymer morphology near the solid substrate in the cast block copolymer film having a perpendicular cylinder morphology on the surface.     

Surface and bulk ordering in thin films of a symmetrical diblock copolymer

Amphiphilic diblock copolymers have the ability to adapt their surface's molecular composition to the hydrophilicity of their environment. In the case of about equal volume fractions of the two polymer blocks, the bulk of these polymers is known to develop a laminar ordering. We report here our investigation of the relationship between bulk ordering and surface morphology/chemical composition in thin films of such an amphiphilic diblock copolymer. Upon annealing in vacuum, the expected lamella ordering in the bulk of the film is observed and we find the morphology of the film surface to be defined by the thickness of the as‐deposited film: If the as‐deposited thickness matches the height of a lamella stack, then the film exhibits a smooth surface. Otherwise, an incomplete lamella forms at the film surface. We show that the coverage of this incomplete layer can be quantified by X‐ray reflectivity. To establish the lamella ordering in the bulk, the film needs to be annealed above the glass temperature of the two blocks. Molecular segregation at the film surface, however, is already occurring at temperatures well below the glass temperature of the two blocks. This indicates that below the glass temperature of the blocks the bulk of the thin film is “frozen,” whereas the polymer chains composing the surface lamella have an increased mobility. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys., 2013 , 51, 1282–1287     

Mechanism and kinetics of ordering in diblock copolymer thin films on chemically nanopatterned substrates

Lamellae forming diblock copolymer domains can be directed to assemble without defects and in registration with chemically nanopatterned substrates. Initially, thin films of the lamellar poly(styrene-b-methyl methacrylate) block copolymer form hexagonally close-packed styrene domains when annealed on chemical nanopatterned striped surfaces. These styrene domains then coalesce to form linear styrene domains that are not fully registered with the underlying chemical surface pattern. Defects coarsen, until defect-free directed assembly is obtained, by breaking linear styrene domains and reforming new structures until registered lamellae have been formed. At all stages in the process, two factors play an important role in the observed degree of registration of the block copolymer domains as a function of annealing time: the interfacial energy between the blocks of the copolymer and the chemically nanopatterned substrate and the commensurability of the bulk repeat period of the block copolymer and the substrate pattern period. Insight into the time-dependent three-dimensional behavior of the block copolymer structures is gained from single chain in mean field simulations. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3444–3459, 2005     

Atomic force microscopy study of salivary pellicles formed on enamel and glass in vivo

This study was performed to evaluate the use of atomic force microscopy (AFM) in examining the surface of the adsorbed layer of salivary proteins (salivary pellicle) formed in vivo on dental enamel and glass surfaces. Enamel and glass test pieces were attached to the buccal surfaces of the upper first molar teeth in two adults using removable intraoral splints. The splints were carried intraorally over periods ranging from 10 min to1 h. Using the contact mode of AFM, pellicle structures could be recognised on intraorally exposed specimens compared to nonexposed enamel and glass surfaces. The surface of the adsorbed salivary pellicle was characterised by a dense globular appearance. The diameter of the globulelike protein aggregates adsorbed onto enamel and glass varied between 80 and 200 nm and 80 and 150 nm, respectively. The structure of the adsorbed protein layer was clearly visible on glass surfaces, even though minor differences in the protein layer between glass and enamel specimens were observed. This study indicates that AFM is a powerful tool for high-resolution examination of the salivary pellicle surface structure in its native (hydrated) state. AFM avoids artefacts due to fixing, dehydration and sputter-coating which occur with scanning electron microscopic analyses. Received: 29 November 2000 Accepted: 14 December 2000     

Structures and electrical properties of ferroelectric copolymer ultrathin films

Ultrathin films of ferroelectric copolymer vinylidenefluoride and trifluoroethylene, P(VDF-TrFE), were successfully obtained by spin-coating and their nanoscale structures and electrical properties were studied utilizing atomic force microscopy (AFM). We succeeded in obtaining ultrathin copolymer films on graphite whose thickness ranged from 1 nm to several tens of nanometers by controlling concentration of copolymer solutions in methylethylketone. We found that ultrathin films thinner than 4 nm showed layered structures whose layer thickness was about 0.5 nm. On the other hand, films thicker than 4 nm formed typical edge-on lamellar crystal structures. Furthermore, we investigated surface potential distribution and piezoelectric property by AFM-based techniques and discussed interaction between electrical dipoles in the molecular chains and graphite substrate.     

Scanning force microscopy and effective medium theory study of free-standing polyaniline thin films

The technique of scanning force microscopy was used to study the nanometer-scale structure of NMP cast films of polyaniline. Noncontact mode images provide direct evidence that polyaniline prepared in this form is a granular conductor. The films were found to consist of micrograins whose size and density were determined by the pH of the acid solution used to protonate the films. At pH 7, the polyaniline films exhibited a mostly disordered structure, with small 2–10 nm particles visible. Protonation at pH 5 to pH 3 resulted in partial agglomeration of the primary particles into larger bundles, with sizes up to 75 nm. Treatment in solution pHs of 2 or less resulted in films consisting of close-packed bundles of dimension 20–30 nm. The conductivity of the films exhibited a sharp rise beginning with protonation at pH 2 or less. Effective medium theory (EMT), was used to model the macroscopic conductivity of these films based on the SPM measured microscopic film structure. Using the size and size distribution of polymer micrograins or bundles in a modified EMT, we are able to obtain predicted conductivities that are close to the measured values for these films. © 1995 John Wiley & Sons, Inc.     

Atomic force microscopy of extended-chain crystals of polyethylene

Extended-chain crystals of polyethylene grown at elevated pressure and temperature were analyzed for the first time by atomic force microscopy. It was possible to compare the typical fracture surface striation features with those obtained earlier by electron microscopy. High resolution atomic force microscopy on flat surfaces enabled the recording of an atomic scale regularity that could not be fully indentified. © 1993 John Wiley & Sons, Inc.     

Morphologies of microphase-separated conformationally asymmetric diblock copolymers

The equilibrium morphological behavior of a series of conformationally asymmetric linear diblock copolymers is characterized via small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The linear diblock molecules of polyisoprene and poly(t-butylmethacrylate) (PtBMA) are prepared anionically over a range of PtBMA volume fractions 0.17 to 0.85. Solution light-scattering experiments are performed on PtBMA homopolymer at theta conditions, and the results were compared with PI data in the literature in order to characterize the degree of conformational asymmetry between the respective blocks. This conformational asymmetry is quantified by an ε of 0.75. The experimental results are compared with morphological behavior calculated utilizing self-consistent mean field theory for a diblock system with ε = 0.75. At middle to high PtBMA volume fractions, ϕ_PtBMA > 0.30, the experimental morphological behavior agrees well with the calculated behavior; the microphase boundaries are slightly shifted to higher volume fractions of the PtBMA block due to its larger Kuhn length. At ϕ_PtBMA < 0.30, however, discrepancies are found in the volume fraction dependence of experimentally determined morphological behavior and that calculated theoretically. Interestingly, extremely well-ordered cylindrical microstructures were observed for PI cylinder domains embedded in PtBMA matrices; these samples were prepared by solvent casting with no treatment, such as shearing, to enhance long-range order. These well-ordered cylinder structures contrast with PtBMA cylinders in a PI matrix on the opposite side of the phase diagram that have very poor long-range order. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2629–2643, 1997