Microdomain Morphology Research Articles

Self‐assembled structures in d8‐polystyrene‐block‐polyisoprene/polystyrene blends in the weak segregation regime: SAXS and TEM study

AbstractBACKGROUND: This paper reports an investigation of the microphase‐separated morphology and phase behaviour in blends of d‐polystyrene‐block‐polyisoprene with homopolystyrene in the weak segregation regime, using small‐angle X‐ray scattering and transmission electron microscopy, as a function of composition, weight‐average molecular weight and temperature. The chain length ratio parameter rM = MH/MC (where MH and MC are the weight‐average molecular weights of the homopolymer and corresponding block copolymer chain) was selected to encompass all possible types of mutual homopolymer/block copolymer sizes.RESULTS: In the weak segregation regime the polystyrene block chains behave as a ‘wet brush’ for rM < 1 similarly to the intermediate and strong segregation regimes. For rM > 1 a macroscopic phase separation occurs. The domain spacing D increases systematically in the range 0 < rM ≤ 1 with increasing concentration of homopolymer wP and increasing rM regardless of the implemented specific morphology, but the slope of the periodicity D versus wP relation is smaller than in the intermediate and strong segregation regimes.CONCLUSION: The criterion for ‘wet and dry brush’ morphologies has been applied to explain the changes in microdomain morphology during the self‐assembly process. It has been shown that the parameters rM and χ3/2N (where χ is the Flory–Huggins parameter and N the number of segments per chain) characterize the slope of the D versus wP relation in the weak and intermediate segregation regimes. Copyright © 2009 Society of Chemical Industry

Self-Assembly of Cylinder-Forming Block Copolymers on Ultrananocrystalline Diamond (UNCD) Thin Films for Lithographic Applications

AbstractBlock copolymers (BCPs) consist of two or more chemically distinct and incompatible polymer chains (or blocks) covalently bonded. Due to the incompatibility and connectivity constraints between the two blocks, diblock copolymers spontaneously self-assemble into microphase-separated nanoscale domains that exhibit ordered 0, 1, 2 or 3 dimensional morphologies at equilibrium. Commonly observed microdomain morphologies in bulk samples are periodic arrangements of lamellae, cylinders, or spheres. Block copolymer lithography refers to the use of these ordered structures in the form of thin films as templates for patterning through selective etching or deposition. The self-assembly and domain orientation of block copolymers on a given substrate is critical to realize block copolymer lithography as a tool for large throughput nanolithography applications. In this work, we survey the morphology of cylinder-forming block copolymers by atomic force microscopy (AFM). Three kind of block copolymers were studied: a) poly(styrene-block-ferrocenyldimethylsilane), PS-b-PFS b) poly(styrene-block-methylmethacrylate), PS-b-PMMA and c) poly(styrene-block-dimethylsiloxane) PS-b-PDMS. Block copolymers were dissolved in a neutral solvent for both blocks (toluene) in order to obtain solutions of various concentrations (1 and 1.5 wt %). From these solutions, films were prepared by spin casting on ultrananocrystalline diamond (UNCD) thin film substrates. Results indicate that PS-b-PFS exhibits chemical and morphological compatibility to the UNCD surface in terms of wetting and domain control. A systematic comparison of self-assembly of these polymers on silicon nitride substrates demonstrates that UNCD thin films would require pre-treatment to be considered as a substrate for BCP lithography.

Self‐assembled structures in blends of disordered and lamellar block copolymers: SAXS, SANS and TEM study

AbstractBACKGROUND: The phase behaviour of copolymers and their blends is of great interest due to the phase transitions, self‐assembly and formation of ordered structures. Phenomena associated with the microdomain morphology of parent copolymers and phase behaviour in blends of deuterated block copolymers of polystyrene (PS) and poly(methyl methacrylate) (PMMA), i.e. (dPS‐block‐dPMMA)1/(dPS‐block‐PMMA)2, were investigated using small‐angle X‐ray scattering, small‐angle neutron scattering and transmission electron microscopy as a function of molecular weight, concentration of added copolymers and temperature.RESULTS: Binary blends of the diblock copolymers having different molecular weights and different original micromorphology (one copolymer was in a disordered state and the others were of lamellar phase) were prepared by a solution‐cast process. The blends were found to be completely miscible on the molecular level at all compositions, if their molecular weight ratio was smaller than about 5. The domain spacing D of the blends can be scaled with Mn by D ∼ Mn2/3 as predicted by a previously published postulate (originally suggested and proved for blends of lamellar polystyrene‐block‐polyisoprene copolymers).CONCLUSIONS: The criterion for forming a single‐domain morphology (molecularly mixed blend) taking into account the different solubilization of copolymer blocks has been applied to explain the changes in microdomain morphology during the self‐assembling process in two copolymer blends. Evidently the criterion, suggested originally for blends of lamellar polystyrene‐block‐polyisoprene copolymers, can be employed to a much broader range of block copolymer blends. Copyright © 2008 Society of Chemical Industry

Supramolecular structure of crosslinked polyurethane elastomers based on well‐defined prepolymers

AbstractThis article provides both quantitative and qualitative evaluations of supermolecular structures present in segmented polyurethane elastomers. This work presents our recent results from investigation of the microdomain morphology of poly‐(ether‐urethanes) based on polyethylene glycol and poly‐(ester‐urethanes) based on poly‐ε‐caprolactone diol with different molecular weight as soft segments and hard segments based on the combination of TDI and triethanolamine or 4,4′,4″‐triphenylmethane triisocyanate (Desmodur RE). Those PU elastomers were obtained not only in a typical one‐step method but also in the prepolymer method with the use of urethane oligomers with controlled molecular weight distribution (MWD). Prepolymers with well‐defined chain structures and narrow distributions of their molecular weights (MWD) $ \overline {M}_w / \overline {M}_n $ = 1.1–1.6 were produced in a multistage method, i.e., in step‐by‐step polyaddition of 2,4‐ and 2,6‐TDI with polyoxyethylenediols or polycaprolactonediols of different molecular weights. Isocyanate oligomers obtained at individual stages were then crosslinked with triethanolamine, whereas hydroxyl‐terminated oligomers were crosslinked with Desmodur RE. Microdomain morphology of the soft and hard‐segments structure was characterized by means of wide‐angle X‐ray scattering and scanning electron microscope. The phase morphology and microdomain crystallinity were investigated. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Lithium Ion Induced Nanophase Ordering and Ion Mobility in Ionic Block Copolymers

Diblock copolymers of poly(styrene-b-methacrylic acid) (PS−PMAA) of various compositions have been synthesized by anionic polymerization and were employed as templates for the development of “single-ion” polymer electrolyte systems. Lithium ions were introduced as the effective component that alters the phase state and contributes to ionic conductivity of these block polyelectrolytes. The local structure, microdomain morphology, and ion dynamics of the pure block copolymers and of the block polyelectrolytes were investigated by infrared spectroscopy, small-angle X-ray scattering, differential scanning calorimetry, and dielectric spectroscopy. Investigation of the microdomain structure revealed complexation of lithium ions with the carboxylate units of the PMAA phase in two types of coordination (chelating vs bringing) that alters the phase state of the copolymers and results in a change from the weak- to strong-segregation limit. The investigation of dynamics revealed a new dielectrically active process associated with a local relaxation of lithium ions coupled to the MAA segments via the two types of coordination. The ionic conductivity of the block polyelectrolytes was found enhanced by about 3 orders of magnitude relative to that of the parent copolymers. These results contribute to a better understanding of the changes in microdomain morphology as well as of the ion dynamics in systems resulting from the association of lithium ions with the charged PMAA block.

Ozonolysis efficiency of PS‐b‐PI block copolymers for forming nanoporous polystyrene

AbstractA PS‐b‐PI diblock copolymer has been synthesized and blended with a homopolystyrene of a variable molecular weight at a variable weight ratio, resulting in different volume fractions of PS to PI and various morphologies. After being cast as a film, the double bonds in the PI microdomains were cleaved via ozonolysis resulting in the formation of a nanoporous PS. The ease of ozonolysis depended strongly upon the morphology of the PI microdomains. The degree of ozonization after 24‐h reaction was about 90% for lamellar microdomains, about 80% for bicontinuous microdomains, about 70% for cylindrical microdomains, and about 50% for spherical microdomains. These variations were attributed to the decrease in the contact area of PI microdomains with ozone and the total volume of PI microdomains accessible to ozone. All PS‐b‐PI/homopolystyrene samples have turned into nanoporous materials after the removal of PI nanodomains. SEM, AFM, and TEM images indicated that the resulting PS had a pore size of 20–30 nm, and thus was potentially useful for photonic crystals or fuel cell applications. © 2008 Wiley Periodicals, Inc. JPolym Sci Part A: Polym Chem 46: 1964–1973, 2008

Crystallization behavior of dry‐brush PEO‐PS block copolymer and PEO homopolymer blend

AbstractThe crystallization behavior of semicrystalline PEO homopolymer/triblock PS‐PEO‐PS copolymer blend system, which exhibited “Dry‐Brush” in the melt. A symmetric polystyrene–poly(ethylene oxide)–polystyrene triblock copolymer was blended with PEO homopolymer (h‐PEO) having the same molecular weight as that of the PEO block in the copolymer. Considering the composition of the blend (Wps ≥ 0.8), PEO spheres were formed in the blend. Because of the dry‐brush phase behavior of this blend, h‐PEO added was localized in the PEO microdomains, which increases the domain size without changing the microdomain morphology. The crystallization of PEO block was confined within the microdomains and the crystallization temperature was about 60°C lower than normal. Self‐seeding tests were performed to clarify the nucleation mechanism of the blend. Because the droplets size varies greatly, multicrystallization peaks were witnessed in the self‐seeding process. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Microstructural Organization of Three-Phase Polydimethylsiloxane-Based Segmented Polyurethanes

Microphase separation is investigated for segmented polyurethane copolymers synthesized from 4,4‘-methylenediphenyl diisocyanate and 1,4-butanediol as the hard segments and poly(hexamethylene oxide diol) and hydroxyl-terminated polydimethylsiloxane as soft segments. The neat PDMS-based diol exhibits two segmental relaxations corresponding to the principal repeating unit, Si(CH3)2O, and to the hydroxyl end-group segments. When incorporated in the polyurethanes, the siloxane units form their own phase, while the end-group segments are mixed with the second macrodiol and some short hard segment sequences. This is demonstrated by the results of dynamic mechanical analysis and wide-angle X-ray diffraction experiments. The microdomain morphology was characterized principally by small-angle X-ray scattering, and the scattering data were analyzed with a pseudo-two-phase model as well as a modified three-phase core−shell model.

Self‐Assembled Structures in Block Copolymer Blends: SAXS, SANS and TEM Study

AbstractPhenomena associated with the order‐disorder transition, microdomain morphology and phase behavior of a deuterated block copolymer (BCP) blend I/II (where I is dPS‐block‐dPMMA and II is dPS‐block‐PI) were studied by SAXS, SANS and TEM. The studied, almost symmetric, copolymers differ essentially in microdomain morphology. One of them (I) is in disordered microdomain state, while the other (II) displays lamellar morphology at ordinary temperatures. Self‐assembled structures in blends were investigated as a function of concentration of the added microphase‐separated copolymer and temperature. The ODT positions were located in all the blends, the position of ODT depending only slightly on the concentration of the ordered copolymer. A systematic increase in long period D of the lamellar phase is observed with the growing content of the disordered copolymer. The evaluation of TEM shows the gradual diminishing of macrophase separated regions of disordered copolymer I with growing content of the lamellar copolymer II.magnified image

Relationship between composition and properties for stable chitosan films containing lipid microdomains

AbstractBiocompatible, biodegradable films composed of a hybrid blend of chitosan and egg phosphatidylcholine (ePC) were characterized in terms of composition, morphology, and performance‐related properties. The miscibility between chitosan and ePC for blends of 1 : 0.2 to 1 : 2.5 chitosan : ePC (wt/wt) was examined by differential scanning calorimetry and X‐ray diffraction analysis. The partial miscibility exhibited between chitosan and ePC provided an understanding of the microdomain morphology that was visualized by laser scanning confocal fluorescence microscopy of the films. The stability of the films in physiologically relevant media was assessed by percent weight loss over time. The mechanical properties of the chitosan–ePC films were determined by dynamic mechanical analysis and tensile tests. Interestingly, the dry film composed of a high lipid formulation (1 : 2.5 (wt/wt) chitosan: ePC) had the lowest tensile strength, contained lipid microdomains (10–30 μm in size), and provided the highest degree of stability. Following immersion in phosphate buffer solution, the Young's modulus of the film was found to decrease by more than two orders of magnitude and could be further manipulated by decreasing the lipid content within the film. In this way, relationships between the composition and the physical as well as mechanical properties of the chitosan–ePC blends were established. Furthermore, this study demonstrates the potential usefulness of partially miscible chitosan‐based blends for biomedical purposes. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 3453–3460, 2007

Kinetics of the thermally induced precursor curing to polymer via statistical analysis of TEM images

Thermally induced curing of the precursor to the cross-linked polymer was studied with transmission electron microscopy. The cured polymer was observed to have micro-domain morphology. Analysis of the microscopic images resulted in the statistical size distributions of micro-domains described subsequently using principles of irreversible thermodynamics. The statistical distribution parameters were determined to change with the curing temperature. A correlation of micro-structural and macroscopic parameters (the mean size of micro-domains and the dynamic Young’s modulus), both varying during the precursor curing, was found and explained in terms of the kinetic concept of fracture.

Self-Assembled Morphologies of a Diblock Copolymer Melt Confined in a Cylindrical Nanopore

We systematically investigate the microdomain morphologies that self-assemble in a diblock copolymer melt confined to a narrow cylindrical nanopore using real-space self-consistent mean-field theory. We observe, for a bulk cylinder-forming copolymer, that a variety of novel microdomain structures emerge under confinement. As the pore diameter increases, the following sequence of stable structures occurs: single cylinder, stacked disks, single helix, double helix, toroid−sphere, and helix−cylinder. We accurately locate the pore diameters where first-order phase transitions between these morphologies occur. We vary the interaction of the diblock copolymer with the pore wall from being preferential for the minority block to neutral to being preferential for the majority block and find that the sequence of structures is unchanged, for these interactions. The pitch angle of the helices and the pore diameter can be related using a simple geometrical argument. Over the range of pore diameters we examine, phases...

Self‐Assembled Structures in Blends of Block Copolymer A‐block‐B with Homopolymer A: SAXS and SANS Study

AbstractSummary: The study describes the investigation of the microphase‐separated morphology of a block copolymer A‐block‐B with the parent homopolymer A, where A is polystyrene and A‐block‐B is poly(perdeuterated styrene)‐block‐poly(methyl methacrylate) (dPS‐block‐PMMA). The microdomain morphology and phase behavior in blends of dPS‐block‐PMMA with polystyrene of $\overline M _{\rm w}$ = 8 000 and 35 000 were investigated. Binary blends of the diblock copolymer and homopolymers were prepared with various amounts of homopolymers. The criterion for “wet and dry brush” taking into account different solubilization of homopolymer has been applied to explain the changes in microdomain morphology during the self‐assembling process. magnified image

Fabrication of microdomains at the +Z surface of near-stoichiometric lithium tantalate crystals

Scanning force microscopy is used to investigate microscale ferroelectric domains at the +Z surface of near-stoichiometric lithium tantalate (SLT) single crystals. Both line and lattice patterns of microdomains are fabricated at the +Z surface of SLT wafers, which are switched by scanning these wafers with the external field exceeding the coercive field. The +Z polar surface in these patterns is etched by a mixture of acid aqueous solution (HF : HNO3 = 1 : 1), which reveals the precise microdomain structure. The analysis of the experimental data indicates that the rearrangement of the ferroelectric domain structure occurs during the chemical etching process; the effect of both defect and crystal structure on the microdomain growth process is also discussed. The present results are helpful for us to realize the fabrication of the expected microdomain morphology.

Monte Carlo simulation of AB diblock copolymer between surface and substrate and comparison of experimental results

The morphologies of AB diblock copolymer film between the substrate and surface were investigated via Monte Carlo simulations on simple cubic lattices. The morphological dependence of the diblock copolymer thin film on the thickness, as well as the composition and interactive intensity has been mainly studied. With the increase of A-segments fraction, various microdomain morphologies including regular parallel stripe-like, mesh-like, and normal lamella near the region of the surface were generated in this work. The morphology of thin films of asymmetric diblock copolymer was found to form cylinders in a bulk system when Lz was equal to 30. The morphologies of PS-b-PDMS diblock copolymer films have been studied via atomic force microscopy (AFM) and transition electron microscopy (TEM) measurements. The surface morphology of the PS-b-PDMS copolymer thin film shows a mesh-like microphase separated structure, and PDMS continuous phase protruded on the PS dispersed phase. The surface composition of PS-b-PDMS copolymer thin films was measured by means of X-ray photoelectron spectroscopy (XPS) and ATR-IR. The comparison results show that the experimental observations are in good agreement with the simulation results. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1835–1845, 2006

The influence of confinement and curvature on the morphology of block copolymers

AbstractIn the bulk, at equilibrium, diblock copolymers microphase separated into nanoscopic morphologies ranging from body‐centered cubic arrays of spheres to hexagonally packed cylinders to alternating lamellae, depending on the volume fraction of the components. However, when the block copolymers are forced into cylindrical pores, where the diameter of the pores are only several repeat periods of the copolymer morphology or less, then commensurability of the copolymer period and the pore diameter can impose a frustration on the microdomain morphology. In addition, due to the small pore diameter, a curvature is forced on the microdomain morphology. In combination with interfacial interactions between the blocks of the copolymer and the pore walls, the preferential segregation of one component to the walls, spatial confinement and forced curvature are shown to induce transitions in the fundamental morphology of the copolymers seen in the bulk. Lamellar morphologies transformed into torus‐type morphologies, cylinders are forced into helices, and body‐centered cubic arrays of spheres are force into helical arrays of spheres due to these restraints. The novel morphologies, not accesssible in the bulk, open a large array of nanoscopic structures that can be used as templates and scaffolds for the fabrication of inorganic nanostructured materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3377–3383, 2005

Blends of a 3-Miktoarm Star Terpolymer (3μ-ISD) of Isoprene (I), Styrene (S), and Dimethylsiloxane (D) with PS and PDMS. Effect on Microdomain Morphology and Grain Size

The morphologies of a 3-miktoarm star terpolymer of isoprene (I), styrene (S), and dimethylsiloxane (D), 3μ-ISD, and its blends with PDMS and PS homopolymers, having a lower molecular weight than the corresponding blocks of the star, were studied. Combined EF-TEM and 3D electron tomography clearly revealed that three kinds of cylindrical structures, composed of three different blocks, joined to fill the space and that these cylinders were curved collectively into an arc. The grain size of this star terpolymer was extremely small compared with that of other known linear block copolymers. By blending the neat 3μ-ISD with a small quantity of PDMS homopolymer (5 wt %), the morphology remained the same, but the grain size increased dramatically. It seems that the PDMS homopolymer inserts into the PDMS phase of the star terpolymer and straightens the bent cylinders, thereby increasing long-range order. However, the extra PDMS accumulates in line defects, which if controlled could be beneficial as waveguides in photonic crystals. Blending with PS leads to smaller grain sizes and unstable hexagonal symmetry, which disappears upon the addition of extra PS. Therefore, tetragonal symmetry is inherent in the neat star.

A Thermal and Manufacturable Approach to Stabilized Diblock Copolymer Templates

Stabilization of the morphology in thin diblock copolymer films is critical for the generation of robust templates for subsequent device fabrication. Copolymers containing thermally cross-linkable groups, such as benzocyclobutene (BCB), can be annealed to induce an ordered microdomain morphology and then subsequently heated to cross-link the system, locking in the morphology and eliminating the need for multistep processing. Poly[(styrene-r-BCB)-b-lactic acid] (PSBCB-b-PLA) was synthesized by a combination of living free radical and ring-opening polymerizations. Thin films (∼30 nm) of PSBCB-b-PLA annealed at 170 °C, followed by cross-linking at 200 °C, showed that the ordered and oriented microdomain morphology was preserved. For PSBCB-b-PLA, with PLA volume fractions of 0.24 and 0.36, arrays of cylindrical microdomains of PLA oriented normal to the surface were produced in a cross-linked PS matrix. Washing these films with a weak base degraded and removed the PLA, producing a nanoporous cross-linked template.

Pattern Formation and Dewetting in Thin Films of Liquids Showing Complete Macroscale Wetting: From “Pancakes” to “Swiss Cheese”

Based on the complete 3D numerical solutions of the nonlinear thin film equation, we address the problems of surface instability, dynamics, morphological diversity and evolution in unstable thin films of the liquids that display complete macroscale wetting. The twin constraints of complete macroscale wettability and nanoscale instability produce a variety of microscopic morphological phases approximating sharp crystal surfaces with flat tops resembling a mesa or a micro "pancake" or a slice of Swiss cheese. While the maximum thickness of flat regions is found to be independent of the initial film thickness, the precise lateral morphology of microdomains formed depends on the film thickness. As the film thickness is increased, the initial pathway of evolution changes from the formation of small spherical droplets, to long mesas (parapets) and islands, to circular holes, all of which eventually resolve by ripening into a collection of round pancakes at equilibrium. However, beyond a certain transition thickness, a novel metastable honeycombed morphology, resembling a membrane or a slice of Swiss cheese, is uncovered, which is produced by an abrupt "freezing" of the evolution during hole growth. In contrast, the spinodal dewetting in thin films of partially wettable systems always engenders spherical droplets at equilibrium. The equilibrium dewetted area from simulations, as well as from simple mass balance, is shown to decline linearly with the initial film thickness.

Block Copolymers under Cylindrical Confinement

Microphase-separated block copolymers were introduced as melts into nanoscopic cylindrical pores in alumina membranes via capillary action. The geometric confinement of both lamellar and cylindrical microdomain morphologies of styrene/butadiene block copolymers, PS-b-PBD, was investigated by transmission electron microscopy. Well-developed microphase-separated structures were formed within the resulting nanorods. Polymers that exhibit cylindrical microdomains in the bulk orient with cylindrical microdomains along the nanorod axis due to the preferential segregation of the PBD block to the walls of the pores. The period and packing of the microdomains differ from those observed in the bulk due to an incommensurability between the pore geometry and the natural period and hexagonal packing of the copolymer microdomains. With polymers exhibiting bulk lamellar morphology, confinement forces the formation of concentric cylinders oriented along the nanorod axis. The number of concentric cylinders depends on the ratio of the nanorod diameter to the equilibrium period of the copolymer. Because of the preferential segregation of PBD at the alumina surface, either PBD or PS can form the central core. These results indicate a method by which copolymer microdomains can be manipulated in a simple manner for the fabrication of isolated nanostructures.