PSE@UMASS Thomas Russell Research Group

Russell Group Research Overview

In the bulk, block copolymers microphase separated into nanoscopic morphologies including spheres, cylinders and lamellae depending on the volume fraction of the components. The size of the domains is defined by the molecular weight of the copolymer and the strength of the segmental interactions between the blocks. If a block copolymer thin film is confined between two solid interfaces, the copolymer chains will either stretch or compress. The incommensurability between the copolymer period and the interface separation distances will alter the fundamental repeat period. When the block copolymers are forced into cylindrical nanopores, where the diameter of the nanopores are several repeat periods of the copolymer morphology, both commensurability and imposed curvature influence the morphology. We have used porous alumina membrane to provide the cylindrical confinement. Confinement effects are found to distort the natural packing of the microdomains and cause an apparent deviation of the repeat period from the bulk values. Under severe confinement, where the pore diameters are comparable to the equilibrium period of the copolymer, morphologies different than those observed in the bulk are seen. Stacked torus-type structures, helical cylinders and helical strings of spherical structures were seen for lamellar, cylindrical, and spherical copolymers, respectively.



Porous alumina membranes were also used to generate amorphous carbon nanotubes(CNTs). Polyacrylonitrile(PAN) and polystyrene-block-polyacrylonitrile(PS-b-PAN) were used as carbon precursors. Uniformly aligned amorphous carbon nanotubes were synthesized by the wetting and sequential pyrolysis. The thickness of the carbon nanotube walls can be controlled by either changing the concentration of the precursor solution or by using multiple wetting and pyrolysis steps. When diblock copolymers of PS-b-PAN were used, it was found that, nanopores were created within the nanotube wall after pyrolysis. The CNTs in this study were found to be amorphous from Raman spectroscopy and x-ray diffraction. However, the graphitization level of the CNTs is expected to increase by adjusting processing conditions, in particular the pyrolysis temperature. The CNTs can also be used as a shrouding to confine other materials, such as block copolymers.

When the porous alumina membranes are wetted by polymer melts, two wetting regions (partial wetting and complete wetting) are observed. These regions are similar to the case of a flat solid substrate. In the partial wetting region, polymer melts are drawn into the nanopores via capillary force, forming polymeric nanorods. In the complete wetting region, polymer melts form precursor films and spread on the inner walls of nanopores in alumina membranes, forming polymeric nanotubes. We studied the wetting behavior of polystyrene melts using nanoporous alumina membrane by systematically changing the annealing temperature and polymer molecular weight. A transition from the partial to complete wetting was observed when the annealing temperature was raised above a critical temperature Tw, which is related to the increase of spreading coefficient S with increasing temperature. The difference in wetting rate between two wetting regions was also used to fractionate polystyrene with different molecular weight.

Block copolymers (BCPs) are emerging as promising candidates as scaffolds and templates for the fabrication of nanoscopic structures. Key to the use of block copolymers is the control of the lateral ordering and orientation of the microdomains in BCP thin films. Electric field is proved to be able to orient the microdomains either in the plane of the substrate or perpendicular to it. For in-plane alignment of BCPs, electrodes need to be fabricated by standard optical lithography and electron-beam evaporation of a metal. Initially, the axes of the cylinders are lying parallel to the substrate due to surface interactions. The axes can then be induced to stretch along the electric field lines when film is simultaneously heated above the glass transition temperature of both blocks. Since electric fields with an order 30 kV/cm were necessary to orient the domains, the patterning of an entire wafer would require the use of multiple electrodes and a careful understanding of the electric field formed when applying a voltage between electrodes on top of a thin insulating layer on a conducting substrate. When an external eclectic field is applied to across the film, PMMA cylinders in PS metric can be oriented perpendicular to the substrate and these ordered arrays can be used to grow nanowires either by electrodepositing through the porous template or by using it as a mask for etching or patterning the underlying substrate, as schematically illustrated in Fig. 1. Analysis of scattering patterns indicated that during alignment of the lamellar microdomains, the material goes through an intermediate state with substantially reduced long-range order. After orientation, the sample consists of many small grains with the lamellae being oriented parallel to the electric field and a random orientation in the plane perpendicular to the field.

Furthermore, our most recent results show that lithium ions coordinate with the carbonyl groups in PMMA block to form lithium-PMMA complexes, resulting in a significant increase of the Flory-Huggins segmental interaction parameter ? as well as dielectric contrast between PS and PMMA blocks. The increased segmental-segmental interaction parameter X drives PS-b-PMMA block copolymer from a weak segregation regime into a strong segregation regime. Thus the local order of the microdomains was markedly enhanced both in bulk and in films without applying any external fields. The increased dielectric contrast significantly amplified the driving force to align the BCP microdomains in films under an external electric field and reduced the critical electric field strength required to overcome the preferential interactions of the blocks with the substrate. Therefore, the complete alignment of lamellar microdomains in PS-b-PMMA block copolymer thin films has been achieved by using an electric field (Fig. 2).

Fig.1 Schematic representation of high density nanowire fabrication in a polymer matrix. (A) An asymmetric diblock copolymer annealed above the glass transition temperature of the copolymer between two electrodes under an applied electric field, forming a hexagonal array of cylinders oriented normal to the film surface. (B) After removal of the minor component, a nanoporous film is formed. (C) By electro-deposition, nanowires can be grown in the porous template, forming an array of nanowires in a polymer matrix.

Fig 2. (A) Left: TEM images of PS-b-PMMA and PS-b-PMMA with lithium complexes after annealing at 170 °C for 2 days (no electric field was applied) followed by quenching to room temperature. Right: Domain spacing, D, vs chain length, N, in a double-logarithmic plot. Triangle: data from PS-b-PMMA copolymers; diamond: data from corresponding PS-b-PMMA copolymers with lithium complexes. Full lines: nonlinear least-squares fitting with an R2 of 0.999 for each line in the range N = 535-1102. (B) Dielectric constants of PMMA with different molar ratios of added LiCl to the carbonyl group 0 (black), 1:10 (red), and 1:6 (green) at 170 oC. (C) Cross-sectional TEM image of PS-b-PMMA thin film with almost saturated lithium-PMMA complexes after annealing at 175±5 °C in N2 environment under a dc electric field of ~ 40 V/mm for 24 h.

The fabrication of functional nanostructured materials for sensing, encapsulation and delivery requires practical approaches to self-assembly on multiple length scales and the synthesis of tough yet permeable structures. Ligand-stabilized nanoparticles assembled into three-dimensional constructs at fluid-fluid interfaces driven by the reduction in interfacial energy were investigated. Studies on the dynamics of the nanoparticles and the self-assembled structures formed at the interface suggest a liquid-like behavior and ordering at the interfaces. Cross-linking of the nanoparticle assembly using functional ligands, affords robust membranes that maintain their integrity even when they are removed from the interface. These composite membranes, nanometers in thickness, are elastic yet permeable. Combining other self-assembly processes on different length scale, i.e. "breath figures", with the self-assembly of nanoparticles at the oil-water interfaces lead to the formation of hierarchically structured nanoparticle arrays. The assembly of virus and other biological complexes at fluid interfaces was also investigated where interfacial assembly rendered an easy route to direct and assemble the bioparticles into 2-D and 3-D constructs with hierarchical ordering. These assemblies enable the potential use of the bioparticles as a natural supramolecular building block to obtain materials with well-defined bio-functionalities.



The organization of inorganic nanostructures within self-assembled organic or biological templates is receiving the attention of scientists interested in developing functional hybrid materials. Recent theoretical arguments have suggested that synergistic interactions between self-organizing particles and a self-assembling matrix material can lead to hierarchically ordered structures. Here we show that mixtures of diblock copolymers and either cadmium selenide- or ferritin- based nanoparticles exhibit cooperative, coupled self-assembly on the nanoscale. In thin films, the copolymers assemble into cylindrical domains, which dictate the spatial distribution of the nanoparticles; segregation of the particles to the interfaces mediates interfacial interactions and orients the copolymer domains normal to the surface, even when one of the blocks is strongly attracted to the substrate. Organization of both the polymeric and particulate entities is thus achieved without the use of external fields, opening a simple and general route for fabrication of nanostructured materials with hierarchical order.


In the last few decades, a lot of research has been done on phase separation of polymers. It can be either phase separation of homo-polymer blends or diblock copolymers. The polymers phase separate because of a mismatch in the Flory-Huggins parameter of two polymers. The progress in controlling and understanding phase separation processes has led to bottom-up approach to synthesize new functional materials. This approach has been further advanced and has been applied to phase separation of inorganic nanorods in polymer matrix. The interaction between polymer and nanorods is governed by the nature of ligands attached to nanorods. When nanorods are functionalized with ligands which have unfavorable interaction with polymer and electric field is applied while drop casting, normally oriented nanorods in a polymer matrix are observed. This is shown in the following TEM micrograph. This phase separation of nanorods is termed as "Self-corralling". The process of self corralling can be directed by using chemically patterned substrate where nanorods preferably go on one part of pattern and polymer goes to other. Further, by using nanorods functionalized with photoactive ligands and self corralling them; efficient photovoltaic devices can be foreseen.


Figure. CdSe nanorods covered with alkane ligands "self-corralled" in PMMA matrix.

Multilayer composites, which combine ductile polymers with brittle films, constitute vital components for optical communications, microelectronics and bio-engineering applications. However, crack formation is a critical problem in these materials; thus, designing layered systems that can respond to environmental changes and undergo self-healing is particularly important for a range of technologies. Here, nanoparticles dispersed in a polymer matrix were found to migrate to a crack generated at the interface between the polymer and a glassy layer. Segregation of the nanoparticles to the crack depended on both the enthalpic and entropic interactions between the polymer and nanoparticles. In particular, poly(ethylene oxide)-covered 5.2-nm spherical nanoparticles in a poly(methyl methacrylate) matrix diffused to cracks in the adjoining silicon oxide layer, whereas tri-n-octylphosphine oxide-covered nanoparticles did not. These results point to a simple means of fabricating systems that can self-heal, improving the durability of multilayered systems, or form the basis for auto-responsive materials.


The self-assembly of materials is emerging as a key process in the fabrication of functional nanostructured materials, since it provides a powerful "bottom-up" route to well organized structures on the nanoscopic scale. Block copolymers, one class of self-assembling materials, offer an attractive route to fabricate nanometer-scale structures, since they spontaneously assemble a range of well-defined, well-ordered structures including spheres, cylinders, and lamellae, depending on the volume fractions of the components. In addition, the molecular weight of the copolymer provides control over the size and separation distances of the microdomains, and the specific functionality cane be incorporated into the structure by varying the chemical nature of the copolymer. This versatility makes block copolymers ideal candidates for use as templates and scaffolds for applications ranging from magnetic storage to displays and sensors. However, control of well-organized structures over large scale is still challenging. For applications requiring addressability, long-range lateral order and orientation of elements are key, and therefore self-assembly is no longer sufficient. Rather, a directed self-assembly is required wherein the precise location of each nanoscopic element is defined. Normally this requires external fields, such as electric field, shear, temperature gradient, crystallization, chemically patterned substrate or controlled interfacial interactions. Very recently, we showed that solvent annealing and solvent evaporation at a controlled rate can provide a very simple but robust route to generating almost defect-free microstructures over large areas in block copolymer thin films. Solvent evaporation is a strong, highly directional field. Strong repulsion between the copolymer blocks, combined with the directionality of solvent evaporation, where ordering is initiated at the surface of the film and propagates through the entire film, leads to high degree of long range order with few defects. Studies were performed on a polystyrene-block-poly(ethylene oxide) copolymer (PS-b-PEO), where the minor component is the PEO. After spin coating, cylindrical structures were generated normal to the surface, having poor lateral order. After some thermal annealing, the order improves such that micron size grains can be observed after 96 hours. However, by solvent annealing with controlled evaporation, the ordering of the copolymer was found to be superior to thermal routes. The solvent imparts a high degree of mobility in the microphase-separated copolymer that enables a rapid removal of defects and, a high degree of lateral order. After only 48 hours of solvent annealing in benzen vapor under controlled humidity, the order is greatly improved such that the ordering of the film is nearly perfect over the 2 ? 2 ?m2 area. (Figure 1). [S Kim, M Misner, M Kimura, T Xu, T Russell. Highly Oriented and Ordered Arrays from Block Copolymers Via Solvent Evaporation. Advanced Materials. 16: 226-231 (2004)]

Even with solvent evaporation and solvent annealing techniques, it is difficult to achieve grains sizes that are greater than tens of microns. Generally, there is no preferential macroscopic gain orientation and limited translational order. However, it has been shown that by using a patterned substrate, such as parallel lines etched in an oxide layer, that have features on the order of microns one can achieve improved lateral order. We have been able to use this to provide a preferential macroscopic gain orientation where the gain orientation is defined on large length scales. A solution of PS-PEO was spin coated onto a silicon oxide surface containing 875 nm wide, 325 nm deep channels. Same solvent annealing strategy was applied. As can be seen in Figure 2, essentially a defect-free array of cylindrical domains oriented normal to the surface is obtained within each of the channel. This order propagates along the length of the channels over hundreds of micrometers.

Recently we uncovered a unique mean of inducing a robust orientation of microdomains in PS-b-PEO by the use of a wide variety of salts. In particular, by incorporating dilute concentrations (1%) of M+X- , where M can be K, Li or Cs and X can be Cl or I, into the solution used to prepare the thin PS-b-PEO films, a pronounced orientation of the cylindrical PEO microdomains with lateral long range order after solvent annealing was observed. Unlike the preparation of the pure PS-b-PEO films which is extremely sensitive to changes in the humidity, with the addition of the salt, humidity has very little, if any, influence on the orientation of the microdomains. More recently, we show that by placing a cleavable juncture, a triphenylmethyl (trityl) ether linkage, between PS and PEO, highly ordered nanoporous thin films could be prepared directly from PS-b-PEO diblock copolymers. Combining the above two strategies, a cleavable PS-PEO complexed with salt was prepared and used for generating thin films with lateral long range order on a topographically patterned surface. As seen in Figure 3, a highly ordered nanoporous thin film can be readily generated.


Figure 1: SFM phase image of PE-PEO thin films after annealing for 48h in a benzen vapor under controlled humidity.

Figure 2: SFM image of PS-PEO on a topographically patterned surface

Figure 3: SFM phase image of cleavable PS-PEO with slat on topographically patterned surface.

A fluid or any substance can be in a supercritical state above its critical temperature (Tc) and critical pressure (Pc). Tc and Pc denotes the highest temperature and pressure at which the fluid co-exists as liquid and vapor in equilibrium. The distinguishing characteristics of Supercritical fluids (SCF) are adjustable solvent strength and gas-like transport properties. Following table shows a comparison between liquid and SCFs.

  Diffusivity (cm2/sec) Viscosity (cps) Density (g/cm3) Surface Tension (dynes/cm)
SCF 10-3 0.02 - 0.05 0.2 - 1.0 0
Liquid 10-5 1 0.8 20 - 50

We use supercritical carbon dioxide (SC CO2) for most of our study, because of its moderate critical properties (Tc = 31? C; Pc = 73.8 bar). Most of the polymers are insoluble in SC CO2, excluding amorphous fluoropolymers, polysiloxanes and some polycarbonate copolymers. Though most of the polymers fail to dissolve, they can be swollen in SC CO2. Sorption of CO2 has significant effect on the properties of the polymer. The most important effects are the (i) depression of Tg (ii) enhancement of small molecules and polymer chain diffusivity within dilated or swollen polymers and (iii) increase in the free volume present in the polymer. Research being done in our group involves exploring and utilizing the above mentioned effects in various polymeric systems. Our recent work can be broadly divided into three categories. They are listed below with a representative data and a brief description.

(i) Effects of SCF on phase behavior and morphology of block copolymers:
The influence of CO2 sorption on the closed-loop phase diagram of Polystyrene-block-poly(n-pentyl methacrylate) (PS-b-PnPMA) copolymers1 are shown below. Differential dilation of the constituent blocks was used to induce microphase separation of the PS-b-PnPMA. With increasing carbon dioxide loading an appreciable expansion in the size of the closed loop was found. At a carbon dioxide activity of 0.1 the upper order-to-disorder transition temperature (UODT) was elevated by more than 20 °C, whereas the lower disorderto-order transition temperature (LDOT) was depressed by the same magnitude. This arises from the entropic nature of the closed-loop miscibility gap in this weakly interacting system, where microphase separation is driven by disparity in free volumes between dissimilar segments of the chain.


Figure 1: UODT and LDOT temperatures in 50 kDa PS-b-PnPMA at CO2 activities of 0.0 (circles), 0.1 (squares), 0.2 (triangles), and 0.3 (diamonds) for volume fraction ratios of 50:50 (closed symbols) and 42:58 (open symbols).

(ii) Effect of enhanced mobility on morphology of novel block copolymers:
Using a series of AnBn miktoarm star block copolymers with different numbers of arms (n = 1, 2, 4, and 16), the effect of molecular architecture on the grain growth kinetics was investigated by annealing in supercritical carbon dioxide2. The grain growth kinetics was monitored in real space by transmission electron microscopy (TEM), followed by subsequent micrograph image analysis. It was found that the molecular architecture influenced the grain growth kinetics of these AnBn star block copolymers significantly. The grain growth kinetics for the AnBn stars with n = 2, 4, and 16 were fount out to be similar for both supercritical CO2 and thermal annealing. However, the grain growth kinetics of the diblock (AnBn with n = 1) was dramatically enhanced in supercritical CO2 relative to thermal annealing.


Figure 2: Comparison of the growth of correlation lengths in the A1B1 star block copolymers between supercritical CO2 annealing and thermal annealing. The lines in the plot are the least-squares, power law fits to the corresponding correlation length.

(iii) Phase selective chemistry in block copolymer domains:
Enhanced diffusivity of small molecules in CO2- swollen polymer matrices can be used advantageously to deliver reactants into the block copolymer domains without disturbing the microphase separated nanostructures. Specific affinity between the reactants and one of the block copolymer domains can be manipulated to carry out phase selective reactions in block copolymer domains. Figure 3 shows the cross-section of the nanocomposite film having silver particles deposited selectively in PVP domains of the PS-b-PVP block copolymer film3. Figure 4 shows AFM image of the patterned mesoporous silicate structures obtained by performing domain selective condensation of precursors within self-assembled block copolymer templates by using supercritical CO2 as a delivery medium4. The domain selectivity is imparted by the segregation of acid catalyst into hydrophilic domains. Further, by using a photo acid generator, the presence of acid within the film can be controlled spatially via photolithography. Thus patterns at two different length scales i.e., at nanoscale from self-assembled block copolymer and microscale from photolithography can be generated simultaneously.


Figure 3: TEM cross-sectional image of the PS-b-P2VP (54K-b-50K) film metallized with Silver particles in P2VP domains. Silver particles are the result of reduction of an organometallic precursor(Ag(COD)hfac).

Figure 4: AFM image of the patterned mesoporous silicate structures templated from chemically amplified block copolymer films. Silicate network was obtained by hydrolysis and condensation of silicate precursor(Tetra ethyl orthosilicate)