Confining block copolymers to thin films can have a dramatic effect on the microdomain structure, a topic we are investigating in collaboration with Professor Paul Chaikin (Physics, New York University). Preferential wetting of the surfaces by one of the blocks will generally drive “parallel” alignment of cylinders (cylinder axis in film plane) and lamellae (lamellar normal perpendicular to film plane). However, for film thicknesses which are incommensurate with the microdomain spacing, “perpendicular” orientations can be observed. In the case of cylinders, both the parallel and perpendicular structures can be useful templates for nanofabrication (of line and dot structures, respectively), but controlling the film to yield the desired structure for a particular application is clearly essential. We have constructed a flow-coating device which allows the preparation of substrate-supported polymer thin films with a predefined thickness gradient, allowing the effect of film thickness on domain structure to be probed in a high-throughput way by atomic force microscopy (AFM). The figure below shows the evolution of structure with film thickness t for a particular polystyrene-poly(n-hexylmethacrylate) diblock, PS-PHMA, forming PS cylinders; depending on the film thickness, either all dots (perpendicular cylinders) or all lines (parallel cylinders) can be observed in the AFM images. Note that PS and PHMA actually have rather different surface tensions and interactions with the substrate, so the broad range over which perpendicular cylinders is observed is quite unexpected. A collaboration with the group of Professor Athanassios Panagiotopoulos is providing insight into how the microscopic interaction energies can influence the domain structure.
Left: tapping-mode AFM phase images of a PS-PHMA 24-89 thickness gradient film, imaged at various film thicknesses t as indicated. Right: fractional coverage of parallel cylindersvs. t; letters (a-e) correspond to panels at left. Note strong oscillation of perpendicular vs. parallel orientation, until film is approximately three microdomain spacings thick, above which parallel orientation dominates (Davis et al., Macromolecules, 47, 5277 (2014)).
Shearing the film transforms the “fingerprint” texture spontaneously formed by parallel cylinders (panels b, d, f in the figure above) into one of aligned stripes with long-range order, extending over centimeters or larger (indeed, as large as the PDMS pad used for the shearing). While the extent of order is truly macroscopic, the order is not perfect: though there must be zero unpaired disclinations over the entire sheared area (otherwise, the cylinder director would vary over the sheared region), there are still isolated dislocations (typically of order ~1 per μm2), as well as long-period undulations in the cylinder trajectory. Panel (a) in the figure below shows an AFM image revealing both a few dislocations, as well as cylinder undulations; panel (b) shows a magnified view of a dislocation core. The impact that these defects have on the orientational order parameter ψ2 is shown in panel (c); though the values of ψ2 are uniformly high (>0.99), they never exceed 0.999 even in regions of the film which are free from dislocations, due to the cylinder undulations (y-intecept of best-fit solid line). The impact that dislocations have on ψ2 can be quantitatively captured with a simple continuum model for the displacement of the lattice lines, developed originally for edge dislocations in three-dimensional atomic solids. Simulations incorporating both dislocations and undulations (example simulated image shown in inset to (c)) semiquantitatively capture the observed effects.
In our current work, we are looking beyond the structure of these thin block copolymer films, towards the dynamics of the polymer chains contained therein. In a collaboration with Professor Rodney Priestley, we are investigating how confinement of polymer chains, both by block copolymer microdomains and by the substrate and air surfaces, influences their glass transition.
Supported by the National Science Foundation, Materials Research Science and Engineering Center (MRSEC) Program, through the Princeton Center for Complex Materials, and Graduate Research Fellowship Program
Current/Recent Group Members, and Their Project Titles:
Dane Christie PhD *19 – “Influence of Confinement and Interfaces on the Structure and Dynamics of Amorphous Polymers”
Raleigh Davis PhD *15 – “Shear Alignment of Block Copolymer Thin Films to Produce Well-Ordered Microdomains for Use in Nanofabrication”
Saswati Pujari PDRA – “Alignment of Perpendicular Lamellae in Block Copolymer Thin Films by Shearing”
Andrew Marencic PhD *11 – “Well-Ordered Block Copolymer Thin Films Using Shear-Alignment Techniques”
Sahana Jayaraman ‘16 – “Creating Controlled Thickness Gradients in Polymer Thin Films via Flowcoating”
Ha-Kyung Kwon ‘13 – “Synthesis and Characterization of Polystyrene-block-Poly(2-ethylhexylmethacrylate) Diblock Copolymers”
Siobhan Galligan ‘13 – “Gradient Measurements of Phase Behavior in Thin Films of Block Copolymer and Homopolymer Blends”
Michael Keaton '11– "Terpolymer Brushes for Substrate Neutralization"
Brian Michal ‘10 – “Synthesis and Characterization of Polystyrene-Poly(n-hexylmethacrylate) Diblock Copolymers”
Maria Phillip ‘10 – “Statistical Terpolymers for Surface Modification”
Selected Recent Publications:
R.L. Davis, P.M. Chaikin, and R.A. Register, "Cylinder Orientation and Shear-Alignment in Thin Films of Polystyrene-Poly(n-hexylmethacrylate) Diblock Copolymers", Macromolecules, 47, 5277-5285 (2014).
S. Pujari, M.A. Keaton, P.M. Chaikin, and R.A. Register, "Alignment of Perpendicular Lamellae in Block Copolymer Thin Films by Shearing", Soft Matter, 8, 5358-5363 (2012).
A.P. Marencic, P.M. Chaikin, and R.A. Register, "Orientational Order in Cylinder-Forming Block Copolymer Thin Films", Phys. Rev. E, 86, 021507 (2012).
A.P. Marencic, D.H. Adamson, P.M. Chaikin, and R.A. Register, "Shear Alignment and Realignment of Sphere-Forming and Cylinder-Forming Block Copolymer Thin Films", Phys. Rev. E, 81, 011503 (2010).