Sunday, May 19, 2013

Creation of triblock copolymer thin films by combining vapor annealing with a raster spray

Adapted from "Spatial and Orientation Control of Cylindrical Nanostructures in ABA Triblock Copolymer Thin Films by Raster Solvent Vapor Annealing", ACSNano,
Jonathan E. Seppala, Ronald L. Lewis, III, and Thomas H. Epps, VOL. 6 NO. 11 98559862 2012)

This nanosciece paper describes a novel approach to annealing polymeric thin films, particularly block copolymer thin films.  Self-assembly is a significant phenomenon in these materials because they open the floodgates for designer nanoscale materials for nanoporous membranes, lithographic masks, and nanopatterning/templating applications (the last two have huge implications for the electronics industry).  These three nanotechnologies often exploit morphologies often found in AB diblock and ABA triblock copolymers (spheres, gyroid, and lamellae) because the thermodynamics of bulk self-assembly is relatively well established.    The morphology of bulk block copolymers is influenced by three major factors: the degree of polymerization (N), the interaction parameter (c), and the volume fraction of the blocks (f). 
With thin films, surface energy becomes an additional factor; it can be exploited by thermal annealing to facilitate copolymer self-assembly via the bestowment of mobility to amorphous regions that are trapped upon casting.  However, thermal annealing is limited to copolymer systems where the components have similar γ‘s and are thermally insensitive.  Another technique is solvent vapor annealing (SVA), which grants mobility by effectively reducing the Tg of the copolymers.  It is a powerful technique, but is limited to small quantities that are typical of research labs. 
The objective is to devise a faster method for large-scale production of block copolymer (BCP) nanotechnologies.  The method must enable control over morphology and orientation of BCP thin films.  The authors propose raster solvent vapor annealing (RSVA): solvent vapor from a bubbler system is directed onto a BCP surface, which then creates a SVA zone.  The zone is modified/expanded by a motorized stage moving in a raster fashion.  

RSVA was performed with a THF-rich vapor stream in single or multiple passes over a 100 nm thick poly(styrene-b-isoprene-b-styrene) (SIS) film, with domains of 29 nm.  The RSVA speeds ranged from 500 µm/s to 3 µm/s.   The RSVA process swelled the films due to hydrolysis, so the film thickness was measured by spectral reflectometry.  The swelling increased the thickness to 160 nm, and eventually dried down to nearly the original thickness, although some samples were reported to have residual solvent. 
Several approaches to RSVA were performed.  One was single-pass, where the stage moved under the nozzle once, with only the speed varying. The as-cast film had a lamellar structure consisting of cylinders oriented parallel to the substrate, but with minimal long-range order.   Varying the speed affected the ordering of the lamellar cylinders.  The slower the speed, the longer the order-range; at 10 µm/s, the cylinders have mostly perpendicular orientation.  The 10 µm/s speed corresponds to an annealing time of 50 s.  The cylinders even looked slightly swollen, which was confirmed by azimuthally integrated 1D profiles from FFTs of the AFM images.    Even reducing the nozzle diameter still induced the ^cylinders, although at lower speeds. 

The post-RSVA morphology is an imbroglio of competing forces. The lower g for polyisoprene (32.0 mJ/m2 vs. 40.7 mJ/m2), the majority block, enables wetting of both the free and substrate surfaces, which leads to the propensity for the cylinders to possess parallel orientation.  High RSVA speeds do not change the orientation (see 2a–2c) because the surface energy difference was too large for entropy to take effect.  The slow raster allowed enough solvation to lower the differences in g, which lets a) entropic effects to manifest, and to compensate for the stretching experienced by the cylinders during swelling and deswelling. 
              Briefly, Seppela et al tried two more approaches.  One is multiple passes under the nozzle.  Retracing the RSVA pathways altered the cylindrical orientation toward perpendicularity.  The other is a crossed-path approach; two orthogonal passes cross each other, and the result is a domain dominated by perpendicular cylinders.  Both approaches are supported by crisp AFM phase images. 
            This is a wonderfully written paper, but it helps that I have significant background in polymer chemistry.  Note that only THF was used in the vapor stream; it’s natural to ask if this approach has been done with other solvents, and other BCPs.  dTHF = 18.1 (MPa)1/2, dpolyisoprene = 16.2 (MPa)1/2, dpolystyrene =  18.6 (MPa)1/2, so utilizing similar solubilities is a probable reason for the RSVA setup described in the paper.  One should also ask if this technique can be done for other self-assembling polymeric thin films. Seppela et al noted that this annealing method can be altered according to slit geometries, solvent quality, and substrate temperature–indications of much promise for RSVA.  

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