Crystallization of Poly(ethylene oxide) Patterned by Nanoimprint Lithography Brian C. Okerberg, † Christopher L. Soles,* Jack F. Douglas, Hyun Wook Ro, and Alamgir Karim Polymers DiVision, National Institute of Standards and Technology, 100 Bureau DriVe, Gaithersburg, Maryland 20899-8541 Daniel R. Hines Laboratory for Physical Sciences, UniVersity of Maryland, 8050 Greenmead DriVe, College Park, Maryland 20740 ReceiVed February 2, 2007 ReVised Manuscript ReceiVed March 12, 2007 Crystallization of polymeric materials in confined geometries is a subject of intense recent interest. It has been shown that confinement of semicrystalline polymers in thin films can significantly affect primary nucleation, the crystal morphology, crystal growth rates, and the crystal orientation. 1-10 Nanoimprint lithography (NIL) has recently emerged as a powerful patterning tool capable of producing nanoscale patterns in a variety of materials 11-18 and, as such, provides new directions for studies of confinement effects on crystallization. A schematic of the nanoimprint process is shown in Figure 1. Bulk semicrystalline polymers typically exhibit a spherulitic morphology, consisting of lamellae radiating from a central nucleation site. These crystalline structures span a range of length scales, from angstroms for the unit cell of the crystal to millimeters for the spherulitic superstructure. Nanoscale patterning clearly has the potential to influence the crystallization process at several length scales, affecting growth of individual lamellae as well as the development of the larger spherulitic morphologies. Here we present the effects of patterning via nanoimprint lithography on the crystal morphology of poly(ethylene oxide) (PEO). PEO is an ideal model system because its crystallization behavior has been extensively studied and is well-understood. 19-40 PEO has a low melting temperature and simple chain architec- ture and forms large spherulites. The crystal lamellae in PEO are on the order of 10 nm thick and can extend for several microns in the lateral directions. In films thicker than 300 nm, spherulitic morphologies are observed and the lamellae usually have an edge-on orientation. In thinner films, the lamellae tend to adopt a flat-on orientation, with their fold surfaces parallel to the substrate. 33,36 As the initial film thickness nears the preferred lamellar thickness, dense-branched morphologies (DBM) are often reported, which are suggested to result from a depletion zone near the growth front. 3 The nanoimprint mold used in this study consists of parallel line-grating patterns with a periodicity of 400 nm etched into a silicon oxide substrate. The cross section of the lines is approximately trapezoidal, with an average line width of 160 nm, an average line height of 350 nm, and an average sidewall angle of 5° (the sidewall angle is the deviation from a rectangle; 0° would be a rectangle). The mold is treated with a fluorinated self-assembled monolayer (SAM) of tridecafluoro-1,1,2,2- tetrahydrooctyltrichlorosilane to facilitate release of the PEO patterns. PEO films (molar mass ≈ 100 000 g/mol) are spun- cast from 1,2-dichloroethane onto silicon wafers and dried for 2 h at 50 °C to produce a 230 nm thick film. Using a Nanonex NX 2000 imprint tool, 41 the samples are placed under vacuum and imprinted in the melt state at 75 °C for 1 min at 3450 kPa. The samples are crystallized by cooling slowly in the imprint mold to room temperature before releasing the pressure and separating the mold from the sample. An optical micrograph of an imprinted PEO sample is shown in Figure 2, with a set of three vertical lines indicating the orientation of the grating. AFM scratch tests reveal that the imprinted lines are ∼350 nm tall sitting on a 35 nm thick residual layer. Despite the fact that the continuous portion of the film between the lines is only 35 nm thick, a bulklike spherulitic morphology is observed. This result is surprising given that DBM is normally observed in planar 35 nm thick films. 34 Since the DBM morphology has been claimed to result from a depletion zone at the growth front, we suspect that the molten PEO in the imprinted lines above the thin residual layer acts as a “reservoir” of crystallizable material that feeds the crystallization front. This process facilitates the development of a three-dimensional bulklike spherulite by allowing “com- munication” through the continuous residual layer and growth of the spherulite into the thick, but discontinuous, patterned region. It is also striking that the radial growth of the spherulites does not appear to be significantly perturbed by the presence * To whom correspondence should be addressed. E-mail: christopher.soles@nist.gov. † National Research Council Postdoctoral Associate. Figure 1. Schematic of the nanoimprint process. Figure 2. Optical micrograph of a nanoimprinted PEO film with a spherulitic morphology. The direction of the imprint lines is indicated in the upper left. 2968 Macromolecules 2007, 40, 2968-2970 10.1021/ma070293h CCC: $37.00 © 2007 American Chemical Society Published on Web 03/28/2007