pubs.acs.org/Macromolecules Published on Web 11/11/2009 r 2009 American Chemical Society 9332 Macromolecules 2009, 42, 9332–9337 DOI: 10.1021/ma9015505 Triggering Mesophase Order in Melts of Metastable, Ultrathin Diblock Copolymer Films through Microstretching: Effect of Melt Film Thickness Susana Moreno-Flores,* ,†,§ Rainer Nehring, † Roberto Raiteri, ‡ and Wolfgang Meier* ,† † Department of Chemistry, University of Basel, Klingelbergstrasse 80, CH-4056 Basel, Switzerland, and ‡ Department of Biophysical and Electronic Engineering, Via Opera pia 11a, University of Genova, I-16145 Genova, Italy. § Current address: Biosurfaces unit, CIC BiomaGUNE, Paseo Miramon 182, E-20009 San Sebastian, Spain. Received July 16, 2009; Revised Manuscript Received October 13, 2009 ABSTRACT: By means of a sharp atomic force microscopy (AFM) probe, we have induced the formation of lamellar structures with nanoscale size in ultrathin films of melted poly(1,4-butadiene)-block-poly(ethylene oxide) (BD 84 -EO 110 , f PB = 0.55) at temperatures above the melting temperature of the PEO block and below the order-disorder transition temperature of the copolymer (T m <T<T ODT ). The morphology of films 30-50 nm thick is mainly featureless except for the presence of a few monolamellar nanopatches. The absence of an overall lamellar structure, the typical mesophase encountered in this kind of copolymers, suggests that the ultrathin films are arrested in the disordered state. However, repetitive loading-unloading cycles exerted with the AFM probe in the structureless regions lead to the formation of multilamellar islands consisting of 2-9 layers 10-12 nm thick at precise locations. The size, shape, and number of layers of these islands can be controlled by tuning the size of probed area, the loading-unloading rate, and the overall film thickness. Introduction Amphiphilic block copolymers composed of hydropho- bic-hydrophilic blocks have been attracting great interest in basic research, and (bio)technology. 1 The large variety of self- assembled structures and morphological transformations exhib- ited by these polymers, either in bulk or dispersed in selective solvents, is the result of the interplay between the molecular structure and the interaction of the blocks. Both the mesophase symmetry and the polymer’s capacity to respond under selective physical or chemical conditions have been exploited in lithogra- phy 1 and as sensors. 2 Block copolymers composed of a crystallizable (C-) block and an amorphous (A-) block have received considerable attention because they represent a further step in the degree of structural complexity in block copolymer self-assembly. 3 Below the or- der-disorder temperature (T ODT ), they undergo microphase separation into ordered structures with a defined symmetry called mesophases; however, the order may be altered by the crystal- lization of the C-block, which can template- or breakout-crystal- lize, preserving or destroying the mesophase, respectively. 3 Extensive studies have been devoted to the influence of C-block crystallization in the mesophase morphology and physical beha- vior of C-A copolymers (T g A , T m C < T ODT ). 3 In particular, block copolymers of poly(butadiene)-block-poly(ethylene oxide) (the former block in either its hydrogenated or its unsaturated form) have been systematically investigated as bulk model systems for confined crystallization, 4,5 and in blends with poly- (butadiene) 6,7 or with other diblock copolymers. 8 As films, the complexity increases because thickness imposes a further spatial confinement that may alter both crystallization and mesophase ordering. Despite that, the mechanism of C-block recrystallization in thin and ultrathin films has been studied in detail by Reiter et al., 9,10 whereas other studies rather focused on the influence of C-block crystallization upon the mesophase morphology. 11-15 Nanotechnological applications abound where ultrathin copolymer films are used as lithographic masks, as templates, or as photonic crystals to create a great variety of nanopatterns. This is possible because of the high control over the microdomain morphology and orientation, which is achieved by manipulating the chemistry of the substrate or by applying electrical fields and mechanical perturbations. 16-18 To this effect, atomic force microscopy (AFM) has not only been an invaluable tool for imaging film morphology on the micro- and nanoscale. 19 As nanolithographic tool, the AFM probe has been used to create patterns on polymer films via indentation, 20 local heating, 21 or electrostatic nanolithography. 22 In this work, we show new insight into the creation of mesophase ordering in ultrathin melt films of a symmetric diblock copolymer of poly(butadiene)-block-poly(ethylene oxide) (PB-b-PEO) using temperature-dependent AFM as both an imaging and a nanoconstruction tool. Ultrathin melt copolymer films are mainly structureless except for a few monolamellar patches of small size that appear only above a certain thickness threshold. The featureless regions are actually pseudodewetted areas 10 coated with a residual, nanometer thick polymer layer arrested in the disordered state. Through stretching with the microsized AFM tip, piles of a few lamellae can be generated within these regions at desired locations. The mechanically induced formation of the mesophase from the disordered melt depends on the overall film thickness, whereas the size and shape of the mesophase can be controlled by tuning the size of the probed area and the stretching rate. Triggering mesophase order in ultrathin films not only introduces a new kind of nanopattern- ing, but it can also generate user-defined geometries for investiga- tion of simultaneously occurring template and breakout C-block crystallization in thin films. Experimental Section Chemicals. All reagents used for polymer synthesis had the highest purity grade and were used as received, unless otherwise *Corresponding authors.