Controlling Nanowear in a Polymer by Confining Segmental Relaxation Bernd Gotsmann* and Urs T. Duerig IBM Research GmbH, Zurich Research Laboratory, 8803 Ru ¨schlikon, Switzerland Scott Sills, Jane Frommer, and Craig J. Hawker ² IBM Research, Almaden Research Center, 650 Harry Road, San Jose, California 95120 Received October 17, 2005 ABSTRACT Molecular relaxation of a copolymer designed for nano-electromechanical systems was chemically confined by varying the spacing between cross-links, δ c . A critical cross-link spacing of 1-3 nm marks a transition in the nano-mechanical properties evaluated by atomic force microscopy. The transition reveals an interplay between the cross-link spacing and the length scale for backbone relaxation,  r , in cooperatively rearranging regions. For δ c .  r , the natural backbone relaxation process is relatively unaffected by the cross-links and a ductile, low hardness behavior results. For δ c <  r , the cross-links directly interfere with backbone relaxation and confine segmental mobility, leading to a brittle, high hardness response. Nano-electromechanical systems (NEMS) are receiving considerable attention in both science and technology. Friction and wear are among the most critical problems to be solved in order to fabricate commercially viable devices with moving parts. 1 Extensive efforts have been devoted to controlling and minimizing wear. However, when device applications require nanoscale phase homogeneity, specialty coatings, composite reinforcement, and third body lubrication become unattractive approaches to tackling wear issues. Examples of such applications include scanned-probe data storage 2,3 and scanned-probe lithography and patterning. 4 Wear management in NEMS must evolve from the molecular scale. Macroscopic wear phenomenology, with descriptors such as yield stress, hardness, wear volume, etc., fails to completely describe molecular and nanoscale wear processes. Nanoscopic wear characteristics of polymers are regulated by the interaction of critical molecular length scales, including relaxation length scales, the spacing between cross-links, and the size of external contacts. Many proposed NEMS schemes incorporate sharp probes for transport, sensing, and patterning operations. Under loading forces of a few nanonewtons, a nanometer sharp tip typically produces stresses of hundreds of megapascals. A polymer surface beneath the tip accommodates these stresses through both viscoelastic and plastic modes. The frictional energy is dissipated either internally, i.e., through viscoelastic molecular relaxation processes, 5-7 or through pseudoplastic mechanisms such as chain pull-out, scission, 8 or crazing. 9 The wear characteristics of the surface inevitably depend on which relaxation modes are available under the sliding conditions and on the extent to which the relaxation processes are spatially correlated throughout the material. In this Letter, we focus on controlling the wear charac- teristics of synthetic polymers for contact mechanical opera- tions in NEMS. By reducing the cross-link spacing, we confine natural molecular relaxation and reveal a transition between the dominant wear mechanism: from viscoelastic rippling in a ductile material to pseudoplastic, abrasive wear in a brittle material. The transition between wear modes occurs when the spacing between cross-links matches the length scale required for segmental backbone relaxation. Random copolymers of styrene and benzocyclobutene (PS-BCB) 10 were selected as a model system, based on the developing technology of scanned-probe data storage. 2,3 Latent cross-links are provided via the BCB groups, with the primary reaction pathway proposed in Figure 1: PS-BCB films (20 nm thick) were spin cast from cyclohexanone solutions (1 wt %) onto silicon wafers coated with an epoxy (SU8) buffer layer (80 nm). The buffer serves to prevent tip wear if penetration through the surface film occurs. The films were thermally cross-linked under nitrogen at 220 °C for 1 h. 10 The BCB content was varied during synthesis from 0 to 30 mol %, with overall molecular weights M w and molecular weight between cross-links M c ranging from 14 to 123 kDa and 243-17400 Da, respectively. 11 The * Corresponding author, bgo@zurich.ibm.com. ² Present address: University of California, Santa Barbara, CA 93106- 9510. NANO LETTERS 2006 Vol. 6, No. 2 296-300 10.1021/nl0520563 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/11/2006