IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 19 (2008) 495703 (9pp) doi:10.1088/0957-4484/19/49/495703 Nanoscale thermal–mechanical probe determination of ‘softening transitions’ in thin polymer films* Jing Zhou, Brian Berry, Jack F Douglas, Alamgir Karim, Chad R Snyder and Christopher Soles Polymers Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, MD 20899-8541, USA E-mail: alamgir.karim@nist.gov and csoles@nist.gov Received 27 August 2008, in final form 8 October 2008 Published 19 November 2008 Online at stacks.iop.org/Nano/19/495703 Abstract We report a quantitative study of the softening behavior of glassy polystyrene (PS) films at length scales on the order of 100 nm using nano-thermomechanometry (nano-TM), an emerging scanning probe technique in which a highly doped silicon atomic force microscopy (AFM) tip is resistively heated on the surface of a polymer film. The apparent ‘softening temperature’ T s of the film is found to depend on the logarithm of the square root of the thermal ramping rate R. This relation allows us to estimate a quasi-equilibrium (or zero rate) softening transition temperature T s0 by extrapolation. We observe marked shifts of T s0 with decreasing film thickness, but the nature of these shifts, and even their sign, depend strongly on both the thermal and mechanical properties of the supporting substrate. Finite element simulations suggest that thin PS films on rigid substrates with large thermal conductivities lead to increasing T s0 with decreasing film thickness, whereas softer, less thermally conductive substrates promote reductions in T s0 . Experimental observations on a range of substrates confirm this behavior and indicate a complicated interplay between the thermal and mechanical properties of the thin PS film and the substrate. This study directly points to relevant factors for quantitative measurements of thermophysical properties of materials at the nanoscale using this nano-TM based method. S Supplementary data are available from stacks.iop.org/Nano/19/495703 (Some figures in this article are in colour only in the electronic version) 1. Introduction Materials patterned into nanostructures or multiphase materials with nanoscale domains (e.g., block copolymers, polymer blends [6, 28, 17, 3, 21, 27], or nanocomposites [8, 1, 2, 15]) are the cornerstone for many emerging applications in nanotechnology. In many instances, one would like to probe the thermal properties of these systems that are heterogeneous at the nanoscale. This requires a local thermal analysis technique with nanometer resolution. Conventional calorimetry techniques such as differential * The error bars presented throughout the manuscript indicate the relative standard uncertainty of the measurement. scanning calorimetry (DSC) require several milligrams of sample to obtain a reasonable signal-to-noise ratio. Localized thermal probe measurements using a Wollaston wire based scanning tip have been reported [18, 19, 7]. Typically, a 5 μm diameter wire is bent into a tip with 20 μm radius of curvature. This system is not suitable for thermal analysis at sub-μm length scales. Nano-thermomechanometry (nano-TM) has recently emerged as a possible technique for quantitative nano-thermal analysis [12, 11]. King et al have pioneered this technique, which utilizes a highly doped Si AFM tip that is resistively heated [16]. With proper calibration, this nano- TM technique has the potential to determine the melting point (T m ) or the glass transition temperature (T g ) of a material at the nanoscale. Nano-TM measures the cantilever deflection 0957-4484/08/495703+09$30.00 © 2008 IOP Publishing Ltd Printed in the UK 1