Time-Temperature Superposition for Viscoelastic Properties of Regioregular Poly(3-hexylthiophene) Films A. Robert Hillman,* ,² Igor Efimov, ² and Magdalena Skompska ‡ Contribution from Department of Chemistry, UniVersity of Leicester, Leicester LE1 7RH, U.K., and Department of Chemistry, Warsaw UniVersity, 02 093 Warsaw, Poland Received October 14, 2004; E-mail: arh7@le.ac.uk Abstract: Shear moduli were determined for chemically polymerized and solvent cast regioregular poly- (3-hexylthiophene) films, using thickness shear mode acoustic wave resonators. The results are strikingly different to those for electropolymerized regiorandom poly(3-hexylthiophene) films. The time scale of the measurement was varied directly by use of higher harmonics of the acoustic wave resonator and indirectly via temperature. The significant variations in shear modulus with effective time scale can be “normalized” onto a stress master relaxation curve by using the concept of time-temperature superposition; this is the first time this has been demonstrated for electroactive films. The shift factors required to effect this normalization do not follow the classical Williams-Landel-Ferry (WLF) equation developed for long-range backbone motions of bulk polymers. Instead, they follow an Arrhenius-like behavior, commonly used to describe secondary motions of polymer side-chains. The activation enthalpy associated with this is independent of applied potential, is the same as for as cast (undoped) films, and is similar to that for rotation about a carbon-carbon single bond. These all point to the hexyl side-chains as the origins of the observed phenomena, consistent with the “melting point” separating two temperature-dependent phases and with the different molecular packing arrangements that would necessarily apply to regioregular and regiorandom materials. Introduction Overview. In this paper we explore, for the first time, the extent to which the principle of time-temperature superposition can be applied to a thin electroactive polymer film. The goal is to combine data acquired under different conditions (of time scale and temperature) in such a manner as to place them on a common “time scale” axis and thereby explore the master relaxation curve; typically, 1,2 this extends over a wide range of time scales, such that the window provided by a single data set is inadequate to provide useful insight. Indeed, even the minority of studies of electroactive polymer film dynamics that are quantitative (in terms of shear modulus components) are generally restricted to a single value at fixed temperature and observational frequency. The unique fundamental feature of the present study is the manipulation of the normalized time scale of the system through three control parameters: applied potential, acoustic wave device frequency, and temperature. Respectively, these operate indirectly by influencing electrostatic stiffening and solvent plasticization, directly via observational time scale and directly through variation of polymer relaxation rate. Together, these three control parameters open an unusually wide window on the master relaxation curve. The practical significance of this is the extent to which it is appropriate to take a macroscopic concept from polymer science and apply it to materials used on microscopic scales relevant to nanoscience- based applications. Motivation. The principle of time-temperature superposition is a powerful and well-established concept for rationalizing relaxation phenomena in bulk polymeric materials. Qualitatively, it expresses the notion (described below and elsewhere 1,2 ) that polymer dynamics data acquired at different temperatures, and for which different relaxation rates apply, can be placed upon a single (master) relaxation curve by means of a “shift” factor, a T . If the polymer dynamics are quantified in terms of the shear modulus, G(T,t), acquired at two temperatures T 1 and T 2 , for which different relaxation rates apply, then: In terms of the angular frequency (ω) in a modulation experi- ment and the characteristic relaxation time (τ) of the system, which is a function of temperature, this means that one can find a corresponding frequency of mechanical excitation at which G measured at two different temperatures will be the same. In the most common case, the mechanical properties depend on the product ωτ. If one knows the manner in which relaxation rate varies with temperature, the shift factor can be predicted. Conversely, if the shift factor required to place the responses on a single curve is determined, then one has access to the energetics of the system and thereby insight into the underlying physicochemical processes. Classically, the procedure has been ² University of Leicester. ‡ Warsaw University. (1) Ferry, J. D. Viscoelastic Properties of Polymers, 2nd ed.; Wiley: New York, 1970. (2) Aklonis, J. J.; MacKnight, W. J. Introduction to Polymer Viscoelasticity, 2nd ed.; Wiley: New York, 1983. G(T 1 ,t) ) G(T 2 ,t/a T ) (1) Published on Web 03/01/2005 10.1021/ja0437508 CCC: $30.25 © 2005 American Chemical Society J. AM. CHEM. SOC. 2005, 127, 3817-3824 9 3817