Materials Science and Engineering A 444 (2007) 227–235 Thermal, electrical and magnetic studies of magnetite filled polyurethane shape memory polymers Muhammad Yasar Razzaq, Mathias Anhalt, Lars Frormann, Bernd Weidenfeller ∗ Faculty of Natural and Materials Sciences, Clausthal University of Technology, Adolph-Roemer-Straße 2A, 38678 Clausthal-Zellerfeld, Germany Received 14 June 2006; received in revised form 22 August 2006; accepted 23 August 2006 Abstract Thermal, electrical and magnetic properties of polyurethane shape memory polymer (SMP) samples filled with 0–40 vol% magnetite particles prepared by mixing and injection molding were investigated. Shape recovery in the shape memory polymer was initiated by a magnetizing field of H = 4.4 kA/m at a frequency f = 50 Hz. Electric resistivity was decreased by magnetite particles from ρ el ≈ 10 10  m to ρ el ≈ 10 6  m. The percolation threshold is achieved at a magnetite concentration of approximately 30 vol%. Thermal conductivity increases from 0.19 W/m K to 0.60 W/m K with magnetite fraction in the polymer. Thermal conductivity values are compared with several theoretical and semi empirical models. The Agari–Uno model shows a very good correlation to measured values. Changes in specific heat capacity with temperature were also measured and could be correlated with the morphology of the polymer. With a hysteresis recorder the power losses in magnetization reversal of the filled SMP were estimated. Using measured specific heat capacity and power losses the time for an increase of sample temperature from room temperature up to shape recovery temperature was calculated to be t ≈ 4 min. Time dependent photographic pictures of the shape recovery process showed a good accordance between the calculated and observed time for the shape recovery. © 2006 Elsevier B.V. All rights reserved. Keywords: Shape memory polymer; Magnetite; Thermal properties; Electrical properties; Magnetization reversal 1. Introduction Recently active research has been done on so called smart materials such as shape memory polymers capable of responding to an external stimulus [1–6]. Shape memory polymers (SMP) are showing a wide range of temperatures for shape recovery, high recoverable strain up to 400%, and low costs in manufac- turing [7–11] which enables their usage in medical devices and biological micro electrical systems [12–17]. The typical sequence for thermally stimulated shape mem- ory polymers is the application of an initial deformation to the polymer at an elevated temperature (pre-deformation), fixing the shape by cooling the pre-deformed SMP under strain to a lower temperature (storage), and then heat the material to recover the original shape (recovery) [12,18]. Conventionally, the shape recovery in a SMP is actuated by external heating. If the external heating process is not applicable, ∗ Corresponding author. Tel.: +49 5323 723708; fax: +49 5323 723184. E-mail address: bernd.weidenfeller@tu-clausthal.de (B. Weidenfeller). an other mechanism for triggering the shape recovery has to be considered. If magnetic particles are incorporated into the SMP, an inductive heating of shape memory polymers by electromag- netic fields can be used to induce the shape memory effect [19]. Because of their small sizes heat generation in magnetic nanoparticles as they are used in [19] under rotational magnetic field is very low due to Brownian and N´ eel relaxation losses [20,21]. Therefore, high magnetizing frequencies have to be used to generate an appropriate heat by power losses in appli- cations with magnetic nanoparticles. However, in micro sized magnetic particles magnetization reversal losses are usually divided into hysteresis loss, eddy current losses and anomalous losses. Due to increased eddy current losses as well as due to increased number of domain walls micro sized particles show higher heat generation by increased power losses under low mag- netizing frequencies. Recognizing this we have pursued the enhancement of thermal, electrical and magnetic properties of polyurethane SMP by using micro sized magnetite as a filler. Due to its typical magnetic and electrical properties, magnetite (Fe 3 O 4 ) is 0921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.08.083