Thermal, electrical and magnetic studies of magnetite filled polyurethane shape memory polymers

Thermal. electrical and magnetic properties of polyurethane shape memory polymer (SMP) samples filled with 0-40vol% 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 P-el approximate to 10(10) Omega m to P-el approximate to 10(6) Omega 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 approximate to 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. (c) 2006 Elsevier B.V. All rights reserved.