Initiation of shape-memory effect by inductive heating of magnetic nanoparticles in thermoplastic polymers

Abstract

In shape-memory polymers, changes in shape are mostly induced by heating, and exceeding a specific switching temperature, T(switch). If polymers cannot be warmed up by heat transfer using a hot liquid or gaseous medium, noncontact triggering will be required. In this article, the magnetically induced shape-memory effect of composites from magnetic nanoparticles and thermoplastic shape-memory polymers is introduced. A polyetherurethane (TFX) and a biodegradable multiblock copolymer (PDC) with poly(p-dioxanone) as hard segment and poly(epsilon-caprolactone) as soft segment were investigated as matrix component. Nanoparticles consisting of an iron(III)oxide core in a silica matrix could be processed into both polymers. A homogeneous particle distribution in TFX could be shown. Compounds have suitable elastic and thermal properties for the shape-memory functionalization. Temporary shapes of TFX compounds were obtained by elongating at increased temperature and subsequent cooling under constant stress. Cold-drawing of PDC compounds at 25 degrees C resulted in temporary fixation of the mechanical deformation by 50-60%. The shape-memory effect of both composite systems could be induced by inductive heating in an alternating magnetic field (f = 258 kHz; H = 30 kA x m(-1)). The maximum temperatures achievable by inductive heating in a specific magnetic field depend on sample geometry and nanoparticle content. Shape recovery rates of composites resulting from magnetic triggering are comparable to those obtained by increasing the environmental temperature.

Fig. 1.

Chemical structures of thermoplastic shape-memory polymers. (a) Polyetherurethane TFX (28), which is synthesized from methylene bis(p-cyclohexyl isocyanate) (H₁₂ MDI), 1,4-butanediol (BD), and poly(tetramethylene glycol) (PTMG). (b) Multiblock copolymer PDC. PPDO, poly(p-dioxanone); TMDI, 2,2(4),4-trimethylhexanediisocyanate; PCL, poly(ε-caprolactone).

Fig. 2.

Transmission electron microscopy pictures of TFX100 with 10 wt % particle content. (Scale bars: a, 2 μm; b, 200 nm.)

Fig. 3.

Heating curves for samples with different S/V ratios. f = 258 kHz, H = 12.6 kA·m⁻¹. – · – · – · –, S/V = 1.4; ——, S/V = 1.5; - - - -, S/V = 1.6; – · · – · · –, S/V = 8.1.

Fig. 4.

Temperatures achieved for TFX composites when exposed to magnetic field depending on particle content and magnetic field strength. f = 258 kHz, sample size length = 30.0 mm; thickness = 0.3 mm; width = 4.0 mm; T_max is the maximum temperature achieved by inductive heating. —■—, 5 wt % particle content; --•--, 7.5 wt % particle content; ··▴··, 10 wt % particle content.

Fig. 5.

Series of photographs showing the macroscopic shape-memory effect of TFX100 composite with 10 wt % particle content. The permanent shape is a plane stripe of composite material, and the temporary shape is a corkscrew-like spiral. The pictures show the transition from temporary to permanent shape in a magnetic field of f = 258 kHz and H = 30 kA·m⁻¹ in an inductor. Movie 1 shows the shape change.