Organic solvent—driven activation

Deriving from the same physical principles as used in water-driven actuation, organic solvents have been proposed for the purpose of shifting the Tg to initiate actuation. Diffusion of solvents is dependent on the mutual

(A) Example of a water-actuated SMP foam applied to biomedical occlusion device

Figure 5.9 (A) Example of a water-actuated SMP foam applied to biomedical occlusion device. Utilizing an SMP with a Tg well above body temperature, recovery of up to 100 times volume expansion is achieved when brought into contact with the patient's blood. (B) The plasticizing effect of the moisture absorption on the SMP reduces the glass transition temperature allowing for the patient's own body temperature to induce actuation [109].

Schematic explanation of a uniaxially deformed cylindrical specimen's tendency toward buckling as a result of nonuniform radial actuation as modeled by Zhao et al

Figure 5.10 Schematic explanation of a uniaxially deformed cylindrical specimen's tendency toward buckling as a result of nonuniform radial actuation as modeled by Zhao et al. For a column fixed in tensile strain the dependence of actuation on diffusion of solvent molecules into the bulk material results in the elongated specimen experiencing a compressive load prior to complete recovery. The fixed interior of the specimen initially serves as a support;however, as the solvent penetrates deeper along the radius the compressive load increase while the support radius is reduced. Eventually the rigid core collapses resulting in kinks in the column prior to complete recovery. Displayed sequence: (A) as fabricated specimen, (B) programming step, (C) fixing step, (D) initial solvent diffusion into outermost layers with beginning of recovery, (E) partially recovered specimen buckling due to instability, (F) fully recovered specimen [110].

solubility of the two compounds, which will influence the activation rate [107]. In order for spontaneous diffusion of the solvent into the polymer, the Gibbs free energy of the components must be negative.

The consequences of relatively slow diffusion speed and geometric dependence was demonstrated by Zhao et al. in their work on buckling of prestrained SMP columns recovering in ethanol (Fig. 5.10) [110]. During immersion in the solvent the outer layers of the cylindrical columns actuated prior to the interior material in effect creating a compressive load on the column. After the ethanol reaches a critical penetration depth, the rigid and continuously diminishing interior-supporting structure becomes unstable and the column collapses.

 
Source
< Prev   CONTENTS   Source   Next >