Infrared actuation in aligned polymer-nanotube composites

Rubber composites containing multiwalled carbon nanotubes have been irradiated with near-infrared light to study their reversible photomechanical actuation response. We demonstrate that the actuation is reproducible across differing polymer systems. The response is directly related to the degree of uniaxial alignment of the nanotubes in the matrix, contracting the samples along the alignment axis. The actuation stroke depends on the specific polymer being tested; however, the general response is universal for all composites tested. We conduct a detailed study of tube alignment induced by stress and propose a model for the reversible actuation behavior based on the orientational averaging of the local response. The single phenomenological parameter of this model describes the response of an individual tube to adsorption of low-energy photons; its experimentally determined value may suggest some ideas about such a response.

Figure 1

(Color online) Scheme of the apparatus; see text for detail.

Figure 2

Spectral data of the light source (left axis, arbitrary units), and the normalized absorption of the PDMS0.3 composite and the control pristine PDMS elastomer (right axis).

Figure 3

(Color online) (a) The x-ray scattering image showing key reflections; the outer ring $\left(3.4\mathrm{\AA }\right)$ is the signal from the multiwall nanotubes. The inner ring $\left(7.5\mathrm{\AA }\right)$ represents the PDMS mesh size; see Sec. 3B. The arrow shows the direction of the uniaxial aligning strain. (b) The typical azimuthal intensity variation $I\left(\beta \right)$ at a scattering angle of $3.4\mathrm{\AA }$ reflection. The data are fitted by the model Ref. 29.

Figure 4

(Color online) Young modulus $Y$ for PDMS nanocomposites at increasing MWCNT loading. The arrow points at the value for control PDMS rubber.

Figure 5

(Color online) The change in the orientational order parameter $S_d$ of nanotubes in PDMS7 composite, as a function of imposed uniaxial strain, obtained from the x-ray scattering data (엯, data points; dashed line is a guide to the eye). Solid line shows the affine rigid-rod model prediction.

Figure 6

The scheme of an affine incompressible extension, changing the orientation of an inflexible rod embedded in the medium.

Figure 7

(Color online) The response of a $1\mathrm{wt}\%$ nanocomposite PDMS1 to IR radiation at different levels of prestrain $\epsilon$. Stress is measured at fixed sample length (different prestrain curves labeled on the plot).

Figure 8

The speed of actuation response, illustrated by plotting the actuation stress in PDMS3 nanocomposite, $\Delta \sigma$ in kPa, as a function of time for different prestrain values (labeled on the plot).

Figure 9

(Color online) The normalized stress response plotted alongside the normalized change in temperature, as functions of time (PDMS3, prestrain $\epsilon =20\%$); see text for discussion.

Figure 10

(Color online) The magnitude (in kPa) of exerted actuation stress (the height of steps in Fig. 7, $\Delta {\sigma }_{\max }$), as a function of prestrain. Different PDMS composites are labeled on the plot by their wt  % value. The right $y$ axis shows the corresponding actuation stroke: the change in natural length $L_0\left(\mathrm{IR}\right)$.

Figure 11

(Color online) The magnitude of the actuation stroke at $\epsilon =40\%$ as a function of filler concentration $n$. The maximum of the response at $\sim 2\mathrm{wt}\%$ is evident. The single square symbol gives the value for $3\mathrm{wt}\%$ carbon black filler in PDMS.

Figure 12

Summary of IR actuation stroke from LCE composites (PLCE0.15 and MLCE0.2) and a SIS0.1 composite, as a function of applied pre-strain. Note that MLCE data do not have a crossover at $\epsilon \sim 10\%$, since the tubes are aligned there at preparation. For comparison, the similar data for two PDMS samples, 0.5 and 0.02, are shown by dashed lines.

Figure 13

The scheme illustrating how the distortion (kinking) of an individual tube, lying at an angle $\theta$ to the macroscopic alignment axis, projects on the $z$ axis to contribute to the average uniaxial strain, Eq. (10).

Figure 14

(Color online) The result of the affine theoretical model, Eq. (10); the dashed line shows the linear approximation at small prestrain. The nanotube contraction factor is chosen to be $\Delta =0.8$, as suggested by the crossover strain value ${\epsilon }^{*}\sim 0.1$.