Near-infrared-triggered Meniscus Climbing of a Shape Memory Polymeric Object

Abstract

Near-infrared (NIR)-triggered artificial meniscus climbing was realized using a shape memory polymer (SMP) object. The SMP was coated with polypyrrole (PPy) overlayer, which can convert NIR light into heat. The flat PPy-coated SMP became curved upon NIR irradiation owing to the photothermal conversion of PPy. The curved PPy-coated SMP demonstrated meniscus climbing.

The larvae of Pyrrhalta nymphaeae, a leaf beetle species that live on land, can reach the shore by bending their elongated bodies to change the shape of the surrounding water surface when they fall into a pond because of wind or other factors.¹ This phenomenon is called meniscus climbing. Meniscus climbing occurs by changing the shape of water surrounding a curved object.^2–5 In this study, we realized artificial meniscus climbing due to the change in the shape of a shape memory polymer (SMP) from flat to curved using near-infrared (NIR) light as the external stimulus (Figure 1).

SMP can change its shape above its glass transition temperature (T_g) as it transitions into a rubbery state (Figure S1).^6–10 When the temperature is lower than T_g, the shape can be fixed. When it is heated above T_g again, the deformed SMP returns to its original shape. To change the shape of an SMP floating on the water, it is necessary to increase the temperature of the SMP by heating the water surface. However, heating the water requires much energy. In addition, it is difficult to heat only the SMP floating on the water surface. Therefore, we performed local heating with NIR light irradiation.¹¹ Photothermal conversion materials such as conjugated polymers, including polypyrrole (PPy),^12–19 gold nanoparticles,²⁰ and so on²¹ can efficiently convert NIR energy to heat. Coating of SMP with light-to-heat photothermal transducing material can realize site-selective and on-demand heating by NIR irradiation.

The SMP was prepared via the copolymerization of stearyl acrylate (SA), methyl acrylate (MA), and bismethylene acrylamide (MBAA) (Scheme 1).²² The SMP coated with PPy (PPy-coated SMP) was prepared via the chemical oxidation polymerization of pyrrole on the surface of the SMP. Heat generation and the shape change of the PPy-coated SMP upon NIR irradiation were studied. Furthermore, NIR was irradiated to the flat PPy-coated SMP floating on the water surface in a petri dish. The PPy-coated SMP was deformed from flat to curved shapes due to heat generated via the photothermal conversion of PPy. Then, the curved PPy-coated SMP moved toward the wall of the petri dish by meniscus climbing.

SMPs were prepared via conventional free-radical copolymerization of SA, MA, and MBAA (crosslinking agent). The molar ratio of SA/MA was kept constant at 2/8. In advance of the synthesis of SMP-PPy, SMPs were prepared with 1, 5, and 10 mol % MBAA concentrations to investigate the effect of the cross-linking density on the shape memory properties (The detailed synthesis method is provided in Supporting Information). Rod-shaped SMPs were prepared in a Teflon tube. The obtained SMPs were washed with ethanol and immersed in water at 40 °C to deform into a spiral shape. The temperature was returned to room temperature at 25 °C to fix their shapes (Figure S2). Consequently, SMPs were immersed in water at 40 °C again to confirm that they returned to their original rod shape. The SMP prepared with 1 mol % MBAA could not maintain its shape and adhered to the beaker in water at 40 °C, which did not exhibit the shape memory behavior. The SMP prepared with 10 mol % MBAA could not be deformed because of too high crosslinking density. The SMP prepared with 5 mol % of MBAA could be deformed in water at 40 °C, and the shape was fixed at room temperature. The SMP prepared with 5 mol % MBAA returned to its original rod shape when immersed in water at 40 °C again (Video S1). Therefore, the composition of MBAA was fixed at 5 mol % to prepare the SMP.

Chemical oxidative seeded polymerization of pyrrole (Py) was performed in the presence of the rod-shaped SMP to coat a PPy overlayer that is a photothermal conversion material. ATR-IR spectra for the SMP and the PPy-coated SMP using [Py] = 3.85 mM were measured (Figure S3). The IR spectra for the SMP and the PPy-coated SMP were almost the same. This is not surprising considering the thickness of PPy overlayer was 0.5–5 µm²³ and the loading amount of PPy was very low. To confirm PPy coating using another method, the surfaces of the SMP and the PPy-coated SMP were observed using SEM (Figure S4). The surface morphology changed after PPy coating, suggesting that PPy was coated on the SMP surface. To confirm the change in the hydrophilicity of the SMP surface due to PPy coating, the contact angles (θ) of the SMP and the PPy-coated SMP were measured (Figure S5). θ of bare SMP was 78.5°, and θ of the PPy-coated SMP varied in the range of 89.1°–95.2° independent of [Py]. θ of PPy-coated SMP was higher than that of bare SMP because the surface roughness increased owing to the PPy coating based on the Cassie–Baxter model.^24,25

Differential scanning calorimetry was measured to determine the transition temperature of the SMP and the PPy-coated SMP (Figure S6a). Osada et al.²² reported the order-disorder transition (melting) temperature of the SMP prepared with [SA]/[MA]/[MBAA] = 20/79/1 mol % was 34 °C. In this study, the melting temperatures of the SMP prepared with [SA]/[MA]/[MBAA] = 18/77/5 mol % and the PPy-coated SMP prepared with [Py] = 3.85 mM were 37.9 °C and 41.6 °C, respectively. The glass transition temperature (T_g) of PPy is approximately 100 °C.^26–28 However, the T_g of PPy in the PPy-coated SMP could not be detected owing to the small amount of PPy coating. Thermogravimetric analysis (TGA) results showed that the weight losses of the SMP and the PPy-coated SMP at 500 °C were 97.0% and 96.6%, respectively (Figure S6b). The decomposition of PPy starts at above 230 °C.²⁹ Due to the small amount of PPy coating, the difference between the SMP and the PPy-coated SMP could not be detected with TGA.

The surface temperature of the PPy-coated SMP prepared with [Py] = 38.5 mM on a glass substrate (Figure S7) reached 250 °C after 1-min NIR light (800 nm, 2.0 W/cm²) irradiation (Figure S8). Smoke was observed upon NIR irradiation, indicating that the sample was decomposed because of local heating. The surface temperature of bare SMP irradiated by NIR was constant. The TGA results for the PPy-coated SMP indicated 2.14% weight loss at 250 °C (Figure S6b), verifying the decomposition upon NIR irradiation even though a small amount.³⁰ The photothermal conversion effect of PPy was too high at [Py] = 38.5 mM; thus, [Py] was reduced to prepare the PPy-coated SMP.

The PPy-coated SMP was prepared by changing [Py] from 0.967 mM to 9.67 mM ([Py] is the Py concentration for the chemical oxidative polymerization). The surface temperature of each PPy-coated SMP was monitored after 1 min of NIR irradiation (Figure 2). No change was observed in the appearance of PPy-coated SMPs upon NIR irradiation. The surface temperature upon NIR irradiation increased with increasing [Py]. Immediately after NIR irradiation, the SMP coated with [Py] ≥ 3.85 mM could be deformed by hand, but not with [Py] = 0.967 mM. The surface temperature increased to 108 °C at [Py] = 0.967 mM; however, the temperature inside the sample may be below T_g (= 45.6 °C). The surface temperature of the PPy-coated SMP at [Py] ≥ 5.77 mM increased to over 190 °C. Samples may decompose at above 190 °C based on the TGA results. Thus, 3.85 mM [Py] was found to be optimal to deform the sample without decomposition. Therefore, the PPy-coated SMP prepared at [Py] = 3.85 mM was used in the subsequent study.

A curved PPy-coated SMP with a radius of 2.06 mm was prepared from the rod-shaped SMP. The rod-shaped SMP was cut into a curved shape, and PPy was polymerized on the surface of the curved SMP. The curved PPy-coated SMP piece with [Py] = 3.85 mM was heated to deform into a flat plate with a length of 8.7 mm, a width of 3.1 mm, and a thickness of 1.3 mm. The surface temperatures of plate-shaped PPy-coated SMP samples were monitored after 1-min NIR irradiation using varying NIR intensities from 0.02 W/cm² to 2.0 W/cm² (Figure S9). The temperature increased with increasing irradiation intensity. At 0.02 W/cm², the surface temperature was 21.4 °C, and the deformation of the PPy-coated SMP was not observed. At 0.12 W/cm², the surface temperature was 24.1 °C, and the plate-shaped PPy-coated SMP was curved a little. However, the shape was not fully recovered to the original curved shape. At 0.5 W/cm², the temperature was 41.5 °C, and the sample returned to its original curved shape completely. Based on these results, the NIR light intensity must be at least 0.5 W/cm².

The PPy-coated SMP deformed into a plate shape was located on the water surface, and its NIR-responsive behavior was observed (Figure 3, Video S2). The PPy-coated SMP was exposed to NIR light irradiation, and the irradiation was stopped when it finished curving. The PPy-coated SMP returned to its original curved shape, which moved to the wall of the petri dish. The curved PPy-coated SMP moved from 1.28 cm away from the wall. The PPy-coated SMP deformed from a flat shape to its original curved shape upon NIR irradiation owing to the photothermal conversion of PPy. Furthermore, the curved PPy-coated SMP demonstrated meniscus climbing.

The shape of the PPy-coated SMP is related to meniscus climbing. Therefore, we investigated the effects of the degree of curvature (D_c) of the PPy-coated SMP on acceleration (a) and travel distance (l) (Figure 4). D_c is the reciprocal of the radius of the circle (r) when the curved PPy-coated SMP plate is considered an arc (Figure 5a). The PPy-coated SMP was heated and bent by hand to vary D_c. a was determined from the video taken starting from 1 cm from the wall until the sample was on the wall (Figure 5b). l was defined as the distance from the position where the PPy-coated SMP plate started to move to the wall after being placed on the water surface. a decreased with increasing D_c. We focused on the horizontal force because the vertical gravity and the buoyancy of the PPy-coated SMP plate floating on the water surface were balanced. The direction approaching the wall was defined as the +x direction. The force applied to the object due to shape change of water surface applied in the positive and negative directions of x were defined as F_R (>0) and F_L (<0), respectively (Figure 5c and d). The components of the horizontal force to water were F_Rx and F_Lx. The components of the vertical force to water were F_Ry and F_Ly. Then, the horizontal force applied to the polymer plate was F_Rx + F_Lx. When the plate was flat, |F_Lx| > |F_Rx|, so F_Rx + F_Lx < 0, and the plate moved in the negative direction of x, i.e., away from the wall. On the other hand, when the plate bent, |F_Rx| > |F_Lx|, so F_Rx + F_Lx > 0, and the plate moved in the positive direction of x, i.e., toward the wall.² When D_c increased, a decreased because the difference between forces approaching and moving away from the wall decreased with increasing D_c. Therefore, a can be controlled by the plate shape. On the other hand, l was almost constant, independent of D_c. Generally, the spread of the meniscus in the x-axis direction depends on the wettability of the wall. The l value indicates that the meniscus is affected up to ca. 2.3 cm from the wall of the glass. Meniscus climbing may also be affected by parameters other than D_c, such as the meniscus shape and the hydrophilicity of the plate surface.

In conclusion, meniscus climbing of a PPy-coated SMP was demonstrated upon irradiation with NIR light on a water surface within 2.3 cm from a wall. a decreased with increasing D_c. The PPy-coated SMP is expected to be applied to energy-saving transport using sunlight containing the NIR wavelength range as the external stimulus.

Acknowledgment

This research was partially supported by KAKENHI grants (21H02005, 21K19931, 21H05027, and 21H05535) from the Japan Society for the Promotion of Science (JSPS), JSPS Bilateral Joint Research Projects (JPJSBP120203509), the Cooperative Research Program of Network Joint Research Center for Materials and Devices (20214044), the International Collaborative Research Program of Institute for Chemical Research, Kyoto University (2022-121), and MEXT Promotion of Distinctive Joint Research Center Program (JPMXP 0621467946).

Supporting Information is available on https://doi.org/10.1246/cl.220541.

References