Abstract
Soft polymer actuators have myriad applications and have therefore gained considerable attention in recent years. However, it remains challenging to prepare soft actuators with predefined shapes. Here, a bilayer polymer actuator with a (re)programmable shape is prepared from a microporous anisotropic polypropylene scaffold and a thin, pH-responsive liquid crystalline network (LCN) layer. The hydrogen bonds between dimerized benzoic acid derivatives in the LCN can be disrupted by an alkaline treatment, resulting in a pH-responsive LCN hydrogel layer. The pH-responsive actuation is governed by both the anisotropic mechanical properties of the scaffold and the cross-link density of the LCN hydrogel. Ca2+ ions can be used to chemically cross-link the actuator resulting in an initial programmed shape. The shape fixing can be reversed by removing the Ca2+ ions with an ethylenediaminetetraacetic acid (EDTA) solution. The shape fixing can be performed locally, resulting in pH-responsive actuators with three-dimensional initial configurations of choice.
Highlights
The pH-responsive anisotropic microporous PP− liquid crystalline network (LCN) composite soft actuators were made by filling a thin LCN layer in the top side of the PP scaffold according to the method described in our previous work.[29]
Previous work has shown that single-layer LCN actuators based on 10 wt % diacrylate II (10% Cl) swell mainly in the direction perpendicular to the molecular director upon converting the H-bonded carboxylic acids into the carboxylate, forming a water-absorbing polymer hydrogel salt.[17,30]
The 40% Cl polymer salt sample still has an isotropic ring near the lamellar spacing, while in the 10% Cl sample the diffraction patterns disappear. The transition to this “X-ray silent” morphology is linked to an amorphous morphology, in which most of the order is lost.[33]. These results show that the 40% Cl sample is able to partially maintain its order, while the 10% Cl loses the majority of its order when brought into the K+ salt form
Summary
Soft polymeric actuators have garnered much interest over the past decade due to their potential applications, including microfluidic devices, artificial muscles, and soft robotics.[1−5] Various stimuli-responsive polymeric systems were developed, which change shape when exposed to light,[6−10] electric or magnetic fields,[1,11] humidity,[12−15] pH,− or temperature.[19,20] More recently, examples of combined actuators and programmable shape-memory materials have been reported, enabling (re)programming and reuse of these materials.[7,15,21−23]Functionalized anisotropic polyolefins were used to fabricate soft actuators by either incorporation of responsive small organic molecules or a secondary responsive polymer network. Light-,24−27 humidity-,14 and temperature27-responsive anisotropic polyolefin actuators have been reported.[14,24,26,28] Expanding the types of stimuli and enabling shape reprogramming of the actuators may increase the range of applications for these types of soft polymer actuators. The hydrogen bonds (H-bonds) between benzoic acid moieties are disrupted and a hydrogel is formed This allows the LCN layer to swell primarily in the direction perpendicular to the molecular director, resulting in a pH-responsive shape deformation (Figure 1c). This shape deformation is highly dependent on the cross-link density.[17] Calcium ions can be used to render the benzoic acids irresponsive to changes in pH by binding two carboxylate salts
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