Liquid crystal elastomers (LCEs) with intrinsic anisotropic strains are reversible shape‐memory polymers of interest in sensor, actuator, and soft robotics applications. Rapid gelation of LCEs is required to fix molecular ordering within the elastomer network, which is essential for directed shape transformation. A highly efficient photo‐cross‐linking chemistry, based on two‐step oxygen‐mediated thiol–acrylate click reactions, allows for nearly instant gelation of the main‐chain LCE network upon exposure to UV light. Molecular orientation from the pre‐aligned liquid crystal oligomers can be faithfully transferred to the LCE films, allowing for preprogrammed shape morphing from two to three dimensions by origami‐ (folding‐only) and kirigami‐like (folding with cutting) mechanisms. The new LCE chemistry also enables widely tunable physical properties, including nematic‐to‐ isotropic phase‐transition temperatures (TN‐I), glassy transition temperatures (Tg), and mechanical strains, without disrupting the LC ordering. 相似文献
Thermal reprogrammability is essential for new‐generation large dry soft actuators, but the realization sacrifices the favored actuation performance. The contradiction between thermal reprogrammability and stability hampers efforts to design high‐performance soft actuators to be robust and thermally adaptable. Now, a strategy has been developed that relies on repeatedly switching on/off thermal reprogrammability in liquid‐crystalline elastomer (LCE) actuators to resolve this problem. By post‐synthesis swelling, a latent siloxane exchange reaction can be induced in the common siloxane LCEs (switching on), enabling reprogramming into on‐demand 3D‐shaped actuators; by switching off the dynamic network by heating, actuation stability is guaranteed even at high temperature (180 °C). Using partially black‐ink‐patterned LCEs, selectively switching off reprogrammability allows integration of completely different actuation modes in one monolithic actuator for more delicate and elaborate tasks. 相似文献
A novel heat‐curable silicone rubber (MCSR/Si‐PAMAM) was prepared by using siloxane polyamidoamine (Si‐PAMAM) dendrimers as cross‐linkers and polysiloxane containing γ‐chloropropyl groups as gums. The chemical cross‐linking occurs through the reaction between Si‐PAMAM dendrimers and polysiloxane containing γ‐chloropropyl groups. The effect of various amounts of cross‐linkers on mechanical properties of MCSR/Si‐PAMAM was discussed in this paper. MCSR/Si‐PAMAM exhibits favorable mechanical properties with a tensile strength of 10.06 MPa and a tear strength of 47.9 kN/m when the molar ratio r of [N‐H]/[CH2CH2CH2Cl] is 1:1. These excellent mechanical properties can be attributed to the formation of concentrative cross‐linking from Si‐PAMAM dendrimers in the cross‐linking networks, along with the introduction of Si–O–Si units in the internal structure of dendrimers. The introduction of Si–O–Si units reduces the steric hindrance of molecular structure, which facilitates the N–H bonds in the interior layers of dendrimers to react with γ‐chloropropyl groups. In addition, thermogravimetric analysis results indicate that MCSR/Si‐PAMAM is thermally stable even at high temperatures in a nitrogen atmosphere. Differential scanning calorimetry analysis reveals that the glass transition peak of MCSR/Si‐PAMAM is not identified in the temperature range −150 to −30°C, only a melting endothermic peak at −40°C. 相似文献
Summary: The reaction of triphosgene with poly(ethylene glycol) yielded poly(ethylene glycol) dichloroformate. This difunctional cross‐linker was allowed to react with poly(ε‐caprolactone) bearing carbanionic sites obtained by activation with lithium diisopropylamide. The reaction resulted in the cross‐linking of poly(ε‐caprolactone) chains by poly(ethylene glycol) segments, giving copolymer networks that gel in both organic and aqueous media.
Schematic of the PCL‐g‐PEG copolymers synthesized here. 相似文献
Liquid crystalline elastomers (LCEs) have been actively investigated as stimuli‐controlled actuators and soft robots. The basis of these applications is the ability of LCEs to undergo a reversible shape change upon a liquid crystalline (LC)‐isotropic phase transition. Herein, we report the synthesis of a novel LCE based on a side‐chain liquid crystalline polymer (SCLCP). In contrast to known LCEs, this LCE exhibits a striking anomalous shape change. Subjecting a mechanically stretched monodomain strip to LC‐disorder phase transition, both the length and width of the strip contract in isotropic phase, and both elongate in LC phase. This thermally induced behaviour is the result of a subtle interplay between the relaxation of polymer main chain oriented along the stretching direction and the disordering of side‐group mesogens oriented perpendicularly to the stretching direction. This finding points out potential design of LCEs of this peculiar type and possible applications to exploit. 相似文献
Summary: Conjugated poly(p‐phenylene ethynylene) networks with interesting optoelectronic properties were synthesized by the palladium‐catalyzed polycondensation of 2,5‐diiodo‐4‐[(2‐ethylhexyl)oxy]methoxybenzene, and 1,4‐diethynyl‐2,5‐bis‐(octyloxy)benzene, with 1,2,4‐tribromobenzene as cross‐linker. The cross‐linker concentration was varied and materials with different cross‐link densities were prepared. The materials were processed into films by simultaneous polymerization and shaping. An alternative approach is to synthesize these cross‐linked polymers in the form of spherical particles, which can be processed from dispersions.
Schematic representation of the cross‐linking process. 相似文献
Exploiting the tremendous potential of the recently discovered reversible bidirectional shape‐memory effect (rbSME) for biomedical applications requires switching temperatures in the physiological range. The recent strategy is based on the reduction of the melting temperature range (ΔTm) of the actuating oligo(ε‐caprolactone) (OCL) domains in copolymer networks from OCL and n‐butyl acrylate (BA), where the reversible effect can be adjusted to the human body temperature. In addition, it is investigated whether an rbSME in the temperature range close or even above Tm,offset (end of the melting transition) can be obtained. Two series of networks having mixtures of OCLs reveal broad ΔTms from 2 °C to 50 °C and from −10 °C to 37 °C, respectively. In cyclic, thermomechanical experiments the rbSME can be tailored to display pronounced actuation in a temperature interval between 20 °C and 37 °C. In this way, the application spectrum of the rbSME can be extended to biomedical applications.
Thermal reprogrammability is essential for new-generation large dry soft actuators, but the realization sacrifices the favored actuation performance. The contradiction between thermal reprogrammability and stability hampers efforts to design high-performance soft actuators to be robust and thermally adaptable. Now, a strategy has been developed that relies on repeatedly switching on/off thermal reprogrammability in liquid-crystalline elastomer (LCE) actuators to resolve this problem. By post-synthesis swelling, a latent siloxane exchange reaction can be induced in the common siloxane LCEs (switching on), enabling reprogramming into on-demand 3D-shaped actuators; by switching off the dynamic network by heating, actuation stability is guaranteed even at high temperature (180 °C). Using partially black-ink-patterned LCEs, selectively switching off reprogrammability allows integration of completely different actuation modes in one monolithic actuator for more delicate and elaborate tasks. 相似文献