Bicontinuous vitrimer heterogels with wide-span switchable stiffness-gated iontronic coordination

Abstract

Currently, it remains challenging to balance intrinsic stiffness with programmability in most vitrimers. Simultaneously, coordinating materials with gel-like iontronic properties for intrinsic ion transmission while maintaining vitrimer programmable features remains underexplored. Here, we introduce a phase-engineering strategy to fabricate bicontinuous vitrimer heterogel (VHG) materials. Such VHGs exhibited high mechanical strength, with an elastic modulus of up to 116 MPa, a high strain performance exceeding 1000%, and a switchable stiffness ratio surpassing 5 × 10³. Moreover, highly programmable reprocessing and shape memory morphing were realized owing to the ion liquid-enhanced VHG network reconfiguration. Derived from the ion transmission pathway in the ILgel, which responded to the wide-span switchable mechanics, the VHG iontronics had a unique bidirectional stiffness-gated piezoresistivity, coordinating both positive and negative piezoresistive properties. Our findings indicate that the VHG system can act as a foundational material in various promising applications, including smart sensors, soft machines, and bioelectronics.

Fig. 1.. Design and structural feature of bicontinuous vitrimer heterogels.

(A) Chemical structures of reacted precursor components within VHGs. (B) VHGs have the bicontinuous structure of vitrimer (VFP) and ILgel (IFP) framework phases owing to the orthogonal polymerization-induced phase separation. (C) Nano CT image demonstrating the bicontinuous phase structure of VHG₅. Two distinct VFP (orange) and IFP (blue) domains seamlessly interpenetrated, with each phase domain forming a continuous path. (D) SEM image demonstrating the continuous morphologies of the VFP network in VHG following the removal of the ILgel phase. (E) AFM nanomechanical mapping revealed the bicontinuous morphology of VHGs and exhibited distinct mechanical properties between VFP and IFP. (F) Surface modulus and surface height variation of VHG₅.

Fig. 2.. High mechanical performance and wide-span switchable stiffness.

(A and B) Tensile stress-strain curves and elastic modulus (E_stiff) of the brittle PCL vitrimer, the soft ILgel, and VHGs with different VFP and IFP components at 20°C, respectively. (C) VHG_7.5 with a thickness of 1 mm can easily withstood a tensile load of 10 kg, even when undergoing temporary thermoplastic deformation. (D and E) Tensile stress-strain curves and elastic modulus (E_soft) at 80°C for the different VHGs. (F) Switchable stiffness ratios of the PCL vitrimer, the ILgel, and the different VHGs, respectively. (G) Stable transitions between high and low storage modulus (G′) of VHG₅ at 20° and 80°C. (H) Tensile stress-strain curves of VHG₅ under cooling conditions (−20°, −40°, and −60°C). (I) Analysis comparing the switchable stiffness ratio to strain_max in VHGs with that of existing polymer materials with switchable mechanics, including vitrimers, shape memory polymers and gels, organohydrogels, stimuli-responsive gels, and ILgels. (J) Comparative analysis of VHGs and other typical ILgels with different structures (i.e., single/double network, nanocomposite, phase-separated network, and bicontinuous network), focusing on the relationship between E_stiff and strain_max.

Fig. 3.. Ion liquid–enhanced dynamic bicontinuous network reconfiguration.

(A) Schematic illustration of the enhancement of both transesterification and transcarbamoylation processes by ion liquid in VHG systems. (B) Consecutive reprocessing of VHG₅ with a dynamic bicontinuous reconfiguration. (C) Stress relaxation behaviors of the PCL vitrimer and VHGs with different VFP and IFP components at 130°C. (D) Stress relaxation and Arrhenius analysis of VHG₅ as the temperature increased from 110° to 140°C. (E) E_stiff and shape reconfiguration ratios of VHG₅ during consecutive reprocessing, with processing strains progressively increasing from 100 to 500%. (F) In VHG systems, various ILs contained [NTf₂] as the anion enabled the enhancement of bicontinuous network reconfiguration. Scale bar, 1 cm.

Fig. 4.. Programmable shape memory morphing property.

(A) Nano CT images exhibiting variations in the bicontinuous structure of VHG₅ during the programmable shape memory process. (B) Highly programmable shape memory morphings were based on two specific techniques: Miura origami and square-patterned kirigami. Scale bar, 2 cm.

Fig. 5.. Bidirectional stiffness-gated iontronic piezoresistivity.

(A) Schematic illustration of VHG iontronic piezoresistive sensor (4 × 4) arrays that had the capability of the bidirectional stiffness-gated piezoresistivity. Scale bar, 1 cm. (B) When the VHG iontronic sensor units were pressed, the corresponding feedback was reflected on the monitor. (C) Ionic conductivity of the ILgel iontronics and VHG_7.5 iontronics ranged from −20° to 100°C. (D) Negative piezoresistive signal (ΔI/I_o) responses of the conventional ILgel iontronics under loading (loading pressure, 50 kPa) and unloading cycles, respectively. (E) Distinct negative and positive piezoresistive signal responses of the VHG_7.5 iontronics. (F) Three-dimensional finite element analysis of VHG solid mechanics, demonstrating the variations of the ion transport pathway during the bidirectional stiffness-gated piezoresistivity process. (G) Negative and positive piezoresistive signal responses of the VHG_7.5 iontronics against different pressures, respectively. (H) Distinct from the unidirectional negative/positive features of any existing piezoresistive systems, VHG iontronic systems exhibited a bidirectional stiffness-gated piezoresistivity.