Reconfigurable and orthogonal stiffness‐structure patterning of dynamically crosslinked amphigels

Patterning diversified properties and surface structure of polymer materials are of great importance toward their potential in biology, optics, and electronics. However, achieving both the patternability of stiffness and microstructure in a reconfigurable manner remains challenging. Here, we prepare amphigels crosslinked by dynamic disulfide bonds, which can be reversibly swollen by immiscible water or liquid paraffin. In the paraffingel form, the materials exhibited a high modulus of 130 MPa due to densified hydrogen bonds. Whereas swollen by water, the modulus fell over two orders of magnitude owing to the destruction of the hydrogen bonds. Via regionalized swelling of the solvents, well‐controlled and rewritable soft/stiff mechanical patterns can be created. On the other hand, the dynamic exchange of the disulfide crosslinking enables mechanophoto patterning to fabricate sophisticated macrogeometries and microstructures. The reconfigurable stiffness‐structure patterning can be manipulated orthogonally, which will create more application opportunities beyond conventional hydrogels and organogels.

A straightforward strategy for pattern stiffness is masked photopolymerization.The crosslinking density can be controlled by light exposure time, thus manipulating the modulus. 30Although simple and efficient, it is likely to get into troubles of mechanical failure on the interfaces.Besides, it will cause undesired deformation of the material entirety owing to the differential in crosslinking densities.Alternatively, materials that can be postcured by light were synthesized.][33][34][35] However, most secondary curable polymers suffer from instability of the residual unreacted components or functional groups, and the patterned materials may become uniform due to some unavoidable dark reactions.To solve this issue, innovative material systems and fabrication methods have been developed.For instance, localized oxidation, which can be directly combined with lithographic fabrications, was developed to pattern stiff regions on a thioether-contained elastomer. 18Apart from the complicated chemical design of these materials, patterning diversified physical features on one material requires further investigations to achieve multifunctionalities.
In comparison with static features, reconfiguration of surface structure and material properties is also desired, since it offers a customizable and highly tunable fashion.To achieve pattern reconfiguration, unexpected strategies have been invented based on dynamically crosslinked polymer networks.Mechanophoto patterning is an elegant example. 36Elastomers containing photoreversible dynamic bonds were spatially irradiated to enable localized plastic deformation upon stretching.After the release of the external force, microstructures emerged due to the light-defined nonuniform strain recovery.For swollen gel systems enabling photo-induced bond exchange, structure patterning can be realized in the absence of mechanical stretching when solvent freezing or migration is applied as the deformation mechanism. 37,38In addition to surface structures, spatialized crystallinity distribution and the resulting modulus patterning of polymer sheets were achieved via isomerization of network structure from, for instance, a grafting topology to a brush-like topology. 39,40he above discussion clarifies the strategies and problems in patterning a single surface property.In a more adaptive manner, the different properties (e.g., stiffness and microstructures) are expected to be orthogonally patterned, which provides more freedom to manipulate materials in diverse situations.For this purpose, the major challenge is to decouple the patterning processes and avoid mutual influence.
Here, we present a dynamically crosslinked amphigel enabling orthogonal reconfiguration of both stiffness patterning and surface structures.The amphigel can interchangeably and spatially swell in water or liquid paraffin to form stiffness patterns due to the large modulus difference between the hydrogel and paraffingel states.On the other hand, the amphigel is mechanically tough in both solvents, which ensures that the material can withstand large deformation during the mechanophoto patterning process.As such, the stiffness and the microstructure can be patterned independently.

| Synthesis of 2,2′-dithiodiethanol dimethacrylate (MASS)
The disulfide crosslinker MASS was synthesized according to the previous literature. 41Typically, 2,2′dithiodiethanol (2.0 g) and triethylamine (5.4 mL) were dissolved in dichloromethane (30 mL).After cooling to 10 °C, methacryloyl chloride (3.14 mL) was added gradually.The reaction proceeded at room temperature for 24 h.The product was washed with deionized water and saturated sodium chloride aqueous.Afterward, magnesium sulfate was used to absorb the residual water.Following filtration, the liquid was collected, and the final product was obtained after volatilization at 25 °C and 1 atm for 48 h.The final yield was 72.5%.

| Preparation of crosslinked polymers
MASS (50 mg), DA (1.0 g), and benzoyl peroxide (10 mg) were mixed and stirred at 50 °C for 30 min.After cooling to room temperature, AAc was added to form the precursor at different AAc:DA mass ratios (0%, 20%, 40%, 60%, and 80%).The precursor was transferred to a glass mold and polymerized in a 70 °C oven.After 24 h, a crosslinked polymer film was obtained.Besides, the HDDA crosslinked polymer film was synthesized by replacing MASS with HDDA.Unless otherwise noted, the AAc:DA mass ratio was fixed at 60% for the following procedure.

| Transformation between the liquid paraffin-swollen paraffingels and waterswollen hydrogels
A polymer sheet was swollen in liquid paraffin for 24 h to form a paraffingel.To switch the solvent, the paraffingel was placed in a 70 °C oven for 48 h to remove the liquid paraffin.The obtained sheet was then swollen in water for 24 h to acquire a hydrogel, which can be dried in a 70 °C oven for 48 h to start another cycle.

| Patterning of stiffness
The process is shown in Section 3. Specifically, the acquired sheets were firstly swollen in liquid paraffin for 24 h using polyimide tapes as physical masks providing specific patterns.Afterward, the sheet was covered with a complementary polyimide mask and soaked in water for another 6 h to achieve the patterning.For better visualization, the paraffingel parts were dyed in red by Sudan Ⅲ (0.1 g/L), and the hydrogel parts were dyed in blue by Gentian Violet (0.1 g/L).

| Pattern erasing
A patterned gel was soaked in n-hexadecane and then acetic acid, respectively, for 24 h to remove Sudan III and Gentian Violet.Afterward, the gel was heated at 70 °C for 48 h.

| Patterning of conductivity
The circuit pattern is obtained by replacing the water in the stiffness patterning process with PBS.

| Light-induced surface imprinting
Silicon wafers were used as the imprinting templates.A polymer sheet was swollen in I819 liquid paraffin solution (2 mg/mL) for 24 h.The paraffingel was pressed on a silicon wafer and exposed to ultraviolet (UV) light (Intelliray 600 by Uvitron) for give times to achieve replication of microstructures from wafer template.
Unless otherwise noted, the exposure time was fixed at 15 min.

| Replication
I819 (1 wt%) was dissolved in PEGDA 1000, and the obtained precursor was coated and cured on the surfacestructured paraffingel after UV exposure for 30 s.By peeling off the two layers, surface microstructures were replicated on the PEGDA layer.Variable surface structures can be constructed via the same procedure upon the deformation of the amphigel template.

| Photo-induced stress relaxation
Paraffingel strips were folded in half and exposed to UV light at various times, and the samples in the control group were fixed for the same time without UV exposure.The shape retention was calculated as (180°− φ)/ 180°× 100%, where the φ was the angle of the sample after programming as shown in the Supporting Information.

| Mechanophoto patterning
The paraffingel containing I819 was stretched to 100% strain and exposed to UV light for 3 min under a photomask.Then, the photomask was removed, and another 3 min exposure was applied to eliminate residual stress.Finally, the gel was annealed in a 70 °C oven for half an hour.

| Photo-induced macroscopic shape-changing
The paraffingel with I819 was folded into threedimensional (3D) structures and exposed to UV light for 5 min.

| Characterization
Tensile tests were performed on a universal tensile testing machine (Zwick/Roell Z005) under a stretching speed of 20 mm/min at room temperature.The dimension of all the samples was 20.0 mm × 2.0 mm × 1.0 mm.The transmittance was measured by a UV spectrophotometer (UV-2550PC).The two-dimensional smallangle X-ray scattering analysis (2D SAXS) was carried out on a Xeuss system (Xenocs SA) equipped with a multilayer focused Cu Kα X-ray source (GeniX3D Cu ULD by Xenocs SA) and a semiconductor detector (Pilatus 100 K; DECTRIS).The wavelength of the X-ray radiation was 1.5148 Å.The distance from the sample to the detector was 570 mm.The phase diagram was obtained by atom force microscopy (AFM, Jupiter XR by OXFORD).The morphology of the Janus film was acquired by scanning electron microscope (SU-8010) after lyophilization.The conductivity of the PBS hydrogel and the paraffingel were measured by four-point probe method using a digital multimeter (34465A by Keysight), in which a DC power supply (3610DS by Maisheng) was used to generate voltage. 42Surface microstructures were characterized by a white light interferometer (NT9100 by Veeco).Thermogravimetric curves were obtained using a thermogravimetric analyzer (PE Pyris 1).A ramping rate of 20 °C/min was adopted, followed by an isothermal program at 200 °C for 1 h.The swelling ratio was defined as (Q t − Q 0 )/Q t × 100%, where Q 0 refers to the mass of the dried polymer sheet and Q t refers to the mass of the swollen polymer film.

| Molecular dynamics (MD) simulation
MD simulations were carried out using Materials Studio 2016 with COMPASS II force field.The polymer model was built using 6 DA units and 12 AAc units, as shown in the Supporting Information.Liquid paraffin and water models were built, respectively.The amorphous cell suite was applied to construct polymer-liquid paraffin model with 100 polymers and 250 liquid paraffin molecules and a polymer-water model with 20 polymers and 10,000 water molecules, respectively.The size of the model box was 8.0 nm × 8.0 nm × 8.0 nm.After this, the Forcite suite was used to run 10,000 steps of geometry optimization to eliminate the undesirable contacts.Then, the polymer systems were relaxed adequately using the Forcite anneal module with five cycles from 300 to 600 K and back to 300 K.The dynamics simulation under NVT ensemble was performed for 100 ps with a time step of 1 fs at 300 K using an Andersen thermostat.Finally, MD was performed under NPT ensemble at 300 K for 1400 ps using a Nose thermostat and Berendsen barostat.For all the MD simulations, the electrostatic and van der Waals interactions were calculated by Ewald-and Atom-based summation methods, respectively.The hydrogen bond (HB) was defined to be less than 3.5 Å for O-H distance and more than 150°for O-H-O angles.The number of HBs of the whole system, polymer-polymer, and solute-solute was calculated, respectively.Thus, the number of HBs of the polymer-solute was obtained by the following equation: HBs (polymersolute) = HBs (system) − HBs (polymer-polymer) − HBs (solute-solute).

| RESULTS AND DISCUSSION
The material was synthesized by thermal polymerization of DA, AAc, and disulfide crosslinker MASS, as shown in Figure 1A.The obtained polymer network can be swollen by immiscible liquid paraffin (n-hexadecane, t m = 18 °C) or water to form either a paraffingel or a hydrogel.At the paraffingel state, liquid paraffin would trigger the aggregation of the poly(acrylic acid) (PAAc) segments, which offer dense hydrogen bonding to achieve high stiffness (Figure 1B).The phase separation would be reversal when the network is swollen by water.The internal HBs would be destroyed and replaced by polymer-water HBs, which leads to a greatly softened hydrogel.In this case, the mechanical properties of the material can be manipulated via liquid selections.In another aspect, the surface microstructures of the acquired gels can be further regulated through mechanophoto patterning.The specific procedure includes the spatial UV exposure to the gels during stretching.Owing to the disulfide bond exchange, 36 the local network is relaxed, which causes the formation of surface microstructures after removing the external force.Accordingly, the stiffness and the surface structures can be orthogonally patterned as schematically shown in Figure 1C.
In the paraffingel state, the mass ratio of AAc to DA is essential for mechanical properties.As shown in Figure 2A, the paraffingel is soft and brittle without AAc.It gradually becomes stiff and tough with increasing AAc contents.When the ratio rises to 60 wt%, the tensile modulus reaches 130 MPa with a toughness of 27.5 MJ/m 3 (Supporting Information S1: Table S1).Further increase of the AAc ratio can elevate the modulus while reducing the toughness.Therefore, 60 wt% was chosen for the following explorations.It is speculated that the excellent mechanical property originated from the nanoscale phase separation.PAAc segments would form clusters, which provide densified HBs for efficient stress dissipation.2D SAXS was employed to investigate the phase separation.A diffraction ring appears on the intensity map as shown in Figure 2B, which confirms the existence of the ordered structures.Combined with the scattering intensity curves shown in Figure 2C, the feature size is 5.7 nm (q = 1.1 nm −1 ) when the ratio of AAc to DA is 60 wt%. 43At this ratio, the paraffingel is highly transparent as shown in Figure 2D, which confirms that the scale of the microphase separation is much smaller than the wavelength of visible light.During stretching, the feature size decreases to 4.0 nm, which indicates that the deformation of aggregated PAAc domains is essential for mechanical strengthening (Supporting Information S1: Figure S1).Besides, the MD simulation was also conducted.The result in Figure 2E is consistent with the above characterization and clearly shows the enriched HBs in the aggregated PAAc domains.For further investigation, the phase diagram was acquired by AFM as shown in Figure 2F.The aggregated PAAc domain is the hard area presenting a relatively low phase angle (blue areas), which directly confirms the phase separation in our paraffingel. 44Meanwhile, the mechanical hysteresis in Supporting Information S1: Figure S2 indicates the excellent energy dissipation capacity of the paraffingel owing to the rearrangement of the HBs.In addition, the stretched paraffingel can fully recover to its original shape because of the network elasticity (Supporting Information S1: Figure S3).
The phase separation is inverted when replacing the liquid paraffin with water.As indicated in Supporting Information S1: Figure S4, hydrogels with different AAc/ DA mass ratios are phase-separated and show good mechanical properties owing to the aggregation of poly (dodecyl acrylate).Specifically, the feature size for a 60% AAc/DA mass ratio hydrogel is 5.2 nm (q = 1.2 nm −1 ) (Figure 2G).The hydrogel is also of high transparency in visible wavelengths (Supporting Information S1: Figure S5).At this hydrogel state, the long alkyl chains of the DA component will aggregate due to the hydrophobic interaction, and the PAAc segments will be swollen by water.The internal HBs are destroyed by the solvent (water), thus the modulus reduces.As shown in Figure 2H, the tensile modulus and toughness of the hydrogel respectively fall to 0.9 MPa and 0.45 MJ/m 3 , both showing a two-order decrease compared to the paraffingel.Nevertheless, the hydrogel is still mechanically robust and tough compared to the 0% AAc/DA mass ratio paraffingel, which does not have the HBs to act as the toughening mechanism.The MD simulation was also conducted (Supporting Information S1: Figure S6).The counted polymer-polymer HBs decreased to 0.04%, which confirms that water destroys the internal HBs.
By switching the solvents, the modulus of the amphigel can be repeatedly adjusted between 130 and 0.9 MPa (Figure 3A).Notably, the repeated paraffingel-hydrogel switching did not show an obvious impact on the good mechanical stability of the material.The amphigel sheet can lift a 1 kg weight under both two states with paraffingel dyed in red and hydrogel dyed in blue (Figure 3B).Owing to the immiscibility of liquid paraffin and water, stiffness patterns can be created on the amphigel taking advantage of physical masks.The specific process is shown in Figure 3C.When swollen in liquid paraffin, the hydrogel region is covered by a polyimide mask and vice versa.Consequently, the stiffness pattern is achieved.Figure 3D shows belt patterns with various widths on the amphigel, providing a minimum feature size of 200 μm.In situ measurement using a Shore hardness tester clearly shows a significant difference in the stiffness at various regions.Anisotropic mechanical properties can be easily endowed upon specific pattern designs (Figure 3E).The loading weight is 0.4 kg.Furthermore, the patterns can be erased upon solvent evaporation by heating, and the material can be rewritten in another fashion (Figure 3F).Meanwhile, the dyes that are only used for visualization are washed with n-hexadecane and acetic acid.It is noted that water can certainly be used to soften some hydrophilic polymers (e.g., hydrogels after drying).However, water patterning is commonly unfeasible since water diffusion would largely compromise the patterning accuracy.Moreover, the solvent absorption would result in undesired deformation due to the inevitable volume expansion.For the presented amphigel in contrast, the water diffusion is restrained by its immiscibility with the paraffin-swollen regions, and the solvent contents of the paraffingel and the hydrogel were both around 26% (Supporting Information S1: Figure S7) avoiding uncontrollable shape transformation.The specific swelling kinetics for both paraffingel and hydrogel is shown in Supporting Information S1: Figure S8.Forgoing the assistance of the physical mask, the dried material can stay at the interface of liquid paraffin and water, which creates a Janus amphigel (Supporting Information S1: Figure S9).Overall, the controllable distribution of the two solvents can make the amphigel with sophisticated mechanical properties.
In addition to the mechanical properties, the electroconductivity of the amphigel can also be patterned via replacing the water with conductive PBS.The conductivity of PBS-swollen hydrogel is 3.73 × 10 −3 S/m, while the paraffingel cannot be detected in the present testing mode because of its extremely low conductivity.As shown in Figure 3G, a circuit is created by patterning the conductive saline around the nonconductive paraffingel.The LED lamps ⅰ and ⅱ are connected in a series circuit, and the LED ⅲ is in an open circuit.
Hereafter, we establish microstructure fabrication of the amphigels by mechanophoto patterning, 36 which can further be combined with the oil-water stiffness patterning orthogonally.We first show the photodynamic feature of our design by developing a light-induced imprinting process.Specifically, the surface microstructure of the paraffingel can be imprinted by replicating from the silicon wafer via the light-induced disulfide bond exchange (Figure 4A).The height of the imprinted structure increased with the exposure time, which reached an equilibrium after 15 min as shown in Figure 4B.A patterned array of square holes with a feature size of 10 μm can be fabricated (Figure 4C), showing high resolution and good fidelity of the imprinting method.Owing to the independent manipulations, an amphigel can be further patterned with liquid paraffin and water after micromanufacturing (Figure 4D).After the orthogonal patterning, the surface microstructures undergo nonuniform deformation during stretching.The stiff paraffingel regions maintain the feature size of the microstructures, and the soft hydrogel regions transform along the stretching direction with deformed microstructures.As such, adaptive shifting of microstructures can be realized on a single amphigel.Due to the low adhesion feature provided by the paraffin and the long alkyl chain (Supporting Information S1: Figure S10), the amphigel template can mold another material with heterogeneous surface microstructures as shown in Figure 4D.Furthermore, we employ mechanophoto patterning to fabricate the surface microstructures.First, the photo-induced stress relaxation behavior was studied (Supporting Information S1: Figure S11).The procedure was conducted as Supporting Information S1: Figure S12.The shape retention of a folded paraffingel increases gradually with the exposure time and reaches a plateau value of 80% after exposure for 15 min.In contrast, the unexposed sample can be fully recovered.For further comparison, a nondynamic paraffingel was fabricated by using HDDA, which could not permanently alter into the bent shape under UV exposure as shown in Supporting Information S1: Figure S13.Taking advantage of local stress relaxation by using a photomask, the surface structure of a belt array was constructed due to the recovery of nonuniformity between the exposed and unexposed regions after removing the external force (Figure 5).The mechanical properties do not show obvious change after UV exposure (Supporting Information S1: Figure S14).Going through the mechanophoto patterning process once again along the vertical direction, the generated microstructures were accumulated to the ones fabricated in the former process.The sequential manipulation and the accumulative effect enable sophisticated and diversified patterns using a single photomask.After solvent patterning subsequently, the regionalized stiffness was independently added to the microstructures, reflecting the merit of the amphigel.Finally, the 3D geometry of the acquired amphigel can also be altered owing to the solid-state plasticity. 45asically, the planar gel sheet is folded into a flowerlike shape.During UV irradiation, the disulfide bond exchange will cause the stress relaxation of the polymer network, thus retaining the 3D shape.Through these procedures, constructions of both surface microstructures and macroscopic geometries can be achieved for an amphigel.

| CONCLUSION
In summary, dynamically crosslinked amphigels enabling reconfigurable and orthogonal stiffnessstructure patterning were synthesized.The phase separation induced by liquid paraffin densified the internal HBs, which led to high modulus (130 MPa) and toughness (27.5 MJ/m 3 ).On the contrary, the polymerpolymer HBs were replaced by polymer-water HBs in a hydrogel form of the material, resulting in a two-order decrease of the modulus.Due to the immiscibility of the two solvents, the stiffness can be patterned by spatial-selective swelling of liquid paraffin or water.Meanwhile, mechanophoto patterning of surface structure was feasible by triggering the dynamic bond exchange upon light exposure.The stiffness and surface-structure patterning can be manipulated orthogonally, realizing reconfigurable and hieratical patterns in a single material system.Inspired by this patterning approach, further incorporation of functional liquids is believed to spatially switch more diverse physical properties (e.g., conductivity, light transparency, and adhesion) beyond the stiffness to extend the versatility of the amphigel.

F I G U R E 1
Preparation and patterning of amphigels.(A) Precursors to synthesize the amphigel.(B) Mechanism illustrating the invertible phase separation.(C) Schematic of the microstructure-stiffness orthogonal patterning.

F I G U R E 2
Characterization of the amphigels swollen by either liquid paraffin or water.(A) Mechanical properties of the paraffingels with different acrylic acid/dodecyl acrylate (AAc/DA) mass ratios.(B) Two-dimensional small-angle X-ray scattering analysis (2D SAXS) images of the paraffingels to illustrate the phase separation status.(C) One-dimensional (1D) scattering intensity curves.(D) Transmittance of the paraffingel with 60 wt% AAc/DA ratio.(E) Molecular dynamics simulation of the paraffingel.(F) Phase diagram characterized by atom force microscopy (AFM) for the paraffingel with 60 wt% AAc/DA ratio.(G) 1D scattering intensity curve of the hydrogel at 60 wt% AAc/DA ratio with an inserted 2D SAXS image.(H) Mechanical properties of the amphigel at 60 wt% AAc/DA ratio swollen by either liquid paraffin or water.

F
I G U R E 3 Patterning of amphigels by solvent selections.(A) Repeated modulus switching of an amphigel.(B) Photographs showing the mechanical strength of an amphigel in liquid paraffin and water-swollen states.The inset images indicate the gel dimensions.(C) The illustration of stiffness patterning procedure.(D) The belt-like stiffness pattern of an amphigel.For visualization, the paraffingel and the hydrogel locations were dyed in red and blue, respectively.(E) A patterned amphigel with anisotropic mechanical properties.(F) Erasing and rewriting the oil-water stiffness patterns on an amphigel.(G) An amphigel circuit with patterned conductivity using phosphate-buffered saline.

F I G U R E 4
Orthogonal patterning of the amphigels.(A) Illustration of the imprinting process.(B) Increase in feature height during the imprinting process.(C) Paraffingel with a pattern array of square holes with a feature size of 10 μm.(D) Adaptive and hieratical pattern transformation of an orthogonally patterned polymer film upon stretching.