Application of Multilayered Blend Films as Soft, Stretchable, Self‐Adhesive, and Self‐Healing Absorption‐Dominant EMI Shielding and Microwave Absorber

Solution‐processable organic conductor–supramolecular elastomer blends are emerging materials for intrinsically stretchable and autonomously self‐healing organic electronics. Herein, the feasibility of a heterogenous multiphase polymer blend is demonstrated for soft, ultraflexible, self‐adhesive, and self‐healing multilayered film structures for electromagnetic interference (EMI) shielding and microwave absorber (MWA) purposes. The developed soft multilayered films achieve a 5–40 fold improvement in the thickness of the MWA compared to the current state‐of‐the‐art. The thickness normalized reflection loss (RL) is up to 65.26 dB mm−1 with 8.5 GHz bandwidth at 18–26.5 GHz. The maximum thickness normalized EMI shielding effectiveness peaks at up to 175 dB mm−1. The EMI shielding and MWA properties are maintainable up to 150% tensile strain with only a small decrease in the overall attenuation and RL. Furthermore, the developed films are capable of fully autonomously self‐healing and achieve a tough adhesion in temperatures of −30–145 °C, and underwater with maximum single‐lap shear adhesion strength of ≈481.5 kPa to soft thermoplastic polyurethane films. Thus, the developed multilayered films can be utilized for absorption‐dominant EMI shielding or MWA purposes as stretchable coatings. The developed materials also show considerable potential for emerging damage and puncture‐resistant organic soft electronics with autonomous material‐level self‐healing.


Introduction
Thin, lightweight, soft, and ultraflexible material structures are needed for electromagnetic interference (EMI) shielding and DOI: 10.1002/admi.202300960microwave absorber (MWA) purposes in order to meet the requirements of electromagnetic (EM) compatibility, reduce radiative coupling, and support the multimodal sensing schemes in soft and flexible electronics and display technologies, in robotics or medical electronic devices. [1,2]For sensors and display technologies capacitive sensing is favored over other electrical transduction mechanisms as they enable multi-touch operations, and have lower power consumption, near-linear responses, high precision, low drift, and non-transient behavior for sensors. [3][6] Thus, robustness and immunity from environmental noises must be sought in the abovementioned applications by means of device or component design in order to achieve the required performance level.
To reduce EM radiation and coupling to and from other nearby electronic components and devices, MWAs would be needed to avoid back reflection.[9] On the other hand, absorption-dominant EMI shielding, or MWAs, are possibly better alternatives, but their design can be significantly more complex, and achieving good dissipation by absorption with thin structures is more challenging.[15] However, typical MWAs are too bulky (i.e., several millimeters thick) to be applicable for soft and wearable electronics.Furthermore, many material structures lack several important features benefiting these applications, such as Young's modulus (E) matching with the softer materials or tissues (in the 1-600 kPa range), stretchability, self-adhesiveness, and autonomous self-healing. [7,16,17]Moreover, the EMI shielding and MWA material should also provide strong adhesion to other functional materials and soft substrates for ease of fabrication, robust integration, and installation.
Thus, it would be important to develop thin, intrinsically soft, stretchable, self-adhesive, and autonomously self-healing MWAs, for the 6-300 GHz frequency range, that can be fabricated at low cost, are applicable to large areas and compatible with printed electronics manufacturing methods.Such material or multimaterial structures could be applied as a coating to any 3D surface or design of the stretchable electronic device while obtaining good adhesion with the underlying surface and conforming the structure precisely regardless of the form factor.
[26][27][28][29][30][31] Although self-healing supramolecular materials are very promising, for example, due to their extraordinary stretchability, selfadhesiveness, and self-healing characteristics, [32][33][34][35] the level of properties required for EMWs absorption is not often possible to achieve at 1-30 GHz range with a thickness of less than 1 mm.For example, in the case of non-patterned conductive materials, this can be due to initial surface reflection resulting from improper impedance matching at the air interface, [14,21,22] and with resistive gradients introduced to the structure the overall thickness increases.Thus, achieving efficient absorption and attenuation of EMWs with soft polymer composites and their multilayered or multimaterial assemblies often requires also blending of a relatively high content of electric and/or magnetic solid filler particles to the polymer matrices.However, the blending of immobile solid filler particles influences the viscoelastic characteristics of the composites and thus may compromise the other important material features, such as stretchability, toughness, notch-insensitivity, and self-healability.For soft electronics, another challenge with elastomers and their composites is that their toughness remarkably decreases when, e.g., notched, [36,37] and without self-healing functionality these materials are extremely prone to rupture under dynamic and static mechanical loading conditions.These conditions are crucial for soft and wearable electronic devices to endure in order to achieve reliable operation for a longer term.
In this work, for the first time, we show the application of organic conductor-supramolecular elastomer blend films as soft, stretchable, self-adhesive, and autonomously self-healing absorption-dominant EMI shielding and MWA with excellent performance. [36,38]The intrinsically soft and stretchable multilayered films were optimized for the purpose by also considering the orientation and number of the electrically anisotropic film layers in order to achieve excellent performance.The resulting absorption-dominant EMI SE reached ≈175 dB mm −1 which was maintainable at ≈122 dB mm −1 for ≈150% uniaxial tensile strain.The corresponding thickness normalized reflection loss (RL) was up to 65.26 dB mm −1 with full 8.5 GHz absorption bandwidth (ΔB) at K-band frequencies (18-26.5 GHz) important for 5G telecommunication technologies.The unprecedented combination of intrinsic softness and elasticity, high toughness, stretchable adhesion, excellent microwave absorption characteristics, and attenuation of EMWs at low film thicknesses enable absorption-dominant EMI shielding coatings for complex 3D surfaces, and soft electronics devices and circuits in the future.

Material Design and Film Preparation
Multilayered film structures were developed by utilizing previously developed heterogenous multicomponent elastomer blends with optimized composition (Figure S1, Supporting Information).The electrical anisotropy and orientation of the individual films were further considered and controlled (Figure S2, Supporting Information) in order to achieve good EMI shielding and MWA properties with the multilayered films at the Kband frequency range (18-26.5 GHz).The overall thickness of the structure was controlled only by the number of individual films to maintain a desirable morphology and structure in the blend films (as discussed previously in ref. [36]).
The electrical and mechanical properties of the prepared multilayer films was controlled by the composition of the polymer blend to achieve electrical conductivity () of 0.06-130 S m −1 (Figure S1a, Supporting Information), and a lowE in the range of 158-484 kPa (without further modification of the supramolecular elastomer), [38] high stretchability and toughness up to ≈2500% and ≈50 MJ m −3 , respectively, (Figure S1b, Supporting Information) The E of the films can also be further decreased by several orders of magnitude in order to match the modulus of softer materials or tissues (heart, brain, etc.) by controlling, e.g., the chain length of the hydroxy-terminated poly(dimethylsiloxane). [38] On the other hand, the maximum achievable E of the films is dominated by the choice of electrically insulative supramolecular elastomer as shown by the experimental results and theoretical values derived on the basis of the equivalent Takayanagi model. [36]he multilayer film structures were made from self-bonding individual films with segregated structure and electrical anisotropy (Figure 1).The first step was to prepare the poly (3,4ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) to form PEDOT-rich nanofibrils by coalescing the cores of the PEDOT:PSS microgel particles with the use of dimethyl sulfoxide (DMSO) at or above the 16 vol.%critical volume concentration. [36]This was done by directly mixing the aqueous PEDOT:PSS with the predefined amount of DMSO.The PEDOT-rich nanofibrils can then be directly blended with the other elastomer components [36] by the use of amphiphilic surfactant, such as Triton TM X-100.The use of Triton TM X-100 allows dispersion between hydrophobic and hydrophilic chain segments which then enables the formation of a homogenous mixture at the initial stage. [36]The choice of the starting materials and synthesis method has been previously explained in our previous works. [36,38]he mixing time of the multicomponent blend (1) was then precisely controlled before the solution was cast to a substrate to evaporate excessive water from the mixture and to achieve desirable segregation in the film during the solidification.41] The heterogeneity and co-continuity between the existing phases can be further controlled with the multiphase conductor composition. [36]To achieve a desirable co-continuous morphology with large domain sizes (Figure S3a,b, Supporting Information) several compositional and processing parameters must be carefully controlled as discussed in our previous work (such as the weight of the PEDOT:PSS/DMSO solution, mixing time of the third solution, and thickness of the solution-casted films). [36]he resulting films are non-porous, although the surface roughness can be relatively high depending on the composition.After casting and solidification (4-5), individual films with segregated structures can be self-bonded together to form a multilayered film structure or a larger film with multiple layers (as shown in Figure S4, Supporting Information).When these films are self-bonded, they cannot be physically separated by any means as the polymer phases become entangled and interlaced at the molecular scale.The anisotropy and formation of non-conductive and conductive regions inside the multilayer film structures can be further controlled with the assembly of the individual films (Figure S2, Supporting Information).
Typically, only one side of the film becomes highly conductive due to vertical phase separation.Whether the  after the solidification is the best on the surface at the air or substrate interface would be dependent on multiple factors, including substrate material, blend composition, processing conditions, and thick-ness of the deposited films. [36]As illustrated in Figure 1, on the conductive side of the film, the PEDOT-rich domains and hard phase-rich domains form a dual-rich region on the film structure in the lateral direction, [36] while the soft phase tends to occupy the other side of the film.Thus, giving rise to a double-layered film with anisotropic , where the  is the worst along the thickness direction.However, by adjusting the composition (e.g., by further increasing the ratio of conductive to insulating phase) the conductivity along the thickness direction can be considerably increased as a three-layered film can be formed.

Self-Adhesiveness and Interfacial Bonding Strength
The adhesion properties of the developed films are important for coating materials conforming 3D shapes, or in soft and stretchable electronics having multilayered device or component designs with solid-state surface mountable components.Thus, a single-lap shear test was made by double-sided deposition of the films to the target materials (Figure 2a; Figures S5a-c, S6a-c, and S7, Supporting Information).Because the outer surfaces of the multilayered film structures were identical, and with better adhesion in all cases (Figures S2 and S6a, Supporting Information), the film structure made of four films (denoted as 2PEC7-F4) was used for all testing purposes.
The adhesion strengths of the films were recorded on various inorganic and organic substrates having different E and surface chemistries to demonstrate the wide applicability and elastic deformability of the films (Figure 2b,c; Figures S5a,b and S6c, Supporting Information).The single-lap shear adhesion strength (in kPa) was measured as force (N) applied per surface area of the film (cm 2 ) plotted against applied strain calculated from the displacement, while considering the change in cross-sectional area as a function of strain.For nonporous materials obtained values for adhesion strength were 46.5-100.2kPa with 50%s −1 strain rate at the ambient conditions (Figure 2b; Figure S6c, Support-ing Information) while for porous nylon fabric it remarkably increased up to 480.3 kPa (Figure 3c).In latter case, the multilayer film embeds into the porous fabric leading to a very strong interfacial bond between the materials due to the large surface area of interaction and formation of polymer chain entanglements around the nylon threads which both increase effective adhesion strength. [42]Due to good wetting by the adhesive, the present dipole-dipole interactions (O─H and O─B bonds) can rapidly form large number of adhesive interactions with any of the target materials leading to quick saturation of the interfacial adhesion due to high segmental mobility of the polymer chains and high free volume of the network. [36,38]The blend film composition achieves a good balance between the dissipative properties and cohesion of the network which are important for the adhesion properties under shear strain.More importantly, high molecular weight polymer chains may enable the formation of physi-cal entanglements with some other polymer substrates, without breaking the covalent bonds, while the chemisorption of boron oxide nanoparticles can also form a bridging interface between the polymer chains of the adhesive and substrate, which both can contribute to increase of the interfacial adhesion.
No cohesive debonding failures occurred as residues were not found in any of the target materials excluding the nylon (Figure S5c, Supporting Information).The detachment occurs on either side of the multilayer film at the material interfaces.At strain rates equal to or less than 100% s −1 , the adhesive failures were found to be due to adhesive creep when the long polymer chains of the soft phase become easily un-entangled by the reversible bond disassociation of hydrogen bonds and Si─O:B dative bonding.Thus, at low strain rates (<100% s −1 ), the elastomeric network can easily flow as the strain energy is effectively dissipated by the reversible bond breaking.As shown, the adhesive creep was prevented by the shear stiffening behavior (Figure 2b; Figure S6c, Supporting Information) as the films became both stiffer and tougher with the increasing rate of deformation. [36]Then, the failure of adhesion occurred either due to peeling off the film from either side of the target material caused by the adhesion anisotropy, or due to the cohesion failure when the elastomeric network ruptures.
The single-lap shear adhesion strength was also measured as a function of both strain rate and temperature with commercial TPU film (Platilon U 4021) (Figure 2b; Figure S6c, Supporting Information).The single-lap shear adhesion strength remarkably increased from 64.1 to 410.4 kPa when the strain rate and the temperature increased from 1.67 to 150% s −1 and 20 to 145 °C, respectively.Thus, the mechanical loading conditions and effect of environmental factors, including temperature and humidity, should be taken into an account when the adhesion of the films needs to be considered as these will influence the overall elasticity, strength, and toughness the material. [36,38]urthermore, the adhesion strength was measured after mechanical failure and self-healing of the film (Figure 2c; Figure S7, Supporting Information).After mechanically breaking the adhesion, the fractured surfaces of the films were manually aligned for self-healing.Remarkably, the films fully recovered (≈100%) the original bonding strength (67.3-479.9kPa) for all tested target materials in ≈60 s (Figure 2c).The bonding strength did not increase over time indicating that not only the bonding strength was recovered, but also the mechanical properties of the film.More importantly, the adhesion strength did not decrease after a longer period in dynamic testing and similar strength could be achieved after ten consecutively tested failures (Figure S7, Supporting Information) due to efficient self-healing of the material. [36,38]However, it should be noted that the interdiffusion mechanism occurring over the time on the scale of weeks, months, or longer might change the adhesion strength not only because the toughness of the material changes, but because the water molecules are absorbed. [36,38]Due to high water diffusion, the polymer blend would require an additional layer made of different material to provide the hermetic barrier.However, minimizing the effect of humidity would require additional research also under different conditions and aging of the films.
Maintaining good adhesion in wet conditions is critical for the soft electronics, when interfaced with the skin or bioelectronics integrated inside the human body, due to exposure to moisture, humidity, sweat, and body fluids which can degrade the original dry adhesion of the films and even lead to delamination of bonded layers.Thus, the wet adhesion of the film was also tested underwater.As shown by the simplistic demonstration, the multilayer films can also obtain a strong interfacial bonding under water with a metal block (Figure 3d; Movie S1, Supporting Information).In the demonstration, the applied pressure (≈48.4 kPa) was lower than the elastic modulus (in the range of 150-500 kPa) of the elastomer film structure when using a contact force of 15 N with adhesive contact area of 3.1 cm 2 and multilayer film thickness of 1.5 mm.The metal block weighting of ≈0.50 kg was eas-ily lifted off without the failure of the adhesion during the testing that lasted for ≈30 s.In this demonstration, the soft film structure was capable of lifting over 2300 times of its own weight underwater and with only 10 s of physical contact to the metal block.It should be noted that the adhesion of the film is pressure sensitive due to nature of the supramolecular elastomer, thus the bonding strength can decrease over time after removal of the contact force with the dynamic relaxation of the elastomeric network.The time scale on which the adhesion can be maintained can be increased, e.g., by increasing the contact force and time on which the force is applied, while swelling of the film should be also considered upon longer immersion times.
The excellent adhesion and self-healability of the films were further demonstrated in cold environment in mid-January 2024 at Oulu, Finland (Figure 2e; Figure S8, Supporting Information).2PEC7-F2 film structure with area of ≈400 × 500 mm (width × length) was made first by self-bonding multiple individual films together via self-healing (Figure S4, Supporting Information).Then, the film was adhered outside at −25 °C and in ≈70% relative humidity (at 65°03′ 25.6″ N, 25°27′ 55.1″ E) to the left side of the aluminum hood of the Volvo XC60 (Figure S8, Supporting Information).The adhesion of the film was activated in the cold by gently pressing the film against the surface of the car.During the first day of the application, the film adhesion was maintained while driving 42.9 and 38.6 km at highway E8 and short distances in urban areas in Oulu with average speeds of 73.5 and 89.1 km h −1 , respectively, when the outside temperature was −23 to −27 °C and relative humidity (RH) was 67% to 73%.During and after driving two times, there was negligible temperature change in the surface where the film was adhered.On the second day, the only significant change in the film was the reduced number of air pockets and disappearance of the clear interfaces between the films after removal of snow from the surface of the car.After the application of the film, there were no dimensional changes nor weakening of the adhesion when the car was also driven from outside to a warm environment (>20 °C, RH ≈20%), and then back to the outside (−16 to −30 °C, RH 65-80%).
As shown, the flexibility and mobility of polymer chains can be maintained in extreme temperatures due to low glass transition temperature of poly(dimethylsiloxane)-backbone [38] which not only allows wetting the surface to build adhesion but also selfhealing of the film structure resulting in a strong and seamless bonding between the individual films over time.

Anisotropic Swelling
Because the structural and morphological stability of the material are crucial characteristics due to exposure to humidity over longer period, the swelling kinetics of the multilayered film structures were studied underwater with varied insulating to conducting phase ratios (Figure 3a-c; Figure S9a-d, Supporting Information).For the swelling tests, the film structures were prepared as previously (Figure S3a, Supporting Information) and then aged for at least 20 days in ambient conditions.This was done to neglect the effect of aging for the swelling behavior.Previously it has been shown that aging can result to disappearance of the vertical phase separation and homogenization especially with large insulative to conducting phase ratios. [37]he presence of hydrophobic and hydrophilic chain segments with variable swelling rates results in both compositional and film structural dependent anisotropic swelling behavior in the film structures (Figure 3c; Figures S9b-d and S10, and Movies S2-S4, Supporting Information).Although all chain segments are known to participate in the swelling, anisotropic swelling behavior of the film structures was mainly dominated by the highly hydroscopic nature of PSS-chain segments (which form the shell of the coalesced PEDOT-rich cores in the said nanofibrils).This relates to deprotonation of sulfonate groups [43] and generates selective swelling in the films along the thickness direction with the vertically phase-separated IPN and multilayered structure.As illustrated (Figure 3a), the PEDOT-rich nanofibrils tend to swell along the thickness of the film (up to 350% along z-axis).Although the swelling along thickness direction results in a compression along the x-and y-axis (up to −60%; Figure 3c), the overall volume change was still positive.
The reversible condensation/hydrolysis equilibrium increases the supramolecular interactions of Si:O─B dative bonding which then can generate a complex internal stress build-up along with the swelling of the segments. [36][46] The shape-change was found to be controllable to a certain degree by the composition of the multiphase conductor and formed multilayered structure before the aging.
As shown (Figure 3b), at the initial stages of swelling the strain from the build-up internal stress may concentrate on the outer edges of the multilayered films.This results in a small deflection that increases over time and becomes more pronounced when the ratio of insulating to conducting phase increases in the multiphase conductor (Movies S2-S4, Supporting Information).The multilayered films may deform from the initial "─"shape (when t 0 = 0 s) to "L" or "V"-shape (at t 2-3 ≈ 3450 s), and with higher insulating to conducting phase ratios may eventually change shape from "V"-shape to "I"-shape (at t 3 ≈4000 s) (Movie S3, Supporting Information).The deflection increases until the swelling equilibrium is reached ≈t 4 >4600 s.Interestingly, it was also observed that a physical contact with the surface of water through swelling can increase the characteristic deformation rate underwater (Figure S10, Supporting Information).However, further investigation would be required to fully understand this phenomenon.Because the water-triggered actuation is driven by diffusion of water rather than a snap-through instabilities (capable of a rapid self-actuation) the characteristic deformation is in most cases slow.45] The hydration/dehydration process is reversible with the film structures, but dehydration is a rather slow process without external intervention in ambient conditions. [38]n the basis of the adhesion tests and swelling behavior, the interfacial adhesion and cohesion mainly relates to the dipoledipole interactions present in the supramolecular elastomer (Figure 3d).The hydroscopic nature of the PSS-chain segments, resulting swelling along the thickness of the film, and water sensitivity of the dipole-dipole interactions can be assumed to be responsible for achieving the initial physical contact with the target material under water.This then prevents the formation of a thin water layer in between the film and the target material that would interfere with the adhesion.Thus, when the films are kept under water for a longer time, the swelling of the film can negatively affect the bonding strength to the target material as the strength and toughness of the films will decrease over time until swelling equilibrium is achieved. [36,38]However, in the used demonstration for the underwater adhesion, (Figure 2d; Movie S1, Supporting Information) this can be completely neglected due to relatively short immersion times to water.Because the chain mobility of the supramolecular elastomer is influenced by the water molecules and applied force, the material can form time-dependent interfacial adhesion to the target material in extremely short periods of time as shown.

Pristine PEDOT:PSS Films
The microwave absorption characteristics are largely dependent on the achieved impedance matching at the air interface with the desired frequency bandwidth.It is well known that intrinsically highly conductive polymers, such as PEDOT:PSS, [43] poly(pyrrole), or polyaniline, have large R coefficients at the microwave frequencies.To further verify this, the scattering parameters of flexible and water-stable PEDOT-rich nanofibril films were measured with and without electrically isolative coatings made of polyimide (PI) to calculate the EMI shielding and microwave absorption characteristics (Figure 4a-c; Figure S11a,b, Supporting Information).Thicknesses of the freestanding films were measured to be ≈25 μm (Figure 4a).The  were then ≈1100 ± 300 S cm −1 (expressed as mean ± STD; n = 3).
On the basis of the , the skin depth ( = √(1/fμ r μ 0 , where f is the frequency, μ r is the relative permeability, and μ 0 is the permeability of free space) for the PEDOT-rich nanofibrils films were calculated to be in the range of ≈9.4-11.3μm at the K-band frequency range (18-26.5 GHz).For good conductors ( >>wɛ, where w is the angular frequency and ɛ is the permittivity), the intrinsic impedance () can be written as  = √((jwμ)/(+jwɛ)) ≈ √(jwμ/), where μ is the permeability. [47]The theoretical calculated attenuation by absorption (−8.688d/,where d is the thickness of the shield) was 19.2-23.1 dB and reflection (20 log( 0 /4), where  0 is the impedance of free space) was 37.1-38.7 dB on the basis of measured  (Figure 4c; Note S1, Supporting Information).The waveguide measurements, based on scattering parameters, for total EMI shielding effectiveness (SE) show similar values (maximum of 55.1 dB) that was expected on the basis of  (56.3-61.8dB) for the film (Figure 4c).The total EMI SE further increases to a maximum value of ≈66.9 dB at 23.2 GHz frequency with multiple films.The corresponding values for maximum specific shielding effectiveness (SSE) of the films reached ≈22 000 dB cm 2 g −1 which is typical value for highly conductive polymers.As shown, the main shielding mechanism of PEDOTrich nanofibril films is by surface reflection (Figure 4b) as also indicated by the low impedance ratio (expressed as │Z in /Z 0 │) without electrically isolative coating (Figure S11b, Supporting Information).The use of PI coating leads to a better impedance match as the film can achieve RLof 28.4 dB at 21.01 GHz frequency with ΔB of 5.9 GHz (Figures S11b and S12, Supporting Information).As shown, the other side of the conductive polymer film is still highly reflective in the reverse measurement direction due to absence of PI coating (Figure S11b, Supporting Information).

Multilayered Films Made of Organic Conductor-Elastomer Blends
With the consideration of the discussion above, it was supposed that any type of vertically phase separated polymer blend made from organic conductor and elastomer having electrical anisotropy could achieve a similar interaction with the EMWs at the millimeter wavelengths.Thus, the EMI shielding and MWA properties of the developed multilayer films made of the polymer blend with the optimized composition (Figure 5) were analyzed by measuring the scattering parameters at the K-band (18-26.5 GHz) with the vector network analyzer without (Figure 6a-e) and with uniaxial tensile strain (Figure 7a-f).The performance of the films was also compared to the state-of-the-art polymer composite-based MWAs.
Both EMI shielding and microwave absorption properties were measured (Figure 6a,b; Figure S13, Supporting Information) for the aged multilayered films (Figure 5) similarly as when testing the swelling behavior.The EMI shielding measurements were done for comparative purposes as existing self-healing MWAs are scarce (please see the performance comparison in the Supporting Information).As shown by the contour plots (Figure 6a), the electrical anisotropy present in the vertically phase-separated multiphase conductor films was visible in the scattering parameters measured in both forward (S 11 , S 21 ) and reverse measurement directions (S 22 , S 12 ) for the different film structures.The multilayered film structures 2PEC7-F2 and 2PEC7-F3 achieved excellent impedance matching conditions in both measurement directions with wide ΔB ≈ 5.25-8.5 GHz and at extremely low film thicknesses of 500-750 μm (Figure 6a), while the achieved RL in the forward direction were up to −31.83 to −32.83 dB@ 21.01-21.04GHz (Table S1, Supporting Information).
The autonomous self-healability of the films were further demonstrated with the tensile measurements, and measurement of scattering parameters that were then used for calculating the EMI shielding and MWA performance.The self-healing efficiencies calculated for toughness were in the range of 32.3-115.1% when measured after ≈120 s after the manual alignment of the cut surfaces in various environments (Figure S14a, Supporting Information).The multiphase blend films reversibly soften in wet conditions, thus this also influences the measured tensile properties after self-healing in comparison to a pristine sample. [36,38]The self-healing mechanism of the blend films is based on supramolecular elastomer, with dipole-dipole interactions (O─H and O─B bonds), heterogeneity and high free volume of the network, which are further explained in more details in our previous works. [36,38]he calculated ΔB for 2PEC7-F2 film decreases from 6.38 to 6.22 GHz after the initial cut, while measured minima was −32.03 dB@20.76GHz before a cut and then −29.39 dB@20.76GHz after the cut (Figure S14b, Supporting Information).During the self-healing, the ΔB fully recovers to 6.38 GHz, while the minima was −32.55 dB@20.76GHz after the selfhealing.In comparison, the calculated EMI shielding effectiveness (for the 2PEC7-F2 with thickness of ≈250 μm) decreases from ≈22.4 dB to 20.53 dB after the initial cut was made with a razor, and then gradually increases to value of 22.72 dB after ≈800 s (Figure S15, Supporting Information).The results indicate that a single cut with a razor has negligible effect to the EMI shielding or microwave absorption properties of the blend film structures, while there was still visible change in the properties when measured over time.The self-healing efficiency and time taken to repair the damage corresponds to the recovery time for the . [38]hus, calculated self-healing efficiency for toughness gives a better overview of the overall self-healability of the blend film structures as the structural damages left unrepaired are more easily reflected in the mechanical properties.
Typically, several millimeters thick wave absorber material structures are required to achieve such values at these frequency ranges.On the basis of the matching theory (Note S2, Supporting Information), the relative permittivity would be required to be extremely large to explain the minima at the measurement frequency (K-band) with the thickness of the films.However, the │Z in /Z 0 │indicate a good impedance matching with the air interface (Figure S16, Supporting Information).The approximation of theoretical reflection based on conventional isotropic and homogenous films assembled into structure that has clear , where the first part (2PEC7) indicates the blend composition, and the second part (F2-F7) the total number of films/layers) and waveguide measurement directions.The thickness of the samples increased from ≈250 to 1750 μm as a consequence of stacking more films together.The assembled film structures were then aged in ambient conditions.Note that the schematic illustrations are drawn after the assembly and before the aging.
interfaces between the individual films would not be adequate for several reasons.For example, the anisotropic films consist of conductive and non-conductive interphase regions with nondefinable thicknesses by currently available measurement techniques.Furthermore, these would be expected not only to vary with volume but as a function of time with the increasing number of film layers (as further discussed later).
Due to absence of discrete layered structure with clear interfaces between the individual films (similar to the large film applied as coating), only a single resonance peak was found in all cases at the K-band (Figure S13, Supporting Information).
Hence, this indicates that the films are not structured as would be expected (Figure S5, Supporting Information).We suppose that the morphology and structure of the multilayer film has been spontaneously shifted over time to an energetically more favorable conformation by the interdiffusion mechanism [36] which partly results from the swelling by the absorption of water molecules.As shown, the RL minima typically occur at the 19.10-20.76GHz frequencies in both measurement directions (Table S1, Supporting Information).For instance, the minima for 2PEC7-F2/F3 and 2PEC7-F4/F5/F6 occurs at 21.01-21.04GHz and 20.76 GHz frequencies, respectively.The ΔB decreases at  the K-band frequency range by increasing the number of the films (≥ 4) as indicated by the threshold of −10 dB for the RL (Figure 6a).Similarly, the EMI shielding was not showing an increasing trend as a function of number film layers (Figure 6b).The EMI SE value increased from 2PEC7-F2 to 2PEC7-F6, but it was lower with 2PEC7-F7 than in 2PEC7-F4, -F5, and -F6 (which have smaller thickness).We suppose that the larger structural thickness and increased number of film layers results in degradation over time by the high degree of domain coarsening and reduced degree of interpenetration of the multiphase polymer network as a consequence of increased free volume. [36]This can reduce the overall performance of the film structure and decrease microwave absorption.
The reflection (R), absorption (A), and transmission (T) coefficients, and corresponding EMI shielding effectiveness (SE) was also calculated (Figure S17, Supporting Information) to better compare with previously reported self-healing composites.The results show that the A, R, T coefficients were highly dependent on the measurement direction or orientation of the film structure, frequency, and the overall film structure (Figure S16, Supporting Information).The calculated mean values for the coefficients at K-band are shown in Table S2 (Supporting Information).The film structures show strong absorption-dominant EMI shielding in the forward measurement direction at 18-24 GHz, and at 18-19.5 GHz and/or 24-26 GHz for the reverse measurement direction (excluding 2PEC7-F2, -F3) In forward measurement direction, R and A coefficients were ≈0.033-0.587and 0.441-0.973at 18-22.5 GHz, respectively.R coefficients were found to increase as a function of frequency from 0.110-0.418at ≈23 GHz to R ≤ 0.893 at 26.5 GHz.The corresponding calculated EMI shielding by reflection were 0.15-3.84dB and <9.76 dB, respectively.As expected, the obtained R, A, and T coefficients were completely different in the reverse measurement direction.R coefficients achieved peak values of 0.808-0.973at 20.6-22.0GHz with 2PEC7-F3, -F4, -F5, -F6, and -F7.The A coefficients were the largest near the lower and higher end of the frequency range (i.e., at 18 and 26.5 GHz) when the orientation of the film was same, but measurement direction was different.The calculated EMI SE values were in the range of 22.45-36.13dB at 18.0-21.6GHz with film thicknesses of at least 250 μm (Figure 4b).The value further increases to a peak value of 38.2-54.7 dB at the ≈24 GHz frequency which are a result of low T coefficients.The maximum EMI SE values achieved correspond to 99.985-99.998%attenuation of EMWs at the K-band frequency range.
On the basis of assuming a good conductor (i.e.,  >>wɛ), the theoretical calculations on the basis of  were found to poorly correlate with the waveguide measurements for the blend films (Figure 4c) by also considering the large theoretical R coefficient (0.89).It is known that redistribution of charges and strong interfacial polarization can be a consequence of anisotropic layered structure (in this case, made of heterogenous co-continuous elastomer), with multiple interphase regions and distinct phase-rich regions, which then result in differences in dielectric characteristics and  mismatches.This is also beneficial for the dielectric loss mechanisms and conversion of EM energy into heat.Regardless, multiple reflections occurring at the material interfaces between the highly conductive PEDOT-rich nanofibril network and surrounding electrically insulative polymer chains is the dominant mechanics which increases the path of propagation of EMWs (Figure 6d) and results in a good attenuation. [9,15]or the soft, stretchable, self-adhesive, and autonomously selfhealable multilayered blend films, the SSE value can reach ≈1800 dB cm 2 g −1 (or up to ≈176 dB mm −1 ).The values are good considering the extremely low film thicknesses and the small R coefficients at wide frequency band (Figure S17, Supporting Information).Moreover, the developed material obtains excellent overall tensile and self-healing properties and provides functionality by working as a soft, stretchable, and tough adhesive for different materials.Thus, the developed solution-processable multiphase conductors show considerable potential for absorptiondominant EMI shielding and as MWA that are needed in the applications of soft electronics and robotics (as illustrated in Figure 6c).

Overall Performance Comparison
The overall performance of the developed multilayer films was compared to other non-healable and self-healing EMI shielding and MWA material structures (Figure 6e; Figure S18 and Tables S3-S5, Supporting Information).The state-of-the-art absorptiondominant EMI shielding and MWA material structures are often several millimetres thick, non-stretchable, solid particle-polymer matrix composites relying on the resonance at the microwave frequencies (Table S4, Supporting Information).Thus, total ΔB in many cases is limited to 1-5 GHz with a narrow single resonance peak.The developed thin, soft, and ultraflexible MWA material structures, based on organic conductor-elastomer blends, achieved a considerable improvement in the thickness of the material structure (by a factor of 5 to 40) at which sufficient attenuation by absorption occurs (≥ 90%).The thickness normalized RL was up to 65.26 dB mm −1 which was improved by factor of ≈1.5-12.0 in comparison to those previously reported with other self-healing MWA materials.In comparison, the EMI SE values divided by thickness have been in the range 1.6-61.5 dB mm −1 with many other polymer composite-based EMI shielding materials (regardless of their large R coefficients).The material structures reported in this work achieved values up to ≈175 dB mm −1 while having extremely low R coefficients < 0.1.
Many previously reported materials and developed structures have been only capable of self-healing with external energy input or certain type of stimuli, by heating, applying pressure or electric field, or after being immersed to water (Tables S4 and S5, Supporting Information) which may hinder their feasibility for onskin or body-integrated soft electronics.In this work, the polymer blends and their multilayered film structures were capable of selfhealing autonomously initiated solely by the induced deformation of the film upon mechanical damage.The self-adhesiveness and good adhesion strength in dry, wet, and cold conditions to different surfaces has not been previously reported in any of the material structures developed for the MWA purposes so far (Tables S2-S4, Supporting Information).This further limit the use of other material structures for the proposed applications as tough, stretchable adhesion to other functional materials, polymers and textile substrates is favorable when designing any kind of soft or wearable components, and circuits with improved performance and EM compatibility, or using the using the materials as functional coatings.

Performance Under Uniaxial Tensile Strain
The EMI shielding and MWA properties of the best performing 2PEC7 film structures with thickness of ≈250 μm were also measured with the waveguide at 18 GHz-26.5GHz frequencies when applying uniaxial tensile strain (Figure 7a-e).The non-strained and strained 2PEC7-F2 films were attached to the waveguide sample holder (as illustrated in Figure S19, Supporting Information).The theoretical attenuation by absorption A ɛ and reflection R ɛ under tensile strain were calculated with Equations ( 1) and (2) (assumption of  >>wɛ and Poisson's ratio of ≈0.5): where d 0 is the initial thickness of the structure, Δd 0 is the change of thickness under strain, f is the frequency,  ɛ is the electrical conductivity under specific strain, μ r is the relative magnetic permeability, μ 0 is the magnetic permeability of free space, and  0 is an impedance of free space (≈377 Ω).
The typical Poisson's ratio for homogenous materials is ≈0.30 but for poly(dimethylsiloxane)-based elastomers it has been shown to be typically in the range of 0.45-0.50.This value can be used in this case also with the consideration of earlier experiments. [36,38]he electrical resistance of the soft and ultraflexible multilayer film increased from ≈2.15 to 7.58 kΩ, from 7.58 to 19.13 kΩ, and then from 19.13 to 350.23 kΩ by applying uniaxial tensile strains of 0-75%, 75-200%, and 200-300% (Figure 7b), respectively.The decrease of  cannot be avoided due to reversible dislocation of the percolated PEDOT-rich nanofibril network even though the microphase-separated interfacial regions can provide stable junction points for the entropic recovery of properties (Figure 7c). [36]hus, the  decreased from ≈104.5 to 0.6 S m −1 while the film thickness decreased in polynomial fashion from 250 to 125 μm due to the Poisson's effect.
The theoretical  on the basis of  increased from ≈368 to 993 μm at 18 GHz frequency as the films were uniaxially stretched up to 125%.However, the experimental results show that the total EMI SE value only decreased from 21.6 to 19.9 dB due to decrease of A ɛ .The maximum thickness normalized EMI SE value was achieved at 150% tensile strain (122.3 dB mm −1 ) as the cross-sectional area decreased with the stretching while the EMI SE by absorption decreased to a lesser degree.The measured EMI SE value was still 6.53 dB at 300% tensile strain (Figure 7e) which cannot be explained by the theoretical calculations for A ɛ and R ɛ based on  derived directly from , or by assumption of a good conductor thus other mechanisms takes place.In theory, the reflection should slightly decrease with the uniaxial tensile strain (due to ), but the experimental results show that the reflection was unchanged.
In the case of non-strained and ordinary nanofibril film the theoretical calculated values on the basis of  (Figure 4c) were more closely related to the calculated values based on the measured scattering parameters.In comparison, for the multilayered polymer blend films, the theoretical calculations show a near linear decrease of total EMI SE at 25-300% tensile strain range which was not the case with the calculated values based on the measured scattering parameters (Figure 7e).As shown, there is only a significant decrease in the total EMI SE after applying ≈150% tensile strain which is further supported by the change in resistance (Figure 7b) and  measurements obtained in previous work. [36]The RL peak, measured in forward direction, shifts from 20.76 to 22.04 GHz with the application of the tensile strain (Figure 7f) which is supported by decreasing thickness under strain.Because the EMI shielding can be maintained at the minimum required for commercial applications (≈20 dB) and the ΔB is not affected by the applied strain, the developed films would be suitable for the soft electronics and sensor applications requiring stretchable coatings with absorbing-dominant EMI shielding.

Conclusion
So far, it has been difficult to achieve thin, soft, stretchable, self-adhesive, and autonomously self-healing material structures for MWA purposes due to the highly reflective nature of nonpatterned conductive supramolecular materials.In this work, the applicability of the developed organic conductor-supramolecular elastomer blend was demonstrated for the above mentioned purposes achieving good overall performance highlighted by the absorption-dominant shielding with maximum EMI SE value of ≈175 dB mm −1 maintainable at ≈122 dB mm −1 for ≈150% uniaxial tensile strain with capability to fully recover the properties after mechanical damage.It should be noted that the EMI shielding, and MWA performance of film coatings, with large surface area, should be evaluated in practice with radar sensors/systems to further verify the results.The multilayer films also showed a tough adhesion to common soft substrates, including TPU films, by achieving a single-lap shear strength up to ≈480 kPa.The strong adhesion was also maintainable in cold environments (−16 to −30 °C), elevated temperatures (20-145 °C), and under water.With the consideration of provided functionality and performance, the developed multilayer films are promising as thin, non-reflective, absorption-dominant EMI shielding coatings for complex 3D surfaces and soft electronics devices and circuits.
Preparation of Flexible PEDOT-Rich Nanofibril Films: The films were prepared on glass substrates.First, DMSO was added to PH1000.The PEDOT-rich nanofibrils were formed with DMSO volume content of ≈16 vol.% and a specific amount of amphiphilic surfactant (1.3-13.2wt%) was added to the solution to further improve flexibility of the film.The asprepared solution was drop-casted to a glass substrate and then polymerized at elevated temperatures (70 °C for 24 h).Due to anisotropic swelling behavior of PEDOT-rich nanofibrils, the films were easily removable from the glass substrate without breaking after swelling underwater.
Preparation of Single-Layer and Multilayer Multiphase Conductors: The single-layer multiphase conductor films with optimized composition based on the previous work were prepared as described. [22,23]The first solution consisted of a mixture of DMSO (≈16 vol%) and PH1000.The weight of the first solution was decreased to ≈59% from the initial weight.Triton TM X-100 (≈13.2 wt% in relation to the weight of the first solution), PDMS-OH and B 2 O 3 NPs (≈0.82 wt% in relation to the insulating selfhealing elastomer components) were added to the first solution (then referred as the second solution).The second solution was mixed for a predefined time (preferable for ≈10-15 min) and then referred as a third solution.After mixing, the third solution was cast into a substrate.The multicomponent blend was simultaneously and independently polymerized at elevated temperature (70 °C for 24 h).The thickness of single-layer solution-casted films after the solidification were ≈250 μm.The films were tape-cast with semi-automatic small-scale equipment (Figure S4, Supporting Information).The multilayer film structures were prepared by selflaminating a predefined number of individual films together with specific surfaces facing each other (Figure 1; Figures S1 and S10, Supporting Information).The overall thicknesses of the film structures were then controlled by only increasing the number of films.This was done in order to maintain a desirable morphology and structure for each individual film as previously discussed. [36]haracterization: Morphology of the films was assessed with Bruker Multimode 8 Atomic Force Microscopy in a tapping mode.Tensile and adhesion properties were recorded with Stable Microsystem Texture Analyser (TA750) with a 50 N-load cell at ambient conditions (Note S1, Supporting Information).Rectangular film geometries with dimensions of ≈25.0 × 12.0 × 1.0 mm (length × width × thickness) were used for tensile testing.The true stress was calculated from the nominal stress with the assumption of isotropy and incompressibility.Swelling ratios for x-, y-, and z-axis were recorded with a camera system and plotted as a function time.Electrical conductivities ( DC ) were calculated as reciprocal of resistivity ( DC =  −1 = RA/l), where R is the resistance, A is the measured crosssectional area, and l is the distance between the probes.The resistance was measured with two-probe setup using Keysight Electrometer B2987A.EMI shielding performance and microwave absorption characteristics were evaluated by measuring scattering parameters (S 11 , S 12 , S 22 , S 21 ) with DAK-3.5TL2connected to a Rohde & Schwarz ZVT20 Vector Network Analyser (VNA), and Agilent 8510C VNA with Keysight K11644A Calibration Kit (WR-42) (Note S1, Supporting Information).For waveguide VNA measurements, two-port calibration was done by using through, short, and load for both ports, transmission, and isolation.The EMI shielding characteristic and RL in forward and reverse direction (Figure S10, Supporting Information) was calculated from the measured scattering parameters using the Equations S1-S13 (Supporting Information).The scattering parameters were also measured under uniaxial tensile strain by fixing the stretched films to the sample holder (as illustrated in Figure S15, Supporting Information).
Single Lap Shear Adhesion Test: Individual films were first prepared on the chosen carrier substrate (as described above) and then self-bonded together to form 2PEC7-F4 film structures (illustrated in Figure S4, Supporting Information).The single-lap shear tests were performed for following target materials: glass, flexible printed circuit board (PCB), silver-plated nylon fabric (MedTex P130), TPU film (Platilon 4201AU), carbon black filled polymeric foil (Velostat), poly(dimethylsiloxane) (Sylgard 184, Dow Corning), polyethylene terephthalate (PET), and PI film with a silicone adhesive (Kapton, 3M).The adhesion areas were ≈15.0 × 10 mm (length × width) for PCB, PET, TPU, Velostat, and Kapton, and 25.0 × 20.0 mm (length × width) for glass and Sylgard 184.The cross-sectional area change under strain was recorded with a camera for calculating the true stress from the nominal stress.
Statistical Analysis: Continuous variables were expressed as mean ± SD (n = 3; Figure 2c) and mean (n = 3, Figures 2b and 3c; n ≥ 3, Figure S9, Supporting Information).The data were not preprocessed, no statistical tests or any software were used for the analysis of significance.

Figure 1 .
Figure 1.Film preparation process.Schematic illustration for preparation of a soft, ultraflexible, self-adhesive and self-healing multilayered film structures for EMI shielding and MWA purposes.a) Multicomponent blends consist primarily of a bimodal polydimethylsiloxane-based self-healing supramolecular elastomer and PEDOT:PSS with a conductivity enhancer that are cast to a substrate.a,c) Solvent and water are evaporated from the blend during the simultaneous and independent polymerization of phases and film solidification.Phase separation and coarsening occurs resulting in the formation of vertically phase separated IPN with heterogenous, co-continuous morphology.d) Individual films are then self-bonded together by gently pressing the solution-cast films together to form multilayered film structures.e) To achieve a freestanding multilayered film structure, the supporting substrates can be gently peeled off.

Figure 2 .
Figure 2. Adhesion properties.a) Schematic illustration of single-lap shear test for the stretchable substrate.b) Photograph of the multilayered films in-between two thermoplastic polyurethane (TPU) films and the single-lap shear adhesion strength on TPU film as a function of temperature and deformation rate.Data expressed as mean values (n = 3) for the 42 measurement points.Discrete stress-strain curves are shown in the Supporting Information.c) Single-lap shear adhesion strength for pristine and self-healed multilayered films on various substrates in ambient conditions.Data expressed as mean values (n = 3).d) Force plotted as a function of time during lifting of a metal block underwater and photographs taken during the test.e) Film coating made of two films (denoted as 2PEC7-F2) self-bonded to a car at temperature of −27 °C.

Figure 3 .
Figure 3. Anisotropic swelling behavior.a) Schematic illustration of the anisotropic swelling behavior underwater.The presence of PEDOT-rich nanofibrils results in a swelling along the z-axis that results in a volume increase in the multiphase conductor.b) Photographs of 2PEC7-F4 film structure placed underwater.c) Swelling ratios measured along z-, x-, and y-axis as a function of time with varied insulating to conducting phase ratios (2:1 to 10:1).The data expressed as a mean (n = 3) for nine compositions.Discrete data points can be found in the Supporting Materials.d) Schematic illustration of the interfacial adhesion mechanism underwater.

Figure 4 .
Figure 4. a) Photograph of freestanding, flexible and water-stable PEDOT-rich nanofibril film.b) Illustrated EMI shielding mechanism of the film.c) EMI shielding effectiveness (SE) values for stacked PEDOT-rich nanofibril films with total thickness of ≈25, 50, and 75 μm.Theoretical (filled markers) and experimental (non-filled markers) EMI SE values as a function of film thickness for PEDOT-rich nanofibril films and blend film structure (2PEC7-F2).

Figure 5 .
Figure 5. Schematic illustrations of the assembled multilayered film structures (denoted as 2PEC7-F2, -F3, etc., where the first part (2PEC7) indicates the blend composition, and the second part (F2-F7) the total number of films/layers) and waveguide measurement directions.The thickness of the samples increased from ≈250 to 1750 μm as a consequence of stacking more films together.The assembled film structures were then aged in ambient conditions.Note that the schematic illustrations are drawn after the assembly and before the aging.

Figure 6 .
Figure 6.EMI shielding and MWA properties.a) Measurement direction dependent RL in the forward and reverse direction (expressed in dB) as a function of film structure.Please see Supporting Information for detailed information and additional data.b) EMI shielding properties for the film structures at the K-band frequency range.c, d) Photograph and schematic illustration of the skin-like MWA and the microwave cloaking mechanism.The adhesion to the skin demonstrates the ultraflexible nature and good adhesion of the blend films.e) Performance comparison of the state-of-the-art non-healable and self-healable MWA material structures.The thickness normalized RL (denoted as RL/d 0 ) is plotted as a function of ΔB.The individual data points and references are shown in the Supporting Information.

Figure 7 .
Figure 7. EMI shielding and MWA properties under tensile strain.a) Stretching of a multilayer film.b) Change of electrical resistance under uniaxial tensile strain.c) Schematic illustrations of the elastic recovery under strain and d) waveguide measurement with and without strain.e) EMI shielding properties and f) microwave absorption characteristics under uniaxial tensile strain.