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Sulfonic SiO2 nanocolloid doped perfluorosulfonic acid films with enhanced water uptake and inner channel for IPMC actuators

This study provides a facile and effective strategy to fabricate sulfonic SiO2 nanocolloid (HSO3–SiO2) doped perfluorosulfonic acid (PFSA) films with enhanced water uptake and inner channel for high-performance and cost-effective ionic exchange polymer metal composite (IPMC) actuators. A commercial precursor of mercaptopropyl trimethoxysilane was hydrolyzed to form thiol functionalized SiO2 nanocolloids (SH–SiO2, ∼25 nm in diameter), which were further oxidized into sulfonic SiO2 nanocolloids (HSO3–SiO2, ∼14 nm in diameter). Both SiO2 nanocolloids were used as additives to dope PFSA film for fabricating IPMC-used matrix films. Due to difference of compatibility, the SH–SiO2 nanocolloids take phase separation in the cocrystallization course, and aggregate into huge, regular spherical particles with a mean diameter of ∼690 μm; while the HSO3–SiO2 nanocolloids are completely compatible with PFSA, forming a very homogeneous hybrid matrix film. Related physiochemical investigations by analytical tools revealed that, the resultant HSO3–SiO2 hybrid film shows better IPMC-related properties compared to the SH–SiO2 hybrid film: 1.59 folds in water uptake, and 2.37 folds in ion exchanging capacity, thus contains an increased number of cations and possesses larger and better interconnected inner channels for IPMC bending. Consequently, the HSO3–SiO2 hybrid IPMC actuator exhibits remarkably higher levels of actuation behaviours such as higher force output, higher displacement output, and longer stable working time, which could be used as a valuable artificial muscle for flexible actuators or displacement/vibration sensors at low cost.


Introduction
Ion-exchange polymer metal composites (IPMCs) are exible actuators made of smart materials that respond to deformation based on external electrical stimulation. [1][2][3][4] Due to high energy density and large displacement output under very low driving voltages (usually less than 5.0 V), they are widely used in biomimic robotic actuators, articial muscle, and dynamic sensors. [5][6][7][8][9][10][11] For conventional IPMC actuators, an ion exchanging lm such as peruorosulfonic acid (PFSA, a commercial name of Naon) or peruorocarboxylic acid, is sandwiched between two inert metal (typically Pt) sheets as working electrodes. When an electric eld is applied, force and displacement are generated due to the migration of hydrated cations in inner channels. 12 According to this actuation mechanism, IPMC's electromechanical response is closely related to migration rate of the hydrated cations, which depends on water uptake (WU) of the matrix lm. However, the pure PFSA lm always exhibits a poor hydrophilicity, cannot save much water for supporting IPMC actuation longtime in air, thus limits its commercial applications. Hydrophilic silicate species such as SiO 2 particles and montmorillonite (MMT) akes were commonly used additives to improve the WU of PFSA matrix as well as its mechanical performance, and those hybrid IPMC actuators therefore exhibited enhanced force output and elongated stable actuation time. [13][14][15][16][17] The main chain of DuPont PFSA is a linear hydrophobic structure, which takes microphase separation and crystallizes into a non-ionic phase; conversely, the branch chains of sulfonic groups are hydrophilic, and form an ionic phase with adsorbed water molecules. 18,19 Ionic domains aggregate to form inner channels, allowing hydrated cations to migrate. Thus the matrix lm containing more sulfonic acid or carboxylic acid groups can produce larger and better interconnected inner channels for ion migration. 20 DuPont PFSA possesses a varying ion exchanging capacity (IEC) of 0.90-1.05 mmol g À1 , usually forms inner channels with varying diameters of 4-5 nm. 21 To further enhance IEC, many sulfonic additives such as sulfonic acid functionalized polystyrene, [22][23][24] graphene oxide, 25 carbon nanotubes, 26,27 poly(ether ether ketone), 28 and polysulfone 29,30 were doped to fabricate hybrid IPMC actuators with improved electromechanical behaviours.
Sulfonic acid functionalized SiO 2 might be a most promising additive considering its high WU and IEC. Former papers had reported the preparation techniques of Naon hybrid lms containing sulfonic SiO 2 additives for applications in direct methanol fuel cells. [31][32][33] Li's group doped fumed silica into Naon and then graed sulfonic acid on the silica surface by sulfonation of concentrated H 2 SO 4 , the resultant composite lm exhibited good ionic current and proton conductivity. 32 Ren' group introduced active thiol group into the silica doped Naon, and then conversed the -SH group to the -SO 3 H group by oxidization. 33 The resultant hybrid lm exhibited enhanced mechanical properties, but endured shortages of low IEC and proton conductivity, when compared to the pure Naon lm.
Herein, we developed a facile and effective strategy to fabricate sulfonic SiO 2 doped PFSA (HSO 3 -SiO 2 /PFSA) lm with enhanced WU and IEC. As shown in Fig. 1, a commercial thiol precursor of mercaptopropyl trimethoxysilane was hydrolyzed to form thiol functionalized SiO 2 nanocolloids (SH-SiO 2 ), which were further oxidized into sulfonic SiO 2 nanocolloids (HSO 3 -SiO 2 ) for using as additive of PFSA. Because the sulfonic acid group is rather compatible with PFSA, no evident phase separation happened during the cocrystallization course, and a very homogeneous copolymer lm of HSO 3 -SiO 2 /PFSA was yielded. Related physiochemical investigations by analytical tools revealed that, this new hybrid lm exhibited high WU, strong IEC, and fast Li + ion conductivity, without a sacrice of mechanical strength, which can be used to fabricate highperformance and cost-effective IPMC actuators. We therefore provide a practical technique to fabricate commercialized actuators or displacement/vibration sensors at low cost.

Preparation and characterization of sulfonic silica nanocolloids
Thiol functionalized SiO 2 nanocolloids (SH-SiO 2 ) were prepared by hydrolysis of the precursor KH590. 2.0 mL of pure KH590 was added into a mixture containing 18.0 mL of ethanol and 2.0 mL of deionized water (pH ¼ 2.0), this mixture was permitted to react for 90 min at 80 C under violently stirring. Aer complete hydrolysis of the KH590, the product was dialysed in anhydrous ethanol three times to remove excess water (weight cutoff of the dialysis bag was 300). Finally, the solution was concentrated to 2.0 mL by a rotating evaporator, thus obtaining a colorless and transparent colloid solution containing around 1.08 g SH-SiO 2 nanocolloids.
Prior to concentrating, 20.0 mL of hydrogen peroxide (30%) was added into the above mixture for oxidizing the -SH. The oxidation reaction was performed under 60 C for 24 h, then the resultant mixture was concentrated to 2.0 mL. A colorless and transparent colloid solution containing around 1.59 g HSO 3 -SiO 2 nanocolloids was yielded.
Both the SH-SiO 2 and HSO 3 -SiO 2 nanocolloids were observed by transmission electron microscopy (TEM, JEM-2100) for morphology characterization. To obtain their chemical component information, they were dropped onto a glass slide, drying for X-ray photoelectron spectroscopy (XPS, ESCA Multilab 2000) and (ATR-FTIR, Bruker IFS66/S) analyses.

Preparation and characterization of PFSA hybrid lms
The PFSA hybrid lms were prepared by cocrystallization of PFSA with the functionalized SiO 2 nanocolloids. 1.0 mL concentrated SH-SiO 2 or HSO 3 -SiO 2 colloid solution was added into two mixtures containing 18.0 mL of PFSA (20 wt%) and small amount of DMF. The mixtures were vigorously stirred for uniformly mixing, and respectively poured into two self-made PDMS-based containers (75 Â 80 Â 50 mm 3 ), followed by crystallizing via solvent removal at 60 C for 48 h. A SH-SiO 2 /PFSA Fig. 1 Formation of HSO 3 -SiO 2 doped PFSA film. Thiol precursor of KH590 is hydrolyzed and oxidized to form the HSO 3 -SiO 2 nanocolloids, which are doped into PFSA to form a hybrid matrix film after cocrystallization. Due to introduction of substantial sulfonic acid groups, the hybrid film exhibits higher hydrophilicity, larger and better interconnected inner channels, which benefit to improve IPMC's electromechanical performance. hybrid lm (white color, $0.46 mm thickness) and a HSO 3 -SiO 2 /PFSA hybrid lm (light yellow color, $0.38 mm thickness) were obtained (Fig. 2). For comparison, 18.0 mL of PFSA solution was poured into the above container to make a pure PFSA lm (control, white color, $0.31 mm thickness). To study the relationship between lm thickness and electromechanical property, two other HSO 3 -SiO 2 /PFSA hybrid lms with different thicknesses of 0.34 and 0.41 mm were prepared by using 0.50 and 1.50 mL HSO 3 -SiO 2 solution. All PFSA lms were annealed at 150 C for 5 min before use.
Surface chemical component and electronic structure of PFSA family lms were detected by high resolution XPS. Young's modulus, IEC, WU, and water contact angle (CA) of the matrix lms were determined refer to our former reports. 34,35 The PVDF lms were grounded into pieces for thermogravimetric analyses (TG, SDTQ600). All samples were heated from the room temperature to 800 C at a heating rate of 10 C min À1 under N 2 .

Morphology characterizations and performance measurements of IPMCs
Refer to former literatures, the PFSA matrix lms were deposited Pt nano electrodes for fabricating IPMCs. 17,36 To generate a stronger electrochemical response, IPMCs were incubated in a LiOH (0.05 mol L À1 ) solution before usage. Surface morphologies of IPMCs were detected by a eld emission scanning electron microscope (FESEM) (S4800, Hitachi, Japan) using a Schottky eld emission source. The IPMC samples were placed into the liquid nitrogen for 30 min, and then broken. Before SEM observation, samples were deposited by gold particles ($5 nm) for conductivity.
The electromechanical behaviours of the three IPMC actuators were determined by our custom system, which consists of a signal generator, a signal amplier, a force sensor, and a laser displacement sensor. The signal generator has a multifunction data acquisition card (NI, 6024E), which can produce electrical signals with frequencies in the range of 0.1-100 Hz, voltages in the range of 0-10 V, and different waveforms (square, sinusoid and triangle). A force sensor with a sensitivity of 0.1 mN (CETR-UMT) and a laser displacement sensor with a sensitivity of 0.1 mm (LK-G80) were used to acquire force and displacement data. The IPMC end was aligned with the tip of the force sensor in an equilibrium position and collected the blocking force from one side. All IPMC samples were cut in same size of (30 Â 5 mm 2 ) to detect and compare their electromechanical properties.

Results and discussion
Morphology and component characterizations of functionalized SiO 2 nanocolloids Fig. 3a and b present similar, amorphous capsule structures of the SH-SiO 2 and HSO 3 -SiO 2 nanocolloids. Because the latter possesses a higher dispersibility in the polar environment (mixture of ethanol and water) as compared to the former, its mean dimension ($14 nm) is lower than that of the former ($25 nm), showing much ner nanocolloids. IR and XPS analyses conrm the chemical component of both nanocolloids, and are shown in Fig. 3c and 37 Additionally, sharp peaks in the range 2800-3000 cm À1 are resulted from the propyl chain of the organic linker; a stronger, broader O-H band appears at the curve of HSO 3 -SiO 2 , showing that the sulfonic acid group has a stronger affinity to water compared to its precursor.
The successful oxidation of thiol group is further conrmed by XPS detection. Fig. 3d and Table 1 recorded the high resolution S 2p XPS data of both SiO 2 nanocolloids. For the SH-SiO 2 nanocolloids, the S 2p spectra consists of two components at 163.1 and 164.4 eV, resulting from the sulfur with a valence of À2, the former peak is derived from the S-H bond, while the latter peak is derived from the C-S bond. 38,39 For the HSO 3 -SiO 2 nanocolloids, a broad peak belongs to the sulfur with a valence of +6, resulting from sulfonic acid group. Collectively, the above results conrm the successful preparation of the sulfonic SiO 2 nanocolloids.    Table 1 compare the XPS data from the PFSA family lms. Peaks of C 1s, S 2p, O 1s, F 1s, and Si 2p are shown in their survey scan curves (Fig. 4a). For the high resolution S 2p spectrum (Fig. 4b), a peak of sulfonic acid group appears at 168.6 eV, resulting from the exposed PFSA; aer doping the SH-SiO 2 nanocolloids, a new peak of S-H group appears at 163.2 eV; aer doping the HSO 3 -SiO 2 nanocolloids, only an enhanced peak of sulfonic acid group remains with enhanced S element concentration increasing from 2.74% to 3.07% (Table 1), this increase is resulted from the doping of sulfonic additives. For the high resolution Si 2p spectrum (Fig. 4c), no Si signal is seen for the control, while a strong Si-O-Si signal appears at 103.0 eV in both hybrid lm curves, generating from the additives of SiO 2 nanocolloids. 40 Si element concentration varies from 1.52% of the SH-SiO 2 /PFSA lm to 2.22% of the HSO 3 -SiO 2 /PFSA lm, the change was derived from the different aggregation states of Si. Additional information such as decreases of C and F elemental concentrations and increase of O elemental concentration further conrm the doping of functionalized SiO 2 nanocolloids (Fig. 4d-f, Table 1).

IPMC-related property characterizations
Introduction of functionalized SiO 2 nanocolloids signicantly changes the matrix lm's polarity, WU, IEC, and mechanical property, thus affecting IPMC's electromechanical performance. [31][32][33][34][35]41 The detected water CAs are 84.1, 75.7, and 54.5 for the pure PFSA, SH-SiO 2 /PFSA, and HSO 3 -SiO 2 /PFSA lms, respectively ( Fig. 5a and Table 2), thus both hybrid lms become more hydrophilic due to additions of SiO 2 nanocolloids, whose surfaces widely distribute hydrophilic Si-OH and Si-O-Si groups. The HSO 3 -SiO 2 /PFSA lm exhibits the best hydrophilicity among those lms due to the strong polarity of sulfonic acid function. Data of WUs exhibit similar changing trend to CAs: WU of the HSO 3 -SiO 2 /PFSA lm is higher than that of the SH-SiO 2 / PFSA lm, which is higher than that of the control ( Table 2). The detected WU of the HSO 3 -SiO 2 /PFSA lm is 32.2%, increasing by 1.80 folds when compared to the control. Notably, waterdriven IPMC actuators always experience water shortage due to signicant evaporation and electrolysis when the hydrated cations move fast between two electrodes. 34,35 Thus, high WU is important to ensure their actuation stability.  Because the sulfonic acid group possesses a strong ionization ability, the pure HSO 3 -SiO 2 has a remarkably theoretic IEC of 6.76 mmol g À1 , given the SH group is completely conversed to the SO 3 H group. In our experiment, the HSO 3 -SiO 2 /PFSA lm exhibits a detected IEC of 1.66 mmol g À1 , which is around 1.90 folds of the self-casting pure PFSA, or 1.86 folds of the commercial Naon lm. 42,43 But for the SH-SiO 2 /PFSA lm, because Si-OH and Si-SH hardly provide ion exchanging ability, its detected IEC is only 0.70 mmol g À1 , which is even lower than the control of 0.89 mmol g À1 . It should be noted that, the enhanced IEC would provide more inner channels, which benets numerous ions migrating at the same time, thus might produce strong force and power outputs. 44 To evaluate the compatibility between SiO 2 additives and PFSA matrix lms, mechanical properties of the resultant hybrid lms were detected by a nanoindenter. Fig. 5b and c show the typical stress-strain curves and the calculated Young's moduli, while their detailed parameters are shown in Table 2. Clearly, all PVDF lms exhibit a linear elastic region followed by a nonlinear deformation. In the dry state, the self-casting, pure control lm shows a compressive Young's modulus of 714 MPa, which is higher than the commercial Naon lm of around 220 MPa in same membrane thickness. 45 Aer doping functionalized SiO 2 nanocolloids, the mechanical properties get apparently improved, the detected Young's moduli are 804 MPa for the SH-SiO 2 /PFSA lm and 1135 MPa for the HSO 3 -SiO 2 / PFSA lm, respectively increasing to 1.13 and 1.59 folds compared to the control. Thus the HSO 3 -SiO 2 /PFSA lm shows the strongest mechanical strength among these lms, indicating a good compatibility between the HSO 3 -SiO 2 nanocolloids and the PFSA. Aer absorbed water, all three matrix lms exhibit attenuated mechanical properties. Due to high hydrophilicity and WU, the HSO 3 -SiO 2 /PFSA lm exhibits the lowest yield modulus of 83 MPa in the wet state, only 7.3% remains compared to the value in the dry state, suggesting a high exibility, therefore the HSO 3 -SiO 2 /PFSA IPMC is expected to generate larger deformation. Fig. 4 compares the thermal properties of the three matrix lms. For the control, three stages of weight losses, respectively beginning at 285, 399, and 438 C, correspond to the thermal decomposition temperatures (T dec ) of sulfonic group, branch chain, and C-F backbone. Aer embedded the SH-SiO 2 particles, those T dec data get evidently increased, shiing to 355, 405, and 446 C, respectively. But for HSO 3 -SiO 2 /PFSA, only one strong weight loss at 464 C is found, showing a high thermal stability of the HSO 3 -SiO 2 /PFSA lm. Increases of T dec hint a presence of strong interactions between the HSO 3 -SiO 2 nanocolloids and the PFSA, further demonstrates a high compatibility between them.   4 ] 2+ , which act as counter cations to balance the sulfuric acid anions, were absorbed and reduced into the Pt nanograins. As a result, the HSO 3 -SiO 2 /PFSA based IPMC presents a thicker Pt electrode. Notably, increasing the electrode thickness of IPMC benets to overcome the electrode defects, and improve the electronic conductivity and "granular damming effect". 46,47 What is important, an evident transition layer generates from the interface between Pt electrode and the polymer matrix in HSO 3 -SiO 2 /PFSA based IPMC (Fig. 6e), while nearly nothing is found in SH-SiO 2 /PFSA based IPMC (Fig. 6b). This result hints that, a more stable connection was yielded between the Pt nanosheets and the HSO 3 -SiO 2 /PFSA matrix, which would prevent the separation of Pt nanograins from the matrix surface and ensure a stable electromechanical response. 48 A striking difference generates from the Fig. 6c and f the SH-SiO 2 /PFSA, many isolated spherical particles and closed pores with a mean dimension of around 690 mm are observed (Fig. 6c). Those huge spherical particles should be the SH-SiO 2 particles, which were generated from the microphase separation during cocrystallization course. Because the SH-SiO 2 nanocolloids are not compatible with the PFSA, they would aggregate together forming the huge spherical particles. 35 Induced by the exterior polar, protonic environment, the hydrophilic Si-OH group should be exposed outside and produces H-bond interaction with the sulfonic acid group from PFSA, while the lower hydrophilic group of thiol should be buried inside. Supposed this hybrid lm is oxidized by H 2 O 2 in situ, only the naked, small amount thiol groups get opportunity of oxidation and turn into sulfonic acid groups, most of thiol groups can not be oxidized. Clearly, this strategy can not produce high IEC lm.
Those closed pores generated from the removal of the SH-SiO 2 particles, which might happen during the nitrogen cooling fracture process. The presence of pores also demonstrates the poor compatibility between the SH-SiO 2 nanocolloids and the PFSA. That is why an opaque white color lm of SH-SiO 2 /PFSA is seen in Fig. 2. However, due to introduction of sulfonic functions, the HSO 3 -SiO 2 nanocolloids exhibit a high chemical compatibility with the PFSA, thus a very homogeneous copolymer of HSO 3 -SiO 2 /PFSA is yielded, no evident particles or pores is seen in Fig. 6f, and a transparent lm is observed in its optical image (Fig. 2).

Investigation of possible electromechanical response of IPMCs
Actuations of IPMCs were captured and shown in Fig. 7. Clearly, all IPMCs from the pure PFSA, SH-SiO 2 /PFSA, and HSO 3 -SiO 2 / PFSA lms generate stable electromechanical bending, and their maximum swing angles are 65, 28, and 110 , respectively. According to the electromechanical response mechanism, driven by the electric eld, hydrated Li + ions migrate from the anode to the cathode, thus forms a water concentration gradient, causing IPMC to bend towards the anode. 1,49,50 Therefore, the amounts of hydrated Li + ions and inner channels are critical to IPMC bending. 17,49 Due to introduction of the hydrophilic sulfonic acid groups, the HSO 3 -SiO 2 /PFSA lm possesses a higher IEC and a higher WU when compared to the control, those data potentially increase to 1.87 and 1.80 folds, respectively ( Table 2). Thus the doping of HSO 3 -SiO 2 nanocolloids benets to improve IPMC actuation as discussed below.
(1) because the polar sulfonic acid group takes part in the formation of inner channel, thus the hybrid lm would possess enlarged and more interconnected inner channels. 46 (2) Substantial Li + ions are introduced into the hybrid lm as counter cations to balance the sulfonic acid anions, thus provides enough movable hydrated cations to migrate. (3) Driven by the electric eld, the hydrated cations (i.e., Li + ) migrate from one end of sulfonic acid group to another end along inner channels, therefore a large number of sulfonic acid groups provide a large number of ion exchanging sites, supporting the fast migration of hydrated cations. 51 Additionally, high WU of the matrix lm also brings two evident benets for IPMC as discussed below. (1) High WU makes the matrix lm exible, which benets to generate larger deformation; (2) high WU benets to maintain stable concentration of the hydrated cations, and subsequently stable actuation, because IPMC cannot work without electrolyte solution. 52 Overall, increasing IEC and improving the hydrophilicity of the matrix lm are effective strategies to enhance IPMC electromechanical deformation.

Electromechanical performance analyses of IPMCs
Displacement and force outputs are two important parameters for assessing IPMC electromechanical response. 53 Typical displacement and force output curves are recorded in Fig. 8a and d. By using a sinusoid signal as the driving source, both  (Table 3). Their total deection angles including le and right swing angles are 65, 28, and 110 , respectively (Table  3). Collectively, IPMC of HSO 3 -SiO 2 /PFSA exhibits the strongest displacement and force outputs, resulting from the high IEC, WU, Li + ion conductivity, and exibility of the matrix lm.
Proles of displacement and force outputs shown in Fig. 8c and f conrm the above results. Under the same actuation voltage of 2.5 V, all three displacement curves exhibit an initial rise followed by a relatively stable displacement output, corresponding to the stable working time for the actuator. This result might be explained by the fact that, all the inner channels are not completely interconnected and the hydrate Li ions migrate and gradually open the inner channels driven by electric eld. 34,35 Finally, the electromechanical response goes recession with the water loss (Fig. 8c). As the blocking force of IPMC originates from its deformation, force proles shown in Fig. 8f correlate well with displacement proles shown in Fig. 8c. Based on HSO 3 -SiO 2 /PFSA lm with varying thicknesses of 0.34-0.41 mm, the detected forces of IPMCs increase from 26.31 mN to 44.45 mN, but the deection angles decrease from 118 to 89 under 2.5 V driven voltage, showing a competitive correlation between force output and displacement output, where a thicker matrix lm always leads to smaller deformation and larger blocking force.
Additional evidence shows improved working time performance of HSO 3 -SiO 2 /PFSA compared to control. With continuous actuation in air, IPMC actuators may lose almost all water and subsequently stop actuation. 52 Under the driving voltage of 2.5 V, the HSO 3 -SiO 2 /PFSA based IPMC exhibits a stable bending time of more than 580 s, its force output also keeps stable for 440 s, both data are much better than other two IPMCs (Table 3). Collectively, the HSO 3 -SiO 2 hybrid IPMC actuator exhibits better electromechanical properties compared  to the pure PFSA lm IPMC actuator: 1.51 folds in displacement output, 3.33 folds in force output, and 4.46 folds stable working time. Based on above results, we conclude that: introduction of sulfonic SiO 2 nanocolloids can signicantly enlarge the inner channels, increase the density of inner channels, and increase ion exchange rate, thus enhancing the IPMC's electromechanical properties.

Conclusions
Sulfonic SiO 2 nanocolloids (HSO 3 -SiO 2 ) were fabricated by initial hydrolysis of the commercial precursor of (3-mercaptopropyl)trimethoxysilane, thus forming thiol functionalized SiO 2 nanocolloids (SH-SiO 2 ), followed by oxidization of thiol group into sulfonic acid group. IR and high resolution XPS spectra monitored the total reaction courses, and demonstrated the successful preparation of the title nanocolloids. High resolution TEM observations revealed that, both SiO 2 nanocolloids presented similar, amorphous capsule structure, their mean diameters were $14 nm for the HSO 3 -SiO 2 nanocolloid, and $25 nm for the SH-SiO 2 nanocolloid. Both SiO 2 nanocolloids were used as additives to modify the PFSA lm for fabricating IPMC-used matrix lms. FESEM observations of hybrid lms revealed that, the SH-SiO 2 nanocolloids took evident phase separation in the cocrystallization course, and aggregated into huge, regular spherical particles with a mean diameter of $690 mm, which separately dispersed in the matrix lm; while the HSO 3 -SiO 2 nanocolloids were rather compatible with the PFSA, thus forming a very homogeneous copolymer. Other related physiochemical investigations by analytical tools revealed that, the HSO 3 -SiO 2 hybrid IPMC actuator exhibits better electromechanical properties compared to the pure PFSA lm IPMC actuator: 1.51 folds in displacement output, 3.33 folds in force output, and 4.46 folds stable working time. Such highperformance and cost-effective IPMC could be used as a valuable articial muscle for exible actuators or displacement/ vibration sensors at low cost.

Conflicts of interest
There are no conicts to declare.