Highly Stretchable and Self-Healing âSolidâLiquidâ Elastomer with

Subscriber access provided by UNIV OF SOUTHERN QUEENSLAND

Applications of Polymer, Composite, and Coating Materials

A highly stretchable and self-healing "solidliquid" elastomer with strain-rate sensing capability Qi Wu, Hui Xiong, Yan Peng, Yi Yang, Jian Kang, Guangsu Huang, Xiancheng Ren, and Jinrong Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05230 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

A highly stretchable and self-healing “solid-liquid” elastomer with strain-rate sensing capability Qi Wu, Hui Xiong, Yan Peng, Yi Yang, Jian Kang, Guangsu Huang, Xiancheng Ren, and Jinrong Wu* State Key Laboratory of Polymer Materials Engineering, College of Polymer Science and Engineering, Sichuan University, Chengdu 610065, China.

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

Abstract To mimic the velocity-sensitive ability of human skin, we fabricate a class of “solid-liquid” elastomers (SLEs) by interpenetrating polyborosiloxane (PBS) with polydimethylsiloxane (PDMS). PBS forms a dynamic network through boron/oxygen dative bonds, while PDMS is covalently crosslinked to form a permanent network. The permanent network affords a scaffold for the dynamic network, imparting SLEs with high elasticity and structural stability, thereby overcoming the inherent drawbacks such as fluidity and irreversible deformation of conventional solid-liquid materials. Meanwhile, the dissociation and association of dynamic network is time dependent. Thus, the modulus of SLEs varies with strain rates, and if the SLEs contain carbon nanotubes their electric conductivity is also responsive to strain rates. This property can be utilized to fabricate skin-like sensors with the ability to distinguish different contact velocity. Moreover, the dynamic network can dissipate energy and be repaired, leading to the high stretchability and self-healing performance of SLEs.

Keywords: solid-liquid, high stretchability, elastomers, self-healing, strain-rate sensing.


2

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Human skin, a multilayer interface between tissues and the environment, enables human to feel ambient stimulations such as temperature, pressure, touch and pain.1 In particular, when receiving two nuanced external stimuli with rapid and slow touch, the mechanoreceptors in the skin can distinguish the different contact velocity by generating different electrical signals.

2-4

Such velocity-sensitive ability also exists in the skin of other genera including the hagfish Myxine glutinosa,5 the leech Hirudo medicinalis,6 the catfish Ictalurus nebulosu7 and so forth. While mimicking the capabilities of the human skin to detect temperature, pressure and touch has been of a great success in wearable devices and artificial robots,8-9 fabricating a skin-like velocity-sensitive device is still challenging. It requires complicated designs involving lithography, in-fiber Doppler velocimetry or ion channels to simulate the morphologies or sensory channels of biological systems.10-12 Up to date, a bulk material with an inherent ability of sensing mechanical-stimulus velocity has not been reported, despite that some materials show strain-rate dependent mechanical properties.13-20 Polyborosiloxane (PBS), nicknamed as “bouncing putty” or “silly putty”21, is a well-known solid-liquid material because of its fascinating viscoelastic properties.22 It behaves as a viscous liquid at a low strain rate but like an elastic solid at a high strain rate under room temperature. Such solid-liquid behavior is enabled by the reversible association/dissociation of the triple and quadruple bonds between the boron and oxygen atoms.23-24 Moreover, like many other supramolecular polymers, PBS also exhibits an intrinsic self-healing property due to the dynamic boron/oxygen dative bonds. Thanks to the attractive viscoelastic and self-healing characteristics, electrically conductive sensors capable of detecting pressure and flexion were developed by


3


Page 4 of 25

compounding PBS with reduced graphene oxide,24 graphene nanosheets,25 or multiwalled carbon nanotubes,26 and gas sensors were fabricated by 3D printing of methanol-liquefied electrochemically-exfoliated graphene/PBS composites.27 Moreover, PBS is also applied as selfhealing coating for corrosion protection of metals28 or self-healing adhesive layers for construction of brick and mortar structures.29 Despite these efforts, PBS has not been exploited to fabricate intrinsic velocity-sensitive materials or sensors. One major reason for preventing such advance is that PBS is structurally unstable: due to the lack of permanent network and rubbery elasticity, it gradually flows with increasing time at room temperature, and cannot recover its original shape after deformation. Herein, we develop a class of “solid-liquid” elastomers (SLEs), which combine the properties of both PBS and covalently crosslinked elastomers. Such “solid-liquid” elastomers are enabled by a structural

design

of

interpenetrating

double

networks:

the

permanently

crosslinked

polydimethylsiloxane (PDMS) network and the dynamically crosslinked PBS network. The PDMS network acts as a scaffold to keep shape stability of PBS and provide elasticity, while the PBS network still manifests solid-liquid behavior. As a result, SLEs show mechanical properties highly dependent on strain rates and super stretchability with a strain at break up to 2100%, yet can recover their initial shape upon unloading. Moreover, since the PBS network is dynamic, SLEs possess self-healing ability to cracks. By harnessing the solid-liquid like property of the elastomers, a sensor mimicking the touching-rate sensibility of skin is fabricated by introducing carbon nanotubes into the elastomers, which is the first intrinsic velocity-sensitive bulk material to the best of our knowledge.

Results and Discussion


4

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Preparation of “Solid-Liquid” Elastomers The fabrication of “solid-liquid” elastomers involves two steps, as shown in Figure 1a and b. In the first step, PBS is prepared by heating the mixture of polydimethylsiloxane (PDMS) and boric acid at 190oC, leading to PDMS chain scission and generating new short PBS chains terminated by boric acid. The number-average molecular weight (Mn) of PBS is 2398 g/mol, which is nearly one order of magnitude lower than that of PDMS (25000 g/mol), implying the success of the chain scission and the short chain structure of PBS. In the second step, PBS is mixed with vinylmethylsiloxane-dimethylsiloxane copolymers (vinyl PDMS, Mn ≈ 530000 g/mol) and curing agent. The mixture is then hot pressed at 160 oC to crosslink the vinyl PDMS, resulting in the “solid-liquid” elastomers, which are designated as SLE-n where n is the weight ratio between PBS and vinyl PDMS. FTIR spectra of PBS, uncured vinyl PDMS and SLEs are shown in Figure S1a. The appearance of new characteristic bands of Si‒O‒B moieties at 1340 cm‒1 and B‒OH out-of-plane bending at 890 cm‒1 in PBS suggests the successful introduction of boric acid on the PBS chains.30 These characteristic bands also exist in SLEs, yet no peak at 1380 cm‒1 assigned to the B‒O‒B can be detected, demonstrating no distinct condensation between borono terminals of PBS in the composites during hot pressing in a short time. To further confirm that PBS is not covalently crosslinked, SLEs are extracted with toluene to remove uncrosslinked components. FTIR measurement (Figure S1b) of the extracted residues shows similar spectra to that of crosslinked PDMS, without the characteristic peaks (1340 and 890 cm‒1) of PBS, indicating that PBS molecules are not crosslinked or grafted onto the crosslinked PDMS network.


5


Page 6 of 25

Figure 1. Design concept and mechanical stability of “solid-liquid” elastomers. a) Chemical route to the synthesis of polyborosiloxane (PBS) through heating polydimethylsiloxane (PDMS) with boric acid (BA) at 190oC. b) Schematic process to fabricate “solid-liquid” elastomers with synthetic PBS, PDMS and curing agent through mixing, compression molding and vulcanizing at 160°C for 12 min. c) Optical images of the different shape preserving behaviors of PBS (colored in cyan, left) and SLE-2 (colored in red, right) with the same size and shape (scale bar: 3 cm). d) Photographs of the distinct shape-recovery behaviors between PBS (colored in cyan, left) and SLE-2 (colored in red, right) with the same size and shape. Through putting and removing a weight of 200 g on the samples, the PBS forms a concave shape and cannot recover to its origin structure while the SLE-2 can keep its shape (scale bar: 3 cm). Although PBS is not covalently crosslinked, it can form a dynamic network through boron/oxygen dative bonds with oxygen atoms not only from the PBS chains but also from the


6

Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PDMS chains. Peel test of a sandwich structure composed of a PBS inner layer and two PDMS outer layers (Figure S2) reveals that fracture occurs in the PBS layer but not the interface between the PBS and PDMS substrate, illustrating that there is strong interaction between PBS and PDMS by dint of boron/oxygen dative bonds.26 Therefore, SLEs are actually composed of two networks: one is the permanent network formed by covalently crosslinked PDMS, and the other is the dynamic network formed by PBS. Due to the introduction of permanent PDMS network into the dynamic PBS network, SLEs show good shape stability over a long time, while PBS flows gradually (Figure 1c). Moreover, SLEs are elastic, since they can recover its original shape after being deformed by a mechanical loading. By contrast, PBS deforms permanently upon mechanical loading (Figure 1d). Therefore, the crosslinked PDMS network acts as a framework, allowing the PBS matrix to overcome its natural limitation of “liquid” properties. In addition, the thermostability of SLEs is improved by introducing of PDMS network (Figure S3), which will broaden the application temperature range of SLEs. Rheological Properties of “Solid-Liquid” Elastomers To study the viscoelastic properties of the elastomers, rheological measurements are performed on SLE-n (n = 1, 2, 3), crosslinked PDMS and PBS. Dynamic mechanical properties as a function of frequency ranging from 0.01 to 100 Hz at 25 oC are shown in Figure 2a, b and c. For the crosslinked PDMS, the storage modulus (G′) is always higher than the loss modulus (G′′) in the whole frequency range, and moreover both of them are independent of frequency (Figure 2b). Such phenomena indicate that the crosslinked PDMS is elastic at room temperature.31-32 On the contrary, PBS shows liquid-like property at low frequency, as G′ is lower than G′′, and G′ ∝ ω2 and G′′ ∝ ω1 (Figure 2c). Above a certain frequency, G′ becomes higher than G′′, and G′


7


Page 8 of 25

shows a rubbery plateau, indicating that PBS shows elastic behavior at high frequency.33-34 Interestingly, SLE has similar frequency-dependent viscoelastic behavior to that of PBS (Figure 2a), suggesting that SLE also shows solid-liquid like behavior dependent on the frequency.35-36 The crossover point of the transition from viscous state to elastic state is indicated by the frequency where G′ = G′′. This frequency increases with increasing PBS weight fraction in SLEs. Thus, the corresponding characteristic relaxation time of the network (τc, which is the reciprocal of the crossover angular frequency)37-39 decreases with increasing PBS weight fraction, as shown in Figure 2d. Such phenomenon suggests that higher content of PBS leads to faster rate of dynamic association and disassociation of boron/oxygen dative bonds within the elastomers.40 In addition, the viscoelastic properties of SLEs, crosslinked PDMS and PBS are also examined by stress relaxation tests. When stretched to 5% strain and held constant, crosslinked PDMS barely shows detectable stress relaxation, as the permanent network does not change its topology under loading; while the stress of PBS is quickly relaxed to 0 within 0.5 s due to the fast dynamics of boron/oxygen dative bonds (Figure S4). Introduction of PDMS into SLE retards the relaxation process, and thus more time is needed for the normalized relaxation modulus to reach its equilibrium value with the increasing content of PDMS in the elastomers. Such results are consistent with the relaxation time determined by oscillatory frequency sweeps.


8

Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Rheological properties of “solid-liquid” elastomers. Frequency dependence of storage modulus and loss modulus for a) SLEs with different weight fractions of PBS (n = 1, 2, 3), b) covalently crosslinked PDMS and c) pure PBS. d) Characteristic relaxation times of the network (τc) obtained from frequency sweeps of SLEs and pure PBS. All measurements were performed at 25oC. Mechanical Properties of “Solid-Liquid” Elastomers To investigate the mechanical properties of the elastomers, tensile tests are carried out for SLEs and crosslinked PDMS. The stress-strain curves at stretching speeds of 10 mm min−1 and 100 mm min−1 are presented in Figure 3a and S5, respectively. It is clear that neat crosslinked PDMS is quite fragile with a low strain at break of 180% (Figure 3a). Incorporating PBS into PDMS significantly enhances the strain at break of the resulting SLEs. SLE-3 even can sustain a strain of 1777% before fracture. Meanwhile, the tensile strength decreases with increasing PBS weight fraction. Thus, the mechanical properties can be easily tuned from the stiff state to the highly stretchable state by simply changing the ratio of PBS to PDMS. To further study the


9


Page 10 of 25

effect of PBS on the mechanical properties of SLEs, cyclic tensile tests to a strain of 100% are performed on SLEs and crosslinked PDMS (Figure S6). Clearly, crosslinked PDMS is quite resilient, as its hysteresis loop is very small. With increasing PBS, the hysteresis loop becomes larger and larger. This increased hysteresis can be attributed to the association and dissociation of the boron/oxygen dative bonds.41 Thus, these bonds can act as sacrificial bonds to dissipate energy, while the PDMS network distributes stress and maintains the integrity of the materials. As a result, SLEs are highly extensible with a strain at break up to 1777%.

Figure 3. Mechanical properties of “solid-liquid” elastomers. a) Representative stress–strain curves measured with the strain rate of 10 mm min−1 for crosslinked PDMS and SLEs with different weight fractions of PBS. b) Tensile strength and strain at break of SLEs with different weight fractions of PBS at two distinct strain rates (100 mm min−1 and 10 mm min−1). c) Tensile stress/strain curves (the left) of the SLE-2 sample with the maximum strain of 200% at different strain rates and Young’s modulus as a function of strain rate (the right). d) High stretchability


10

Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and recoverability of SLE-2. d1) Representative stress–strain curves with the strain rate of 100 mm min−1 for SLE-2 with different contents of curing agent (0.1 wt% and 0.5 wt%). d2) The residual strain as a function of time of SLE-2 with 0.1 wt% curing agent after unloading the deformation of 1500% strain. d2, inset) Photographs of the shape recovery of the sample at the same scale: before stretching (the top), after releasing the tensile load with the residual strain of 58% (the middle), entirely recovery after 2 days (the bottom). d3) Photographs of manual horizontal stretching test of SLE-2 with 0.1 wt% curing agent at 1500% strain: before sideways stretching (the left), stretching (the middle), after releasing the manual stretching (the right). e) Schematic description of network change in the “solid-liquid” elastomers during the slow stretching or fast stretching process. The extensibility of SLEs can be further improved by adjusting the amount of curing agent (2,5bis(tert-butylperoxy)-2,5-dimethylhexane, DHBP, from 0.5 wt% to 0.1 wt%). Figure 3d1 shows the stress–strain curves of SLE-2 with different contents of curing agent at a stretching speed of 100 mm min−1. It is noteworthy that SLE-2 with 0.1 wt% DHBP is super stretchable with a strain at break of about 2100%, which is nearly two-fold higher than that of SLE-2 with 0.5 wt% DHBP (about 1100%); meanwhile, the tensile strength of SLE-2 decreases from 0.15 to 0.07 MPa upon decreasing the usage of DHBP. Unlike many other super stretchable elastomers which show large residue strain after deformation, SLEs are elastic and can completely recover their original shape even though they are subjected to large deformation.42-43 To demonstrate this ability, a sample of SLE-2 crosslinked with 0.1 wt% DHBP is stretched to a strain of 1500%. After releasing the stress, the instant residual strain is 58%, and it gradually decreases as a function of time (Figure 3d2 and S7). After two days, the sample completely recovers its original shape without evident residue strain. By contrast, the PBS shows no recovery ability


11


Page 12 of 25

after deformation (Figure S8). Moreover, when the sample immobilized at 1500% strain is subjected to a manual horizontal stretching, it can recover immediately to the straightened state after the horizontal stretching is released (Figure 3d3 and video S1), indicating that SLEs have excellent elasticity under the stretched state.44 It is found that the mechanical properties of SLEs are strongly dependent on the tensile strain rates (Figure 3b). In particular, the strain at break of the SLEs samples stretched at 10 mm min−1 is much higher than that at 100 mm min−1, suggesting that the dynamic PBS network has enough time to reorganize at small strain rate and thus delays the fracture of the materials. By contrast, the crosslinked PDMS is elastic without energy-dissipation ability, thus its strain at break does not depend on the strain rates. Most interestingly, SLEs can change its stiffness under different strain rates. To investigate this performance, the samples of SLE-2 are stretched to 200% strain at various strain rates (Figure 3c). The resulting stress-strain curves are remarkably different under different strain rates. The Young’s modulus obtained from the initial linear region of the stress-strain curves increases from 0.033 to 0.64 MPa when the strain rate varies from 1 to 500 mm min−1. Moreover, this increase nearly follows a linear manner in the double-log plot corresponding to the linear area in rheological measurement, as shown in Figure 3c. Such a phenomenon demonstrates that the elastomers possess self-stiffening ability with increasing strain rate, which is similar to the behavior of traditional solid-liquid materials.45-46 To explain such behavior, we propose a mechanism for the rate-responsive property of SLEs, as shown in Figure 3e. Under slower stretching rate, the physical crosslinking points have enough time to disassociate and relax; as such, they do not contribute to the modulus, and instead dissipate energy, leading to the more liquid-like property and higher extensibility. Under higher stretching rate, the physical crosslinking points do not have enough time to relax, which increases the total


12

Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


amount of effective crosslinking density, resulting in the more solid-like property and relatively lower stretchability. Self-Healing Property of “Solid-Liquid” Elastomers

Figure 4. Self-healing properties of “solid-liquid” elastomers. a) SEM images of healing process of SLE-1 after fracture: a1) the original state after cut, a2) the healing state after 4 h, a3) the healed state after 24 h. b) Schematic illustration of the self-healing process of “solid-liquid” elastomers showing the dynamic crosslinked network can be gradually reformed on the fractured surface by reassociation between boron and oxygen atoms. The dynamic boron/oxygen dative bonds can be reformed after dissociation, allowing SLEs to spontaneously repair themselves when suffering cracks or fractures at room temperature. To illustrate the self-healing property, the SLE samples are cut into two halves by a blade, and the healing process of the fractured samples is monitored (Figure 4a, S9 and S10). It is found that the fractured surfaces quickly merge together and the merging zone completely disappears after healing, showing that the SLEs can fully heal the fractures due to the existence of dynamic PBS network. To quantify the self healing, dumbbell shaped samples are cut in the middle into two


13


Page 14 of 25

pieces, which are then brought into contact and mended at room temperature. The stress−strain curves of the healed samples are acquired by tensile tests at a controlled strain rate of 100 mm min−1 (Figure S11 and S12) and the corresponding healing efficiencies are shown in Figure S13. The mended samples can still be stretched to strains from 100% and 200%, despite the healing efficiency is about 20%. The incomplete healing of the fractured samples can be attributed to the destroyed covalent network which cannot be reformed, shown in Figure 4b. Strain-Rate Responsive Sensor Due to the intrinsic solid-liquid like behavior, the Young’s modulus of SLEs increases linearly with strain rate. We can envision that if such change can be converted into electrical signal, a strain-rate responsive sensor mimicking the mechanoreceptors of human skin can be obtained. To explore this possibility, 5 wt% of carbon nanotubes (CNT) is blended with SLE-2 by physical compounding to fabricate a SLE-2/CNT composite. We first examine the viscoelastic property of the composite. Due to the reinforcement effect of CNT,47 the composite becomes stiffer and stronger than SLE-2, as shown by tensile tests in Figure S14a. Meanwhile, rheological measurements reveal G′ > G″ in the whole frequency range under room temperature for the composite (Figure S14b), which suggest that introduction of CNT increases the “solid” property of the material. However, both G′ and G″ are strongly dependent on frequency in the range from 0.01 Hz to 10 Hz, demonstrating that the composite still possesses liquid-like property. Stress relaxation tests show that the normalized relaxation modulus can quickly relax to an equilibrium value, and cyclic tensile tests reveal that the hysteresis loops are very large (Figure S14c and d), indicating that the PBS network in the composite is also highly dynamic and dissipative. Moreover, the Young’s modulus of the composite also increases with increasing strain rate (Figure S14e), thus the rate-responsive property is still retained when CNT is introduced into


14

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SLEs. Collectively, these results confirm that the SLE-2/CNT composite has the similar viscoelastic properties to SLE-2.

Figure 5. Performance of strain-rate responsive sensor. a) Relative resistance versus time for cycling stretch at the 10% strain with different strain rates of 1, 10, 50, 100 mm min−1. b) The relative resistance change under different strain rates. c) The relative resistance response to human motions of bending and releasing the elbow with different time duration of 1, 10, 20, 30 s. d) The relative resistance change with different time duration of elbow bending and releasing. We then examine the electrical properties of the SLE-2/CNT composite. It is found that the composite has an electrical conductivity of 1.58 × 10‒3 S/cm. More intriguingly, like the case of Young’s modulus, the relative resistance change (ΔR/R0, where R0 is the resistance of the original sample, ΔR is the change in resistance when the sample is deformed) at 10% strain also increases with increasing strain rate; this increase nearly follows a linear manner, seen in Figure


15


Page 16 of 25

5a and b. Such a phenomenon can be interpreted in terms of the influence of matrix rigidity and time duration of applying the strain on the reorganization of the conductive CNT network. Upon deformation, the conductive CNT network is partially disrupted; meanwhile the disrupted CNT network can reform during the stretching process. At high strain rates, the matrix is quite rigid and the time duration of applying the strain is short, thus the CNT is difficult to reorganize to reform the conductive network, leading to high ΔR values. By contrast, at low strain rates, the matrix is soft and the time duration of applying the strain is long, which allow the CNT to reorganize to repair the conductive network, leading to small ΔR values. In addition, the SLE2/CNT composites are also stretched to 10% strain with different strain rates and then held constant. The relative resistance shows a faster relaxation during the holding process after being stretched at slower strain rates, suggesting that under such circumstance the disrupted conductive CNT network is easier to be repaired (Figure S15). Thus, the resistance change shows a strong dependence on the strain rate, which can be utilized to fabricate the strain-rate responsive sensor based on the material itself. Due to the high strain-rate sensitivity of the sensor, monitoring the different speeds of the same human movements can be realized. As such, the sensor is attached to an elbow to track the speed of elbow bending and releasing. Upon increasing the motion duration from about 1s to 30 s, that is, decreasing the movement speed, the relative resistance change decreases from 0.517 to 0.260 (Figure 5c and d). Meanwhile, the uniform curve and resistance change of several cycles for fast and slow motions indicate good repeatability of the rate responsive sensor (Figure S16). What’s more, when quickly and slowly repeating the gestures of bending and releasing on fingers, the sensor also shows the good rate-resistance correlation and repeatability (Figure S17).


16

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


To mimic the feeling of skin to the rate of touch, the sensor is installed 1.5 mm away from the surface of the wooden model hand, enabling the same magnitude of elastic deformation when the hand is touched at different rates. Subsequently, a pendulum ball at different heights swings down and hits on the sensor to generate different touching rates. It is clear that the relative resistance change at faster touching is larger than that at slower touching, as shown in Figure S18. In addition, the sensor directly attached to the real human skin can also feel the different touching rates through the resistance change, as shown in Figure S19 and Movie S2. Collectively, these results reveal that SLEs can sense the various touching rates, which can simulate a part function of real skin.

Conclusion In summary, we develop a series of “solid-liquid” elastomers with high stretchability, selfhealing property and fascinating viscoelastic performance by interpenetrating PBS with PDMS. The covalently crosslinked network formed by PDMS affords a scaffold to overcome the inherent drawbacks of PBS such as time-dependent fluidity and structural instability, and offers elasticity and shape-recovery ability for the SLEs. The dynamic crosslinked network generated by boron/oxygen bonds show time-dependent dissociation/association and strong energy dissipation, imparting SLEs with high stretchability, self-healing and strain-rate responsive abilities. More interestingly, the electronic conductivity of SLEs containing carbon nanotubes is responsive to strain-rate, therefore, they can serve as sensors for strain-rate monitoring and artificial skin for touching-rate sensing. Our work provides a strategy in overcoming the internal disadvantage while not losing the fantastic viscoelastic performance of traditional solid-liquid materials. The resulting SLEs are the first generation of bulk materials which can mimic the


17


Page 18 of 25

touching-rate responsive ability of human skin, providing an attractive platform to develop smarter soft robotics, wearable electronic devices and multifunctional sensors.

Experimental Section Synthesis: Polyborosiloxane (PBS) was prepared by heating PDMS and boric acid at 190oC for 4 h. The “solid-liquid” elastomers were synthesized by roll-milling PBS, vinyl-PDMS and DHBP at room temperature followed by compression molding and vulcanizing at 160°C for 12 min. The strain-rate responsive sensor was fabricated by dispersing CNT in uncured PBS/PDMS composites with roll-mill method and then compression molding at 160°C for 12 min. Characterization: The molecular weight was determined using a Tosoh HIC-8320GPC with tetrahydrofuran (THF) as the eluent. FTIR spectra were recorded on Thermo Scientific Nicolet iS50 FTIR by an attenuated total reflection mode at room temperature. The thermal stability was characterized with a thermal gravimetric analyzer (TA Q500). Rheological measurements were conducted on a TA AR2000ex rotational rheometer. Tensile experiments were performed on an Instron 5967 tensile tester. The structures of self-healing process were characterized by a scanning electron microscopy (SEM, Nova nanoSEM450) and a polarizing optical microscope (Olympus BX-51). Electro-mechanical tensile measurements were performed on a Keithyley 6487 in a resistance mode assisted with a tensile tester (SANS CMT 4503).

ASSOCIATED CONTENT Supporting Information. FTIR spectra and TGA curves of PDMS, PBS and SLEs, additional rheological and mechanical properties of SLEs, SEM images, polarizing microscope map and stress-strain curves for self-


18

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


healing ability of SLEs, rheological and mechanical properties of SLE-2/CNT, additional performance of strain-rate responsive sensor (PDF) Excellent elasticity of SLE-2 with 0.1 wt% DHBP under the stretched state (Movie S1), touching rate sensing of the sensor attached to the real human skin (Movie S2)

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (J. R. W.) Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant Nos. 51673120 and 51873110) and State Key Laboratory of Polymer Materials Engineering (Grant No. sklpme2017-3-05).

ABBREVIATIONS PDMS, polydimethylsiloxane; PBS, polyborosiloxane; SLE, “solid-liquid” elastomer; DHBP, 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane; CNT, carbon nanotube.

REFERENCES (1). Rushmer, R. F.; Buettner, K. J. K.; Short, J. M.; Odland, G. F. The Skin. Science 1966, 154 (3747), 343-348. (2). Loewenstein, W. R. Excitation and Changes in Adaptation by Stretch of Mechanoreceptors. J. Physiol. 1956, 133 (3), 588-602.


19


Page 20 of 25

(3). Konietzny, F.; Hensel, H. Response of Rapidly and Slowly Adapting Mechanoreceptors and Vibratory Sensitivity in Human Hairy Skin. Pflüg. Arch. 1977, 368 (1), 39-44. (4). Maksimovic, S.; Nakatani, M.; Baba, Y.; Nelson, A. M.; Marshall, K. L.; Wellnitz, S. A.; Firozi, P.; Woo, S.-H.; Ranade, S.; Patapoutian, A.; Lumpkin, E. A. Epidermal Merkel Cells are Mechanosensory Cells that Tune Mammalian Touch Receptors. Nature 2014, 509, 617. (5). McVvean, A. Velocity and Displacement Receptors in the Skin of the Hagfish Myxine Glutinosa. J. Zool. 1989, 219 (2), 251-267. (6). Carlton, T.; McVean, A. The Role of Touch, Pressure and Nociceptive Mechanoreceptors of the Leech in Unrestrained Behaviour. J. Comp. Physiol. A 1995, 177 (6), 781-791. (7). Biedenbach, M. A. Functional Properties and Projection Areas of Cutaneous Receptors in Catfish. J. Comp. Physiol. 1973, 84 (3), 227-250. (8). Trung, T. Q.; Lee, N.-E. Flexible and Stretchable Physical Sensor Integrated Platforms for Wearable Human-Activity Monitoringand Personal Healthcare. Adv. Mater. 2016, 28 (22), 43384372. (9). Wang, X.; Dong, L.; Zhang, H.; Yu, R.; Pan, C.; Wang, Z. L. Recent Progress in Electronic Skin. Adv. Sci. 2015, 2 (10), 1500169. (10). Ha, M.; Lim, S.; Park, J.; Um, D.-S.; Lee, Y.; Ko, H. Bioinspired Interlocked and Hierarchical Design of ZnO Nanowire Arrays for Static and Dynamic Pressure-Sensitive Electronic Skins. Adv. Funct. Mater. 2015, 25 (19), 2841-2849. (11). Jin, M. L.; Park, S.; Lee, Y.; Lee, J. H.; Chung, J.; Kim, J. S.; Kim, J.-S.; Kim, S. Y.; Jee, E.; Kim, D. W.; Chung, J. W.; Lee, S. G.; Choi, D.; Jung, H.-T.; Kim, D. H. An Ultrasensitive, Visco-Poroelastic Artificial Mechanotransducer Skin Inspired by Piezo2 Protein in Mammalian Merkel Cells. Adv. Mater. 2017, 29 (13), 1605973. (12). Chun, K.-Y.; Son, Y. J.; Jeon, E.-S.; Lee, S.; Han, C.-S. A Self-Powered Sensor Mimicking Slow- and Fast-Adapting Cutaneous Mechanoreceptors. Adv. Mater. 2018, 30 (12), 1706299. (13). Sun, T. L.; Kurokawa, T.; Kuroda, S.; Ihsan, A. B.; Akasaki, T.; Sato, K.; Haque, M. A.; Nakajima, T.; Gong, J. P. Physical Hydrogels Composed of Polyampholytes Demonstrate High Toughness and Viscoelasticity. Nat. Mater. 2013, 12, 932. (14). Mei, J.-F.; Jia, X.-Y.; Lai, J.-C.; Sun, Y.; Li, C.-H.; Wu, J.-H.; Cao, Y.; You, X.-Z.; Bao, Z. A Highly Stretchable and Autonomous Self-Healing Polymer Based on Combination of Pt···Pt and π–π Interactions. Macromol. Rapid Commun. 2016, 37 (20), 1667-1675.


20

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(15). Wu, J.; Cai, L.-H.; Weitz, D. A. Tough Self-Healing Elastomers by Molecular Enforced Integration of Covalent and Reversible Networks. Adv. Mater. 2017, 29 (38), 1702616. (16). Peng, Y.; Zhao, L.; Yang, C.; Yang, Y.; Song, C.; Wu, Q.; Huang, G.; Wu, J. Super Tough and Strong Self-Healing Elastomers Based on Polyampholytes. J. Mater. Chem. A 2018, 6 (39), 19066-19074. (17). Hu, X.; Zhou, J.; Vatankhah-Varnosfaderani, M.; Daniel, W. F. M.; Li, Q.; Zhushma, A. P.; Dobrynin, A. V.; Sheiko, S. S. Programming Temporal Shapeshifting. Nat. Commun. 2016, 7, 12919. (18). Zhang, G.; Zhao, Q.; Zou, W.; Luo, Y.; Xie, T. Unusual Aspects of Supramolecular Networks: Plasticity to Elasticity, Ultrasoft Shape Memory, and Dynamic Mechanical Properties. Adv. Funct. Mater. 2016, 26 (6), 931-937. (19). Hu, X.; Zhou, J.; Daniel, W. F. M.; Vatankhah-Varnoosfaderani, M.; Dobrynin, A. V.; Sheiko, S. S. Dynamics of Dual Networks: Strain Rate and Temperature Effects in Hydrogels with Reversible H-Bonds. Macromolecules 2017, 50 (2), 652-659. (20). Konda, A.; Urayama, K.; Takigawa, T. Strain-Rate-Dependent Poisson’s Ratio and Stress of Polymer Gels in Solvents Revealed by Ultraslow Stretching. Macromolecules 2011, 44 (8), 3000-3006. (21). Juhász, A.; Tasnádi, P.; Fábry, L. Impact Studies on the Mechanical Properties of Polyborosiloxane. Phys. Educ. 1984, 19 (6), 302. (22). Goertz, M. P.; Zhu, X.-Y.; Houston, J. E. Temperature Dependent Relaxation of a “Solid– Liquid”. J. Polym. Sci. B: Polym. Phys. 2009, 47 (13), 1285-1290. (23). Cross, R. Elastic and Viscous Properties of Silly Putty. Am. J. Phys. 2012, 80 (10), 870875. (24). D'Elia, E.; Barg, S.; Ni, N.; Rocha, V. G.; Saiz, E. Self-Healing Graphene-Based Composites with Sensing Capabilities. Adv. Mater. 2015, 27 (32), 4788-4794. (25). Boland, C. S.; Khan, U.; Ryan, G.; Barwich, S.; Charifou, R.; Harvey, A.; Backes, C.; Li, Z.; Ferreira, M. S.; Möbius, M. E.; Young, R. J.; Coleman, J. N. Sensitive Electromechanical Sensors Using Viscoelastic Graphene-Polymer Nanocomposites. Science 2016, 354 (6317), 1257-1260.


21


Page 22 of 25

(26). Wu, T.; Chen, B. Synthesis of Multiwalled Carbon Nanotube-Reinforced Polyborosiloxane Nanocomposites with Mechanically Adaptive and Self-Healing Capabilities for Flexible Conductors. ACS Appl. Mater. Interfaces 2016, 8 (36), 24071-24078. (27). Wu, T.; Gray, E.; Chen, B. A Self-Healing, Adaptive and Conductive Polymer Composite Ink for 3D Printing of Gas Sensors. J. Mater. Chem. C 2018, 6 (23), 6200-6207. (28). Puneet, P.; Vedarajan, R.; Matsumi, N. Electrochemical Evaluation of the Rapid SelfHealing Behavior of Poly(borosiloxane) and Its Use for Corrosion Protection of Metals. Electrochem. Commun. 2018, 93, 1-4. (29). D’Elia, E.; Eslava, S.; Miranda, M.; Georgiou, T. K.; Saiz, E. Autonomous Self-Healing Structural Composites with Bio-Inspired Design. Sci. Rep. 2016, 6, 25059. (30). Liu, Z.; Picken, S. J.; Besseling, N. A. M. Polyborosiloxanes (PBSs), Synthetic Kinetics, and Characterization. Macromolecules 2014, 47 (14), 4531-4537. (31). Lemmers, M.; Spruijt, E.; Akerboom, S.; Voets, I. K.; van Aelst, A. C.; Cohen Stuart, M. A.; van der Gucht, J. Physical Gels Based on Charge-Driven Bridging of Nanoparticles by Triblock Copolymers. Langmuir 2012, 28 (33), 12311-12318. (32). Appel, E. A.; Tibbitt, M. W.; Webber, M. J.; Mattix, B. A.; Veiseh, O.; Langer, R. SelfAssembled Hydrogels Utilizing Polymer–Nanoparticle Interactions. Nat. Commun. 2015, 6, 6295. (33). Seetapan, N.; Fuongfuchat, A.; Sirikittikul, D.; Limparyoon, N. J. J. o. P. R. Unimodal and Bimodal Networks of Physically Crosslinked Polyborodimethylsiloxane: Viscoelastic and Equibiaxial Extension Behaviors. J. Polym. Res. 2013, 20 (7), 183. (34). Tang, M.; Wang, W.; Xu, D.; Wang, Z. Synthesis of Structure-Controlled Polyborosiloxanes and Investigation on Their Viscoelastic Response to Molecular Mass of Polydimethylsiloxane Triggered by Both Chemical and Physical Interactions. Ind. Eng. Chem. Res. 2016, 55 (49), 12582-12589. (35). Zhang, Q.; Archer, L. A. Poly(ethylene oxide)/Silica Nanocomposites: Structure and Rheology. Langmuir 2002, 18 (26), 10435-10442. (36). Bourlinos, A. B.; Ray Chowdhury, S.; Herrera, R.; Jiang, D. D.; Zhang, Q.; Archer, L. A.; Giannelis, E. P. Functionalized Nanostructures with Liquid-Like Behavior: Expanding the Gallery of Available Nanostructures. Adv. Funct. Mater. 2005, 15 (8), 1285-1290.


22

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37). Bode, S.; Enke, M.; Bose, R. K.; Schacher, F. H.; Garcia, S. J.; van der Zwaag, S.; Hager, M. D.; Schubert, U. S. Correlation between Scratch Healing and Rheological Behavior for Terpyridine Complex Based Metallopolymers. J. Mater. Chem. A 2015, 3 (44), 22145-22153. (38). Tan, C. S. Y.; Agmon, G.; Liu, J.; Hoogland, D.; Janeček, E.-R.; Appel, E. A.; Scherman, O. A. Distinguishing Relaxation Dynamics in Transiently Crosslinked Polymeric Networks. Polym. Chem. 2017, 8 (35), 5336-5343. (39). Yan, T.; Schröter, K.; Herbst, F.; Binder, W. H.; Thurn-Albrecht, T. What Controls the Structure and the Linear and Nonlinear Rheological Properties of Dense, Dynamic Supramolecular Polymer Networks? Macromolecules 2017, 50 (7), 2973-2985. (40). Wang, D.-P.; Lai, J.-C.; Lai, H.-Y.; Mo, S.-R.; Zeng, K.-Y.; Li, C.-H.; Zuo, J.-L. Distinct Mechanical and Self-Healing Properties in Two Polydimethylsiloxane Coordination Polymers with Fine-Tuned Bond Strength. Inorg. Chem. 2018, 57 (6), 3232-3242. (41). Li, C.-H.; Wang, C.; Keplinger, C.; Zuo, J.-L.; Jin, L.; Sun, Y.; Zheng, P.; Cao, Y.; Lissel, F.; Linder, C.; You, X.-Z.; Bao, Z. A Highly Stretchable Autonomous Self-Healing Elastomer. Nat. Chem. 2016, 8, 618. (42). Zhang, B.; Zhang, P.; Zhang, H.; Yan, C.; Zheng, Z.; Wu, B.; Yu, Y. A Transparent, Highly Stretchable, Autonomous Self-Healing Poly(dimethyl siloxane) Elastomer. Macromol. Rapid Commun. 2017, 38 (15), 1700110. (43). Lv, C.; Zhao, K.; Zheng, J. A Highly Stretchable Self-Healing Poly(dimethylsiloxane) Elastomer with Reprocessability and Degradability. Macromol. Rapid Commun. 2018, 39 (8), 1700686. (44). Cao, P.-F.; Li, B.; Hong, T.; Townsend, J.; Qiang, Z.; Xing, K.; Vogiatzis, K. D.; Wang, Y.; Mays, J. W.; Sokolov, A. P.; Saito, T. Superstretchable, Self-Healing Polymeric Elastomers with Tunable Properties. Adv. Funct. Mater. 2018, 28 (22), 1800741. (45). Cross, R. The Bounce of a Ball. Am. J. Phys. 1999, 67 (3), 222-227. (46). Li, X.; Zhang, D.; Xiang, K.; Huang, G. Synthesis of Polyborosiloxane and Its Reversible Physical Crosslinks. RSC Adv. 2014, 4 (62), 32894-32901. (47). Moniruzzaman, M.; Winey, K. I. Polymer Nanocomposites Containing Carbon Nanotubes. Macromolecules 2006, 39 (16), 5194-5205.


23


Page 24 of 25


24

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Contents


25

Highly Stretchable and Self-Healing âSolidâLiquidâ Elastomer with

Recommend Documents

Highly Stretchable and Self-Healing âSolidâLiquidâ Elastomer with