Functionalization-Directed Stabilization of Hydrogen-Bonded Polymer Complex Fibers: Elasticity and Conductivity
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Elastic, repairable and conductive fibers are desirable in the newly emerging field of soft electronic and wearable devices. Here, we design a multifunctional fiber by incorporation of different components to optimize its performance. The combination of the poly(acrylic acid) (PAA) and poly(ethylene oxide) (PEO) through hydrogen bonding endows the fiber with high elasticity and repairability. Polydopamine (PDA) significantly increases the stability of the fiber, thus the fiber will not dissolve in alkaline solutions and still keep the repairable ability. The fiber shows a reversible swelling-shrinking property as pH values go up and down. Further, the conductive component, carbon nanotube, is adsorbed at the swelling state and then is fastened with fiber shrinking.
KeywordsHydrogen bond Polymer complex fiber Self-healing Elastic Conductive
The combination of elasticity and the ability to undergo large deformation is a special property of selected polymeric materials. These characteristics have led to the wide adoption of such materials in applications such as tires, seals, dampers, gloves, tubes, and fibers [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Polymers exhibiting high elasticity should (i) possess flexible chain segments to ensure large deformation under load ; and (ii) undergo cross-linking to prevent flow and creep, thereby guaranteeing recovery to original dimensions upon stress removal . Different strategies have been used to form cross-links, including covalent bonds, and/or secondary interactions that induce aggregation, glass or crystalline domains [14, 15, 16, 17, 18, 19].
Recently, a new elastic system with no distinguishable cross-linking junction has been reported, namely, a hydrogen bonded polymer complex of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) [20, 21, 22]. While PEO has a flexible polymer chain with a Tg of − 60 °C, it crystallizes easily and the bulk material is rigid [23, 24]. The carboxylic acid groups of PAA (Tg > 100 °C) can readily hydrogen bond with the ether moieties of PEO . After complexation, the crystallization of PEO is restricted while the polymer chains retain their flexibility, thus the system becomes highly stretchable [20, 21, 22]. After removal of the stress, the system is shown to have good dimensional recovery . The key factor associated with the elastic properties of the polymer complex is the dynamic nature of the hydrogen bonded network that can effectively act like permanent cross-linking junction points. The hydrogen-bonded PAA/PEO complex, however, has major weaknesses; it swells in neutral water and dissolves quickly in aqueous alkaline media [26, 27, 28].
In this work, we overcame the weaknesses associated with the PAA/PEO complex, particularly in fiber form, whereby the hydrogen bonded structure was toughened, stabilized and functionalized via catechol chemistry. The resultant (PAA/PEO)@PDA fiber exhibits gradient chemical and dynamic hydrogen bonded cross-linked structure. In addition to retaining the elasticity and mendable (repairable) characteristics of the original PAA/PEO complex fiber, (PAA/PEO)@PDA becomes stable, strong and tough. Notably, carbon nanotubes (CNTs) can be adsorbed onto the fiber surface when the fiber is in the swollen state and fastened to the fiber during shrinking, resulting in a wrinkled structure on the fiber surface. Introduction of the CNTs affords a conductive fiber, whose conductivity is tensile sensitive. These characteristics suggest potential future applications in flexible electronics and wearable devices.
Poly(acrylic acid) (PAA; Mw = 450,000 g mol−1), and poly(ethylene oxide) (PEO, Mw = 600,000 g mol−1) were purchased from Sigma-Aldrich. Dopamine hydrochloride (Dopa) was obtained from Alfa Aesar. Multi-walled carbon nanotubes (inner diameter: 3–5 nm, outer diameter: 8–15 nm, length: about 50 μm, purity: 95%, conductivity: 106,000 μS cm−1) were bought from Aladdin Chemical Co., Ltd. (China). Hydroxide (NaOH) and hydrochloric acid (HCl, 36.5 wt%) were supplied by Kunshan Jinke Microelectronics Material Co., Ltd. Other chemicals were analytical grade.
Preparation of PAA/PEO Fiber
Preparation of PAA/PEO fibers was following a procedure reported by our group . Briefly, PAA and PEO mixtures (3.48 g) with a molar ratio of 1: 2 were dissolved in 40 mL of aqueous NaOH and stirred for 12 h. After removal of air bubbles, these solutions were extruded through a 100 μm spinneret (single hole) into a 0.1 M HCl coagulation bath at an extrusion speed of 0.1 mL min−1 to obtain fiber.
Preparation of (PAA/PEO)@PDA fiber
The PAA/PEO fibers were washed using pH 2 aqueous solution. Subsequently, these fibers were immersed in a dopamine solution (pH 2, and 5 mg mL−1) for at least 15 min to let dopamine fully diffused into them. An oxidant (NaIO4) was then added to induce dopamine polymerization. After 6 h polymerization, the fibers were washed three times with a pH 1 aqueous solution to remove NaIO4 and the unstable polydopamine. (PAA/PEO)@PDA fibers were naturally dried in an ambient environment.
Adsorption of CNT
0.1 g L−1 CNT solution was dispersed in 1 g L−1 SDS solution under sonication for 20 min. The (PAA/PEO)@PDA fibers were immersed into neutral water for pre-swelling and then immersed in the CNT dispersion for 15 min. After washing by DI water, the fibers were immersed into pH 1 aqueous solution for 30 min. Finally, the fibers were dried in an ambient environment for 12 h.
Mechanical properties tests were performed using an electronic single fiber strength tester (XS(08)XG-3, China). (PAA/PEO)@PDA fiber with a gap of 20 mm was subjected to tensile tests at an elongation rate of 40 mm min−1. Before testing, all the (PAA/PEO)@PDA fibers were incubated for a certain time to reach an equilibrium state in an automatically controlled humidity chamber (Testsky, Nanjing, China) with a designated humidity and temperature. The morphology of the fibers was observed by scanning electron microscopy (SEM, Hitachi, S4800) equipped with an EDS system. The resistance of the fibers was measured as a function of tensile strain by a two-probe method (Keithley 2000 source meter, China). The conductive fibers are connected to copper wires through silver paste. The probe heads of Keithley are attached to the two grippers of the copper wires. Differential scanning calorimetry (DSC, Netzsch, 204F) was performed under nitrogen atmosphere (flow rate: 60 mL min−1) with 15 °C min−1 heating rate from − 60 to 120 °C. Fourier transform infrared (FTIR) spectra (Nicolet 8700) were recorded using a Single-Bounce ATR attachment. The LSCM imaging was recorded by a laser scanning confocal microscopy (Leica, TCS SP5II). The X-ray diffraction (XRD) spectra of the fibers were obtained on a diffractometer (Bruke, D8) with a Cu Ka radiation. The X-ray photoelectron spectra (XPS) were collected using a spectrometer (Thermo Fisher Scientific, Escalab 250Xi) with a dual Al Kα X-ray source.
Results and Discussion
The Structure and Mechanical Properties of (PAA/PEO)@PDA Fibers
While no melting transition was observed for the PDA treated fiber, the observed glass transition temperature was slightly higher than that of the parent PAA/PEO analog: 26 °C vs. 20 °C, respectively. Differences in FTIR spectral features were also evident in the PDA modified fiber vs. the pristine sample (Fig. 2b). Specifically, the carbonyl (C=O) vibration peak of (PAA/PEO)@PDA is shifted to 1723 cm−1 from its initial 1726 cm−1. The cause of the observed differences in Tg and C = O vibrations are discussed below. The cross-section of (PAA/PEO)@PDA was also analyzed by SEM–EDS (Fig. S4), which confirmed the presence of nitrogen derived from PDA on the fiber surface and within.
According to the characterization results presented above, the following mechanism was proposed to interpret the chemical processes taking place during the fabrication of the modified fiber. It is known that the catechol containing dopamine molecules can be easily transformed into their reactive quinone counterparts in the presence of an alkaline solution or a strong oxidizing agent, and then induced to polymerize [30, 31, 32]. This oxidation-induced polymerization leads to the production of the cross-linked polymers baring catechol and quinone functionalities, which can be exploited for further modifications [33, 34]. In the present work, PAA/PEO fibers were immersed into the dopamine acidic solution containing NaIO4 as the oxidant, whereby dopamine was oxidized to the corresponding quinone. The resulting oxidation product, which was adsorbed onto the fiber surface and diffused into the fiber, subsequently polymerized. A PDA skin layer was then formed on the fiber surface, as well as a loose covalently network structure was believed to be formed within the fiber matrix. In addition, PDA can undergo hydrogen-bonding with both PAA and PEO due to the exist of phenol groups in the structure. The presence of both covalent and hydrogen bond induced cross-linking within the fiber matrix most likely results in the increase of Tg and the red shift of the C=O IR vibration [35, 36].
As with the parent PAA/PEO fibers, the (PAA/PEO)@PDA analog displays a typical elastic behavior: the strain–stress curve shows initial stiffening, followed by a rubbery plateau, and a strain-induced hardening until break (Fig. 3a). The low-strain properties remain unchanged (initial stiffening region), and the initial Young’s modulus is 9.3 ± 2.3 MPa, similar to that of PAA/PEO fibers (Table S1, Supporting Information). A less rubbery plateau and more hardening are observed for (PAA/PEO)@PDA fibers. Such a nonlinear hardening behavior is more favorable for resisting the propagation of cracks . These characteristics suggest that the PDA induced network is loose and soft, which normally affects the mechanical response at a higher strain. In the present fibers, the crosslink density is very critical. When dopamine treatment was extended to more than 12 h, the fiber lost its rubber-like elastic behavior and yielded under very low strain. Its initial modulus and break strength greatly increased (Fig. S6, Supporting Information).
According to this standard, the recovery rate of (PAA/PEO)@PDA fibers was determined to be 90%, higher than that of PAA/PEO fibers (90% vs. 80%).
We further investigated the time dependence of the fiber recovery rate. The fiber was extended to 300% strain and then unloaded. After resting for different times, the fiber was reloaded and released, as presented in Fig. 3c. The permanent set is denoted as the least strain in the reloading curve that relates to a positive engineering stress. The value of permanent set reflects the recovery ability of the fiber. Figure 3d shows the permanent set as a function of resting time. As the resting time increases, the permanent set decreases, indicating an increased recovery rate. After 6 h resting time, the permanent set is approaching to zero, i.e., the fiber almost exhibits a 100% recovery. The results suggest that the recovery process includes both a quick and a slow process.
Progressive loading–unloading cycles with 100% strain increments were performed until break. The results are shown in Fig. 3e. The area between the loading and unloading curves is defined as the damping energy. The damping energy divided by the mechanical work (the area under the load curve) yields the damping capacity, which reflects the ability of a material to absorb energy by converting mechanical energy into heat. Figure 3f presents the damping capacity of (PAA/PEO)@PDA as a function of the applied strain. At low applied strain, the fibers display small damping capacity and little hysteresis. At high applied strain, the fibers exhibit high damping capacity with pronounced hysteresis, indicating that the fibers can dissipate energy efficiently.
Again, the mechanical behavior of the dopamine treated fibers appears dependent upon the interior characteristics of the fibers. Given the preparation process, (PAA/PEO)@PDA fibers are expected to have a hierarchical network structure, mainly comprised of a hydrogen bonded PAA/PEO network and a PDA induced covalent network which distributed gradually from the outer fiber surface to the center axis. While the dynamic hydrogen bonds can reversibly break, reorient, and re-form, which helps the energy dissipation during loading and reloading, the PDA generated covalent network serves to bridge cracks and stabilize deformation, enabling hydrogen bonds to unzip for large deformations.
Further, the fibers were healed in RH 90% at 30 °C for different time to systematically investigate their self-healing properties. Stress–strain curves show that a long healing time results in a high recovered ultimate strain (Fig. 4e). Even when contact time is as short as 1 h, the healed fiber can be stretched up to about 350%. The fibers retain a 748 ± 85% strain with a healing efficiency of 71% after 24-h healing. Because the fibers are too thin, it is difficult to match them perfectly when the two pieces are brought into contact together. Thus, the minor scars are observed in SEM images (Fig. 4c), resulting in the incomplete self-healing. In addition, the damaged hydrogen bonding can re-form but the covalent bonds cannot. This may explain why the ultimate stress of the 24 h healed fibers is similar to that of the uncross-linked PAA/PEO fibers.
Reversible Swelling–Shrinking Behavior
Construction of Conductive Fibers
The electrical performance of the CNT wrapped fibers were investigated as a function of tensile strain on a high precision load/displacement measurement apparatus at a fixed strain rate of 20 mm min−1. As shown in Fig. 6c, the resistance is linearly dependent upon the applied strain, especially after the second stretch/release cycle. A simple circuit was used to connect a blue-colored LED to the conductive fibers. It was observed that the intensity of light emitted by the LED decreased when the fibers were stretched up to 300% strain (Fig. 6d) but recovered upon release. The cycle was repeated 5 times (Fig. 6e), pointing to the excellent cycling stability and repeatability for strains less than 300%. Furthermore, 200 cycles of 200% strain drawing were performed, and the fibers showed an excellent repeatability (Fig. S9, Supporting Information). In contrast, fibers with CNT absorbed in the non-swelling state (at pH 1) exhibited a much high initial resistance and its resistance was unstable when the strain was higher than 60% (Fig. S10, Supporting Information).
To illustrate one of many potential applications, the conductive CNT wrapped (PAA/PEO)@PDA fiber was used to construct wearable sensors to detect motion. The wearable sensors were then attached to different positions of the human body by tape. Various physiological signals could be thus obtained by monitoring changes in the electrical resistance. For example, a quick wrist bend resulted in a sharp pulse signal (Fig. 6f). Five independent strain sensors were also affixed to a glove to identify different human gestures (Fig. 6g). Different gestures induced differential resistance responses. For example, when fingers were bent, the resistance increased suddenly and then remained stable. More importantly, the strain sensors exhibited a good response rate and cycling stability. Hence, such conductive, elastic fibers can be used as an effective sensor for monitoring physiological movement.
In summary, tough, highly elastic polymer fibers based upon a complex of PAA with PEO were introduced. Upon treatment with dopamine, the produced (PAA/PEO)@PDA fibers combined the advantages associated with a hydrogen-bond network and a chemically gradient PDA network that distributing from the outer skin layer to the fiber center axis. The hierarchical structure could effectively dissipate energy during stretching and enhance the toughness, strength, and elastic recovery rate. The fibers could be easily induced to reversibly swell and shrink in basic and acidic aqueous media, respectively. A large amount of CNTs could be adsorbed to the fiber surface on in its swollen state and fastened to the fiber upon shrinkage, which generated a compact conductive CNT layer on the fiber surface. The CNT-wrapped fibers exhibited a tensile sensitive resistance and were shown to have potential in applications such as wearable strain sensors. Moreover, further chemical modification or immobilization of other functional nanoparticles is expected to endow (PAA/PEO)@PDA fibers with alternative specific functionalities, such as fluorescence, magnetic or photothermal properties.
This wok was financially supported by Science and Technology Commission of Shanghai Municipality (Grants No. 16JC1400700) and Fundamental Research Funds for the Central University (102552017045).
Compliance with ethical standards
Conflict of interest
There are no conflicts of interest.
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