Highly Elastic Fibers Made from Hydrogen-Bonded Polymer Complex

Jun 23, 2016 - In this letter, we put forward an approach to prepare hydrogen-bonded complex fibers. First, a spinnable fluid is obtained by restricti...
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Highly Elastic Fibers Made from Hydrogen-Bonded Polymer Complex Jiefu Li,†,‡ Zhiliang Wang,†,‡ Lingang Wen,†,‡ Jing Nie,†,‡ Shuguang Yang,*,†,‡ Jian Xu,§ and Stephen Z. D. Cheng‡,∥ †

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials & College of Material Science and Engineering, Donghua University, Shanghai 201620, China ‡ Center for Advanced Low-dimension Materials, Donghua University, Shanghai 201620, China § Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ∥ Department of Polymer Science, The University of Akron, Akron, Ohio 44325, United States S Supporting Information *

ABSTRACT: In this letter, we put forward an approach to prepare hydrogen-bonded complex fibers. First, a spinnable fluid is obtained by restricting hydrogen bonds, and then it is extruded through a spinneret into a coagulation bath where hydrogen bonds are built to induce fiber formation. The hydrogen-bonded poly(acrylic acid)/poly(ethylene oxide) (PAA/PEO) complex was prepared into fibers. PAA/PEO fiber shows excellent elastic behavior and can be drawn to more than 12× its original length without breaking, which is much higher than Spandex fiber or natural rubber fiber. In the fiber, PAA and PEO are miscible in the molecular level. Dynamic hydrogen bonding between PAA and PEO restricts the crystallization of PEO, retains flexibility of polymer chains, and also provides recovery forces when removing stress. hydrogen-bonded PAA/PEO film which is flexible and could be stretched up to 450%.19 LbL assembly is a controlled polymer complexation process at interfaces. Many hydrogen-bonded polymer complex systems have been prepared as thin films, coatings, and capsules with LbL assembly.20−26 Here we put forward an approach to prepare hydrogen-bonded complex fibers. First, a spinnable fluid is obtained by restricting hydrogen bonds and then it is extruded through a spinneret into a coagulation bath where hydrogen bonds are built and the fiber is formed (Figure 1a). NaOH is deliberately added into the solution to break hydrogen bonds between PAA and PEO.27,28 Due to the restriction of hydrogen bonding, the solution is homogeneous and the viscosity is fit for the spinning process.29 The solution is extruded into a 0.1 M HCl solution. In an acidic environment, PAA is protonated and hydrogen bonds between PAA and PEO are formed (SI, Figures S1 and S2), which leads to fiber formation. SEM images show that the hydrogenbonded PAA/PEO fiber is smooth and dense (Figure 1b,c). The fiber exhibits large elongation (Figure 1d). It can be drawn to more than 12× its original length without breaking (SI, Figure S3). Compared with the LbL assembled PAA/PEO film,19 the elongation of PAA/PEO fiber is much high.

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lastomers and highly elastic materials are essential to human life.1−6 The polymeric materials that can be used as elastomers should have flexible polymer chains, weak abilities for crystallization, relatively low glass transition temperatures, and mechanisms for cross-linking.7 Polybutadiene has a flexible chain, but without vulcanization it is gummy, easy for oxidation, and does not show resiliencies.8 Polysiloxane has a pliable chain, and the cross-linking of siloxane elastomers is obtained by cohydrolysis of dichlorosilanes with alkyl-trichlorosilanes.9,10 Polyurethane is composed of soft and hard segments. The soft segments make the polymer show a high elongation, while hard segments are hydrogen-bonded tie-points that ensure the recovery force.11,12 Low-dimensional elastic materials, such as fibers and thin films, are essential for developing the flexible electronics and intelligent wearable products.13,14 Traditional elastomers, such as natural rubber and polyurethane, are exploited to make functional flexible fibers and films.15−18 In this work, we report a new elastic fiber that is made from the hydrogen-bonded polymer complex of poly(acrylic acid) (PAA) and poly(ethylene oxide) (PEO). PAA is an amorphous polymer and its glass transition temperature is around 100 °C. PEO has flexible chain and relatively low glass transition temperature, but PEO is easy to crystallize. Neither PAA nor PEO shows elastic behavior under ambient conditions. However, through hydrogen-bonding complexation, the PAA/ PEO system shows elastic behavior. For example, Hammond et al. applied layer-by-layer (LbL) assembly to fabricate a © XXXX American Chemical Society

Received: May 5, 2016 Accepted: June 21, 2016

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DOI: 10.1021/acsmacrolett.6b00346 ACS Macro Lett. 2016, 5, 814−818

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ACS Macro Letters

Figure 1. Scheme of fiber preparation (a); SEM images of the PAA/PEO fiber ([AA]/[EO] = 1), external surface (b), and cross-sectional morphology (c); Photographs of a PAA/PEO fiber drawn to various degree (d).

Figure 2. (a) Stress−strain curves obtained by stretching a 20 mm long fiber at a strain rate of 40 mm/min at 25 °C. (b) DSC curves of five polymer complex fibers. 815

DOI: 10.1021/acsmacrolett.6b00346 ACS Macro Lett. 2016, 5, 814−818

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ACS Macro Letters

Figure 3. (a) Stress−strain curves of fiber F1−2 after incubation in relative humidity (RH) 30%, 45%, 60%, 75%, and 90% at 30 °C. (b) TGA traces of fiber F1−2 at a heating rate of 15 °C/min from room temperature to 200 °C under a nitrogen atmosphere after 12 h incubation at different RH.

Figure 4. F1−2 fiber’s diameter (a), ultimate stress (b), ultimate strain (c), and fracture energy (d) as a function of water content (wt %).

and its melting peak is 62 °C. The lack of any endothermic peak indicates that the crystallization of PEO is totally suppressed. XRD pattern further demonstrates that the PEO crystallization in PAA/PEO fibers is prevented (SI, Figure S7). Also, observation of only one glass transition demonstrates that PAA and PEO are intimately mixed. The Tg of the hydrogen-bonded PAA/PEO fiber is measured during the second heating scan under constant nitrogen flow, that is, in a relatively dry state. The Tgs of fibers F3−1 and F2− 1 are much higher than room temperature. Thus, under ambient conditions, these two fibers exhibit mechanical behavior similar to amorphous polymers in the glassy state. As the PEO content in the fiber increases, the Tg gradually decreases. Tgs of fibers F1−1, F1−2, and F1−3 are much close to room temperature. PAA/PEO fibers are hygroscopic and adsorb water from the environment. This absorbed water acts as a plasticizer and lowers the Tgs of the fibers. In an ambient environment, due to water absorption, the actual Tgs of fibers F1−1, F1−2, and F1−3 are expected to be lower than room temperature, which explains their elastic rather than glassy properties.

The elastic properties depend on the molar ratio of PAA and PEO (hereafter the molar ratio of repeating units is shown as [AA]/[EO]: for example, F3−1 and F1−1). The stress−strain curves of the fibers prepared with different molar ratios are shown in Figure 2a. Fibers F3−1 and F2−1 show a yield point and then gradually harden until break, which is typical for materials in the glassy state. Fibers F1−1, F1−2, and F1−3 have no stress yield point, but rather a rubbery plateau and show recovery rate about 80−85% (SI, Figures S4 and S5). As the PEO molar ratio increases, PAA/PEO fibers will change from ductile plastic to rubber. PAA/PEO fibers prepared with different molar ratios have been characterized by differential scanning calorimetry (DSC). In the first heating curve, the fiber exhibits an endothermic peak near the boiling point of water, indicating the hygroscopic nature of the fiber. In the additional heating scan, the fiber shows only one glass transition and no melting peak (Figure 2b). The glass transition temperatures (Tg) of fibers F3−1, F2− 1, F1−1, F1−2, and F1−3 are 80, 73, 33, 20, and 14 °C, respectively. Tg of PEO and PAA are −53 and 105 °C, respectively (SI, Figure S6). PEO is a semicrystalline polymer 816

DOI: 10.1021/acsmacrolett.6b00346 ACS Macro Lett. 2016, 5, 814−818

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hygroscopic. Uptake of water by the fiber lowers the glass transition temperature and the fiber can exhibit an elongation up to 12 times. PAA/PEO fibers will disintegrate in basic water and chemical cross-linking can be considered to enhance the stability of the fibers. The good biocompatibility of PAA and PEO implies that the fibers have potential applications in tissue engineering scaffolds, drug controlled release fabrics, and so on. The favorable dispersibility of spinning dope makes it possible to incorporate various functional molecules and nanoparticles and, hence, endows the elastic fiber with specific functions, such as conductivity or fluorescence.

To further demonstrate that the absorbed water content affects the elastic behavior, hydrogen-bonded PAA/PEO fibers (F1−2) were incubated in different humidity environments. The stress−strain curves obtained after 12 h incubation are shown in Figure 3a. As the humidity increases, the fiber has a higher elongation but lower strength. Specifically, after incubation in RH 30%, the initial modulus is 100 MPa, the ultimate stress is 10.2 MPa and the ultimate strain is 500%. After incubation in RH 90%, the initial modulus decreases to 1.5 MPa and the stress at break is only 2.2 MPa, while the elongation at break is as high as 1200%. Clearly the mechanical properties of the PAA/PEO fibers are very sensitive to the environmental humidity. The water taken up in a humid environment can be removed by heating, as illustrated in Figure 3b. The water release (weight loss) is determined by TGA, and water contents in the fibers after incubation in different humidity environments are shown in Supporting Information (SI, Figure S8). The fiber’s diameter, stress at break, strain at break, and fracture energy as a function of water content are shown in Figure 4. As the water uptake increases, the fiber’s diameter and elongation increase, whereas the ultimate stress decreases significantly. The area under the stress−strain curve represents the fracture energy. As the water uptake increases, the fracture energy will gradually decrease, indicating that water acts as a plasticizer and enhances the mobility of polymer chains. Two other hydrogen-bonded polymer complexes, namely, poly(methacrylic acid)/PEO ([MAA]/[EO] = 1) and PAA/ poly(vinylpyrrolidone) ([AA]/[VPON] = 1) were also spun to prepare fibers. They do not show elastic behavior, contrary to the PAA/PEO fiber. Their elongations at break are less than 5% (SI, Figure S9). The initial modulus of PAA/PEO fiber (F1−1) is 4−9 MPa in an ambient environment. By contrast, the moduli of PMAA/PEO fiber and PAA/PVPON fiber are almost 3 orders of magnitude higher, 2.3 and 2.0 GPa, respectively. Compared with PAA, PMAA has an additional methyl group and shows strong complexation ability with PEO.30 For the PMAA/PEO system, hydrogen bonding is coupled with hydrophobic interaction between the CH3 groups of PMAA and the CH2 groups of PEG, and massive CH3 groups increase the steric hindrance31 which make the chain movement extraordinarily difficult compared with the PAA/PEO system. PVPON is a stronger proton-acceptor and its complexes with PAA are more stable than that formed by PEO and PAA.30 In both cases, one of the components (either PMAA or PVPON) is less water absorbent than the component it replaces. Thus, water-induced plastification is weaker. Tgs of PAA/PVPON and PMAA/PEO are 204 and 142 °C, respectively. The underlying water plastification is insufficient for the fibers to display elastic behavior. A delicate balance of interactions between PAA and PEO explains the elasticity of the fiber. Hydrogen bonding between PAA and PEO is not strong enough to forbid polymer chain movements but it prevents the crystallization of PEO. Since noncrystallized PEO chains are flexible enough to change their conformation during drawing and hydrogen bonds can act as dynamic cross-linking points to guarantee the recovery after the removal of stress, PAA/PEO fiber displays elastic behavior. In summary, highly elastic hydrogen-bonded PAA/PEO fibers were prepared by wet spinning. Hydrogen bonding between PAA and PEO effectively suppresses the crystallization of PEO. By adjusting the molar ratio of AA and EO a plastic to elastic transition can be generated. PAA/PEO fibers are

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EXPERIMENTAL SECTION

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ASSOCIATED CONTENT

Material. Poly(acrylic acid) (PAA; Mw = 450000 g/mol), poly(ethylene oxide) (PEO; Mw = 600000 g/mol), and poly(vinylpyrrolidone) (PVPON; Mw = 360000 g/mol) were purchased from Sigma-Aldrich. Poly(methyl methacrylate) (PMAA; Mw = 100000 g/mol) was supplied by Polyscience. NaOH and HCl were obtained from Kunshan Jinke Microelectronincs Material Co., Ltd. All chemicals were used without further purification. Fiber Preparation. PAA and PEO mixtures (3.48 g) of different molar ratios are dissolved in 40 mL of aqueous NaOH ([AA]:[NaOH] = 10:1) and stirred for 12 h in an ambient environment to ensure complete dissolution. After 48 h of static standing to remove air bubbles, these solutions are extruded through a 100 μm spinneret (single hole) into a coagulation bath of 0.1 M HCl at an extrusion speed of 0.1 mL/min. The fibers are wound up and dried in an ambient environment for 12 h. Characterization. Mechanical properties tests are performed using an electronic single fiber strength tester (China, XS(08)XG-3). PAA/ PEO fiber with a gap of 20 mm is subjected to tensile tests at an elongation rate of 40 mm/min. DSC (Netzsch, 204F) is performed under a nitrogen atmosphere (flow rate: 60 mL/min−1) with 15 °C/ min heating rate from −60 to 150 °C. The relative humidity and temperature condition used for PAA/PEO fibers are maintained with an automatically controlled chamber (Testsky, Nanjing, China). TGA (TA, Q5000) is conducted at a heating rate of 15 °C/min from room temperature to 200 °C under a nitrogen atmosphere. The weight loss before 130 °C is considered as the water content in the PAA/PEO fiber. The morphology of the fibers is investigated by a scanning electron microscopy (Hitachi, SU8010).

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00346.

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Additional supporting figures and experimental details (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

S.Y. gratefully acknowledges the support from National Natural Science Foundation of China (NSFC, Grant No. 51373032), Innovation Program of Shanghai Municipal Education Commission, Fundamental Research Funds for the Central University, and DHU Distinguished Young Professor Program. 817

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DOI: 10.1021/acsmacrolett.6b00346 ACS Macro Lett. 2016, 5, 814−818