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Sustainable, Reshapable Surfactant−Polyelectrolyte Plastics Employing Water as a Plasticizer Zhangjun Huang, Haiyan Jia, Antoine P. van Muyden, Zhaofu Fei, and Paul J. Dyson* Institut des Sciences et Ingénierie Chimiques, Ecole Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
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ABSTRACT: Natural polymers such as those present in foods contain abundant noncovalent intra- and intermolecular interactions, notably hydrogen bonds, which make them rigid when dry, but on exposure to water soften, due to disruption of these interactions. This softening process allows them to be reshaped. Food-derived materials, however, have limited practical use due to their high brittleness and gradual degradation. Nevertheless, inspired by such properties, surfactant−polyelectrolytebased polymers that contain abundant ionic interactions and can be repeatedly reshaped using water as plasticizer are described. The polymers, on the basis of main chain anionic poly(styrene sulfonates) combined with phosphonium surfactant, are readily synthesized with well-defined lamellar domains through interfacial metathesis reactions. The polymers present typical stress−strain characteristics of plastics, and their modulus undergoes a decrease of ca. 3 orders of magnitude upon shear and stretch forces after plasticizing with water. Since recycling of plastics generally involves complicated and energy-intensive processes (that leads to the majority of plastics being land-filled or incinerated), it is envisaged that reshapable polymers, such as those described here, could reduce the amount of plastic waste as they can be remolded as and when required, thus reducing pollution and the depletion of resources, ultimately contributing to a more sustainable society. KEYWORDS: recycling plastics, reshapable, water plasticizer, surfactant−polyelectrolytes, sustainable encountered in foods such as doughs.9 Doughs are composed of stretched proteins and aggregated starches,10 and the abundant noncovalent interactions (largely hydrogen bonds) cross-linking the two components provide doughs with rigidity when dry but become soft and flowable after being plasticized by water. Although some dry doughs exhibit considerable strength under ambient conditions, they are unsuitable for applications as plastics as they gradually degrade and are generally too brittle when dry. Consequently, macromolecules linked by ionic interactions rather than hydrogen bonds have been studied to overcome the limitation of natural reshapable materials. Surfactant−polyelectrolyte (SUPE)-type polymers are fabricated readily from polyelectrolytes (PEs) and oppositely charged surfactants through entropy-driven based salt metathesis reactions.11 Note, it is also possible to fabricate the SUPE polymers from monomers.12 SUPEs are insoluble in water and typically have self-ordered lamellar structures,13 in which the PE and surfactant phases interact strongly with each other through electrostatic interactions. Polymers with such electrostatically cross-linked structures tend to exhibit tough mechanical properties in the bulk state.14−18 SUPEs have been used in self-healing materials19 and can undergo light-induced shape transformations20 or pH-responsive rearrangements.21
1. INTRODUCTION Plastics are lightweight and durable materials that can readily be molded into a variety of products that find use in a vast range of applications.1 However, the production of new plastics hugely surpasses recycling efforts, leading to a huge accumulation worldwide and considerable environmental problems.2 On the basis of the versatility and ubiquity of synthetic polymers in the world today, simply eliminating them would be highly problematic. Polymers based on renewable feedstocks are more sustainable than synthetic polymers derived from petrochemical feedstocks, but their level of production corresponded to only 1.7 megatons from nearly 300 megatons of plastics produced globally in 2014.3 In addition to substituting polymers derived from fossil sources by those derived (or partly derived) from renewable feedstocks, reducing the amount of polymers used is also important and, consequently, reshapable plastics that can be remolded in a facile manner could significantly contribute to this end. Self-healing polymers can be partially reshaped or even extensively reshaped in the presence of external stimuli, including temperature, 4 pressure,5 or application of a plasticizer.6,7 These polymers tend to be soft; thus, they are not suited to the everyday applications of many polymers that must be both rigid and robust over a range of conditions. Plasticizers have been extensively used to improve the reshapability of polymers.8 The phenomenon of plastic-toviscoelastomer transfer triggered by water is commonly © XXXX American Chemical Society
Received: June 3, 2019 Accepted: July 22, 2019
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DOI: 10.1021/acsami.9b09426 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 1. Synthesis, structure, and characterization of the SUPEs P26S20 and P26S100. (A) Illustration of the synthetic procedure. (B) 1H NMR spectrum (CD2Cl2) of P26S100, with the peak integration for aromatic protons of PSS (red and orange arrows) and the terminal aliphatic protons of P26 (blue arrows). (C) 31P NMR spectra (CD2Cl2) of P26Cl, P26S20, and P26S100. (D) WAXRD spectra of P26S20 and P26S100 containing a main peak at 2θ = 20.8° and a higher order peak at 2θ = 42.6°. (E) SAXRD spectra of P26S20 and P26S100 containing a broad peak with a periodical distance of 34 Å. (F) Photograph of a tensile bar (size: 36 × 10 × 1 mm3) constructed from P26S100 and an illustration of the proposed domains in the SUPEs based on the WAXRD and SAXRD spectra.
Figure 2. Mechanical and rheological properties of P26S20 and P26S100 and slip motions of the polymer chains. (A) Stress−strain curves of P26S20 and P26S100 at 23 ± 2 °C and RH = 44%. (B) Master curves for P26S20 and (C) master curves for P26S100 at a reference temperature of 23 ± 2 °C, prepared by the time−temperature superposition treatment of the storage modulus (G′) and loss modulus (G″) values obtained in the range of 30−80 °C for P26S20 and 30−90 °C for P26S100 in frequency-dispersion tests [frequency (ω) = 1−628 rad/s] at a constant applied strain of 0.1%. (D) Relaxation times (τ) of P26S20 and P26S100 for the flow transition, estimated from the crossover points of G′ and G″ at various temperatures (15−30 °C) (see Figures S5−S7 for further details). (E) Arrhenius plots of the intersection frequencies ( f = ω/2π) for the G′ and G″ curves of P26S20 and P26S100, according to the frequency sweep tests [ω = 1−628 rad/s] at a constant applied strain of 0.1% in the temperature range for the flow transition (Figure S8). (F) Apparent activation energies (Ea) for the slippage of the polymer chains in P26S20 and P26S100.
They have been explored in biomedical applications,22 used in combination with contrast agents for magnetic resonance imaging,23 and as drug delivery systems.24 SUPEs have been evaluated in many other applications, including water purification,25 catalysis,26 organic solar cells,27 etc. Herein, we describe the synthesis of rigid SUPE materials derived from medium (Mw = 200 000 Da) or high (Mw = 1 000 000 Da) molecular weight poly(sodium styrene sulfonate) (PSSNa) and a phosphonium chloride surfactant (P[4,4,4,14]-
Cl, termed P26Cl)28,29 via salt metathesis and elimination of NaCl. The resulting SUPEs (termed P26S20 and P26S100) display water-triggered plastic-to-viscoelastomer properties, allowing reshaping, but when dry, they exhibit strong mechanical properties and stabilities. The characterization, properties, and application of these unique reshapable polymers are also described. B
DOI: 10.1021/acsami.9b09426 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Figure 3. Influence of humidity and water on P26S20 and P26S100. (A−H) Reshaping processes of water-saturated P26S100. (I) The water content in P26S20 and P26S100 at ambient RHs (40−67%), extreme RH (>90%), and water-saturated at 23 ± 2 °C. G′ and G″ of P26S100 (J) and P26S20 (K) under ambient conditions (23 ± 2 °C, RH = 44 and 65%), extreme humidity (23 ± 2 °C, RH > 90%), and water saturation at frequency (ω) = 1−628 rad/s and a constant applied strain of 0.1%. The stress−strain of P26S20 (L) and P26S100 (M) under ambient conditions (23 ± 2 °C, RH = 44 and 65%), extreme humidity (RH > 90%), and water saturation. (N) Illustration of the proposed interaction of water molecules with the self-organized structure of the SUPE under an ambient environment and following saturation with water.
2. RESULTS AND DISCUSSION The SUPEs, P26S20, and P26S100, composed of the PSS anion (Mw of corresponding PSSNa = 200 000 and 1 000 000 Da, respectively) and the P26 cation, were prepared via a biphasic interfacial salt metathesis reaction (Figure 1A), between corresponding PSSNa and P26Cl (see the Experimental Section for full details). The SUPE and NaCl byproducts form at the interface of the two immiscible solvents and autoseparate, with the SUPEs migrating into the organic phase and the NaCl by-product into the aqueous phase. The organic phase is then extracted and repeatedly washed until no chloride is detected in washings. The P26S100 and P26S20 products are obtained as solids following removal of ethyl acetate under vacuum. 1H NMR spectra of P26S100 (Figure 1B) and P26S20 (Figure S1) confirm the 1:1 stoichiometry between the styrene sulfonate group of the PSS anion and the P26 cation (on the basis of peak integration of the aromatic protons of the polymer (red arrow, Figure 1B) and the terminal aliphatic protons of the phosphonium cation (blue arrow, Figure 1B). The 13C NMR spectra of P26S20 and P26S100 exhibit peaks for the aliphatic carbon atoms of the cation and polyanion between 13 and 40 ppm and signals for aromatic carbon atoms of the polyanion centered at 126 and 145 ppm (Figure S2). Moreover, the position of the peak of the phosphorus center in the 31P NMR spectrum of P26Cl
changes quite significantly in P26S100 and P26S20 (δ = 0.31 ppm), indicative of the different environments of the phosphonium cation (Figure 1C). Elemental analysis (Figure S3 and Tables S1 and S2) indicates that the polymers are of high purity and free from residual NaCl impurities. Film prepared from the polymers are flexible, highly transparent, and thermally stable, decomposing at 340 °C (P26S20) and 351 °C (P26S100) under a nitrogen atmosphere (Figure S4). Neither a glass transition nor any other phase transitions were observed by differential scanning calorimetry, consistent with related materials.11 Wide angle X-ray diffraction (WAXRD) spectra of P26S20 and P26S100 (Figure 1D) show an absence of crystallinity, although the diffraction peak at 2θ = 20.8°, corresponding to a Bragg spacing of ca. 4.3 Å that is close to that of a densely packed alkane layer30 but is narrower than that expected for an amorphous polymer and a higher order peak, is also observed at 2θ = 42.6°. The small angle X-ray diffraction (SAXRD) spectra of P26S20 and P26S100 (Figure 1E) contain a peak, which corresponds to a long period of about 34 Å. The WAXRD and SAXRD spectra of P26S20 and P26S100 suggest the presence of domains with lamellar-like structures that presumably correspond to alternating alkane and ionic layers in the materials, as illustrated in Figure 1F. Tensile strips and plastic cylinders were fabricated from P26S20 and P26S100 and subjected to mechanical and C
DOI: 10.1021/acsami.9b09426 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces rheological tests, respectively, at ambient temperature (23 ± 2 °C) and a relative humidity (RH) of 44%. The stress−strain curves (test rate: 10 mm/min) display an abrupt increase in stress reaching 3.5 MPa for P26S20 (pink, Figure 2A) and 19.5 MPa for P26S100 (green, Figure 2A). P26S20 and P26S100 yield at an applied strain of 16 and 28%, respectively. After the yielding stage, the specimens elongate by up to 682 and 526%, respectively, and then fracture. Young’s modulus of P26S20 and P26S100 is 46 and 282 MPa, respectively. According to the tensile tests, both polymers show typical stress−strain curves of plastics, with an increase of 5.6 times in tensile strength before yielding and an increase of 6.1 times in the modulus achieved as the molecular weight of the polyelectrolyte is increased from 200 000 Da in P26S20 to 1 000 000 Da in P26S100. The higher molecular weight leads to more extensive intermolecular interaction, presumably resulting in the higher mechanical and Young’s modulus and relatively smaller elongation. A similar strengthening effect is observed when shear force is applied. On the basis of the frequencydispersion properties of the storage modulus (G′) and loss modulus (G″) values of P26S20 and P26S100 between 15 and 90 °C with angular frequency (ω) from 1 to 628 rad/s, the relaxation times (τ) for the flow transition of P26S20 and P26S100 were estimated (Figure 2B,C) from their master curves between 15 and 30 °C (Figures S5, S6, and S7).31 The τ values for P26S20 (pink squares, Figure 2D) are in the range of 106 s (on the order of weeks) to 108 s (on the order of years), whereas for P26S100 (green circles, Figure 2D), the τ values are in the range of 108 s (on the order of years) to 1010 s (on the order of hundreds of years). The τ values of both samples fluctuate to a lesser extent than amorphous plastics physically cross-linked by hydrogen bonds in the same temperature range, in which τ values drop from years to days as the temperature is raised from 20 to 30 °C.5 The dramatic improvement in τ for P26S100, compared with P26S20, is presumably due to the additional accumulated electrostatic and Van de Waal forces in the lamellar structure due to the longer polymeric backbone, restricting the flow of the domains. The apparent activation energies (Ea) required for the flow of the domains were estimated from the Arrhenius plots (Figure 2E) of the intersection frequencies of the G′ and G″ curves (Figure S8) as 304 and 230 kJ/mol for P26S100 and P26S20, respectively (Figure 2F). The SUPE plastics are relatively rigid under ambient conditions and become soft after soaking water overnight (P26S100 is shown as an example in Movie S1). This watertriggered plastic-to-viscoelastomer property enables the SUPE (Figure 3A) to be rolled (B), stretched (C), cut (D), molded into stripes (E), circled to close the loop (F), and baked (G). The entire process is reminiscent of reshaping and baking a dough, and the reshaped and baked (dried) P26S100 material recovers rigidity (Movie S2). Note that the water content of the SUPEs in air is 90% (the upper limit of the humidity detector is 90%), the decrease in G′/G″/stress values are 43, 46, and 41% for P26S100 and 61, 66, and 63% for P26S20. It is noteworthy that PSSNa is used clinically as a sequestrant32 and P26Cl can be used as a nonoxidizing broadspectrum biocide, and both are essentially non-biodegradable.33 Hence, the SUPEs composed of these components are likely to be nontoxic and non-biodegradable.
3. CONCLUSIONS In summary, we demonstrate that P26S20 and P26S100 behave as robust plastics under ambient conditions and soften after soaking in water, allowing them to be reshaped. The water-softened SUPEs recover their robustness after baking (e.g., at 50 °C). Although the mechanical properties of the SUPEs are somewhat less strong than commercially available plastics, increasing the molecular weight of the polymer backbone could lead to improvements in their mechanical properties. Their water-triggered plastic-to-viscoelastomer properties could help decrease the amount of plastic waste used and, in turn, reduce energy expenditure and pollution. 4. EXPERIMENTAL SECTION 4.1. Materials. Experimental details. All chemicals were obtained for commercial sources and used as received unless otherwise stated. PSSNa with weight average molecular weight of 200 000 and 1 000 000 were purchased from Sigma. P26Cl was obtained from CYTEC and used directly. All organic solvents were of analytical grade, and Mili-Q water (18 MΩ cm) was used. 4.2. Instrumentation. 1H, 13C, and 31P NMR spectra were recorded on a Bruker 400 MHz instrument; CD2Cl2 was used as solvent. Thermal degradation was determined using a Perkin Elmer TGA-7 thermogravimetric analyzer (TGA) at a heating rate of 10 °C/ min under a nitrogen atmosphere. Field emission SEM images were obtained using a Merlin instrument equipped with an EDS detector. Elemental distributions were determined by EDS analysis. Small angle and wide angle X-ray diffractions (SAXRD and WAXRD) were recorded on an Empyrean machine; the scanning range and rate for SAXRD and WAXRD were 0.2−5° (at 0.1°/min) and 5−70° (at 6°/ min), respectively. The tensile stress of samples with a size of 36 × 10 D
DOI: 10.1021/acsami.9b09426 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces × 1 mm3 was tested on a Zwick Z010 instrument equipped with a 5 kN detector. Young’s modulus was determined using tensile strength at 5% strain. The rheology of the samples with a wafer shape (25 mm in diameter and 2 mm thick) was determined on an Anton Paar model MCR-102 rheometer. Both tensile and rheological tests were conducted under 23 ± 2 °C; different relative humidities were as follows: RH = 44, 65, and >90% and water saturation. The temperature and humidity were measured by a mini hydrometer with a detecting range of RH = 20−90%. 4.3. Synthesis of P26S20 and P26S100. P26Cl (12.65 g) was dissolved in ethyl acetate (200 mL). The solution was slowly poured into an aqueous solution of PSS (120 g, 5 wt %, Mw = 200 000 or 1 000 000, pH 8.9). The mixture was stirred for 4 h, then transferred to a 500 mL separating funnel, and, after phase separation, the organic phase was collected and repeatedly washed using an aqueous solution of PSS (0.1 wt %), until the water phase was chloride free (tested with AgNO3 aqueous solution). The ethyl acetate phase was then collected and dried under vacuum to obtain the products. Isolated yields of P26S20 and P26S100 exceeded 95%. P26S20: 1H NMR (400 MHz, CD2Cl2, δ): 7.45 (br, 2H), 6.46 (br, 2H), 2.25−2.17 (m, 8H), 1.47−1.28 (m, 36H), 0.90−0.80 (m, 12H). 13 C NMR (101 MHz, CD2Cl2, δ): 145.8, 127.2, 40.2, 32.0, 30.9, 29.8, 29.4, 29.1, 23.7, 22.7, 21.8, 18.6, 18.1, 13.9, 13.4. 31P NMR (162 MHz, CD2Cl2, δ): 32.90. P26S100: 1H NMR (400 MHz, CD2Cl2, δ): 7.37 (br, 2H), 6.38 (br, 2H), 2.17−2.10 (m, 8H), 1.40−1.20 (m, 36H), 0.83−0.72 (m, 12H). 13C NMR (101 MHz, CD2Cl2, δ): 145.8, 126.5, 40.2, 32.1, 31.0, 29.9, 29.5, 29.2, 23.7, 22.8, 22.0, 18.9, 18.4, 14.0, 13.5. 31P NMR (162 MHz, CD2Cl2, δ): 32.90.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b09426.
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NMR spectra; SEM and EDS data; TGA; characterization; thermal stability and rheological and tensile properties of SUPEs (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: paul.dyson@epfl.ch. ORCID
Paul J. Dyson: 0000-0003-3117-3249 Notes
The authors declare no competing financial interest.
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REFERENCES
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