Soy Protein Film with Fully

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An Ultraelastic Poly(ethylene oxide)/Soy Protein Film with Fully Amorphous Structure Jianying Ji, Bin Li, and Wei-Hong Zhong* School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164, United States S Supporting Information *

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than 50% after addition of 20 wt % SPI.15 However, considering the special helical structure similar to that of PEO and the large amount of polar groups involved,16 we explored the possibility of protein products, such as SPI, acting as candidate materials for improving the intrinsic crystal of PEO. Recently, great interest has been aroused in flexible/bendable electronics such as roll-up displays and wearable devices. In order to fully achieve functionality of these devices, compliant batteries with high energy and power density should be developed to complement them. However, it is a great challenge to fabricate flexible/bendable power sources due to the lack of materials that combine superior electrical conductivity and high mechanical flexibility. One attempt to achieve highly elastic ion-conductive materials was by pursuing ionic conductivity in common elastomers. For example, the ionic conductivity of natural rubber/PEO blends has been studied. However, the ionic conductivity was too low at room temperature for practical applications.17 In this paper, we present our recent exploratory studies on a new type of ultraelastomeric ion-conductive PEO/SPI film, which was produced by blending the denatured SPI (55 wt %) with PEO. The resultant film has a fully amorphous structure with ultraelasticity. Preliminary studies indicate that the ionic conductivity was dramatically enhanced compared with that of pure PEO-based film. The amorphous thin films are easy to fabricate, and both processing and material usage are environmental friendly, so that this bio-based PEO/SPI film has a great potential for applications in foldable/flexible electronics. Solid protein isolate is a mixture of soy proteins, which are mainly composed of glycinin and β-conglycinin.18,19 Glycinin consists of one basic and one acidic polypeptide, which are linked to each other by a single disulfide bond. β-Conglycinin is a trimeric glycoprotein consisting of three types of subunits α, α′, and β, in seven different combinations. The subunits are associated via hydrophobic and hydrogen bonding (Figure 1a). During the denaturation, the bonding interactions responsible for the secondary and tertiary structure are disrupted, leading to an unfolded structure. After denaturation, a transparent material resulted (see Supporting Information Figure S1),18,20 which indicates that a fine-stranded network structure has formed. The lithium ions are strongly adsorbed onto the surface of the SPI due to the negative acid group in the SPI (Figure 1b). A preferential protein−salt binding results in an

oly(ethylene oxide) (PEO) is a semicrystalline polyether often terminated with hydroxyl groups. It is available with average molecular weights ranging from 200 to 8 × 106 g/mol (according to the Sigma-Aldrich product list), and a relatively narrow molecular weight distribution can be achieved compared with many other polymers; i.e., the polydispersity of PEO is less than 1.1 prepared by anionic ring-opening polymerization.1 The important properties, such as lack of color and odor, inertness to many chemical agents, biocompatibility, and lack of immunogenicity, make it as a preponderant polymer for biomedical applications.2 Moreover, fast ion transportation in PEO doped with sodium salts was found by Wright and coworkers 30 years ago.3 The possibility of PEO-based electrolytes for applications in solid-state electrochemical devices was suggested a few years later by Armand and co-workers.4 It was clear soon after that PEO is the most successful host material for electrolytes due to the suitable distance between each hanging ether oxygen, which is very important in the salt dissociation and charge transport.5 Either too large or too small of a distance between polar groups will lower the charge transport possibilities. Also, it has been proved that fast ionic conduction takes place in the amorphous electrolyte phases, in which conductivity is 2/3 orders of magnitude higher than in the crystalline phases due to the increased mobility of the chains in the amorphous phase.6,7 Thus far, most research efforts, such as cross-linking, copolymerization, comb formation, polymer alloy, and inorganic filler addition, have been adopted directly toward the achievement of films containing large and stable amorphous phases, possibly with a low glass transition temperature, Tg, in order to obtain good mobility of the polymer chains which are responsible for the ion transport.8−12 Therefore, a PEO film with highly, even fully amorphous structure is in great demand, especially through using a biomaterial and an environmentally benign approach. Soy protein existing in various soy products is one of the most abundant renewable resources and is receiving burgeoning interest for nonfood usage due to their low cost, biodegradability, and environmental friendliness. However, materials made from soy products including the type with the highest percentage protein (90%), soy protein isolate (SPI), have intrinsic disadvantages such as poor processability and brittleness.13 Currently, those soy products are primarily utilized as plastic components in blending, or as simple fillers in various polymer matrices, in order to improve the mechanical properties of the polymeric systems.14 However, the addition of these soy protein products into polymers inevitably decreases the flexibility of polymers. For example, the strain of poly(ester urethane) film decreased from 750% to less © 2011 American Chemical Society

Received: October 20, 2011 Revised: November 23, 2011 Published: December 7, 2011 602

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Figure 1. Structure of (a) received SPI, (b) denatured SPI, and (c) PEO/SPI.

Figure 2. DSC curves for PEO and PEO/SPI films (a) and polarized optical images showing the crystalline structure of PEO (b) and amorphous structure of PEO/SPI (c).

effective protein−protein repulsion.18 When loading the PEO, the protein prefers to be surrounded by the PEO chains rather than making a protein−protein contact, which has been confirmed by the following DSC results. The “electron-rich” sites in PEO absorbed to the lithium ions or bonded to a positive ammonium group, which highly disturbs the order of the PEO chains, resulting in a fully amorphous structure, while the cross-linking or entanglements between PEO and SPI contribute to the ultraelasticity.

Miscibility is a quite important factor in determining both ionic conductivity and mechanical properties for the resulting films, which can be analyzed by the Tg and morphology of the blend system.21 DSC (Figure 2a) and optical microscopy (Figure 2b,c) were exploited to characterize the miscibility of the system. As shown in Figure 2b, distinct spherulites are visualized in the micrographs of PEO film. After addition of the SPI, the microstructure of the film was severely changed. Significantly, no spherulite was observed (Figure 2c), which indicated the amorphous state of the PEO/SPI film. It is 603

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Figure 3. (a) Photograph of the tensile tests; (b) stress−strain curves of PEO/SPI film; (c) load−penetration depth curves of nanoindentation results.

unloading cycles in both films.23 For the PEO film, the minimum load was below zero after unloading, which implies viscous flow is dominant in PEO. However, in the film of PEO/ SPI, the minimum load remains above zero, and its hysteresis loop is still as obvious as pure PEO film which indicates that the involvement of SPI made the film less viscous or more elastic. The PEO/SPI film at room temperature displays a highly elastic nonlinear behavior, typical characteristic of elastomeric materials. The elongation at break is greater than 700% and the tensile modulus is lower than 0.1 MPa, which are nearly the same as that of the raw natural rubber.24 The ionic conductivity and dielectric behavior of a polymer are directly related to the inclusion of an electrically conductive material, which were studied by the impedance and dielectric measurements at room temperature. Each spectrum shows a compressed semicircle in the high frequency range, which is obtained by plotting the imaginary part vs the real part of the impedance, as shown in Figure 4a. The bulk resistance (Rb) can

obviously seen that there was no phase separation and SPI particles were distributed in the matrix, with the blend of SPI and PEO at a ratio of 55:45, which is indicative of good miscibility between PEO and SPI. This conclusion is further confirmed by DSC results (Figure 2a); the DSC curve for PEO/SPI does not exhibit a crystallinity peak. A clear and welldefined single Tg, −39 °C, was observed in PEO/SPI that is an indication of miscibility between PEO and SPI. The Tg of the denatured SPI was not found in the test temperature range (−100 to 130 °C), and we can know it is lower than −100 °C (see Supporting Information Figure S2). Assuming the Tg of denatured SPI is less than −101 °C, the Kwei equation can be applied22

Tg =

Tg1 + ω2(kTg2 − Tg1) + q(ω2 − ω2 2) 1 + ω2(k − 1)

(1)

where Tg1 and Tg2 are the glass transition temperatures of the components, ω2 is the weight fraction of second component, q is a measure of strength for specific interactions between the two kinds of macromolecules (when q > 0, the interaction between the different molecular chains is larger than the interactions from the same molecular chains), and k is the ratio of the two special heat increments before and after glass transition (normally k = 1). Our calculated result is that q > 0, which indicates the interaction between PEO and denatured SPI is larger than the interaction from the interior PEO and denatured SPI. The elasticity of the film was examined by tensile tests and the nanoindentation method. Figure 3a shows digital photos of the PEO/SPI in tensile processes. The PEO/SPI films can be stretched from 0.64 to 5.4 cm (the original length is confined by the equipment test range). A typical stress−strain curve of the PEO/SPI film in tension is presented in Figure 3b. The average ultimate tensile strength, σu, is 0.98 ± 0.07 MPa, whereas the elastic modulus, E, is 0.08 ± 0.02 MPa. The strain at the yield point is 146%. The film with pure PEO as the matrix is too sticky to be peeled off from the substrate without damage, and thus we could not test the mechanical properties by tensile method. The nanoindentation approach was thus applied to compare the mechanical properties between the PEO/SPI film and PEO film, and can be seen from the load− penetration depth curves in Figure 3c. A lower load is needed in PEO/SPI than in PEO for the same indentation depth, while at same load level, the indentation depth in PEO/SPI is much bigger, which suggests the PEO/SPI film is softer and the fully deformation recovered during unloading indicates the film is more elastic. Hysteresis loops, as a result of the remarkable viscoelastic properties, are formed between different loading−

Figure 4. (a) Impedance spectra for PEO and PEO/SPI; (b) frequency-dependent ionic conductivity: (c) dielectric constant; and (d) tan δ of the as-received, stretched, and retracted PEO/SPI films.

be obtained by the cross section of the semicircle and the real impedance.25 Figure 4a shows the Rb of the PEO/SPI film is significantly reduced compared to the PEO film. Further, a significant enhancement in ionic conductivity has been found in the PEO/SPI film. The conductivities of PEO and SPI/PEO are 2.52 × 10−8 and 2.63 × 10−6 S/cm, respectively. Both an increase in charge carrier and ion mobility are responsible for this enhancement. Regarding the increased number of carriers, the SPI includes a large amount of polar groups. After denaturation, the well-defined folded protein structure is 604

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PEO-related material studies, since no fully amorphous solid PEO system with such high elasticity has been reported. Although further in-depth studies on this new elastic material are ongoing (in particular, strict conductivity testing is in progress in our lab), our exploratory studies suggest that the soy protein-based PEO film may have great potential for application in foldable/flexible devices of next-generation electronics and high power density.

transformed to an unfolded state, which favors the lithium salt dissociation.26 Additionally, the fully amorphous phase, in which a large degree of polymer chain segment mobility above Tg exists offering a free environment for the ion transport, allowed ions to migrate easily, resulting in high ionic mobility. Thus, the large amount of polar groups included in the PEO/ SPI material and the fully amorphous structure of the PEO/SPI film resulted in the higher ionic conductivity enhancement compared with that of PEO film. Figure 4b shows the conductivities as a function of frequency from 10 Hz to 3 MHz for a 0.02 mm thick PEO/SPI film under unstretched, stretched to 100%, and retracted conditions. Samples were sandwiched between gold-coated copper electrodes at room temperature. The conductivity of the samples increased with frequency.27 Ionic conductivity in PEO/SPI is governed by the complex interplay of two mechanisms: one associated with ion transport along directed molecular structure (intrachain transport); the other strongly dependent on host segmental motions, which is controlled by ion hopping between such structures (interchain transport). It has recently been found that intrachain transport is far more efficient than interchain hopping.28 In other words, preserving some parts of the helical structure (ion movement channel) reduced the coulumbic interactions between ions.29 Stretching the films at room temperature (much higher than the Tg = −39 °C), the polymer chains become more organized or ordered, and the alignment of the polymer chains was followed by an increase in ionic conductivity as shown in Figure 4b. Figure 4c shows the real parts of the dielectric constant as a function of frequency at room temperature. It is evident from the figure that the values of dielectric constants are significantly higher in the low-frequency region. This indicates that the electrode polarization phenomenon occurred as a result of an accumulation of ions near the electrodes.30 The dielectric loss curves (Figure 3d) for the PEO/SPI reveal prominent relaxation processes. The strength and frequency of relaxation depend on the characteristic property of dipolar relaxation. When stretched to 100%, the dielectric constant and loss tangent values with respect to frequency were unchanged, which are favorable for high energy density electronics. In summary, this paper describes a highly elastic ionconductive film obtained through employing a soy protein product, SPI, blended with PEO with weight ratio 55:45. The film possesses a fully amorphous phase with high ionic conductivity at room temperature. Our study results showed that the conductivity and elasticity are both significantly improved with 55 wt % SPI involvement. Certain kinds of characteristics, such as strong interaction and miscibility between the components and the full amorphous structure with very low Tg, contribute to both an increase in the ionic conductivity and mechanical properties. Remarkably, this PEO/ SPI film has been elongated up to 700% without mechanical damage and 100% without loss of ionic conductivity. This exploratory work initially reveals that a highly elastic ion-conductive film can be obtained through employing a biomaterial blended polymer. To our knowledge, it is the first time to obtain such high elasticity without incorporation of elastomer filler but using only a conventional amount of lithium compound.7 In addition, the highly elastic film was achieved counterintuitively from applying a rigid soy protein isolate, a very low cost bio mass product, to form a polymer blend. More significantly, the soy protein-based elastic film we prepared possesses a fully amorphous phase, which is a breakthrough in

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ASSOCIATED CONTENT * Supporting Information Experimental details; Figures S1−S3. This material is available free of charge via the Internet at http://pubs.acs.org. S

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AUTHOR INFORMATION Corresponding Author *Tel +1 509 335 7658; fax +1 509 335 4662; e-mail Katie_ [email protected].

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ACKNOWLEDGMENTS The authors gratefully acknowledge ADM Co. for providing soy protein isolate (SPI). REFERENCES

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