J. Phys. Chem. C 2010, 114, 14281–14289
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Polymer Nanocomposites as Solid Electrolytes: Evaluating Ion-Polymer and Polymer-Nanoparticle Interactions in PEG-PU/PAN Semi-IPNs and Titania Systems Md. Selim Arif Sher Shah, Pratyay Basak,* and Sunkara V. Manorama* Nanomaterials Laboratory, Inorganic and Physical Chemistry DiVision, Indian Institute of Chemical Technology, Hyderabad 500 607, India ReceiVed: March 27, 2010; ReVised Manuscript ReceiVed: July 15, 2010
This paper is an investigation on the physicochemical properties and Li ion conductivity behavior of a new class of polymer nanocomposites, which holds promise for its potential use as solid polymer electrolytes (SPE). A family of nanocomposite semi-interpenetrating polymer networks of titania (TiO2)/poly(ethylene glycol)-polyurethane (PEG-PU)/poly(acrylonitrile) (PAN) incorporated with LiClO4 was synthesized. The effects of titania loading, nanoparticle surface functionalization, and ion-polymer and polymer-nanoparticle interactions were evaluated in detail using XRD, FT-IR, TEM, DSC, TG-DTA, and dc conductivity studies. At low degree of TiO2 loading, reasonably good dispersion and encapsulation within the semi-IPN matrix was achieved. FT-IR studies strongly indicate that analogous to the ion-polymer interaction, a strong interaction coexists between the polymer matrix (C-O-C) and the surface of TiO2 nanoparticles. Electron micrographs of the semi-IPN nanocomposite films reveal nanophase separation within the polymer matrix. The larger poly(acrylonitrile) domains (∼50-400 nm) along with interspersed titania nanoparticles (∼5-20 nm) are well distributed throughout the PEG-PU parent networks. The even dispersion and low nanoparticle agglomeration apparent from the micrographs also indicate a reasonable degree of polymer-nanoparticle interaction. Interestingly, no substantial change in the thermal properties of the polymer-nanocomposite matrices is observed even with increased amounts of TiO2 loading (up to 5%) and the glass-transition temperature remained well below the ambient (∼-38 °C). The degradation onset temperature for the semiIPNs and TiO2 nanocomposites (T0 ∼ 250 °C) suggests good thermal stability of the matrix. An enhancement in ionic conductivity for all the semi-IPN/nanocomposite was observed. The presence of a very disordered polymer phase in the near vicinity of the nanoparticle surface, interfacial dynamics possibly creating smaller domains of PEG segments wherein the cations (Li+) are more loosely cross-linked under the circumstances of competitive interactions, may help faster ionic transport contributing to the overall increase in the bulk conductivity. Overall, tailoring the polymer-nanocomposite matrices further holds promise toward addressing the practical challenges in the success of solid polymer electrolytes in Li+ ion batteries/dye-sensitized solar cells. Introduction The global initiative for clean and alternative sources of energy has motivated research on energy generation and storage technologies.1 Of the numerous alternatives proposed, harvesting solar energy using dye-sensitized photovoltaic devices2,3 and photoelectrochemical cells4 for power generation, coupled with high-density Li+ ion rechargeable batteries for storage and delivery,5,6 are important tools to be considered. This has translated into reintensified efforts to find low-cost options and easily processable materials as device components. One of the key constituents of these mutlicomponent systems is the electrolyte sandwiched between the two active electrodes. To enhance the durability and performance of such systems, the concept of solid polymer electrolytes (SPEs) has hence emerged as an exciting interdisciplinary area in polymer science and electrochemistry.7,8 A long-standing goal of polymer electrolyte research over the past three decades is the preparation of an electrolyte that combines the processing characteristics of * To whom correspondence should be addressed. Tel.: +91-40-27193225. Fax: +91-40-27160921. E-mail:
[email protected] (P.B.), manorama@ iict.res.in (S.V.M.).
conventional thermoplastics with the ionic conductivity of low molar mass liquids.9-11 Given the broad range of polymer structures that have been investigated as potential electrolytes, it is unlikely that new polymers will be prepared that have significantly higher conductivities. Nevertheless, structural modifications of the existing systems remain the most viable alternative to enhance the electrolyte properties. Among the structural modifications like blending,12,13 copolymerization,14-16 grafting,17,18 and cross-linking,19-21 the formation of polymer networks is suggested to be the most effective strategy to achieve low degree of crystallinity as well as good dimensional stability. In this perspective, interpenetrating polymer networks (IPNs) are a special class of polymer networks that can be custom designed to suit specific end uses.22 In an attempt to increase the conductivity of these materials, it has been recently proposed to introduce inorganic oxides into the polymer matrix.23-28 In these materials the incorporated oxide particles create grain boundaries, which are responsible for the formation of highly conductive layers of polymer ceramic interfaces and prevent the polymeric chains from crystallizing. Researchers have narrowed down to nanostructured titanium dioxide (TiO2) as the most viable material in recent years,
10.1021/jp105450q 2010 American Chemical Society Published on Web 07/30/2010
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because of its photovoltaic properties, stability, and other physical properties such as high refractive index combined with high degree of transparency in the visible region of the spectrum.29,30 Thus, incorporating nanostructured TiO2 as inorganic oxide filler in the solid polymer electrolytes is of interest for such photochemical devices not only to increase the conductivity of the SPEs but also to enhance the efficiency of electron transfer in the medium by effectively reducing the interfacial barrier between the TiO2 layer and the electrolyte. Hence, a successful formulation of nanocomposite solid polymer electrolytes would be ideal for dye-sensitized solar cell applications. In our previous reports, the feasibility of poly(ethylene oxide)-polyurethane/poly(acrylonitrile) (PEO-PU/PAN) semiIPNs doped with various alkali metal salts with improved conductivity for SPE applications has been demonstrated.31-34 Further attempts to modify and enhance the properties of these semi-IPNs by forming nanocomposites was intended to suit their application as possible electrolytes in Li ion batteries/dyesensitized solar cells. In this endeavor, nanostructured titania was incorporated into the semi-IPN matrix and the consequential effects on physicochemical properties and ion-conductivity behavior of the electrolyte matrix was extensively evaluated. The effects of loading bare and surface modified nanoparticles on the properties of the semi-IPN systems were also assessed in parallel. The titania nanoparticles were functionalized using 4-tert-butyl catechol following the procedure reported by Niederberger et al.35 Our rationale during the preliminary stages for selecting 4-tert-butyl catechol modification were primarily based on (1) ease and effectiveness of the surface modifications along with good colloidal stability during the processing and curing stages of the nanocomposite films, and (2) the absence of other reactive functional groups, such as amines, which prevents undue complications by participating directly in the polymerization process. In this paper, we report our interesting findings on the ion-polymer and nanoparticle-polymer interactions in the synthesized PEG-PU/PAN semi-IPN titania nanocomposites. The results presented herein provide a more comprehensive understanding of the complex matrix behavior and give important leads toward developing an ideal SPE for practical purposes. Experimental Section Materials. Castor oil (CO) (BSS grade), diphenylmethane diisocyanate (MDI) (Merck), acrylonitrile (AN) (SRL), lithium perchlorate (LiClO4) (Aldrich), benzyl alcohol (Aldrich), titanium tetrachloride (TiCl4) (RANKEM), hydrochloric acid (HCl) (RANKEM), and tetrahydrofuran (THF) (RANKEM) were used for synthesis. Poly(ethylene glycol) (PEG) (Mw ∼ 4000), hydrazine monohydrate (N2H4 · H2O), benzoyl peroxide (BPO), N,N-dimethyl aniline (DMA), 4-tert-butyl catechol, dichloromethane, and acetonitrile were purchased from S.D. FineChem Ltd., India. All the chemicals were reagent grade and used without any further purification. Synthesis of Titania Nanoparticles. A 60 mL portion of TiCl4 dissolved in HCl (50% v/V) was first diluted in deionized water (total volume 500 mL). An aqueous solution of hydrazine monohydrate (50% v/v) was added dropwise under vigorous stirring to adjust the solution pH to 8. The reaction mixture was left under stirring for another 12 h at room temperature and then allowed to settle. The obtained precipitate was washed and centrifuged several times until the supernatant was free of chloride ions. The product was oven-dried, ground, and then
Shah et al. calcined at 350 °C for 10 h before further characterizations. These synthesized bare titania nanoparticles is denoted as “BT” in the text. Synthesis of 4-tert-Butyl Catechol Functionalized Titania Nanoparticles. The synthesis of 4-tert-butyl catechol functionalized titania nanoparticles was achieved following the procedure as described elsewhere.35 In brief, 0.5 g (3.0 mmol) of 4-tertbutyl catechol was dispersed in 60 mL of benzyl alcohol in a round bottomed flask. To this solution, 3.0 mL (27.2 mmol) of TiCl4 was added slowly under vigorous stirring at room temperature. The dark red reaction mixture so obtained was stirred at room temperature for about 2 h and then heated to 70 °C. The temperature was maintained to age the reaction mix for 5 days. The resulting brown suspension after aging was centrifuged, and the precipitate was thoroughly washed with methylene chloride twice. The product was left to dry at room temperature followed by oven drying at 60 °C before characterization. Henceforth, this catechol-functionalized titania nanoparticles will be designated as “CT” in the text. Synthesis of PEG-PU Polymer Networks. The poly(ethylene glycol)-polyurethane (PEG-PU) polymer networks were synthesized as described elsewhere.31,34 For a typical reaction, the total -OH/NCO ratio was maintained at 1:0.8. Initially, CO (-OH value ∼2.6) and the required amount of MDI solution in THF were taken in a round-bottomed flask, degassed, and stirred for an hour at room temperature under nitrogen atmosphere. To this prepolymer, a solution of the macromonomer (PEG) in THF along with DMA as catalyst was added and the stirring was continued further for half an hour before casting on a glass Petri dish. The mix was then allowed to dry at room temperature for 24 h followed by oven curing at 80 °C for another 24 h to obtain a free-standing film of ∼0.4 mm thickness. The prepared PEG-PU polymer film is coded as “P3” in the text. The schematic pathway for the prepolymer followed by network formation along with the FTIR collected at different stages is provided in the Supporting Information. Synthesis of PEG-PU/PAN Semi-IPNs. Semi-IPNs of PEGPU/PAN (in weight ratio of 60/40) were synthesized as described in refs 31 and 34. Central to the synthesis of simultaneous IPNs is the identification of two polymerization reactions which are mutually exclusive. Thus, the synthetic strategy of PEG-PU/PAN semi-IPN has been the one-step interpenetration of PEG-PU network and PAN. The PEG-PU/ PAN semi-IPNs have been achieved using two simple and mutually exclusive reactions wherein the formation of polyether-polyurethane networks proceeds via polycondensation reaction while poly(acrylonitrile) has been interpenetrated by free radical polymerization of the respective monomers. In a typical procedure for PEG-PU/PAN semi-IPNs, the prepolymer was formed as detailed in the preceding section. To this PEG (monomer-I), BPO (initiator), DMA (catalyst), and AN (monomer-II) were added, stirred for half an hour, and cast on a glass Petri dish. The polymer was cured similarly to take the reaction to completion and obtain free-standing films of ∼0.4 mm thickness. These samples are designated as “P32” in the text. The schematic details are provided in the Supporting Information. Synthesis of PEG-PU/PAN Semi-IPN Salt Complexes. The reaction procedure followed to obtain salt complexed semi-IPNs remained the same with the addition of LiClO4 in the required amount (EO/Li ratio ) 20) in the second step. The polymer samples are designated as “P321” in the text. Synthesis of Semi-IPNs/Titania Nanocomposites. The synthesis method remains the same as above with an added step
Polymer Nanocomposites as Solid Electrolytes where the desired amounts of bare titania (BT) or 4-tert-butyl catechol functionalized titania (CT) nanoparticles are incorporated into the salt-complexed semi-IPNs. The reaction mixtures for these nanocomposite samples were, however, stirred for ∼20 h prior to casting for achieving a homogeneous distribution of TiO2 nanoparticles. The characterized samples of these polymer nanocomposites are denoted as “P321-xBT” and “P321-xCT” for the bare and functionalized titania nanoparticles respectively, where x denotes the percentage weight of titania nanoparticles loaded. Characterization. X-ray diffraction (XRD) patterns were taken in reflection mode on a Rigaku MiniFlex tabletop X-ray diffractometer using a Cu KR source (λ ) 1.5406 Å). XRD patterns were obtained in the 2θ range, 2°-80° by continuous scanning with a step size of 0.01°. The FT-IR spectra were recorded at room temperature in the range of 4000-400 cm-1 using a Bruker ALPHA-T spectrometer after 256 scans with a wavenumber resolution of 4 cm-1. The HRTEM images were collected on a JEOL TEM 2010 microscope operating at 200 kV. The synthesized nanoparticles were dispersed in ethanol by ultrasonication prior to loading onto a carbon-coated copper grid. For TEM analysis of the semi-IPN nanocomposites, sample preparation was achieved by spin-coating the reaction mix at 3000 rpm on a pristine glass substrate, cured, and lifted on copper grids to obtain suitable thin films (
