interaction between iodine and ethyl cellulose

Interaction Between Iodine and Ethyl Cellulose

YANG WANG, ALLAN J. EASTEAL

Department of Chemistry, The University of Auckland, Private Bag 92019, Auckland, New Zealand

Received 6 May 1998; accepted 25 July 1998

ABSTRACT: Interaction between iodine and ethyl cellulose was investigated by immers-ing ethyl cellulose membranes in aqueous iodine–iodide solution (iodine doping) andincorporating iodine in the solution from which the membrane was cast. Oxygen andnitrogen permeabilities in iodine-doped ethyl cellulose decrease with an increase in theconcentration of iodine in the dopant solution up to 0.003 mol L21 and increase sharplyat higher iodine concentrations, and ultraviolet–visible and far-infrared spectra indi-cate formation of a charge transfer complex. Differential scanning calorimetry of bothtypes of membrane shows changes in the characteristic phase transitions of ethylcellulose after iodine treatment, including the crystal–liquid crystal transition that hasbeen reported to occur in ethyl cellulose. Further evidence for liquid crystal phases hasbeen found from circular dichroism. X-ray photoelectron spectroscopy of iodine-dopedethyl cellulose films indicates that the iodine is present in two different chemical states,probably l3

2 and l2. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1303–1314, 1999

Key words: ethyl cellulose; charge transfer complex; iodine doping

INTRODUCTION

Iodine complexes of polymers are important formany reasons.1 The doping of polyacetylene filmsby iodine has been widely studied,2–5 and yieldsgood electrical conductors. Nylon 6–iodine1,6 com-plexes and poly(vinylpyridine)–iodine complexescan be used as electrode materials in lithium–iodide batteries.

The physicochemical effects of iodine on a num-ber of polymers have been investigated in somedetail. It has been reported that iodine sorptionproduces a significant structure change in poly-mers such as poly(vinyl alcohol),7,8 poly(acrylo-nitrile)9 (PAN), and nylon 6.1,6 The observed diffrac-tion pattern suggests that iodine penetrates intothe crystalline phase of PAN and becomes ar-ranged regularly with PAN chains.8 Pseudohex-agonal crystals of PAN are expanded uniaxially

by replacement of PAN molecular cylinders by thepolyiodine columns. In iodinated nylon 6, iodineions form sheets intercalated between sheets ofnylon 6 chains to form a paracrystalline lattice.1

In the case of polyvinyl octal and iodine complexformation, it was found that iodine induces poly-meric chain widening and changes the order inthe molecular lattice.10

The cellulose derivative, ethyl cellulose (EC),has been extensively studied, but there are veryfew reports on the interaction or complex forma-tion of EC and iodine. Khare and Chardok11 havestudied the films formed on aluminium by dippingan aluminium plate in a solution of EC 1 iodineand found that the conductivity increases withiodine content of the film.

The EC structure is as follows:

Correspondence to: A. J. Easteal (telephone: 64-9-3737599;fax: 64-9-3737422; E-mail: [email protected]).

Contract grant sponsor: University of Auckland ResearchCommittee and the New Zealand Lottery Grants Board.Journal of Applied Polymer Science, Vol. 71, 1303–1314 (1999)© 1999 John Wiley & Sons, Inc. CCC 0021-8995/99/081303-12

1303

The addition of iodine to EC is likely to formcharge transfer complexes (CTC) by donor–accep-tor interaction between the ether oxygens of ethylcellulose and iodine molecules. In addition, ECforms both lyotropic and thermotropic liquid crys-tals under certain conditions due to the rigidity ofits anhydroglucose backbone,8 and it was antici-pated that the mesogenic properties can be af-fected by iodine doping.

In the present work, gas permeability mea-surements have been used to probe structuralchanges of EC membranes after iodine dopingand following iodine incorporation from the solu-tion from which membranes were cast. The io-dine–EC membranes have been further charac-terized by means of ultraviolet–visible (UV–vis),infrared (IR), and far-IR, and X-ray photoelectron

spectroscopy (XPS), differential scanning calorim-etry (DSC), and circular dichroism (CD).

EXPERIMENTAL

Materials

Ethyl cellulose was supplied by Acros Organics(Belgium). The ethoxy content was nominally

Table I Oxygen and Nitrogen PermeabilityConstants for Iodine-Doped Ethyl Cellulose

C(l2)/mol L21 P(O2) P(N2)

0 10.2 2.242 3 1023 8.9 2.173 3 1023 6.7 1.984 3 1023 166 1795 3 1023 4.66 3 103 7.48 3 103

The permeability constant is expressed in Barrer.

Figure 1 Effect of iodine concentration in the dopantsolution on EC membrane weight gain after dopingwith iodine.

Figure 2 Effect of iodine dopant solution concentra-tion on oxygen and nitrogen permeability at 20°C: (F)oxygen; (■) nitrogen.

Figure 3 Effect of iodine dopant solution concentra-tion on oxygen to nitrogen selectivity.

1304 WANG AND EASTEAL

48%, and the viscosity of a 5% solution in 80/20toluene to ethanol was nominally 100 mPa s. Io-dine and potassium iodide were reagent gradematerials. Dichloromethane, chloroform, and tet-rahydrofuran were used as received.

Membrane Preparation

Ethyl cellulose films were cast from 3% w/w solu-tion in dichloromethane, chloroform, and tetrahy-drfuran (THF) in 13.6-cm-diameter glass petridishes. The solvent was allowed to evaporate atambient temperature for 24 h, and the mem-branes were then dried in vacuum for 24 h toremove residual solvent. The membranes soformed were of the order 50–100 mm thick.

Iodine Doping

Ethyl cellulose films were immersed in aqueousl2–Kl solutions of different concentrations at roomtemperature for 24 h to attain equilibrium sorption.

Figure 4 UV spectra of iodine-doped ethyl cellulose:(1) pure EC; (2) 0.002; (3) 0.003; (4) 0.004; (5) 0.005 molL21 iodine solution.

Figure 5 Far-IR spectrum of EC and EC doped in 0.004 mol L21 iodine solution.

IODINE–ETHYL CELLULOSE 1305

The iodinated EC films were rinsed with water anddried between sheets of filter paper in vacuum for24 h at ambient temperature. The amount of iodinesorption was determined from the increase inweight of a dry film before and after doping.

Iodine Incorporation During Casting

Different amounts of iodine were incorporated inEC casting solutions by dissolution of appropriateamounts of elemental iodine. The resulting mem-branes were rinsed with water and dried in vac-uum as for the iodine-doped membranes.

Characterization

UV–vis spectra of iodine-doped membranes wererecorded using a SHIMADZU UV 2101 PC UV–VIS scanning spectrophotometer. Far-IR spectrawere recorded on a Digilab FTS-60 Fourier trans-fer infrared (FTIR) spectrometer. Spectra wereobtained in the absorbance mode between 60–500cm21, and 64 scans were averaged to give a con-sistent spectrum at a resolution of 2 cm21.

Thermal analysis was performed using DSC(Polymer Laboratories Model 12000 instrument)at a heating rate of 10°C min21, in an atmosphereof dry, oxygen-free nitrogen.

Circular dichroism (CD) spectra of films weredetermined with a Jasco model 720 instrument inthe wavelength range of 200–800 nm.

X-ray photoelectron spectroscopy (XPS) wascarried out using a Kratos XSAM 800 X-ray pho-toelectron spectrometer.

Gas permeability constants were determinedusing equipment designed and constructed in ourlaboratory. Sample membranes in the form of cir-cular discs 9.0 cm in diameter were mounted in athermostatted permeation cell operating on thevolume-increase principle. The downstream sideof the cell was maintained at atmospheric pres-sure. The pressure on the upstream side was ad-justed with a needle valve and read by a pressuresensor of type Sen Sym 618A SCX 100 DN.Steady-state conditions were attained in about 30min, after flushing both sides of the membranewith test gas for 8–10 min.

RESULTS AND DISCUSSION

Iodine-Doped Ethyl Cellulose Film

When ethyl cellulose membranes were immersedin iodine–potassium iodide solution, iodine was

Figure 6 DSC scans of iodine-doped EC films: (upper) undoped EC; (middle) ECdoped in 0.002 mol L21 iodine solution; (lower) EC doped in 0.004 mol L21 iodinesolution.


readily absorbed into the polymer. The treatedmembranes developed a characteristic orange tobrown color, depending on the iodine concentra-tion. The intensity of the color increased withdoping time, suggesting formation of a charge

transfer complex, which remains after rinsingand vacuum drying.

Figure 1 shows the amount of iodine absorbedby EC films as a function of the iodine concentra-tion in the doping solution, relative to the initial

Figure 7 Induced CD spectra for EC (lower) and EC doped in 0.004 mol L21 iodinesolution.


Figure 8 XPS scans of iodine-doped EC film.


weight of the film. Iodine absorption increasesrapidly with iodine concentration, at concentra-tions smaller than about 0.01 mol L21, and muchless rapidly at higher concentrations.

Gas Permeability

Ethyl cellulose has been the subject of researchfor oxygen enrichment applications since the1950s. The permeability of a polymer is deter-mined primarily by the magnitude of the freevolume, and it was anticipated that incorporationof a significant proportion of large iodine mole-cules in EC should change the free volume andinfluence the magnitude of the permeability. Inaddition, it was expected that the distribution offree volume could be modified, leading to a changein the selectivity of the membrane.

Oxygen and nitrogen permeability constants ofthe iodine-doped membrane are given in Table Iand are shown graphically in Figure 2. Both theoxygen and nitrogen permeabilities initially de-crease slightly with the iodine concentration inthe dopant solution and, hence, with the amountof iodine absorbed. This trend can be attributed tothe formation of charge-transfer complexes thatrestrict polymer chain mobility and reduce freevolume. It is possible that a reduction in the sol-ubility of O2 and N2 contributes to lower gas per-meabilities, but it is more likely that the principalfactor is reduced diffusion coefficients for oxygenand nitrogen.

A striking feature of Figure 2 is the dramaticincrease in permeabilities of both gases when theiodine concentration of the dopant solution ex-ceeds 0.003 mol L21. The increase in permeabilityis exponential in iodine concentration at concen-trations above 0.003 mol L21. There appears to bea threshold iodine concentration (around 0.003mol L21) above which the mechanism of gastransport through the membrane changes suchthat a more facile route for gas permeation be-comes available.

Somewhat similar behavior has been found forpolyacrylonitrile. Kim found that high iodine do-pant levels cause a striking change in PAN,9

whose crystalline structure expands. According tothe intercalation model,12 it is possible that at ahigher concentration, iodine intercalates betweenpolymer chains in the form of a polyiodine columnand opens the polymer structure, allowing gasmolecules to diffuse more rapidly.

The gas selectivity is defined as the ratio of gaspermeabilities measured under the same condi-tions. Figure 3 shows the effect of the iodine dop-

ing solution concentration on oxygen to nitrogenselectivity. The iodine-doped membrane becomesmore permeable to nitrogen than to oxygen whenthe iodine concentration is higher than 0.003mol L21.

UV, Far-IR, and DSC Characterization of Iodine-Doped Ethyl Cellulose

Figure 4 shows a series of UV–VIS absorptionspectra of the l2–EC complex formed by ECfilms immersed in l2–Kl solutions with differentiodine concentrations. Ethyl cellulose exhibitsno band in the visible region, and the spectrumof iodine-doped ethyl cellulose shows bands thatare attributable to a charge transfer complex.The absorption maxima at around 280 and 350nm increase with increasing iodine concentra-

Figure 9 Typical crystalline structures produced byincorporation of iodine in EC during casting from chlo-rinated alkane solution: (a) single crystals; (b) crystal-line microfibers. The magnification is about 2003 for(a) and (b).


tion. The 280 and 350 nm bands arise from theCT complex with l2–l3

25, and the broad absorp-tion band in the 480 – 600 nm range is due tounreacted l2.9

The far-IR spectrum of iodine-doped EC (Fig. 5)show a strong absorption at 138 cm21, assigned tothe complex13,14 EC–l3

2. No absorption at 180cm21 (expected for l–l stretch) appears.

Figure 10 CD spectra of EC films with iodine incorporated during casting: (a) pureEC; (b) 40% iodine, cast from tetrahydrofuran; (c) 40% iodine, cast from dichlorometh-ane.


DSC scans of pure EC films exhibit three phasetransitions due to its ability to form a thermo-tropic liquid crystal. According to Chen and co-workers,15 the transition at 188°C is a solid–me-sophase transition, and that at 228°C is the me-sophase–isotropic liquid transition. The othertransition at 135°C is the glass transition. Afteriodine doping, it was found that the endotherm at228°C became hard to detect, while a sharp endo-therm appears at 183°C (Fig. 6). This could be theresult of EC becoming more crystalline due to CTcomplex formation and chain ordering, and to io-dine sublimation, since the iodine sublimationtemperature is 180°C. The endotherm at 183°Cvanished from a second DSC scan on the samesample, suggesting that the endotherm is associ-ated with iodine sublimation.

CD Spectra

EC film cast at room temperature from solution isan orientationally ordered glass, where the chiralnematic structure, initially present in the liquidcrystalline solution, is retained on drying.16 Fig-ure 7 shows CD spectra for the films. The EC film(lower spectrum) exhibited a positive peak at eachwavelength, that is, the cotton effect, indicatingthat the cast film retained left-handed cholesteric

liquid crystalline order. The wavelength of thepeak is directly proportional to the pitch.17 Io-dine-doped EC film (upper spectrum) shows twoCD bands. The first arises from EC cholestericorder and occurs near 250 nm, and the second,near 490 nm, may be assigned to CD induced iniodine.

Iodine-doped EC film exhibits induced opticalactivity due to association of iodine via a strongoriented interaction with chiral centers on thecellulose chain. Evidently, iodine also adopts thecholesteric helicoidal arrangement. Induced opti-cal activity has previous been observed18 for thedye Congo red bound to regenerated cellulose.

Chemical State of Iodine

The nature of the iodine species in iodine-dopedpolymers depends on the polymer. It may be anionic (l3

2, l52, l7

2, polyiodide) or neutral (l4, l6) spe-cies.19 X-ray photoelectron spectroscopy has beenused to determine doping levels and the nature ofdoping species. In Figure 8 (upper spectrum), awide scan of iodine doped EC shows strong iodineabsorption. The iodine 3d3/2–3d5/2 core level nar-row scan spectrum is shown in the lower spec-trum. The splitting is indicative of two types ofiodine. The iodine 3d5/2 levels can be deconvoluted

Figure 10 (Continued from the previous page)


into two components, at 618.8 and 619.9 eV, pos-sibly attributable to l2 and l3

2.20,21

Based on the experimental observations, it issuggested that the interaction between iodine andethyl cellulose proceeds as follows:

1. iodine sorption;2. iodine molecules forming a charge transfer

complex with ether oxygen of ethyl cellu-lose;

3. iodine changing from an achiral to a chiralform;

4. iodine penetration into the liquid crystal-line phase of EC, changing the structure ofthe parent polymer.

Ethyl Cellulose with Iodine IncorporatedDuring Casting

Murthy and co-workers reported2 that immersingpolyacetylene in an aqueous Kl–l2 solution pro-duces polyacetylene–iodine complexes, which arevery similar to those obtained both by vapor-phase doping, and by doping with iodine dissolvedin organic solvents, such as heptane and pentane.

To better understand the behavior of iodine-doped ethyl cellulose membrane, iodine was in-corporated into ethyl cellulose solution beforecasting. Iodine is retained by the resulting ECfilm, giving orange–brown membranes.

When the proportion of iodine in the castingsolution is greater than 40% (w/w) of the EC con-tent, the films cast from solution change frombeing homogeneous and amorphous, into par-tially crystalline, multiphase structures. Opticalmicroscopy shows iodine-induced crystallinity ap-pearing as either single crystals [Fig. 9(a)] orcrystalline microfibres [Fig. 9(b)] imbedded in anamorphous matrix. The single crystals formedwere not recrystallized iodine; they were colorlessand had a sharp melting point (determined with amicroscope with heating stage) of 230°C, veryclose to that of pure EC. It seems that wheniodine is incorporated in the casting solution, thestrong interaction that leads to CTC formationdecreases side chain mobility, changes the ECconformation, and brings about alignment of thehighly flexible and mobile chains, thus facilitat-ing crystallization. A complicating factor is thatthe single crystals produced by casting from anEC–iodine solution (in chloroform, for example)gave no X-ray reflections, suggesting some form ofmolecular disorder. That the single crystals arenot chemically different from the matrix EC wasconfirmed by IR microscopy. Reflectance spectra

of the single crystals were not discernibly differ-ent from spectra of the adjacent matrix.

EC membrane with iodine incorporated duringcasting has different CD spectra (Fig. 10) fromiodine-doped membranes. The intensity decreasesas the amount of incorporated iodine increases,and the spectrum shows a fine structure. In con-trast to iodine-doped EC, no discrete band at 490nm appears.

DSC scans of EC and two membranes withdifferent proportions of iodine present in the cast-ing solution are shown in Figure 11. The glasstransition temperature is reduced only slightly byincorporation of iodine, but both the crystal 3mesophase and mesophase 3 isotropic liquidtransition temperatures are substantially re-duced. Furthermore, for the membrane cast froma solution with 40% iodine, there is a markedchange in the shape of the melting endotherm.The associated phase transition appears to occurin two stages, a gradual first stage that leads to avery sharp endotherm in the second stage.

The solvent influences the interaction betweeniodine and ethyl cellulose. No crystalline phaseappears at even higher amounts of iodine incor-porated into EC–THF solution, and no cotton ef-fect was observed for the membranes cast fromTHF solution. The solvent effect can be explainedby the fact that formation of charge transfer com-plexes between the two components is stronglyenhanced in polar chlorohydrocarbons22 becausethe hydrogen atom from the chlorohydrocarbonenhances the electrophilic nature of l4 and l2.

Using XPS, a very weak iodine peak was de-tected in films made by incorporating high levelsof iodine in EC at the casting stage. The dopinglevel is determined from the integrated intensityof the peaks. Table II gives the elemental analysisfrom XPS for I, C, and O for iodine-doped EC filmsand chemical microanalytical data for C, H, and Ofor films with iodine incorporated during casting.It appears that iodine is lost by evaporation fromfilms with iodine incorporated during casting.The implication is that the EC-iodine interactionis stronger, and the iodine less labile in the io-dine-doped membranes.

CONCLUSIONS

Iodine, when doped in polymers, may reside atvarious sites.23 It may go substitutionally into thepolymer chain or reside at the amorphous–crys-talline boundaries and diffuse preferentiallythrough the amorphous regions and form charge


transfer complexes (CTC), or it may exist in theform of molecular aggregates.

In case of ethyl cellulose, it is quite likely thatmolecular iodine may be diffusing in the amorphousphases of the polymer and forming CTC with thepolymer chains. The formation of CTC is usuallycharacterized by the appearance of a new charge-transfer absorption band in the absorption spec-trum of the doped polymer, and that is the case foriodine-doped EC. Moreover, iodination of ethyl cel-lulose films produces a significant change in the

structural order of EC, while iodine-doped EC filmexhibits induced optical activity due to associationof iodine with chiral centres on the cellulose chain.

In the case of iodine incorporated during casting,the EC structure is affected by iodine and solvent.At a high level of iodine in the casting solution,apparently, crystalline structures are formed. Itseems that the strong interaction between EC andiodine facilitates ordering of EC molecules, allowingnucleation and crystal growth to occur as the sol-vent evaporates.

Figure 11 DSC scans of pure EC (lower trace), EC with 20% (middle), and EC with40% (upper) iodine in the casting solution.

Table II Elemental Analysis (Wt %) of Iodine-Doped EC and EC with IodineIncorporated During Casting

Membrane C H O I

ECa 56.9 8.9 34.2 —EC (l2-doped)b 50.2 (7.9) 30.2 11.8EC (l2 incorporated, 20%)c 55.2 8.8 (35.4) 0.65

EC (l2 incorporated, 40%)d 55.4 8.8 (34.9) 0.90

The parenthesized figures were obtained by the difference.a Calculated.b From XPS, assuming that relative proportions of C, H, and O are unchanged.c From microanalysis (20% iodine in casting solution).d From microanalysis (40% iodine in casting solution).


The authors are indebted to Lawrie Creamer (of theNew Zealand Dairy Research Institute) for CD spectra;and to Trish Shaw (Defence Scientific Establishment)for IR microscopy. Funding for thermal analysis instru-mentation was provided by the University of AucklandResearch Committee, and the New Zealand LotteryGrants Board.

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interaction between iodine and ethyl cellulose

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