Proton conducting interpenetrating polymer networks

Solid State Ionics 31 t 1988) 117-125 North-HoUand, Amsterdam

PROTON CONDUCTING INTERPENETRATING POLYMER NETWORKS S. PETTY-WEEKS, J.J. ZUPANCIC and ;.R. SWEDO Allied-Signal Engineered Materials Research Center, 50 E. Algonquin Road, P.O. Box 5016, Des Plaines, IL 60017-5016, USA Received 18 April 1988; accepted for publication 25 July 1988

In order to improve stability and performance of the polymer electrolyte-based hydrogen sensor developed for on-line analysis, modifications to the PVA/H3PO4 proton conducting polymer blend were made. Water insoluble, increased bulk modulus, and higher conductivity polymer membranes have been fabricated by development of interpenetrating polymer networks that incorporate the PVA/H3PO4 (host) blend. The guest polymer is a three dimensional polymer network composed of methacrylic acid and methylenebisacrylamide. High protonic conductivity results from the phosphoric acid and water: the poly (methacryl:,c acidmethylenebisacrylamide) contributes to the increased modulus and water insolubility. Hydrogen sensors have been demonstrated using these membranes.

I. Introduction

The proton conducting polymer blend of poly(vinyl alcohol~, PVA, and phosphoric acid has been used as a "solid-state" electrolyte in hydrogen sensors [ 1 ] and polyaniline-based microelectrochemical transistors [2 ]. This material has two limitations in these applications: the polymer blend is ve~ soft (rubbery) and water soluble. The hydrogen sensor is designed to operate in the temperature range - 40 ° C to 55 ~C. The lower temperature limit is due to the reduced ionic conductivity of the polymer [ 3 ], while the upper lim;.t represents the highest temperature at which the p,~lymer blend is stable. There are two major considerations when examining temperature effects on the physical properties of the polymer: changes in conductivity and mechanical integrity. Temperature limitations will include" (1) Loss of water, which will increase the resist l v i l v n f l h o m o r n h r n n o n n d i~ o n l y n a r t i a l l v rever.~,A

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ible upon reexposure to water vapor. This will be offset partially or totally by the increase in ion mobility, which results in increasing conductivity of the film, as temperature increases. (2) Chemical reactions, such as degradation or crosslinking of the polymer, which affect both conductivity and mechanical stability. Discoloration of

the polymer blends with time and elevated temperatures has been noted. (3) Loss of mechanical integrity as the glass transition temperature of the polymer is approached, potentially damaging the membrane or disrupting the electrodes. Extensive differe, ial scanning calorimetry, DSC, characterization of proton conducting blends based on these and other proton conducting polymer blends indicate that although the glass transition temperature Tg, of the polymer is above the operating temperature of the membrane, the polymer is plasticized by a second phosphoric acid/water conducting phase [ 3]. Dynamic mechanical analysis DMA, confirms that the majority of modulus loss in the polymer, approximately two orders of magnitude, is due to this effect. Thus, for polymer blends of comparable conductivities, introducing a polymer with a higher heat distortion temperature will not address these issues due to the highly plasticiTing nature of the conductive phase. These problems are mitigated by the development of a proton conducting semi-interpenetrating peiymer network (IPN). Interpenetrating networks of polymers provide a means of altering a given polymer's physical properties by incorporating a second, cross-linked polymer network. The former is termed

0 167-2738/88/$ 03 50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division )

118

S. Petty- Weeks et al. / Proton conductirt;y "nterpenetrating polymer networks

the host polymer and the latter the guest polymer. This technique results in a material distinct from a blend of the two polymers in that it possesses properties which are a combination of those possessed by the host and guest polymers. In this case, the high ionic conductivity is due to the PVA/H3PO4 host polymer [blend ], and the guest polymer, a three dimensional network of poly(methacrylic acid-methylenebisacrylamide), provides most of the mechanical stability [4]. Properties such as resistance to solvents, glass transition temperature, modulus above Tv creep and dimensional stability can be controlled by the cross-linked portion of the membrane [5]. However, since proton conduction in these films occurs by a percolation mechanism [ 3 ], care must be taken not to disrupt the continuity of the electrolyte conducting phase [6 ]. A similar approach was successful for improving the mechanical properties of ion-conducting PEO-LiCIO4 polymersalt complexes [ 7 ]. The proton conducting IPN's discussed in this report are formed by mixing the host polymer with a monomer, a crosslinking agent, and an initiato: if necessary, in a common solvent. This will then be polymerized to forrr, the guest polymer network after removal of the solvent, In semi-IPN systems, only one polymer is cross-linked. The host and guest polymers are permanently entangled, but are nc)t linked by covalent bonds. Some degree of miscibility of the components are needed to form the IPN; however, most of the proton conducting IPN's show some amount of inhomogeneity and phase separation is observed after casting and evaporation of solvent. This report presents the results of initial synthesis, characterization, and sensor experiments. For the proton conducting IPN's discussed here, the film is prepared by polymerizing the guest polymer in the presence of the host polymer PVA/H3PO4 blend, ~tlt\

t

~,..a, o t x a t ~

Lll*,.,

alal.tt

:,~ t 1 ~,tL

t,~,/taX/v.ta~:~

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guest polymer is composed of both difunctiona! and monofunc~ional monomers, methylenebisacrslamide and methacrylic acid, respectively, plus initiator for thermaily polymerized samples, to form a crosslinked polymer network. The resulting polymer membrane has properties attributaole to each polymer: proton conductivity due to the PVA/H3PO4 components and water resistance and increased bulk modulus due to ~he poly(methacDli= acid-methy-

lenebisacrylamide). This does not imply, however, that the IPN will be useful for solution applications as it is expected that the phosphoric acid will leach out ,of the membrane under such conditions. A number of films of different compositions have been prepared, varying the mode of polymerization of the guest polymer and the conductivity of the membranes. Polymerization was accomplished by either exposure to an electron (E) beam or by thermal treatment. Irradiation has the disadvantage of being a surface treatment in the case of thick films, although the E-beam will penetrate the polymer completely if the membrane is thin enough. Heating should provide uniform polymerization of the monomer throughout the film. Ionic conductivity was varied by changing the acid content of the film. It should be noted that increased acid content decreases the bulk modulus of the membrane. Increased guest polymer content increases the modulus, but decreases the conductivity. Thus, these materials can be tailored to specific applications by changing the relative amounts of host and guest polymers. The cross-link density of the guest polymer network can also be varied, although in all IPN's made for this study they are approximately equal. Mechanical properties were characterized by DMA; conductivity was determined by dc resistance measurements. In some cases, the bulk conductivity was also estimated from complex plane impedance techniques.

2. Experimental

2. I. Polymer characterization Water insolubility of the IPN films were checked by boiling them in water for 15 rain. The unmodified PVA/H3PO4 membrane dissolves in approximately one minute m:der these conditions. :~,lthcm~hthe fully cross-linked 1PN's were water insoluble, the membranes swell a~ter boi|ing. While not specifically noted, all membranes co~,.~ rain an undetermined amount of residual water. Because of the pronounced effect of water on the physical properties of the membranes, all measurements were made ,m films that had been dried at least 24 h in d~' gas at room temperature. This ensures that the membrants a~e of comparable "dryness",

S. Petty- 14"eeks et ai. / Proton conductivity interpenetrating polymer networks

although the absolute amount of water in the film will depend on the PVA and H3PO4 contents since both are very hygroscopic. dc resistance was determined by measuring the initial current and the potential drop across a m e m brane with sputtered platinum electrodes on each side separating two regions of differing hydrogen partial pressures. A schematic of the test set-up is shown in fig. 1. ac resistance measurements are discussed below. Sensor testing and response are discussed in more detail in [ 1 ]. Sensor performance was determined from the Nerst equation (eq. ( 1 ) ): (1)

E M F = m ( R T / 2 F ) .In ( P ' / P " ) .

P' and P" are the hydrogen partial pressures on each side of the membrane and m is the isothermal response factor. For PVA/HaPO4 films with sputtered Pt electrodes, m is generally 0 . 9 8 - 0 . 9 9 + . E beam polymerized proton conducting IPN's, whose compositions are listed in table l, had responses (summarized in table 2) ranging from 0.98+ to 1.00; m for the thermally polymerizeo polymer was 0.96, probably due to the physical inhomogeneity in the cast films. The membranes cast on porous supports for hydrogen sensors are generally much thinner. 20 microns as opposed to 50-100 microns, and may contain small defects such as pinholes. The isothermal response factor varies from 0.95-0.99, where m now reflects performance of the total structure. Mechanical properties for several films were measured using a Rheovibron dynamic mechanical ~nalyzer over the temperature range - 80 °C to 240 ° C. The Rheovibron applies a small sinusoidal strain on a sample in tension and measures the amplitude and phase of the sinusoidal stress response as a function of temperature. The strain frequency was 3.5 Hz for ro

rH 2

t,-~',

rH 2 \

119

all samples. The complex tensile modulus F* is defined by: E*=E' +iE" ,

(2)

where the storage modulus E' is a measure of the elastically stored energy and the loss modulus E" is associated with the dissipation of energy as heat. These are a mechanical analog to the dielectric constant and dielectric loss, respectively [8 ]. The loss tangent, tan 0, is defined in eq. (3)" tan ~= E" /E' .

(3)

A~I measurements were performed in dff nitrogen on polymer samples that had been dried in nitrogen at room temperature for at least 24 h for reasons noted above. Glass transition temperatures are reported as the midpoint of the inflection. Polymers containing acid were black and brittle after exposure to high temperatures in the DMA. Because improvement of the mechanical properties was the main impetus for this work, only a limited amount of DSC analysis was performed to supplement the mechanical characterization data and will only be discussed qualitatively. Complex impedance measurements were made on membranes with platinum (ohmic) electrodes using a Gen Rad Precision RLC Digibridge in an RC parallel circuit configuration over a freauency range of 12 Hz to 100 kHz with an applied 5 mV amplitude ac signal. The real and imaginary parts of the admittance can De calculated and plotted in the complex plane. These experiments and calculations are described more fully in [ 3]. All samples were dried for at least 24 h in dry gas at room temperature and run in 100% hydrogen. Resistivities were calculated from the conductivity values extrapolated to the x axis intercept (of the real component of the complex admittance) at the high frequency end. This has been confirmed as a reasonable estimate for the admitlance to caicuiate "~". . . . ,~,,~, - .;,,. r ,~,,, ~,,,~, ierial for PVA/H3PO4 blends. A typical example of the resulting complex admittance plot is shown in fig. 8 and resistivity values are denoted in parentheses in table 1.

2.2. IPN's polymerized by irradiation Fig. I. S c h e m a t i c

of

proton conducting polymer membrane

for i s o t h e r m a l r e s p o n s e ( ,1 ).

test

IPN films were prepared by mixing aqueous solutions of PVA/phosphoric acid and methylenebis-

S. PetO,-Weeks et al. / Proton conductiviO, interpenetrating polymer networks

120

Table 1 Compositions and resistivities of proton conducting IPN's polymerized by irradiation. Sample

PVA (g)

H 3PO., (g)

Methacrylic acid

Methylenebisac~'lamide

(g)

(g)

142-1A

0.50

0.29

0.54

0.04

142-1B

0.50

0.29

0.54

0.04

142-2

0.50

0.29

2.69

0.18

146-3B

0.50

0.56

0.54

0.04

146-3C

0.50

0.56

0.54

0.04

146-4

0.50

0.56

2.69

0.18

146-1

0.50

-

0.54

0.04

146-2

0.50

-

2.69

0.18

E beam dose " '

one pass-one side one pass-both sides one pass-both sides one pass-both sides one pass-both sides @ 7.5 Mrad/pass one pass-both sides one pass-one side one pass-one side

Resistivity "' (D.. cm)

2.0× 106 6.9× l0 s 5.1 × 106 1.9× ( 1.8X 2.6X (8.4X

10 S 105) 104 103)

2.3× l0 s ( 1.2× l0 s ) ( 1.5 ) × 10s )

" ~ 5 Mrad/pass, 175 keY. t,, Resistivities estimated from dc measureaaents: values in parentheses are determined from complex admittance plots.

acrylamide/methacrylic acid, and allowing the solvent to evaporate at ambient conditions. The films were then exposed to the E-beam (Energy Sciences Electrocurtain) for yawing times and doses. The IPN's were insoluble in all cases. Table 1 gives exact compositions, doses, and resistivities estimated from dc measurements for these films. Films that were irradiated on one side only showed physical differences between the two sides. In general, the nonirradiated side was much tackier and the sputtered platinum electrode was more fragile and was easily damaged during handqng. In general, the membranes were brown and translucent; discoloration increased with increasing acid content and increasing dose. B1ov~ning over time also occurs with the high acid pro 'on conducting polymer blend~, which does not appear t~ impair short-term ~evsor performance in either case. Sensors based on the PVA/H~PO4 blend remain operational in dr)' gas at room tempera~.ure for over a year. Long term stability for IPN-based sensors has not been determined. The conductivity appears to be dependent on do~e as well as acid content of the membrane. Films with higher methacryl~c acid/methylenebisacrylamide contents ,,,,'ere white and visually inhomo~eneeus

with some crystalline areas, but were still flexible. Table 2 compares the proton conducting IPN properties with the PVA/H3PO4 blend.

2.3. IPN'~ polymeJ ized by thermal treatment Thermally polymerized IP' 's were cast from aqueous solutions of the compt,nents above with the addition of azobisisobutryonitrile ~(AIBN ) initiator, which did not completely dissolve in the monomer matrix solution. Two hours at 75 °C in e nitrogen atmosphere were sufficient to polymerize the dried fiims, i.e., they swelled, but did not dissolve in boiling water after 15 min. The films were white and translucent and in some cases, the final IPN polymer film contained crystalline material imbedded in the matrix. This crystalline material is believed to be tetramethylsucdnodinitrile which is the predominant cage recombination product from thermal decomposition of AIBN in the solid state [9]. Since this mixture is really an emulsion, casting from concentrated aqueous solutions resulted in increased film inhomogeneity. A solution of 2 grams PVA, 2.2 g 85% H~PO4, 5.3 g methac~lic acid, 0.4 g methylenebisac~,lamide,

S. Petty-Weeks et ui. / P~'oton conductivity interpenetrating polymer networks

121

Table 2 Physical properties of proton conducting polymer blends and IPN's.

wt% H3PO4 a~ wt% IPN '~ polymerization method p(X-cm) E' ( M N / m - ' ) @ 25°C @ 50°C soluble in hot water?

PVA/HsPO4 blend

IPN 146-3B

IPN 146-3C

IPN 55-1

IPN 142-1B

IPN 146-4

IPN 142-2

IPN 146-2

IPN 146-1

37 -

34 35 E beam

34 35 E beam t,~

23 57 thermal

21 42 E beam

14 73 E beam

8 78 E beam

0 54 E beam

0 85 E beam

4X 105c~

2X 10s¢~

8X 10 "~¢~

5 X 10 4

7X l0 s

i × 10 s'~

5X 106

26 20

11 10

9 7

71 65

321 151

1120 1155

1880 1620

yes

no

no

no

no

no

no

no

no

2 × 108"~

~ On a water free basis. t'~ One pass each side at 7.5 Mrad/pass, 175 keV. "~ Calculated from ac impedance measurements.

0.06 g AIBN, and approximately 50 mL DI water was cast into a film. After the solvent evaporated, the membrane was heated in a nitrogen atmosphere at 75 °C for two hours. Slight browning occurred. After 18 h in a dry gas stream, the membrane had a room temperature resistivity of 4.6:< 104 ~ cm. The potential between two platinum electrodes was 28.2 mV at a hydrogen partial pressure ratio of approximately 10/1. The theoretical EMF is 29.3 mV at 22°C based on the Nernst equation; m equals 0.96.

2.4. Sensors fabricated with proton conducti':g IPN'~ A hydrogen sensor was fabricated from a porous glass bead support with a platinum electrode sputterdeposited on it. An aqueous solution of poly(vinyl alcohol), phosphoric acid, methacrylic acid, and methylenebisacrylamide was cast on the substrate. When casting the polymer solution on a porous substrate, one must consider wetting interactions between the solution and the s~,bstrate as well as viscosity. Electrode sputter-deposition conditions were adjusted to minimize wetting. After the solvent had evaporated, the senso: (p,Aymer film side up) was irradiated at 7.5 Mrad and !75 keV per pass for one r~a~ fcdlr~ .... A b~' a P~. c~c~:t~uuc -' . . . . . . . . sputtered onto the polymer membrane. Sensor performance is summarized in table 3 and fig. 2. The device resistance at room tempera!ure was ap-

Table 3 Response for sensor with E beam polymerized iPN proton conducting polymer. 10:1 hydrogen partial pressure ratio across membrane. Temp.

mV actual

mV theor.

/tl

31.2 30.8 30.1 29.1 29. l 26.2

3 !.3 30.8 30. I 29.5 29.4 27.4

0.997 !.00 1.00 0.986 0.990 0.956

(:c) 42.8 37.7 32.6 24.5 22.7 3.0

proximately 14.5 k.fL The major contribution to the resistance at this temperature is the electrode sputtered on the substrate. As the temperature drops, the increased resistivity of the film supersedes the resistance due to the electrodes. The sensor performed as predicted by the Nernst equation above room tern-

E 25,[- =

I I 20!

o

~ THEORETICAL mV A ACTUAL mV ; I-In (mV) = 3.262T + .0043; r=0.989 i

10

20 3'o TEMPERATURE CC~

4'0

so

Fig. 2. Response for hydrogen sensor with IPN proton conducting polymer, 10:1 hydrogen partial pressure ratio across sensor.

122

S. Petty. Weeks et al. / Proton conductivity interpenetrating polymer networks

perature, but never developed the theoretical potential below, compared to the P V A / H 3 P Q based sensors which develop the full Nernstian response to - 4 0 ° C. The voltage readings below room temperature drifted in the direction of the theoretical voltage for the duration of the measurements. Given the limited amount of samples and data, it is not clear whether this is due to sensor performance or experimental design. Sensors can also be fabric=ted by impregnating a porous support with a similar solution, polymerizing the guest polymer network, then applying platinum electrodes to either side of the substrate. In this case, a thermally polymerized IPN will be the preferred method to insure polymerizatie,a of the guest network throughout the entire membrane. The device impedance will be much higher due to the increased thickness of the polymer electrolyte, but will still operate satisfactorily with the appropriate electronics. Care must be taken to insure that the film remains continuous throughout the substrate and that there is adequate contact between the electrodes and the electrolyte. Prototype sensors followed the Nerst equation. No attempt was made to optimize the sensor properties or performance.

3. Discussion Based on these experiments, modifying the properties of the proton conducting PVA/H3PO4 blend by forming an IPN is an easy and successful approach. Tailoring the membranes to specific conductivity and mechanical requirements by varying the relative amounts of host and guest polymer has been demonstrated, Because the PVA/H~PO4 conducts by a percolation mechanism, i.e., a critical volume of electrolyte must be reached to form a continuous conducting phase before bulk conductiv" " itv o c c u r s , ~mk c.,. .c. . . , , ,:11 , , be a l l m l l I o '~' . . . . . . . * " , ¢ ~ -,,', conducting guest pobmer network that can be incorporated into the membrane. Plots of conductivity versus •Q.~' I.l.P. .~. I nr o l~v-mj o r * - . " - = t~a ~~ ,. ~, p-u. ,.,',~ j ]X ¢l V, -l, . .tLi l-e tL I %: r_".,Il Ur ' x 4 -

Lllt~.

(.,I. I I I U I ~ I I L

v~j I

issue of disrupting the conducting phase is the same for both IPN cases. Similarly, there is a critical volume for the guest polymer network above ~hich the modulus does not increase significantly. The IPN's show the same general trends as the proton conducting polymer blends: conductivity above the percolation threshold increases with increasing acid content and the electrolyte phase plasticizes the polymer matrix. The conductivity also appears to be dose dependent for the E-beam polymerized samples (see table l ). Inhomogeneity of the cast films contributes to scatter in the conductivity data. Because of the mechanical stability introduced by the network structure, much more acid can be added to the IPN than to the PVA/H3PO4 blend. Differential scanning calorimetry and dynamic mechamcal analysis should yield similar, but not necessarily identical, results. Discrepancies between the two techniques are due to the differences in the physical properties being measured, i.e., heat flow versus mechanical relaxation. In both cases, phase separation is indicated by more than one second order transition :n either the heat flow to the sample (DSC) or in the storage modulus (DMA), or by multiple tan fi peaks in the polymer sample (DMA). Just as the T g ' s determined by DSC are dependent on the temperature program rate, the transition temperatures in DMA decrease and resolution between transitions increases with decreasing applied frequency [ 10 ]. The PVA/H3PO4 blend exhibits two second order transitions by DSC and a first order transition cor1

1'1

0 vo

m

z m

ll~Jll-

poly (ethylene oxide)/ep_~xy system discussed in ref. [7] clearly show a percolation threshold. It should be noted that the LiCIO4--PEO polymers conduct by a free volume mechanism, where ion motion is linked to movement of the pol~ met backbone; however, the

-1

-22 -40

-20

0

20

TEMPERAiHRE

40

60

80

100

(C)

Fig. 3. Storage m o d u ; u s and tan (i d a l a for a P V A / H d~()~ m e m brane: 37 ~ t % p h o s p h o r i c acid on a water-frcc basis.

S. Petty-Weeks et al. I Proton conductivity interpenetrating pol.vmer networks

•EolO

"~'~,=

>" 9 _=

\

',._

tkl Z

~

\ ~_~

IPN

(8 WT~ AC,O)

PVA/H3P04 IPN - ~ - - - - ' ~ ' ' ' - - ' - - ~ _

-1 i,o

_

(~4 wT~ AC=O)

iu

PVA/H3PO 4

123

~

"

~

;=

0

0,8

PVA/H3P04 (37 WT% ACIO)

7 -50

i

i

"1'0

i i ' '3'0 7'0 TEMPERATURE (C)

9~

-2 i

li0

150 -3

Fig. 4. DMA analyses comparing storage moduli of proton conducting IPN's to a PVA/H3PO4 blend.

responding to the melt of crystalline PVA (see [ 3 ] ). The glass transitions correspond to two partially resolved tan ~ peaks by DMA, although the storage modulus for this material shows only one transition (see fig. 3). It can be postulated that only one transition is evident in the storage modulus because the mechanical properties are dominated by the continuous polymer phase. As the sample is dried, the tan 5 peaks become less distinct and pGorly resolved. Compared to the PVA/H3PO4 polymer blend, the IPN samples with similar conductivifies show vastly improved bulk modulus properties, especially above room temperature (see fig. 4). In addition, the Ebeam polymerized IPN's maintain mechanical integrity above 100°C, where the polymer h!end f~!ls. Table 2 compares conductivity and mechanical propert~,s of E-beam and thermally polymerized I?N's ~..~a PVa,/H3PO4 blend. Since gross phase separation, in the form of crystalline material, were

-50

-10

30 70 110 TEMPERATURE (C)

150

190

Fig. 6. Storage modulus and tan 6 data for proton conducting IPN sample 142-2, 8wt% phosphoric acid on a water-free basis.

present in the thermally polymerized samples, no DMA's were run. Fig. 5 compares IPN's of varying acid contents, but with comparable guest network cross-link density. Phase separation occurs between the host and guest polymers, indicazed by the two glass transitions in E'. Both E' and tan ~ for sample 142-2 are shown in fig. 6. The DSC data for sample 142-2 contains three second order transitions, presumably due to the conducting phase, host, and guest polymers (see fig. 7). Based on the DSC and DMA data for the PVA/H3PO4 blend, one would expect phase separation between the electrolyte and PVA in this temperature region. Increasing acid/water content decreases the modulus of the IPN's, with most of the loss occuring below room temperature. All the IPN samples tested have a single wide, or perhaps several partially resolved, asymmetric tan ~ peak corresponding to this Tg. Te,e experimental co:.-litions may

111 ~

"~,

i~6-2 'NO AC,O)

~.~...~...

',,

\

""

c=o)

.~

z 9~146-4 ".. . . . . . . . >.-, .I .~.lq'. .w. .I "0 . .A;'-wlUI ...............

"~ I

- - ' ~ ~

o

|

7

[

-8o

~

~

-4o

.... ~

L

o

~

A

~

~

40 8'0 ~ o TEMPERATURE (C)

A

,

~6o

-J

1

200

Fig. 5. DMA analyses comparing storage moduii of proton conducting IPN's with varying phosphoric acid contents.

-100

-50

~.

0

r...

50

100

150

TEMPERATURE (C)

Fig. 7. DSC analysis of proton conducting IPN sample 142-2:8 wt% phosphoric acid on a water-free basis.

124

S. Petty- Fl'eeks et al. / Proton conductivity interpenetrating polymer networks

not be optimum for examining transitions in the lower temperature range; however, these experimental limitations do not present a significant problem since performance at or above room temperature is of major interest. Above 120-130 °C, the modulus increases in the acid containing IPN's, probably due to chemical reaction and degradation in the samples. As noted earlier, the polymers were black and brittle following exposure to these temperatures. Two scenarios can be postulated to account for the discrepancies between the DSC and DMA results (run for the 8% acid IPN sample only), especially for the lowest temperature transition, which does not appear on the DMA. The low temperature transition, which corresponds to the electrolyte conducting phase, is not expected to contribute mechanically to the polymer matrix at temperatures well below the £~'s of the (plasticized) polymers, although it does contribute to the decrease in modulus due to plasticization at higher temperatures. Alternatively, since resolution is also affected by frequency, generally increasing as frequency decreases, the two low temperature transitions may not be resolved. More experimental work should be done before drawing any firm conclusions. Complex admittance measurements have been used for many years to model ion conducting systems by equivalent circuit analysis [ 11 ]. In general, the model comprises resistors and capacitors in a lumped parameter system, although one no',able exception is the Wavburg impedance. Zx~. which is represented by an infinite RC line and is used as an analog for diffusive processes. When the circuit element response does not fall into one of these categories, it can be modeled by a "constant phase element" (CPE), whose frequency dependent admittance. Y, is given by Y=A,,(ioJ)"--A ~(io)r)",

pretation of these systems is much more difficult [ 13,14 ]. Materials that conduct via a percolation mechanism can be modeled using fractal mathematics, which results in a system mathematically equivalent to constant phase elements, distribution ..,,'~', relaxation times, or' distribution of activation energies often used to represent this behavior. This approach is hampered by the lack of a mathematically unique equivalent circuit for a given impedance-frequency relation [ 15 ], requiring other independent t~hysical measurements to supplement this data. A representative, complex admittance plot is shown in fig. 8. A more detailed description of the experiment can be found in [ 31. As mentioned previously [ 3 ], the PVA/H3PO4 blends can be adequately modeled by a lumped RC circuit below the percolation thr,:shold, but show distributed behavior above. Both the ~ow frequency (electrode and electrode/electrolyte interface) and high frequency (bulk material) regions are affected. The complex admittance plot for sample 146-3B shows the distributed behavior associated with the highly conductive PVA/H3PO4 samples. The center of the low frequency arc is depressed below the real axis. In cases where the high frequency spur is apparent, it ibrms an angle less than 90 ° (lumped RC), but greater than 45 ° (Warburg impedance) with the real axis. The conductivity of the bulk material can be calculated from the high frequency intercept to the x axis. This should be a more accurate value than the dc measurements, where polarization of the system occurs. The scatter in resistivity values as a function of composition for both dc and ac measurements is probably due to the highly inhomogeneous nature of G b = 7 . 2 9 x 1 0 - 3 0 -1 5

(4)

where o) is the frequency, z is the relaxation time, and A,, and .4~ are frequency independent coefficients. The expor~ent n is 1.0 for the lumped RC equivalent ci~cui~ and 0.5 for the Warburg distributed RC transmission line. However. the response of many ion conductors, especially inhomogeneous materials, falls outside these two values [ 121. While these distributed frequency responses are readily amenable to mathematical treatment, physical inter-

1=115x

10-4cm

A = 1.539 crn 2 74 C

¸ 0=~=1.8

~a

x 1050.cm

I

1 0 0

1

2

3 4 5 Gx103(0-1 ,

6

7

8

Fig. 8. ac admittance of proton conducting IPN sample 146-3B with platinum (ohmic} clectrodcs.

S. Pelt),- Weeks et al. / Proton conductivity interpenetrating polymer net works

the materials under study. Membrane inhomogeneity is not necessarily a problem in this case, but may be critical for other applications. One approach to minimize phase separation would be to incorporate oligomers ~n the guest polymer to prevent monomer segregation. For a more complete picture of the polymer composition/property relationships several experiments are suggested by the data. These include: ( l ) a more thorough examination of conductivity and bulk modulus versus composition, varying both acid and guest polymer network content, (2) correlating DSC and DMA results, and ( 3 ) addressing membrane inhomogeneity by improved fabrication procedures, especially for the thermally polymerized IPN's. With a thorough understanding of this approach to materials by design, these polymers can be adapted for many other applications.

Acknowledgement We would like to thank C.A. Mendyk for assistance with the DMA experiments, J.F. Parmer for helpful discussions with the former and UOP Monirex Systems (Allied-Signal) for funding the work.

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