DSC and high pressure conductivity and electrical relaxation measurements in PPO and PPO complexed with lithium salts

Solid State Ionics 18 & 19 (1986) 253-257 North-Holland, Amsterdam

253

DSC AND HIGH PRESSURE CONDUCTIVITY AND ELECTRICAL RELAXATION MEASUREMENTS IN PPO AND PPO COMPLEXED WITH LITHIUM SALTS* J. J. FONTANELLA, M. C. WINTERSGILL, J. P. CALAME, M. K. SMITH Physics Department, U. S. Naval Academy, Annapolis, MD, USA 21402 C. G. ANDEEN Case Western Reserve U n v e r s i t y ,

Cleveland, OH, USA 44106

D i f f e r e n t i a l scanning c a l o r i m e t r y and high pressure e l e c t r i c a l c o n d u c t i v i t y s t u d i e s have been c a r r i e d out on p o l y ( p r o p y l e n e o x i d e ) a f t e r the a d d i t i o n o f v a r i o u s l i t h i u m s a l t s . In a d d i t i o n , high pressure e l e c t r i c a l r e l a x a t i o n measurements have been performed in the glass t r a n s i t i o n r e g i o n on uncomplexed PPO. The vacuum c o n d u c t i v i t y measurements show t h a t To i s 30-40°C lower than the DSC Tg. Next, i t is found t h a t both To and Tg increase several K/kbar. In terms of the c o n f i g u r a t i o n a l e n t r o p y model, the i s o b a r i c data y i e l d a reasonable value f o r the a c t i v a t i o n volume which is c o n s i s t e n t w i t h values p r e v i o u s l y r e p o r t e d f o r PEO complexed w i t h a l k a l i metal ions. The i s o t h e r m a l data y i e l d a n e g a t i v e a c t i v a t i o n volume f o r PPO complexed w i t h LiSCN. F i n a l l y , DSC and isothermal c o n d u c t i v i t y measurements show t h a t n e i t h e r Tg nor the apparent a c t i v a t i o n volume scale d i r e c t l y w i t h ion s i z e . I.

INTRODUCTION P o l y ( p r o p y l e n e o x i d e ) (PPO) has a Tg comp a r a b l e t o t h a t o f PEO and t h e r e have been seve r a l s t u d i e s to determine i t s complexing p r o p erties. I-4 It is significant that highly amorphous PPO can be prepared. ticularly

methanol at about 55°C and cast onto a t e f l o n p l a t e in a glass r e t a i n i n g r i n g . The samples were i n i t i a l l y d r i e d a t room temperature in f l o w i n g d r y n i t r o g e n and then t r a n s f e r r e d t o a vacuum oven f o r f i n a l d r y i n g a t about 65°C. The e l e c t r i c a l

This i s p a r -

i m p o r t a n t when c o n s i d e r i n g the e f f e c t

measurements were c a r r i e d out

of pressure on the i o n i c c o n d u c t i v i t y . Previous work on the pressure dependence of the

using techniques described elsewhere. 5 Two t e r m i n a l high pressure measurements were performed using S p i n e s s t i c 22 as the pressure

c o n d u c t i v i t y of PEO complexed w i t h v a r i o u s s a l t s , p a r t i c u l a r l y the e v a l u a t i o n of the

f l u i d and an o i l bath was used f o r temperature control. I s o b a r i c measurements were performed

a c t i v a t i o n volumes, was c o m p l i c a t e d by the e x i s t e n c e of d i f f e r e n t phases in e q u i l i b r i u m w i t h one another. 5 In the present work, the e l e c t r i c a l c o n d u c t i v i t y o f samples of amorphous PPO complexed w i t h several Li s a l t s was s t u d i e d a t high pressure. Also, e l e c t r i c a l r e l a x a t i o n

w i t h about 30 min. e q u i l i b r a t i o n

time w h i l e f o r

isothermal data the e q u i l i b r a t i o n time was I0 min. DSC was performed w i t h a DuPont 990 DSC i n t e r f a c e d w i t h a microcomputer.

and d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) work were c a r r i e d out.

3. RESULTS AND DISCUSSION The c o n d u c t i v i t y data were analysed using standard complex impedance techniques a l l o w i n g the d e t e r m i n a t i o n of the bulk r e s i s t a n c e as a

2. EXPERIMENT PPO in the form of PAREL elastomer was

f u n c t i o n of temperature. p l o t is shown in f i g . I .

o b t a i n e d from Hercules, Inc. A l l o f the sample p r e p a r a t i o n procedures were c a r r i e d out in a dry box o r vacuum oven. The polymer and s a l t s ,

used, in c o n j u n c t i o n w i t h g e o m e t r i c a l measurements, to c a l c u l a t e the bulk c o n d u c t i v i t y . N e i t h e r thermal expansion nor c o m p r e s s i b i l i t y i s included in the data a n a l y s i s . The r e s u l t s

LiCF3SO3, LiClO 4, and LiSCN were d i s s o l v e d in

*Work supported in p a r t by the O f f i c e of Naval Research. 0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

A t y p i c a l impedance These values were

254

J.J. Fontanella et al.

%-

/ PPO and PPO c o m p l e x e d with lithium salts

12

-4

9.6

~--"-5.2 I

E

E

?-6.4

~7.2 0 LO 0

E Io

4.8

v

"~-7.6 o

~

2.4

0

D~ D ;3

Z' (1605

~-8.8

. 9

ohms)

15

12

of t y p i c a l i s o b a r i c data runs are shown in f i g . 2. The c u r v a t u r e o f t e n observed f o r amorphous polymer systems i s obvious, That the samples were amorphous is confirmed by the DSC r e s u l t s shown in f i g . 3. Consequently, the conductdata were analyzed v i a the VTF eq,: 6

o = AT- I / 2

e x p - [ E a / k ( T - T o )]

(I)

w i t h the a d j u s t a b l e parameters A, Ea, and To , A l e a s t squares f i t of eq. 1 was c a r r i e d out and Table 1 c o n t a i n s the b e s t - f i t parameters, Fig. 2 shows the r e s u l t s of these f i t t i n g procedures f o r PPO:LiCF3SO 3 a t t h r e e pressures. The glass t r a n s i t i o n temperatures, Tg, obtained from the DSC r e s u l t s shown in f i g ,

3, are a l s o

l i s t e d in Table I . Tg was obtained by standard s t r a i g h t l i n e a n a l y s i s . The estimated uncerc e r t a i n t i e s are ATg=~5 K, AEa=+O.O05 eV, and &loglOA=~O.3. While the u n c e r t a i n t i e s are l a r g e , some trends can be discerned. First,

2.5

2.7

2.9

(.1-1) 3.3

3.5

1000/T

FIGURE 1 Complex impedance p l o t f o r PPO8LiCF3SO3 at 308K and O.l GPa,

ivity

-10

the vacuum value f o r TO i s always

30-40°C lower than T8, This is r e a s s u r i n g since Tg-T o i s u s u a l l y on the o r d e r of 50°C f o r polymer systems 7-9 and t h i s is c o n s i s t e n t w i t h

FIGURE 2 E l e c t r i c a l c o n d u c t i v i t y vs, r e c i p r o c a l temperature f o r PPO8LiCF3SO3 at three pressures: (a) vacuum; (b) 0. I GPa ( I k b a r ) ; (c) 0,2 GPa. which the c o n f i g u r a t i o n a l entropy of the whole system becomes very small, 7,8 The second t r e n d i s t h a t both Ea and T o i n crease w i t h pressure. Since both q u a n t i t i e s increase, t h i s is u n l i k e l y t o be an a r t i f a c t of the f i t t i n g procedure, since in t h a t case one parameter would tend t o increase at the expense of the o t h e r . A t y p i c a l example of t h i s behavi o r i s shown in Table 1 where the r e s u l t s f o r two samples of PPO8LiCF3SO3 are given, Also, the d i f f e r e n c e between the present r e s u l t s and t h a t of Armand et a l . 1 e x h i b i t s t h i s e f f e c t . The r e s u l t t h a t To increases several K/kbar is c o n s i s t e n t w i t h the c o n t e n t i o n t h a t To i s somehow r e l a t e d to the glass t r a n s i t i o n , since Tg n o r m a l l y increases by t h i s amount. Some evidence f o r t h i s behavior in uncomplexed PPO i s given in f i g . 4 where e l e c t r i c a l r e l a x a t i o n r e s u l t s are shown. That r e l a x a t i o n is associ a t e d w i t h the glass t r a n s i t i o n and the s h i f t

temperature v a r i a t i o n of the size of the c o o p e r a t i v e l y r e a r r a n g i n g region of the amorphous m a t e r i a l in which ion t r a n s p o r t

in the maximum temperature, Tm, i s about 17 K/kbar which is c o n s i s t e n t w i t h the above result, U n f o r t u n a t e l y , s i m i l a r measurements cannot be c a r r i e d out on complexed m a t e r i a l s because of the high DC c o n d u c t i v i t y . DSC measurements under pressure which would y i e l d t h i s i n f o r m a t i o n are c u r r e n t l y being under-

occurs and i d e n t i f i e s

taken,

the c o n f i g u r a t i o n a l entropy model (CEM) o f Gibbs and co-workers, l O ' l l This considers the

TO as the temperature at

Still

o t h e r evidence t h a t T o is r e l a t e d

J.J. Fontanella et al. / PPO and PPO complexed with lithium salts

TABLE I .

DSC r e s u l t s f o r Tg and best f i t

S a l t in

RMS

PPO ( 8 : 1 )

loglo A

Deviation

255

parameters in equations I , 5, and 6.

Ea

To

Tg

(eV)

(K)

(K)

C1

C2

loglOO(Tg)

l o g l o A'

(K)

E'a

T'o

(eV)

(K)

LiCF3SO3 Vacuum #I Vacuum #2 0. I GPa 0.2 GPa

0.021 0.020 0.0063 0.016

-0.49 -0.94 -I.02 -0.73

0.092 0.086 0.098 0.104

210 214 216 233

238 238 255 272

16.9 18.7 12.6 13.3

25.9 22.0 37.1 38.1

-18.8 -21.0 -15.1 -15.4

-I.90 -2.34 -2.41 -2.11

0.087 0.082 0.093 0. I01

212 216 218 234

LiCIO 4 Vacuum

0.013

-0.04

0.081

235

263

14.8

26.4

-16.2

-I.44

0.077

237

LiSCN Vacuum

0.011

+0.047

0.126

203

245

15.2

39.9

-16.1

-0.95

0.120

205

t o Tg i s t h a t PPO8 LiClO 4 gave r i s e to a value

VAr r = -kT dlnG/dP

of 235 K f o r To . This is s i g n i f i c a n t l y h i g h e r than any of the r e s u l t s f o r l i t h i u m t r i f l a t e or t h i o c y a n a t e , and i s c o n s i s t e n t w i t h the DSC

and are a l s o l i s t e d in Table 2. F i n a l l y , values f o r v * / g were c a l c u l a t e d using:

r e s u l t s f o r Tg.

v*

Also, Armand e t a l . 1 have

shown t h a t To increases w i t h i n c r e a s i n g s a l t c o n c e n t r a t i o n as i s observed f o r Tg.12 Usin 8 the CEM, the s h i f t in Ea can be used t o c a l c u l a t e the a c t i v a t i o n volume, v*. With the usual assumptions. 5 i t f o l l o w s t h a t : 1 dEa - -

Ea dP

1 dT o -

v* + - -

TO dP

(2)

y i e l d s v * / g = 37±40 cm3/mol-eV f o r PP08: LiCF3SO 3, This value i s the d i f f e r e n c e between two l a r g e , r e l a t i v e l y u n c e r t a i n q u a n t i t i e s , however the mean value is reasonable since, f o r a value of g=leV, f o r example, the a c t i v a t i o n volume is s i m i l a r t o the values quoted metals in PEO.5

(T-To)[dlnA -

No

L ~

dlna] -

dR

T

(dTo~

- To(T-To) \ ~ - /

(4)

where the assumptions f o r a l l m a t e r i a l s are t h a t dlnA/dP=O and t h a t dTo/dP=9.9 K/GPa, which was observed f o r PPO8LiCF3SO3, The remaining parameters are those l i s t e d in Tables 1 and 2. The pressure v a r i a t i o n of the c o n d u c t i v i t y f o r PPO8LiCIO 4 i s c o n s i s t e n t l y l a r g e r than f o r

g

previously for alkali

g

(3)

Conse-

q u e n t l y , the comments made f o r PEO concerning the t r a n s p o r t mechanism 5 a l s o hold f o r PPO. In o r d e r t o compare the e f f e c t s of pressure on PPO complexed w i t h the various s a l t s , i s o t h e r m a l data were also o b t a i n e d . L i n e a r l e a s t square best f i t s were obtained and the r e s u l t s at 50 ° , 70 ° and 90°C are shown in Table 2. The 70°C data and best f i t curves are shown in f i g . 5. Also, the values f o r an " A r r h e n i u s " a c t i v a t i o n volume were c a l c u l a t e d from:

PPO8LiCF3SO3 which, in t u r n , i s l a r g e r than t h a t f o r PPO8LiSCN. Since t r i f l a t e i s the l a r g e s t anion, the slope does not scale w i t h the s i z e of the anion. S i m i l a r measurements on PEO c o n t a i n i n g LiClO 4 and LiSCN were the basis f o r the suggestion t h a t the slope scales w i t h ion s i z e . 5 However, LiCF3SO3 was not s t u d i e d in t h a t case. In the PEO work i t was p o i n t e d out t h a t the s i m p l e s t e x p l a n a t i o n of the ion s i z e e f f e c t s was t h a t the anions were mobile. That e x p l a n a t i o n could s t i l l hold i f a l a r g e r f r a c t i o n of the c u r r e n t i s c a r r i e d by L1 ions in the case of LiCF3SO3 complexed m a t e r i a l . However, as was also pointed out in the PEO work. i t may be t h a t o t h e r e f f e c t s such as morphology changes may be the e x p l a n a t i o n . Tg a l s o shows the same general t r e n d as the e f f e c t of pressure on the c o n d u c t i v i t y in t h a t

J.J. Fontanella et al. / PPO and PPO complexed with lithium salts

256

0.6

l

0.8

0.48

0.6

0.36 ]

E

0.24]

f-

"'

0.4

I 0.2

0.12]

0

0 L 50

-80

60

-10

130

200

210

222

T(°C) FIGURE 3 DSC thermograms f o r : (a) uncomplexed PPO; (b) PPO8LiCF3S03; (c) PPO8LiSCN; (d) PPO8LiClO4. LiClO 4 e x h i b i t s the highest Tg. A possible e x p l a n a t i o n f o r these e f f e c t s may be r e l a t e d to the f a c t t h a t both the t h i o c y a n a t e and t r i f l a t e ions have a d i p o l e moment, in c o n t r a s t to the p e r c h l o r a t e ion which does not. For example the added d i p o l e i n t e r a c t i o n could lead to a c o n f i g u r a t i o n which decreases both Tg and the e f f e c t of pressure on the c o n d u c t i v i t y . It w i l l be of i n t e r e s t to study PPO complexed with other l i t h i u m s a l t s to t e s t t h i s idea. Next, the value of v*/g l i s t e d in Table 2 f o r PPO8LiSCN is negative. This r e s u l t is reminiscent of an observation f o r PEO.5 That was considered an anomaly p r i m a r i l y due to an improper choice of To . That is o b v i o u s l y not the e x p l a n a t i o n in the present work and presumably not f o r PEO e i t h e r . There are several assumptions in the c a l c u l a t i o n s based on eq. (4) and thus t h i s r e s u l t is not definitive. Some of those assumptions are e l i m i n a t e d via i s o b a r i c data and such experiments are c u r r e n t l y underway. F i n a l l y , in the In(G) vs P p l o t s f o r PPO, there appears to be a small curvature at lower temperatures which is opposite in sense from t h a t reported f o r PEO. The e f f e c t s of t h i s curvature a r e a l s o apparent in f i g . 2. I t is l i k e l y t h a t the d i f f e r e n t curvature f o r the PEO p l o t s arises because the PEO r e s u l t s were

234 246 T(K)

258

270

FIGURE 4 Conductance vs. temperature f o r uncomplexed PPO at various pressures. The curves from l e f t to r i g h t are: (a) I00 MPa (I arm); (b) 0.03 GPa; (c) 0.06 GPa; (d) 0.09 GPa; (e) 0.12 GPa; ( f ) 0.15 GPa; (g) 0.18 GPa. s i n g l e frequency data r a t h e r than the r e s u l t of complex impedance a n a l y s i s , In f a c t , a decrease in the curvature with frequency is obvious from f i g . 1 of t h a t paper. Presumably. the downward curvature at lower temperatures in PPO is due to the s h i f t of Tg with pressure. For comparison, the data were reanalyzed in terms of the WLF equation: 13

lOglO

--

o(T)

O(Tg)

-

Cl(T-Tg)

(5)

C2+(T-Tg)

The r e s u l t a n t parameters are l i s t e d in Table 1. The values of C1 and C2 are as close to the " u n i v e r s a l " values of 17.4 and 51.6 as are those of Watanabe et al. 2 F i n a l l y , f o r completeness, the data were analyzed via the VTF eq. in the form: o = A' exp-[Ea/(T-To )]

(6)

The r e s u l t s are also l i s t e d in Table 1. 4. CONCLUSIONS In summary, then, the e f f e c t of pressure on the e l e c t r i c a l c o n d u c t i v i t y f o r PPO complexed with l i t h i u m s a l t s has been determined and DSC measurements have been c a r r i e d out. In

J.J. Fontanella et al. / PPO and PPO complexed with lithium salts

257

TABLE 2. Pressure v a r i a t i o n of the c o n d u c t i v i t y and a c t i v a t i o n volumes f o r PPO complexed with l i t h i u m salts.

-4.6

70°C ~-~-5.2 I

Salt in PPO (8:1)

E

? -5.8

T (°C)

E

r"

dlna v*/g V~r r dP (GPa)- I (cm3/mol-eV) (cm3/mol)

0

"~'-6.4

a

o

LiClO 4

50 70 90

-24.0 -17.0 -14.2

68 60 72

64 58 43

LiCF3SO3

50 70 90

-18.9 -13.3 -10.5

67 47 41

51 38 32

LiSCN

50 70 90

-13.0 -I0.0 -9.1

-23 -23 -II

35 29 24

b G -7.6

0

0.07

0.14 0.21 P(GPe)

0.28

0.35

FIGURE 5 Conductivity vs. pressure for: (a) PPO8LiSCN; (b) PPO8LiCF3S03; (c) PPO8LiCIO4,

addition, high pressure e l e c t r i c a l r e l a x a t i o n measurements have been performed in the glass t r a n s i t i o n region on uncomplexed PPO. The vacuum c o n d u c t i v i t y measurements show that To is 30-40°C lower than the DSC Tg. Next. both To and Tg increase several K/kbar. Also. the isobaric data y i e l d a reasonable value f o r the a c t i v a t i o n volume which is consistent with values previously reported for PEO complexed with a l k a l i metal ions. The isothermal data y i e l d a negative a c t i v a t i o n volume f o r PPO8-LiSCN. F i n a l l y . i t is found that neither Tg nor the Arrhenius a c t i v a t i o n volume scales d i r e c t l y with ion size. ACKNOWLEDMENTS The authors would l i k e to thank John Schultz for his technical assistance and Hercules, Inc. f o r supplying the PAREL elastomer.

T. Kobayashi and Z. Ohtaki. Polym. J. 16 (1984) 711. 3.

M. Watanabe, K. Nagaoka, M. Kanba and I. Shinhara, Polym. J. 14 (1982) 877.

4m

M. Watanabe, J. Ikeda and I. Shinohara, Polym. J. 15 (1983) 65.

5.

J.J. Fontanella, M.C. W i n t e r s g i l l , J.P. Calame, F.P. Pursel, D.R. Figueroa and C.G. Andeen, Sol. St. lonics 9&lO (1983) 1139.

6.

H. Vogel, Physik Z. 22 (1921) 645; V.G. Tammann and W. Hesse. Z. Anorg. A l l g . Chem. 156 (1926) 245; G.S. Fulcher. J. Am. Ceram. Soc. 8 (1925) 339.

7.

B.L. Papke, M.A. Ratner and D.F. Shriver, J. Electrochem. Soc. 129 (1982) 1694.

8.

C.A. Angell. Sol. St. lonics 9&lO (1983) 3.

9.

C.A. Angell and J.C. Tucker, J. Phys. Chem. 78 (1974) 278.

I0. G. Adam and J.H. Gibbs, J. Chem. Phys. 43 (1965) 139. REFERENCES I.

2.

M.B. Armand. J.M. Chabagno and M.J. Duclot, in "Fast lon Transport in Solids", eds. P. Vashishta. J. N. Mundy and G. K. Shenoy. (North Holland, New York, 1979) pp 131. M. Watanabe. K. Sanui. N. Ogata, F. Inque,

II.

J.H. Gibbs and E.A. DiMarzio, J. Chem. Phys. 28 (1958) 373.

12. J. Moacanin and E.F. Cuddihy, J. Polym. Sci. C14 (1966) 313. 13. M.L. Williams. R.F. Landel, and J.D. Ferry, J. Am. Chem. Soc. 77 (1955) 3701.6