LiCF3SO3 polymer electrolyte blends including morphology

SOLID STATE

Solid State lonlcs 60 (1993) 113-117 North-Holland

IONICS

The physical characteristics of PPG/PMMA/LiCF3SO3 polymer electrolyte blends including morphology T. Mani, R. Mare and J.R. Stevens Guelph- Waterloo Programfor Graduate Work m Chemtstry, Umverstty of Guelph, Guelph, Ontarto N1G 2W1, Canada

Adhesive and transparent polymer blend electrolytes were synthesized by the free radical polymerization of methyl methacrylate (MMA) in poly(propylene glycol) (PPG) complexed with lithium trlflate (LiCF3SO3) Polymer electrolytes with differing PPG molecular weights, PMMA composmon, and O/Li ratios were prepared The physical charactensUcs of the polymer blend electrolytes were found to depend on the PPG molecular weight, salt concentration and PMMA content SEM studies indicated the polymer blends to have a phase separated morphology The salt, LICF3SOa, was found to act as an emulsifier in stabilizing the two phase microstructure of the blends

I. Introduction

2. Experimental

In recent years a number of articles have been published describing the use of polymer electrolytes in various applications VlZ., high energy density batteries [ 1-4 ], solid state electrochromlc displays [ 5 ], photoelectrochemical cells [6 ], and smart wmdows [7,8] Extensive structural modifications have been carried out on the host polymer (generally a polyether) in order to achieve better conductlVltles [911 ] However reports on the blending of other polymers with polyethers to achieve optimum properties are scarce. We have recently reported [12,13 ] the blending of PMMA with PPG complexed with L1CF3SO3 by a novel method developed in our laboratory i e, free radical polymerization of MMA in the polyether/salt matrix Excellent properties like improvement in the stability of polymer electrolytes to atmospheric moisture, optical clarity and adhesiveness have been achieved without detrimentally effecting the conductivity. They were found to be suitable materials for "smart windows" The physical characteristics of these materials have been found to depend on the molecular weight of PPG, salt concentration and the weight percent of PMMA Incorporated [12,13] This paper rationalizes these changes and also describes the morphological characterist~cs of these blends

PPG (molecular weights 2000 and 425) obtained from Dow Chemical were dried under reduced pressure (10 -4) Torr) by repeated freeze-thaw cycles The degassed PPG was stored and handled in a dry box under dry argon atmosphere MMA (Fisher Scientific) was dried and distilled over calcium hydride under reduced pressure. LICF3SO 3 (100%, 3M, USA) was vacuum dried for 24 h at 120°C prior to use AIBN was recrystalhzed from methanol The synthesis of polymer blend electrolytes has been described in detail earlier [ 12,13 ] To determine the molecular weight and molecular weight distribution of the PMMA the polymer electrolyte was repeatedly extracted with hot methanol PPG and the salt dissolved in hot methanol while the PMMA separated. It was then filtered, washed with hot methanol and dried in vacuum prior to characterizatlon Gel permeation chromatography measurements were performed at 25 °C using THF as the eluent (flow 1 m£/mln), on a Waters 590 GPC equipped with a model R-401 DRI detector The detector was connected to an IBM compatible microcomputer for data acquisition The system was operated with 4 PL-gel columns with 100000, 1000, 500 and 100 A pore sizes Both polystyrene (Pressure Chemical Co and Polymer Laboratories) and PMMA (Polymer Laboratories) standards were used

Elsevier Science Pubhshers B V

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T M a n t el al / P P G / P M M A / L I ( b 3SO ~p o h met ele¢ troh te I~lend~

m cahbratlng the system A umversal cahbration was used The water content of the polymer electrolyte samples determined by a 684 Karl Fischer Coulometer (Metrohm L t d , Switzerland) was found to be less than 1% Thermogravlmetrlc analysis on degassed P P G using a Perkxn-Elmer TGAA-7 showed a change m weight of 0 06% after raising the temperature from 30°C to 150°C at 2 0 ° C / m l n SEM studies were carried out on an Hitachi S-570 T h i n films of the samples prepared by a solvent evaporation technique were m o u n t e d onto 1 cm diameter a l u m i n u m stubs, the P P G was stained with O s m i u m tetroxlde vapour for 8-12 h so that the distribution of the P M M A could be seen Some of the samples were sputter-gold coated (20 n m ) All specimens were examined at a working voltage of 10 KV Ionic eonductlVltles were determined using a computer controlled HP 4192A complex impedance analyzer over a frequency range of 5 Hz to 13 Hz The dimensions of the samples sandwiched between two cylindrical stainless steel electrodes were 7 8 m m diameter and 1 6 m m thickness, they were placed in a temperature controlled furnace The glass transition temperature (Tg) of the samples was determined usm g a D u P o n t 2910 DSC instrument, under a nitrogen atmosphere

3. Results and discussion A comparison of the optical clarity of P M M A / PPG-L1CF3SO3 blends are shown in table 1 All samples irrespective of the P P G molecular weight visibly phase aggregated in the absence of salt While all the samples based on PPG-425 were optically clear in the various compositions studied, the PPG-2000 based samples were clear only at higher O/L1 ratios (8 and 16) (fig l b and c) The actual a m o u n t of L1CF3SO 3 required to form an optically clear blend was found to be ~ 15% by weight on PPG-2000 (corresponding to O / L I = 18) At lower salt concentrations the blends were found to be visibly phase aggregated or cloudy Fig l a with O / L 1 = 2 5 in P P G 2000 with P M M A (4%) is cloudy The P M M A molecular weights of the PPG-425 based samples were lower than those of the corresponding PPG-2000 based polymer electrolytes (table 2) The higher VlS-

Table 1 Composmons and opucal clarat~ ot PMMA/PPG/LI(~F~SO3 polymer electrolyte samples Sample number

PPG

O/M raUo -

PMM~ (%)

1

2000

4

2 3

2000 2000

25 1 25 1

4 8

4 5 6 7 8

2000 2000 2000 2000 425

16 16 8 8 -

4 8 4 8 4

9

425

-

8

10 11 12 13 14 15 16 17 18 19 20

425 425 425 425 425 425 425 425 425 425 425

25 25 25 25 25 16 16 16 8 8 8

4 8 12 16 20 4 8 12 4 8 12

l 1 I 1

Appearance phase aggregated cloudy phase aggregated clear clear clear clear phase aggregated phase aggregated clear clear clcar clear clear clear clear clear clear clear clear

cosity of the reaction m e d i u m in PPG-2000 compared to PPG-425 can be expected to hinder the term i n a t i o n rate thereby enhancing propagation resulting in higher molecular weights in these samples The lower n u m b e r average molecular weights of the blend components in the PPG-425 electrolytes thus leads to enhanced miscibility m this system Such t h e r m o d y n a m i c factors as molecular weight in faclhtatmg miscibility of the blend components has been reported in the literature [14] Scanning electron mlcrographs of the clear blends (fig l b and c) indicated a phase separated microstructure The P M M A appears to be well dispersed in the PPG-salt matrix for all the samples in fig 1 Including the cloudy sample (fig l a ) Similar results were observed w~th all the clear blends Although the salt has been found to play an important role in mlsclbihzlng the two immiscible polymers by mteractmg with the ether oxygens of the polyether and ester groups of PMMA [13], 1t was not known earlier whether a t h e r m o d y n a m i c single phase was formed

T Mant etal /PPG/PMMA/LtCFsSOjpolymerelectrolyteblends

115

(

C

,

....

Fig 1 Scannmg electron mlcrographsofPPG-2000blends (a) PMMA (4%), O/L1=25, (b) PMMA (4%), O/L~=16, (c) PMMA (8%), O/L~=8

116

T Mare et al / PPG/PMMA/LICFsS03 polymer electtoh te blends

Table 2 Tg o f the p o l y m e r electrolytes a n d m o l e c u l a r wetghts o f P M M A o b t a i n e d by G P C a n a l y s i s Sample number

Tg (°C)

Mn

Mw/Mn

A a) 2 3 B b)

-56 -57 -58 -- 50

0 2 1 7

X 104 163 200 --

2 l0 3 06

4 5 C ~ 6 7 D d)

-50 -50 -23 -23 -24 -59

2 1 6 3 8 0

113 173 161 194 -

2 40 1 73

l0 11 12 13 14 E ~)

-59 -595 -59 -58 -58 -51

3

44 58 100 100 124 -

1 90 1 73

15

- 52 3

78

16 17 F el

-51 9 -51 5 -32 5

113 121 -

1 63 1 78 1 67 1 72 2 01 1 68 1 60 1 80 -

18 19 20

-32 4 -33 6 -32 7

61 101 120

2 00 1 86 17

3 0 1 9

") A = P P G ( 2 0 0 0 ) / L 1 C F s S O 3 ( 2 5 1), ~) B = P P G ( 2 0 0 0 ) / L I C F s S O 3 ( 1 6 1), c) C = P P G ( 2 0 0 0 ) / L I C F s S O 3 (8 1), d~ D = P P G ( 4 2 5 ) / L l C F 3 S O 3 ( 2 5 1 ), ~) E = P P G ( 4 2 5 ) / L l C F 3 S O 3 ( 1 6 1 ), fl F = P P G ( 4 2 5 ) / D C F 3 S O a (8 1)

or the blends had a mtcrophase separated morphology It is now clear from SEM studies that the salt serves as an emulsifier to the multxphase morphology by l m p r o v m g the dispersion o f P M M A m the P P G matrix and prevents the coalescence o f the phases The effect o f the salt is similar to that of a block copolymer which is known to reside at the interface [ 15 ] between the p o l y m e r phases and reduce mterfaclal tension [ 16 ] The role of the salt as a c o m p a t l b l h z e r to the muln p h a s e morphology can also be clearly seen by comparing the results of P P G - 2 0 0 0 / L I C F 3 S O 3 / P M M A system (table 1) At an O / L I ratio of 25 the blend contamlng 4% P M M A r e m a m s cloudy but mcreasing the P M M A composltton to 8%, O / L t remaining the same, results in coalescence of the two phases

Increasing the P M M A concentration d i d not effect the glass transition temperature o f the P P G / L I CF3SO3 mixtures (table 2) but had a shghtly negative effect on the ionic c o n d u c t l v m e s (table 3) Nevertheless the lomc conductlVltles are still in the range of a p p h c a b l h t y of these materials m " s m a r t windows" The PPG-425 samples had better ionic conductlvltms than the PPG-2000 samples which can be attributed to the lower glass t r a n s m o n temperature o f these samples The incorporation o f P M M A considerably enhanced the stabdlty o f the polymer electrolytes to atmosphertc moxsture and increased the adhesiveness of these materials [ 12,13 ] None of the clear samples of the polymer electrolytes was found to phase aggregate even after storage for two years The blends were also found to be stable m the t e m p e r a t u r e range - 10 to 100°C We have not investigated the blends at t e m p e r a t u r e lower than -10°C The electrolyte has been cycled at room temperature under ambtent condlttons in an electrochemical cell with p l a t m u m reference and counter electrodes The working electrode was a plate o f glass coated wtth tin d o p e d i n d i u m oxide and tungsten oxide (WO3) After 10 5 cycles the conductivity was reduced by 10% and the moisture content Increased to Table 3 I o m c conductlVltles o f the p o l y m e r electrolytes

a) A t 301 K

Sample number

a ( S / c m ) "~

2 3 4 5 6 7 10 11 12 13 14 15 16 17 18 19 20

6 4 X 1 0 -6 4 9 × 1 0 -6 6 0 X 10 _6 5 0 X 10 _6 1 3×10 -6 1 1×10 -6 3 9 × 1 0 -5 3 1×10 -s 2 4 × 1 0 -5 20xlO -s 18xIO -s 3 4 X 10 - 5 2 9 X 1 0 -5 2 3 X 1 0 -5 8 6 × 1 0 -6 8 Ixl0 -6 6 6 X 1 0 -6

T Mant et al / PPG/PMMA/LtCFaSOa polymer electrolyte blends

4%. The electrolyte was stdl clear After further cychng the WO3 deteriorated due to the moisture and/ or hthlum depletion so that the colour change was &mmlshed considerably

4. Conclusions Stable polymer blend electrolytes based on PMMA/PPG/L1CF3SO3 have been synthesxzed with properttes suttable for their apphcatlon m "smart windows" The morphological characteristics of the blends and the influence of polyether molecular weight, salt concentration and PMMA content on physical properties of these materials have been mvest,gated. We have shown previously that the conductlv~ty and the wscoslty of the electrolyte are a function of molecular weight and salt concentraUon [ 17 ]. In this and previous papers [ 12,13 ] we found that the in s~tu polymenzed PMMA is compatible with the PPG in the presence of a sufficient concentration of the salt which acts as an emulsifier m stabilizing the two phase blend m~crostructure Other than the adhesive properties which are enhanced w~th increased concentration of PMMA the PMMA has httle effect on the glass transmon temperature up to 20 wt% PMMA, the conductlwty is effected only at the h~gher temperatures

117

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