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Structure and ionic conductivity of graft polymer network electrolytes containing some star-like side chains and LiClO4
SOLID STATE
Solid State lonics 72 ( 1994 ) 172-176 North-Holland
IONICS Structure and ionic conductivity of graft polymer network electrolytes containing some star-like side chains and LiC104 D a n Li, C h u n P u H u *, S h e n g K a n g Y i n g Institute of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
Poly(ethylene oxide) (PEO) 800 macromers (PEO800-MA) with different number of average functionalities (f) have been synthesizedto copolymerizewith unsaturated polyester having PEO 600 segmentsin macromolecularchains and LiCIO4to form graft polymer networksincludingboth sample PEO grafts and star-likePEO side chains. The copolymerizationbetween these two "monomers" went quite well. All these network systemswere completelyamorphous, and the ionic conductivities of the network were measured over a range of temperatures and found to increase with decreasing the fof PEO800-MA for same [EO unit]/ [Li + ] ( = 50 ). For the lowerf 0r= 1.2 ) system,a= 3.8 × 10- s S cm- ~at ambient temperature and for the higherf (f= 2.0 ) system, a= 1.4× 10-s S cm -~. The high ambient conductivities for these systems were attributed to more PEO grafts existed in the network. Another graft polymer network was also prepared with similar polyester, as mentioned above, but having PEO800 segments in the polyesterchains and [EO unit ] / [Li+ ] ratio of 30. It gave even higher conductivity of 4.4 × 10- s S cm- ~at 298 K.
I. Introduction For enhancing the ionic conductivity at a m b i e n t temperature and mechanical properties as well as the dimensional stability, network polymer electrolytes have been studied and paid more attention, such as polyurethane ( P U ) networks [1,2] and polyester networks [ 3,4 ], to enable balance of these physical and mechanical properties. In the previous papers [ 5,6], a new kind of polymer electrolyte called "graft polyester network" was synthesized by using macromer technique and it was found the a m b i e n t conductivity was 3 . 3 × 10 -5 S cm - t , as there were some poly(ethylene oxide) (PEO) grafts in the network. In this article, another graft polymer network, which has not only PEO grafts but some star-like PEO side chains as well, prepared by copolymerizing PEO macromer with unsaturated polyester from PEO600 or PEO800-maleic anhydride is reported. The complexes between these polyester-macromer and LiCLO4 were formed before copolymerizing. All the graft network electrolytes were flexible and had good mechanical properties. The conductivities at a variety of temperatures were * To whom all correspondenceshould be addressed.
measured and correlated with the structure and morphology of the graft networks.
2. Experimental All monomers, salts and solvents etc. were standard laboratory chemicals obtained from various suppliers. An unsaturated polyester was prepared by reacting PEO600 or PEO800 with maleic anhydride (the molar ratio of PEO600 or PEO800 and maleic anhydride was 2). The molecular weights of these two polyesters measured using a Kanuer-11.00 vapour pressure osmometer ( V P O ) were 1040 and 1600 respectively. PEO800 macromers (PEO800MA) were synthesized by reacting PEO800 and methacryloyl chloride with triethylene amine as catalyst in toluene. The procedures of synthesis for these polyesters and macromers have been described elsewhere [4,6 ]. The n u m b e r of average functionalities of these macromers were characterized by VPO and tH nuclear magnetic resonance (Spectrospin WP100SY) and found to be in the range of 1.2 to 2.0. To prepare the network films, polyester was mixed with macromer (the weight ratio of macromer and polyester was 1.5), 5% solution of LiCLO4 in ace-
0167-2738/94/$ 07.00 © 1994 ElsevierScienceB.V. All rights reserved.
D. Li et al. / Graftpolymer network electrolytes tonitrile and redox initiator (methyl ethyl ketone peroxide and cobalt naphthenate). The films were formed by casting the mixture in a silicon rubber mould and copolymerizing them at 353 K for 6 h. The sample films ( ~ 1 m m thick) were then dried under vacuum for 24 h at 333 K and were finally cut into shape for characterization and conductivity measurement. Differential scanning calorimetry (DSC) was carried out using a D u P o n t Instrument 1090 thermal analyser at a heating rate of 20 K min -1. Thermal behaviour of the specimens was also observed using a Leitz hot-stage polarizing microscope at a heating rate o f 2 K m i n - ~. Conductivity measurements were carried out by using two polished stainless steel electrodes for graft polymer network specimens and made by means of a Model YY2812 L C R Meter at 1 KHz. All measurements were conducted under vacuum in the presence o f phosphorous pentoxide. The insoluble parts of samples were measured by extracting them in acetone for 24 h.
173
Table 1 Some physical parameters for conducting graft polymer networks prepared with PEO800 macromer having different functionalities. f
[ EO unit ] / [Li + ]
Insoluble fraction
0298K (S cm- ~)
1.2 1.5 2.0
50 50 50
0.28 0.50 0.68
3.8X 10-5 3.1XI0 -5 1.4X 10-5
3. Results and discussion -80-60-4,0-20
3.1. Structure and morphology Since the molar ratio of PEO600 or PEO800 and maleic anhydride is 2 in the feed for preparing polyester, and the molecular weight o f polyester is approximately twice as large as that of PEO600 or PEO800, it is reasonable to indicate that each macromolecule of polyester only has one double bond existing in the middle of polyester chain. As the number of average functionalities (f) o f all PEO800 macromers prepared here are more than one, the graft polymer networks should be formed by copolymerizing such polyesters with PEO macromers [ 5 ]. Thus, both simple PEO side chains and some star-like PEO grafts in these network systems should be expected. The later case must result from the PEO600 or PEO800 segments in the polyester chains. Table 1 shows the PEO800-MA could copolymerize with the unsaturated polyester quite well, as the insoluble fraction of the cured products (0.50 ~ 0.68) are very close to that o f similar unsaturated polyester systems crosslinked by using styrene (around 0.6)
0
20 4.0 60 80
TEMPERATURE ( *C )
Fig. 1. Profiles of DSC curves of graft polymer networks prepared with PEO800-MA (f= 1.5 ) and PEO600 polyester containing various contents of LiCIO4. [EO unit]/[Li ÷ ]: (a) ~, (b) 100, (c) 50, (d) 30, (e) 20, (f) 10. [4], which is a very commonly used co-monomer for industrial unsaturated polyesters. However, the value of the insoluble fraction for PEO800-MA ( f = 1.2) system is low (around 0.3). The low crosslinkage for this network may be attributed to the low molar ratio o f double bonds of macromer and polyester in the feed. This suggests that there are more graft copolymers having both simple PEO grafts and star-like PEO side chains in such a network. Fig. 1 shows the profile of DSC curves for the PEO800-MA 0 % 1.5)/PEO600 polyester networks containing various concentrations of LiCLO4. All graft networks display glass transition temperatures (T~), but the salt free and less salt ( [ E O u n i t ] / [ L i + ] = 1 0 0 ) networks also show a broad exothermic peak and endothermic peak from 242 K to
D. Li et al. / Graft polymer network electrolytes
174
295 K on the DSC plot. The DSC scan results are presented in table 2. However, the observations of polarizing microscope reveal that these network systems are completely amorphous. Thus, the broad exothermic and endothermic events for these two networks could be attributed to the rearrangement of PEO grafts with exotherm (higher than Tg) and endotherm in the amorphous phase during the heating process so as to be in a mesomeric state for these PEO side chains in the networks, as the PEO grafts and star-like grafts in the network can be easily entangled in each other. With increasing the content of LiCLO4 in the network, more complexes between the PEO grafts and LiCLO4 must be formed and the rearrangement ability for those entangled grafts should be suppressed. The DSC plot clearly shows that these exothermic and endothermic peaks do disappear for more LiCLO4 systems, which exhibit the typical amorphous DSC scan curves. Another evidence for this explanation is that the values of Tg increase with increasing the content of salt for these network systems (see table 2).
3.2. Conductivity Fig. 2 shows that the conductivity of graft polymer network synthesized by using PEO600 polyester and P E O 8 0 0 - M A ( f = l . 5 ) but containing various concentrations of LiCLO4 goes through a maximum with lithium salt content. The conductivity is highest for [EO unit] / [Li] = 50. This observation is broadly consistent with the observations of other workers for similar graft polyester networks having PEO400 or PEO800 side chains [ 5,6] and PU/vinyl ester resin interpenetrating polymer network systems [ 7 ] as well as some graft copolymers having PEO grafts [8]. Thus, the conductivities in these networks may de-
pend on both the degree of aggregation of the salts [ 8 ] and the built-up of charge carriers [ 10 ]. Table I indicates that the conductivity increases with decreasing the f of PEO800 macromer in these graft network systems. The enhancement of ionic conductivity must result from the increase of PEO grafts in the networks, which could connect the ionic conducting pathways, and this means that these PEO side chains could play an important part in mobility of charge carriers in the ionic conducting pathways, as reported in the previous papers [ 5,6 ]. The high conductivity for f = 1.2 system could be therefore attributed to the lowering of crosslinking density as well as the increase of PEO grafts existed both in the network and in the graft copolymers, as mentioned before. This experimental behaviour is consistent with the DSC scan results. As there are more PEO grafts and star-like PEO chains in such a network than in other systems, the entanglement between these grafts must be so strong that it exhibits the highest conductivity at 298 K for f = 1.2 and [EO u n i t ] / [ L i + ] ratio of 50 ( 6 = 3.8 X 10 -5 S c m - i ). Furthermore, a similar conducting graft polymer network also containing some star-like PEO side chains has been synthesized with PEO800-MA (f= 1.5) and another unsaturated polyester having same structure as that of PEO600 polyester, but containing PEO800 segments in macromolecular chains and LiCLO4 ( [EO unit]/ [ L i + ] = 3 0 ) . The conductivity for such a graft network has given rise to the quite high level, 6 = 4 . 4 × 10 _5 S cm -1 at 298 K. This system reveals even higher conductivity at 298 K than the graft network consisting of similar structure but only containing simple PEO800 grafts (CYegsK=3.3)< 10 -5 S c m - l ) [ 6 ]. This clearly shows that the PEO800 starlike chains in the network are more important for the mobility of charge carriers in the ionic conducting
Table 2 Some physical parameters for PEO800-MA ( f = 1.5 )/polyester networks containing various concentrations of LiCIO4. [ EO unit ] / [ Li + ]
Insoluble fraction
Tg
Ea
(K)
(K J m o l - t )
100 50 30 20 10
0.46 0.50 0.52 0.48 0.44
225 229 232 235 242
7.0 7.7 7.8 8.5 9.5
C1 7.2 7.8 7.8 8.7 9.9
C2 (K )
~r298K (S c m - ~)
49.4 48.2 48.5 48.4 49.1
2.7>( 3.1 × 2.3)< 1.5× 5.4×
10 -5 10 -5 10 5 10 5 10 -8
175
D. Li et al. / Graft polymer network electrolytes
-3
~_-2
\ -5 2.5
2.7
2.9
3.1
I O 0 0 / T ( K~
3.3
6
3.5
)
Fig. 2. Log tr versus 1/ T for graft polymer networks prepared with PEO 800-MA 0c= 1.5) and PEO600 polyester containing various contents of LiCIO4. [EO unit]/[Li÷]: (O) 100, (A) 50, (r-l) 30, ( 0 ) 20, (A) 10.
pathways than those simple PEO grafts, although the length of the grafts is the same. The same interpretation of strong entanglement between these more longer PEO star-like grafts can be applied to this high conductivity system. The temperature dependence of ionic conductivity for all these PEO800-MA 0c= 1.5)/PEO600 polyester systems gives rise to some curves, as shown in fig. 2. For an amorphous polymer electrolyte, a Vogel-Tamman-Fulcher (VTF) equation usually provides a better fit to the experimental data: or=AT °5 exp[ - E a / ( R ( T - To) ] ,
7 ~000/(T-
( 1)
where To is a temperature which can be associated with the Tg of the electrolyte and Ea is an apparent activation energy. It is clear that these curves can be linearized satisfactorily in accordance with VTF eq. ( 1 ) taking To= T g - 50 K (ref. [ 11 ] ), as shown in fig. 3. The calculated apparent activation energies (Ea) are listed in table 2. The values of Ea for the networks increase with the increase of the content of salt as well as the raising of Tg, but these values do not differ significantly. The same behaviour was also
8
9
~0
To )
Fig. 3. Log (aT i/2) versus 1/ ( T - To) for graft polymer networks [EO unit]/[Li+]: (O) 100, (A) 50, ([3) 30, (O) 20, (A) 10.
observed in a similar polyester network containing LiCLO4 [4]. Although the interpretation of Ea is uncertain, Hu and Wright [4] have suggested that it perhaps represent the energy required to create adequate space for mobility in conducting pathway. Thus, higher content of salt, higher Tg and higher value of Ea should be obtained for those amorphous graft networks. According to free-volume considerations, the Williams, Landel and Ferry (WLF) equation has also been applied to the conductivity of amorphous polymer electrolytes: Iog(77./log(TT~=Cj(T-Tg)/[C2+(T-Tg)]
.
(2)
As tTT~values of present specimens were too hard to be measured at lower temperature, the data sets of C1, C2 were calculated using the non-linear least squares method and are given in table 2. The values of C2 are close to the universal value of 51.6, but the C] values are less than the universal value of 17.4 [ 12 ]. Variations in C~ have also been observed by other workers in PEO-PU networks [ 2,13 ] and PEOpolyester networks [4]. The reasons for these deviations might be complex [ 14 ], and will be studied further.
176
D. Li et al. / Graft polymer network electrolytes
The Project was supported by National Natural Science Foundation of China.
References [ 1 ] J.F. LeNest, A. Gardini and H. Cheradame, Br. Polymer J. 20 (1988) 253. [2]M. Watanabe, M. Itoh, K. Sanui and N. Ogata, Macromolecules 20 (1987) 569. [3] P.V. Wright, Polymer Preprints 30 (1989) 402. [4] C.P. Hu and P.V. Wright, Br. Polymer J. 21 (1989) 421. [5]B. Fang, C.P. Hu, H.B. Xu and S.K. Ying, Polymer Commun. 32 (1991) 382. [ 6 ] B. Fang, C.P. Hu and S.K. Ying, Eur. Polymer J. 29 ( 1993 ) 799.
[ 7 ] J. Shi, C.P. Hu, R. Chen and S.K. Ying, in: Recent Advances in Fast Ion Conducting Materials and Devices, eds. B.V.R. Chowdari, Q. Liu and L. Chen (World Scientific, Singapore, 1990) pp. 267-271. [8 ] J.M.G. Cowie, A.C.S. Martin and A-M. Firth, Br. Polymer J. 20 (1988) 247. [9]J.R. MacCallum, A.S. Tomlin and C.A. Vincent, Eur. Polymer J. 22 (1986) 787. [ 10 ] C.A. Angell, Solid State lonics 9/10 ( 1983 ) 3. [ 11 ] M.B. Armand, J.M. Chabagno and M.J. Duclot, in: Fast Ion Transport in Solids, eds. P. Vashishta, J.N. Mundy and G.K. Shenoy (North-Holland, New York, 1979 ) pp. 131 - 136. [ 12] M.L. Williams, R.F. Landel and J.D. Ferry, Am. Chem. Soc. 77 (1955) 3701. [13] J.F. LeNest, A. Gandini, H. Cheradame and J.P. CohnAddad, Polymer Commun. 28 ( 1987 ) 302. [ 14 ] K. Xu, Q. Xu and G. Wan, ISSI Lett. 3 ( 1992 ) 3.
Solid State lonics 72 ( 1994 ) 172-176 North-Holland
IONICS Structure and ionic conductivity of graft polymer network electrolytes containing some star-like side chains and LiC104 D a n Li, C h u n P u H u *, S h e n g K a n g Y i n g Institute of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China
Poly(ethylene oxide) (PEO) 800 macromers (PEO800-MA) with different number of average functionalities (f) have been synthesizedto copolymerizewith unsaturated polyester having PEO 600 segmentsin macromolecularchains and LiCIO4to form graft polymer networksincludingboth sample PEO grafts and star-likePEO side chains. The copolymerizationbetween these two "monomers" went quite well. All these network systemswere completelyamorphous, and the ionic conductivities of the network were measured over a range of temperatures and found to increase with decreasing the fof PEO800-MA for same [EO unit]/ [Li + ] ( = 50 ). For the lowerf 0r= 1.2 ) system,a= 3.8 × 10- s S cm- ~at ambient temperature and for the higherf (f= 2.0 ) system, a= 1.4× 10-s S cm -~. The high ambient conductivities for these systems were attributed to more PEO grafts existed in the network. Another graft polymer network was also prepared with similar polyester, as mentioned above, but having PEO800 segments in the polyesterchains and [EO unit ] / [Li+ ] ratio of 30. It gave even higher conductivity of 4.4 × 10- s S cm- ~at 298 K.
I. Introduction For enhancing the ionic conductivity at a m b i e n t temperature and mechanical properties as well as the dimensional stability, network polymer electrolytes have been studied and paid more attention, such as polyurethane ( P U ) networks [1,2] and polyester networks [ 3,4 ], to enable balance of these physical and mechanical properties. In the previous papers [ 5,6], a new kind of polymer electrolyte called "graft polyester network" was synthesized by using macromer technique and it was found the a m b i e n t conductivity was 3 . 3 × 10 -5 S cm - t , as there were some poly(ethylene oxide) (PEO) grafts in the network. In this article, another graft polymer network, which has not only PEO grafts but some star-like PEO side chains as well, prepared by copolymerizing PEO macromer with unsaturated polyester from PEO600 or PEO800-maleic anhydride is reported. The complexes between these polyester-macromer and LiCLO4 were formed before copolymerizing. All the graft network electrolytes were flexible and had good mechanical properties. The conductivities at a variety of temperatures were * To whom all correspondenceshould be addressed.
measured and correlated with the structure and morphology of the graft networks.
2. Experimental All monomers, salts and solvents etc. were standard laboratory chemicals obtained from various suppliers. An unsaturated polyester was prepared by reacting PEO600 or PEO800 with maleic anhydride (the molar ratio of PEO600 or PEO800 and maleic anhydride was 2). The molecular weights of these two polyesters measured using a Kanuer-11.00 vapour pressure osmometer ( V P O ) were 1040 and 1600 respectively. PEO800 macromers (PEO800MA) were synthesized by reacting PEO800 and methacryloyl chloride with triethylene amine as catalyst in toluene. The procedures of synthesis for these polyesters and macromers have been described elsewhere [4,6 ]. The n u m b e r of average functionalities of these macromers were characterized by VPO and tH nuclear magnetic resonance (Spectrospin WP100SY) and found to be in the range of 1.2 to 2.0. To prepare the network films, polyester was mixed with macromer (the weight ratio of macromer and polyester was 1.5), 5% solution of LiCLO4 in ace-
0167-2738/94/$ 07.00 © 1994 ElsevierScienceB.V. All rights reserved.
D. Li et al. / Graftpolymer network electrolytes tonitrile and redox initiator (methyl ethyl ketone peroxide and cobalt naphthenate). The films were formed by casting the mixture in a silicon rubber mould and copolymerizing them at 353 K for 6 h. The sample films ( ~ 1 m m thick) were then dried under vacuum for 24 h at 333 K and were finally cut into shape for characterization and conductivity measurement. Differential scanning calorimetry (DSC) was carried out using a D u P o n t Instrument 1090 thermal analyser at a heating rate of 20 K min -1. Thermal behaviour of the specimens was also observed using a Leitz hot-stage polarizing microscope at a heating rate o f 2 K m i n - ~. Conductivity measurements were carried out by using two polished stainless steel electrodes for graft polymer network specimens and made by means of a Model YY2812 L C R Meter at 1 KHz. All measurements were conducted under vacuum in the presence o f phosphorous pentoxide. The insoluble parts of samples were measured by extracting them in acetone for 24 h.
173
Table 1 Some physical parameters for conducting graft polymer networks prepared with PEO800 macromer having different functionalities. f
[ EO unit ] / [Li + ]
Insoluble fraction
0298K (S cm- ~)
1.2 1.5 2.0
50 50 50
0.28 0.50 0.68
3.8X 10-5 3.1XI0 -5 1.4X 10-5
3. Results and discussion -80-60-4,0-20
3.1. Structure and morphology Since the molar ratio of PEO600 or PEO800 and maleic anhydride is 2 in the feed for preparing polyester, and the molecular weight o f polyester is approximately twice as large as that of PEO600 or PEO800, it is reasonable to indicate that each macromolecule of polyester only has one double bond existing in the middle of polyester chain. As the number of average functionalities (f) o f all PEO800 macromers prepared here are more than one, the graft polymer networks should be formed by copolymerizing such polyesters with PEO macromers [ 5 ]. Thus, both simple PEO side chains and some star-like PEO grafts in these network systems should be expected. The later case must result from the PEO600 or PEO800 segments in the polyester chains. Table 1 shows the PEO800-MA could copolymerize with the unsaturated polyester quite well, as the insoluble fraction of the cured products (0.50 ~ 0.68) are very close to that o f similar unsaturated polyester systems crosslinked by using styrene (around 0.6)
0
20 4.0 60 80
TEMPERATURE ( *C )
Fig. 1. Profiles of DSC curves of graft polymer networks prepared with PEO800-MA (f= 1.5 ) and PEO600 polyester containing various contents of LiCIO4. [EO unit]/[Li ÷ ]: (a) ~, (b) 100, (c) 50, (d) 30, (e) 20, (f) 10. [4], which is a very commonly used co-monomer for industrial unsaturated polyesters. However, the value of the insoluble fraction for PEO800-MA ( f = 1.2) system is low (around 0.3). The low crosslinkage for this network may be attributed to the low molar ratio o f double bonds of macromer and polyester in the feed. This suggests that there are more graft copolymers having both simple PEO grafts and star-like PEO side chains in such a network. Fig. 1 shows the profile of DSC curves for the PEO800-MA 0 % 1.5)/PEO600 polyester networks containing various concentrations of LiCLO4. All graft networks display glass transition temperatures (T~), but the salt free and less salt ( [ E O u n i t ] / [ L i + ] = 1 0 0 ) networks also show a broad exothermic peak and endothermic peak from 242 K to
D. Li et al. / Graft polymer network electrolytes
174
295 K on the DSC plot. The DSC scan results are presented in table 2. However, the observations of polarizing microscope reveal that these network systems are completely amorphous. Thus, the broad exothermic and endothermic events for these two networks could be attributed to the rearrangement of PEO grafts with exotherm (higher than Tg) and endotherm in the amorphous phase during the heating process so as to be in a mesomeric state for these PEO side chains in the networks, as the PEO grafts and star-like grafts in the network can be easily entangled in each other. With increasing the content of LiCLO4 in the network, more complexes between the PEO grafts and LiCLO4 must be formed and the rearrangement ability for those entangled grafts should be suppressed. The DSC plot clearly shows that these exothermic and endothermic peaks do disappear for more LiCLO4 systems, which exhibit the typical amorphous DSC scan curves. Another evidence for this explanation is that the values of Tg increase with increasing the content of salt for these network systems (see table 2).
3.2. Conductivity Fig. 2 shows that the conductivity of graft polymer network synthesized by using PEO600 polyester and P E O 8 0 0 - M A ( f = l . 5 ) but containing various concentrations of LiCLO4 goes through a maximum with lithium salt content. The conductivity is highest for [EO unit] / [Li] = 50. This observation is broadly consistent with the observations of other workers for similar graft polyester networks having PEO400 or PEO800 side chains [ 5,6] and PU/vinyl ester resin interpenetrating polymer network systems [ 7 ] as well as some graft copolymers having PEO grafts [8]. Thus, the conductivities in these networks may de-
pend on both the degree of aggregation of the salts [ 8 ] and the built-up of charge carriers [ 10 ]. Table I indicates that the conductivity increases with decreasing the f of PEO800 macromer in these graft network systems. The enhancement of ionic conductivity must result from the increase of PEO grafts in the networks, which could connect the ionic conducting pathways, and this means that these PEO side chains could play an important part in mobility of charge carriers in the ionic conducting pathways, as reported in the previous papers [ 5,6 ]. The high conductivity for f = 1.2 system could be therefore attributed to the lowering of crosslinking density as well as the increase of PEO grafts existed both in the network and in the graft copolymers, as mentioned before. This experimental behaviour is consistent with the DSC scan results. As there are more PEO grafts and star-like PEO chains in such a network than in other systems, the entanglement between these grafts must be so strong that it exhibits the highest conductivity at 298 K for f = 1.2 and [EO u n i t ] / [ L i + ] ratio of 50 ( 6 = 3.8 X 10 -5 S c m - i ). Furthermore, a similar conducting graft polymer network also containing some star-like PEO side chains has been synthesized with PEO800-MA (f= 1.5) and another unsaturated polyester having same structure as that of PEO600 polyester, but containing PEO800 segments in macromolecular chains and LiCLO4 ( [EO unit]/ [ L i + ] = 3 0 ) . The conductivity for such a graft network has given rise to the quite high level, 6 = 4 . 4 × 10 _5 S cm -1 at 298 K. This system reveals even higher conductivity at 298 K than the graft network consisting of similar structure but only containing simple PEO800 grafts (CYegsK=3.3)< 10 -5 S c m - l ) [ 6 ]. This clearly shows that the PEO800 starlike chains in the network are more important for the mobility of charge carriers in the ionic conducting
Table 2 Some physical parameters for PEO800-MA ( f = 1.5 )/polyester networks containing various concentrations of LiCIO4. [ EO unit ] / [ Li + ]
Insoluble fraction
Tg
Ea
(K)
(K J m o l - t )
100 50 30 20 10
0.46 0.50 0.52 0.48 0.44
225 229 232 235 242
7.0 7.7 7.8 8.5 9.5
C1 7.2 7.8 7.8 8.7 9.9
C2 (K )
~r298K (S c m - ~)
49.4 48.2 48.5 48.4 49.1
2.7>( 3.1 × 2.3)< 1.5× 5.4×
10 -5 10 -5 10 5 10 5 10 -8
175
D. Li et al. / Graft polymer network electrolytes
-3
~_-2
\ -5 2.5
2.7
2.9
3.1
I O 0 0 / T ( K~
3.3
6
3.5
)
Fig. 2. Log tr versus 1/ T for graft polymer networks prepared with PEO 800-MA 0c= 1.5) and PEO600 polyester containing various contents of LiCIO4. [EO unit]/[Li÷]: (O) 100, (A) 50, (r-l) 30, ( 0 ) 20, (A) 10.
pathways than those simple PEO grafts, although the length of the grafts is the same. The same interpretation of strong entanglement between these more longer PEO star-like grafts can be applied to this high conductivity system. The temperature dependence of ionic conductivity for all these PEO800-MA 0c= 1.5)/PEO600 polyester systems gives rise to some curves, as shown in fig. 2. For an amorphous polymer electrolyte, a Vogel-Tamman-Fulcher (VTF) equation usually provides a better fit to the experimental data: or=AT °5 exp[ - E a / ( R ( T - To) ] ,
7 ~000/(T-
( 1)
where To is a temperature which can be associated with the Tg of the electrolyte and Ea is an apparent activation energy. It is clear that these curves can be linearized satisfactorily in accordance with VTF eq. ( 1 ) taking To= T g - 50 K (ref. [ 11 ] ), as shown in fig. 3. The calculated apparent activation energies (Ea) are listed in table 2. The values of Ea for the networks increase with the increase of the content of salt as well as the raising of Tg, but these values do not differ significantly. The same behaviour was also
8
9
~0
To )
Fig. 3. Log (aT i/2) versus 1/ ( T - To) for graft polymer networks [EO unit]/[Li+]: (O) 100, (A) 50, ([3) 30, (O) 20, (A) 10.
observed in a similar polyester network containing LiCLO4 [4]. Although the interpretation of Ea is uncertain, Hu and Wright [4] have suggested that it perhaps represent the energy required to create adequate space for mobility in conducting pathway. Thus, higher content of salt, higher Tg and higher value of Ea should be obtained for those amorphous graft networks. According to free-volume considerations, the Williams, Landel and Ferry (WLF) equation has also been applied to the conductivity of amorphous polymer electrolytes: Iog(77./log(TT~=Cj(T-Tg)/[C2+(T-Tg)]
.
(2)
As tTT~values of present specimens were too hard to be measured at lower temperature, the data sets of C1, C2 were calculated using the non-linear least squares method and are given in table 2. The values of C2 are close to the universal value of 51.6, but the C] values are less than the universal value of 17.4 [ 12 ]. Variations in C~ have also been observed by other workers in PEO-PU networks [ 2,13 ] and PEOpolyester networks [4]. The reasons for these deviations might be complex [ 14 ], and will be studied further.
176
D. Li et al. / Graft polymer network electrolytes
The Project was supported by National Natural Science Foundation of China.
References [ 1 ] J.F. LeNest, A. Gardini and H. Cheradame, Br. Polymer J. 20 (1988) 253. [2]M. Watanabe, M. Itoh, K. Sanui and N. Ogata, Macromolecules 20 (1987) 569. [3] P.V. Wright, Polymer Preprints 30 (1989) 402. [4] C.P. Hu and P.V. Wright, Br. Polymer J. 21 (1989) 421. [5]B. Fang, C.P. Hu, H.B. Xu and S.K. Ying, Polymer Commun. 32 (1991) 382. [ 6 ] B. Fang, C.P. Hu and S.K. Ying, Eur. Polymer J. 29 ( 1993 ) 799.
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