- Email: [email protected]
Structural and dynamical characterization of melt PEO–salt mixtures
Physica A 304 (2002) 308 – 313
www.elsevier.com/locate/physa
Structural and dynamical characterization of melt PEO–salt mixtures A. Trioloa; ∗ , F. Lo Celsob , V. Arrighic , P. Strunz a , R.E. Lechner a , M. Mastragostinod , S. Passerinie , B.K. Annisf , R. Triolob a Hahn-Meitner
Institut, BENSC, Glienicker Str. 100, D-14109, Berlin, Germany di Chimica Fisica, Universita’ di Palermo, V.le delle Scienze, Parco d’Orleans II, 90128 Palermo, Italy c Department of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS Scotland, UK d Universit3 a di Bologna, Bologna, Italy e ENEA Electrochemical Energy Conversion Division, Rome, Italy f Chemical and Analytical Sciences Division, ORNL, P.O. Box 2008, Oak Ridge, TN 37831-6197, USA
b Dipartimento
Abstract Salt doped poly ethylene oxide (PEO) mixtures were investigated by means of both small angle neutron scattering and QENS techniques aiming to characterize morphological and dynamical features in the melt state. These experimental evidences provide support to the proposed heterogeneous scenario for polymer electrolytes. In particular, the existence of PEO– cation complexes is proposed to play a major role in intramolecular cooperation and interc 2002 Elsevier Science molecular transient crosslinks, which a9ects the mixture properties. B.V. All rights reserved.
1. Introduction In the last decades a large amount of e9orts has been applied to understanding and development of polymer electrolytes [1,2]. These systems have been shown to constitute a novel class of materials with high potentials for applications in the
Corresponding author. Tel.: +49-30-8062-3072; fax: +49-30-8062-3094. E-mail address: [email protected] (A. Triolo).
c 2002 Elsevier Science B.V. All rights reserved. 0378-4371/02/$ - see front matter PII: S 0 3 7 8 - 4 3 7 1 ( 0 1 ) 0 0 5 3 6 - 2
A. Triolo et al. / Physica A 304 (2002) 308 – 313
309
the salt cation (generally Li+ or Na+ ) and the polar portion of the polymer chain [3]. In particular, poly ethylene oxide (PEO) is an almost ideal system for these kind of applications due to the possibility that transient complexes might form between its ether–oxygen atoms and the cation [1,2]. The existence of such complexes has been detected both experimentally (see e.g. Ref. [4]) and theoretically, by means of molecular dynamics simulations [3,5,6], and is supposed to be responsible for the formation of transient crosslinks between neighbor chains. It has also been observed that most of the electric performances of these mixtures are related to a continuous solvation–desolvation process involving formation and breakage of the complex, as a consequence of the polymer chain segmental motion [4]. The latter process is then assumed to play a major role in determining the relevant performances of polymer electrolytes and accordingly has been the subject of a variety of investigations. In the present communication, we report a characterization of both structural and dynamical properties of PEO–salt mixtures. In particular, we report the
2. Experimental SANS measurements on PEO–LiI were run at the SANS instrument at the BENSC reactor [7]. In order to characterize labile clusters induced by the existence of the complexes, we decided to compare the scattering pattern from fully deuterated PEO (D-PEO) and two mixtures D-PEO20 –LiI and D-PEO10 –LiI. D-PEO had a molecular weight of 26,700, with a polydispersity of 1.04 and a degree of deuteration higher than 99%. LiI was an Aldrich product with purity higher than 99%. The materials were chosen to minimize the incoherent signal arising from H atoms, so to detect the presence of D-PEO–Li clusters embedded in a matrix of pure D-PEO. Previously, similar measurements were run on a conventional SANS instrument, which did not allow covering the Q-range corresponding to the expected size of the clusters [8]. Moreover, SANS measurements were collected on mixtures of hydrogenated and deuterated PEO (H=D = 70=30) both with and without salt, in order to probe the scattering from the single chain in the random coil con
310
A. Triolo et al. / Physica A 304 (2002) 308 – 313
3. Results and discussion 3.1. Structural characterization In Fig. 1, a comparison between the SANS patterns from D-PEO and D-PEO doped with LiI is reported. Apart from di9erences in the constant incoherent term depending on the di9erent sample compositions, there are no major di9erences in the large Q range. In this Q range, in the case of PEO–salt mixtures, we would expect to
D-PEO D-PEO 15 /LiI
d∑(Q)/dΩ (cm- 1 sr- 1)
100
10
1
0.1
0.1
1 -1
Q (nm )
Fig. 1. SANS patterns from pure fully deuterated PEO (D-PEO) and LiI-doped D-PEO (D-PEO15 =LiI) at ◦ 75 C. By using fully deuterated PEO, we aimed minimizing the incoherent scattering to detect any evidence of clusters due to the wrapping of PEO chains around the Li+ cation. No major di9erences can be detected M in the large Q range of the patterns, thus indicating that the clusters, if any, have sizes smaller than 20 A.
A. Triolo et al. / Physica A 304 (2002) 308 – 313
311
d∑(Q)/dΩ (cm- 1sr- 1)
1000
D-PEO + H-PEO (30/ 70) D/H(30/70)-PEO15 /LiI 100
10
0.1
1 -1
Q (nm )
Fig. 2. Comparison between SANS patterns from a pure mixture of D-PEO and H-PEO (ratio H-PEO=D-PEO = 70=30) and LiI doped 70=30 H=D-PEO (with composition PEO15 –LiI). The lines correspond to a
1000/T (1/K) 2.0
2.5
3.0
3.5
4.0
4.5 3 2 1 0 -1
0.1
log τ (ns)
QENS This work DS NMR
-2
S(ω )
-3 -4
0.01
1E-3 -2
Data Fit Elast i c KWWPEO Lorentzian 0
2
4
6
8
10
Energy (meV)
◦
−1
M . The model used for Fig. 3. Representative
performances of polymer electrolytes, it is of relevance to apply this technique aiming to detect the microscopic details of the relationship between segmental dynamics and complexes. In Fig. 3, the QENS pattern from pure PEO is reported, together with
312
A. Triolo et al. / Physica A 304 (2002) 308 – 313
S(Q=2.16 Å-1,ω)
1
PEO15-LiBETI Pure PEO
0.1
0.01 0
2
4
6
8
10
Energy (meV) ◦
−1
M . The Fig. 4. Comparison between QENS data from pure PEO and PEO15 –LiBETI at 80 C and Q = 2:16 A two patterns di9er only for the elastic portion of the spectra, this indicating di9erent populations of chains relaxing according to a slow motion not resolved at the current setup. Data are reported on a logarithmic scale.
chain in the melt phase [11]; Lor(Q; !) being a Lorentzian function accounting for the fast librational motions occurring on a ps time scale and ⊕Res(Q; !) corresponding to the convolution with the instrumental resolution; A, B and C being
A. Triolo et al. / Physica A 304 (2002) 308 – 313
313
1 Data Fit Elastic KWWPEO KWWPEO-Salt Lorentzian
S(ω)
0.1
0.01
1E-3 -2
0
2
4
6
8
10
Energy (meV) ◦
−1
M . Data have been Fig. 5. Representative
4. Conclusion Polymer electrolytes are presently the subject of large research e9orts. In this communication, we report recent structural and dynamical characterization of the behavior of PEO=Li-based salts mixtures. The SANS technique did not allow probing the existence of micro-heterogeneities due to the wrapping of PEO chains around the Li+ ion. However, indications of a collapse of the random coil con
F.M. Gray, Polymer Electrolytes, Royal Society of Chemistry, Cambridge, 1997. J.R. MacCallum, C.A. Vincent, Polymer Electrolytes Reviews, Vols. I and II, Elsevier, New York, 1987. F.M. ller-Plathe, W.F. van Gunsteren, J. Chem. Phys. 103 (1995) 4745. J.P. Donoso, et al., J. Chem. Phys. 98 (1993) 10 026. O. Borodin, G.D. Smith, Macromolecules 31 (1998) 8396. O. Borodin, G.D. Smith, Macromolecules 33 (2000) 2273. U. Keiderling, A. Wiedenmann, Physica B 213–214 (1995) 895. B.K. Annis, M.H. Kim, G.D. Wignall, O. Borodin, G.D. Smith, Macromolecules 33 (2000) 7544. R.E. Lechner, R. Melzer, J. Fitter, Physica B 226 (1996) 86. P. Debye, J. Appl. Phys. 15 (1944) 338. V. Arrighi, et al., Macromolecules, submitted for publication. T.M. Connor, B.E. Read, G.J. Williams, J. Appl. Chem. 14 (1964) 74. C.H. Porter, R.H. Boyd, Macromolecules 4 (1971) 589. A. Dekmezian, D.E. Axelson, J.J. Dechter, B. Borah, L. Mandelkern, J. Polym. Sci.: Polym. Phys. Ed. 23 (1985) 367.
www.elsevier.com/locate/physa
Structural and dynamical characterization of melt PEO–salt mixtures A. Trioloa; ∗ , F. Lo Celsob , V. Arrighic , P. Strunz a , R.E. Lechner a , M. Mastragostinod , S. Passerinie , B.K. Annisf , R. Triolob a Hahn-Meitner
Institut, BENSC, Glienicker Str. 100, D-14109, Berlin, Germany di Chimica Fisica, Universita’ di Palermo, V.le delle Scienze, Parco d’Orleans II, 90128 Palermo, Italy c Department of Chemistry, Heriot-Watt University, Edinburgh EH14 4AS Scotland, UK d Universit3 a di Bologna, Bologna, Italy e ENEA Electrochemical Energy Conversion Division, Rome, Italy f Chemical and Analytical Sciences Division, ORNL, P.O. Box 2008, Oak Ridge, TN 37831-6197, USA
b Dipartimento
Abstract Salt doped poly ethylene oxide (PEO) mixtures were investigated by means of both small angle neutron scattering and QENS techniques aiming to characterize morphological and dynamical features in the melt state. These experimental evidences provide support to the proposed heterogeneous scenario for polymer electrolytes. In particular, the existence of PEO– cation complexes is proposed to play a major role in intramolecular cooperation and interc 2002 Elsevier Science molecular transient crosslinks, which a9ects the mixture properties. B.V. All rights reserved.
1. Introduction In the last decades a large amount of e9orts has been applied to understanding and development of polymer electrolytes [1,2]. These systems have been shown to constitute a novel class of materials with high potentials for applications in the
Corresponding author. Tel.: +49-30-8062-3072; fax: +49-30-8062-3094. E-mail address: [email protected] (A. Triolo).
c 2002 Elsevier Science B.V. All rights reserved. 0378-4371/02/$ - see front matter PII: S 0 3 7 8 - 4 3 7 1 ( 0 1 ) 0 0 5 3 6 - 2
A. Triolo et al. / Physica A 304 (2002) 308 – 313
309
the salt cation (generally Li+ or Na+ ) and the polar portion of the polymer chain [3]. In particular, poly ethylene oxide (PEO) is an almost ideal system for these kind of applications due to the possibility that transient complexes might form between its ether–oxygen atoms and the cation [1,2]. The existence of such complexes has been detected both experimentally (see e.g. Ref. [4]) and theoretically, by means of molecular dynamics simulations [3,5,6], and is supposed to be responsible for the formation of transient crosslinks between neighbor chains. It has also been observed that most of the electric performances of these mixtures are related to a continuous solvation–desolvation process involving formation and breakage of the complex, as a consequence of the polymer chain segmental motion [4]. The latter process is then assumed to play a major role in determining the relevant performances of polymer electrolytes and accordingly has been the subject of a variety of investigations. In the present communication, we report a characterization of both structural and dynamical properties of PEO–salt mixtures. In particular, we report the
2. Experimental SANS measurements on PEO–LiI were run at the SANS instrument at the BENSC reactor [7]. In order to characterize labile clusters induced by the existence of the complexes, we decided to compare the scattering pattern from fully deuterated PEO (D-PEO) and two mixtures D-PEO20 –LiI and D-PEO10 –LiI. D-PEO had a molecular weight of 26,700, with a polydispersity of 1.04 and a degree of deuteration higher than 99%. LiI was an Aldrich product with purity higher than 99%. The materials were chosen to minimize the incoherent signal arising from H atoms, so to detect the presence of D-PEO–Li clusters embedded in a matrix of pure D-PEO. Previously, similar measurements were run on a conventional SANS instrument, which did not allow covering the Q-range corresponding to the expected size of the clusters [8]. Moreover, SANS measurements were collected on mixtures of hydrogenated and deuterated PEO (H=D = 70=30) both with and without salt, in order to probe the scattering from the single chain in the random coil con
310
A. Triolo et al. / Physica A 304 (2002) 308 – 313
3. Results and discussion 3.1. Structural characterization In Fig. 1, a comparison between the SANS patterns from D-PEO and D-PEO doped with LiI is reported. Apart from di9erences in the constant incoherent term depending on the di9erent sample compositions, there are no major di9erences in the large Q range. In this Q range, in the case of PEO–salt mixtures, we would expect to
D-PEO D-PEO 15 /LiI
d∑(Q)/dΩ (cm- 1 sr- 1)
100
10
1
0.1
0.1
1 -1
Q (nm )
Fig. 1. SANS patterns from pure fully deuterated PEO (D-PEO) and LiI-doped D-PEO (D-PEO15 =LiI) at ◦ 75 C. By using fully deuterated PEO, we aimed minimizing the incoherent scattering to detect any evidence of clusters due to the wrapping of PEO chains around the Li+ cation. No major di9erences can be detected M in the large Q range of the patterns, thus indicating that the clusters, if any, have sizes smaller than 20 A.
A. Triolo et al. / Physica A 304 (2002) 308 – 313
311
d∑(Q)/dΩ (cm- 1sr- 1)
1000
D-PEO + H-PEO (30/ 70) D/H(30/70)-PEO15 /LiI 100
10
0.1
1 -1
Q (nm )
Fig. 2. Comparison between SANS patterns from a pure mixture of D-PEO and H-PEO (ratio H-PEO=D-PEO = 70=30) and LiI doped 70=30 H=D-PEO (with composition PEO15 –LiI). The lines correspond to a
1000/T (1/K) 2.0
2.5
3.0
3.5
4.0
4.5 3 2 1 0 -1
0.1
log τ (ns)
QENS This work DS NMR
-2
S(ω )
-3 -4
0.01
1E-3 -2
Data Fit Elast i c KWWPEO Lorentzian 0
2
4
6
8
10
Energy (meV)
◦
−1
M . The model used for Fig. 3. Representative
performances of polymer electrolytes, it is of relevance to apply this technique aiming to detect the microscopic details of the relationship between segmental dynamics and complexes. In Fig. 3, the QENS pattern from pure PEO is reported, together with
312
A. Triolo et al. / Physica A 304 (2002) 308 – 313
S(Q=2.16 Å-1,ω)
1
PEO15-LiBETI Pure PEO
0.1
0.01 0
2
4
6
8
10
Energy (meV) ◦
−1
M . The Fig. 4. Comparison between QENS data from pure PEO and PEO15 –LiBETI at 80 C and Q = 2:16 A two patterns di9er only for the elastic portion of the spectra, this indicating di9erent populations of chains relaxing according to a slow motion not resolved at the current setup. Data are reported on a logarithmic scale.
chain in the melt phase [11]; Lor(Q; !) being a Lorentzian function accounting for the fast librational motions occurring on a ps time scale and ⊕Res(Q; !) corresponding to the convolution with the instrumental resolution; A, B and C being
A. Triolo et al. / Physica A 304 (2002) 308 – 313
313
1 Data Fit Elastic KWWPEO KWWPEO-Salt Lorentzian
S(ω)
0.1
0.01
1E-3 -2
0
2
4
6
8
10
Energy (meV) ◦
−1
M . Data have been Fig. 5. Representative
4. Conclusion Polymer electrolytes are presently the subject of large research e9orts. In this communication, we report recent structural and dynamical characterization of the behavior of PEO=Li-based salts mixtures. The SANS technique did not allow probing the existence of micro-heterogeneities due to the wrapping of PEO chains around the Li+ ion. However, indications of a collapse of the random coil con
F.M. Gray, Polymer Electrolytes, Royal Society of Chemistry, Cambridge, 1997. J.R. MacCallum, C.A. Vincent, Polymer Electrolytes Reviews, Vols. I and II, Elsevier, New York, 1987. F.M. ller-Plathe, W.F. van Gunsteren, J. Chem. Phys. 103 (1995) 4745. J.P. Donoso, et al., J. Chem. Phys. 98 (1993) 10 026. O. Borodin, G.D. Smith, Macromolecules 31 (1998) 8396. O. Borodin, G.D. Smith, Macromolecules 33 (2000) 2273. U. Keiderling, A. Wiedenmann, Physica B 213–214 (1995) 895. B.K. Annis, M.H. Kim, G.D. Wignall, O. Borodin, G.D. Smith, Macromolecules 33 (2000) 7544. R.E. Lechner, R. Melzer, J. Fitter, Physica B 226 (1996) 86. P. Debye, J. Appl. Phys. 15 (1944) 338. V. Arrighi, et al., Macromolecules, submitted for publication. T.M. Connor, B.E. Read, G.J. Williams, J. Appl. Chem. 14 (1964) 74. C.H. Porter, R.H. Boyd, Macromolecules 4 (1971) 589. A. Dekmezian, D.E. Axelson, J.J. Dechter, B. Borah, L. Mandelkern, J. Polym. Sci.: Polym. Phys. Ed. 23 (1985) 367.