Free volume and Phase Transitions of 1-Butyl-3-Methylimidazolium Based Ionic Liquids: Positron Lifetime

Transcription

1 Free volume and Phase Transitions of 1-Butyl-3-Methylimidazolium Based Ionic Liquids: Positron Lifetime Positron Annihilation Laboratory Yu, Yang Oct. 12th. 211

2 Outline 2 Introduction to free volume Positron annihilation lifetime spectroscopy (PALS) Ionic liquids Experiment results and discussion Conclusion

3 Introduction to free volume Hole Free volume in molecular materials: Vf =V Vocc: Vf: free volume Vocc: occupied volume. 3 Vf=<vh> Nh : <vh>: average hole volume Nh : hole number density per gram Structural, static and dynamic disorder. Viscosity, molecular transport, structural relaxation and physical aging. Vw: van der Waal volume; VI: interstitial volume; Vc: crystalline volume; Vocc: occupied volume; Vf: free volume; Tg: glass transition temperature; Tc: crystallization temperature; Tm: melting temperature

4 Free volume influence to molecular transport property Permeation properties (small molecules in polymer), viscosity, viscoelasticity, glass transition, volume recovery, mechanical properties 4 Fluidity: Doolittle A bv v f exp[ / Mobility: Cohen-Turnbull equation D A T exp( v / v f ) Permeability coefficient P SD Selectivity: A/ B PA/ PB ( SA/ SB)( DA/ DB) c Ionic conductivity: T * exp[ ( v ) / vf Dynamics: Williams-Landel-Ferry (WLF) equation f g = f(t g ) =.25 Ref: Doolittle, Journal of Applied Physics, Cohen and Turnbull, Journal of Chemical Physics, Williams, Landel, and Ferry, Journal of the American Chemical Society, 1955.

5 Positron Annihilation Lifetime Spectroscopy 5

6 Positronium interaction with molecular material 6 Ref: G. Dlubek, Positron Annihilation Spectroscopy, in: Encyclopedia of Polymer Science and Technology, ed. by. A.Seidel, John Wiley&Sons, Hoboken, 28.

7 Free volume from positron lifetime 7 Theory:Tao-Eldrup model.5ns o Ps pickoff rh 1 2 r h 1 sin rh r 2 rh r o r 1.66 A o-ps lifetime po (ns) Tao-Eldrup Standard Model 7 6 r h threshold hole radius r h (Å)

8 Ionic Liquids (ILs): Definition: organic salts with melting points below 1 o Cor even room temperature (RTILs). 8 Structure: organic cations paired with organic or inorganic anions. Property: excellent solvating properties; no measurable vapor pressure; non-flammability; high thermal stability; low melting temperature. Application: green replacement for classical organic solvents, electrolytes in batteries, solar cells and fuel cells, lubricants and heat transfer fluids.

9 Ionic Liquids (ILs): 9 [BMIM + [BF 4 - [NTf 2 - [OTf - [PF 6 - [Cl - [B(hfip) 4 - Ionic formulae of the ionic liquids studied in this work.

10 Experiment results and discussion: PALS 1 [BMIM[BF 4 : (ns) < > (ns) [BMIM[BF 4 cooling heating T g =19K < T k =28K 3 > T (K) The mean, < >, and the standard deviation,, of the o-ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM[BF 4. T g indicates the glass transition temperature and T k the knee temperature. I 3 (%) [BMIM[BF 4 cooling heating T (K) The intensity I 3 of the o-ps lifetime as a function of temperature T during cooling and heating of [BMIM[BF 4.

11 11 [BMIM[NTf 2 : (ns) (ns) [BMIM[NTf 2 black: cooling red : heating T g =19K T c =25K T (K) T k =27K T m =272K The mean, < > (squares), and the standard deviation, (spheres), of the o- Ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM[NTf 2. DSC, Jin et al., T g =186K T cr =232K T m =271K I 3 (%) [BMIM[NTf 2 filled: cooling empty: heating T (K) The o-ps intensity I 3 as a function of temperature during cooling and heating of [BMIM[NTf 2

12 12 [BMIM[Cl: (ns) < >(ns) 4 (ns) [BMIM[Cl black: cooling red: heating 23 K T g T (K) T cr 29 K The mean, < >, and the standard deviation,, of the o-ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM[Cl. 4 shows an additional o-ps lifetime which appears after crystallization. T k 335 K 4 35 K < > T m I 4 (%) I 3 (%) [BMIM[Cl cooling heating T (K) The two o-ps intensities I 3 and I 4.

13 13 [BMIM[PF 6 : (ns) < > (ns) 4 (ns) h2, glass h1 cooling 1 heating 1 heating 2 heating 3 h3 T g cr-ii c1 cr-i [BMIM[PF 6 liquid < > T m T (K) The two o-ps intensities I 3 and I 4. 4 I 4 (%) I 3 (%) The mean, < >, and the standard deviation,, of the o-ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM[PF 6. 4 shows an additional o-ps lifetime, which appears after transformation of the cr-ii into the cr-i phase. 5 [BMIM[PF 6 h1 h2, glass h3 cr-ii c1 cr-i T (K) I 3 T m liquid I 4 cooling 1 heating 1 heating 2 heating 3

14 14 [BMIM[OTf: (ns) < > (ns) BMIM-OTf cooling heating T cr T m =285K T (K) The mean, < >, and the standard deviation,, of the o-ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM[OTf. T cr and T m show the temperatures of crystallization (during cooling) and melting. I 3 (%) BMIM-OTf cooling heating T cr T m T (K) The o-ps intensity I 3.

15 15 [BMIM[B(hfip) 4 : [BMIM[B(hfip) 4 liquid (ns) (ns) heating cooling crystalline solid The mean, < >, and the standard deviation,, of the o-ps lifetime distribution as a function of temperature T during cooling and heating of [BMIM[B(hfip) T (K)

16 Occupied volume and number density 16 V sp (cm 3 /g) Cooling, [BF 4 Heating, [BF 4 Cooling, [NTf 2 Cooling, [PF 6 Heating, [PF 6 Cooling, [Cl Heating, [Cl Plot of the specific volume from pressure-volume-temperature (PVT) experiment under MPa vs the mean hole volume at supercooled liquid state (between T g and T k ). The line is a linear fit of the data V <v h > (Å 3 ) [C 4 MIM + [Cl [BF 4 [PF 6 [NTf 2 V occ_sp (cm 3 /g) (PALS) N f (1 21 g -1 )

17 17 Summarized parameters from experiment results for the ionic liquids. [BMIM + [Cl - [BF 4 - [PF 6 - [OTf - [NTf 2 - [B(hfip) 4 - T g (K)(DSC) T m /T cr (DSC) 341/29 283/22 286/ / /3 T g (PALS) 23 ± 5 K 19±3 K 188 ± 3 K 19±5K T k 335 ± 5 K 28±5 K 285 ± 5 K 27±5 K T g /T k V occ_sp (cm 3 /g) (PALS) N f (1 21 g -1 ) V occ (Å 3 )(PALS) f h (T g ) f h (T k ).25 (23 K).7 (335 K).3 (19 K).79 (28 K).34 (188 K).88 (285 K).22 (19 K).61 (27 K)

18 Dynamic spectroscopy 3 18 Ln (s) [BF 4 [NTf 2 [PF 6 [Cl VFT fitting Vogel-Fulcher-Tamman (VFT) equation: ln ln T (K) [C 4 MIM + [Cl [BF 4 [PF 6 [NTf 2 Ln(t )(s) B T (K) max_o-ps (ns) T( = max_o-ps ) (K) T k (K) 335±5 28±5 285±5 27±5 Ref: Vogel, Phys. Z., Fulcher, Journal of the American Ceramic Society, Tammann, G. and W. Hesse, Zeitschrift für anorganische und allgemeine Chemie, 1926

19 Hole volumes comparison with molecular volume [BMIM + [Cl [BF 4 [PF 6 [OTf [NTf 2 [B(hfip) 4 19 V m = V(A + X )(Å 3 ) V([X )(Å 3 ) 47± liquid (< >, ns; <v h >, Å 3 ) glass, 14 K (,ns; <v h >, Å 3 )) crystal (< > ns) / <v h > (Å 3 ) V m (Å 3 ) The hole volumes of various ILs in the liquid (filled circles) and in the glass (14 K, empty circles) states as function of the molecular volume V m = V(A + X ).

20 Fürth s hole theory: The energy required for the formation of a hole of spherical shape of radius r in a continuum is equal to the sum of the work to be done against the surface tension ( and the work to be done against the pressure (p). 4 Relation between hole volume ( ) and surface tension ( )..68 / P 2 Ts Fig. Comparison of hole volume from Fürth theory (squares) and PALS (circles). V (stars) is specific volume from PVT experiment. Ref: Dlubek, G., Yu, Yang, et al., Free volume in imidazolium triflimide ([C 3 MIM[NTf 2 ) ionic liquid from positron lifetime: Amorphous, crystalline, and liquid states. The Journal of Chemical Physics, (12): p [Fürth, R. Mathematical Proceedings of the Cambridge Philosophical Society, 1941.

21 Hole volume comparison with Fürth theory 21 <v h > (Å 3 ) [NTf 2 [BF 4 [PF 6 [Cl - B(hipf) 4 - NTf 2 OTf - - PF 6 - BF 4 1 Cl T (K) Comparison of the mean hole volumes <v h > for the liquid or supercooled liquid and glassy states of the ionic liquids under investigation. Filled symbols: cooling, empty symbols: heating. Free volume calculated by Fürth theory is shown as line in the graph. [Fürth, R. Mathematical Proceedings of the Cambridge Philosophical Society, 1941.

22 Ln( T -1/2 ) (Pa s/k.5 ) Viscosity and conductivity [C 4 MIM[BF 4 1/V f (g/cm 3 ) CT: = CT 1/2 e ( V*/V f ) T: 188 K ~ 293 K VFT: = T 1/2 e B/(T-T ) Conductiity: 1/T (K -1 ) CT: σ / VFT: / exp / Ln( T -1/2 ) (Pa s/k.5 ) Ln( T 1/2 ) (ms/cm) 22 Viscosity: CT: / exp / VFT: / exp / 1/V f (g/cm 3 ) [C 4 MIM[BF CT: = CT -1/2 e V*/V f VFT: = T -1/2 e B/(T-T ) -2-4 T: K ~ K /T (K -1 ) Ln( T 1/2 ) (ms/cm)

23 23 Ln( T -1/2 ) (Pa s/k.5 ) /V f (g/cm 3 ) [C 4 MIM[NTf 2 CT: = CT 1/2 e ( V*/V ) f VFT: = T 1/2 e B/(T-T ) /T (K -1 ) Ln( T -1/2 ) (Pa s/k.5 ) 9 Viscosity: CT: / exp / VFT: / exp / [C 4 MIM[NTf 2 1/V f (g/cm 3 ) Conductiity: CT: σ / VFT: / exp / Ln( T 1/2 ) (ms/cm) CT: = CT -1/2 e V*/V f VFT: = T -1/2 e B/(T-T ) Ln( T 1/2 ) (ms/cm) /T (K -1 )

24 Ln( T -1/2 ) (Pa s/k.5 ) [C 4 MIM[PF 6 1/V f (g/cm 3 ) CT: = CT 1/2 e ( V*/V f ) VFT: = T 1/2 e B/(T-T ) /T (K -1 ) Ln( T -1/2 ) (Pa s/k.5 ) Ln( T 1/2 ) (ms/cm) [C 4 MIM[PF [C 4 MIM[Cl 1/V f (g/cm 3 ) 1/V f (g/cm 3 ) VFT: = T -1/2 e B/(T-T ) CT: = CT -1/2 e V*/V f /T (K -1 ) Ln( T 1/2 ) (ms/cm) Ln( T -1/2 ) (Pa s/k.5 ) CT: = CT 1/2 e ( V*/V ) f VFT: = T 1/2 e B/(T-T ) /T (K -1 ) Ln( T -1/2 ) (Pa s/k.5 )

25 25 [BMIM + [Cl [BF 4 [PF 6 [NTf 2 Ln( )(Pa*s) B T Viscosity_VFT Ln(C) γ Viscosity_CT Ln( )(ms/cm) B T Conductivity_VFT Ln(C) γ Conductivity_CT γ /N M /V m

26 Conclusion Important information of the local free volume in the amorphous (glass, supercooled liquid, true liquid) and crystalline phases of ionic liquids as well as the corresponding phase transitions can be obtained from PALS. 26 The o-ps mean lifetime < > shows different behaviour indicating different phases (smaller values in crystalline phase due to dense packing of the material). The parameters I 3 also responds to phase transition by sharp value change. Low value in supercooled and true liquid, due to solvation of e +, precursor of Ps. The knee temperature T k coincides with melting temperature of corresponding crystalline structure for [NTf 2, [PF 6 and [Cl samples. The local free volume from PALS displays a systematic relationship with molecular volume. Fitting result of viscosity and conductivity by CT equation shows the free volume influence to molecular transport property.

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