SAXS/DSC/WAXD Study of the Temperature Evolution in Nanocomposite Polymer Electrolytes  with Different Nanofillers

A. Turković1, P. Dubček1, K. Juraić1 and S. Bernstorff2

1 Institute "Ruđer Bošković", P.O.Box 180, HR-10002 Zagreb, Croatia
2 Sincrotrone Trieste, 34149 Basovizza /Trieste, Italy

Polymer electrolytes as nanostructured materials are very attractive components for batteries and opto-electronic devices. (PEO)8ZnCl2 polymer electrolytes were prepared from PEO and ZnCl2. The nanocomposites (PEO)8ZnCl2 themselves contained TiO2, Al2O3, MgO, ZnO and V2O5 nanograins. In this work, the influence of the ZnO and TiO2 nanograins on the morphology and ionic conductivity of the nanocomposite was systematically studied by transmission small-angle X-ray scattering (SAXS) simultaneously recorded with wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). SAXS experiments are suitable to determine the structure of nanocomposite polymer electrolyte. Polymeric complexes of (PEO)n with ZnCl2 have been used, due to their stability and very high conductivity [1,2]. 
The measurements yielded insight into the temperature-dependent changes of the grains of the electrolyte. The  heating and cooling rates were 1°C/min, ½ °C/min and ¼ °C/min. 
Our research was aimed to study the temperature behavior of the nanocomposite (PEO)8ZnCl2 electrolyte, and to optimize its properties [3-5] as these materials are attractive as electrolytes for second generation of polymer-based rechargeable batteries [6, 7]. This structural investigation provided informations about the behavior of the nanosizes through the superionic phase transition, which occurs at ~65 °C. 

Figure 1.  SAXS, DSC and WAXD results for samples A, B, C and D. 

Figure 1 shows the results from the simultaneous SAXS, DSC and WAXD measurements in the temperature range from 20°C to 100°C to 20°C at rate of ½ C°/min on the polymer electrolytes: (PEO)8ZnCl2, (PEO)8ZnCl2/TiO2, (PEO)8ZnCl2 irradiated with a dose of 309 KGy and (PEO)8ZnCl2/TiO2 + 309 KGy (denoted as A, B, C and D respectively). In Fig. 1 graph A shows the results from the SAXS/WAXD/DSC measurements in the temperature range from 20°C -> 100°C -> 20°C at the rate of ½ C°/min performed on polymer electrolyte (PEO)8ZnCl2. The intensity close to Is (for s=0) is falling at 68.3°C indicating the phase transition temperature in the heating cycle. The phase transition temperature in the cooling cycle is at 47.6°C due to hysteresis. The average grain radius varies from 4.0 nm to 4.4 nm in the region below the phase transition temperature and then from 3.5 nm to 2.6 nm in the highly conductive phase. The cooling cycle in the SAXS data shows a change of the grain sizes in the range from 2.6 nm to 1.9 nm. SAXS measurements for (PEO)8ZnCl2/TiO2 (Fig. 1 B), result in changes of grain sizes from 4.6 to 3.7 nm, the third sample (PEO)8ZnCl2 irradiated with a dose of 309 KGy (Fig. 1 C), registers changes from 3.3 to 0.7 nm and during the fourth run on the sample (PEO)8ZnCl2/ TiO2 + 309 KGy (Fig. 1 D), grain sizes change from 4.4 to 2.7 nm. From these we can generally conclude that the average grain sizes in all four samples remained in the same range from 0.7 to 4.6 nm. In a lamellar picture of PEO [8] these grain sizes would correspond to the lamellae LP2 with no integrally folded (NIF) chains [9] combined with salt and TiO2.
The SAXS, WAXD and DSC data show a hysteresis, i.e. the phase transition temperature in the cooling cycle is much lower than 65°C, which is the melting temperature of the PEO crystallites, i.e. the spherulites [10]. In the case of the nanocomposite polymer electrolyte, combined forms of PEO and ZnCl2, or both in combination with TiO2 crystallites, influence the melting temperature. The combined WAXD, SAXS and DCS results are summarized in the Table 1. Environment friendly galvanic cells as well as solar cells of the second generation are to be constructed with nanocomposite polymer as electrolyte.

Table 1. Changes of the average grain radius R (nm) as obtained by SAXS, R=D/2 as determined from WAXD and DSC-obtained phase transition temperatures t (in °C) in (PEO)8ZnCl2/TiO2 nanocomposite polyelectrolyte during heating and cooling with a rate of ½ °C/min.

Sample  	Heating	
	SAXS	WAXD	DSC	
	t (°C)	R (nm)	t (°C)	R (nm)	t (°C)	
A	68.3	4.0-4.4
3.5-2.6	68.9;82.2	34-45; 95-96	65.3	
B	68.7	4.6-4.5
3.8-3.9	68.9	35-47	65.0	
C	62.5	2.4-3.3
1.7-0.8	63.0; 75.5	45-51; 82-82	59.0	
D	63.4	4.2-4.4
3.0-3.0	63.4; 74.7	45-58; 109-111	58.4	

Sample	Cooling	
	SAXS	WAXD	DSC	
	t (°C)	R (nm)	t (°C)	R (nm)	t (°C)	
A	47.6	2.6-2.0
2.0-1.9	30.3; 87.0	57-61; 68-85	56.2	
B	49.2	3.7-3.65
3.65-3.7	35.0	50-102	44.6	
C	42.2	0.7-1.5
0.8-0.7	43.0; 85.6	45-45; 109-111	46.6	
D	28.8	2.9-2.8
2.7-2.7	37.6; 85.1	54-62; 66-111	46.4	

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