X-ray diffraction at elevated temperatures

Powder X-ray diffraction patterns were recorded at temperatures either side of the thermal events indicated by DSC. Figure 5-9 shows some of the patterns obtained for PE0 gCa(Cp3S0 3)2. A t room temperature, the pattern consisted o f peaks due to the

crystalline complex and PEO. A t 100°C, the peaks due to the crystalline PEO had disappeared from the pattern but those due to the complex remained. As the temperature was further increased, the intensity of peaks due to the crystalline complex decreased and the sample became increasingly amorphous. Eventually, at 207°C, peaks due to the pure salt were detected. The intensity of these salt peaks increased with temperature.

Figure 5-10 shows some of the diffraction patterns obtained upon cooling the PE0 gCa(CF3S0 3 ) 2 sample. The salt gradually redissolved in the polymer until at

18QOC a completely amorphous sample was obtained. The complex took several days to recrystallise completely from the sample. The peaks labelled P I and P2 on figures 5-Id and 5-9a were ascribed to PEO since they were in the positions corresponding to the most intense PEO peaks (figure 5-2d) and then disappeared from the diffraction pattern upon heating.

The behaviour of the complex, PE0 5Ca(CF3S0 3)2, was somewhat different upon

heating. The patterns obtained are shown in figures 5-11 and 5-12. A t room temperature, the pattern consisted of peaks due to the crystalline complex. The pattern showed no significant change upon heating to lOCPC due to the absence o f uncomplexed PEO in the sample.

W ith increasing temperature, the peaks due to the complex shifted to slightly lower 29 values as a result o f thermal expansion of the complex and concommitant increase in the d spacings of the lattice. Above 18CPC ( the onset temperature for peak 2 in the DSC spectrum fo r PE06Ca(CF3S03)2), a peak at 29 = 20.8° appeared. There was also a change in the relative intensities of the group of peaks in the region 29 = 22.3 -

23.5° on the 180°C diffractogram as compared to the room temperature diffractogram. Above 180°C, the intensity of the peaks due to the pure salt increased and those due to the crystalline complex gradually decreased in intensity. Eventually, at 201°C, the peaks due to the crystalline complex completely disappeared and those of the salt remained. The presence of peaks labelled SI and S2 on figure 5 -lc and 5-11 indicated that for this particular sample, small traces of salt were present. The position of S I

was identical to that o f the PEO peak P I in the 8:1 sample however, the intensity %

remained unchanged upon heating to 100°C. It was believed that peak S2 was 4

coincident with a small complex peak since a peak in this position was also present in the 8 :1 composition.

The behaviour of compositions richer in salt than PE0 ^Ca(CF3SC^ ) 2 was identical

to that for the crystalline complex with the exception that peaks due to the salt were present w ith significant intensities at all temperatures. This is illustrated for the PE0 4Ca(CF3Sp3)2 composition in figure 5-13.

For compositions more dilute in salt than PEO9 2Ca(CF3S0 3)2, patterns were

obtained at temperatures between the first and second endotherms that could possibly indicate the occurrence of a completely amorphous sample. A t these compositions however, very little if any crystalline complex was detected at room temperature. It was therefore not possible to say w ith any certainty that the absence o f observable complex peaks in the patterns was due to the absence of crystalline complex in the sample. It may simply have been present in insufficient quantities to be detected by the diffractometer. For all compositions, peaks due to the pure salt were obtained at the highest temperatures. This is illustrated for the PEO20,5Ca(CF3SO3)2 in figure 5-14.

The reversibility of the salt precipitation process is illustrated in figure 5-15 for a heating-cooling-reheating cycle of PEO92Ca(CF3S0 3)2. This figure also illustrates

5.2.4 Conductivity Measurements

The conductivities o f polym er electrolyte film s w ith c o m p o s itio n s PE0 xCa(CF3S0 3 ) 2 (where x = 5.3 6.5, 12.0, 20.5 and 50.4) were determined as a

function of temperature, as illustrated in figures 5-16 to 5-21. A ll conductivities were significantly enhanced compared to the pure polymer. A ‘ knee’ was observed in the Arrhenius plot for compositions where x > 12.0. This corresponded to the melting of uncomplexed PEO. The conductivity o f PEO5 3Ca(CF3S0 3 ) 2 was only slightly

enhanced compared to the pure polymer. The absence of uncomplexed PEO in the sample accounted for the absence of a ‘ knee’ in the conductivity plot.

•4 Ig Sigma ■5 6 ■7 ■8 ■9 -10 2.6 2.7 2.6 2.9 3.0 3.1 3.2 3.3 3.4 3.5 3.6 lOOOT(K)

Ig Sigma

1000/T(K)

Figure 5-17 Conductivity of PEO20.5^^(CP3SO3 )2

Ig Sigma

1000/T(K)

-5.0 Ig Sigma _{□ Q} -5.2 - -5.4 - -5.6 - -5.8 - -6.0 2.1 2.2 2.3 2.4 2.5 2.6 1000/T(K)

Figure 5-19 Conductivity of PE0 6^Ca(CFgS0 g)2

-6.0 Ig Sigma -6.5 - -7.0 - -7.5 - lOOOn’(K)

Figure 5-20 Conductivity of PEO53Ca(CF^S0 3 ) 2

log Sigma + ♦ oo □ o □ t * -10 " 3.5 3.2 2.9 2.3 2.6 2.0 □ Ig Sigma (50) ♦ Ig Sigma (20) + Ig Sigma (12) o Ig Sigma (5) ■ Ig Sigma (6.5) ê o Ig Sigma (PEO) 1000/T(K)

Figure 5-21 A comparison of the conductivities of the polymer electrolyte films PEOxCa(CF^S(33)2