MERCADO NACIONAL DE YUCA Y ALMIDÓN DE YUCA

The detailed morphology of polymer electrolytes is highly complex and dependent not only on the system and its composition but on the method o f preparation used. Currently, the two most common methods of preparation are the solvent casting method and the cryogrinding / hot pressing method ( described in more detail in sections 4.1.1 and 4.2.2 ). It is found that the morphology of a system can be affected by the characteristics of the solvent casting process, e.g. the nature of the solvent and rate of removal, as well as by the temperature and thermal history of the sample.

As for the simple polymers, described in section 1.2.4, most polymer electrolyte systems which crystallise, form spherulites of well defined stoichiomery. It is often found that the interlamella amorphous regions contain small amounts of dissolved salt. Spherulites of more than one composition may be observed in any given system, the proportion o f each type o f spherulite depends on the overall composition o f the system. Thus Neat et al 24 observed three types of spherulite in the PEO (M W 4x10^) : LiC lO ^ system cast from acetonitrile. Each type of spherulite was characterised by a different melting point and salt content. No more than two types of spherulite were observed for any one composition. The same authors25 also observed sim ilar behaviour in the PEO : LiCFgSOg system.

The affect of the nature of the solvent on the morphology of polymer electrolytes has been investigated by Payne and W right 26 for the PEO (MW 5x10^) : LiBF^ system. Films cast from methanol were observed to be semi-crystalline w ith low melting points. Those cast from a chloroform /acetone mixture were highly crystalline w ith well defined spherulites melting at much higher temperatures. X-ray diffraction studies however, indicated that the short range structure was similar for both cases. It was postulated that the morphology of the film was affected by the extent o f pre-

association of the ions and the polymer in solution prior to removal of the solvent. For example, in dilute solutions where the salt is only weakly solvated, it is possible for 'precursor complexes' to be formed w ith the ether. Very little reordering is subsequently required to form the final crystalline structure. In such instances, a highly crystalline material containing high melting point spherulites is expected to form as exhibited by the acetone/chloroform mixture. For solutions where the salt is highly solvated, complex formation with the ether is not initiated until the bulk of the solvent has been removed. For such concentrated solutions, there is a high degree of polymer chain entanglement thus reducing the long range order of the resulting complex, as seen from the methanolic solution.

Wendsjo and Yang 27 in their study of factors affecting the degree o f crystallinity of PEO based electrolytes containing PW2 observed that different complexes formed

when the solvents dimethyl sulphoxide (DMSG) and dimethyl formamide (DMF) were used to dissolve the lead iodide. Furthermore, it was found that the use o f DM F inhibited the formation of crystalline PEO. The importance of specifying the solvents used in the preparation procedure is thus emphasised.

The affect o f water on the morphology and electrical properties o f polymer electrolytes has been the subject of several studies. Wendsjo and Y ang 27 observed that

the degree of water uptake vai ies greatly with the nature o f the ions in the electrolyte. Their X-ray diffraction studies of PE0 4ZnCl2 and PEOgNiBr2 clearly indicated that

the uptake o f water destroys the crystalline complex phase. Upon subsequent dehydration o f PEOgNiBr2, peaks due to the crystalline complex reappeared.

Farrington et al28,4 observed that upon controlled hydration / dehydration of PEOxNiBr2 electrolytes, the conductivities and N i2+ transport numbers were greatly

enhanced. Spectroscopic studies indicated that upon exposure to moisture, the water preferentially coordinated w ith N i(ll) to form the hexa-aquo ion. The increased m obility o f the hydrated N i2+ ion was ascribed to the fact that it was no longer

conductivity of the electrolyte upon dehydration could be due to a significant change in structure and noted that TGA data indicated that all of the N i(II) forms a complex with the PEO. The X-ray diffraction patterns of Wendsjo et al do not indicate a significant change in structure upon controlled hydration / dehydration. The explanation for the concommitant enhanced conductivity is thus still the subject o f debate.

G ra y 16 has cautioned that it is d ifficu lt to generalise as to the effect of solvent type

and its rate o f removal since this w ill be highly dependent on the individual polymer electrolyte system. In some cases, spherulite growth may not be initiated until after removal o f the solvent, in which case, the morphology w ill not be affected by its rate of removal.

It is d ifficu lt to make generalisations as to the effect of heat treatment and thermal history on polymer electrolyte morphology. This is also observed to vary w ith the individual system and composition. Lee and Wright^^.is have investigated the effect o f heat treatment on the morphology of some PEO (MW 10,000) : NaSCN and PEO (M W 10,000) : Nal complexes. They observed that the solution deposited materials all contained crystalline lamellae o f thickness 150 - 200 Â (1Â = lO '^^^j), Annealing at temperatures just below the melting point gave rise to significant lamella thickening which increased with time. If the same material was heated to above its melting point and subsequently cooled, a lower melting point material was produced fo r the PEO:Nal system. This process was irreversible and not observed fo r the PEO : NaSCN system.