STRUCTURE OF POLY(VINYLIDENE FLUORIDE) GEL ELECTROLYTES

Polymer gel electrolytes have three constituents. polymer, salt and so lvent. This paper gives structural information on polymer gel electrol ytes made from poly(vinylidene fluoride), lithium triflate and tetragl yme. These electrolytes exhibit a room-temperature ionic conductivity in the region of 10(-3) S cm(-1), while maintaining sufficient mechani cal rigidity to form self-supporting films (having elastic moduli in t he region of 100 kPa). Differential scanning calorimetry and dynamic m echanical analysis have been used to show that the majority of the net work junctions of the gel ale crystalline in Mature. Wide angle X-ray diffraction has revealed that when no salt is included in the gel, the se crystal junctions are almost an order of magnitude larger in their lateral dimensions than when salt is present. The salt is thought to n ucleate crystallisation. The modulus is significantly reduced by inclu sion of salt; however, DSC suggests that apparent crystallinity is onl y slightly reduced by the presence of salt. This discrepancy is attrib uted to either the uncertainty in the heat of fusion of PVDF, or to th e formation of small crystalline particles that are not incorporated i n the network junctions. Gels with polymer concentrations between 15 a nd 40% (by weight) maintain their mechanical rigidity lip to temperatu res around 100 degrees C. However, once melted, the gel structure only reforms at much lower temperatures. The variation of ionic conductivi ty of salted gels with temperature shows no such hysteresis, and it is concluded that the ionic conductivity is independent of the mechanica l state of the gel. (C) 1997 Elsevier Science Limited.