Graft copolymers and polymer electrolytes based on polyethers
This thesis is concerned with the preparation, characterisation and applications of butyl rubber (BR) with poly(tetrahydrofuran) (PTHF) and polymer electrolytes based on polyethers. The first part of this thesis is concerned with the preparation of rubber-PTHF graft copolymers by a cationic "grafting-from" reaction initiated by silver perchlorate at allylic bromine sites on the rubber. In order to optimise the content of grafted PTHF a series of experiments was carried out, in which reaction variables such as reaction temperature and time, and ratio of silver salt to active Br, were changed systematically. It was shown that an initiation period at -20C for 30 min. , followed by polymerisation at 0C with a [AgClO4]/[Br] ratio of ca. unity, produced near optimum reaction conditions with grafting efficiencies > 90%. The properties of the new BR-g-PTHF copolymers were analysed by Differential Scanning Calorimetry (DSC), Dynamic Mechanical Thermal Analysis (DMTA) and Thermogravimetry Analysis (TG). The DSC shows that all the graft copolymers have two-phase morphology comprising amorphous BR with crystalline domains of PTHF. There appears to be little interpenetration of the two phases. The DMTA suggests that the crystalline PTHF domains exert a reinforcing effect which persists from 195K up to ca. 300K at which the PTHF domains melt. TG shows that the PTHF side chains on BR function as "weak links" and reduce the thermal stability of the original rubber, This is not unexpected because it is. well known that polyethers including PEO, PPO and PTHF are susceptible to hydroperoxidation. The second part of this thesis deals with LiClO4-containing electrolytes based on PTHF and the BR-g-PTHF copolymers described above as well as a liquid EO/PO copolymer. Arrhenius plots of the conductivity and viscosity (where appropriate) are non-linear but follow the Vogel-Tamman-Fulcher (VTF) equation. PTHF-LiClO4 systems show two distinct regions in the Arrhenius plots of the conductivity ?, corresponding to temperatures below Tm (ca. 30 C) where K is very low, and above Tm where there is a marked increase in K. In the curves of molal conductivity (?) versus ?c. of both liquid polymers, ? passes through a maximum accompanied by a steep rise in viscosity, typical of coupled systems. However, the minima seen in related systems at low concentrations are absent. EO/PO-LiClO4 complexes yield electrolytes of much higher conductivities compared with the corresponding PTHF systems. Addition of 10% by weight of the plasticisers tetrahydrofuran (THF) and propylene carbonate (PC) produces a decrease in viscosity and an increase in conductivity in both types of polymer electrolyte. However, the increase in ? is relatively small on addition of THF, but very large for PC, especially at low salt concentration c. In the curves ? versus ?c the usual maximum is absent when PC is added, at a level of 10% by weight as plasticiser, and these cosolvent-salt systems appear to be "decoupled". Both PC and THF plasticise the polymer, but PC, being the solvent of higher permittivity, also interacts strongly with the salt and apparently reduces ion pairing. The conductivity of BR-g-PTHF systems depends on the type of salt and its concentrations as well as on the temperature. Thus, LiClO4 leads to complexes with BR-g-PTHF of higher conductivities than those based on NaCSN. Polymer electrolytes based on pure PTHF have higher conductivity than those derived from BR-g-PTHF. Finally, the trial prototype "supercapacitors" constructed with polyether-based electrolytes and carbon cloth TCM 128 have a higher useful voltage, ca. 2.3V, compared with those based on aqueous electrolytes. It is possible to use "solvent free" polymer electrolytes in such devices.