Polymer/salt complexes formation

While oxygen-containing polymers have received more attention other heteroatom-containing polymers have also been studied. In addition to homopolymers, copolymers containing more than one monomer has also received attention. Further, modifications of homopolymers by plasticizers, or crosslinking, or grafting to improve the properties of the polymers towards polymer-salt complex formation or increasing the dimensional stability of the materials has also been a focus of research. 

Since the polymer and the metal salt involved are both solid materials, the preparation of a polymer salt complex is achieved by the dissolution of the two materials in a common solvent such as acetonitrile, methanol or THF followed by a slow removal of the solvent in vacuum. This results in either the bulk polymer-metal salt complex or a thin film depending upon the method of preparation. It is essential to ensure that no traces of moisture are present and hence the operations are carried out by using Schlenk techniques or glove box methods. The essential reaction that occurs in the formation of a polymer-metal salt complex can be written as... 

Just as the dissolution of ionic salts in a solvent system requires that the solvation energy of the ions in solution overcome the lattice energy of the ionic salt, similarly,polymer-metal salt complex formation proceeds,provided the polymer matrix effectively solvates the ions and overcomes the lattice energy of the ionic salt. Three essential criteria for this process have been identified [37] ... 

While small ions such as Li+ which can be strongly solvated, lead to formation of polymer salt complexes even up to LiCl (lattice energy 853 KJ mob1) other larger cations such as Na+, K+ etc., require bulky counter anions such as I, SCN, or CF3SO3 in order to be solvated by PEO [38]. 

The relative advantages of drying at different temperatures have been discussed in detail elsewhere [73]. In summary, drying at high temperatures modifies the morphology and the amorphous/crystalline ratio and favours the formation of high temperature polymer-salt complexes in which the ions are too tightly bound to be mobile. 

Polymer metal complex formation of different polyvinylpyridines in solution, in hydrogels and at interfaces were investigated [83]. In aqueous solution linear or crosslinked polyvinylpyridines in the interaction with H2PtCl6 results in reduced viscosities and reduces swelling coefficients, respectively. Complexation leads to molecular bridges and folding of the polymer. Film formation was observed at the interface of poly(2-vinylpyridine) dissolved in benzene and metal salts dissolved in water. 

These observations have been interpreted in terms of complex formation between polymer and salt in the solid state. The change in Tg was shown to be an unusual function of metal ion concentration (i.e. Tg increases with increasing metal concentration up to... 

When a cupric salt is added to a polymer-ligand solution, a greenSpectroscopic study indicated that complex formation between Cu and a polymer-ligand was not a step-by-step mechanism and that the composition of the complex, Cu(ligandK, in a polymer ligand remained constant throughout the course of the reaction ( ). 

Phosphate -ester cross-linked polyethylene glycol)s [60] are obtained from the condensation of glycols with POCl3. Partition of Li-trifluoro-methanesulfonate-LiCF3S03 between acetone solution and the polymer gel results in the formation of the electrolyte salt complexes [60],... 

Some papers60-61 have been devoted to phase separation of polyionic complexes from partially furated (PVA-S) and aminoacetylated (PVA-AAC)poly(vinyl alcohol) in aqueous salt solutions. The separation liquid-liquid or complex coacervation occurs at a definite value of the charge density on the macromolecule. From the concentration dependence of the reduced viscosity of the initial components PVA-S, PVA-AAc and their equivalent mixture in water it follows that the viscosity of the components noticeably increases with dilution, and the curve of the equivalent mixture is concentration independent. This fact confirms the formation of the neutral polymer salt, due to electrostatic interactions of PVA-S (strong polyadd) and PVA-AAc (weak polybase). 

Ichimura et al. [76] have described photoresists based on soluble polymers bearing methacrylated side-chain groups which are photocross-linked by a free radical mechanism using a diphenyliodonium salt as initiator, with a p-dimethylaminobenzylidine sensitizer. These investigators found spectroscopic evidence for in situ ground state charge transfer complex formation between the sensitizer and initiator, and that such complexes are involved both in the photocross-linking process as well as in the thermal instability of the photoresist films. 

Various methods have been employed to find out about the structure of polymer electrolytes. These include thermal methods such as differential scanning calorimetry (DSC), differential thermal analysis (DTA), X-ray methods such as X-ray diffraction and X-ray absorption fine structure (XAFS), solid state NMR methods particularlyusing7LiNMR,andvibrationalspectroscopicmethodssuch as infrared and Raman [27]. The objective of these various studies is to establish the structural identity of the polymer electrolyte at the macroscopic as well as the molecular levels. Thus the points of interest are the crystallinity or the amorphous nature of materials, the glass transition temperatures, the nature and extent of interaction between the added metal ion and the polymer, the formation of ion pairs etc. Ultimately the objective is to understand how the structure (macroscopic and molecular) of the polymer electrolyte is related to its behavior particularly in terms of ionic conductivity. Most of the studies have been carried out, quite understandably, on PEO-metal salt complexes. In comparison, there has been no attention on the structural aspects of the other polymers particularly at the molecular level. 

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