Reverse mechanochemical system

The expansion and contraction of the polymer chain which accompanies the redox of Cu ions can also be visually confirmed by means of the mechanochemical system proposed by Kuhn161), as illustrated in Fig. 31. A film is prepared with a poly(vinylalcohol)-Cu(II) complex and is suspended with a sinker in water. The film is extended by about 20% on the reduction of Cu(II) to Cu(I) and shrinks back to its original length on the oxidation of Cu(I). The poly(vinylalcohol) chain is densely crosslinked by the extremely stable Cu(II) chelate, but is loosened when Cu ion forms the unstable Cu(I) chelate. This change is reversible as may be observed. 

The history of the creation of the first smart polymeric system dates back to the works of Kuhn and Katchalsky (88, 89). They demonstrated that collagen fibers changed dimension reversibly on transition from cyclic helices to random coils when immersed cyclically between salt solution and water. This was referred to as a mechanochemical system capable of transforming chemical energy to mechanical work. 

It will be shown in a forthcoming review that a reversible change of conformation of linear macromolecules in solution may result in an appredable change of solution properties, and particularly of their viscosity. Such conformational changes and the resulting effects can be displayed by chemical means, and were actually described several years ago by Kuhn, Katchalsky and coworkers " and more recently by Osada and Saito. They are responsible for the mechanochemical behavior of polymer systems in the solid state, i.e. the conversion of chemical into mechanical energy . ... 

This reaction system is classified as Type II because coordination polymers with large channels and no channels are reversibly switched depending on the present solvents, which function as chemical stimuli. For this conversion, the coordinations of the solvent molecules, H2O and DMF in this case, to Zn(II) centers at the first step would largely contribute to switch the structures. Similar structural interconversions have been observed in Zn-oba system by using mechanochemical method. In this system, PCP [Zn2(bdc)2(H20)2] DMF and non-PCP [Zn(bdc)(H20)2] are switched by grinding the sample in the presence of small amount of solvent. ... 

The reversible conversion of the system properties caused by the conformational changes is described by the curves having approximately the same shape, irrespeetive of the stimulus nature, Figure 4.11. The plotted curves differ eaeh to another by the amplitude and the frequency of the changes of measured property. Presently, the efforts are focused on the inerease both of amplitude of dimensional changes and developed foree, and shortening of mechanochemical cycle. 

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