Polymer chains desolvation

As the poly(alkenoic acid) ionizes, polymer chains unwind as the negative charge on them increases, and the viscosity of the cement paste increases. The concentration of cations increases until they condense on the polyadd chain. Desolvation occurs and insoluble salts precipitate, first as a sol which then converts to a gel. This represents the initial set. 

In addition to size and molecular weight, one of the most important factors which determines plasticizer efficiency is the rate of diffusion of the plasticizer in the polymer matrix. In view of the dynamic solvation-desolvation between the plasticizer molecules and the polymer chains, the higher the diffusion rate, the greater the efficiency of the compound as a plasticizer. However, high diffusion rates are usually encountered with small molecules the smaller the plasticizer molecule, the greater its volatility and, therefore, the rate at which it is lost from the plasticized product. 

Two major changes are associated with the redox switching process in ECPs one is the charge of the polymer chains that induce ionic motion to maintain electroneutrality (and in the particular case of PANI, possibly proton exchange [179]) the other one is related to the hydrophobic/hydrophUic balance of the polymer matrix that makes the solvent play an important role in the doping process and often induce swelling phenomena [180]. It has been shown that counterion insertion during oxidation of PPy is accompanied by desolvation processes as evidenced by variation of the diffusion coefficient of the counterion [181]. These desolvation processes could be involved in the achievement of the quasi-metallic state of ECP [182]. 

Thermodynamic or Mechanistic Theory. From the observation of migration of plasticized polymers it is clear that plasticizer molecules are not boimd permanently to the polymer, but rather a dynamic equilibrium exists between solvation and desolvation of the polymer chains by plasticizer. Different families of plasticizers are attracted to the pol5uner by forces of different magnitude but the attraction is not permanent. There is a continuous exchange where a plasticizer molecule becomes attached to an active group on the polymer chain only to be dislodged and replaced by another plasticizer molecule. 

The mechanism of plasticization is not completely understood. There are many theories aimed at explaining the specific interactions between the plasticizer and PVC, e.g., the gel theory, the grease theory, and the theory of equilibrium between the processes of solvation and desolvation. Based on the most recent studies, it is assumed that the plasticizer particles permeate into polymer chains during the swelling process. The... 

Fig. 2.22 Schematic illustration of the relation between the frequency shift (A/) and the solvation/desolvation of the grafted polymer chains and the relation between the dissipation shift (AD) and the stretching/collapse of the grafted polymer chains...

This theory also explains plasticization by nonsolvents (softeners). When introduced into the polymer mass, these molecules act by holding apart the polymer molecules and so breaking some unions between active centers on the polymer. It was also explained why internally plasticized systems behave worse with the temperature than the externally plasticized, since molecules of a separate plasticizer are free to solvate and desolvate the active centers on the resin macromolecules to a given extent, determined by the concentration, the temperature and the equilibrium involved in the system. Permanently bound side chains have no such freedom. Other properties such as the tear strength or the creep behavior of plasticized systems were also explained. 

Figure 4. Schematic illustration of the PEAA-driven vesicle-to-micelle transition in DPPC Route a (upper) Acidification causes conformational collapse of PEAA and subsequent partitioning of the collapsed chain into the bilayer. Route b (lower) Acidification causes polymer adsorption without conformational collapse. Further protonation causes desolvation of PEAA and reorganization of the aggregate into mixed micelles. The latter route appears to describe the pseudoequilibrium experiments discussed in the text. The sequence of events that follows rapid acidification has not been determined.

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