The Gel theory

The solvation-desolvation process described by Aiken to explain plasticization matches the basic ideas of the gel theoiy. But the first concepts, that anticipated the gel theory, are considered to be brought forward by Manfred and Obrist. These authors understood plasticization as the disaggregation of the polymer molecules in the presence of the plasticizer. Some years later, Busse had called attention to the possibility of for- 

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. 

---

The mode of action of plasticizers can be explained using the Gel theory [35 ]. According to this theory, the deformation resistance of amorphous polymers can be ascribed to the cross-links between active centres which are continuously formed and destroyed. The cross-links are constituted by micro-aggregates or crystallites of small size. When a plasticizer is added, its molecules also participate in the breaking down and re-forming of these cross-links. As a consequence, a proportion of the active centres of the polymer are solvated and do not become available for polymer-to-polymer links, the polymer structure being correspondingly loosened. 

By 1950 two theories had evolved to account for the major "flexibilizing" effect of plasticization the "lubricity" theory and the "gel" theory ( ). In addition, the "free volume" theory devised to explain fluid flow was also being adapted to plasticization (10). 

The lubricity theory explains the resistance of a polymer to deformation. Stiffness and rigidity are explained as the resistance of intermolecular friction. The plasticizer acts as a lubricant to facilitate movement of macromolecules over each other, thus giving the resin an internal lubricity. The gel theory is applied to predominantly amorphous polymers. It proposes that their rigidity and resistance to flex are due to an internal three-dimensional honeycomb structure or gel. The spatial dimensions of the cell in a brittle resin are small because their centers of attraction are closely spaced and deformation cannot be accommodated by internal movement in the cell-locked mass. Thus, the elasticity limit is low. Conversely, a thermoplastic or thermosetting polymer with widely separated points of attachment between its raacroraolecules is flexible without plasticization. 

The gel theory of Carothers [4] in which the critical degree of conversion at the gel point (pgei) is defined as pg = 2jf, with /being the average functionality of the monomers in the system. 

In the case of the formulation of mixed resins for adhesives for which extensive formulation experience does not exist, the gel theory approach is also particularly useful. Le us examine, as an example, what would be the degree of conversion at the gel point of a resin based on the reaction of resorcinol (R) with acetaldehyde (A) and formaldehyde (F) in relative molar proportions of, respectively, R A F = 1 1 0.5. The two aldehydes can react and do react with resorcinol while under the conditions used they are considered as not being able to react with each other (aldol condensation is indeed minimal under the conditions used to prepare adhesive resins). In short to avoid any gelling of our prereacted resin in the reactor, during manufacture, we want to know at what degree of conversion we can advance reaction of the resin without gelling it. 

The first two theories which try to explain the plasticization of polymers were the gel theory and the lubricity theory. Both were developed simultaneorrsly and sometimes the authors to whom the theories are attributed were not aware of the differences between both... 

The lubricity theory is attributed to Kirkpatrick, Clark, and Houwink among others but, as mentioned above, some concepts common to the lubricity and to the gel theories appear in their works. 

The free volume theory originated some years later than the lubricity and the gel theories, when the evolution of different properties of polymers as a function of temperature, specific volume, thermal expansion coefficients, or viscosity was attempted to be explained.The relationships between these properties and some variables corresponding to polymer stracture, such as molecular weight or terminal groups content, the presence of another monomer and, of course, the presence of plasticizers, was also explained. For plasticized polymers the theory attempted to explain the diminution of the glass transition temperature with the plasticizer content. This theory is a contribution of different authors, but it was postulated by Fox and Floiy. The theory is still being used to explain some properties of plasticized polymers, i.e., viscoelastic properties. ... 

Afterwards, based on the hypothesis that the elasticity development in plasticized PVC was due to the enhancement of micro-Brownian motions of the polymer, Doolittle, Stickney and Cheyney and Alfrey et developed the Gel Theory. They considered that the mechanical properties of a polymer were due to the elastic resistance of entangled segments of the macromolecules structured as a three-dimensional network. The plasticizer molecules which are dissolved into the polymer matrix break some points of attachment between macromolecules leading to an easier glide. Moreover,... 

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... 

---