Antiplasticization mechanisms

For many years, it has been known that a small quantity of plasticizer acts as an anti plasticizer for polyvinyl chloride (PVC). During a recent search for effective plasticizers for polycarbonate, W. J. Jackson and J. R. Caldwell found several groups of compounds which acted as antiplasticizers. They increased the tensile modulus and strength and reduced the elongation of polycarbonate films. In contrast to plasticizers, these antiplasticizers affected glass transition temperature quite differently. Their mechanism is explained by the fact that they either increase crystallinity or reduce the mobility of the polymer chain through the bulkiness of their molecules. 

Carbon-13 rotating-frame relaxation rate measurements are used to elucidate the mechanism of gas transport in glassy polymers. The nmr relaxation measurements show that antiplasticization-plasticization of a glassy polymer by a low molecular weight additive effects the cooperative main-chain motions of the polymer. The correlation of the diffusion coefficients of gases with the main-chain motions in the polymer-additive blends shows that the diffusion of gases in polymers is controlled by the cooperative motions, thus providing experimental verification of the molecular theory of diffusion. Carbon-13 nmr relaxation... 

Plasticization, whose main manifestation is the decrease of the glass transition temperature (a transition in dynamic mechanical spectra), is generally accompanied by an increase of the glassy modulus in the temperature interval between Tp and T. , an effect is known as antiplasticization. 

Studies on the mechanical properties of glassy polymer-solvent or, more generally, polymer-diluent mixtures have been primarily concerned with the deformation behavior at small strains which is governed by the viscoelastic properties of the material. From these studies it is well known that diluents significantly affect relaxation processes in glassy polymers, as clearly evidenced by phenomena such as plasticization and antiplasticization... 

The effect of diluents on the viscoelastic behavior of amorphous polymers is more complex at temperatures below T, i.e., in the range of secondary relaxation processes. Mechanical, dielectric and NMR measurements have been performed to study the molecular mobility of polymer-diluent systems in this temperature range (see e.g. From extensive studies on polymers such as polycarbonate, polysulfone and polyvinylchloride, it is well known that diluents may suppress secondary relaxation processes. Because of the resulting increase in stiffness, these diluents are called antiplasticizers . Jackson and Caldwell have discussed characteristic properties... 

Guo, J.-H. Effects of plasticizers on water permeation and mechanical properties of cellulose acetate antiplasticization in slightly plasticized polymer film. Drug Dev. Ind. Pharm. 1993, 19 (13), 1541-1555. 

Figure 13 illustrates the elongation profile of the composition when three different plasticizers are used at increasing concentrations (37). Dioctyl adipate emerges from the antiplasticization area at relatively low concentrations and the elongation property builds steeply. The composition responds intermediately with increasing amounts of dioctyl phthalate, and dicyclohexyl phthalate is quite inefficient. When different plasticizers are substituted in a vinyl composition that must still maintain specific mechanical properties, the concentration of the substitutions must be adjusted to accommodate each particular plasticizer s "efficiency factor."... 

The concentration of plasticizer is important since too low a concentration results in rigid and brittle membranes. This is termed the antiplasticization effect, and the minimum plasticizer concentration required is dependent on the plasticizer type and the base polymer used. For example, for PVC PIMs, this concentration is around 20 wt%. If the plasticizer concentration is too high, then excess plasticizer can exude to the PIM surface and the thin film formed can inhibit transport across the membrane. Also, such PIMs are normally mechanically weak and difficult to use. 

Figure 7.17. Mechanism of antiplasticization, a. high, b. low [Adapted, by permission, from Anderson S L Grulke E A DeLassus P T Smith P B Kocher C W Landes B Q Macromolecules, 28, No.8, 10th April 1995, p.2944-54.]...

Table 7. Mechanical Property Data for Various Antiplasticized Aromatic Polymer Systems ...

Lamm MS, Simpson A, McNevin M, Ftankenfeld C, Nay R, Variankaval N (2012) Probing the effect of drug loading and humidity on the mechanical properties of solid dispersions with nanoindentation antiplasticization of a polymer by a drug molecule. Mol Pharm 9(11) 3396— 3402... 

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