59th Annual Report on Research 2014

In addition to forming micelles, surfactants adsorb at solid/liquid and liquid/gas interfaces and decrease the interfacial free energy. The adsorbed surfactant aggregates, which we will assume to be bilayers, act as a two dimensional solvent allowing the partitioning of organic molecules into the admicelle in a phenomenon termed “adsolubilization.” In the late 1980’s, researchers began to investigate the application of the adsolubilization of polymerizable organic compounds to produce polymeric films on surfaces. Since the initial reports, admicellar polymerization, the surface analog of emulsion polymerization, has been utilized to synthesize a thin polymeric film on substrates for a number of applications ranging from modified silica fillers for tires to stain and fire resistant coatings for fibers. Despite the growing number of reports, there still exists a lack of fundamental knowledge regarding the admicellar polymerization process that must be overcome before the technique may be extended to more advanced applications. The first year of research included the efforts of one graduate student and three undergraduate students and has been devoted to answering two of these underlying questions: i) What is the effect of oxygen on admicellar polymerization?, and ii) What is the effect of monomer to initiator ratio on the kinetics of admicellar polymerization?

One of the major challenges leading to a lack of fundamental knowledge regarding the admicellar polymerization process is the difficulty of synthesizing enough polymer for detailed kinetic studies and the difficulty of completely extracting the polymer from the substrate without degradation occurring; thus the properties (i.e., molecular weight, tacticity, branching) of polymers synthesized by admicellar polymerization are virtually unknown. A significant effort in the first year was devoted to investigate proper techniques for synthesizing sufficient polymer for analysis. This included investigating both porous silicas of varying surface area and non-porous glass beads as the substrate. In our studies, we have utilized cetyltrimethylammonium bromide (CTAB) as the surfactant, precipitated silica (Hi-Sil 233) as the substrate, styrene as the monomer, and either 2,2'-azoisobutyronitrile (AIBN) or 4,4-azobis(4-cyanopentanoic acid) (V-501) as a radical initiator. We utilized a monomer to surfactant ratio of 2:1 and varied the monomer to initiator (M:I) from 15:1 to 1000:1. After polymerization, the formed polymer film was isolated from the silica via Soxhlet extraction with THF.

Effect of Oxygen on Admicellar Polymerization

In previous investigations, admicellar polymerizations utilized a low monomer-to-initiator ratio relative to emulsion polymerization. Our hypothesis is that the requirement for a high initiator concentration stems from the presence of oxygen (a known radical inhibitor) and can be overcome by the removal of oxygen from the polymerization solution. In order to investigate this hypothesis, we deoxygenated the solution by purging the headspace with nitrogen gas. Immediately prior to removing the nitrogen purge, deoxygenated styrene was added to the solution and the reaction flask sealed. After polymerization, the polymer modified silica was dried and analyzed by TGA to determine the mass of the polymer film. Additionally, the polymer film was extracted from the silica and analyzed by gel permeation chromatography.

Figure 1 shows the results of preliminary studies on the effect of deoxygenation on the molecular weight of the polymer formed by admicellar polymerization using AIBN as the radical initiator. At M:I ratios of 15:1 and 150:1, the molecular weight of the isolated polymer formed in the deoxygenated system is larger than that formed in the presence of oxygen. At a M:I ratio of 1000:1, insufficient polymer was extracted from the polymerization mixture containing oxygen for analysis. These results are thought to be due to the termination of polymerization by oxygen diffusing into the adsorbed bilayer. Since AIBN is water-insoluble, the initiator partitions to the hydrophobic core of the bilayer. Once the polymerization is started, monomer adds to the active radical until the growing polymer chain is terminated by either another radical or a radical inhibitor such as oxygen. In the non-deoxygenated polymerization solution, oxygen may diffuse into the adsorbed bilayer, prematurely terminating polymerization and leading to suppressed molecular weight as shown in Figure 1.

Figure 1. Effect of deoxygenation on the molecular weight of polymer films formed by admicellar polymerization.

The effect of deoxygenation may also be seen by thermogravimetric analysis (TGA) on the modified silica. Figure 2 shows the weight loss associated with the polystyrene film formed on the surface on the silica. Based on system loading, the weight loss expected if all styrene monomer is incorporated into the polymer film is roughly 5%. At an M:I of 15:1, both systems show that essentially all the styrene is incorporated into the polymer film. At 150:1 and 1000:1, the deoxygenated system generates a greater mass of polymer film than the polymerization performed in the presence of oxygen. The TGA reaffirms the observation of very little polymer formed in the presence of oxygen at an M:I ratio of 1000:1.

Figure 2. Effect of deoxygentation on polymer film mass at varying monomer to initiator ratios.

Kinetics of Admicellar Polymerization

Another focus for the first year of the project has been to investigate the kinetics of admicellar polymerization. Utilizing the same system described above, we utilized TGA to determine the weight loss attributed to polystyrene for M:I ratios of 15:1 and 1000:1 as a function of polymerization time. As Figure 3 shows, polymerizations performed at a 15:1 ratio reach completion after two hours while polymerization utilizing a M:I ratio of 1000, require between 8 and 24 hours to reach completion.

Figure 3. Kinetics of the admicellar polymerization of styrene for M:I ratios of 15:1 and 1000:1.