60th Annual Report on Research 2015

In addition to forming micelles, surfactants adsorb to form aggregates at the solid/liquid interface. These adsorbed surfactant aggregates (admicelles) act as a two dimensional solvent allowing the partitioning of organic molecules into them 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 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 can hinder the technique being extended to more advanced applications.

Effect of Oxygen on Admicellar Polymerization

In previous investigations, admicellar polymerizations utilized a low monomer-to-initiator ratio relative to emulsion polymerization. In order to investigate the effect of deoxygenation on the polymer yield and molecular weight, we examined M/I of 15, 150, and 1000 for six hour polymerizations initiated by either AIBN or V-501 (Table 1). If all of the added styrene polymerized, it would result in a 4.9% weight loss above 300°C in the modified silica. The apparent monomer conversion (polymer weight loss/amount of monomer added) varied from 11% to 98%. The weight-averaged molecular weight (M_w) of the polymers extracted from the polystyrene-modified silica samples are presented in Figures 1 and 2. As expected, the M_w increases with increasing M/I. Deoxygenated samples have slightly higher M_w than the control samples. Extraction of the control samples at M/I of 1000 resulted in insufficient polymer recovery for GPC analysis, likely due to the low conversion in these samples as evidenced by TGA. The polymer with highest M_w (~1.8 x 10⁶) was formed on the silica sample at M/I of 1000 in a deoxygenated system.

TABLE 1. Polymer weight loss of modified silica and weight-averaged molecular weight M_w of extracted polymer for deoxygenated and control admicellar polymerizations initiated by AIBN and V-501.

FIGURE 1. Weight-averaged molecular weight of extracted polymer from modified silica in AIBN-initiated deoxygenated (red) and control (blue) admicellar polymerization at a M/I of 15, 150, and 1000.

FIGURE 2. Weight-averaged molecular weight of extracted polymer from modified silica in V-501-initiated deoxygenated (red) and control (blue) admicellar polymerization at a M/I of 15, 150, and 1000.

The polymer synthesized at an M/I of 15 showed a significantly lower M_w when polymerized in the presence of oxygen. Additionally, the mass of polymer formed under an inert environment was greater than the comparable polymerization system containing oxygen. This effect can best be observed in the samples at an M/I of 1000. At M/I of 1000, the AIBN deoxygenated admicellar polymerization system achieved an apparent conversion of 84% while the control sample had an apparent conversion of 11%. The presence of oxygen did not show a significant effect on the mass of the polystyrene formed at an M/I of 15 (Table 1), likely due to excess initiator available to compensate for the loss of active radicals to oxygen.

Kinetics of Admicellar Polymerization

Another focus for the project has been to investigate the kinetics of admicellar polymerization. The effect of varying the M/I on the kinetics of admicellar polymerization was examined by determining the apparent conversion at varying polymerization times (Figure 3). Results show that increasing the M/I reduced the admicellar polymerization rate, as expected. The polymerization at a M/I of 15 achieved ~100% conversion in two hours. In contrast, the samples with a M/I of 1000 achieved 63% monomer conversion at two hours and achieved a maximum conversion of 88% at six hours. These results are consistent with the mechanism of free radical polymerization in which a lower M/I provides fewer polymerization sites and therefore results in a slower polymerization with higher molecular weights (Figure 1).

FIGURE 3. Apparent conversion versus polymerization time in AIBN-initiated, deoxygenated admicellar polymerization. (red circles are M/I of 15; blue squares are M/I of 1000)

Effect of Initiator Solubility

We investigated the effect of using a water-soluble and a water-insoluble initiator on the yield and molecular weight of polymer formed via admicellar polymerization. The apparent conversion was similar for comparable M/I regardless of the initiator used (Table 1). Higher M_w polymer, however, was formed in admicellar polymerization with the water-soluble initiator (V-501) especially at an M/I of 15 (Figure 4). These results agree with emulsion polymerization systems in which high molecular weight polymer can be achieved when using water-soluble initiator. Water-soluble initiators partition into the admicelle to a lower degree than the more insoluble initiators. Therefore, fewer polymerization sites were available resulting in higher molecular weight polymer using a water-soluble initiator.

FIGURE 4. Weight-averaged molecular weight of extracted polymer from modified silica in AIBN- (orange) and V-501-initiated (green) deoxygenated admicellar polymerization at M/I of 15, 150, and 1000.

Being the first research grant awarded to the PI, the ACS PRF award enabled significant progress in understanding the fundamental parameters controlling admicellar polymerization. This project has produced two manuscripts currently under review and another in preparation. The support received from this award was instrumental in providing the resources for two graduate students and four undergraduates to have contributed to this work. The students’ work has resulted in one graduate and one undergraduate student presentations at national conferences.