Journal of Student Research 2015

159

Photopolymerization of Methylmethacrylate:

vial. After irradiation, it was observed that some of the reaction mixtures still remained water-like. After sitting overnight (without exposure to air), addi tional polymerization could occur and the mixture would “set” and form a highly viscous gel. The mechanism of this “living” phenomena is elusive, and a proper treatment is beyond the scope of this work. The optimal irradiation time for polymerization was determined by varying the irradiation time (from 15 minutes to 13 hours) and visually monitoring the reactor. Decomposition of the initiator occurs within minutes of irradiation. The reaction mixture changes from colorless to bright yellow quickly, long before a change in vis cosity is noticeable. From these observations, it was determined that a 1 hour irradiation period is sufficient to break down the initiator. The high molar absorption of MK/BDEABP leads to yellowing even in low concentrations. Because the absorption is very high, yellowing could not directly measured and correlated with radical yield or conversion. The measured intensity as shown in Figure 4 was 415 mW/cm2 and 494 mW/cm2 for the individual LEDs at 1mm from the source, which decays exponentially as the distance increases. FIGURE 4

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Obtaining a high molecular weight polymer is often difficult with photoinitiated systems, as reported by McGinniss et al. (1978). Generally many radicals result in short polymer chains due to radical termination by combination. The addition of benzophenone was found to significantly decrease the molecular weight of the polymer. Raw chromatograms presented in Figure 5 and summary data in Table 2 show this trend. For the polymer produced without benzophenone, a bimodal distribution was observed, possi bly due to competing mechanisms of radical formation or chain growth.