Synthesis of Polymer-Titanium Dioxide Composites
Through Reversible Addition-Fragmentation Chain Transfer (RAFT) Polymerization
Matthew Flack 1, Dr. Devon Shipp 2
Department OF Chemistry, Clarkson University
Introduction
Synthetic polymers are used in numerous scientific applications as well as everyday life, and are becoming more important by the day. Gaining control of the characteristics and functionality of polymers is becoming increasingly important especially with applications emerging in biological and medicinal fields. Surface modification of of inorganic particles with polymers has become widely explored particularly in the field of nanotechnology. TiO 2 is an extensively used inorganic particle with many uses in research and industry including UV light stabilizers, thermal stabilizers, and solar cells. 1 Furthermore, TiO 2 is thermally stable, nontoxic, and inexpensive.
Gratzel et al. have shown that TiO 2 surfaces modified by ruthenium-based dyes can be incorporated into solar cells. 3 These photovoltaic cells are called dye sensitized solar cells (DSSC). One issue facing these devices is that they exhibit a reduced efficiency when operating at slightly elevated temperatures (e.g. 80 oC). To overcome this problem, work in the Shipp group has begun on creating new TiO 2 based DSSC’s that have improved thermal stability through the use of polymer surface modification. However, no work has been done on making such polymer-TiO 2 nanocomposites in which the polymer has the desired molecular weight, low polydispersity or chemical functionality. Therefore, we want to explore the use of reversible addition-fragmentation chain transfer (RAFT) polymerization to create polymer-TiO 2 nanocomposites.
RAFT polymerization uses dithioester compounds to mediate the polymerization. This results in predictable molecular weights and low polydispersities (Pd). In recent years there has been extensive research into RAFT polymerization, and a detailed discussion on the mechanism can be found in pertinent articles. 4,5
Methods
RAFT Polymerization of MMA- Poly(methyl methacrylate) (PMMA) was prepared by adding 0.058g of P 4S 10, 0.067g of benzoic acid and 0.25g of 2,2-azo-bis-isobutyronitrile (AIBN) into a 10ml Schlenck flask containing 3ml of toluene. The mixture was purged with nitrogen to remove any oxygen and the solution was heated at 110 oC for 1hr. After heating 0.01g of AIBN and 6ml of methyl methacrylate (MMA) were added to the solution. This was followed by freeze pump thawing three times and nitrogen purging. The solution was then heated at 60 oC. Samples were taken in situ and analyzed using gas chromatography (GC) and gel permeation chromatography (GPC) in order to monitor kinetics and molecular weight development.
Synthesis of PMMA with TiO 2 at 5% 10% and 20%- The procedure is the same for the RAFT polymerization of PMMA above with the exception that the weight % TiO 2 was added with the AIBN and MMA and the entire mixture was dispersed at room temperature by placing in a ultrasonic bath for 10min in order to obtain a good dispersion of the oxide particles into the polymer matrix. The final product was analyzed using thermogravimetric analyzer (TGA), GPC, GC and differential scanning calorimetry (DSC).
Results and Discussion
Using GC and GPC we are able to compare the number average molecular weight (M n) and polydispersity (Pd) to the % conversion as seen in Figures 1 and 2 respectively. Figure 1 is for PMMA without TiO 2 while Figure 2 is PMMA with TiO 2 at 5%. The M n slope for Figure 1 is linear with the exception of two discernable points at 45% conversion and 85% conversion. This could be due to the error involved in the GPC and GC or possible air contamination. Figure 2 also shows a linear slope for M n with negligible divergence from the slope.
Figure 1. M n and Pd as a function of % monomer conversion during the RAFT polymerization of MMA (without TiO 2).
Figure 2. M n and Pd as a function of % monomer conversion during the RAFT polymerization of MMA with 5 wt.% TiO 2.
TGA and DSC are currently being used to analyze the Thermal stability of PMMA with TiO 2. The results will be presented at the SURE conference.
Conclusion
The analysis of the inclusion of TiO 2 in the RAFT polymerization of MMA has shown that polymerization can proceed to give high molecular weights (70,000 – 80,000) and low polydispersities (<1.5). We anticipate that the inclusion of the TiO 2 into the PMMA matrix will result in an increased thermal stability. This analysis will hopefully provide valuable insight on surface modification with RAFT polymerization and may demonstrate important information for the future development of dye-sensitized solar cells.
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