Enhancement of glass transition temperature of polar polymer using salt

Introduction: Poly(methyl methacrylate) (PMMA) has various attractive properties such as transparency, mechanical toughness, light weight and good weatherability [1]. Therefore, it is a candidate as a substitute for inorganic glass especially in the automobile industry due to high demand for a lightweight vehicle. In order to replace an inorganic glass, however, the thermal stability must be improved as it has a low glass transition temperature (Tg) (ca. 105 oC). Generally, the introduction of cross-linking and hydrogen bonding is known to enhance Tg of a polymer. Nevertheless, these methods have disadvantages such as losing thermoplastic nature. In this study, a salt with low lattice energy is mixed with PMMA by the solution blend technique. The salt used in this study is well mixed with PMMA and enhances the Tg without sacrificing the transparency and flowability of the material. Experiment: PMMA and 5 to 60 wt% of lithium trifluoromethanesulfonate (LiCF3SO3, LiFMS) or lithium perchlorate (LiClO4) were dissolved in the mixture solvent of dichloromethane (90 wt%) and methanol (10 wt%) and stirred for 1 hour. The solution was cast on the Teflon plate and dried at 160 ºC for 30 hours. The obtained mixtures were compressed into flat sheets with a thickness of 300 μm by a compression-molding machine for 3 min at 240 ºC under 10 MPa and subsequently cooled at 25 ºC. Tg of the samples was measured by using a dynamical mechanical analyzer and a differential scanning calorimeter. 3. Results and Discussion: Figure 1 shows Tg, measured by differential scanning calorimeter of the blends containing various amounts of the salts, in which the salt amount is expressed by the molar ratio of the lithium to the carbonyl group in PMMA. The result indicates that the salt addition greatly enhances Tg without scarifying its transparency. The Tg enhancement is presumably caused by the electrostatic interaction between Li-cation and carbonyl group in PMMA as indicated in attenuated total reflection results. As a result, the blends show high Tg. Moreover, all the blends are found to be transparent for PMMA/ LiFMS in the experimental range, while the blends with more than 50 wt% of LiClO4 are opaque. 4. Conclusion: The Tg enhancement of PMMA was observed by the addition of the lithium salts. Electrostatic interaction between lithium cation and the carbonyl group is responsible for the Tg enhancement.Supervisor:山口　政之マテリアルサイエンス研究科修

Enhancement of glass transition temperature for poly(methyl methacrylate) by salt

We investigated the effects of two metal salts, lithium trifluoromethanesulfonate (LiCF_3SO_3) and lithium bromide (LiBr), on the glass transition temperature (Tg) of poly(methyl methacrylate) (PMMA). Both LiCF_3SO_3 and LiBr greatly enhanced the T_g of PMMA under dry conditions. However, once the sample films were exposed to humidity, the PMMA containing LiCF_3SO_3 absorbed a large amount of water, which acts as a plasticizer. As a result, the T_g shifted to a lower temperature, which limits the utility of this polymer in industrial applications. In contrast, the T_g of PMMA containing LiBr was minimally affected by the absorption of water. This phenomenon can be explained by the ion–dipole interactions with the small number of dissociated lithium cations