We report the preparation and characterization of physical gels using mixtures of poly(vinyl alcohol) (PVOH) and a high melting ionic liquid (IL), 1-ethylpyridinium bromide. Similar to PVOH hydrogels, these mixtures show elastic responses in uniaxial tension tests due to the propensity to form specific interactions between PVOH and IL. The tensile modulus of the mixtures decreases significantly as the IL content in the mixture increases, indicating that the IL not only promotes physical association of PVOH but also acts as a plasticizer. Thermal analysis shows that both the melting temperature (Tm) and the glass transition temperature (Tg) of the as-cast mixtures decrease noticeably as IL content increases. For mixtures that were aged for one week at room temperature, a secondary endothermic peak appears at almost constant temperature irrespective of the mixture composition, suggesting the formation of a polymersolvent (IL) compound that is typically observed in solvent-induced physical gelation. Wide angle X-ray scattering results indicate that the IL is highly miscible with PVOH for IL-lean mixtures (up to 40% by mass of IL) and forms a modified amorphous phase. IL-rich mixtures exhibit new diffraction peaks at high scattering angles which are absent in neat IL, indicating the development of small crystalline order (0.35 nm to 0.45 nm in size). FT-IR analysis shows that the O-H stretching vibration band shifts to higher frequency as IL content increases, suggesting that part of the hydrogen bonding in PVOH is replaced with the interaction between the IL and hydroxyl group of PVOH, which disrupts PVOH crystallization and/or promotes formation of physical junctions that enhances the elastic behavior. These observations demonstrate that simple mixtures of polar polymers and compatible ionic liquids can be used to develop thermoplastic elastomers with widely tunable thermal and mechanical properties.
Pub Type: Journals
gels, polymers, ionic liquid, mechanical properties, physical gel