Synthetic 'Finger' to Pick Up High Temperature Object?

New findings about ionic liquids (ILs) keep reminding us their analogous properties to common solvents, especially water. In particular, the hydrophobic interaction, which has been proposed to mainly occur in solvents with highly structured network of polar molecules (e.g. water), may also exist in ionic liquids which also represent dynamically structured network. Therefore theoretically all hydrophobic driven phenomena could be repeated in ILs. Recent evidences include the self-assembly of block copolymer and inclusion complexation between cyclodextrin and guest molecules in 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]), a typical IL.

Hydrophobic segments of a amphiphilic block copolymer will aggregate into a core in water due to hydrophobic interaction, leaving the hydrophilic segments on the surface to form a corona, a process known as 'micellization'. Similarly, cyclodextrin, a macrocyclic molecule with a hydrophilic outside and a hydrophobic cavity, may spontaneously include hydrophobic guests of proper size in water. These processes are thermodynamically favored considering the breakage and re-formation of the structured solvent molecular network.

Now a third well-known hyrophobic-driven phenomena, the thermoresponsive sol-gel transition of amphiphilic polymer in water and the resultant volume phase transition of the crosslinked hydrogel, was reported alternatively in an IL, 1-ethyl-3-methylimidazolium bis(trifluoromethane sulfone)imide ([C₂mim][NTf₂]) (Langmuir 2007. DOI: 10.1021/la062986h). Poly(benzyl methacrylate) (PBzMA) and its copolymers show sharp lower critical solution temperature (LCSTs) in this IL. For linear polymer samples, the PBzMA homopolymer show a T_c (phase transition temperature) at about 105°C. Copolymerization with styrene (St), a 'sovatophobic' or insoluble component lowers the T_c, and that with methyl methacrylate (MMA), a 'sovatophilic' or soluble component raises the T_c, an analogous effect to the PNIPAAm/water system (but the T_c here ranges from ca. 70°C to ca. 140°C). It is also reported that changing the alkyl chain length of the imidazolium substituent, the n in [C_nmim] with a fixed [NTf2] anion, could also alter the T_c significantly (from ca. 90°C to ca. 240°C!), increasing it with longer alkyl chain. But the reason of the phenomenon is poor explain in the paper (involving the balance between the entropy and enthalpy changes which are not yet probed, and resorting to analogue to PNIPAAm's behavior in an aqueous solution but not necessarily relevant). I prefer to understand this based on the polarity concept of solvent; increasing the alkyl chain length may lead to decrease of polarity of the solvent and thus the tendency to form networks, unfavorable for the phase transition of the polymer and consequently rising the T_c. The author also evaluate the equilibrium swelling state of the crosslinked gel of PBzMA. The phase transition is discontinuous and reversible with a hysteresis loop, similar to the first-order volume change of PNIPAAm hydrogel (in water), yet the transition temperature is 105°C which can be further raised.

Considering the traditional responsive hydrogel which can only work under the boiling point of water, this ion-gel can broaden the application of smart, soft and wet materials beyond biomedical area. Just think of two synthetic 'fingers' that can pick up objects at above 200°C!