Thermoresponsive Dynamers: Thermally Induced, Reversible Chain Elongation of Amphiphilic Poly(acylhydrazones)
2011-07-20T00:00:00Z (GMT) by
A nanostructured poly(acylhydrazone), which is reversibly formed in acidic aqueous solution from di(aldehyde) and di(acylhydrazine) monomers with appended hexaglyme groups, was found to display lower critical solution (LCS) behavior. Remarkably, under acidic conditions in which polymerization is reversible, large and reversible molecular weight (<i>M</i><sub>w</sub>) increases were observed in response to elevated temperatures, both below and above the LCS temperature. No variation in <i>M</i><sub>w</sub> was evident under neutral and alkaline conditions, in which the acylhydrazone condensation is essentially irreversible. Results of turbidometry studies, size-exclusion chromatography–multiangle laser light scattering (SEC–MALLS), and transmission electron microscopy (TEM) suggest that heating the polymer below the LCS temperature leads to polymer growth with preservation of the characteristic nanostructured morphology, whereas the onset of the microphase separated state causes a fundamental change in morphology, in which the polymer chains aggregate into larger bundles and fibers. van’t Hoff analysis of a small molecule model system indicates that the acylhydrazone condensation is enthalpy driven (Δ<i>H</i><sub>eq</sub> = −8.2 ± 0.2 kcal·mol<sup>–1</sup> and Δ<i>S</i><sub>eq</sub> = −11.1 ± 0.4 = cal·mol<sup>–1</sup>·K<sup>–1</sup>), which suggests that the observed polymer growth with temperature is not a consequence of the intrinsic thermodynamics of the intermonomer linkage but is likely the result of the thermoresponsive characteristics conferred by the multiple hexaglyme groups. The system described displays double control of the dynamer state by two orthogonal agents, heat and protons (pH). It also represents a prototype for dynamic materials displaying multiple control adaptive behavior.