The influence of two thermoresponsive polymers, poly(N-isopropylacrylamide) (PNIPAM) and poly(N-vinylcaprolactam) (PVCL), on the phase transition behavior of a thermoresponsive ionic liquid, tributylhexylphosphonium 3-sulfopropylmethacrylate ([P4,4,4,6][MC3S]), was investigated. An obvious distinction was observed in the LCSTs and morphologies of [P4,4,4,6][MC3S]–PNIPAM and [P4,4,4,6][MC3S]–PVCL aqueous solutions, indicating their large differences in dynamic transition processes. In general, PNIPAM can “break” the water structure of [P4,4,4,6][MC3S] to decrease the transition temperature, while PVCL can “make” the water structure to increase it. Surprisingly, [P4,4,4,6][MC3S] has an unusual over-hydration behavior before dehydration while PNIPAM experiences a two-step transition process in [P4,4,4,6][MC3S]–PNIPAM aqueous solution, which has never been reported so far. Further studies revealed that the formation of strong intra-/inter-molecular hydrogen bonds CO⋯D–N in PNIPAM is the driving force for the LCST phenomenon of [P4,4,4,6][MC3S]–PNIPAM solution, while it is the [P4,4,4,6][MC3S] that dominates the phase separation of [P4,4,4,6][MC3S]–PVCL solution.
The self-aggregation behavior of poly(N-isopropylacrylamide)-b-poly(N-vinylcaprolactam) (PNIPAM-b-PVCL) during the thermal-induced phase transition in water was explored by a combination of calorimetric, turbidity, dynamic light scattering (DLS) and FTIR measurements. Only one transition can be observed via all detecting methods, revealing the cooperative aggregation of the two distinct temperature-sensitive segments. What is more, the combination of strong hydrophobic interactions among the entire polymer chains and hydrogen bonds of CO…H–N within PNIPAM segments results in the sharp variations at the LCST and gradual variations above the LCST during the phase transition of PNIPAM-b-PVCL aqueous solution upon heating. Additional analysis by perturbation correlation moving window (PCMW) and two-dimensional correlation spectroscopy (2Dcos) indicates that the hydrophobic C–H groups have an earlier response than the relatively hydrophilic CO groups during the LCST transition.
The volume phase transition (VPT) behavior of poly(N-vinylcaprolactam-co-4-vinylpyridine) P(VCL-co-4VP) microgels and their hybrid counterparts containing Au nanoparticles (AuNPs) is explored and compared by FTIR measurements in combination with the perturbation correlation moving window (PCMW) technique and two-dimensional correlation spectroscopy (2Dcos) analysis. It is demonstrated that the incorporation of AuNPs inside the microgels cause a slight dehydration effect on the microgels in the swollen state and an increase of the VPT temperature. Moreover, the interaction between CO and AuNPs can be confirmed by combining the conventional IR and 2Dcos analysis. It is with the help of the additional cross-links between AuNPs and the polymer chains (CO and pyridine groups) in the microgel network and the tendency of AuNPs to aggregate that the hybrid microgels exhibit a much greater dehydration during the VPT, as compared with the nonhybrid ones.
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