PNIPAmx–PPO36–PNIPAmx thermo-sensitive triblock copolymers: chain conformation and adsorption behavior on a hydrophobic gold surface

Abstract

The chain conformations and adsorption behaviors of four thermo-sensitive poly(N-isopropylacrylamide)_x–poly(propylene oxide)₃₆–poly(N-isopropylacrylamide)_x (PNIPAm_x–PPO₃₆–PNIPAm_x) triblock copolymers with x values of 15, 33, 75, and 117 in dilute aqueous solutions were investigated by combined techniques of micro-differential scanning calorimetry (micro-DSC), static and dynamic light scattering (SLS & DLS), and the quartz crystal microbalance (QCM). PNIPAm₁₅–PPO₃₆–PNIPAm₁₅ only exhibited the lower critical solution temperature (LCST) of the PPO block, i.e. 25 °C, because the PNIPAm block with x = 15 was too short to maintain its own LCST. With middle lengths x of 33 and 75, the LCSTs of PPO and PNIPAm blocks were observed, respectively. For the longest PNIPAm block with x = 117, only LCST of PNIPAm block dominated, i.e. 32.3 °C. DLS results revealed that the four PNIPAm_x–PPO₃₆–PNIPAm_x triblock copolymers formed “associate” structures in their dilute aqueous solutions at 20 °C, which was well below the LCSTs of the PPO and PNIPAm blocks. QCM results indicated that the adsorption time constant decreased with increasing adsorption temperature but tended to increase with increasing length x of the PNIPAm block. A complex adsorption behavior with large adsorption amounts was only observed at the corresponding LCST of the PNIPAm block for PNIPAm_x–PPO₃₆–PNIPAm_x with longer PNIPAm blocks with x = 33, 75, and 117. Furthermore, the adsorbed PNIPAm_x–PPO₃₆–PNIPAm_x layers obtained at 20 °C were rigid with less energy dissipation.