Small dop of comonomer,giant shift of cloud point: Thermo-responsive behavior and mechanism of poly(methylacrylamide) copolymers with an upper critical solution temperature

Poly(methylacrylamide) (PMAM) is a thermo-responsive polymer with an upper critical solution temperature (UCST). Its cloud point (T_cp) is around 60 °C, unsuitable for certain biomedical and industrial applications. This study brought up a copolymerization strategy to tune the T_cp of PMAM with hydrophilic comonomers. Surprisingly, with a small portion of hydrophilic monomer doped, the T_cp of the PMAM copolymer can be significantly shifted. For instances, with ≤7 mol% of acrylamide or 1 mol% of oligo(ethylene glycol) methacrylate, the T_cp can be shifted in a wide range from ~69 to ~0 °C. Microdifferential scanning calorimetry demonstrated that the enthalpic effect during the phase transition of the solutions is indistinctive, while fluorescence measurement with pyrene as a probe revealed that the hydrogen-bonding within polymer chains is enhanced by the hydrophobic aggregation of methyl groups. Therefore, the doped hydrophilic monomer could remarkably alter the ordering of water-molecules and the extent for the aggregation of methyl groups, leading to the pronounced shifting in the T_cp of the copolymers. This work would facilitate the application of PMAM as smart polymer materials and guide the inventions of functional materials based on UCST polymers.