Chain movements at the topmost surface of polymer films at high temperature or under water directly evaluated by in-situ real-time AFM in the tapping mode

It is believed that due to the limited resolution of atomic force microscopy (AFM), no meaningful information concerning chain movements can be obtained by AFM observations of polymer bulk films, such as of poly(methyl methacrylate) (PMMA) and polystyrene (PS). However, this is not true. (I) We observed the surfaces of PS and PMMA bulk films in situ at high temperature by AFM in the tapping mode and found that the chain movements at the topmost surfaces can be observed [1]. PMMA and PS chains started moving at ~97 °C and ~50 °C, respectively, that were slightly and significantly below the values of their bulk T_g (PMMA: 108 °C, PS: 104 °C). The activation energies of the apparent diffusion constants of PMMA and PS, derived by a particle image velocimetry (PIV) analysis, were 193 and 151 kJ mol^-1, respectively, and reasonable for the surface T_g. (II) We also observed the bulk films of PMMA, poly(2-hydroxyethyl methacrylate)(PHEMA), and poly(2-methoxyethyl methacrylate)(PMEMA) under water in real time and found that the chain movements observed under water and in air at room temperature were quite reasonable considering their T_g and water absorption. (III) We further evaluated the chain movements in each domain of a PS-b-PMMA film and found that the chain movements were significantly affected by the microphase separation.

[1] Kouki, K.; Kumaki, J. Langmuir 2022, 38, 13707-13719.