Local thermomechanical analysis of a microphase-separated thin lamellar PS- b-PEO film

We use atomic force microscopy (AFM) and hot tip AFM (HT-AFM) to thermophysically characterize a 30 nm thick film of poly(styrene-block-ethylene oxide), PS-b-PEO, and to modify its lamellar patterns having spacing of 39 ± 3 nm. AFM tip scans of the polymer film induce either abrasive surface patterns or nanoscale ripples, which depend upon the tip force, temperature, and number of scans. The evolution of the lamellar patterns is explained by the polymer film molecular structure and mode I crack propagation in the polymer combined with the stick-and-slip behavior of the AFM tip. The HT-AFM measurements at various tip-sample temperatures and scanning speeds yield several thermophysical quantities: the PEO melting temperature of 54 ± 12 °C, the PS glass transition temperature of 54 ± 12 °C, the PS-b-PEO specific heat of 3.6 ± 2.7 J g^-1 K^-1, the PEO melting enthalpy of 111 ± 88 J g^-1, and the free energy of Helmholtz for PEO unfolding (and melting) of 10^-20 J nm ^-2. These quantities are obtained for PS-b-PEO volumes of 30 000 nm³, which correspond to 30 ag of the polymer.