Local Thermomechanical Analysis of a Microphase-Separated Thin Lamellar PS-b-PEO Film

Abstract

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 nm(3), which correspond to 30 ag of the polymer.

Velocity dependence of nano-abrasive wear of amorphous polymers obtained using a spiral scan pattern

“…Finally, we have also observed the formation of ripples inside all spiral tracks, resulting from the stick-slip mechanism mentioned above [19]. An example is given in Fig.…”

“…In order to interpret our results we first notice that stationary heat flow below by the cantilever reaches a critical value and the tip suddenly jumps in a stick-slip fashion [19].…”

Surface rippling induced by periodic instabilities on a polymer surface

“…For an overview on these and related processes, the reader is referred to a recent review by Li et al [4]. On the nanoscale, the formation of wavy ripple patterns on compliant surfaces has been frequently reported in atomic force microscopy (AFM) measurements in contact mode [5][6][7][8][9][10][11][12][13][14][15][16][17]. In this case, the ripple periodicity λ is found to be slightly larger than the size of the tip apex, and strongly influenced by external conditions such as normal force, scan velocity and temperature [13,[17][18][19], while none of the two aforementioned macroscopic processes has been observed.…”

“…Elkaakour et al assumed that the surface rippling of polymers is caused by a peeling process in which the material is pushed ahead of the contact by crack propagation [6]. This hypothesis was recently revisited by Rice et al [17], who ruled out the presence of peeling effects on copolymers locally heated and scraped by a dedicated 'hot tip' AFM. Filippov et al reproduced various ripple patterns on ionic crystals assuming that the substrate is removed atom-by-atom and randomly displaced aside at an angle depending on the tip shape [20].…”

Heterogeneity of spiral wear patterns produced by local heating on amorphous polymers

“…Thermal stabilization of the polymers has been showed to occur at the time scales of less than 1 µs and AFM tip-polymer contact radii are of the order of only tens of nanometers below the T g [23]. At such conditions, plastic deformations and/or viscous matter flow do not affect the consecutive spiral windings.…”