Nanoscale Viscoelastic Behavior of the Surface of Thick Polystyrene Films as a Function of Temperature

Abstract

The variation in the glass transition temperature of thin polymeric films represents a phenomenon yet to be fully explained. To date, it is widely agreed that it is linked to the interfaces that the film forms with the air and the supporting substrate. Herein, we address one of the main issues regarding the viscoelastic behavior of the region near the free interface of thick polystyrene films where a reduction in the glass transition temperature is expected to occur. We have measured, as a function of the temperature, the elastic and viscous responses of polystyrene films with molecular weights above and below the critical value for the occurrence of molecular entanglement. The experiments have been carried out by means of scanning probe microscopy in a configuration combining the acquisition of force versus distance and indentation versus time curves. We show that the viscoelastic behavior of polystyrene films with thickness down to 30 nm can be successfully evaluated in the time scale from tenths to tens of seconds. In particular, we observe that the viscoelastic behavior of thick film surfaces has a similar dependence from the temperature as the viscoelastic behavior of the bulk, independently if the molecular weight is above or below the critical value. We estimate that the region at the free interface with a reduced glass transition temperature, if present, has a thickness below 3 nm.

Use of polystyrene brushes to investigate the role of interface between substrates and thin homogeneous films

“…As shown in a previous publication, z increases immediately after the feedback is switched on and flattens out at a value of around 25 nm, depending on the history of the applied voltage to the piezoactuator. This is a typical creep effect and can be evaluated on a rigid sample such as a piece of silicon wafer and its contribution then considered in the data analysis: it has the opposite sign from the tip penetration.…”

“…In a recent article, we have addressed the issue regarding the decrease in T g at the air interface and the depth of this region . Our approach was based on a direct measurement of the surface indentation as a function of T .…”

“…Previously, the viscoelastic properties of films have been measured using various techniques, but mainly indirectly . We use the same setup above mentioned for the measurements of surface indentation, which is based on an atomic force microscope and a custom‐developed lithographic software capable of controlling the relative position of the tip ( z ) with respect to the sample surface while fixing parameters such as applied load ( F N ), temperature ( T ), and time of contact (Δ t ) . As the prefixed deflection value is reached, the feedback circuit is switched on in order to maintain F N constant while monitoring the tip penetration and the cantilever torsion ( ϕ ) during a given Δ t (Fig.…”

“…Quantitative data on the viscosity of the film are beyond reach due to the complexity in modeling the tip/surface system. However, via measuring the occurrence of viscoplastic deformation as a function of temperature, one can infer the T g value of the film under investigation . In particular, we have focused our study to films of a thickness value low enough (30 nm) so that when the films are spin‐coated on SiO x , the T g measured is below the bulk value ( ).…”

Interfacial and Annealing Effects on Primary α-Relaxation of Ultrathin Polymer Films Investigated at Nanoscale

“…1−3 It is suggested that at the free surface of a polymer film a layer of few nanometers of thickness exists, 1,2,4 in which chain motions are faster than in the bulk system. Moreover, depending on the degree of the polymer/substrate interaction, the mobility of polymer chains at the interface with the substrate can be increased, decreased, or remain the same as in the bulk.…”