Characterisation of glass transition temperature of polymeric materials using an AFM Scanning Thermal Microscopy (SThM) probe technique

A. Dawson1, M. Rides1, A. Cuenat1 and L. Winkless1

1National Physical Laboratory, U.K.

Keywords: micro
property: glass transition temperature
material: polymer

This study describes the measurement of the glass transition behaviour of polymers on the micro-scale using an atomic force microscope (AFM). Micro-moulded polymeric parts and polymeric thin films are of increasing importance in the manufacture of miniaturized devices for applications such as drug delivery systems, cellular communications and as coatings on medical implants. Poor mixing of additives and non-homogeneities within materials need to be detected on this scale to improve product quality and reliability and to reduce wastage. The scanning thermal microscopy (SThM) probe technique is based on a thermally active probe mounted on an AFM. Heating of the specimen can be achieved either by using a temperature-controlled hot stage on which the specimen is mounted, or by passing a current through the resistive tip of the SThM probe thereby heating the specimen locally. The SThM probe can be used as a resistance thermometer to measure the temperature of the specimen's surface whilst, simultaneously, operated in force-distance mode to obtain thermal and mechanical properties data for the specimen. The glass transition temperature behaviours of poly(methyl methacrylate) and poly(vinyl acetate) films were investigated by analyzing the gradients of the approach force-distance curves, obtained over a range of temperatures where the specimens were heated using the hot stage. Also, pull off force, snap in force and adhesion energy data were obtained from the approach and retract force-distance curves. The SThM probe was also used as a localized heating source for further force-distance measurements on the poly(methyl methacrylate) and poly(vinyl acetate) films. The SThM results show good agreement with differential scanning calorimetry results for the transition of the lower Tg poly(vinyl acetate) (» 43 °C), whereas the agreement was not as good for the higher Tg poly(methyl methacrylate) (» 111 °C), presumed to be due to heat losses from the probe tip. Initial results also indicate that the observed onset of changes in mechanical properties at the glass transition temperature was dependent on the mechanical property in question, with pull-off values changing before snap-in force and adhesion energy. Further investigation is necessary to elucidate these observations. The SThM-AFM is a powerful technique enabling access to micro-scale thermomechanical properties of polymers.

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