Surface effects on phase transition behaviour and thermal polymerisation of long-chain compounds in micro DSC measurements

Abstract

The surface effects of the aluminium sample pan on the melting behaviour of stearic acid and also on the phase transition behaviour and thermal polymerisation of N-octadecyI acrylamide during differential scanning calorimetry (DSC) were investigated. The surface effects in the micro DSC measurements can be classified into two types. In the case of stearic acid, when the weight of sample was smaller than 0.01 mg, the molar heat of fusion estimated from the peak area in the first heating curves increased abruptly up to two or three times the normal value (64.3 kJ mol −1); however, in the second heating curves obtained with samples stored for a long time after the first heating at room temperature, the molar heat of fusion tends to decrease. In contrast, in the DSC curves for N-octadecyl acrylamide with sample weights of around a milligram, an endothermic peak for melting appeared at 75°C, followed by a broad exothermic peak for thermal polymerisation ranging from 115 to 155°C; when the sample weight is smaller than a critical value of around 0.2 mg, the thermal polymerisation and also the polymorphic transition are suppressed strongly. The surface effects are attributed to the strong interaction, such as chemisorption, of the polar head group with the surface of the pan which restricts the conformational freedom of the functional groups.