Assessment of nicotinamide polymorphs by differential scanning calorimetry

The thermal behaviour of nicotinamide was investigated by differential scanning calorimetry (DSC). Nicotinamide was heated to 140°C, cooled and subsequently re-heated using DSC. Nicotinamide displayed four polymorphs. The melting points of Forms I, II, III and IV were 126–128, 112–117, 107–111 and 101–103°C, respectively. Untreated nicotinamide composed only of Form I. When molten and subsequently cooled, a mixture of polymorphs recrystallised. When the molten liquid was cooled to temperatures <80°C, the main product was Form I. In these scans, conversion from the metastable forms to the stable one without accompanying melting was observed at low re-heating rates. At higher rates, the melting of the metastable form became more obvious and it occurred concomitantly with recrystallisation of the stable form. When molten samples were cooled to 80 or 85°C and held isothermally, Form II was predominantly formed and the formation of Form I could not be detected. Form II was converted to Form I by cooling to 30°C. Form I was the stable form; others were metastable.