In situ AFM observation of polymorphic transition of caffeine form I (stable at high temperatures) to form II (stable at room temperature) was conducted to obtain fundamental information on the mechanism with which the solid state transition progresses. From the observation of formation of projections of a new phase (form II) followed by their growth inward, the crystalline body of form I was found. On the basis of this observation, the kinetics was analyzed by the penetration model proposed by Matsuo and Matsuoka,~(1) and the transition was found to be well expressed by the model.
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