We
present experimental evidence that the transformation from one
solid phase to another solid phase within a faceted polymer single
crystal passes through a transient mesomorphic phase. This phase exists
in the boundary zone separating the two crystalline phases of isotactic
polybutene-1 (PB-1), i.e., Form II and Form I of
PB-1. Employing real-time atomic force microscopy in tapping mode
(TM-AFM), we followed the crystal–crystal transformation in
time. TM-AFM measurements performed at about 20 °C under a nitrogen
atmosphere with high humidity have allowed through differences in
viscoelastic properties and associated changes in thickness to distinguish
the two crystalline phases as well as the boundary zone. Measurements
at an elevated temperature of 90 °C (which was far below the
melting temperatures of both Form II and Form I crystals) further
revealed that this boundary zone shows a transient and diffuse character
and a distinctly increased thickness. Using high-resolution phase
imaging of TM-AFM, we observed that within this rather extended boundary
zone of up to some hundred nanometers at elevated temperatures, Form
I crystals with a diameter of around 30 nm were growing in a softer
and featureless matrix phase. The boundary zone and so the growth
front of the Form I crystal advanced at an almost constant rate, suggesting
that chains in the mesomorphic phase were considerably mobile. Our
observation of an extended boundary zone probably indicates that compared
to a direct crystal–crystal transformation, the corresponding
free energy barrier for this transformation is lower within such a
zone. Hence, our here presented model study sheds light on the poorly
understood and complex pathways involved in a solid–solid transformation
within a polymer single crystal.