Identifying a Mesomorphic Boundary Zone within Polybutene-1 Single Crystals Emerging During a Crystal–Crystal Transformation

Abstract

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.

Effects of nucleating agents on the II–I transition of iPB-1

“…Furthermore, in one of our previous works, 37 a medium packed-mesophase between forms II and I has been proposed. Zhang et al 61 distinguished a mesomorphic boundary zone within the iPB-1 single crystal emerging during the II–I transition via differences in viscoelastic properties and associated changes in thickness by employing real-time atomic force microscopy (AFM) in a tapping mode. The existence of the observed mesomorphic boundary zone between forms II and I regions in the iPB-1 single crystal not only excludes a direct solid–solid transition from form II to form I, but also provides evidence for the existence of the medium packed-mesophase state between forms II and I.…”

Utilizing 3-methyl-1-butene co-units to tailor phase transition behavior in butene-1 copolymers

“…7,8,13 Thus, to effectively regulate the mechanical properties, the study of the phase transition process is attracting more and more attention. 15–25 Over the past decades, various methods have been reported to tailor the phase transition process. 15,25–46 Qiao et al systematically investigated the correlation between temperature and phase transition kinetics.…”