Polymorphic Crystallization and Melting−Recrystallization Behavior of Poly(3-hydroxypropionate)

Using wide-angle X-ray diffraction, high-resolution solid-state 13 C NMR (SNMR), attenuated total reflection infrared (ATR/IR), and Fourier transform infrared (FTIR) spectrometry, we investigated the crystalline structure and morphology of melt-crystallized poly(3-hydroxypropionate)s (PHPs). The melt-crystallization of the β-form crystal and that of the γ-form crystal are found to be favored respectively thermodynamically at higher temperature and kinetically at lower one. In addition, the PHPs with higher molecular weight more readily crystallize into the γ-form crystal. In this way, the polymorphism leads to the bimodal growth behavior for PHP spherulites, attending by a discontinuously temperature-dependent morphology phenomenon. PHP chains take on a similar conformation (the trans) in both type crystals but are packed with different efficiency. In the γ-form, the tighter packing of the atoms along the a axis make the adjacent chains strongly interact with each other, resulting in the crystal field splitting. The corresponding splitting of IR absorption distinguishes the γ-form from the β-form, which endows us with the capability to in-situ monitor the structure evolution during the crystallization and melting. In the crystallization, PHP chains are always crystallized into the γ- and β-form crystal at a constant ratio, which value depends on the temperature. In the melting, only the γ-form crystal can be reorganized via the melting−recrystallization, resulting in the enrichment of the γ-from in the recrystallized phase and the split melting of the two forms for the γ-rich sample.