Polymorphic crystallization and melting-recrystallization behavior of poly(3-hydroxypropionate)

Using wide-angle X-ray diffraction, high-resolution solid-state C-13 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 beta-form crystal and that of the gamma-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 gamma-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 gamma-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 gamma-form from the beta-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 gamma- and beta-form crystal at a constant ratio, which value depends on the temperature. In the melting, only the gamma-form crystal can be reorganized via the melting-recrystallization, resulting in the enrichment of the gamma-from in the recrystallized phase and the split melting of the two forms for the gamma-rich sample.