Structural characterization and enzymatic degradation of α-, β-, and γ-crystalline forms for poly(β-propiolactone)

A structural comparison of three different crystalline forms of poly(beta-propiolactone) (PPL) was carried out by wide-angle X-ray diffraction. Fourier transform infrared spectroscopy, and differential scanning calorimetry. The alpha-form in a hot-drawn and annealed film represents a 2(1) helix conformation. The beta-form in a cold-drawn and annealed film represents a planar zigzag conformation. The gamma-form in an oriented sedimented mat of solution-grown chain-folded lamellar crystals also implies a planar zigzag conformation. The solution-cast film depicts similar outlines with the gamma-form in lamellar crystals in all the experimental measurements, suggesting that the molecular chain in the solution-cast film has a planar zigzag conformation. While elongation at break decreased, tensile strength and Young's modulus increased with an increase in the crystallinity, independent of the crystalline forms. The influence of the enzymatic degradation of these crystal structures has been investigated by using an extracellular PHB depolymerase purified from Ralstonia pickettii T1. The rate of degradation was in the order of beta-form > alpha-form > solution-cast (gamma-form) film, and the different surface morphologies after partial enzymatic degradation were observed in scanning electron micrographs. It is suggested that the crystal structure is one of the important factors for determining the rate of degradation together with crystallinity.