Polypentadecalactone prepared by lipase catalysis: Crystallization kinetics and morphology

Background: This work addresses the need to better understand the crystallization kinetics and morphology of poly (ω-pentadecalactone) (PPDL). This polyester has promising mechanical properties and a unique structure that resembles that of polyethylene. PPDL is a member of the poly(ω-hydroxy fatty acid) family, which can be derived from biobased feedstocks. Results: PPDL (M_n = 34 000 g mol^-1 and dispersity D = M_w/M_n = 2.7) was synthesized using enzyme catalysis. Equilibrium melting enthalpy and equilibrium melting point were determined using extrapolation techniques, being 227 J g^-1 and 101 °C, respectively. In addition, the equilibrium melting point (T_m⁰) was found to be 109.3 °C by the nonlinear Hoffman-Weeks plot. For T_m ⁰ = 101.0 °C, the lateral surface free energy (σ), fold surface free energy (σ_e) and foldwork(q) are 10.4 erg cm^-2, 47.5 erg cm^-2 and 2.6 kcal mol^-1, respectively; while for T_m ⁰ = 109.3 °C, they are 25.1 erg cm^-2, 46.6 erg cm^-2 and 2.6 kcal mol^-1, respectively. The results indicated the existence of a regime I to regime II transition during crystallization at about 80 °C. Polarized optical microscopy and AFM provided further evidence for the regime I-II transition. In regime I, coarse spherulites were formed through splaying out and occasional branching of lamellae, as well as stacking of lamellae through screw dislocation. In contrast, in regime II, banded spherulites were formed through crystal twisting. Conclusion: Morphological changes in PPDL at spherulitic and lamellar levels in regimes I and II were confirmed by differential scanning calorimetry, POM and AFM.