Effects of molecular weight on poly(ω-pentadecalactone) mechanical and thermal properties

A series of poly(ω-pentadecalactone) (PPDL) samples, synthesized by lipase catalysis, were prepared by systematic variation of reaction time and water content. These samples possessed weight-average molecular weights (M_w), determined by multi-angle laser light scattering (MALLS), from 2.5 × 10⁴ to 48.1 × 10⁴. Cold-drawing tensile tests at room temperature of PPDL samples with M_w between 4.5 × 10⁴ and 8.1 × 10⁴ showed a brittle-to-ductile transition. For PPDL with M_w of 8.1 × 10⁴, inter-fibrillar slippage dominates during deformation until fracture. Increasing M_w above 18.9 × 10⁴ resulted in enhanced entanglement network strength and strain-hardening. The high M_w samples also exhibited tough properties with elongation at break about 650% and tensile strength about 60.8 MPa, comparable to linear high density polyethylene (HDPE). Relationships among molecular weight, Young's modulus, stress, strain at yield, melting and crystallization enthalpy (by differential scanning calorimetry, DSC) and crystallinity (from wide-angle X-ray diffraction, WAXD) were correlated for PPDL samples. Similarities and differences of linear HDPE and PPDL molecular weight dependence on their mechanical and thermal properties were also compared.