Poly(propylene vanillate): A Sustainable Lignin-Based Semicrystalline Engineering Polyester

Vanillic acid that can be prepared from lignin through vanillin is a relatively new and most promising aromatic monomer for the synthesis of biobased polyesters. Due to its structural similarity to terephthalic acid, vanillic acid-containing polymers have been reported to exhibit comparable properties to poly(ethylene terephthalate). Herein, the nanomechanical and thermal properties and crystallization behavior of poly(propylene vanillate) (PPV), a new alipharomatic biobased polyester synthesized from 4-(2-hydroxypropoxy)-3-methoxybenzoic acid, are reported. As the phase transitions dominate the processing of polymers and determine the processing temperature window, the melting, crystallization, and glass transition temperatures were determined, and the thermodynamics and kinetics of phase changes were studied. The enthalpy of fusion was estimated (ΔHm0 = 162 J/g), and the equilibrium melting temperature (Tm0 = 220.5 °C) was calculated using the Hoffman–Weeks method. The multiple melting behavior of PPV was interpreted based on the melting-recrystallization model. Isothermal crystallization was studied. Since processing is always dynamic in industries, the non-isothermal crystallization profiles from the melt or from glass were investigated; differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), and polarized light microscopy (PLM) were elaborated. Finally, as the mechanical properties of aromatic polymers are of crucial importance for applications, the mechanical behavior of amorphous and annealed PPV samples was examined via nanoindentation.