Phase Diagram, Glassy Dynamics and Crystallization Kinetics of the Biobased Polyester Poly(ethylene 2,5-furanoate) (PEF)

We report the pressure-temperature (<i>P</i>-<i>T</i>) phase diagram, the origin of the subglass dynamics, and the crystallization kinetics of the biobased polyester poly(ethylene 2,5-furanoate) (PEF), through dielectric spectroscopy (DS) measurements performed as a function of temperature and pressure. The phase diagram comprises four different "phases"; glass, quenched melt, crystalline, and normal melt. The cold crystallization temperature, <i>T</i> _cc, increases linearly with pressure (according to the Clausius-Clapeyron equation) as <i>dT</i> _cc/<i>dP</i> _| <i>_P</i> _→0 ∼ 240 K·GPa^-1 and is accompanied by a small change in specific volume (Δ<i>V</i> = 0.028 cm³/g). This contrasts with the stronger dependence of the glass temperature, <i>T</i> _<i>g</i> , with a pressure coefficient, <i>dT</i> _<i>g</i> /<i>dP</i> _| <i>_P</i> _→0, of 383 K·GPa^-1, typical of rigid polymers. With the application of pressure, we address the molecular origin of the subglass β-process through the apparent activation volume, a quantity accessible only by pressure experiments. Moreover, increasing pressure densifies the segmental process but blocks the β-process, with possible implications in the gas-barrier properties. The crystallization kinetics from the quenched melt to the cold-crystallized state was explored by thermodynamics (differential scanning calorimetry, DSC), dynamics (DS), and structure (via simultaneous X-ray scattering at small (SAXS) and wide (WAXS) angles) following different routes within the phase diagram. Interestingly, all probes followed the same sigmoidal kinetics (of the Avrami type) with comparable time scales. Inspection of the evolution of the dielectric strength for the different dynamic processes during isothermal crystallization (at <i>T</i> _c = 402 K; <i>P</i> = 0.1 MPa) revealed the absence of the <i>restricted amorphous fraction</i> (RAF) at the early stages of crystallization. This observation is in line with the proposed <i>mesomorphic phase</i>-an intermediate phase formed during crystallization in the absence of chain folding, as suggested by G. Strobl. Subsequent growth of the RAF followed the same Avrami kinetics as identified by the thermodynamic and structural probes. Shallow quenches within the <i>P</i>-<i>T</i> phase diagram identified experimental routes for keeping PEF in the metastable quenched amorphous state for long times.