Molecular mobility investigation of the biobased Poly(ethylene vanillate) and Poly(propylene vanillate)

Abstract

In this work, the molecular mobility of two relatively new biobased polyesters poly(ethylene vanillate) (PEV) and poly(propylene vanillate) (PPV) is investigate. The importance of these polymers lays on their renewable character, a desired property of today's global needs, along with the fact that are derived from vanillin, trough vanillic acid, a product of lignin. The latter is the second most abundant polymer on earth. Broadband dielectric spectroscopy (BDS) supplemented by differential scanning calorimetry (DSC, TMDSC), polarized light microscopy (PLM) and nuclear magnetic resonance are the main investigation tools. The molecular mobility mapping, for both local and segmental dynamics of PEV and PPV, is shown here for the first time. Beginning with calorimetry, in the amorphous state, the glass transition temperature, Tg, of PEV and PPV was found 69 and 62 °C, respectively, while the corresponding heat capacity change was found 0.54 and 0.47 J/g∙K. A variety of relaxation processes were recorded in BDS, namely the local γ, β1 and β2, the segmental α relaxation, the dielectric analogue of glass transition, and additional processes revealed by the employed analysis. We propose possible molecular origins for the local γ, β1 and β2 relaxations, namely, on side groups rotation and crankshaft motions on the backbone. The latter is suggested by the recorded changes in time scale at specific temperature regions, for example, around Tg for β2, which seems to be a common characteristic between some polyesters. Segmental α relaxation was found to be in accordance with the calorimetric Tg, i.e., faster for PPV, while the dielectric Tgs were found 64 °C and 59 °C, for PEV and PPV, respectively. In the amorphous state, α is less fragile and slightly stronger in PEV, whereas its time-scale is less affected by cold crystallization, in contrast to that of PPV. From the overall results, indications for differences in the interchain distances and chain-chain interactions, along with the effects by the chain length were revealed. These parameters were correlated with the retarded crystals nucleation in the case of the short PEV chains as compared to PPV.