Specific interactions and miscibility are demonstrated in a series of binary miscible blend comprising of bio-compatible/biodegradable polyesters, such as poly(ε-caprolactone) (PCL), poly(ethylene adipate) (PEA), or poly (butylene adipate) (PBA), and a macromolecular ester with polyphenol groups, tannic acid (TA). Thermal analysis and infrared spectroscopy were used for proving existence of favorable interactions, and polarized-light optical microscopy was used for characterizing the changes in crystal growth. The appearance of a single composition-dependent glass transition temperature (Tg) observed by differential scanning calorimetry (DSC) indicated that TA is miscible with PCL, PBA, and PEA, respectively, over the entire range of compositions. Fourier transform infrared (FTIR) spectroscopy confirmed the presence of specific intermolecular hydrogen bonding interactions between the carbonyl groups of polyesters and the phenolic hydroxyl groups of TA. The blend Tg's generally exhibited various extents of positive-then-negative deviation from linearity with the compositions. The Tg-composition relationships for three blend systems could all be fitted by the Kwei equation with large negative q values of -80 to -110 for different polyesters. Significant effects by TA on the spherulitic crystallization growth in the polyester/TA blends were also discussed to support the miscibility and strong interactions. Overall, the behavior of blends of polyesters with TA is similar to that of blends of polyesters with poly(vinyl p-phenol) (PVPh) that have been more widely studied and reported. However, TA is naturally bio-resourceful, bio-compatible, and bio-degradable but PVPh is not. Synergism of miscibility, natural bio-compatibility, and biodegradability in these blends by introducing naturally biodegradable macromolecules such as TA may offer greater potential in intended applications.
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