Synthesis and characterization of pullulan alkyl esters

Fully substituted pullulan (PL) esters with carbon number of acyl group (n) of 2-14 were prepared in carboxylic acid/trifluoroacetic anhydride (TFAA) system. Thermal properties of PL esters were analyzed by differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA). The decomposition temperatures of the PL esters were higher than that of neat PL DSC measurements revealed that the PL esters did not exhibit melting peaks, indicating that PL esters were amorphous. The glass transition temperatures (T(g)s) of PL esters increased from 35 degrees C for PL myristate (PLMy, n = 14) to 174 degrees C for PL acetate (PLAc, n = 2) as the chain length decreased. WAXD diffractogram of the PL esters showed the reflection at small-angle region, indicating alignment of the PL main-chain caused by side-chains which are extended perpendicular to the main-chain. Colorless and transparent PL ester films were obtained by solvent casting and melt-pressing method. DMA analyses indicated a beta-relaxation of alkyl side-chains below room temperature. Tensile strength of the PL ester films decreased with increase in alkyl carbons from 23.4 MPa for PLAc to 1.4 MPa for PLMy. The elongation at break of the PL esters increased with increase in alkyl carbons from 8% for PLAc to 1090% for PLMy. The PL esters exhibited extremely high elongation at break. PL esters could be electrospun into nanofibers. (C) 2015 Elsevier Ltd. All rights reserved.